A thermochromic hydrogel with a predetermined phase-transition temperature for smart window applications

Abstract

The invention provides a solid-state thermochromic hydrogel (10), wherein (A) the thermochromic (10) hydrogel comprises: 10-50 wt.% of water (1); and 50-90 wt.% of a copolymer (2), wherein the copolymer comprises an N,N-dimethylaminoethyl methacrylate moiety (21), a fluorinated monomer moiety (22), and a crosslinker moiety (23); and (B) the thermochromic hydrogel (10) has a transition temperature (TT) selected from the range of 24-40°, wherein the thermochromic hydrogel (10) is configured to be (i) transparent below the transition temperature (TT) and (ii) turbid at or above the transition temperature (TT).

Description

[0001] A thermochromic hydrogel with a predetermined phase-transition temperature for smart window applications

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a solid-state thermochromic hydrogel. The invention further relates to a device and a system comprising said solid-state thermochromic hydrogel. The invention further relates to a method of providing a solid-state thermochromic hydrogel. The invention further relates to a temperature management method for regulating temperature in an indoor space using the solid-state thermochromic hydrogel. The invention further relates to a use of the solid-state thermochromic hydrogel in smart window applications.

[0004] BACKGROUND OF THE INVENTION

[0005] Thermochromic hydrogels are known in the art. W02020097163A1, for instance, describes a hydrogel thin-film device for use in light management, the device comprising: a layer comprising temperature-responsive hydrogel microparticles, the temperature-responsive hydrogel microparticles comprising poly (N- isopropylacrylamide)-2- Aminoethylmethacrylate hydrochloride (pNIPAm-AEMA) microparticles, wherein solar gain is regulated in the device by tuning light scattering in the layer by controlling particle size of the pNIPAm-AEMA microparticles.

[0006] Zhang Qi, et al. (2014): “Oxygen-Nitrogen switchable copolymers of 2,2,2- trifluoroethyl methacrylate and N,N-dimethylaminoethyl methacrylate”, Macromolecular Rapid Communications, describes a novel class of O2 / N2 switchable polymers, especially an aqueous FMA-DMA copolymer solution which becomes transparent with O2 bubbling and turns to turbid with N2 purging. This O2 / N2-responsive switchability between the transparent and turbid states is reversible. The FMA-DMA copolymer is thermosensitive and has a lower critical solution temperature (LCST) of 24.5 °C.

[0007] Rabiee Hesamoddin et al. (2019): “O2 / N2-responsive microgels as functional draw agents for gas-triggering forward osmosis desalination”, Journal of Membrane Science, describes functional O2 / N2 gas-responsive microgels using fluorine monomers of trifluoroethyl methacrylate (FM) or pentafluorostyrene (FS), and water-soluble monomers of diethylaminoethyl methacrylate (DEAEMA), dimethylaminoethyl methacrylate (DMAEMA), hydroxy ethyl methacrylate (HEMA) and N-isopropyl acrylamide (NIP AM).

[0008] US2023366265A1 describes a solar and thermal regulating window structure including: an optically-transparent housing frame; a reversible liquid absorbent material layer positioned in the housing frame; a thermally-reflective layer having high solar transmittance and high thermal reflectance positioned over the liquid absorbent material layer; a liquid, being absorbed in the liquid absorbent material layer below a selected transition temperature, and being positioned over the liquid absorbent material layer above the selected transition temperature, such that when below the selected transition temperature, the window structure facilitates indoor solar heating through solar transmittance during daytime and facilitates indoor heat insulation through thermal reflectance during daytime and nighttime, when above the selected transition temperature, the window structure facilitates indoor heat dissipation through thermal emission; and an optical film with high transmittance for both solar and thermal radiation, configured to seal the liquid absorbent material layer, liquid, and thermally-reflective layer in the housing frame.

[0009] SUMMARY OF THE INVENTION

[0010] To tackle the ever-increasing global greenhouse effect and reach the goal of a carbon-neutral future, one crucial challenge is to decrease the amount of energy consumption via energy saving management of buildings. Approaches like thermochromic smart windows hold practical significance in reducing energy consumption in residential or office buildings, due to their autonomous light management and temperature modulation.

[0011] Known thermochromism devices are mainly based on vanadium dioxide (VO2) or poly(N-isopropylacrylamide) (PNIPAM). VO2 undergoes a reversible metal-semiconductor transition (MST) at a critical temperature (of about 68 °C), causing it to be transparent to infrared light under the critical temperature, while it becomes reflective to infrared light above the critical temperature. However, the critical temperature of these VO2 based transitions may be relatively high compared to the human temperature comfort levels (which levels are approximately between 19.5 °C and 29.5 °C). Strategies such as doping metal ions (e.g., Nb, Ti) in VO2 or fabricating nanoscale VO2 particles could help to decrease the critical temperature and increase the solar transmittance (ATsoiar) to a certain degree, however, such materials are scarce and thus relatively expensive. Hence, conventional thermochromic VO2 devices face limitations, including a too high critical temperature, low luminous transmittance (Tium) and low solar transmittance modulation (ATsoiar), which may not be satisfactory for practical applications.

[0012] Temperature responsive hydrogels have emerged as an alternative; whereby light management may be pursued by maneuvering the light-scattering behavior of the material. PNIPAM hydrogel has been reported for use in smart window systems, as it combines the advantage of a low phase-transition temperature (of about 32 °C), a higher luminous transmittance (Tium) and solar transmittance modulation (ATsoiar) than VO2. The thermochromism performance of PNIPAM has been further improved by preparing microgel suspension, hydrogel-derived liquid, and copolymers. However, the phase-transition temperature of PNIPAM is still relatively high compared to the human temperature comfort zone. In addition, for smart window applications based on microgel suspension, the thermochromic material may need to be encapsulated between rigid and heavy (glass) substrates. Such liquid-based glass devices are not only confronted with possible liquid leakage but may also be lacking in flexibility and may be relatively heavy for certain smart window applications (such as e.g., in cars). Moreover, it may be desired to provide thermochromism devices with further improved coloring / bleaching speed and durability.

[0013] Hence, it is an aspect of the invention to provide an alternative thermochromic hydrogel, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0014] According to a first aspect, the invention provides a thermochromic hydrogel. Especially, in embodiments, the thermochromic hydrogel may be a solid hydrogel, i.e., a solid- state thermochromic hydrogel. The thermochromic hydrogel may, in embodiments, comprise water and a copolymer. Especially, in embodiments, the thermochromic hydrogel may comprise (demineralized) water. Further, in embodiments, the copolymer may comprise one or more of an N,N-dimethylaminoethyl methacrylate (DMAEMA) moiety, a fluorinated monomer moiety, and a crosslinker moiety. Especially, in embodiments, the copolymer may comprise at least the N,N-dimethylaminoethyl methacrylate (DMAEMA) moiety. The thermochromic hydrogel may especially have a transition temperature (TT). In embodiments, the transition temperature (TT) may be selected from the range of 24-40 °C. The thermochromic hydrogel may especially be configured to be transparent (at a temperature) below the transition temperature (TT). Conversely, the thermochromic hydrogel may especially be configured to be turbid (at a temperature) at or above the transition temperature (TT). Hence, in specific embodiments, the invention may provide a solid-state thermochromic hydrogel, wherein (A) the thermochromic hydrogel may comprise: 10-50 wt.% of (demineralized) water; and 50-90 wt.% of a copolymer, wherein the copolymer comprises an N,N-dimethylaminoethyl methacrylate (DMAEMA) moiety, a fluorinated monomer moiety, and a crosslinker moiety; and (B) the thermochromic hydrogel may have a transition temperature (TT) selected from the range of 24-40°, wherein the thermochromic hydrogel may be configured to be (i) transparent below the transition temperature (TT) and (ii) turbid at or above the transition temperature (TT).

[0015] Such embodiments may be beneficial as the proposed solid-state thermochromic hydrogel may have a phase-transition temperature within the human comfort zone. The thermochromic material may be applied in smart windows of for example a green house, a car window, and skyscraper coverings to regulate temperature autonomously. Therefore, the thermochromic hydrogel may be applied in a sandwich-structured smart window, for example composed of two polymer (e.g., polyethylene terephthalate (PET)) films with the thermochromic hydrogel configured in between. Such sandwich- structured device may be relatively easily provided through fast UV-curing. The resulting sandwich-structured device may have a combination of a relatively high solar transmittance modulation and high luminous transmittance, which may allow for efficient sunlight blockage in hot weather and clear vision in cool weather. Furthermore, the solid-state thermochromic hydrogel may have a relatively high durability with high consistency of the transmittance with relatively little to no performance degradation over more than 100 heating / cooling cycles. Moreover, the transition between the transparent and the turbid state may be relatively fast, i.e., a fast coloring and bleaching process may be achieved using the solid-state thermochromic hydrogel of the invention.

[0016] Hence, the invention may provide a thermochromic material that may be transparent when cool and opaque when warm. Especially, the invention may provide a solid- state thermochromic hydrogel with a predetermined phase-transition temperature for smart window applications.

[0017] The solid-state thermochromic hydrogel (or “hydrogel”) may comprise water and a copolymer. Here below, embodiments of the different elements of the solid-state thermochromic hydrogel will be described in further detail.

