Temperature-stable core-shell particle compounds

Abstract

The invention relates to temperature-stable core-shell particles for the production of stimuli-responsive structural color materials. During processing, the cores self-assemble in the soft shell matrix, forming a photonic crystal. High temperature (processing) stability is achieved via optimizations of the particle architecture, regarding an optimized core proportion of 40 to 45 vol% and a reduced amount of cross-linker in the interlayer. The invention is further directed to a process for synthesizing said particles in starved-feed emulsion polymerization and to prepare a particle composition comprising the particles and at least one antioxidant. The invention further includes the use of the thereby obtained particle composition in common polymer processing techniques, especially 3D printing with the fused filament fabrication technique. The use of the invented particles and particle composition allows for preparing geometrically complex objects and detailed 3D structures with structural colors.

Description

Temperature-Stable Core-Shell Particle CompoundsCross-reference to related applications

[0001] This application claims priority to European Application No. EP23210135 which is incorporated by reference herein in its entirety.TECHNICAL FIELD OF THE INVENTION

[0002] The invention relates to temperature-stable core-shell particles for the production of stimuli-responsive structural color materials. During processing, the cores self-assemble in the soft shell matrix, forming a photonic crystal. High temperature (processing) stability is achieved via optimizations of the particle architecture, regarding an optimized core proportion of 40-45 vol% and a reduced number of cross-linking sites in the interlayer. The invention is further directed to a process for synthesizing said particles in starved-feed emulsion polymerization and for preparing a particle composition, comprising said particles and at least one antioxidant. The invention further includes the use of said particle composition in common polymer processing processes, especially 3D printing with the fused filament fabrication technique. The use of inventive particles and particle composition allows for the preparation of geometrically complex objects and detailed 3D structures with structural colors.BACKGROUND ART

[0003] Natural systems and organisms such as opal gemstones, peacocks, or chameleons are known for their vivid and adaptable color. In contrast to common pigments or dies, which selectively absorb certain wavelengths of the visible spectrum, these structural colors are caused by interference of visible light with periodic structures on the nanoscale. Common colorants often contain potentially harmful ingredients and fade over time. Structural colors, on the other hand, are completely harmless and not prone to photobleaching.1'3Potential application fields and research interests include anti-counterfeiting materials, display and monitoring technologies, smart sensing systems, optical coatings, and shape-memory materials.4'7

[0004] Despite tremendous efforts in reproducing structural color synthetically, as of today large- scale production in terms of bulk material fabrication with complex 3D geometry is rare.8'12Bottom-up approaches, based on the self-assembly of building blocks on the nanoscale, represent one of the most promising and feasible techniques to create the required periodic structures at submicron length scale. Self-assembly approaches are further advantageous due to their low-cost building materials, convenient fabrication processes, and ease of scaling up.13'16Within the field of colloidal self-assembly, core-shell-particles (CSP) are considered one of the most promising class of materials, due to their scalable synthesis, tuneable mechano-rheological properties, as well as for the tailorable and stimuli-responsive structural color of derived materials.Moreover, the color of materials based on CSP does not suffer from internal multiple scattering or interfering resonances of individual particles.10’17'22

[0005] Polymeric CSP are usually prepared via stepwise starved-feed emulsion polymerization protocols. Obtained pure organic or hybrid particles consist of a rigid core surrounded by an immobilized shell. The core usually consists of cross-linked polystyrene, whereas the soft elastomeric shell is usually based on polyalkylacrylates. Usually, ethyl acrylate is used as bulk shell polymer, copolymerized with a low proportion of one or more functional comonomers.21’23'25During processing, the non-crosslinked shell polymer forms a continuous matrix around the dimensionally stable core particles. Driven by shear forces, the core particles self-assemble into long-range-ordered periodic lattices, which gives rise to iridescent structural color. Thus, obtained highly ordered systems are referred to as colloidal photonic crystals.14’26’27

[0006] Photonic crystals consisting of crystalline ordered CSP selectively scatter light, due to periodical changes in the refractive index between core and shell, which lead to a photonic bandgap. The reflected wavelength can be calculated with the Bragg-Snell-Law. The lattice constant in Bragg’s Law, and thus the reflected wavelength, is directly proportional to the particle diameter. By adjusting the particle diameter during synthesis, every color within the visible spectrum (and beyond) can be achieved. Ultraviolet or infrared light can also be reflected, if the particles feature a very small or large diameter, respectively. The reflected wavelength is further dependent on the viewing angle and direction of light incidence. This correlation causes the typical brilliant and iridescent appearance of structural colors, as known e.g. from natural opal gemstones. Reflection intensity increases with the level of colloidal order.15’28'30

[0007] Small amounts of carbon black are usually added to the dried particle mass. If homogenously incorporated, carbon black improves structural color quality by absorbing diffuse scattered light.1361

[0008] Macroscopic mechanical deformation of materials based on ordered CSP leads to a microscopic change in the lattice constant, which results in a color change. This phenomenon is known as mechanochromism and makes the materials intrinsically suitable as deformation or force sensors. Besides mechanic load, these materials can also respond to several other triggers from the surrounding environment, if stimuli-responsive copolymers are incorporated into the particle shell. Feasible stimuli include but are not limited to pH value, electric or magnetic fields, solvents, and temperature. This stimuli-responsive behaviour not only enables smart sensing, but also the dynamic switching of coloration172331-33

[0009] This invention aims to broaden the processing options for polymeric materials based on the promising core-shell architecture. Thereby, the production of stimuli-responsive structural color materials with complex 3D geometries via industrially suitable polymer processing techniques shall be made possible, which is, to the best of our knowledge, not possible as of today - neither with CSP nor with other building blocks. State-of-the-art processing techniques for CSP have recently been reviewed by several authors and are briefly summarized in the following.8’21’26’33'37Most common methods like spin-coating or drying techniques rely on a substrate and a dispersion medium. These methods yield highly ordered crystalline structures but cannot produce self-supporting bulk objects. Free-standing films with diameters in the cm range are accessible with the melt-shear organization technique. Opal foils with lengths of several meters can be produced with the bending-induced oscillatory shearing process or similarcalendaring processing. The melt-shearing process and the bending bending-induced oscillatory shearing process were specially developed for CSP. Both processes are scalable and in principle industrially suitable but inherently limited to plain 2D geometries in the form of thin films and foils. Common processing temperatures of both processes are in the range of 60-120 °c,21’26’37-39occasionally up to 140-150 °C.40'45

[0010] Most recently, CSP have also been 3D-printed.40This printing process is best characterized as Direct-Ink-Writing (DIW), which is distinctive to 3D printing with Fused Filament Fabrication (FFF), introduced in the scope of this invention. However, the previously used 3D printer is a custom-made machine, which makes scale-up beyond lab scale difficult to impossible. Moreover, the build volume and object size are restricted to a few grams of material, making it hardly suitable for large-scale production without further adaptation. The therein-developed particles were printed at elevated temperatures of 140 °C; but it was also shown that they were not processable above 180 °C, due to thermal degradation effects. As 180 °C is the lowest temperature that is commonly used in FFF, they are not suitable for this 3D printing technique;46-48in contrast to the particle composition of this invention, which is temperature stable up to at least 250 °C.

[0011] So far CSP have never been processed with established industrial polymer processing techniques, such as profile extrusion, melt casting, rotational molding, blow molding or parison forming, embossing, or FFF and other commercialized 3D printing techniques. All these manufacturing techniques require a top-down approach where a particular macroscopic shape is given to the material. To produce structural color materials, this must be combined with the bottom-up self-assembly of the CSP.10In principle, state-of-the-art particles meet the rheological processing prerequisites and any type of directional shearing should lead to a self-assembly of the particles, as the colloidal crystalline order is thermodynamically favorable.3749-52Despite this potential, as of today large-scale processing options are highly restricted. The presumable reason lies in the poor thermal stability of state-of-the-art CSP, which is too low for the economic and industrial-grade use of the above-described polymer processing techniques.

[0012] Thus, there is a need for CSP with high-temperature stability to enable the manufacturing of geometrically complex bulk objects featuring stimuli-responsive structural color with industrial- suitable polymer processing techniques.SUMMARY OF THE INVENTION

[0013] The present invention relates to particles comprising a) 35 to 50 vol%, preferably 40-45 vol% based on the overall volume of the particle, of a core, comprising a polymer (P1) prepared by radical polymerization; b) 2 to 10 vol%, preferably 4-5 vol%, more preferably 5 vol% based on the overall volume of the particle, of an interlayer comprising a polymer (P2) prepared by radical polymerization, wherein the polymer (P2) comprises 3 to 7 wt%, preferably 3 to 5%, more preferably 5 wt% of a cross-linker based on the overall weight of the polymer (P2); and c) 45 to 60 vol%, preferably 50 to 55 vol% based on the overall volume of the particle a shell, comprising a polymer (P3) prepared by radical polymerization.

[0014] Preferably,(P1) is poly(styrene-co-butanediol diacrylate);(P2) is poly(methyl methacrylate-co-allyl methacrylate);(P3) is poly(ethyl acrylate) or poly(ethyl acrylate-co-methacrylate).

[0015] The invention is further directed to a polymer composition, comprising invented particles, carbon black and / or at least one radical scavenger and / or antioxidant.

[0016] The invention is further directed to the use of the particles or the particle composition in a process of polymer processing, wherein optionally the process comprises at least one process step selected from the group consisting of (co- and / or profile-)extrusion, compounding, direct-ink- writing, fused filament fabrication, uniaxial pressing, embossing, compression molding, calendaring, parison forming, rotational molding, blow molding, injection molding, deep drawing, surface coating; processing preferably involves the preparation of a filament for 3D printing via an extrusion process and / or 3D printing of said filament.

[0017] Moreover, the invention comprises a process for the preparation of invented particles, comprising carrying out at least one of the following steps a) to c), preferably stepwise from a) to c), resulting in a particle comprising a core, an interlayer, and a shell: a) radical emulsion polymerization of a monomer solution (MSa) consisting of one or more monomers containing a terminal alkene group, preferably styrene, and optionally a cross-linker, preferably butanediol diacrylate, resulting in the core polymer (P1) of the particle; b) further radical emulsion polymerization of a monomer solution (MSb) consisting of one or more monomers containing at least one terminal alkene group, preferably methyl methacrylate, and 3- 7 phr, preferably 5 phr of a cross-linker, which is allyl methacrylate, adding the interlayer polymer (P2) to the particle; and c) further radical emulsion polymerization of a monomer solution (MSc) consisting of one or more monomers containing at least one terminal alkene group, preferably ethyl acrylate and / or methyl methacrylate, adding the shell polymer (P3) to the particle.

[0018] The present invention allows for the manufacturing of geometrically complex objects based on core-shell particles with stimuli-responsive, tuneable, and iridescent structural colors. Conventional large-scale polymer processing techniques are not suitable for state-of-the-art coreshell particles since their temperature stability is usually too low for this purpose. State-of-the-art particles usually become increasingly brittle and non-processable at temperatures above 165 °C, due to post cross-linking reactions of unconsumed cross-linking sites (see also example 1.1 -1.3). In contrast, the particle-based polymer compounds of the present invention tolerate much higher temperatures up to 250 °C (example 4 and 5) since the undesired cross-linking reaction is efficiently suppressed via optimizations of the particle architecture (examples 2 and 3) and / or incorporation of antioxidants (example 1.4). Among other polymer processing techniques, this enables the production of filament and subsequent 3D printing of said filament on commercial 3D printers (example 5). The increased temperature stability not only allows for the application of new large-scale industrial methods but is also advantageous for established processing methods, including but not limited to extrusion, direct ink-writing, and the melt-shear organization technique (example 4). Besides the increased temperature stability, the invention also includes an embodiment, wherein the glass transition of the particle shell is above room temperature. Theseparticles show thermoplastic material properties, which is another advantage for filament production and subsequent 3D printing. State-of-the-art core-shell particles usually feature elastomeric shell polymers with glass transitions below room temperature, making thereof produced filaments too soft for 3D printing with the fused filament fabrication technique. The use of the invented particles and particle composition in particular allows for preparing detailed objects and geometrically complex structures with structural colors.BRIEF DESCRIPTION OF THE FIGURES

[0019] Fig. 1 : a) DLS data of particle batch CSP1 for the three individual reaction steps, b) TEM photograph of the core particles C1 . c) TEM photograph of the final CSP1.

[0020] Fig. 2: Analysis of the temperature stability of CSP1. The left column shows temperature sweeps, while the right column shows data obtained from isothermal measurements at 190 °C for 120 minutes, (a) Temperature-dependent rheological examination under ambient atmosphere. In a temperature range from 35 °C to 165 °C measured viscosities were fitted to an Arrhenius- Law, allowing for predicting the estimated course at higher temperatures without a thermally induced cross-linking reaction, (b) Isothermal rheological examination under ambient atmosphere. Therein the estimated viscosity is sketched as a horizontal dashed line, starting from the minimum viscosity after heating to 190 °C was finished, (c) Temperature-dependent DSC thermogram, wherein glass transition temperatures are indicated with a circle, (d) Isothermal DSC thermogram, wherein the base signal without any endo- or exothermic process is indicated as a dashed line, (e-f) Temperature-dependent and isothermal TGA.

[0021] Fig. 3: Processing of CSP1 at unusually high temperatures, showing severe thermally induced damage. At temperatures > 165 °C first deteriorations of surface quality could be observed, as demonstrated in the pictures. At 180 °C for extrusion and DIW and at 200 °C for melt-shearin, the material became essentially impossible to process.

[0022] Fig. 4: Analysis of the influence of antioxidative stabilizers on temperature stability. CSP1 represents the original batch, without additives, (a) Temperature-dependent rheological examinations in ambient atmosphere in terms of dynamic complex viscosities. In a temperature range from 35 °C to 165 °C measured viscosities of CSP1 were fitted to Arrhenius-Law and extrapolated for temperatures > 165 °C to show the expected viscosity without a thermally induced cross-linking reaction, (b) Isothermal rheological examinations in ambient atmosphere. The minimum viscosity after heating to 190 °C is sketched as a horizontal dotted line for each measurement, (c-d) Temperature-dependent and isothermal DSC measurements in synthetic air, wherein the base signals are represented by dotted lines. An additional measurement for CSP1 was conducted under inert nitrogen atmosphere (CSP1 in N2) (e) UV-Vis reflection spectra of opal films made from the respective CSP via application of the melt-shear organization technique.

[0023] Fig. 5: DLS analysis of particles dispersed in water for the test series regarding core-to- shell ratio in example 2. The same batch of core particles C2 was used for all batches, which only differ in the amount of shell polymer.

[0024] Fig. 6: TEM photographs of the dry particles for the test series regarding core-to-shell ratio in example 2. The same batch of core particles C2, shown in picture (a), was used for all batches,which only differ in the amount of shell polymer and are shown in pictures (b) to (e). A figure legend is provided in the right bottom slide.

[0025] Fig. 7: Analysis of the temperature stability of CSP with different core-to-shell ratio for example 2. The core proportion according to TEM measurements (Figure 6) is depicted in brackets behind the respective batch, (a) Temperature-dependent rheological examinations in terms of dynamic complex viscosities. In a temperature range from 35 °C to 165 °C measured viscosities were fitted to Arrhenius-Law and extrapolated for temperatures > 165 °C to show the expected viscosity without a thermally induced cross-linking reaction, (b) Isothermal rheological examinations. The minimum viscosity after heating to 190 °C is sketched as a horizontal dotted line for each measurement, (c-d) Temperature-dependent and isothermal DSC measurements, wherein the base signals are represented by dotted lines, (e) Summary of the measurements, that were conducted in part (a-d), sketched against the core proportion. For all four measurements, the area between the expected course and measured values was integrated concerning the exposition time. Lower values indicate less thermally induced cross-linking, thus higher thermal stability. Integrated viscosities and enthalpies are connected with a linear fit, as a guide for the eye to indicate the decrease at lower core proportions, (f) UV-Vis reflection spectra of opal films made via application of the melt-shear technique, (g-h) Temperature-dependent and isothermal TGA.

[0026] Fig. 8: DLS analysis of particles dispersed in water for the test series regarding the concentration of grafting-anchor in example 3. The same batch of core particles C3 was used for all batches, which only differ in the composition of the interlayer.

[0027] Fig. 9: TEM photographs of the dry particles based on the core particles C3 for the test series regarding the concentration of grafting-anchor in example 3. The same batch of core particles C3, shown in picture (a), was used for all batches, which only differ in the composition of the interlayer and are shown in picture (b) to (e). A figure legend is provided in the right bottom slide.

[0028] Fig. 10: Analysis of the temperature stability of CSP with different concentrations of grafting-anchor in the interlayer for example 3. The ALMA content is depicted in brackets behind the respective batch, (a) Temperature-dependent rheological examinations in terms of dynamic complex viscosities. In a temperature range from 35 °C to 165 °C measured viscosities were fitted to Arrhenius-Law and extrapolated for temperatures > 165 °C to show the expected viscosity without thermally induced cross-linking reactions, (b) Isothermal rheological examinations. The estimated viscosity is sketched as a horizontal dotted line for each measurement, starting from the minimum viscosity after heating to 190 °C was finished, (c-d) Temperature-dependent and isothermal DSC measurements, wherein the base signals without any endo- or exothermic process are represented by dotted lines, (e) Summary of the measurements, that were conducted in part (a-d), sketched against the ALMA content. For all four measurements, the area between the expected course and measured values was integrated concerning the exposition time. Lower values indicate less thermally induced cross-linking, thus higher thermal stability. Integrated viscosities and enthalpies are connected with a linear fit, as a guide for the eye to indicate the decrease at lower ALMA content, (f) UV-Vis Reflection spectra of opal films made from the respective CSP via application of the melt-shear technique, (g-h) Temperature-dependent and isothermal TGA.

[0029] Fig. 11 : (a) DLS data of the temperature stable CSP4 for the three individual reaction steps, (b) DLS data for state-of-the-art particle batch CSP4C, which is based on the same coreparticles and only differs in the composition of the interlayer and the core-to-shell-ratio. (c) TEM photograph of the mutual core particles C4. (d) TEM photograph of the final CSP4. (e) TEM photograph of the final CSP4C.

[0030] Fig. 12: Comparison of the thermal stability of the temperature stable particle composition consisting of CSP4 and an antioxidant with the state-of-the-art particle batch CSP4C. (a) Temperature-dependent rheological examinations in terms of dynamic complex viscosities. In a temperature range from 35 °C to 165 °C measured viscosities were fitted to Arrhenius-Law and extrapolated for temperatures > 165 °C to show the expected viscosity without thermally induced cross-linking reactions, (b) Isothermal rheological examinations. The estimated viscosity is sketched as a horizontal dashed line for each measurement, starting from the minimum viscosity after heating to 190 °C was finished, (c-d) Temperature-dependent and isothermal DSC measurements, wherein the base signals without any endo- or exothermic process are represented by dotted lines, (e) Summary of the measurements that were conducted in part (a-d) compared to the reference batch CSP4C. For all four measurements, the area between the expected course and measured values was integrated concerning the exposition time. Lower values indicate less thermally induced cross-linking reaction, thus having a higher thermal stability, (f) UV-Vis reflection spectra of opal films made from the respective CSP via application of the melt-shear technique. Besides CSP4C, another state-of-the-art batch (CSP1 , see example 1) is shown for comparison, (g-h) Temperature-dependent and isothermal TGA.

[0031] Fig. 13: High-temperature processing at 250 °C of the particle composition composed of CSP4 and an antioxidant. The particle composition was extruded into a homogenous strand (cross-section 1x4 mm) and an opal film (diameter » 100 mm) was prepared by application of the melt-shear technique, both showing smooth surfaces and no signs of thermal damage or deterioration. 3D-printed objects, produced with the direct-ink-writing technique, indicate the obtainable grade of object complexity. Peak-sharpness and monomodality of the UV-Vis reflection spectrum indicate the high quality of the obtained orange structural color. The height of the owl is 21 mm; the height of the worm amounts to 12 mm; and the width of the CSP-gears and styrene ellipse is roughly 40 mm.

[0032] Fig. 14: Synthesis scheme for the preparation of CSP5 via semi-continuous starved-feed emulsion polymerization.

[0033] Fig. 15: (a) DLS data of particle batch CSP5 for the three individual reaction steps, (b) TEM photograph of the core-particles C5. (c) TEM photograph of the final CSP5.

[0034] Fig. 16: Analysis of the thermo-rheological properties of the particle composition consisting of CSP5 and an antioxidant in example 5. (a) DSC measurement and resulting thermogram for the determination of glass transition temperatures and oxidation induction temperatures, (b) TGA revealing the onset of thermal degradation, (c-d) Temperature- and frequency-dependent dynamic complex viscosities. Glass transition temperatures are indicated with a circle. Typical ranges for fused filament fabrication in this example 5 are depicted with a double arrow. Assuming the applicability of the Cox-Merz rule, the herein-determined complex dynamic viscosities as a function of angular frequency are equivalent to the steady-shear viscosity as a function of shear rate, which is the appropriate unit to describe the 3D printing process.

[0035] Fig. 17: Processing scheme for particle synthesis of CSP5, preparation of a filament, consisting of a particle composition of CSP5 and an antioxidant, and subsequent 3D printing of said filament. Starting from polymer synthesis via starved-feed emulsion polymerization, CSP5 were freeze-dried and mixed with an antioxidant. The freeze-dried polymer was composed of disordered CSP (magnified scheme), showing no structural color. In an extrusion process, the composition was homogenized, and a filament was prepared, which was afterward 3D printed by application of the fused filament fabrication technique. During extrusion and subsequent 3D printing, the CSP were exhibited to high shear forces and self-assembled into an ordered colloidal crystalline structure (magnified scheme), resulting in iridescent structural color. A dog bone test specimen (length: 68 mm) and a pirate bust (height: 40 mm) indicate the obtainable level of detail. An UV-Vis reflection spectrum indicates the high quality of the obtained turquoise structural color in terms of monomodality and peak-sharpness.DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention is described in the following, exemplified in the appended examples, illustrated in the figures, and reflected in the claims.

[0037] Definitions****

[0038] It is noted that as used herein, the singular forms “a”, “an”, and “the”, include plural references unless the context indicates otherwise. Thus, for example, reference to “a reagent” includes one or more of such different reagents, and reference to “the method” includes reference to equivalent steps and methods known to those of ordinary skill in the art that could be modified or substituted for the methods described herein.

[0039] Unless otherwise indicated, the term "at least" preceding a series of elements is to be understood to refer to every element in the series. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the present invention.

[0040] The term "and / or" wherever used herein includes the meaning of "and", "or" and "all or any other combination of the elements connected by the said term".

[0041] It should be understood that this invention is not limited to the particular methodology, protocols, material, reagents, substances, etc., described herein and as such can vary. The terminology used herein is to describe particular embodiments only and is not intended to limit the scope of the present invention, which is defined solely by the claims.

[0042] The content of all documents and patent documents cited herein is incorporated by reference in their entirety.

[0043] “phr” means parts per hundred in the respective reaction mixture, “phr” states the relative mass amounts of a component relative to 100 mass parts of the monomer in the respective reaction mixture. Thus, within this reference system, the ”phr” value of the monomer in the reaction mixture is by definition 100.

[0044] The term “solution” is generally used in the context of the present invention for all kinds of mixtures with the presence of a solvent. The term ’’solution” is used for solutions as such, but also when the components do not completely dissolve and in reality, no clear solution is formed, but the components merely form an emulsion or dispersion.

[0045] DTEM is the average diameter of the particle in dry conditions, determined with transmission electron microscopy (TEM). Preferably, “dry conditions” means that the medium in which the particles are present during the determination of C>Tem is air. Preferably, “dry conditions” means that particles exhibit less than 1 wt%, more preferably 0.1 wt%, and most preferably 0% water.

[0046] DDLS is the hydrodynamic average particle diameter, dispersed in water at 25 °C determined with dynamic light scattering measurements (DLS). Preferably, the medium in which the particles are present during the determination of DDLS is water. Data obtained by DLS measurements is evaluated by cumulants analysis, which gives two values, an intensity mean value for the size (DDLS), and a width parameter known as the polydispersity index value (PDI). The calculations for these parameters are defined in the ISO standard document 13321 :1996 E and 22412.

[0047] The effective refractive index netf is calculated based on the volume shares i , and the refractive index of the homopolymers nt, which are obtained from literature.53 54

[0048] The invention is directed to a particle comprising a) 35 to 50 vol%, preferably 40 to 45 vol%, based on the overall volume of the particle and calculated with the average diameter DTEM, of a core, comprising a polymer (P1) prepared by radical polymerization; b) 2 to 10 vol%, preferably 5 vol%, based on the overall volume of the particle and calculated with the average diameter DTEM, of an interlayer comprising a polymer (P2) prepared by radical polymerization, wherein the polymer (P2) comprises 3 to 7 wt%, preferably 5 wt%, of a crosslinker based on the overall weight of the polymer (P2); and c) 45 to 60 vol%, preferably 45 to 58 vol%, more preferably 50 to 55 vol% based on the overall volume and calculated with the average diameter DTEM, of a shell, comprising a polymer (P3) prepared by radical polymerization.

[0049] Wherein the “core” forms the inner core of the particle, and the “interlayer” is set between the core and the shell on the outside.

[0050] A lower content of shell than in the present invention results in lower thermal stability. It is assumed that this is the result of the fact that in this case not all reactive groups of the crosslinker sites in the core and interlayer have been saturated with shell polymer. In contrast, a highercontent of shell polymer than in the present invention has been observed to result in a deterioration of structural color quality. It is assumed that this disturbs the configuration of the colloidal crystalline structure formed by the core particles inside the soft shell matrix since the cores, in this case, have too many degrees of freedom. The impact of the core-to-shell ratio is shown and discussed in detail in example 2.

[0051] A higher content of cross-linker sites in the interlayer as in the present invention has been observed to result in lower thermal stability. It is assumed that in this case, too many reactive groups of the cross-linker that are not saturated such as with shell polymer remain after synthesis, which results in further undesired thermally induced cross-linking reactions during the processing of particles at elevated temperatures. Lower content of cross-linker reagents in the interlayer than in the present invention has been observed to result in insufficient covalent binding of the shell polymer. Free shell polymer may disturb the self-assembly of the cores into the colloidal crystalline structure, resulting in a deterioration of structural color quality. The impact of the interlayer composition is shown and discussed in detail in example 3.

[0052] For example, embodiments featuring the above-described preferred particle architecture are synthesized, analysed and processed in example 4 (CSP4) and example 5 (CSP5)

[0053] The polymer (P1) may be prepared by polymerization of monomers comprising a terminal alkene group, preferably styrene.

[0054] The polymer (P1) may further comprise a cross-linker, preferably selected from the group consisting of butanediol diacrylate (BDDA), diallyl phthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 wt%, more preferably 10 wt% based on the overall amount of polymer (P1).

[0055] Preferably, the use of a cross-linker reagent in (P1) ensures that the cores are not geometrically deformed during processing.

[0056] In one embodiment, the polymer (P1) is poly(styrene-co-butanediol diacrylate) (P(S-co- BDDA)).

[0057] The glass transition temperature of the core polymer (P1) may be 90 to 150 °C, preferably 105 to 120 °C.

[0058] Preferably, the core is dimensionally stable and durable during processing, more preferably with the mechanical properties of a thermoset. Wherein a thermoset polymer is defined as a non-deformable polymer with permanent shape that is obtained by irreversible curing, which includes chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

[0059] The polymer (P2) is prepared by polymerization of monomers comprising a terminal alkene group, preferably selected from one or more of the groups consisting of acrylates and methacrylates, more preferably methyl methacrylate (MMA) and / or ethyl acrylate (EA), most preferably MMA.

[0060] The cross-linker in polymer (P2) may be allyl methacrylate (ALMA).

[0061] Preferably, the cross-linker in polymer (P2) acts as a grafting anchor to covalently immobilize the particle shell, to prevent a detachment and phase separation of core and shell during processing.

[0062] In one embodiment, the polymer (P2) is poly(methyl methacrylate-co-allyl methacrylate) (P(MMA-co-ALMA)).

[0063] The polymer (P3) is prepared by polymerization of monomers comprising a terminal alkene group, preferably selected from one or more of the groups consisting of acrylates, methacrylates, vinylpyridines, acrylonitrile and acrylamides, more preferably EA, MMA, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl methacrylate, and acrylonitrile, most preferably EA and / or MMA.

[0064] The glass transition temperature of the shell polymer (P3) may be -50 °C to 120 °C, preferably -50 °C to 20°C or 21 to 120 °C.

[0065] Preferably, the shell is viscous at room temperature or meltable at elevated processing temperatures, more preferably with elastomeric or thermoplastic properties.

[0066] Preferably, embodiments wherein the glass transition temperature is -50 °C to 20°C, exhibit an elastomeric shell. Embodiments with an elastomeric shell are preferably deformable by mechanical deformation at room temperature, such as 18 to 25 °C, preferably 20 °C, with mechanochromic properties. Preferably the mechanical deformation at room temperature is reversible. See also example 4.

[0067] Preferably, embodiments wherein the glass transition temperature of the shell is 21 to 120 °C, exhibit a thermoplastic shell. Embodiments with a thermoplastic shell are preferably processable as filaments for 3D printing and allow for the preparation of shape memory materials. See also example 5.

[0068] In one embodiment, the polymer (P3) is poly(ethylacrylate) (PEA). The use of PEA preferably results in an elastomeric shell. See also example 4.

[0069] In another embodiment, the polymer (P3) is poly(ethyl acrylate-co-methyl methacrylate) (P(EA-co-MMA)). The use of P(EA-co-MMA) preferably results in the thermoplastic shell. See also example 5.

[0070] The particles may have an average diameter DTEM of 50 to 500 nm. It is known from the literature that the average particle diameter DTEM is adjustable from 50 to 500 nm during particle synthesis.1721

[0071] The particles are monodispersed with a standard deviation of DTEM < 5%, more preferably < 2%.

[0072] Preferably, the monodispersity facilitates self-assembly in a highly ordered colloidal crystalline structure.

[0073] The diameter of the particles of the present invention may further be expressed as hydrodynamic diameter DDLS, wherein DDLS is 55 to 600 nm.

[0074] The hydrodynamic diameter DDLS considers the particles in the swollen state in the respective solvent, preferably in water, and is usually 5 to 20% higher, compared to the dry particle diameter DTEM(see also examples).

[0075] Preferably, the polydispersity index (PDI) of the particle size DDLS is < 5%, more preferably < 2%.

[0076] The effective refractive index of the core and shell differ from each other; preferably the value for the effective refractive index of the core and shell differ from each other by > 0.05, more preferably > 0.1. Preferably, the periodical difference of the refractive index between the core and shell in a crystalline ordered structure causes a photonic bandgap, which gives rise to structural color.28’29

[0077] The core polymer (P1), preferably comprising P(Sgo-co-BDDAio), may have an effective refractive of 1 to 2, preferably 1.58, as calculated from the above-described formula for the effective refractive netf.

[0078] The shell polymer (P3), preferably comprising PEA and / or PMAA, may have an effective refractive index of 1 to 2, preferably 1.47 to 1.48, as calculated from the above-described formula for the effective refractive netf.

[0079] The core-shell particle may have an effective refractive index A of 1-2, preferably 1.4- 1.6, particularly preferred 1.52, as calculated from the above-described formula for the effective refractive netf and taking into account the above-described particle architecture consisting particularly preferred of 40 to 45 vol% of a P(Sgo-co-BDDAio)-core; 5% of a P(MMAgs-co- ALMAs)-interlayer; and 50 to 55% of a P(EAo-ioo-co-MMAo-ioo)-shell.

[0080] The observed structural color of the particles, the composition comprising these particles, or any object made of these particles is directly proportional to the particle size. Preferably, the structural color is a result of Bragg scattering on an ordered crystalline structure of the round core particles in the continuous shell matrix.

[0081] The expected emitted wavelength ^, i.e. the expected structural color, may be calculated with a modified version of the Bragg-Snell law, in dependence on the angle of incident light a, the effective refractive index netf, and the particle diameter Draw.15,21 ,22,55

[0082] For the calculations, the dry particle diameter DTEM, as discussed above, should be used as particle diameter. The hydrodynamic diameter DDLS considers particles in a swollen state and should therefore not be used for calculations with the Bragg-Snell law in order not to overestimate the reflected wavelength.

[0083] In one embodiment the particle diameter DTEM is 50 to 153 nm and the effective refractive index of the particle is netf = 1.52. Under normal light incidence at a = 90°, the expected reflection color according to the calculation with the Bragg-Snell law is in the ultraviolet range with < 380 nm.

[0084] In another embodiment the particle diameter DTEM is 154 to 314 nm and the effective refractive index of the particle is netf = 1.52. Under normal light incidence at a = 90°, the expected reflection color according to the calculation with the Bragg-Snell law is in the visible range with 380 nm < < 780 nm.

[0085] In a further embodiment the particle diameter DTEM is 315 to 500 nm and the effective refractive index of the particle is netf = 1.52. Under normal light incidence a = 90°, the expected reflection color according to the calculation with the Bragg-Snell law is in the infrared range with 780 nm < A.

[0086] Exemplary calculations of the effective refractive index and the expected color according to the Bragg-Snell are provided in detail in example 4 and 5.

[0087] The invention is further directed to a particle composition, comprising the particle as described above and at least one radical scavenger and / or antioxidant. The at least one radical scavenger and / or antioxidant may be selected from the group of primary antioxidants, more preferably quinol, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (AO1010), butylated hydroxytoluene (BHT), Octadecyl3-(3,5-di-t-butyl-4- hydroxyphenyl)propionate. The at least one radical scavenger and / or antioxidant may be present in the composition in an amount of 0.1 to 5 wt%, preferably 0.5 to 2 wt%, more preferably 2 wt%, based on the overall weight of the composition.

[0088] Preferably, the at least one radical scavenger and / or antioxidant increases the temperature stability during processing. The applicants assume that this is the result of scavenging initiating oxygen diradicals and thus suppressing temperature-induced cross-linking reactions. See also example 1.4. To the best knowledge of the applicants, in the prior art, no radical scavenger and / or antioxidant has been added to comparable particles to increase temperature stability.

[0089] Preferably, carbon black is added to the particle composition in an amount of 0.01 to 0.2 wt%, more preferably 0.02 to 0.05 wt%, even more preferably 0.03 wt% based on the overall weight of the composition.

[0090] Preferably, the incorporation of carbon black leads to an improvement in structural color quality, by absorbing diffuse scattered light.

[0091] In one embodiment, the composition is solid, preferably formed as a filament; more preferably as filament for 3D printing such as fused filament fabrication (FFF); preferably the filament has a diameter of 1.00 to 3.00 mm, most preferably 1.75 mm or 2.85 mm, even more preferred 1.75 mm. See also example 5.

[0092] The invention is further directed to the use of the particle or the particle composition as discussed above in a process of polymer processing, wherein optionally the process comprises at least one process step selected from the group consisting of, (co- and / or profile-)extrusion, compounding, direct-ink-writing (DIW), FFF, uniaxial pressing, embossing, compression molding, calendaring, parison forming, rotational molding, blow molding, injection molding, deep drawing, surface coating. See also examples 4 and 5.

[0093] In one embodiment, the particles or the particle composition, preferably the particle composition, is used a) to produce a filament for 3D printing, preferably by extrusion; and / or b) 3D printing of said filament with the FFF technique.An example of this embodiment is sketched in Figure 17 and described in concrete example 5.

[0094] The polymer processing process is carried out at elevated temperatures above 25 °C, preferably at least at 165 °C, more preferably at 165 °C to 200°C, most preferably at 200 to 250 °C. Preferably, the particles of the present invention are stable at these temperatures without being damaged or degradation, thus allowing for the use of the inventive particles in processes applying higher temperatures such as in this range.