[0018] The term hydrogel may herein refer to a biphasic material, i.e., a material having two (or more) phases. Especially, the biphasic material may comprise a mixture of (porous, permeable) solids and an interstitial fluid. In the case of a hydrogel, the interstitial fluid may comprise at least water. In some embodiments, the interstitial fluid of the hydrogel may even be composed of only water. Hence, in embodiments, the hydrogel may comprise water. Such embodiments may be beneficial as water is a relatively cheap and abundant solvent. Moreover, water is nontoxic and has a neutral pH, making it an environmentally and economically friendly solvent. In embodiments, the water may comprise demineralized water (i.e., having an amount of total dissolved solids of less than 10 mg / L). Moreover, in embodiments, the water may comprise distilled water (i.e., having an amount of total dissolved solids of less than 1.5 mg / L).

[0019] The thermochromic hydrogel may, in embodiments, comprise at least 5 wt.% (demineralized) water, such as at least 10 wt.%, like at least 15 wt.%, especially at least 20 wt.%. Additionally, in embodiments, the thermochromic hydrogel may comprise at most 70 wt.% (demineralized) water, such as at most 60 wt.%, like at most 55 wt.%, especially at most 50 wt.%. In embodiments, the thermochromic hydrogel may comprise water selected from the range of 5-65 wt.%, such as from the range of 10-50 wt.%, like from the range of 12-48 wt.%, especially from the range of 15-45 wt.%. Especially, in embodiments, the thermochromic hydrogel may comprise water selected from the range of 30-50 wt.%, like from the range of 35-45 wt.%. A too high weight percentage of water in the hydrogel may cause excessive swelling, which may weaken the mechanical properties and lifetime of the hydrogel. Conversely, a too low weight percentage of water in the hydrogel may cause reduced flexibility and toughness of the hydrogel, which may affect its performance and stability over time.

[0020] Furthermore, in embodiments, the interstitial fluid may, additionally or alternatively, comprise one or more of methanol, ethanol, glyceryl, acetone, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, ethylene glycol, and ethyl acetate. The interstitial fluid may especially have a relatively high polarity, especially, the interstitial fluid may be able to form hydrogen bonds. Such embodiments may be beneficial as the thermochromicity, especially the reversible phase separation (see also further below), of the hydrogel may be based on the formation and breakage of hydrogen bonds between the hydrogel and the interstitial fluid.

[0021] As mentioned above, the thermochromic hydrogel may be a solid-state thermochromic hydrogel. The term “solid-state” may herein refer to a hydrogel that may be self-standing as a result of crosslinking. Such crosslinking may result in a bulk solid elastomer, i.e., the hydrogel, which may be self-supporting rather than being supported through suspension in a liquid. The solid state of the thermochromic hydrogel may provide the advantage that the hydrogel may be configured (or sandwiched) between any type of substrate, rigid and / or flexible, enabling a wide array of applications (such as e.g., smart car-windows, horticulture, and wearables). Hence, the invention may provide a solid-state hydrogel.

[0022] Therefore, in embodiments, the hydrogel may comprise a polymer. Especially, in embodiments, the hydrogel may comprise a polymer comprising at least an N,N- dimethylaminoethyl methacrylate (or DMAEMA) moiety and a fluorinated monomer moiety. Hence, the hydrogel may comprise a copolymer. The copolymer may further, in embodiments, comprise a crosslinker. The crosslinker may especially be configured to crosslink (or interconnect) the DMAEMA moiety and the fluorinated monomer moiety. N,N- dimethylaminoethyl methacrylate has excellent biocompatibility and is biodegradable. Poly (DMAEMA) is known for its use in polymer-based gene delivery systems. Hence, the use of the DMAEMA moiety is advantageous due to its environmentally friendly properties. Moreover, the DMAEMA moiety may provide the thermal responsivity in the thermochromic hydrogel.

[0023] The thermochromic hydrogel may, in embodiments, comprise at least 30 wt.% copolymer, such as at least 40 wt.%, like at least 45 wt.%, especially at least 50 wt.%. Additionally, in embodiments, the thermochromic hydrogel may comprise at most 95 wt.% copolymer, such as at most 90 wt.%, like at most 85 wt.%, especially at most 80 wt.%. In embodiments, the thermochromic hydrogel may comprise copolymer selected from the range of 35-95 wt.%, such as from the range of 50-90 wt.%, like from the range of 52-88 wt.%, especially from the range of 55-85 wt.%. Especially, in embodiments, the thermochromic hydrogel may comprise copolymer selected from the range of 50-70 wt.%, like from the range of 55-65 wt.%. A too high weight percentage of copolymer in the hydrogel may cause reduced flexibility and toughness of the hydrogel, which may affect its performance and stability over time. Conversely, a too low weight percentage of copolymer in the hydrogel may cause excessive swelling, which may weaken the mechanical properties and lifetime of the hydrogel.

[0024] In specific embodiments, the water and the copolymer may together provide at least 70 wt.% of the thermochromic hydrogel, such as at least 80 wt.%, like at least 90 wt.%, especially at least 95 wt.%, including 100 wt.%. Furthermore, in embodiments, the hydrogel may comprise further components. Hence, in embodiments, the water and the copolymer may together provide at most 98 wt.% of the thermochromic hydrogel, such as at most 95 wt.%, like at most 90 wt.%.

[0025] Especially, in embodiments, the hydrogel may also comprise an additive. The additive may for example comprise a dye (or a colorant). For example, in embodiments, the hydrogel may comprise one or more of an azobenzene-based dye, an anthraquinone-based dye, a xanthene-based dye, a triphenylmethane-based dye, etc... Such a dye may be beneficial as different applications may benefit from different hydrogel colors. For example, a horticulture application of the hydrogel may prefer a relatively lighter color of the hydrogel when in the turbid state, whereas a relatively darker color may e.g., be preferred in windows of dark-rooms or bedrooms.

[0026] Additionally or alternatively, in embodiments, the additive may comprise nanoparticles. For example, in embodiments, the hydrogel may comprise reflective nanoparticles, such as light-reflective particles, i.e., particles of nanometer scale having light- reflective properties. The terms “light” and “radiation” are herein interchangeably used, unless clear from the context that the term “light” only refers to visible light. The terms “light” and “radiation” may thus refer to UV radiation, visible light, and IR radiation. Especially, the terms “light” and “radiation” may refer to (at least) visible light. Yet in some embodiments, such as for curing purposes, the terms “light” and “radiation” may refer to (at least) UV light. What type, especially what wavelength, of light the light-reflective particles reflect may depend on the desired application of the hydrogel. For instance, a thermochromic hydrogel dedicated to blocking heat (from sunlight) may comprise light-reflective nanoparticles that may be reflective for light having a wavelength in the IR wavelength range (e.g. >750 nm, such as >780 nm). Conversely, a thermochromic hydrogel dedicated to create a dark environment (in other words “black-out”) may comprise light-reflective nanoparticles that may be reflective for light having a wavelength in the visible wavelength range (e.g. between 380-780 nm). The hydrogel may thus comprise light-reflective nanoparticles. For example, in embodiments, the hydrogel, especially the nanoparticles, may comprise one or more of gold nanoparticles, silver nanoparticles, titania (TiCh) nanoparticles, aluminum nanoparticles, etc... Additionally or alternatively, in embodiments, the hydrogel may comprise thermochromic nanoparticles, i.e., particles of nanometer scale having (additional, e.g., enhancing) thermochromic behavior. For example, in embodiments, the hydrogel may comprise one or more of vanadium oxide (VO2) based nanoparticles, leuco-dye based nanoparticles, titania (TiCh) nanoparticles, DMAEMA- based nanoparticles, etc... Such nanoparticles may further help improve the solar modulation of the thermochromic hydrogel.

[0027] Yet additionally or alternatively, in embodiments, the additive may comprise ionic liquid. In such embodiments, the ionic liquid may provide the advantage of improved modulation of the physical properties of the hydrogel, such as viscosity. Furthermore, the ionic liquid may act as a lubricant.

[0028] Yet additionally or alternatively, in embodiments, the additive may comprise hydroxypropyl cellulose. In such embodiments, the hydroxypropyl cellulose may provide the advantage of stabilizing the hydrogel. The hydroxypropyl cellulose may for example act as a thickener and / or emulsifier. Hence, in embodiments, the thermochromic hydrogel may further comprise an additive selected from the group comprising a dye, reflective nanoparticles, such as light-reflective nanoparticles, thermochromic nanoparticles, ionic liquid, and hydroxypropyl cellulose.

[0029] Yet additionally or alternatively, in embodiments, the additive may comprise an antioxidant, such as ascorbic acid. Yet additionally or alternatively, in embodiments, the additive may comprise a UV blocker, such as e.g. 2-hydroxy-4-acryloyloxyethoxy benzophenone. Such embodiments may be beneficial in improving the functioning of the thermochromic hydrogel and its long-term performance.

[0030] Other ways to improve the long-term performance and stability of the thermochromic hydrogel may include (carefully) washing the thermochromic hydrogel in water and / or buffer solution to remove uncured monomers and reaction byproducts. Hence, in embodiments, the method may comprise washing the thermochromic hydrogel in water and / or buffer solution.