[0095] The invention is further directed to a process for the preparation of a particle according to described above, comprising carrying out at least one of the following steps a) to c), preferably in the same reactor, resulting in a particle comprising a core, an interlayer, and a shell: a) radical polymerization of monomers of a monomer solution (MSa) comprising at least one monomer comprising a terminal alkene group (Ma), preferably styrene, resulting in the core polymer (P1) of the particle; b) further radical polymerization of monomers of a monomer solution (MSb) comprising at least one monomer comprising a terminal alkene group (Mb), preferably selected from one or more of the group consisting of acrylates and methacrylates, more preferably MMA and / or EA, most preferably MMA, and 3 to 7 phr, preferably 5 phr, a cross-linker, preferably ALMA, adding the interlayer polymer (P2) to the particle; and c) further radical polymerization of monomers of a monomer solution (MSc) comprising at least one monomer comprising a terminal alkene group (Me), preferably selected from one or more of the group consisting of acrylates, methacrylates, vinylpyridines, acrylonitrile and acrylamides, more preferably EA, MMA, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl methacrylate and acrylonitrile, most preferably EA and / or MMA, adding the shell polymer (P3) to the particle.

[0096] For example, CSP4 and CSP5, as described in the concrete examples 4 and 5, respectively, were prepared according to the herein described synthesis route. An exemplary synthesis scheme, with the monomers used in the concrete example 5, is provided in Figure 14.

[0097] It is known from literature17’21 56that the ratio of co-monomers to each other and the ratio of monomer(s) to cross-linker in the respective monomer solution (Ma), (Mb), or (Me) is approximately equal to the composition in the polymer (P1), (P2), or (P3), respectively. For an explanatory case concerning the interlayer, it can be assumed that (P2) contains 5 wt% of PALMA, when 5 phr of ALMA were used in (MSb) during synthesis. For another explanatory case concerning the shell, it can be assumed that (P3) consists of a statistical copolymer of 50 wt%PEA and 50 wt% PMMA, when during synthesis (Me) in (MSc) consisted of a 1 :1-mixture of EA and MMA.

[0098] The amounts of the monomers (Ma), (Mb), (Me) relative to each other are: i) the amount of monomer (Ma) is 19 to 59 wt%, preferably 40 to 45 wt%; and ii) the amount of monomer (Mb) is 1 to 15 wt%, preferably 5 wt%; and iii) the amount of monomer (Me) is 40 to 80 wt%, preferably 50 to 55 wt%.

[0099] The amounts of monomer (Ma), (Mb), (Me) relative to each other during synthesis are not necessarily equal to the amounts of (P1 ), (P2), (P3) relative to each other in the final particle. This is due to potentially different yields in each synthesis step and deviations between weigh-in monomer mass and experimentally determined polymer particle volume.

[0100] Preferably, all reaction steps are carried out under continuous stirring; and / or at a temperature of 50 to 100 °C, preferably at 65 to 90 °C, more preferably at 75 °C; and / or under an inert atmosphere, preferably a nitrogen atmosphere.

[0101] In one embodiment, all steps are conducted stepwise in the same reactor directly one after the other. This process technology is explanatory described in the concrete example 1.

[0102] In another embodiment, the reaction solution is drained from the reactor after the core synthesis in step a) and diluted to a defined solids content, which is equal to the core polymer content. The diluted reaction solution is afterwards used as starting reaction solution for the following steps b) and c), i.e. the interlayer and shell synthesis. With this processing technology, it is possible to use the identical batch of cores for several batches of core-shell particles, which improves comparability of these batches. In the concrete examples 2, 3, and 4 this process technology is explanatory described and the enhanced comparability is utilized.

[0103] Preferably, step a) comprises two steps a1) and a2), carried out sequentially; wherein i) in step a1) 1 to 15 wt%, preferably 4 wt% of (Ma) based on the overall amount of (Ma) used in step a1) and step a2) is added; and ii) in step a2) 85 to 99 wt%, preferably 96 wt% of (Ma) based on the overall amount of (Ma) used in step a1) and step a2) is added.

[0104] Preferably, step a1) is carried out as a batch polymerization, leading to the seed of the particle.

[0105] Preferably, step a2) is carried out as semi-continuous emulsion polymerization in starved-feed mode, resulting in the core of the particle.

[0106] The monomer solution (MSa1) in step a1) comprises the above-defined at least one monomer (Ma). (Ma) is per definition present in an amount of 100 phr in (MSa1).

[0107] (MSa1) may further comprise at least one cross-linker, preferably selected from the group consisting of BDDA, diallylphthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 phr, more preferably 10 to 12 phr.

[0108] (MSa1) may further comprise at least one detergent, preferably sodium dodecyl sulfate (SDS), preferably in an amount of 1 to 100 phr.

[0109] Preferably, at least one detergent of step a1) is present above the critical micelle concentration and forms micelles, wherein the polymerization takes place.

[0110] (Msa1) may further comprise a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 3000-20000 phr, more preferably 7500 to 8000 phr.

[0111] (MSa1) may further comprise at least one polymerization initiator, preferably a combination of sodium disulfite (NaDS) and sodium persulfate (NaPS), preferably 0 to 15 phr NaDS and 1 to 100 phr NaPS.

[0112] Preferably, the combination of NaDS and NaPS forms a redox system that acts as an initiator.

[0113] Preferably, at least one polymerization initiator of step a1) is added to (MSa1) after all other components are already present in (MSa1).

[0114] Step a1) is carried out for 0 to 30 minutes, preferably 10 to 15 minutes, more preferably 10 minutes, after all components have been added.

[0115] The monomer solution of step a2), (Msa2) comprises the above-defined at least one monomer (Ma). (Ma) is per definition present in an amount of 100 phr in (MSa2).

[0116] (Msa2) may further comprise at least one cross-linker, preferably selected from the group consisting of BDDA, diallyl phthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 phr, more preferably 10 phr.

[0117] Preferably, the combination of (Ma) as a bulk monomer with the at least one crosslinker leads to a cross-linked seed polymer with mechanical properties of a thermoset.

[0118] (Msa2) may further comprise at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and disodium isododecyl phenyl ether sulfonate (DIPES), preferably each in an amount of 0.1 to 1 phr, more preferably SDS in an amount of 0.33 phr and DIPES in an amount of 0.31 phr

[0119] (Msa2) may further comprise at least one inorganic base and / or buffer system, preferably a base selected from the group consisting of KOH and NaOH, preferably in an amount of 0 to 4 phr, more preferably 0.57 phr.

[0120] (Msa2) may further comprise a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 50 to 500 phr, more preferably 100 to 150 phr.

[0121] (Msa2) may be added continuously over 2 h to 6 h, preferably 4 to 5 h; preferably to the reaction solution of step a1), comprising (MSa1).

[0122] Optionally at least one polymerization initiator, preferably NaPS, preferably in an amount of 0 to 2 phr, is added during the continuous addition of (MSa2); preferably when unconsumed monomer is visually observable.

[0123] Step a2) may be carried out for another 0 to 120 min, after all components of step a2) have been added.

[0124] Optionally the reaction solution comprising (MSa1) and (MSa2) is drained from the reactor after the completion of step a) and diluted to a defined solids content, preferably of 5 to 20 wt%, more preferably 8 wt%, before continuing with step b).

[0125] Preferably, step b) is carried out as semi-continuous emulsion polymerization in starved-feed mode, adding the interlayer to the particle.

[0126] Preferably, at least one polymerization initiator is added before all other components of step b), preferably a combination of NaDS and NaPS; more preferably NaDS in an amount of 0 to 5 phr, and NaPS in an amount of 4 to 30 phr, most preferably NaDS in an amount of 1.7 phr and NaPS in an amount of 10 phr.

[0127] Preferably, the at least one polymerization initiator of step b) is added to the reaction solution comprising (MSa1) and (MSa2).

[0128] Preferably, after the addition of the at least one polymerization initiator in step b) the further components of step b) are added after 0 to 30 min, preferably after 10 to 15 min.

[0129] The monomer solution of step b) (MSb) comprises the above-defined at least one monomer (Mb). (Mb) is per definition present in an amount of 100 phr in (MSb).

[0130] (MSb) further comprises at least one cross-linker, which is ALMA; preferably in an amount of 3 to 7 phr, more preferably 5 phr.

[0131] (MSb) may further comprise at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and DIPES, preferably each in an amount of 0.5 to 5 phr, more preferably SDS in an amount of 0.98 phr and DIPES in an amount of 1.9 phr.

[0132] (MSb) may further comprise a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 200 to 2000 phr, more preferably 400 to 500 phr.

[0133] (MSb) may be added continuously over 10 to 30 min, preferably 15 to 20 min; preferably to the reaction solution of step a) comprising (MSa1) and (MSa2).

[0134] Step b) may be carried out for another 0 to 30 min, preferably 10 to 15 min; after all components of step b) have been added.

[0135] Preferably, step c) is carried out as semi-continuous emulsion polymerization in starved-feed mode, adding the shell to the particle.

[0136] Preferably, at least one polymerization initiator, preferably NaPS, is added before all other components of step c), preferably in an amount of 0 to 1 phr, more preferably in an amount of 0.1 phr.

[0137] Preferably, at least one polymerization initiator of step c) is added to the reaction solution of step b), comprising (MSa1), (MSa2), and (MSb).

[0138] Preferably, after addition of the at least one polymerization initiator in step c) the further components of step c) are added after 0 to 30 min, preferably after 10 to 15 min.

[0139] The monomer solution of step c), (MSc) comprises the above-defined at least one monomer (Me). (Me) is per definition present in an amount of 100 phr in (MSc).

[0140] (MSc) may further comprise at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and DIPES, preferably each in an amount of 0.1 to 1 phr, more preferably SDS in an amount of 0.39 phr and DIPES in an amount of 0.32 phr.

[0141] (MSc) may further comprise at least one inorganic base and / or buffer system, preferably a base selected from the group consisting of KOH and NaOH, preferably in an amount of 0 to 4 phr, more preferably 0.56 phr.

[0142] (MSc) may further comprise a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 50 to 400 phr, more preferably 100 to 150 phr.

[0143] (MSc) may be added continuously over 60 to 180 min, preferably 90 to 150 min; preferably to the reaction solution of step b) comprising (MSa1), (MSa2), and (MSb).

[0144] Step c) may be carried out for another 0 to 120 min, preferably 60 min; after all components of step c) have been added.

[0145] As discussed above and calculable with the Bragg-Snell law, particle size determines the observable structural color. The particle diameter may be controlled by variations in the seed stage a1) via variations of surfactant concentration and / or initiator concentration; and / or in the following stages (a2-c) by variations of the monomer concentration.

[0146] Particle size may be controlled by the correct amount of surfactant in the seed step a1). The higher the concentration of surfactant, the higher the number of micelles that are formed. The higher the number of micelles that are formed, the more seed particles are formed. The more growing particles are present in the system, the less monomer is available and the final particles will thus be smaller. The surfactant concentration in step a1) is therefore anti-proportional to the particle diameter in all stages, including step (a-c) and the final particles.

[0147] Particle size may further or alternatively be controlled by the correct amount of initiator in the seed step a1). The higher the concentration of the initiator, the higher the number of active micelles, wherein the polymerization is initiated. The higher the number of micelles with growing polymer chains, the more seed particles are formed. The more growing particles are present in the system, the less monomer is available per particle and the final particles will thusbe smaller. The surfactant concentration in step a1) is therefore anti-proportional to the particle diameter in all stages, including step (a-c) and the final particles.

[0148] It is known from the literature, that by the described variations of the surfactant and initiator concentration in step a1), the final particle diameter can be adjusted from 50 to 500 nm. The following semi-empirical formula describes how surfactant concentration [E], and initiator concentration [I], are proportional to the number of active micelles (i.e. growing particles) N, the final particle volume V, the final particle diameter D.5657

[0149] Particle size may further be controlled by variations in the amount of monomer (Ma) in (MSa2), (Mb), and (Me). For the same batch of seed material, it can be deduced that the more monomer is added, the bigger particles are obtained. The final particle volume is therefore directly proportional to the overall amount of monomer [M] added.V oc D3oc [M]

[0150] For example, in the concrete examples 4 and 5 referring to batch CSP4 and CSP5, respectively, using 6.67 phr SDS as surfactant and 14.4 phr NaPS in combination with 2.00 phr NaDS as initiator in the seed stage a1), the final particle sizes obtained were DTEM = 239 nm and DTEM = 230 nm, for CSP4 and CSP5 respectively. The smaller diameter of CSP5 resulted from a reduced amount of monomer [M] during the synthesis of CSP5, as explained in example 5.

[0151] CSP4 and / or CSP5 may serve as reference for the tuning of particle diameters. Based on the proportionality between particle diameter and surfactant concentration and / or initiator concentration, it is possible to calculate how the SDS concentration and / or NaPS and NaDS concentration in the seed stage a1) may be adjusted to achieve the appropriate particle sizes. Additionally, or alternatively, particle size may be adjusted by further changes of the monomer content, according to the given proportionality.EXAMPLES OF THE INVENTIONMaterial and Methods

[0152] Materials used’. Styrene (abbreviated with S, in a purity of 99%) was purchased from Fisher Scientific. Butanediol diacrylate (BDDA, 90%), methyl methacrylate (MMA, 99%), and ethyl acrylate (EA, 99.5%) were purchased from Sigma-Aldrich. Allyl methacrylate (ALMA, 98%) was purchased from TCI. Before emulsion polymerization, radical inhibitors were removed from the monomers by passing them through a basic alumina column (50-200 pm, Acros Organics). Potassium hydroxide flakes (KOH, 90% reagent grade), sodium disulfite (NaDS, analysis grade),sodium persulfate (NaPS, > 98%), and sodium dodecyl sulfate (SDS, > 98.5%) were purchased from Sigma-Aldrich. Disodium isododecyl phenyl ether sulfonate (DIPES) was purchased from Ezkem. Carbon Black (Channel Type Black 4) was obtained from Evonik. Pentaerythritol- tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (AO1010) was obtained from Nordmann-Rassmann. Benzene-1 ,4-diol (Quinol, 99.5%) was purchased from Acros Organics.

[0153] Equipment used for Polymer Analysis’. Dynamic light scattering (DLS) measurements were performed with a Zetasizer ZS 90 by Malvern Instruments equipped with a 4 mW, 633 nm HeNe Laser. All measurements were carried out at 25 °C and an angle of 90° with a five-fold determination of an automatically determined number of runs (typically 10-20 runs per measurement). Automated data acquisition in 300 size classes was carried out using a cumulant fit. The hydrodynamic diameter DDLS represents the z-average of this fit, and the polydispersity index (PDI) is indicated with ±. For transmission electron microscopy (TEM) studies, a diluted polymer dispersion was drop-casted on a carbon-coated copper grid. The dispersion medium evaporated at ambient conditions for 24 h. TEM experiments were conducted using a JEOL JEM- 2100 electron microscope at a nominal acceleration voltage of 200 kV with a Gatan Orius SC100 CCD camera in bright field mode. For the determination of average particle diameters DTEM, 25 particles per image were analysed using the software Imaged. The coefficient of variation was calculated as the standard deviation divided by the average particle diameter and is indicated with ±. Temperature-dependent (heating rate: 5 K min-1) and isothermal rheological properties were determined with an Anton Paar MCR501 oscillatory rheometer at a frequency of 1 Hz using a 25 mm plate-plate geometry in an ambient laboratory atmosphere. Temperature-dependent (heating rate: 5 K min-1) and isothermal differential scanning calorimetry (DSC) were carried out with a Netzsch 214 F1 Polyma. Temperature-dependent (heating rate: 10 K min-1) and isothermal thermogravimetric analyses (TGA) were performed with a Netzsch TG 209 F1 Libra system. Unless otherwise stated, DSC and TGA measurements were performed under a synthetic air atmosphere, i.e. in the presence of oxygen. Reflection spectra were recorded using an Ocean Optics fiber FLAME vis / NIR spectrometer USB2000 in reflection mode, combined with an Ocean Optics deuterium / tungsten halogen lamp DT mini 2.

[0154] Equipment used for Polymer Processing’. After synthesis, the emulsified particles were freeze-dried. The dried particle mass was mixed with 0.03 wt% Carbon Black and optionally an antioxidant, as described in the respective examples. Carbon black was always added since it is known to improve structural color by absorbing diffusely scattered light.1361The mixture was extruded at 100-250 °C (unless otherwise stated: 100 °C) and 50-100 rpm, using a Thermo Scientific HAAKE MiniLab 3 Micro-Compounder. This extruder was also used to produce the filament for 3D printing. Opal films were produced with the melt-shear technique: A 2 g portion of the extruded mass was covered with two poly(ethylene terephthalate) foils (Mylar A75, DuPont) and inserted into a laboratory press (Collin P 300 E) for 3 minutes at 100-250 °C (unless otherwise stated: 100 °C) and 20 bar. To produce 3D-Prints with the direct-ink-writing (DIW) technique, the extruded mass was printed on a Saarpricom Delta UpSD 3D-Printer, equipped with a 0.4 mm Nozzle. Slicing was performed with Simplify3D V4.1.2. Unless otherwise stated, printing settings were used as described in a previously published slicing profile.40To produce 3D-Prints with the fused-filament-fabrication (FFF) technique, the extruded mass was printed on a Prusa MK3S+, equipped with a 0.8 mm Nozzle. Slicing was performed with PrusaSlicer 2.6.1. Detailed slicing and printing settings are described in example 5.