[0031] The weight percentages of water and copolymer in the hydrogel may especially be selected such that the hydrogel may have a transition temperature (TT) selected from the range of 24-40°. Hence, in embodiments, the hydrogel may be configured to undergo a transition at its transition temperature. Especially, in embodiments, the hydrogel may be configured to be transparent (at a temperature) below the transition temperature (TT). Additionally, in embodiments, the hydrogel may be configured to be turbid (at a temperature) at or above the transition temperature (TT). In specific embodiments, the hydrogel may be configured to become white (at a temperature) at or above the transition temperature (TT). However, this may not necessarily be the case, e.g., a dye may be added to the hydrogel, see also above. Hence, in embodiments, the hydrogel may be a thermochromic hydrogel. Herein, the term thermochromic may refer to a hydrogel which may undergo a reversible change of color in response to a change in temperature. Especially, the hydrogel may transition from a one phase system to a two-phase system in response to temperature elevation (or the other way around in response to temperature reduction). The thermochromic hydrogel may, in embodiments, thus undergo reversible phase separation. At an environmental temperature above a lower critical transition temperature (LCST) of the hydrogel, the hydrophilic polymer phase would become less miscible with the water phase, forming nanoscale water droplets inside the polymer due to phase separation. As such, light would scatter at the interface of these interlaced pores, appearing to be the turbid state in macroscale. In other words, the LCST of the hydrogel, and thus the transition temperature (TT), may be selected by selecting the water concentration in the hydrogel. Herein, the thermochromic hydrogel may especially be configured to become opaque when the temperature increases to (or over) the transition temperature (TT), whereas it may be (or again become) light-transparent (at or) below the transition temperature (TT).

[0032] In embodiments, the transition temperature (TT) may be selected from the range of > 20 °C, such as from the range of > 22 °C, like from the range of > 24 °C, especially from the range of > 28 °C. Moreover, in embodiments, the transition temperature (TT) may be selected from the range of < 80 °C, such as from the range of < 70 °C, like from the range of < 60 °C, especially from the range of < 50 °C. In specific embodiments, the transition temperature (TT) may be selected from the range of 23-42 °C, such as from the range of 24-40 °C. For example, in embodiments, the transition temperature (TT) may be selected from the range of 26-29 °C. Such embodiments may for example be beneficial for applying the thermochromic hydrogel in temperature-regulating applications for temperate countries. Moreover, in embodiments, the transition temperature (TT) may be selected from the range of 31-35 °C. Such embodiments may for example be beneficial for applying the thermochromic hydrogel in temperature-regulating applications for tropical countries. Hence, in specific embodiments, the transition temperature (TT) may be selected from the range of 26-35 °C.

[0033] The use of N,N-dimethylaminoethyl methacrylate (or DMAEMA) in the hydrogel may provide the benefit that the DMAEMA-based hydrogel may be transparent at room temperature (i.e. around 20 °C), while turning white and turbid (or opaque) at higher temperatures. For instance, in embodiments, the DMAEMA-based hydrogel may turn white and turbid at temperatures around 26-28 °C (also see experiments below). Furthermore, the DMAEMA-based hydrogel may transition at a relatively fast rate without the need for further stimulants or an artificial source of energy.

[0034] The transition temperature (TT) may also be referred to as a lower critical transition temperature (LCST). The combination of a fluorinated monomer moiety with the DMAEMA moiety may provide the benefit of decreasing the lower critical transition temperature (LCST) (and thus the transition temperature) of the hydrogel with respect to a hydrogel without the fluorinated monomer moiety. Such a decrease may be caused by the relative hydrophobicity of the fluorinated monomer moiety. Conversely, the lower critical transition temperature (LCST) (and thus the transition temperature) of the hydrogel may be increased through the addition of a more hydrophilic monomer. Moreover, in embodiments, salts may be included in the hydrogel to tune swelling behavior, ionic interactions and therewith the phase separation of the hydrogel. In embodiments, the copolymer may comprise a mole ratio Ri of the N,N- dimethylaminoethyl methacrylate moiety relative to the fluorinated monomer moiety. Especially, in embodiments, the mole ratio Ri may be selected from the range of 6:3 to 30:3, such as from the range of 7:3 - 27:3, like from the range of 10:3 - 25:3. More especially, in embodiments, the mole ratio Ri may be selected from the range of 8:1 - 10: 1. In specific embodiments, the mole ratio Ri may be 9: 1.

[0035] Hence, in embodiments, the copolymer may comprise a hydrophobic monomer moiety, especially, a fluorinated monomer moiety. Such hydrophobic monomer, especially fluorinated monomer, moieties may provide the advantage that they may be mixed and form crosslinks with the DMAEMA moiety relatively easily. Moreover, the addition of the hydrophobic monomer may help reduce the LCST of the thermochromic hydrogel to a desired transition temperature.

[0036] The fluorinated monomer may, in embodiments, be selected from the group comprising: hexafluoro-iso-propyl methacrylate, 2,2,2-trifluoroethyl acrylate, 2,2,2- trifluoroethyl methacrylate (FMA), lH,lH,2H,2H-heptadecafluorodecyl methacrylate (HDFDMA), and lH,lH,5H-octafluoropentyl methacrylate. Additionally, the fluorinated monomer may, in embodiments, be selected from lH,lH,2H,2H-perfluorodecyl methacrylate and 1,1,1,3,3,3-hexafluoroisopropyl methacrylate. Alternatively, in embodiments, the copolymer may comprise another type of hydrophobic monomer moiety, such as a nonhalogenated analog of the fluorinated monomer. Especially, in embodiments, the copolymer may comprise a hydrophobic monomer moiety selected from the group comprising: tert-butyl methacrylate, cyclohexyl methacrylate, isobomyl methacrylate, lauryl methacrylate, phenyl methacrylate, and benzyl methacrylate.

[0037] In specific embodiments, the fluorinated monomer moiety may comprise 2,2,2- trifluoroethyl methacrylate (FMA). Hence, in such embodiments, the copolymer may thus comprise poly(N,N-dimethylaminoethyl methacrylate / 2,2,2-trifluoroethyl methacrylate). Furthermore, in specific embodiments, the fluorinated monomer moiety may (essentially) consist of 2,2,2-trifluoroethyl methacrylate, and the mole ratio Ri may be selected from the range of 10:3 to 25:3, such as from the range of 8:1 to 10: 1, especially (about) 9: 1. In particular, good results have been obtained with a copolymer of 2,2,2-trifluoroethyl methacrylate and N,N-dimethylaminoethyl methacrylate (see below). Such embodiments may be beneficial as the FMA may be flexible enough (i.e. not too robust / rigid) to efficiently crosslink with the DMAEMA moiety. As described above, the thermochromic hydrogel may be configured for different applications, such as application in different temperature climates. Such configuration may especially be dependent on the water content in the hydrogel. Through careful selection of the weight percentage (or concentration) of water relative to the copolymer in the hydrogel, the transition temperature (TT) may be selected (i.e. tuned) for a desired application.

[0038] In embodiments, thermochromic hydrogel may comprise water selected from the range of 5-35 wt.%, such as from the range of 10-30 wt.%, like from the range of 12-29 wt.%, especially from the range of 15-25 wt.%. Such embodiments may be beneficial as a relatively low water concentration may result in a relatively high transition temperature (TT), which may be desirable for application in more tropical climates. Hence, in specific embodiments, thermochromic hydrogel may comprise 10-30 wt.% water and 70-90 wt.% copolymer.

[0039] Conversely, in embodiments, thermochromic hydrogel may comprise water selected from the range of 28-55 wt.%, such as from the range of 30-52 wt.%, like from the range of 35-48 wt.%, especially from the range of 37-45 wt.%. Such embodiments may be beneficial as a relatively high water concentration may result in a relatively low transition temperature (TT), which may be desirable for application in more temperate (or moderate) climates. Hence, in specific embodiments, thermochromic hydrogel may comprise 30-50 wt.% water and 50-70 wt.% copolymer.

[0040] As described above, the copolymer may further comprise a crosslinker moiety. In embodiments, the crosslinker may comprise one or more of the group comprising: N,N'- methylenebisacrylamide (MBA), polyethylene glycol diacrylate (PEGDA), divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA), trimethylpropane trimethacrylate, 1,3- diisopropenyl benzene, pentaerythritol triacrylate, N,N’-l,4-phenylenediacrylamide, 2,6- bisacryloylamidopyridine, 3,5-bis(acryloamido)benzoid acid, and 1,4-diacryloyl piperazine, especially from the group comprising MBA, PEGDA, ethylene glycol dimethacrylate, trimethylpropane trimethacrylate, 1,3-diisopropenyl benzene, pentaerythritol triacrylate, N,N’- 1,4-phenylenediacrylamide, 2,6-bisacryloylamidopyridine, 3,5-bis(acryloamido)benzoid acid, and 1,4-diacryloyl piperazine. Additionally or alternatively, in embodiments, the crosslinker may comprise one or more of the group comprising: triethylene glycol dimethacrylate, poly(ethylene glycol) dimethacrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanediol diacrylate , and 1,6-hexanediol dimethacrylate. In particular, good results have been obtained with a copolymer of the crosslinker N,N' -methylenebisacrylamide (MBA) with 2,2,2-trifluoroethyl methacrylate and N,N-dimethylaminoethyl methacrylate (DMAEMA) (see below). Good results have also been obtained with a copolymer of the crosslinker ethylene glycol dimethacrylate (EGDMA) with 2,2,2-trifluoroethyl methacrylate and N,N- (dimethylamino)ethyl methacrylate (DMAEMA).

[0041] Furthermore, in embodiments, the thermochromic hydrogel may comprise a buffer. The buffer may especially be prepared using a water-miscible solvent. In embodiments, the buffer may be prepared using a water-miscible solvent selected from the group comprising: 2-[4-(2-hydroxyethyl)piperazin-l-yl]ethanesulfonic acid (HEPES), formamide, and dimethylformamide (DMF).

[0042] According to a further aspect, the invention provides a thermochromic device. In embodiments, the thermochromic device may comprise the solid-state thermochromic hydrogel as described above. Furthermore, the thermochromic device may comprise an encapsulant. In embodiments, the encapsulant may be configured to encapsule (or host) the thermochromic hydrogel. Such an encapsulant may especially prevent drying out of the hydrogel. Furthermore, the encapsulant may protect the hydrogel from contamination, which may extend the lifetime of the thermochromic device.

[0043] In embodiments, the thermochromic device may thus comprise the solid-state thermochromic hydrogel. Note, however, that an uncured precursor solution as described above may also already present a reversible change in opacity in response to temperature variations. Hence, in embodiments, the thermochromic device may comprise the (uncured) precursor solution (thus in its liquid form).

[0044] In embodiments, the encapsulant may comprise a transparent material, i.e., the encapsulant may comprise a transparent encapsulant material. Especially, the encapsulant, such as the encapsulant material, may comprise a light-transparent material. In specific embodiments, the encapsulant may comprise a transparent material selected from the group comprising: a glass, a polymeric material (such as e.g. polyethylene terephthalate (PET), polyestradiol phosphate (PEP), or polyethylene naphthalate (PEN)), a cellophane, and a quartz. Especially, in embodiments, the encapsulant may comprise a polymeric material, such as e.g. polyethylene terephthalate (PET), polyestradiol phosphate (PEP), or polyethylene naphthalate (PEN). In particular, in embodiments, the encapsulant may comprise polyethylene terephthalate (PET). As such, in embodiments, the encapsulant may be configured to transmit light received by the thermochromic device to the thermochromic hydrogel. The encapsulant may especially do so independently of the temperature of the thermochromic device (and / or of the thermochromic hydrogel). Such embodiments may be beneficial as, at a temperature below the transition temperature (TT), the luminous transmittance of the thermochromic device may not be impaired by the encapsulant. Furthermore, an encapsulant comprising a polymeric material may provide the benefit of endowing the thermochromic device with flexibility, light weight, user safety, easy -installation. Hence, in embodiments, the thermochromic device may comprise the solid-state thermochromic hydrogel, as described above, and an encapsulant, wherein the encapsulant may be configured to encapsule (or host) the thermochromic hydrogel, and wherein the encapsulant may comprise a (light-)transparent material. Especially, in embodiments, the thermochromic device may comprise a light-transparent encapsulant configured to encapsule the thermochromic hydrogel.

[0045] The thermochromic device may, in embodiments, be a relatively thin device. Especially, in embodiments, the thermochromic device may have one of a disc-like shape, a flat elongated shape, and a sheet-like shape. Hence, the thermochromic device may have one dimension (e.g. one of length (L), width (W), and thickness (t)), that may be substantially smaller than its other dimensions (e.g. the other ones of length (L), width (W), and thickness (t)). Especially, in embodiments, the thermochromic device may have one dimension (e.g. one of length (L), width (W), and thickness (t)) that may be at most 50% of the other dimensions (e.g. the other ones of length (L), width (W), and thickness (t)), such as at most 40%, like at most 30%, especially at most 20%, more especially at most 10%. However, in such embodiments, that one dimension (e.g. one of length (L), width (W), and thickness (t)) may be at least 0.01% of the other dimensions (e.g. the other ones of length (L), width (W), and thickness (t)), such as at least 0.05%, like at least 0.1%, especially at least 0.5%, more especially at least 1%. Such embodiments may be beneficial as, at a temperature at or above the transition temperature (TT), the thermochromic hydrogel may relatively quickly and efficiently become turbid, therewith providing efficient solar transmittance modulation.

[0046] In particular, in embodiments, the thermochromic hydrogel may have a thickness (t) defined as the smallest dimension of the thermochromic hydrogel. Hence, the thermochromic hydrogel may have a length (L), a width (W), and a thickness (t), wherein L > t, and wherein W > t. In embodiments, the thickness (t) may be selected from the range of 0.4- 3 mm, such as from the range of 0.4-2 mm, like from the range of 0.5-1.8 mm, especially from the range of 0.8-1.5 mm. If the thickness (t) is selected too small (i.e. the gel would become too thin) it may become too difficult to produce a stable hydrogel. Additionally, if the thickness of the hydrogel becomes too small, blocking of solar light by the turbid state of the hydrogel may become less efficient. Conversely, if the thickness (t) is selected too large (i.e. the gel would become too thick), then consistent curing over the whole area of the hydrogel may become difficult, which may lead to inhomogeneity within the device. Inhomogeneity within the hydrogel may also lead to less efficient blocking of solar light by the turbid state of the hydrogel. Hence, in embodiments, the thermochromic hydrogel may have a thickness (t) defined as the smallest dimension of the thermochromic hydrogel, wherein the thickness (t) is selected from the range of 0.4-2 mm.

[0047] Moreover, note that, the terms “transparent” or “light-transparent” may herein refer to the characteristic of a material or a hydrogel that enables light to pass through said material or hydrogel. Especially, herein, light having a wavelength selected from at least the visible wavelength range may pass through (or may be transmitted by) the material and / or hydrogel. Additionally, in embodiments, light having a wavelength selected from one or more of the UV wavelength range and the IR wavelength range may pass through (or may be transmitted by) the material or hydrogel. Hence, the thermochromic hydrogel may comprise a material that may be transparent for, i.e. may substantially transmit, light having one or more of: a wavelength selected from at least the visible wavelength range, a wavelength selected from the UV wavelength range, and a wavelength selected from the IR wavelength range.

[0048] A material being transparent may herein be defined as having a transmittance of at least 50%, such as at least 60%, like at least 70%, especially at least 80% for one or more of: light having a wavelength selected from at least the visible wavelength range, light having a wavelength selected from the UV wavelength range, and light having a wavelength selected from the IR wavelength range. Conversely, a material being turbid (or “opaque”) may herein be defined as having a transmittance of at most 45%, such as at most 40%, like at most 35%, especially at most 30% for one or more of: light having a wavelength selected from at least the visible wavelength range, light having a wavelength selected from the UV wavelength range, and light having a wavelength selected from the IR wavelength range. Moreover, a material may herein be considered transparent for a certain wavelength of light when a ratio of (i) an intensity or (spectral) power of that light being transmitted by the material relative to (ii) an intensity or (spectral) power of that light being reflected by the material is >1.5, like >2, such as >3. Conversely, a material may herein be considered turbid (or opaque) for a certain wavelength of light when a ratio of (i) an intensity or (spectral) power of that light being transmitted by the material relative to (ii) an intensity or (spectral) power of that light being reflected by the material is <0.75, like <0.5, such as <0.33. Transparency and / or turbidity of a material may herein be determined (for example) through (solar) transmittance spectra obtained (especially measured) using a (UV / Visible light / NIR) spectrophotometer and taking into consideration the thickness (t) of the thermochromic hydrogel. Hence, in embodiments, the thermochromic hydrogel may be configured to be (i) transparent below the transition temperature (TT) and (ii) turbid at or above the transition temperature (TT), wherein the transparency and turbidity may be determined based on (solar) transmittance spectra, obtainable using a (UV / Visible light / NIR) spectrophotometer, and the thickness (t) of the hydrogel. Especially, based on (solar) transmittance spectra, obtainable using a (UV / Visible light / NIR) spectrophotometer, and the thickness (t) of the hydrogel a percentage of transmittance of light having one or more wavelengths (when the thermochromic hydrogel is) above the transition temperature may be determined.

[0049] As described above, the thermochromic hydrogel may be configured to be transparent below the transition temperature and turbid at or above the transition temperature. In particular, for a given wavelength, such as for a wavelength in the visible wavelength range, or such as for a wavelength range in the IR wavelength range, may apply that the thermochromic hydrogel is configured to be transparent below the transition temperature and turbid at or above the transition temperature. Hence, in embodiments, the thermochromic hydrogel is configured to be transparent for one or more wavelengths of light, especially for a wavelength range, (when the thermochromic hydrogel is) below the transition temperature and to be turbid for the (same) one or more wavelengths of light, especially for the (same) wavelength range, (when the thermochromic hydrogel is) above the transition temperature, especially wherein (a) below the transition temperature the thermochromic hydrogel has a transmittance of at least 60% for the light and (b) above the transition temperature the thermochromic hydrogel has a transmittance of at most 40% for the one or more wavelengths of light.

[0050] In further embodiments, the thermochromic hydrogel may be configured to have a temperature-dependent transmittance for one or more wavelengths of light. In particular, below the transition temperature the thermochromic hydrogel may have a first transmittance Ti for the one or more wavelengths of light and above the transition temperature the thermochromic hydrogel may have a second transmittance T2 for the one or more wavelengths of light. In embodiments, T1 / T2 > 1.5, such as > 2, especially > 3.

[0051] In further embodiments, the one or more wavelengths, especially the wavelength range, may be selected from the visible wavelength range, such as from the range of 380-780. In further embodiments, the one or more wavelengths, especially the wavelength range, may be selected from the UV wavelength range, such as from the range of 190-380 nm. In further embodiments, the one or more wavelengths of light, especially the wavelength range, may be selected from the IR wavelength range, such as from the range of 780-3000 nm. In further embodiments, the one or more wavelengths of light, especially the wavelength range, may be selected from the IR wavelength range of such as from the range of 700-2500 nm. The relatively highest percentage of solar power is found in this wavelength range, therefore by configuring the thermochromic hydrogel to have a temperature-dependent transmittance for one or more wavelengths of light in the range of 780-2500 nm is beneficial for solar transmittance purposes, e.g. for blocking heat from the sun. Additionally or alternatively, in embodiments, the one or more wavelengths of light, especially the wavelength range, may be selected from the visible wavelength range of 400-780 nm, such as from the range of 400-700 nm. In this wavelength range, the second highest percentage of solar power contribution is found, which accounts for the natural light visible to the human eye. Therefore, by configuring the thermochromic hydrogel to have a temperature-dependent transmittance for one or more wavelengths of light in the range of 400-780 nm is beneficial for luminous transmittance purposes, e.g. for light / blacking out light.