[0155] Calculation of Cross-Linking Indicators and Arrhenius-Law. Thermally induced cross-linking reaction is quantified in several examples according to four indicators: Temperaturedependent and isothermal DSC measurements as well as temperature-dependent and isothermal viscosity measurements. In all four cases, the cross-linking reaction is quantified as an integratedarea between the expected and measured values, concerning the exposition time. For DSC measurements, the expected value is represented by the DSC baseline without any endo- or exothermic process, which is directly obtained from measurement. Integrated values may be interpreted as reaction enthalpies of the cross-linking process. For isothermal viscosity measurements, the expected value is the viscosity after heating to 190 °C was finished, neglecting any thixotropic or rheopexic behaviour. For temperature-dependent viscosity measurements, the expected values were calculated according to an Arrhenius-Law to predict the viscosity decrease at increasing temperatures. To obtain the Arrhenius regression, the temperature-dependent viscosity / }(7 was fitted to measured viscosities in a temperature range from 30 to 165 °C; i.e. the temperature range, were no relevant cross-linking takes place. Therein R is the universal gas constant and r / o the reference viscosity at To = 150 °C. EA describes the activation energy, which was obtained as a fit parameter by minimizing the sum of squared errors. In a temperature range from 165 to 250 °C Arrhenius-law was extrapolated to predict the temperature-dependent viscosity without cross-linking reactions. The formula for Arrhenius-Law is:Example 1

[0156] This example shall illustrate the temperature instability of state-of-the-art core- shell-particles (CSP) and the effect of antioxidative additives. For this purpose, a batch with standard particle architecture is synthesized and the thermo-rheological properties are analysed. The low thermal stability is shown to result from a subsequent cross-linking reaction of unconsumed grafting anchors from synthesis at > 165 °C. It is demonstrated that state-of-the-art processing methods are therefore not suited to produce objects with smooth surfaces and good structural color quality at processing temperatures > 165 °C and are not applicable at all when temperatures are increased to > 180-200 (depending on the processing method). Moreover, the incorporation of antioxidative additives is examined. With 2 wt% of an appropriate antioxidant, processing temperatures can be slightly increased to 180-200 °C, but not beyond.1.1 Synthesis of state-of-the-art Particles

[0157] CSP have been synthesized by different groups from different disciplines.21’37’44’52 58Within this context, stimuli-responsiveness was implemented via copolymers in the shell material. However, particle architecture and basic monomer composition were rarely altered. The common synthesis strategy to achieve this archetypal particle structure was also used in this example to synthesize the batch CSP1. Semi-continuous and stepwise emulsion polymerization was used for this purpose, as described in the following.

[0158] In the first step, poly(styrene-co-butanediol diacrylate) (P(S-co-BDDA)) core particles were synthesized. As given in Figure 1a, the hydrodynamic diameter DDLS of the core particles was adjusted to 166 nm (±3.9%). BDDA was used as a chemical cross-linking reagent to avoid deformation of the core particles during subsequent processing. In the second step, an interlayer of approximately 3 nm (DDLS = 169 nm ±2.1%), consisting of poly(methyl methacrylate-co-allyl methacrylate) (P(MMA-co-ALMA)) was introduced. As ALMA provided two different reactive sites, the soft polymer shell of poly(ethylacrylate) (PEA) could be covalently anchored in the following step. The final CSP1 were found to be monodisperse with an average diameter of DDLS = 223 nm (±1.8%).

[0159] In addition to DLS measurements, the particle size in the dried state DTEM was determined. While DLS analysis determined the hydrodynamic diameter in a swollen state, TEMmeasurements revealed the average particle diameter in the dried state. From the corresponding TEM images in Figures 1 b and c, it can be concluded, that good control over the polymerization was achieved, resulting in an average core diameter of 157 nm (±4.0%) and monodisperse CSP1 of 205 nm (±4.4%).

[0160] The detailed synthesis protocol is described in the following: A 5 L double-wall reactor under nitrogen atmosphere at 75 °C, equipped with a reflux condenser and a stirrer was filled with a monomer solution (Msa1) of 13.5 g S (Ma), 1 .50 g BDDA, 0.812 g SDS, and 1050 g deionized water. While stirring at 250 rpm, the polymerization was initiated with 0.270 g NaDS and 1.95 g NaPS. After a reaction time of 10 minutes, a monomer solution (Msa2) was added continuously with a flow rate of 2.7 mL min-1, using a rotary piston pump. Msa2 contained 132 g S (Ma), 13.2 g BDDA, 0.433 g SDS, 0.752 g KOH, 0.414 g DIPES, and 169 g deionized water. After the complete addition of Msa2, the reaction was kept at a constant temperature and stirred for 15 minutes. While stirring at 250 rpm, the polymerization was reinitiated by adding 1.00 g NaPS. After a reaction time of 15 minutes, a monomer solution (MSb) was continuously added with a constant flow rate of 5.0 mL min-1. MSb contained 17.8 g MMA (Mb), 1.78 g ALMA, 0.345 g DIPES, 0.174 g SDS, and 76.7 g deionized water. After the complete addition of MSb and an additional 15 minutes, a monomer solution (MSc) was continuously added with a flow rate of 5.0 mL min-1using a rotary piston pump. MSc contained 219 g EA (Me), 0.862 g SDS, 0.696 g DIPES, 1.21 g KOH, and 286 g deionized water. After the complete addition of MSc, the reaction mixture was kept at a constant temperature and stirred for an additional hour.1.2 Analysis: Mechanism of Temperature Instability

[0161] To gain insights into the mechanism of the temperature instability in-depth analyses of particle batch CSP1 were conducted. Figure a shows how viscosity rapidly decreased upon heating from room temperature. The viscosity drop at 125 °C relates to the glass transition of the cross-linked core particles. Viscosity then asymptotically flattened off up to 165 °C, resembling the common thermomechanical behaviour of an elastomer with non-covalent physical crosslinking sites. Calculated viscosities for T > 165 °C according to Arrhenius-Law are indicated as dotted lines to visualize the divergence between expected and observed behaviour. In a range from 165-210 °C, measured viscosities deviate from the calculated values, as they stay on a roughly constant level. At first glance, this may be interpreted as a rubbery plateau. Such plateaus are caused by polymer chain entanglements that act like temporary cross-links, starting at temperatures above the glass transition.4950As the glass transition of the shell material is well below room temperature (Figure 2c), an attributable rubbery plateau may be observable in this temperature range, but not at 165 °C. Therefore, the plateau would have to be caused by entanglements of the core polymer between separate particles. This appears highly unlikely, as the cores are thoroughly cross-linked and surrounded by covalently grafted shell polymer. At temperatures > 210 °C the measured viscosity even increased with increasing temperature, further deviating from the expected course. Therewith it was confirmed that the previously observed stagnation was not caused by a rubbery plateau, but another phenomenon.

[0162] Similar results were obtained from isothermal viscosity measurements for 120 min at 190 °C (Figure 2b), wherein viscosity asymptotically increased. The obtained curve shows similarities with a typical vulcanization curve.4950Overall, viscosity increased by about 91% over two hours. It appears unlikely that the observed viscosity increase was a result of gelation or rheopexy, as CSP are known to be shear-thinning, not shear-thickening.37’4052Moreover, such effects would at least partly be reversible, which was not observable.

[0163] To investigate the reaction enthalpy, differential scanning calorimetry (DSC) was conducted (Figure 2c). The glass transitions at -11 °C and 112 °C, can be attributed to the shell and core, respectively. The presence of two glass transitions further proved the successful synthesis and the expected core-shell architecture. At higher temperatures, however, a huge exothermic peak with a peak area of 69 J g-1was observed. The oxidation induction temperature of 165 °C was found to be very similar to the onset of the viscosity stagnation in Figure 2a.

[0164] Figure 2d shows an isothermal DSC measurement at 190 °C for 120 min. The huge exothermic peak area was determined to be 123 J g-1. Compared to Figure 2b, the DSC curve resembles the first derivation of the viscosity curve and may therefore be interpreted as reaction enthalpy.

[0165] The TGA temperature sweep (Figure 2e) revealed a degradation onset of 279 °C, which is well above the previously investigated and herein relevant temperatures. This finding was further confirmed by the isothermal TGA measurement at 190 °C (Figure 2f), illustrating that the mass loss after two hours was well below 1.0 wt%. These TGA proved that oxidative degradation processes or combustions did not contribute to the investigated process.1.3 Processing: Negative Examples for Temperature Instability

[0166] While trying to process CSP1 at temperatures above 165 °C via extrusion, 3D printing, or melt-shearing, recurring observations were made, as visible in Figure 3: The material got brittle and crumbly, processability worsened, structural color quality drastically decreased, surface roughness increased, multiple crack formation occurred, and only short coherent pieces could be produced before the structure broke or teared. At temperatures of 180 °C and higher, the material became completely impossible to process via extrusion, in the form of exceeding the maximum extruder-torque, and DIW, in terms of clogging the 3D printer’s nozzle. The formation of an at least somewhat coherent film by application of the melt-shear technique was found to be impossible at 200 °C (and higher). These observations had to be a result of the particular coreshell architecture, as polystyrene- and polyacrylate-based materials are commonly processed at temperatures in a range of 200 to 250 °C in industry.4959To apply common polymer-processing techniques and to apply established techniques at elevated temperatures (which is advantageous e.g. in terms of increased flowability and reduced extrusion force), the necessity was worked out to increase temperature stability of CSP - which has been performed in this invention, as shown in the following examples.

[0167] In summary of examples 1.2 and 1.3, it was shown that the temperature-instability of CSP is related to the specific core-shell architecture and caused by an increase in viscosity and stiffness. The underlying mechanism was found to be exothermic and does not lead to a loss of polymer mass. This description is well-explainable by a subsequent chemical cross-linking process. Cross-linkable reactive sites were introduced by the incorporation of BDDA as a crosslinker for the core polymer and ALMA as a grafting-anchor for the shell material. Thereby introduced double bonds were likely not quantitatively consumed during CSP synthesis. These non-consumed and chemically available reactive sites could lead to a subsequent cross-linking reaction at elevated temperatures > 165 °C. Such a cross-linking process would follow a radical mechanism with oxygen-diradicals as initiators. Following this idea, thermal stability could be improved, if all cross-linking sites were consumed during synthesis. Alternatively, oxygen radicals could be intercepted before initiation. Both strategies have been successfully applied to develop the present invention, thereby strengthening the proposed theoretical framework, and the latter strategy is discussed in the following example.1.4 Antioxidant Incorporation

[0168] As the herein relevant cross-linking reaction is likely initiated by oxygen diradicals, primary antioxidants, which act as radical scavengers, were assumed to delay or prevent the process. In this example two different antioxidants were examined, Quinol and AQ1010. Both were incorporated in batch CSP1 via extrusion with a proportion of 2 wt%. Quinol is a common radical scavenger and a typical polymerization inhibitor for radical-initiated polymerizations. AQ1010 is one of the most used sterically hindered phenolic antioxidants. Detailed antioxidative mechanisms of those stabilizers have been subject to extensive research and are described elsewhere.60

[0169] The temperature-dependent viscosity measurements of the compounds are illustrated in Figure 4a. While no antioxidant was able to fully suppress the thermally induced cross-linking reaction, the increases in viscosity were not as distinct as for the raw polymer CSP1. The inhibiting effect of both additives was even more pronounced for the isothermal measurement at 190 °C (Figure 4b). Especially AO1010 vastly lowered speed and amount of the viscosity increase, from the original 91% to 39%. The incorporation of quinol also led to an improvement, although less pronounced, lowering the viscosity increase to 62%.

[0170] Temperature-dependent and isothermal DSC measurements (Figures 4c and d) supported previous findings. Exothermic signals, related to thermally induced cross-linking, were reduced upon antioxidant incorporation. The graphs associated with additive-containing CSP are roughly comparable to measurements of the raw CSP1 in inert nitrogen atmosphere, thus proving that the antioxidant mechanism is indeed based on the interception of initiating oxygen diradicals, as intended and expected. The isothermal DSC of the quinol compound showed a peculiarity in the form of a secondary peak after 75 min. Up to this point, all quinol could have been consumed, and the thermally induced cross-linking reaction may have occurred. This thesis is further supported, by the fact that the secondary peak area and shape are very similar to the measurement of CSP1. The curve related to the AQ1010 compound did not show such a secondary peak, indicating that AQ1010 was not fully consumed during the experimental period of 120 min. However, DSC analysis must be interpreted with care. The physical and chemical properties of the additives as well as their antioxidative action may have led to additional exo- or endothermic signals, that could have superimposed the original signals. This was e.g. hinted in the temperature-dependent DSC thermogram of the quinol-containing compound, wherein a small peak at » 160 °C indicated the melting temperature of quinol.

[0171] In direct comparison, it was not surprising that AQ1010 showed higher efficiency. It is known that hindering the phenolic hydroxyl group with at least one bulky alkyl group in the ortho position is necessary for high antioxidant activity. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom - in this case from an unconsumed double bond.60AQ1010 meets this requirement, quinol does not. The long aliphatic chains in AQ1010 further improved performance, by decreasing volatility and increasing solubility in the hydrophobic CSP.60This observation further proved the theoretical framework and justified the selection of stabilizers from the group of primary antioxidants.

[0172] To investigate a possible influence on self-assembly and thereof resulting structural color quality, opal films were prepared via the application of the melt-shear organization technique. UV- Vis reflection peaks of additive-containing compounds were slightly red-shifted (Figure 4e). This indicated that the lattice constant increased. The low-molecular additives are likely dispersed in the soft shell matrix, thereby increasing the distance between the core particles. Structural color quality, in terms of peak sharpness and intensity, hardly changed upon additive incorporation. If there was any influence on the self-assembly of the core particles at all, it must have been minor.

[0173] The rheological analyses of additive-containing compounds implied, that the thermally induced cross-linking reaction was delayed and reduced, but far from being fully suppressed. This thesis was supported by observations made during the processing of the AO1010-containing compound, which showed the most promising results. Processing temperatures could be increased to 180 °C for extrusion and DIW and to 200 °C for the melt-shear technique, which is an improvement, compared to the raw CSP1. Upon further increase of the temperatures above 200 °C, however, processability increasingly worsened, leading to similar problems as portrayed in Figure 3.

[0174] In conclusion to this example, it was shown that the antioxidants worked as expected and intended, thus proving the previously developed theoretical framework. To the best of the inventor’s knowledge, this is the first report of a tailored incorporation of stabilizing additives in CSP. It was shown that thermal stability can be improved by the incorporation of antioxidants, preferably AO1010, but not above 200 °C. As discussed in the following examples (especially example 4), the key feature of the invention, i.e. temperature stability up to at least 250 °C, is only achievable, when the antioxidant incorporation is combined with an optimized particle architecture. The optimized particle architecture includes a tailored core-to-shell ratio (example 2) and interlayer composition (example 3).Example 2

[0175] This example shall illustrate the influence of the core-to-shell ratio on the thermal stability of CSP. For this purpose, four batches with different core proportions were synthesized. It is shown that a core proportion of > 45 vol% is unfavorable for thermal stability and that a core proportion of < 40 vol% leads to a deterioration of structural color quality. With an optimized core proportion of 40 to 45 vol% processing temperatures can be slightly increased to 180-200 °C, but not beyond.2.1 Synthesis

[0176] Following the explanatory approach presented in example 1 , thermal stability is improvable by consuming as many cross-linking sites as possible during particle synthesis. In this invention, this is inter alia achieved by an increase in the amount of soft shell material in the last polymerization step. Due to the grafting-through nature of the shell polymerization step, the more monomer is fed to the particles, the more cross-linking sites are consumed. Moreover, the shell polymer does not feature allylic moieties and therefore acts as a spacer between reactive sites for interparticle cross-linking reactions. Common CSP used in shear-induced self-assembly usually feature core fractions of around 50 to 55 vol%.17’21’37’40’42As demonstrated in the following, thermal stability can be increased by a reduction of the core fraction, which is equivalent to an increase in the shell proportion.

[0177] To demonstrate the feasibility of this approach, four batches of CSP2.1 to CSP2.4 with different core-to-shell ratios were synthesized. The identical batch of core particles C2 was used for all four CSP, to directly compare the influence of the shell material. The average diameter of C2 was found to be DDLS = 177 nm (±3.6%) and DTEM = 153 nm (±4.3%). Starting from CSP2.1 , the proportion of shell monomer solution in starved feed mode was increased for each batch. CSP diameters were determined via TEM and DLS (Figures 5 and 6). Results are summarized in Table 1. Since all batches featured an identical core size, particle sizes increased with higher shell- / lower core proportion. DSC measurements revealed similar glass transitions for all batches of -10 ±1 °C for the PEA shell and 111 ±1 °C for the mutual PS core particles, thus confirming the success of the synthesis and the comparability of the batches.

[0178] Table 1 : Overview of synthesized CSP to investigate the influence of the core-to-shell ratio. The hydrodynamic diameter in the swollen state was determined via DLS measurements. The TEM diameter represents the size of the dried particles. The volumetric core proportion was calculated according to TEM Data.Batch DDLS (nm) DTEM (nm) Core ProportionCSP2.1 213 ±3.9% 181 ±2.6% 61 vol%CSP2.2 233 ±2.5% 202 ±4.5% 44 vol%CSP2.3 252 ±1.9% 217 ±4.2% 36 vol%CSP2.4 271 ±3.3% 224 ±4.5% 32 vol%

[0179] The detailed synthesis protocol to produce the core particles C2 is described in the following: A 5 L double-wall reactor under nitrogen atmosphere at 75 °C, equipped with a reflux condenser and a stirrer was filled with a monomer solution (Msa1) of 18.0 g S (Ma), 2.00 g BDDA, 1.20 g SDS, and 1400 g deionized water. While stirring at 250 rpm, the polymerization was initiated with 0.360 g NaDS and 2.59 g NaPS. After a reaction time of 10 minutes, a monomer solution (Msa2) was added continuously with a flow rate of 4.0 mL min-1, using a rotary piston pump. Msa2 contained 410 g S (Ma), 41.0 g BDDA, 1.34 g SDS, 2.34 g KOH, 1.28 g DIPES, and 527 g deionized water. After the complete addition of Msa2, the reaction was kept at a constant temperature and stirred for 60 minutes. The core dispersion was drained from the reactor and diluted to a solid content (=polymer content) of 8 wt%.