[0052] Herein, UV (ultraviolet) may especially refer to a wavelength selected from the range of 190-380 nm, such as 200-380 nm, though other wavelengths may also be possible. The terms “visible light” or “visible emission”, and similar terms, refer to light having one or more wavelengths in the range of about 380-780 nm. Further, infrared (IR) may refer to radiation having a wavelength selected from the range of 780-3000 nm, such as 780-2000 nm, e.g. a wavelength up to about 1500 nm, though in specific embodiments other wavelengths may also be possible.

[0053] For example, in embodiments, the thermochromic device may be configured for use in a (car) window. In such embodiments, it may be preferred that the thermochromic device may be configured transparent relative to light having a wavelength in the visible wavelength range, while not being configured transparent for light having a wavelength in the UV wavelength range. In another example, in embodiments, the thermochromic device may be configured for use in data encryption. In such embodiments, it may be preferred that the thermochromic device may be configured transparent relative to light having a wavelength in the IR or UV) wavelength range, while not being configured transparent for light having a wavelength in the visible wavelength range.

[0054] In embodiments, the thermochromic device may thus be configured for use in a window, such as e.g. a car or office window. Moreover, in embodiments, the thermochromic device may be configured for use in a greenhouse. In such embodiments, the thermochromic device may be configured parallel to the window or a glass pane of the greenhouse. For example, the thermochromic device may be attached to the window, e.g. through adhesive or means of mechanical attachment. In an alternative example, the thermochromic device may be configured at a distance from the window, e.g. through hanging the device in front of the window like a curtain or shade. Additionally or alternatively, in such embodiments, the thermochromic device may comprise a gradient of thermochromic hydrogels comprising different water concentrations. Hence, in embodiments, the thermochromic device (especially the thermochromic hydrogel) may comprise a gradient of different water concentrations.

[0055] Furthermore, in embodiments, the thermochromic device may be configured for use in a textile and / or in a wearable. For example, the thermochromic device may be configured (i) in the visor of a hat or cap, or (ii) in (or on) a (beach) umbrella. Such embodiments may be possible as the thermochromic hydrogel (and by extension the thermochromic device, when choosing an encapsulant such as a polymer) may be relatively flexible.

[0056] Moreover, in embodiments, the thermochromic device may be configured for use in (or as) a temperature sensor. For example, the thermochromic device may be configured to indicate the temperature of a bottle of milk / formula for an infant.

[0057] Yet further, in embodiments, the thermochromic device may be configured for use in data encryption. For example, the thermochromic device may be configured to only show data or a message under predetermined temperature conditions. Especially, in such embodiments, the thermochromic device may comprise the thermochromic hydrogel over only a part of the area of the thermochromic device. Additionally or alternatively, in such embodiments, the thermochromic device may comprise a gradient of thermochromic hydrogels comprising different water concentrations. Hence, in embodiments, the thermochromic device (especially the thermochromic hydrogel) may comprise a gradient of different water concentrations.

[0058] In specific embodiments, the thermochromic device may thus be configured for use in a window, in a car window, in a greenhouse, in a textile, in a wearable, as a temperature sensor or in data encryption.

[0059] Thus, according to a further aspect, the invention provides a use of the solid- state thermochromic hydrogel, as described herein, in smart window applications. Herein, the term “smart” may refer to the ability of a system or device to sense, respond to, and sometimes learn from its environment or from user input. Moreover, the term “smart window” as used in “smart window applications” may refer to a type of window that can change its light and / or heat transmission properties (such as transparency and / or color) in response to external stimuli such as light, temperature, or electrical voltage.

[0060] Moreover, a “smart window” may also be referred to as a window having

[0061] “switchable glazing” or “dynamic glazing”. Moreover, according to a further aspect, the invention provides a system for regulating temperature in an (at least partially) indoor space. The system may especially comprise the thermochromic device as described herein. In embodiments, the thermochromic device may be arranged (or configured) at an interface between the indoor space and an outdoor space. For example, in embodiments, thermochromic device may be arranged (or configured) at an interface between a room and the outdoor environment. In another example, in embodiments, thermochromic device may be arranged (or configured) at an interface between the interior of a car and the exterior of a car. Especially, in embodiments, the thermochromic device may be configured parallel to a window or a glass pane. For example, the thermochromic device may be attached to the window, e.g. through adhesive or means of mechanical attachment. In an alternative example, the thermochromic device may be configured at a distance from the window, e.g. through hanging the device in front of the window like a curtain or shade. Hence, in specific embodiments, the invention may provide a system for regulating temperature in an indoor space, wherein the system may comprise the thermochromic device, and wherein the thermochromic device may be arranged at an interface between the indoor space and an outdoor space. Such a system, especially the thermochromic device in the system, may become turbid and block sunlight under high temperature without external drivers, which may be highly suitable for autonomous solar modulation.

[0062] According to a further aspect, the invention thus (also) provides a temperature management method for regulating temperature in an (at least partially) indoor space. In embodiments, the temperature management method may comprise configuring the thermochromic device at an interface between the indoor space and an outdoor space (for example as described above). Moreover, in embodiments, the temperature management method may comprise configuring the thermochromic device in a (solar) light-receiving relationship with the external of the indoor space.

[0063] Yet according to a further aspect, the invention provides a method of providing a thermochromic hydrogel, such as e.g. a thermochromic hydrogel as described above. In embodiments, the method may comprise a preparation stage and a curing stage.

[0064] The preparation stage may comprise providing a precursor solution. In embodiments, the precursor solution may comprise N,N-dimethylaminoethyl methacrylate, a fluorinated monomer, a crosslinker, and water.

[0065] In some embodiments, the precursor solution may further comprise a photoinitiator. Hence, in such embodiments, the precursor solution may comprise N,N- dimethylaminoethyl methacrylate, a fluorinated monomer, a photoinitiator, a crosslinker, and water.

[0066] In embodiments, the precursor solution may comprise the N,N- dimethylaminoethyl methacrylate and the fluorinated monomer in a mole ratio Ri selected from the range of 6:3 to 30:3, such as from the range of 7:3 - 27:3, like from the range of 10:3 - 25:3. More especially, in embodiments, the mole ratio Ri may be selected from the range of 8: 1 - 10: 1. In specific embodiments, the precursor solution may comprise the N,N- dimethylaminoethyl methacrylate and the fluorinated monomer in a mole ratio Ri of 9: 1.

[0067] In embodiments, the N,N-dimethylaminoethyl methacrylate and the fluorinated monomer may especially make up at least 45 wt.% of the precursor solution, such as at least 50 wt.%, like at least 55 wt.%, especially at least 65 wt.%. Moreover, in embodiments, the N,N- dimethylaminoethyl methacrylate and the fluorinated monomer may make up at most 95 wt.% of the precursor solution, such as at most 90 wt.%, like at most 80 wt.%, especially at most 70 wt.%. Especially, in embodiments, between 50-90 wt.% of the precursor solution may comprise a copolymer of the N,N-dimethylaminoethyl methacrylate and the fluorinated monomer, such as between 52-88 wt.%, like between 55-80 wt.%.

[0068] Moreover, in embodiments, at least 5 wt.% of the precursor solution may comprise (demineralized) water, such as at least 10 wt.%, like at least 15 wt.%, especially at least 20 wt.%. Additionally, in embodiments, at most 70 wt.% of the precursor solution may comprise water, such as at most 60 wt.%, like at most 55 wt.%, especially at most 50 wt.%. In embodiments, between 5-65 wt.% of the precursor solution may comprise water, such as from the range of 10-50 wt.%, like from the range of 12-48 wt.%, especially from the range of 15- 45 wt.%. The water may especially (in part) be provided in the precursor solution through (or “as part of’) a buffer, as described above.

[0069] Furthermore, in embodiments, between 0.03-0.065 wt.% of the precursor solution may comprise the photoinitiator, such as between 0.035-0.06 wt.%, like between 0.04- 0.055 wt.%. In embodiments, the photoinitiator may comprise 2 -Hydroxy -4 '-(2- hydroxyethoxy)-2-methylpropiophenone (which may also be referred to as Irgacure 2959). Such embodiments may especially be beneficial if the interstitial fluid (see also above) is (demineralized) water. Additionally or alternatively, in embodiments, the photoinitiator may comprise one or more of benzoin methyl ether (BME), 2 -hydroxy-2 -methylpropiophenone (HMPP), 2, 2-dimethoxy-2 -phenyl acetophenone (or Irgacure 651), 1 -hydroxy cyclohexyl phenyl ketone (or Irgacure 184), and 2-methyl-4'-(methylthio)-2 -morpholinopropiophenone (or Irgacure 907). Yet additionally or alternatively, the photoinitiator may comprise lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). Yet additionally or alternatively, in embodiments wherein the interstitial fluid comprises a combination of (demineralized) water with another solvent (see also above), the photoinitiator may comprise one or more of ammonium persulphate (APS), potassium persulphate, and azobisisobutyronitrile (AIBN). The photoinitiator may especially be selected as being soluble in the water-copolymer mixture of the precursor solution. Hence, in specific embodiments, the photoinitiator may comprise 2- Hydroxy-4'-(2-hydroxyethoxy)-2 -methylpropiophenone, wherein the precursor solution may comprise 0.03-0.12 wt.% of the photoinitiator.