[0180] The detailed synthesis protocols to produce CSP2.1-4 is described in the following paragraph and table: The respective amount of diluted core dispersion was filled in in a 1 L doublewall reactor, equipped with a reflux condenser and a stirrer under an N2 atmosphere at 75 °C. While stirring at 250 rpm, the emulsion polymerization was initiated (Ini-MSb). After a reaction time of 15 minutes, a monomer solution (MSb) was continuously added with a constant flow rate of 1.0 mL min-1, to synthesize the interlayer. After the complete addition of MSb and an additional 15 minutes, the emulsion polymerization was reinitiated (Ini-MSc) and continuously stirred for another 10 minutes. Subsequently, a monomer solution (MSc) was continuously added with a flow rate of 1 .0 mL min-1. After the complete addition of MSc, the reaction mixture was kept at a constant temperature and stirred for an additional hour. This synthesis protocol and process technique was used for all batches CSP2.1-4, which only differ concerning quantities of the individual components in terms of core-to-shell ratio, as depicted in Table 2.

[0181] Table 2: Recipes for the synthesis of the CSP-batches based on C2.Component CSP2.1 (g) CSP2.2 (g) CSP2.3 (g) CSP2.4 (g)C2-Dispersion (8%) 253 231 209 176 lni-MSb_NaDS 0.054 0.049 0.044 0.037 lni-MSb_NaPS 0.314 0.286 0.259 0.218MSb_DIPES 0.062 0.056 0.051 0.043MSb_ALMA 0.307 0.281 0.254 0.214MSb_MMA (Mb) 3.07 2.81 2.54 2.14MSb_SDS 0.031 0.028 0.026 0.022MSb_Water 13.6 12.5 11.3 9.50 lni-MSc_NaPS 0.042 0.046 0.057 0.068MSc _SDS 0.220 0.260 0.299 0.360MSc _EA (Me) 55.8 65.9 76.0 91.34MSc _DIPES 0.177 0.209 0.241 0.290MSc _Water 72.9 86.1 99.3 119MSc _KOH 0.310 0.366 0.422 0.5072.2 Analysis of Thermal Stability and Processing

[0182] Figure 7a displays how viscosities increased at temperatures > 165 °C, although the fitted Arrhenius-Laws (dotted lines) predicted a further decrease. CSP2.1 with the highest core proportion revealed the strongest deviation, whereas CSP2.4 with the lowest core proportion showed almost no deviation. Similar observations could be made of the isothermal viscosity measurement (Figure 7b): the lower the core proportion, the lower the increase in viscosity. DSC measurements in Figures 7c and d further verified this result, as the exothermic signal decreased upon reduction of the core proportion.

[0183] A summary of the rheological examinations and DSC measurements is provided in Figure 7 e, wherein integrated areas between expected and measured values are sketched against the core proportion. Integrated viscosities and enthalpies were connected with a linear fit, to visualize the decrease at lower core proportion. The linear fit is not based on chemical or physical considerations, but merely a guide for the eye. Nevertheless, it clearly demonstrates (qualitatively) that the solution approach worked: Remaining cross-linking sites were indeed reducible via a reduction of the core proportion, leading to enhanced thermal stability.

[0184] A lower proportion of the hard PS cores also led to an overall lower viscosity. This had to be expected, as the amount of low viscous shell polymer was higher. Lower viscosities may be advantageous during processing, in terms of increased flowability, but are not advantageous in general. Therefore, alterations of the general viscosity level are neither an argument for nor against low core proportion. Besides, rheo-mechanical properties are easily tuneable in a wide range without sacrificing color quality with comonomers in the shell polymer or incorporation of additives.1740

[0185] Opal films of all particle batches were prepared by application of the melt-shear technique. And analyzed via UV-Vis spectroscopy. The red shift of the peaks in Figure 7f from CSP2.1 to 2.4 resulted from the larger particle sizes (Table 3), by Bragg’s law of diffraction. The structural color quality of the films worsened at core fractions below 40 vol% in terms of peak sharpness, reflection intensity, and visually perceived color. If the proportion of soft shell material is too high, the self-assembly of the cores into a closed-packed structure is likely to be impeded because of the increased space between individual core particles.

[0186] Temperature-dependent and isothermal TGA measurements were conducted for all particle batches CSP2.1 to 2.4. Results are illustrated in Figure 7g and h. In accordance with analogous TGA measurements in examples 1 and 3, thermal degradation started well above 250 °C for all batches, and the average mass loss after 120 min at 190 °C was < 1.0 wt%. Onceagain it was thereby proven that oxidative degradation processes, leading to a mass loss, did not contribute to the investigated phenomena. It was thus further confirmed, that in the relevant temperature range, thermal stability is limited by subsequent cross-linking.

[0187] To optimize temperature stability while maintaining high color quality, a target of a core proportion of 40-45 vol% was thereby found to be favorable. CSP2.2 featured this optimized core proportion, but still showed some cross-linking at elevated temperatures. This was verified by observations made during the processing of CSP2.2 at elevated temperatures. Processing temperatures could be increased to 180 °C for extrusion and DIW and to 200 °C for the meltsheartechnique. Compared to state-of-the-art CSP, this is an improvement. Upon further increase of the temperatures above 200 °C however, processability increasingly worsened, leading to similar problems as portrayed in Figure 3.

[0188] In conclusion of this example, it was shown that thermally induced cross-linking reactions can be reduced by a reduction of the core proportion below 45 vol%, thus proving the previously developed theoretical framework. To maintain structural color quality, however, the core proportion cannot be reduced below 40 vol%. With an optimized core proportion, thermal stability can be improved, but not above 200 °C. As discussed in the other examples (especially example 4), the key feature of the invention, i.e. temperature stability up to at least 250 °C, is only achievable, when the optimized core proportion is combined with antioxidant incorporation (example 1.4) and an optimized concentration of grafting-anchor in the interlayer (example 3).

[0189] The general strategy of a reduced core proportion was reported once before.40In this preliminary work however, core proportions of 34.5 vol% and 36.5% (according to the TEM diameter) were used, which is below the herein-determined optimal range; demonstrating how through further research an optimal range for the core fraction was found, that excludes the old particles. Besides the non-ideal core proportion, the previously reported particles were also not processable above 180 °C. (“According to thermogravimetric analysis (TGA), degradation started at 224 and 230°C [...] We further identified oxidation induction temperatures by using DSC measurements of 194 °C and 196 °C [...] Therefore, printing at Tp > 180°C was avoided.”)Example 3

[0190] This example shall illustrate the influence of the concentration of cross-linker in the interlayer, acting as grafting-anchor for the shell, on the thermal stability of CSP. For this purpose, four batches with different grafting-anchor concentrations were synthesized. It is shown that higher concentrations are increasingly unfavorable for thermal stability, whereas a reduction below 5 wt% leads to a deterioration of structural color quality. With an optimized concentration of 5 wt%, processing temperatures can be slightly increased to 180-200 °C, but not beyond.3.1 Synthesis

[0191] It is well known that the shell polymer must be grafted to the core to obtain an ordered colloidal crystal structure, which leads to the appearance of structural color.21Both stretching and compression are entropically unfavorable for the shell polymer chains, which is why the grafted chains move the cores back to their lattice locations. Without sufficient grafting, the elastomeric matrix represents a highly viscous fluid, wherein the dispersed nuclei can be arbitrarily moved around. Usually, an amount of 10-15 wt% ALMA in the interlayer is used for this purpose.17’21 28By reducing the amount of ALMA in this invention, while still ensuring that the shell is sufficiently grafted onto the core, the number of unconsumed grafting anchors after synthesis can bereduced. As unconsumed grafting anchors act as reactive cross-linking sites during processing, a reduced amount can increase thermal stability.

[0192] To demonstrate the feasibility of this approach, four batches CSP3.1 to CSP3.4 with different amounts of ALMA in the interlayer were synthesized. The same batch of core-particles C3 was used for all four CSP, to improve comparability. The average diameter of C3 was determined as DDLS = 198 nm (±3.9%) and DTEM = 178 nm (±3.6%). CSP diameters were determined via TEM and DLS (Figures 8 and 9), as summarized in Table 3. Particle sizes only vary slightly, thus indicating good comparability of the batches. On average, a core proportion of 55% (according to TEM measurements) was achieved, which is a typical value for state-of-the- art CSP.17'21'37'40'42DSC measurements revealed similar glass transitions for all batches of -10 ±1 °C for the PEA shell and 112 ±1 °C for the mutual PS core particles, thus confirming the success of the synthesis and the comparability of the batches.

[0193] Table 3: Overview of synthesized CSP to investigate the influence of the interlayer composition. The hydrodynamic diameter in the swollen state was determined via DLS measurements. The TEM diameter represents the size of the dried particles.Batch ALMA Content DDLS (nm) DTEM (nm)CSP3.1 10.0 wt% 252 ±3.2% 224 ±4.1 %CSP3.2 7.5 wt% 247 ±1.8% 216 ±4.8%CSP3.3 5.0 wt% 239 ±2.9% 215 ±4.6%CSP3.4 2.5 wt% 243 ±2.2% 218 ±5.0%

[0194] The detailed synthesis protocol to produce the core particles C3 is described in the following: A 5 L double-wall reactor under nitrogen atmosphere at 75 °C, equipped with a reflux condenser and a stirrer was filled with a monomer solution (Msa1) of 18.0 g S (Ma), 2.00 g BDDA, 1.20 g SDS, and 1400 g deionized water. While stirring at 250 rpm, the polymerization was initiated with 0.360 g NaDS and 2.59 g NaPS. After a reaction time of 10 minutes, a monomer solution (Msa2) was added continuously with a flow rate of 4.0 mL min-1, using a rotary piston pump. Msa2 contained 420 g S (Ma), 42.0 g BDDA, 1.38 g SDS, 2.40 g KOH, 1.32 g DIPES, and 540 g deionized water. As unconsumed monomer was visually observable during the addition of Msa2, the polymerization was reinitiated after 120 min with 0.700 g NaPS and again after 195 min with 0.700 g NaPS. 5 minutes before re-initiation and 10 minutes afterward, the addition of MSa2 was paused. After the complete addition of Msa2, the reaction was kept at a constant temperature and stirred for 60 minutes. The core dispersion was drained from the reactor and diluted to a solid content (=polymer content) of 8 wt%.

[0195] The detailed synthesis protocols to produce CSP3.1-4 are described in the following paragraph and table: The respective amount of diluted core dispersion was filled in in a 1 L doublewall reactor, equipped with a reflux condenser and a stirrer under an N2 atmosphere at 75 °C. While stirring at 250 rpm, the emulsion polymerization was initiated (Ini-MSb). After a reaction time of 15 minutes, a monomer solution (MSb) was continuously added with a constant flow rate of 1.0 mL min-1, to synthesize the interlayer. After the complete addition of MSb and an additional 15 minutes, the emulsion polymerization was reinitiated (Ini-MSc) and continuously stirred for another 10 minutes. Subsequently, a monomer solution (MSc) was continuously added with a flow rate of 1 .0 mL min-1. After the complete addition of MSc, the reaction mixture was kept at a constant temperature and stirred for an additional hour. This synthesis protocol and processtechnique was used for all batches CSP3.1-4, which only differ in the ALMA concentration in the interlayer, as depicted in Table 4.

[0196] Table 4: Recipes for the synthesis of the CSP-batches based on C3.Component CSP3.1 (g) CSP3.2 (g) CSP3.3 (g) CSP3.4 (g)C2-Dispersion (8%) 282 282 282 282 lni-MSb_NaDS 0.060 0.060 0.060 0.060 lni-MSb_NaPS 0.349 0.349 0.349 0.349MSb_DIPES 0.068 0.068 0.068 0.068MSb_ALMA 0.349 0.261 0.174 0.0872MSb_MMA (Mb) 3.49 3.49 3.49 3.49MSb_SDS 0.035 0.035 0.035 0.035MSb_Water 15.2 15.2 15.2 15.2 lni-MSc_NaPS 0.049 0.049 0.049 0.049MSc _SDS 0.168 0.168 0.168 0.168MSc _EA (Mc) 42.6 42.6 42.6 42.6MSc _DIPES 0.135 0.135 0.135 0.135MSc _Water 55.6 55.6 55.6 55.7MSc _KOH 0.236 0.236 0.236 0.2363.2 Analysis of Thermal Stability and Processing

[0197] Figure 10a illustrates how viscosities increased at elevated temperatures, although the fitted Arrhenius-Laws (dotted lines) predicted a further decrease. The amount of deviation decreased at lower ALMA content, presumably due to a reduced amount of thermally induced cross-linking reactions. This observation was verified by isothermal viscosity measurements (Figure 10b): The less ALMA was used, the weaker the viscosity increase was. In isothermal mode, the viscosity curve associated with CSP3.4 showed an unusual progression. After the initial drop upon heating to 190 °C, viscosity did not increase immediately but stayed at a roughly constant level for about 30 minutes. This indicated that the thermally induced crosslinking reaction was decelerated and almost fully suppressed by reducing the ALMA content to 2.5 wt%. A lower proportion of ALMA also led to an overall lower viscosity. As discussed in example 2.2, however, this is neither an advantage nor a downside in general.

[0198] Temperature-dependent and isothermal DSC measurements (Figures 10c and d) further supported the results obtained from rheological examinations. The area of the exothermic peaks steadily decreased upon ALMA reduction. In the temperature-sweep, CSP3.4 showed an extraordinarily small signal, which even diminished at higher temperatures. In analogy to the viscosity measurements, this indicated that the cross-linking reaction was almost fully suppressed for this batch.

[0199] A summary of the four discussed measurements in terms of the integrated areas between expected and measured values is provided in Figure 10e. Lower integrated viscosities and enthalpies indicate less thermally induced cross-linking reactions. Thereby obtained values were connected with a linear fit to demonstrate the steady decrease upon reduction of ALMA. The linear fit is not based on chemical or physical considerations, but merely a guide for the eye. Nevertheless, it demonstrates (qualitatively), that the amount of cross-linking reaction could indeed be reduced via a reduction of the amount of ALMA, thus confirming the feasibility of this solution approach. By reducing the ALMA content to very low levels thermally induced crosslinking reactions were almost fully suppressed.

[0200] As indicated by the analytical measurements, the ALMA content should be reduced as much as possible to increase thermal stability. On the other hand, it must be ensured, that every chain of shell polymer is covalently grafted to the core. Insufficient grafting, due to a deficiency of grafting anchors, i.e. too low a content of ALMA, would lead to a loss of structural color quality. To investigate this issue, opal films were prepared via the application of the meltshear technique and investigated with UV-Vis spectroscopy. From Figure 10f it can be concluded that a reduction from the regular 10 wt% to 7.5 wt% or 5 wt% was possible with minor changes regarding color quality. On a side note, the opal film made from CSP3.3 revealed a visible and measurable blue shift of the color; by Bragg’s law, due to the comparably smaller particle diameter. Upon reduction to 2.5 wt% however, structural color quality decreased in terms of peak sharpness, reflection intensity, and visually perceived color. This indicated the presence of nongrafted polymer chains, interfering with the colloidal order of the core particles.

[0201] Temperature-dependent and isothermal TGA measurements were conducted for all particle batches CSP3.1 to 3.4. Results are illustrated in Figure 10g and h. In accordance with analogous TGA measurements in examples 1 and 2, thermal degradation started well above 250 °C for all batches, and the average mass loss after 120 min at 190 °C was < 1.0 wt%. Once again it was thereby proven that oxidative degradation processes, leading to a mass loss, did not contribute to the investigated phenomena. It was thus further confirmed, that in the relevant temperature range, thermal stability is limited by subsequent cross-linking.

[0202] In conclusion to the analytical examinations in this chapter, an ALMA content in the interlayer of 5 wt% was found to be ideal to improve thermal stability, while maintaining high structural color quality. Thereby, the thermally induced cross-linking reaction can be reduced, but not fully suppressed. This thesis was supported by observations made during the processing of CSP3.3 at elevated temperatures. Processing temperatures could be increased to 180 °C for extrusion and DIW and to 200 °C for the melt-shear technique. Compared to state-of-the-art particles, this is an improvement. Upon further increase of the temperatures above 200 °C however, processability increasingly worsened, leading to similar problems as portrayed in Figure 3. As discussed in the other examples (especially example 4), the key feature of the invention, i.e. temperature stability up to 250 °C, is only achievable, when the optimized ALMA-content is combined with antioxidant incorporation (example 1.4) and an optimized core-to-shell-ratio (example 2).

[0203] The general strategy of a reduced ALMA content in the interlayer was reported once before.40In this preliminary work, however, an ALMA concentration of 6 wt% was used, which is above the herein-determined optimal value. Moreover, the previously reported particles did not feature an optimal core-shell ratio and no antioxidants were added. Most likely due to these significant differences to the present invention, the previously reported particles were not processable above 180 °C. (“According to thermogravimetric analysis (TGA), degradation started at 224 and 230°C [...] We further identified oxidation induction temperatures by using DSC measurements of 194 °C and 196 °C [...] Therefore, printing at Tp > 180°C was avoided.”)Example 4

[0204] This example shall illustrate how this invention overcomes the thermal instability of CSP. For this purpose, a particle composition was prepared, consisting of an antioxidant and optimized CSP, in terms of an optimized core proportion and an optimized composition of the interlayer. In other words: all three previously discussed (Example 1-3) improvements were combined. Thermal and rheological analyses demonstrate the vastly improved temperature stability of the composition, compared to state-of-the-art particles. By application of several state- of-the-art processing methods, it is shown that the particle composition can be processed at temperatures of up to at least 250 °C with high structural color quality, which is more than 100 °C higher than usual processing temperatures of CSP. Finally, the advantages of the increased processing temperature are discussed.4.1 Preparation of the temperature-stable particle composition

[0205] Within the previous examples 1 to 3, it was demonstrated that thermal stability can be increased in three different ways, enabling processing at somewhat increased temperatures of 180-200 °C. To produce the preferred particle composition of this invention with processing stability up to 250 °C, however, all three solution approaches have to be combined, as demonstrated in this example. The particle composition, consisting of CSP4 and AO1010, was produced according to the above-described insights comprising particle architecture and antioxidant addition. To enable adequate comparison to state-of-the-art particles, another batch CSP4C was synthesized. CSP4C was based on the identical core particles C4 but featured state- of-the-art particle architecture of the interlayer and the shell. To achieve an optimized core proportion of 40 to 45 vol% for CSP4, the amount of shell-forming monomer was increased during synthesis, compared to CSP4C with a target core proportion of 50 to 55 vol%. The ALMA content inside the interlayer was set to the optimal value of 5% for CSP4 and to the state-of-the-art value of 10% for CSP4C. After synthesis and freeze drying, 2 wt% AO1010 as an antioxidative additive and 0.03 wt% carbon black were incorporated via extrusion in CSP4, whereas no antioxidative additives and only the 0.03 wt% carbon black were incorporated in CSP4C. Synthesis and obtained particle architecture of CSP4, as well as the preparation of the particle composition based on CSP4, represents one of the preferred embodiments of this invention.

[0206] DLS measurements (Figure 11) and TEM data (Figure 12), confirmed the good control over each polymerization step for the formation of CSP4 and CSP4C. The average diameter of the mutual core batch C4 was determined as DDLS = 198 nm (±3.9%) and DTEM = 178 nm (±3.6%). Compared to CSP4C, featuring a particle diameter of DDLS = 252 nm (±3.2%) and DTEM = 224 nm (±4.1 %), a larger particle diameter of DDLS = 274 nm (±2.4%) and DTEM = 239 nm (±4.3%) was achieved for CSP4. This data on particle size was in good agreement with expectations, particularly regarding the optimized core proportion of CSP4 of 41 vol%, compared to the state- of-the-art core proportion of CSP4C of 51 vol% (according to TEM data). Besides, the low ± deviations indicated that monodisperse particles were obtained in both cases.

[0207] The detailed synthesis protocol to produce the core particles C4 is described in the following: A 5 L double-wall reactor under nitrogen atmosphere at 75 °C, equipped with a reflux condenser and a stirrer was filled with a monomer solution (Msa1) of 18.0 g S (Ma), 2.00 g BDDA, 1.20 g SDS, and 1400 g deionized water. While stirring at 250 rpm, the polymerization was initiated with 0.360 g NaDS and 2.59 g NaPS. After a reaction time of 10 minutes, a monomer solution (Msa2) was added continuously with a flow rate of 4.0 mL min-1, using a rotary piston pump. Msa2 contained 420 g S (Ma), 42.0 g BDDA, 1.38 g SDS, 2.40 g KOH, 1.32 g DIPES, and540 g deionized water. As unconsumed monomer was visually observable during the addition of Msa2, the polymerization was reinitiated after 120 min with 0.700 g NaPS and again after 195 min with 0.700 g NaPS. 5 minutes before re-initiation and 10 minutes afterward, the addition of Msa2 was paused. After the complete addition of Msa2, the reaction was kept at a constant temperature and stirred for 60 minutes. The core dispersion was drained from the reactor and diluted to a solid content (=polymer content) of 8 wt%.

[0208] The detailed synthesis protocols to produce CSP4 and CSP4C is described in the following paragraph and table: The respective amount of diluted core dispersion was filled in in a 1 L double-wall reactor, equipped with a reflux condenser and a stirrer under an N2 atmosphere at 75 °C. While stirring at 250 rpm, the emulsion polymerization was initiated (Ini-MSb). After a reaction time of 15 minutes, a monomer solution (MSb) was continuously added with a constant flow rate of 1 .0 mL min-1, to synthesize the interlayer. After the complete addition of MSb and an additional 15 minutes, the emulsion polymerization was reinitiated (Ini-MSc) and continuously stirred for another 10 minutes. Subsequently, a monomer solution (MSc) was continuously added with a flow rate of 1.0 mL min-1. After the complete addition of MSc, the reaction mixture was kept at a constant temperature and stirred for an additional hour. For the synthesis of CSP4C the state- of-the-art ALMA concentration of 10 wt% and state-of-the-art core-to-shell ratio (to yield a core proportion of 50-55%) was used, whereas for CSP4 an optimized ALMA proportion of 5 wt% and an increased proportion of MSc (to yield a core proportion of 40-45 vol%) was used, as depicted in Table 5.

[0209] Table 5: Recipes for the synthesis of the CSP-batches for example 4.Component CSP4 (g) CSP4C (g)C2-Dispersion (8%) 259 282 lni-MSb_NaDS 0.055 0.060 lni-MSb_NaPS 0.321 0.349MSb_DIPES 0.063 0.068MSb_ALMA 0.161 0.349MSb_MMA (Mb) 3.21 3.49MSb_SDS 0.032 0.035MSb_Water 14.0 15.2 lni-MSc_NaPS 0.045 0.049MSc _SDS 0.209 0.168MSc _EA (Mc) 53.0 42.6MSc _DIPES 0.168 0.135MSc _Water 69.2 55.6MSc _KOH 0.294 0.2364.2 Analysis of thermal stability

[0210] The thermal stability of the CSP4, representing the preferred particle composition in this invention, was investigated using rheological examinations and DSC measurements. Results are presented in comparison to CSP4C, representing the state-of-the-art particles. Since CSP4 was based on the same batch of cores as CSP4C, any differences in thermomechanical properties must therefore be a result of the interlayer composition, core-to-shell ratio, or antioxidant incorporation.

[0211] Figures 12a and b show how the viscosity increase of CSP4 was strongly decelerated and overall limited. This observation indicated a restriction of thermally induced cross-linking reaction, as expected and targeted. DSC analysis in Figures 12c and d must be interpreted with care, due to potential interfering signals caused by the antioxidant AO1010. The almost fully suppressed exothermic signals in both measurements nevertheless point in the direction of increased thermal stability. The improved thermal stability was summarized and quantified in Figure 12e. The figure shows the integrated areas between measured and expected values, i.e. integrated viscosities and enthalpies, of all previous measurements on CSP4, divided by the analogously integrated values for CSP4C. Compared to the state-of-the-art batch CSP4C, the lower values of CSP4 demonstrate higher thermal stability in terms of less thermally induced cross-linking. On average, the four cross-linking indicators were reduced by 81 %, thereby proving that thermally induced cross-linking was almost fully suppressed.

[0212] To investigate influences on self-assembly and structural color formation, opal films of CSP4 and CSP4C were prepared by application of the melt-shear technique. The measured reflection spectra in Figure 12f were further compared to the previously prepared opal films made of CSP1 (see example 1). Both CSP1 and CSP4C represented state-of-the-art particle architectures, but the different sizes of the core particles led to different reflection colors. By Bragg’s law, the larger core particles used for CSP4C and CSP4 and thus larger final particle diameter, compared to CSP1 , led to a red shift. The even more pronounced red shift of CSP4, compared to CSP4C can be attributed to the comparably larger particle diameter of CSP4 and the additive incorporation, which both led to an increased lattice distance. However, in terms of peak sharpness, color intensity, and perceived color, the structural color quality of CSP4 was found to be at a comparably high level. It was thereby proven, that the combined strategies to increase the temperature-stability of the inventive particle composition did not result in a deterioration of structural color quality.

[0213] TGA measurements (Figures 12g and h) did not reveal any hints of other thermal deterioration in terms of mass loss in the relevant temperature ranges; neither for CSP4 nor CSP4C. However, the onset of thermal degradation was increased from 265 °C (CSP4C) to 319 °C (CSP4), and the isothermal mass loss was decreased by 0.6 wt% for the invented particle composition. Although these observations are not relevant to the herein investigated cross-linking reaction and polymer processing temperatures, they may be interpreted as a positive side effect, probably caused by the antioxidant incorporation. The shifted onset of thermal degradation indicates that the invented particle composition is probably processable at even higher temperatures than 250 °C.4.3 High-Temperature Processing

[0214] High-temperature processability of CSP4, representing the preferred particle composition of this invention, is demonstrated in the following, using extrusion, melt-shearing, and DIW. In all three cases, the particle composition was conveniently processable at 250 °C. Figure 13 shows thereof obtained structurally colored objects, which featured no signs of deterioration or damage. Regardless of the applied processing technique, color intensity andsurface quality were comparable to previously reported materials, which were processed at much lower temperatures, usually in the range of 60-120 °c,21’26’37’39occasionally up to 140-150 °C.40'45Thus, processing temperatures of the herein-invented particle composition are at least 100 °C higher compared to common CSP.

[0215] The intensity mean of the reflected wavelengths in the UV-Vis spectrum in Figure 13 was found to be 622 nm. This value deviates less than 5% from the calculated wavelength according to the Bragg-Snell law at = 593 nm and is therefore in good accordance with expectations. The expected wavelength was calculated according to the formulas described in the DETAILED DESCRIPTION OF THE INVENTION as follows: neff= ' i ^i - ni= 1.52; wherein n(PS) = 1.59; n(PBDDA) = 1.46; n(PEA) = 1.47; ( / (Core) = ( / / (P(S9o-co-BDDAi0)) = 41 vol%; and ( / (Shell) = ip(PEA) = 59 vol%.The influence of the small interlayer on the effective refractive index was neglected. The volume shares of core and shell were calculated according to C>7Bw(Core) = 198 nm and C>7Bw(Shell) = 239 nm, assuming an ideal spherical geometry.A = 2^DTEMy / ne^2— cos2a = 593 nm; wherein DTEM = 239 nm; netf = 1 .52; and a = 90°This calculation is only valid for light incidence at an angle of 90°, i.e. normal to the surface. At steeper angles, the color blue-shifts, which leads to an iridescent appearance of the materials.

[0216] In direct comparison, CSP1 (see example 1.3) and CSP3.1 , both representing state-of-the-art CSP, were found to be non-processable at elevated temperatures, as described in the following and portrayed in Figure 3. During extrusion at temperatures above 165 °C melt- flow-instabilities were observed, and strand breakage occurred, while the maximum extruder torque was exceeded at 180 °C. Melt-shearing led to multiple crack formation and an inhomogeneous opal film with poor color quality at 180 °C. The pressed mass turned out to be very brittle, which resulted in crumbling and breaking upon removal of the protective PET foils. At 200 °C and above, no coherent film could be obtained. During DIW at temperatures above 165 °C, the material was hardly extrudable, extruded strands were incoherent, color quality was low, and the printed surfaces were rough, inhomogeneous, and crumbly. At 180 °C, the nozzle clogged after a few layers, forcing it to abort the printing process.

[0217] In summary of this example, it was demonstrated that permissible processing temperatures for the herein-invented particle composition are vastly higher, compared to state- of-the-art CSP. While previous reports contained some optimization approaches for thermal stability, processing stability has never been increased to 200 °C or above. In the present invention processing stability up to at least 250 °C has been achieved by the incorporation of antioxidants (example 1.4), combined with an optimized core-to-shell ratio (example 2) and an optimized composition of the interlayer (example 3). It has been shown that, while each optimization on its own leads to a slight improvement, thermal instability due to subsequent crosslinking can only be fully overcome by a combination of all three strategies. It has further been demonstrated that all three factors contribute approximately equally to the increased thermal stability.

[0218] Moreover, indications were obtained that the herein reported 250 °C do not represent the upper limit of processing stability. The invention is therefore explicitly not restricted to the most preferred temperature range of 200-250 °C. Methods that only require short-term temperature exposure, such as the melt-shear technique, may be applicable at further increasedtemperatures. In this temperature ranges, however, other degradation effects besides the crosslinking reaction become the limiting effect. These degradation effects may have been suppressed or delayed as a side effect of the antioxidant incorporation but have not been examined in detail.

[0219] The increased temperature stability generates a variety of advances for established processing methods and enables the application of new methods that are not suitable for state-of-the-art CSP. In the case of extrusion, the lower viscosity at elevated temperatures enables higher throughput and lower engine workload. This is also beneficial for other extrusionbased processes, e.g. injection molding. Regarding the melt-shear technique, higher temperatures facilitate the incorporation of (co-)monomers with relatively high glass transitions in the shell, e.g. all types of methacrylates. Thereby possibilities for the synthesis of CSP and the production of opal films are broadened. The advantages of the melt-shear process also apply to other types of pressing, such as compression molding or coating techniques. For DIW, higher temperatures allow for higher printing speeds and the incorporation of a broader palette of monomers. It also reduces the risk of a clogged nozzle and allows for the use of smaller nozzles to achieve higher resolutions. While only established methods were discussed in this example, the following example 5 demonstrates how the explained advantages facilitate the application of novel processing techniques, that are not applicable for state-of-the-art CSP.Example 5

[0220] This example shall illustrate how this invention enables the production of filament and subsequent 3D printing of said filament with the fused filament fabrication (FFF) technique, which has not been reported before for CSP-based materials. This shows exemplary how the increased temperature-stability enables novel processing options, that have previously not been applicable. Moreover, it is described how a thermoplastic material properties can be achieved, which is a key prerequisite for FFF and another difference from state-of-the-art CSP, which usually feature elastomeric shells. Structural color quality and geometrical object complexity are shown to be on a high level, thus illustrating the application potentials and improvements compared to state-of-the-art methods, which are mostly limited to 2D films and foils.5.1 Preparation of the particle composition

[0221] Within the previous examples 1-4, it was demonstrated how this invention allows for processing stability up to at least 250 °C. Three solution approaches must be combined, including the incorporation of a suitable antioxidant, an optimized core-to-shell ratio, and an optimized interlayer composition. The high-temperature stability is also one key prerequisite for FFF. Therefore, for the synthesis of CSP5, once again an optimized core proportion of 40-45 vol% was targeted and an optimized content of 5 wt% ALMA in the interlayer was used. After synthesis and freeze drying, 2 wt% AO1010 as an antioxidative additive and 0.03 wt% carbon black were incorporated via extrusion.

[0222] Besides high-temperature stability, thermoplastic material properties are another key prerequisite for FFF. The filament must be rigid and buckling-resistant at room temperature, but flowable at elevated printing temperature. Compared to the previous examples and also in distinction to known CSP, the glass transition of the shell had to be increased above room temperature. For this purpose, 50 wt% of MMA was used as a comonomer in the shell, which otherwise consisted of EA. A scheme of the synthesis strategy is provided in Figure 14. Synthesis and obtained particle architecture of CSP5, as well as the preparation of the particle composition, represent one of the preferred embodiments of this invention.

[0223] The detailed synthesis protocol to produce the core particles C5 is described in the following: A 5 L double-wall reactor under nitrogen atmosphere at 75 °C, equipped with a reflux condenser and a stirrer was filled with a monomer solution (MSa1) of 18.0 g S (Ma), 2.00 g BDDA, 1.20 g SDS, and 1400 g deionized water. While stirring at 250 rpm, the polymerization was initiated with 0.360 g NaDS and 2.59 g NaPS. After a reaction time of 10 minutes, a monomer solution (MSa2) was added continuously with a flow rate of 4.2 mL min-1, using a rotary piston pump. MSa2 contained 382 g S (Ma), 38.2 g BDDA, 1.25 g SDS, 2.18 g KOH, 1.19 g DIPES, and 491 g deionized water. As unconsumed monomer was visually observable during the addition of MSa2, the polymerization was reinitiated after 180 min with 0.800 g NaPS. 5 minutes before reinitiation and 20 minutes afterward, the addition of MSa2 was paused. After the complete addition of MSa2, the reaction was kept at a constant temperature and stirred for 60 minutes. The core dispersion was drained from the reactor and diluted to a solid content (=polymer content) of 8 wt%.

[0224] The detailed synthesis protocols to produce CSP5 is described in the following: 552 g of the diluted core dispersion was filled in in a 1 L double-wall reactor, equipped with a reflux condenser and a stirrer under N2 atmosphere at 75 °C. While stirring at 200 rpm, the polymerization was initiated by adding 0.685 g NaPS and 0.117 NaDS. After a reaction time of 15 minutes, a monomer solution (MSb) was continuously added with a constant flow rate of 2.0 mL min’1. MSb contained 7.01 g MMA (Mb), 0.369 g ALMA, 0.134 g DIPES, 0.0676 g SDS, and 29.8 g deionized water. After the complete addition of MSb and an additional 15 minutes, the polymerization was reinitiated with 0.101 g NaPS. After 10 minutes a monomer solution (MSc) was continuously added with a flow rate of 2.0 mL min-1using a rotary piston pump. MSc contained 45.1 g of each EA and MMA (Me), 0.356 g SDS, 0.287 g DIPES, 0.501 g KOH, and 118 g deionized water. After the complete addition of MSc, the reaction mixture was kept at a constant temperature and stirred for an additional hour.

[0225] DLS- and TEM analysis (Figure 15), confirmed the good control over each polymerization step for the formation of C5 and CSP5. The average diameter of the core batch C5 was determined as DDLS = 172 nm (±1.9%) and DTEM = 169 nm (±4.7%). The average diameter of the final CSP5 was determined as DDLS = 248 nm (±3.4%) and DTEM = 230 nm (±4.9%). This data on particle size was in good agreement with expectations, particularly regarding the optimized core proportion of 40 vol%, according to TEM data. Besides, the low ± deviations indicated that monodisperse particles were obtained after each polymerization step.