[0070] Analogously, in embodiments, between 0.03-0.065 wt.% of the precursor solution may comprise the crosslinker, such as between 0.035-0.06 wt.%, like between 0.04- 0.055 wt.%. In embodiments, the crosslinker may be selected from the group comprising: N,N'- methylenebisacrylamide (MBA), polyethylene glycol diacrylate (PEGDA), divinylbenzene (DVB), ethylene glycol dimethacrylate, trimethylpropane trimethacrylate, 1,3-diisopropenyl benzene, pentaerythritol triacrylate, N,N’-l,4-phenylenediacrylamide, 2,6- bisacryloylamidopyridine, 3,5-bis(acryloamido)benzoid acid, and 1,4-diacryloyl piperazine, especially from the group comprising MBA, PEGDA, ethylene glycol dimethacrylate, trimethylpropane trimethacrylate, 1,3-diisopropenyl benzene, pentaerythritol triacrylate, N,N’- 1,4-phenylenediacrylamide, 2,6-bisacryloylamidopyridine, 3,5-bis(acryloamido)benzoid acid, and 1,4-diacryloyl piperazine. The crosslinker may especially be selected as being soluble in the water-copolymer mixture of the precursor solution. Hence, in specific embodiments, the crosslinker may comprise one or more of N,N' -methylenebisacrylamide, and polyethylene glycol diacrylate, wherein the precursor solution may comprise 0.03-0.12 wt.% of the crosslinker.

[0071] The method may further comprise the curing stage. In embodiments, the curing stage may comprise curing the precursor solution to obtain the thermochromic hydrogel. Hence, the invention may provide a method of providing a thermochromic hydrogel, wherein the method comprises: (A) a preparation stage comprising providing a precursor solution, wherein the precursor solution comprises N,N-dimethylaminoethyl methacrylate, a fluorinated monomer, a crosslinker, and 10-50 wt.% of water, wherein the precursor solution comprises the N,N-dimethylaminoethyl methacrylate and the fluorinated monomer in a mole ratio Ri selected from the range of 7:3 to 27:3; and (B) a curing stage comprising curing the precursor solution to obtain the thermochromic hydrogel.

[0072] In embodiments, the curing stage may comprise photoreactive-curing. Therefore, in embodiments, the precursor solution may (further) comprise a photoinitiator. Such curing may further, in embodiments, especially comprise providing light to the precursor solution. Especially, the curing stage may comprise providing light suitable for curing the precursor solution. For instance, in embodiments, the curing stage may comprise providing light having a wavelength selected from the UV wavelength range to the precursor solution. In embodiments, the precursor solution may be prepared (or provided) in a mold. Hence, the curing stage may comprise curing the precursor solution (in a mold) under light, such as under UV light, to obtain the thermochromic hydrogel. In embodiments, the UV light may especially comprise light having a wavelength selected from the wavelength range of 175-400 nm, such as from the wavelength range of 280-370 nm. However, other wavelengths may be possible as well. For example, the curing stage may comprise curing the precursor solution (in a mold) under light having a wavelength selected from the wavelength range of >400 nm, such as a wavelength of 405 nm. In specific embodiments, (for example when the photoinitatior comprises Irgacure 2959) the UV light may especially comprise light having a wavelength of (about) 365 nm. However, in alternative embodiments, (for example when the photoinitiator comprises LAP) the light may especially comprise light having a wavelength of (about) 405 nm. In particular, in embodiments, the wavelength may be selected based on the precursor solution, i.e., the wavelength may be selected to be suitable for curing the precursor solution.

[0073] Hence, in specific embodiments, the method may comprise (i) a preparation stage comprising providing a precursor solution, wherein the precursor solution may comprise N,N-dimethylaminoethyl methacrylate, a fluorinated monomer, a photoinitiator, a crosslinker, and 10-50 wt.% of water, wherein the precursor solution may comprise the N,N- dimethylaminoethyl methacrylate and the fluorinated monomer in a mole ratio Ri selected from the range of 7:3 to 27:3; and (ii) a curing stage comprising curing the precursor solution under UV light to obtain the thermochromic hydrogel.

[0074] Furthermore, in embodiments, the curing stage of the method may comprise a pre-curing step. As the name suggest, the pre-curing step may be performed prior to the curing of the hydrogel. Moreover, in embodiments, the pre-curing step may be performed after (or optionally during part of) the preparation stage. The pre-curing step may especially comprise pre-curing the precursor solution under UV light to obtain a viscous precursor solution. Therefore, in embodiments, a light intensity of UV light onto the precursors solution may be selected from the range of 10-50 mW / cm2, such as from the range of 20-45 mW / cm2, like from the range of 30-45 mW / cm2, especially from the range of 32-40 mW / cm2.

[0075] In embodiments, the viscous precursors solution may have a viscosity selected from the range of 10-100 mPas, such as from the range of 20-90 mPas, like from the range of 30-80 mPas. Such embodiments may be beneficial as a precursor solution with such a viscosity may be relatively evenly dispersed in a mold (due to the reduced flowability relative to the uncured precursor solution), which may result in a more homogeneous thickness of the gel, i.e., may alleviate bulging effects in the gel.

[0076] As described above, in embodiments, the precursor solution may be prepared in or provided in a mold. In embodiments, the precursor solution may be provided in the mold and may subsequently be cured into a thermochromic hydrogel in essentially a one-step curing process. Alternatively, in embodiments, the precursor solution may be pre-cured in the curing step to provide the viscous precursor solution. The pre-curing step may then, in embodiments, comprise arranging the viscous precursor solution in the mold. Such embodiments may be beneficial as the viscous precursor solution may be less prone to leakages and bubbles. Hence, in such embodiments, a relatively flexible mold (or encapsulant) material may be used with reduced risk of leakage of the precursor solution from the mold (or encapsulant) prior to curing.

[0077] In embodiments, the mold may comprise a material selected from the group comprising: a glass, a polymeric material (such as e.g. polyethylene terephthalate (PET), polyestradiol phosphate (PEP), or polyethylene naphthalate (PEN)), a cellophane, and a quartz. In specific embodiments, the mold may comprise the encapsulant as described above.

[0078] The curing stage may furthermore, in embodiments, comprise a curing step comprising (further) curing the viscous precursor solution (in the mold) under UV light to obtain the thermochromic hydrogel. Hence, in such embodiments, the viscous precursor solution may be provided in the mold and may subsequently be cured into a thermochromic hydrogel in a two-step curing process. Hence, in embodiments, the curing stage may comprise: (i) a pre-curing step comprising pre-curing the precursor solution under UV light to obtain a viscous precursor solution; (ii) arranging the viscous precursor solution in a mold; and (iii) a curing step comprising curing the viscous precursor solution (in the mold) under UV light to obtain the thermochromic hydrogel. With such a method only a relatively simple UV-curing process may be required to provide a thermochromic device that can achieve a low and predetermined (especially during fabrication tunable) transition temperature, fast response time, high durability and cyclability, as well as light weight, flexibility and user safety.

[0079] Alternatively, in embodiments, the curing stage may comprise mixing the precursor solution with polymerization initiator to obtain the thermochromic hydrogel. The polymerization initiator may especially comprise an initiator that initiates the formation of radical species. For example, the (radical) polymerization initiator may comprise tetramethylethylenediamine (TEMED), although other polymerization initiators may be possible as well. In such embodiments, the precursor solution may optionally (also) comprise a catalyst. Good results have been achieved while using TEMED as the polymerization initiator together with ammonium persulfate solution (APS) as the catalyst. It will be clear to the skilled person that other initiators and catalysts may function as well. Hence, in embodiments, the invention may provide a method of providing a thermochromic hydrogel, wherein the method comprises: (A) a preparation stage comprising providing a precursor solution, wherein the precursor solution comprises N,N-dimethylaminoethyl methacrylate, a fluorinated monomer, a crosslinker, and 10-50 wt.% of water, wherein the precursor solution comprises the N,N- dimethylaminoethyl methacrylate and the fluorinated monomer in a mole ratio Ri selected from the range of 7:3 to 27:3; and (B) a curing stage comprising curing the precursor solution by adding the polymerization initiator to obtain the thermochromic hydrogel.

[0080] BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Figs. 1A-C schematically depict embodiments of the thermochromic hydrogel of the invention. Figs. 2-3 schematically depict embodiments of the thermochromic device and system comprising the thermochromic hydrogel of the invention. Fig. 4 schematically depicts some aspects of the invention. Figs. 5A-B schematically depict the method of providing a thermochromic hydrogel of the invention. The schematic drawings are not necessarily on scale.

[0082] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0083] Fig. 1A schematically depicts a solid-state thermochromic hydrogel 10. The thermochromic 10 hydrogel may comprise (demineralized) water 1 and a copolymer 2. Especially, in embodiments, the thermochromic hydrogel 10 may comprise between 10-50 wt.% of (demineralized) water 1. Further, in embodiments, the thermochromic hydrogel 10 may comprise between 50-90 wt.% of the copolymer 2. Further, in embodiments, the water 1 and the copolymer 2 together may provide at least 90 wt.% of the thermochromic hydrogel 10.