[0226] Compared to CSP4 in example 4, which also represent the preferred particle architecture of this invention, the amount of monomer (Ma in step a2; Mb; Me) in starved-feed mode was in sum reduced to 91 %; relative to Ma in step a1 and considering that the overall batch size was doubled for the interlayer and shell synthesis of CSP5. The formation of the seed particles in step a1) was however not altered. Therefore, a reduced amount of monomer was available for the growth of the same number of (seed) particles. The final volume of CSP5 compared to CSP4, as calculated from TEM-diameters, was thus reduced to 89%, which is in good accordance with the theoretical value 91 % as predicted from the direct proportionality explained in the DETAILED DESCRIPTION OF THE INVENTION.5.2 Analysis of thermo-rheological properties

[0227] The suitability of the particle composition, consisting of CSP5 and an antioxidant, for FFF was assessed with an analysis of the thermal and rheological properties. The high temperature stability of the particle composition and the efficient suppression of thermally induced cross-linking was inter alia indicated by the absence of an exothermic peak at elevatedtemperatures in the DSC measurement in Figure 16a. Moreover, the DSC thermogram revealed two glass transitions at 36 °C and 111 °C, which are attributable to the particle shell and core, respectively. While the core’s glass transition is comparable to state-of-the-art particles and the previous examples, the comparably higher shell’s glass transition is of utmost importance. Since the rheological properties of the particles are predominantly determined by the shell material and the glass transition is above room temperature, the material is in a glassy state at room temperature. This is a prerequisite for filament-based 3D printing. At room temperature, the filament has to be non-deformable and resistant to buckling. During FFF, the filament acts as a piston to push the heated material out of the nozzle. With a too soft filament, i.e. with the elastomeric shell of state-of-the-art CSP, the developed extrusion forces are too low, and 3D printing is not possible. Moreover, the gears of the filament extruder would penetrate the surface of an elastomeric filament, digging into the material and thus eroding or scraping off the filament structure, instead of pushing it downwards.

[0228] In addition to a high extrusion force, feeding of the filament from the extruder through the heated nozzle is facilitated by a decreased viscosity at elevated temperatures. To achieve a suitable low viscosity, temperature must be increased. For state-of-the-art particles, the processing temperature is limited upwards by a subsequent cross-linking of unconsumed grafting anchors, as demonstrated for CSP1 in example 1 and CSP4C in example 4. The inventive particle composition however overcomes this instability. Since the filament in this example represents an embodiment of the preferred particle composition, other degradation mechanisms determine the upper temperature limit. The onset of thermal degradation was determined to be 307 °C (Figure 16b), indicating that processing at temperatures of up to 250 °C, and probably even substantially higher, are applicable.

[0229] The temperature dependence of the viscosity is shown in Figure 16c. Starting from the glassy state at room temperature, viscosity asymptotically decreased by two orders of magnitude upon heating to 200-250 °C. The steady decrease of viscosity and the absence of a plateau or even increase at higher temperatures indicated the excellent temperature stability of the particle composition and the effective suppression of thermally induced cross-linking reactions. This was to be expected from particle architecture and -composition, since the high temperature stability is the key feature of the particle composition and the invention, as described above.

[0230] The suitability of the invention for FFF is further enhanced by the strong shearthinning behavior, depicted in Figure 16d. In contrast to the high zero-shear viscosity, viscosity drops by one to two orders of magnitude at higher shear rates during 3D Printing (see also Table 6). Owing to the vastly reduced viscosity at elevated temperature under shear force, the material could easily be pushed out of the nozzle during 3D printing, while the downwards pushing filament was in a glassy state and therefore provided a sufficient pushing force and buckling resistance. Once printed, the material was not exposed to shear forces anymore and cooled down. Therefore, viscosity increased rapidly, providing dimensional accuracy and a stable foundation for the following layers.5.3 Processing via Fused Filament Fabrication

[0231] An overview of all processing steps from synthesis to FFF is provided in Figure 17. Before polymer processing, i.e. during synthesis and after freeze-drying, the particles were not ordered and therefore exhibited no structural color, as sketched in the magnified scheme. The lyophilized particle mass was mixed with 2 wt% AO1010 (as antioxidative additive) and 0.03 wt% carbon black (as usual) and homogenized via extrusion. Afterward, a filament composed of thisparticle composition with an average diameter of 1.75 mm was prepared by extrusion. The extrusion temperature of 180 °C during filament production would have been too high for state- of-the-art particles, as described in the previous examples (especially example 1) but did not damage the temperature stable particle composition based on CSP5. Said filament was 3D- printed on an entry-level commercial FFF printer. FFF of pure CSP, CSP-based materials, or particle compositions has not been possible before this invention. As-printed objects showed a high grade of detail and 3D complexity, which has also not been reported for state-of-the-art CSP. Structural color quality of the 3D-Prints was also found to be on a high level, in terms of peak sharpness, intensity, and visual appearance. As expected from Bragg’s law, printed objects showed an iridescent appearance, i.e. , a change of color in dependence on illumination- and / or viewing angle.

[0232] Slicing setting, machine parameters, and a calculation of the shear rates during FFF are provided in Table 6. As the model is built layer by layer, the layer thickness of 0.30 mm dictates the final resolution in the vertical direction. The layer width of 1.00 mm determines the resolution in the horizontal direction. Supports were disabled for the herein shown models but are in general usable with the material. No further processing, finishing, or removal of residues was applied. With the printing speed of 600 mm min-1for all print moves, the shear rates inside the crosssection of the nozzle YNozzie and between the printing surface and nozzle YPrintwere calculated and are indicated in Figure 16d. For the first layer, the printing surface is the heated printing bed, and afterward the current top layer of the printed part.

[0233] Table 6: Slicing settings, machine parameters, and calculations of shear rates during the fused filament fabrication process.Parameter Description ValueFilament Diameter Machine Parameter 1.75 mmBuild Volume Machine Parameter 250*210*210 mmNozzle Diameter dNMachine Parameter 0.80 mmHotend Temperature Filament Setting 220 °CBed Temperature Filament Setting 75 °CPart Cooling Filament Setting Max. Fan SpeedLayer Height h Print Setting 0.30 mmExtrusion width w Print Setting 1 .00 mmPrinting Speed v Print Setting 600 mm min-1Infill Percentage Print Setting 30%Fill Pattern Print Setting HoneycombBottom solid layers Print Setting 4Top solid layers Print Setting 5Flow rate V V = v - w - h 3.00 mm3s'1Shear rate nozzle yNozzie 59.7 s-1Shear rate p i rint V i rpri uinict 33.3 s-1

[0234] 3D printing was found to be possible in a large temperature range of 200 to 250 °C, with high surface resolution, steady flow, sufficient layer adhesion and without nozzle clogging. No signs of thermal degradation or damage were observed in this temperature range, once again demonstrating the excellent temperature stability of the inventive particle composition. In a temperature range from 200 to 220 °C printing speed could continuously be increased, due to the reduction in viscosity. A temperature increase above 220 °C, however, did not enable significantly faster printing, probably due to heat creep, but led to increased stringing and warping. Therefore, the default printing temperature was set to 220 °C. Part cooling was found to massively help with overhangs and therefore enabled.

[0235] The following aspects were considered to set the temperature of the heated printing bed: On the one hand, the temperature of the printing bed should be as high as possible to reduce warping. An increase above 80 °C, however, was found to be not further advantageous for warping, especially with increasing part height. This was probably caused by increasing heat loss, as a printer without enclosure was used. On the other hand, a bed temperature above the glass transition of 35 °C enhances bed- and layer adhesion but also reduces part stability. As shown in Figure 16a, the viscosity at 60-80 °C is still two orders of magnitude higher, compared to the printing temperature, but decreases rapidly with further temperature increase. A bed temperature above 80 °C could therefore lead to an instability of printed parts and dimensional inaccuracy, especially in the bottom layers with low spatial distance to the printing bed. Taking all these aspects into consideration, a temperature of 75 °C was found to be a good compromise to reduce warping and increase adhesion, while ensuring sufficient rheo-mechanical stability during printing.

[0236] The intensity mean of the reflected wavelengths in the UV-Vis spectrum in Figure 17 was found to be 548 nm. This value deviates less than 5% from the calculated wavelength according to the Bragg-Snell law at = 571 nm and is therefore in good accordance with expectations. The expected wavelength was calculated according to the formulas described in the DETAILED DESCRIPTION OF THE INVENTION as follows: neff= ' i ^i - ni= 1.52; wherein n(PS) = 1.59; n(PBDDA) = 1.46; n(PEA) = 1.47; n(PMMA) = 1.48 ( / (Core) = ( / / (P(S90-co- BDDAw)) = 40 vol%; and ( / (Shell) = ( / / (P(EA5o-co-MMA5o)) = 60 vol%.The influence of the small interlayer on the effective refractive index was neglected. The volume shares of core and shell were calculated according to C>7Bw(Core) = 169 nm and C>7Bw(Shell) = 230 nm, assuming an ideal spherical geometry. = 2^DTEMy / ne^2— cos2a = 571 nm; wherein DTEM = 230 nm; netf = 1 .52; and a = 90°This calculation is only valid for light incidence at an angle of 90°, i.e. normal to the surface. At steeper viewing angles or different illumination, the color shows a blue-shift, which leads to an iridescent appearance of the 3D printed objects.

[0237] In summary of this example, it was demonstrated how the increased temperature stability of the invented particle composition enables the application of common industrial polymerprocessing methods, that are not suitable for state-of-the-art materials. Moreover, it was shown, how a thermoplastic shell can be achieved, which has also not been reported before, but is another key prerequisite for FFF and other large-scale polymer processing techniques.REFERENCESKulyk, O., Rocard, L., Maggini, L. & Bonifazi, D. Synthetic strategies tailoring colours in multichromophoric organic nanostructures. Chem. Soc. Rev. 49, 8400-8424 (2020). https: / / doi.org: 10.1039 / c9cs00555bTeyssier, J., Saenko, S. V., van der Marel, D. & Milinkovitch, M. C. Photonic crystals cause active colour change in chameleons. Nat. Commun. 6, 6368 (2015). https: / / doi.org: 10.1038 / ncomms7368Dumanli, A. G. & Savin, T. Recent advances in the biomimicry of structural colours. Chem. Soc. Rev. 45, 6698-6724 (2016). https: / / doi.org:10.1039 / c6cs00129gWang, W. etal. Stimulus-Responsive Photonic Crystals for Advanced Security. Adv. Funct.Mater. (2022). https: / / doi . org : 10.1002 / adfm .202204744Shang, L., Zhang, W., Xu, K. & Zhao, Y. Bio-inspired intelligent structural color materials. Mater. Horiz. 6, 945-958 (2019). https: / / doi.org: 10.1039 / c9mh00101 hQi, Y. & Zhang, S. Recent advances in multifunctional shape memory photonic crystals and practical applications. Nano Res. (2023). https: / / doi . org : 10.1007 / s 12274-023-5801 -0 Hong, W., Yuan, Z. & Chen, X. Structural Color Materials for Optical Anticounterfeiting. Small 16, e1907626 (2020). https: / / doi.org: 10.1002 / smll.201907626Li, M. et al. Bioinspired Colloidal Photonic Composites: Fabrications and Emerging Applications. Adv. Mater, e2110488 (2022). https: / / doi.org: 10.1002 / adma.202110488Vatankhah-Varnosfaderani, M. etal. Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration. Science 359, 1509-1513 (2018). https: / / doi.org: 10.1126 / science.aar5308Schertel, L., Magkiriadou, S., Yazhgur, P. & Demirdrs, A. Manufacturing Large-scale Materials with Structural Color. Chimia 76 (2022). https: / / doi.org: 10.2533 / chimia.2022.833 Li, K., Li, C., Li, H., Li, M. & Song, Y. Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing. iScience 24, 102121 (2021). https: / / doi.org: 10.1016 / j.isci.2021.102121Hou, X., Li, F., Song, Y. & Li, M. Recent Progress in Responsive Structural Color. J. Phys. Chem. Lett. 13, 2885-2900 (2022). https: / / doi.org:10.1021 / acs.jpclett.1c04219Hut, Y., Zhangt, Y, Yang, D., Ma, D. & Huang, S. Self-assembly of colloidal particles into amorphous photonic crystals. Mater. Adv. 2, 6499-6518 (2021). https: / / doi.org:10.1039 / d1 ma00477hCai, Z. et al. From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications. Chem. Soc. Rev. 50, 5898-5951 (2021). https: / / doi .org: 10.1039 / d0cs00706dHynninen, A. P, Thijssen, J. H., Vermolen, E. C., Dijkstra, M. & van Blaaderen, A. Selfassembly route for photonic crystals with a bandgap in the visible region. Nat. Mater. 6, 202-205 (2007). https: / / doi .org: 10.1038 / nmat1841Vogel, N. et al. Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies. Proc. Natl. Acad. Sci. USA 112, 10845-10850 (2015). https: / / doi.org: 10.1073 / pnas.1506272112Schafer, C. G. Stimuli-Responsive Polymer-Opalfilme: Intelligente Materialien furoptische Sensoranwendungen und Sicherheitsmerkmale [dissertation], TU Darmstadt, (2016).Manoharan, V. N. & Stephenson, A. B. A field guide to angle-independent structural color. Phys. Today 74, 62-63 (2021). https: / / doi.org: 10.1063 / pt.3.4663Xuan, Z. et al. Artificial Structural Colors and Applications. Innovation (Camb) 2, 100081 (2021). https: / / doi.org: 10.1016 / i.xinn.2O21.100081Shah, K. W., Huseien, G. F. & Kua, H. W. A State-of-the-Art Review on Core-Shell Pigments Nanostructure Preparation and Test Methods. Micro 1 , 55-85 (2021). https: / / doi.org: 10.3390 / microl 010006Gallei, M. Functional Polymer Opals and Porous Materials by Shear-Induced Assembly of Tailor-Made Particles. Macromol. Rapid. Commun. 39, 1700648 (2018). https: / / doi.org: 10.1002 / marc.201700648 von Freymann, G., Kitaev, V., Lotsch, B. V. & Ozin, G. A. Bottom-up assembly of photonic crystals. Chem. Soc. Rev. 42, 2528-2554 (2013). https: / / doi.org: 10.1039 / c2cs35309aSchafer, C. G. et al. Reversible Light-, Thermo-, and Mechano-Responsive Elastomeric Polymer Opal Films. Chem. Mat. 25, 2309-2318 (2013). https: / / doi.org: 10.1021 / cm400911 jWinter, T., Boehm, A., Presser, V. & Gallei, M. Dye-Loaded Mechanochromic and pH- Responsive Elastomeric Opal Films. Macromol. Rapid. Commun. 42, e2000557 (2021). https: / / doi.org: 10.1002 / marc.202000557Boehm, A. K. etal. Porous Mixed-Metal Oxide Li-Ion Battery Electrodes by Shear-Induced Co-assembly of Precursors and Tailored Polymer Particles. ACS Appl. Mater. Interfaces 13, 61166-61179 (2021). https: / / doi.org:10.1021 / acsami.1c19027Zhao, Q. et al. Large-scale ordering of nanoparticles using viscoelastic shear processing.Nat. Commun. 7, 11661 (2016). https: / / doi.org: 10.1038 / ncommsl 1661Colvin, V. L. From Opals to Optics: Colloidal Photonic Crystals. MRS Bulletin 26, 637-641 (2011). https: / / doi.org:10.1557 / mrs2001.159Spahn, P. etal. Modification of the refractive-index contrast in polymer opal films. J. Mater.Chem. 21 , 8893 (2011). https: / / doi .org: 10.1039 / c1 jm00063bGonzalez-Urbina, L., Baert, K., Kolaric, B., Perez-Moreno, J. & Clays, K. Linear and nonlinear optical properties of colloidal photonic crystals. Chem. Rev. 112, 2268-2285 (2012). https: / / doi.org: 10.1021 / cr200063fMarlow, F., Muldarisnur, Sharifi, P, Brinkmann, R. & Mendive, C. Opals: status and prospects. Angew. Chem. Int. Ed. Engl. 48, 6212-6233 (2009). https: / / doi. org: 10.1002 / anie.200900210Isapour, G. & Lattuada, M. Bioinspired Stimuli-Responsive Color-Changing Systems. Adv. Mater. 30, e1707069 (2018). https: / / doi .org: 10.1002 / adma.201707069Takeoka, Y. Stimuli-responsive opals: colloidal crystals and colloidal amorphous arrays for use in functional structurally colored materials. J. Mater. Chem. C 1 (2013). https: / / doi.org: 10.1039 / c3tc30885ePhillips, K. R. et al. A colloidoscope of colloid-based porous materials and their uses.Chem. Soc. Rev. 45, 281-322 (2016). https: / / doi.org: 10.1039 / c5cs00533gXiong, R. et al. Biopolymeric photonic structures: design, fabrication, and emerging applications. Chem. Soc. Rev. 49, 983-1031 (2020). https: / / doi.org: 10.1039 / c8cs01007b Liu, P. et al. Self-assembled colloidal arrays for structural color. Nanoscale Adv. 1 , 1672- 1685 (2019). https: / / doi.org: 10.1039 / c8na00328aPursiainen, O. L. J. et al. Shear-Induced Organization in Flexible Polymer Opals. Adv.Mater. 20, 1484-1487 (2008). https: / / doi.org: 10.1002 / adma.200701363Finlayson, C. E. & Baumberg, J. J. Generating Bulk-Scale Ordered Optical Materials Using Shear-Assembly in Viscoelastic Media. Materials 10, 688 (2017). https: / / doi.org: 10.3390 / ma10070688Schafer, C. G., Lederle, C., Zentel, K., Stuhn, B. & Gallei, M. Utilizing stretch-tunable thermochromic elastomeric opal films as novel reversible switchable photonic materials. Macromol. Rapid. Commun. 35, 1852-1860 (2014). https: / / doi.org:10.1002 / marc.201400421Li, H. et al. Polychrome photonic crystal stickers with thermochromic switchable colors for anti-counterfeiting and information encryption. Chem. Eng. J. 426 (2021). https: / / doi.org:10.1016 / j.cei.2021.130683Siegwardt, L. & Gallei, M. Complex 3D-Printed Mechanochromic Materials with Iridescent Structural Colors Based on Core-Shell Particles. Adv. Fund. Mater. 33 (2023). https: / / doi.org: 10.1002 / adfm.202213099Rosetta, G. et al. Chromaticity of structural color in polymer thin film photonic crystals. Opt. Express. 28, 36219-36228 (2020). https: / / doi.org: 10.1364 / OE.410338Finlayson, C. E., Rosetta, G. & Tomes, J. J. Spectroscopic Ellipsometry and Optical Modelling of Structurally Colored Opaline Thin-Films. Appl. Sci. 12 (2022). https: / / doi.org: 10.3390 / app12104888Snoswell, D. R. et al. Shear ordering in polymer photonic crystals. Phys. Rev. E. Stat. Nonlin. Soft Matter Phys. 81 , 020401 (2010). https: / / doi.org: 10.1103 / PhysRevE.81 .020401Finlayson, C. E. et al. 3D bulk ordering in macroscopic solid opaline films by edge-induced rotational shearing. Adv. Mater. 23, 1540-1544 (2011). https: / / doi.org: 10.1002 / adma.201003934Imai, Y. et al. Electrically conductive polymeric photonic crystals. Soft Matter s (2012). https: / / doi.org: 10.1039 / c2sm06740dXu, W. et al. 3D printing for polymer / particle-based processing: A review. Composites Part B: Engineering 223 (2021). https: / / doi.org: 10.1016 / j.compositesb.2021 .109102A. Gold, S., Strong, R. & N. Turner, B. A review of melt extrusion additive manufacturing processes: I. Process design and modeling. Rapid Prototyp. J. 20, 192-204 (2014). https: / / doi .org: 10.1108 / rpj-01 -2013-0012Wang, X., Jiang, M., Zhou, Z., Gou, J. & Hui, D. 3D printing of polymer matrix composites: A review and prospective. Compos. B. Eng. 110, 442-458 (2017). https: / / doi.org: 10.1016 / j.compositesb.2016.11.034Cogswell, F. N. Polymer melt rheology: a guide for industrial practice. (Woodhead Publishing Limited, 2003).Tadmor, Z. & Gogos, C. G. Principles of Polymer Processing. 2 edn, Vol. 2 (John Wiley & Sons, 2008).Finlayson, C. E., Rosetta, G. & Baumberg, J. J. An Experimental and Theoretical Determination of Oscillatory Shear-Induced Crystallization Processes in Viscoelastic Photonic Crystal Media. Materials 14, 5298 (2021). https: / / doi.org: 10.3390 / ma14185298 Wong, H. S. et al. The rheology and processing of “edge sheared” colloidal polymer opals.J. Rheol. 58, 397-409 (2014). https: / / doi.org:10.1122 / 1 .4862920Katritzky, A. R., Sild, S. & Karelson, M. Correlation and Prediction of the Refractive Indices of Polymers by QSPR. J. Chem. Inf. Comput. 38, 1171-1176 (1998). https: / / doi.org: 10.1021 / ci980087wXu, J., Chen, B., Zhang, Q. & Guo, B. Prediction of refractive indices of linear polymers by a four-descriptor QSPR model. Polymer 45, 8651-8659 (2004). https: / / doi.org: 10.1016 / i.polymer.2004.10.057Finlayson, C. E. & Baumberg, J. J. Polymer opals as novel photonic materials. Polym. Int.62, 1403-1407 (2013). https: / / doi.org: 10.1002 / pi.4582Lovell, P. A. & Schork, F. J. Fundamentals of Emulsion Polymerization.Biomacromolecules 21 , 4396-4441 (2020). https: / / doi.org: 10.1021 / acs.biomac.0c00769Alexander, A. E. & Napper, D. H. Emulsion polymerization. Prog. Polym. Sci. 3, 145-197 (1971). https: / / doi.org:10.1016 / 0079-6700(71)90004-9Pursiainen, O. L. J. et al. Nanoparticle-tuned structural color from polymer opals. Opt. Express 15, 9553-9561 (2007). https: / / doi.org: 10.1364 / oe.15.009553Brandrup, J., Immergut, E. H. & Grulke, E. A. Polymer Handbook. (John Wiley & Sons, 1999).Richard, T. in Kirk-Othmer Encyclopedia of Chemical Technology (ed Claudia; Ley) Ch. Antioxidants, Polymers, 102-134 (John Wiley & Sons, 2000).