[0084] The copolymer may especially comprise an N,N-dimethylaminoethyl methacrylate (DMAEMA) moiety 21, a fluorinated monomer moiety 22, and a crosslinker moiety 23. The copolymer 2 may comprise a mole ratio Ri of the N,N-dimethylaminoethyl methacrylate (DMAEMA) moiety 21 to the fluorinated monomer moiety 22. Especially, in embodiments, the mole ratio Ri may be selected from the range of 7:3 to 27:3.

[0085] Figs. IB and 1C schematically depict several examples of the molecular structure of a (crosslinked) copolymer. Especially, in embodiments, the fluorinated monomer moiety 22 may comprise 2,2,2-trifluoroethyl methacrylate (FMA). Hence, as depicted in Fig.

[0086] IB, the copolymer 2 thus may comprise poly(N,N-dimethylaminoethyl methacrylate / 2,2,2- trifluoroethyl methacrylate). Note that, in these figures, the parentheses are used to indicate a repeating unit within the (crosslinked co)polymeric structure, where the parameters m, n and p may be individually selected integers.

[0087] Especially, in embodiments, the fluorinated monomer moiety 22 may essentially consist of 2,2,2-trifluoroethyl methacrylate. In embodiments, the mole ratio Ri may be selected from the range of 8: 1 to 10: 1 (especially 9: 1) (in mmol).

[0088] As depicted in Fig. IB, the DMAEMA moiety 21 may be crosslinked with 2,2,2- trifluoroethyl methacrylate as the fluorinated monomer moiety 22 and N,N'- methylenebisacrylamide (MBA) as the crosslinker moiety 23. Alternatively, as depicted in Fig.

[0089] IC, the crosslinker moiety 23 may comprise polyethylene glycol dimethacrylate (PEGDMA). Note that, in embodiments, other combinations, such as e.g. other fluorinated monomer moi eties 22 may be applied as well.

[0090] The thermochromic hydrogel 10 may, in embodiments, have a transition temperature (TT) selected from the range of 24-40°. Especially, the thermochromic hydrogel 10 may be configured to be (i) transparent below the transition temperature (TT) and (ii) turbid at or above the transition temperature (TT).

[0091] In further embodiments, the thermochromic hydrogel 10 may comprise 10-30 wt.% (demineralized) water 1 and 70-90 wt.% copolymer 2. In such embodiments, the transition temperature (TT) may be on the lower side, such as selected from the range of 24-28 °C.

[0092] Alternatively, in embodiments, the thermochromic hydrogel 10 may comprise 30-50 wt.% (demineralized) water 1 and 50-70 wt.% copolymer 2. In such embodiments, the transition temperature (TT) may be on the higher side, such as selected from the range of 30-40 °C.

[0093] Further, in specific embodiments, the transition temperature (TT) may be selected from the range of 26-35 °C. Moreover, in embodiments, the thermochromic hydrogel 10 may further comprise an additive 24. In embodiments, the additive 24 may be selected from the group comprising (i) a dye, (ii) light-reflective nanoparticles such as gold nanoparticles, silver nanoparticles, titania (TiCh) nanoparticles, and aluminum nanoparticles, (iii) thermochromic nanoparticles, (iv) ionic liquid, and (v) hydroxypropyl cellulose.

[0094] Further, in embodiments, the thermochromic hydrogel 10 may have a thickness t, defined as the smallest dimension of the thermochromic hydrogel 10. As depicted in Fig. 1A, the thermochromic hydrogel may have a length L, a width W, and a thickness t. In embodiments, t<2*W, such as t<3*W, like t<5*W. Additionally or alternatively, in embodiments, t<2*L, such as t<3*L, like t<5*L. Especially, in embodiments, the thickness t may be selected from the range of 0.4-2 mm.

[0095] Fig. 2 schematically depicts a thermochromic device 100 comprising the solid- state thermochromic hydrogel 10. In embodiments, the thermochromic device 100 may further comprise an encapsulant 90. The encapsulant 90 may comprise a transparent material 95. Especially, the encapsulant 90 may comprise a transparent material 95 selected from the group comprising: a glass, a polymeric material (such as especially PET, PEP, PEN), a cellophane, and a quartz.

[0096] In further embodiments, the thermochromic device 100 may have a sheet-like shape. Hence, the thermochromic device may have one dimension (e.g. one of length (L), width (W), and thickness (t), here especially the thickness (t)), that may be substantially smaller than its other dimensions (e.g. the other ones of length (L), width (W), and thickness (t)).

[0097] As shown in Fig. 2 subfigure I, the thermochromic device 100 may be warmed using a heat providing object 700 (here as an example a human hand is applied). As depicted by Fig. 2 subfigure II, the thermochromic device 100 may be capable of becoming turbid under higher temperature. Reference 80 especially refers to the turbid hydrogel section. When removing the heat providing object 700, as depicted in Fig. 2 subfigure III, the thermochromic device 100 may again turn transparent when cooling back to room temperature (i.e. about 20 °C), demonstrating successful thermochromism properties for smart window application.

[0098] Hence, the invention provides a use of the solid-state thermochromic hydrogel 10 in smart window applications. Especially, as depicted by Figs. 3A-3B, in embodiments, the thermochromic device 100 may be configured for use in a window 1210, in a car window 1220, in a greenhouse, in a textile, in a wearable, as a temperature sensor or in data encryption. Especially, Fig. 3A may depict a vehicle 1400 comprising the thermochromic device 100 configured as (or in) the car window 1220. Fig. 3B on the other hand schematically depicts an indoor space 1300, such as a room. As depicted here, the thermochromic device 100 may be configured as (or in) the car window 1220. Reference 1310 may here refer to a ceiling, reference 1307 may indicate a wall, and reference 1305 may indicate a floor.

[0099] The invention may further provide a system 1000 for regulating temperature in an (at least partially) indoor space 50. The system may especially comprise the thermochromic device 100. In embodiments, the thermochromic device 100 may be arranged at an interface 1500 between the indoor space 50 and an outdoor space 60.

[0100] Hence, in embodiments, the invention provides a temperature management method for regulating temperature in an (at least partially) indoor space 50. In embodiments, the temperature management method may comprise configuring the thermochromic device 100 at the interface 1500 between the indoor space 50 and the outdoor space 60 (and in a (solar) light-receiving relationship with the external of the indoor space 50).

[0101] Fig 5A schematically depicts a method of providing a thermochromic hydrogel 10. In embodiments, the method may comprise a preparation stage comprising providing a precursor solution 11. The precursor solution 11 may especially comprise N,N- dimethylaminoethyl methacrylate 21, a fluorinated monomer 22, a crosslinker 23, an additive 24, and 10-50 wt.% of water 1. In further embodiments, the precursor solution 11 may comprise the N,N-dimethylaminoethyl methacrylate 21 and the fluorinated monomer 22 in a mole ratio Ri selected from the range of 7:3 to 27:3(, especially 9: 1). Furthermore, in embodiments, the method may comprise a curing stage comprising curing the precursor solution 11 to obtain the thermochromic hydrogel 10.

[0102] The precursor solution 11 may, in embodiments, comprise an initiator 25. In specific embodiments, the initiator 25 may comprise a photoinitiator. In such embodiments, the curing stage may comprise curing the precursor solution 11 under irradiation of light, such as UV light 400, to obtain the thermochromic hydrogel 10.

[0103] In further embodiments, such as depicted in Fig. 5B, the curing stage may comprise a pre-curing step comprising pre-curing the precursor solution 11 under UV light 400 to obtain a viscous precursor solution 15. Furthermore, in embodiments, the curing stage may comprise arranging the viscous precursor solution 15 in a mold 9. Especially, the curing step comprising (further) curing the viscous precursor solution 15 (in the mold 9) under UV light 400 to obtain the thermochromic hydrogel 10.

[0104] Experiments Unless specified otherwise, the experiments described hereinafter were performed using the following materials and methods.

[0105] Materials - N,N-dimethylaminoethyl methacrylate (DMAEMA) (98%, Sigma- Aldrich), 2,2,2-trifluoroethyl methacrylate (FMA) (99%, Sigma-Aldrich), N,N'- methylenebisacrylamide (MBA) (Sigma-Aldrich), 2-Hydroxy-4'-(2-hydroxyethoxy)-2- methylpropiophenone (Irgacure 2959) (98%, Sigma-Aldrich), ethylene glycol dimethacrylate (EGDMA) (98%, Sigma-Aldrich), 4-(2 -hydroxy ethyl)- 1 -piperazineethanesulfonic acid (HEPES) (>99%, Sigma-Aldrich), tetramethylethylenediamine (TEMED) (>99%, Sigma- Aldrich),), ammonium persulfate solution (APS) (98%, Sigma-Aldrich), polyethylene terephthalate (PET) films (thickness: 0.1 mm), double-sided acrylic foam tape (tesa® ACXplus7054, 0.5mm * 6mm), indium tin oxide (ITO) glass, copper tape, silver conductive paint (Electrolube). All the reagents were used as received.

[0106] Monomer stock solution preparation - a stock solution was made containing the monomers and crosslinker. For example, 100 mL of stock solution contained 89.8 mL DMAEMA, 8 mL FMA and 2.2 mL EGDMA. Each time the stock solution was used, it was degassed by sparging with N2 for at least 20 minutes through a silicone tube. The vial was quickly sealed after and allowed to rest for at least 10 minutes. Both of these actions were performed with the vial in an ice bath to keep the solution cold. The stock solution was stored at 4 °C in a sealed tinted glass container.