Claims

CLAIMS1. Particle comprising a) 40 to 45 vol%, based on the overall volume of the particle and calculated with the average diameter DTEM, of a core, comprising a polymer (P1) prepared by radical polymerization; b) 2 to 10 vol%, preferably 5 vol%, based on the overall volume of the particle and calculated with the average diameter DTEM, of an interlayer comprising a polymer (P2) prepared by radical polymerization, wherein the polymer (P2) comprises 3 to 7 wt%, preferably 5 wt%, of a cross-linker based on the overall weight of the polymer (P2); and c) 45 to 60 vol%, preferably 50 to 55 vol% based on the overall volume and calculated with the average diameter DTEM, of a shell, comprising a polymer (P3) prepared by radical polymerization.

2. The particle of claim 1 , wherein i) the polymer (P1) is prepared by polymerization of one or more monomers comprising a terminal alkene group, preferably styrene; and / or ii) the polymer (P2) is prepared by polymerization of monomers comprising a terminal alkene group, preferably selected from one or more of the group consisting of acrylates and methacrylates, more preferably methyl methacrylate (MMA) and / or ethyl acrylate (EA), most preferably MMA; and / or iii) the polymer (P3) is prepared by polymerization of monomers comprising a terminal alkene group, preferably selected from one or more of the group consisting of acrylates, methacrylates, vinylpyridines, acrylonitrile and acrylamides, more preferably EA, MMA, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl methacrylate and acrylonitrile, most preferably EA and / or MMA.

3. The particle of claim 1 or 2, wherein i) the polymer (P1) further comprises a cross-linker, preferably selected from the group consisting of butanediol diacrylate (BDDA), diallyl phthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 wt%, more preferably 10 wt%, based on the overall amount of polymer (P1); and / or ii) the cross-linker in polymer (P2) is allyl methacrylate (ALMA).

4. The particle of claim 1 or 2, wherein i) the polymer (P1) is poly(styrene-co-butanediol diacrylate) (P(S-co-BDDA)); and / or ii) the polymer (P2) is poly(methyl methacrylate-co-allyl methacrylate) (P(MMA-co-ALMA)); and / or iii) the polymer (P3) is polyethyl acrylate (PEA) or poly(ethyl acrylate-co-methyl methacrylate (P(EA- co-MMA)).

5. The particle of claims 1 to 4, wherein i) the dry particle has an average diameter DTEM of 50 to 500 nm, optionally 50 to 153 nm, 154 to 314 nm, or 315 to 500 nm, preferably 154 to 314 nm; and / or ii) the standard deviation of DTEM is < 5%, preferably < 2%; and / or iii) the particle has an average hydrodynamic diameter DDLS of 55 to 600 nm; and / or iv) the PDI of DDLS is < 5%, preferably < 2%; and / or v) the core is dimensionally stable upon particle processing with the mechanical properties of a thermoset; and / or vi) the shell is elastomeric or thermoplastic and viscous at room temperature or meltable at elevated processing temperatures; and / or vii) the refractive index of core and shell differ from each other; preferably the values for the refractive index of core and shell differ from each other by > 0.05, more preferably > 0.1 ; and / or viii) the core polymer (P1), preferably comprising P(S-co-BDDA), has an effective refractive of 1 to 2, preferably 1.58; and / or ix) the shell polymer (P3), preferably comprising PEA and / or PMMA, has an effective refractive index of 1 to 2, preferably 1.47 to 1.48; and / or x) the particle has an effective refractive index of 1 to 2; preferably 1.4 to 1.6; particularly preferred 1.52; and / or xi) the glass transition temperature of the core polymer (P1) is 90 to 150 °C, preferably 105 to 120 °C; and / or xiii) the glass transition temperature of the shell polymer (P3) is -50 °C to 120 °C, preferably -50 °C to 20 °C or 21 to 120 °C.

6. A particle composition, comprising the particles according to claims 1 to 5 and carbon black and / or at least one radical scavenger and / or antioxidant.

7. The particle composition of claim 6, wherein i) wherein carbon black is present in the composition in an amount of 0.01 to 0.2 wt%, preferably 0.02 to 0.05 wt%, more preferably 0.03 wt% based on the overall weight of the composition; and / or ii) the at least one radical scavenger and / or antioxidant, is preferably selected from the group of primary antioxidants, more preferably quinol, Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate) (AO1010), Butylated hydroxytoluene (BHT), Octadecyl 3-(3,5-di-t-butyl- 4-hydroxyphenyl)propionate; and / or iii) wherein the at least one radical scavenger and / or antioxidant is present in the composition in an overall amount of 0.1 to 5 wt%, preferably 0.5 to 2 wt%, more preferably 2 wt% based on the overall weight of the composition; and / or iv) the composition is preferably prepared by extrusion and / or compounding; and / or v) the composition is solid, preferably formed as a filament; more preferably as filament for 3D printing; preferably the filament has a diameter of 1.00 to 3.00 mm, more preferably 1.75 mm or 2.85 mm, and most preferred 1.75 mm.

8. Use of the particle of claims 1 to 5 or the particle composition of claim 6 or 7 in a process of polymer processing, wherein optionally the process comprises at least one process step selected from the group consisting of, (co- and / or profile-)extrusion, compounding, direct-ink-writing (DIW), fused filament fabrication (FFF), uniaxial pressing, embossing, compression molding, calendaring, parison forming, rotational molding, blow molding, injection molding, deep drawing, surface coating; optionally the process involves a) the preparation of said filament of claim 7 v), preferably by extrusion and / or b) 3D printing with the FFF technique of said filament.

9. The use of the particle of claims 1 to 5 or the particle composition of claims 6 or 7, wherein the polymer processing of claim 8 is carried out at elevated temperatures above 25 °C, preferably above 165 °C, more preferably at 200 to 250 °C.

10. A process for the preparation of monodisperse particles according to claims 1 to 5, comprising carrying out at least one of the following steps a) to c), preferably in the same reactor, resulting in a particle comprising a core, an interlayer, and a shell: a) radical polymerization of monomers of a monomer solution (MSa) comprising at least one monomer comprising a terminal alkene group (Ma), preferably styrene, resulting in the core polymer (P1) of the particle; b) further radical polymerization of monomers of a monomer solution (MSb) comprising at least one monomer comprising a terminal alkene group (Mb), preferably selected from one or more of the group consisting of acrylates and methacrylates, more preferably MMA and / or EA, most preferably MMA, and 3 to 7 phr, preferably 5 phr, a cross-linker, preferably ALMA, adding the interlayer polymer (P2) to the particle; and c) further radical polymerization of monomers of a monomer solution (MSc) comprising at least one monomer comprising a terminal alkene group (Me), preferably selected from one or more of the group consisting of acrylates, methacrylates, vinylpyridines, acrylonitrile and acrylamides, more preferably EA, MMA, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, n-butyl methacrylate and acrylonitrile, most preferably EA and / or MMA, adding the shell polymer (P3) to the particle.

11. The process of claim 10, wherein step a) comprises two steps a1) and a2), carried out sequentially; wherein i) in step a1) 1 to 15 wt%, preferably 4 wt%, of (Ma) based on the overall amount of (Ma) used in step a1) and step a2) is added; and / or ii) in step a2) 85 to 99 wt%, preferably 96 wt%, of (Ma) based on the overall amount of Ma used in step a1) and step a2) is added; and / or iii) the reaction in step a1) and / or step a2) is carried out under continuous stirring; and / or iv) the reaction in step a1) and / or step a2) is carried out at a temperature of 50 to 100 °C, preferably at 65 to 90 °C, more preferably at 75 °C; and / or v) the reaction in step a1) and / or step a2) is carried out under an inert atmosphere, preferably a nitrogen atmosphere; and / or vi) the monomer solution of step a1), (MSa1) comprisesa) (Ma); preferably in an amount of 0.5 to 5.0 wt%, more preferably 1.3 wt% based on the overall weight of (MSa1); and / or b) (Ma); in an overall amount of by definition 100 phr; and / or c) at least one cross-linker, preferably selected from the group consisting of BDDA, diallyl phthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 phr, more preferably 10 to 12 phr; and / or d) at least one detergent, preferably sodium dodecyl sulfate (SDS), preferably in an amount of 1 to 100 phr; and / or e) a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 3000 to 20000 phr, more preferably 7500 to 8000 phr; and / or f) at least one polymerization initiator, preferably a combination of sodium disulfite (NaDS) and sodium persulfate (NaPS), preferably 0 to 15 phr NaDS and 1 to 100 phr NaPS; and / or vii) preferably the at least one polymerization initiator is added to (MSa1) after all other components are already present in (MSa1); and / or viii) step a1) is carried out for 0 to 30 minutes, preferably 10 to 15 minutes, more preferably 10 minutes after all components have been added; and / or ix) the monomer solution of step a2), (MSa2) comprises a) (Ma); preferably in an amount of 10 to 60 wt%, more preferably 42 wt%, based on the overall weight of MSa2; and / or b) (Ma); in an overall amount, by definition, of 100 phr; and / or c) at least one cross-linker, preferably selected from the group consisting of BDDA, diallyl phthalate, and divinylbenzene, more preferably BDDA; preferably in an amount of 5 to 20 phr, more preferably 10 phr; and / or d) at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and disodium isododecyl phenyl ether sulfonate (DIPES), preferably each in an amount of 0.1 to 1 phr, more preferably SDS in an amount of 0.33 phr and DIPES in an amount of 0.31 phr; and / or e) at least one inorganic base and / or buffer system, preferably a base selected from the group consisting of KOH and NaOH, preferably in an amount of 0 to 4 phr, more preferably 0.57 phr and / or f) a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 50 to 500 phr, more preferably 100 to 150 phr; and / or x) (MSa2) is added continuously over 2 h to 6 h, preferably 4 to 5 h, preferably to the reaction solution of step a1), comprising (MSa1); and / or xi) optionally at least one polymerization initiator, preferably NaPS; preferably in an amount of 0 to 2 phr, is added during the continuous addition of (MSa2); and / or xii) step a) is carried out for another 0 to 120 min; after all components of step a2) have been added; and / or xiii) optionally the reaction solution comprising (MSa1) and (MSa2) is drained from the reactor after the completion of step a) and diluted to a defined solids content, preferably 5 to 20 wt%, more preferably 8 wt%, before continuing with step b).

12. The process of claim 10 or 11 , wherein in step b)i) the reaction is carried out under continuous stirring; and / or ii) the reaction is carried out at a temperature of 50 to 100 °C, preferably at 65 to 90 °C, more preferably at 75 °C; and / or iii) the reaction is carried out under an inert atmosphere, preferably a nitrogen atmosphere; and / or iv) at least one polymerization initiator is added, preferably to the reaction solution comprising (MSa1) and (MSa2), before all other components of step b); preferably a combination of NaDS and NaPS; more preferably NaDS in an amount of 0 to 5 phr, and NaPS in an amount of 4 to 30 phr, most preferably NaDS in an amount of 1.7 phr and NaPS in an amount of 10 phr; and / or v) after addition of the at least one polymerization initiator in step b) the further components of step b) are added after 0 to 30 min, preferably after 10 to 15 min; and / or vi) the monomer solution of step b), (MSb) comprises a) (Mb), preferably in an amount of 10 to 24 wt%, more preferably 19 wt% based on the overall weight of the monomer solution (MSb); and / or b) (Mb); in an overall amount of by definition 100 phr; and / or c) at least one cross-linker, which is ALMA; preferably in an amount of 3 to 7 phr, more preferably 5 phr; and / or d) at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and DIPES, preferably each in an amount of 0.5 to 5 phr, more preferably SDS in an amount of 0.98 phr and DIPES in an amount of 1.9 phr; and / or e) a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 200 to 2000 phr, more preferably 400 to 500 phr; and / or vii) (MSb) is added continuously over 10 to 30 min, preferably 15 to 20 min, preferably to the reaction solution of step a) comprising (MSa1) and (MSa2); and / or viii) step b) is carried out for another 0 to 30 min, preferably 10 to 15 min; after all components of step b) have been added.

13. The process of claims 10 to 12, wherein in step c) i) the reaction is carried out under continuous stirring; and / or ii) the reaction is carried out at a temperature of 50 to 100 °C, preferably at 65 to 90 °C, more preferably at 75 °C; and / or iii) the reaction is carried out under an inert atmosphere, preferably a nitrogen atmosphere; and / or iv) at least one polymerization initiator, preferably NaPS, is added, preferably to the reaction solution of step b), comprising (MSa1), (MSa2), and (MSb), before all other components of step c); preferably in an amount of 0 to 1 phr, more preferably in an amount of 0.1 phr; and / or v) after addition of the at least one polymerization initiator in step c) the further components of step c) are added after 0 to 30 min, preferably after 10 to 15 min. vi) the monomer solution of step c), (MSc) comprises a) (Me), preferably in an overall amount of 10 to 60 wt%, preferably 43 wt% based on the overall weight of the monomer solution (MSc); and / or b) (Me); in an overall amount of by definition 100 phr; and / orc) at least one detergent, preferably selected from the group of anionic detergents, more preferably SDS and DIPES, preferably each in an amount of 0.1 to 1 phr, more preferably SDS in an amount of 0.39 phr and DIPES in an amount of 0.32 phr; and / or d) at least one inorganic base and / or buffer system, preferably a base selected from the group consisting of KOH and NaOH, preferably in an amount of 0 to 4 phr, more preferably 0.56 phr; and / or e) a dispersion medium, preferably the dispersion medium is a protic solvent, more preferably the protic solvent is water, preferably water in an amount of 50 to 400 phr, more preferably 100 to 150 phr; and / or vii) (MSc) is added continuously over 60 to 180 min, preferably 90 to 150 min; preferably to the reaction solution of step b) comprising (MSa1), (MSa2), and (MSb); and / or viii) step c) is carried out for another 0 to 120 min, preferably 60 min; after all components of step c) have been added.

14. The process of claims 10 to 13, wherein the amounts of the monomers (Ma), (Mb), (Me) relative to each other are: i) the amount of monomer (Ma) is 19 to 59 wt%, preferably 40 to 45 wt%; and ii) the amount of monomer (Mb) is 1 to 15 wt%, preferably 5 wt%; and iii) the amount of monomer (Me) is 40 to 80 wt%, preferably 50 to 55 wt%.

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