[0107] Buffer (HEPES) preparation - 800 mL dH2O was added to a glass bottle together with 23.8 g HEPES powder. The pH of the solution was adjusted to pH 7.4 with 1.5M sodium hydroxide (NaOH). The volume was brought up to 1 L using dH2O. The buffer solution was filtered using a o.2 micron membrane to remove impurities. Before each use, the buffer was degassed by sparging with N2 for at least 20 minutes through a silicone tube, at room temperature (~20 °C).

[0108] Preparation of DMFM precursor solutions and devices through photoreactive curing - The precursor solutions of DMFM hydrogel were prepared through a simple mixing and stirring process. Firstly, 9 mmol of monomer DMAEMA, 1 mmol FMA, 1 mg of photoinitiator Irgacure 2959, 1 mg of crosslinker MBA were mixed together. 1000 pL demineralized (DI) water was then added into the mixture and stirred for 5 mins to obtain precursor solution of DMFM-5 hydrogel. Other recipes including DMFM-1, DMFM-2, DMFM-3, DMFM-4, DMFM-6 and DMFM-7 were prepared with the same ratio of components, but instead adding DI water of 200, 400, 600, 800, 1200 and 1400 pL, respectively. Then, DMFM-5 precursor solution was cured under UV light (365 nm) for 3 minutes as pretreatment to obtain precursor solution with certain viscosity. Thirdly, the precursor solution was injected in between two transparent PET films, where the thickness of the layer was controlled to 0.5 mm by using double-sided acrylic tapes. After UV-curing for 25 minutes, thin-film solid-state thermochromism device with flexibility and transparency was fabricated.

[0109] Radical-polymerization curing - Curing was done by adding tetramethylethylenediamine solution (TEMED) followed by ammonium persulfate solution (APS). For TEMED, a 10%(v) TEMED and 90%(v) dEEO solution was prepared fresh on the day, and kept on ice. For APS also a 10%(v / v) solution in dFFO was prepared, with aliquots stored in the freezer at -20 °C.

[0110] The thermochromic hydrogel was prepared in a container that was under constant N2 flow and in an ice bath to help manage the exothermic nature of the reaction as well as prevent oxygen inhibition.

[0111] For preparing 10 mL of gel, 3.63 mL of buffer was added to the container, followed by 6.37 mL of the monomer stock solution. To this, O.lmL of the 10% TEMED solution was added, followed by 0.1 mL of the APS solution. If the container was open, it was put under constant N2 flow. If the container was closed, N2 was flushed inside if the container was not 100% full. The container was kept on ice, allowing it to slowly warm up as the ice melted over several hours. This resulted in a reproducible transition temperature for the prepared gel.

[0112] Characterization of thermochromism hydrogel - UV / Vis / NIR Spectrophotometer (PerkinElmer LAMBDA 1050+ with integrating sphere, 250 to 2500 nm), power supply (Voltcraft ESP-3005S), digital thermometer (Amprobe TMD-56), water cooling / heating system with transmittance measurement (AlGalnP LED (Kingbright, 660 nm), thermocouple amplifiers (MCP9600), digital light sensor (Adafruit TSL2591), board (Arduino Beetle)).

[0113] Experiment 1

[0114] To quantitatively characterize the thermoresponsiveness of the thermochromic device 100 containing the different hydrogels (i.e., DMFM-1, DMFM-2, DMFM-3, DMFM-4, DMFM-5, DMFM-6, and DMFM-7) as smart window, solar transmittance spectra (250 to 2500 nm) of the hydrogels with layer thickness of 0.5 mm under different temperatures (21 °C to 45 °C) were measured. At temperature of 21 °C for DMFM-5 applied that the solar transmittance Tsoi, IR transmittance TIR and luminous transmittance Tiumwere 82.4 %, 80.3 % and 85.0 %, respectively, demonstrating high transparency in both visible and infrared region at room temperature. When the temperature increased to 27 °C for DMFM-5 applied that the Tsoi, TIR and Tium decreased to 33.7 %, 42.3 % and 27.5 %, respectively, indicating the occurrence of phase transition and LCST of DMFM-5 at around 27 °C. As the temperature further increased to 30 °C, the DMFM-5 hydrogel device became quite turbid with low solar transmittance of 13.0 %, leading to high solar transmittance modulation (ATsoi) of 69.4 %. Under temperatures higher than 33 °C, the solar transmittance of DMFM-5 hydrogel device continued to decrease slightly and reached a plateau of 11.6 % at 45 °C. It could be summarized that the DMFM-5 was capable of changing color at ~27 °C and reaching its saturation at ~30 °C, which is lower than the LCST (~32 °C) of PNIPAM. Testing the other hydrogels (i.e., DMFM-1, DMFM-2, DMFM-3, DMFM-4, DMFM-6, and DMFM-7), it appeared that the phase-transition temperature increased as the water concentration decreased in hydrogel devices from DMFM- 5 to DMFM-1. In particular, there was no color change observed at DMFM-1 hydrogel device; while in DMFM-6 and DMFM-7 hydrogel devices with increasing amount of water concentrations, the hydrogel already turned a bit turbid at room temperature. Hence, it appears that a higher water concentration would result in more easily triggered phase separation.

[0115] Experiment 2

[0116] To further explore the response rate of DMFM-5 hydrogel smart window, a dynamic transmittance spectrum at wavelength of 660 nm was measured. Around 70 heating / cooling cycles were performed with cooling at 20 °C and heating at 35 °C by alternating cool and hot water flow, with each cycle taking around 25 s. The DMFM-5-based thermochromic device 100 turned transparent (light intensity 5069.17 lux) in around 3 seconds with cool water, and then turned turbid (light intensity 856.33 lux) in around 3 seconds under hot water flow. Nearly immediate coloration and bleaching process corresponding with external temperature change could be observed on the DMFM-5 hydrogel smart window. In further testing no significant loss of efficiency was observed when exposing the DMFM-5- based thermochromic device 100 to about 100 heating / cooling cycles.

[0117] Experiment 3

[0118] Fig. 4 schematically depicts luminous transmittances (Tium) of the thermochromic hydrogel 100 with different water concentrations in response to temperatures ranging from 20 °C to 50 °C. Here, LI (or DMFM-1) indicates a thermochromic hydrogel 10 comprising 12 wt.% of water and 88 wt.% of copolymer; L2 (or DMFM-2) indicates a thermochromic hydrogel 10 comprising 21 wt.% of water and 79 wt.% of copolymer; L3 (or DMFM-3) indicates a thermochromic hydrogel 10 comprising 29 wt.% of water and 71 wt.% of copolymer; L4 (or DMFM-4) indicates a thermochromic hydrogel 10 comprising 35 wt.% of water and 65 wt.% of copolymer; L5 (or DMFM-5) indicates a thermochromic hydrogel 10 comprising 40 wt.% of water and 60 wt.% of copolymer; L6 (or DMFM-6) indicates a thermochromic hydrogel 10 comprising 45 wt.% of water and 55 wt.% of copolymer; and L7 (or DMFM-7) indicates a thermochromic hydrogel 10 comprising 48 wt.% of water and 52 wt.% of copolymer. As demonstrated in Fig. 4, the (phase-)transition temperature (TT) of the thermochromic hydrogel 10 may be tuned by altering the water concentration. As can be seen in Fig. 4, the luminous transmission (Tium) of L5 (i.e. DMFM-5) drops at a lower temperature than that of both L4 (i.e. DMFM-4) and L6 (i.e. DMFM-6). Hence, the water concentration of DMFM-5 appears to provide a sort of optimum for reducing the LCST of a hydrogel with a 9: 1 mole ratio of DMAEMA to FMA.

[0119] The results of the thermochromic device 100 of the invention were compared to the prior art in table 1.

[0120] Table 1: Performance comparison of prior art thermochromism devices:

[0121] N / A is used to indicate that the information was not available.

[0122] In conclusion, the tested embodiment of the thermochromic device 100 comprises a predeterminable phase-transition temperature based on the solid-state thermochromic hydrogel 10 which shows excellent solar transmittance modulation (e.g. in the case of DMFM-5 69.4 % and luminous transparency of 85.0 %). By adjusting the hydrogel formulation (e.g. by changing the water concentration), the phase-transition temperature could be lowered to 27 °C with a response rate to ~3 seconds, as well as achieving good cyclability over at least 100 cycles with (essentially) no loss of thermochromic effect At the same time, phase-transition temperature could be tuned between 27 an 44 °C (as evidenced by Fig. 4) by adjusting the amount of water in the thermochromic device, without using unfavorable acid and alkaline chemicals. Compared to the prior art, the thermochromic device 100 of the invention may provide smart window applications in which the phase-transition temperature may be reduced from ~60 °C (VCh-based) and ~32 °C (PNIP AM-based) to, for instance, about 27°C, which may fit well with the human comfort zone. In addition, rather than rigid metal oxide films or microgel suspension that may demand glass substrates, the thermochromic hydrogel 10 of the invention may be encapsulated in flexible materials, such as in PET plastic films, which endows the thermochromic device 100 with light weight and flexibility. Therefore, thermochromic device 100 may be easily installed on existing glass surfaces on curved vehicle ceilings or buildings to form smart window.

[0123] Similar results as those described for experiments (1-3), performed with a thermochromic hydrogel produced using photoreactive curing, were obtained using a thermochromic hydrogel produced using radical-polymerization-based curing (see descriptions for making these hydrogels above). Thermochromic hydrogels produced using the radicalpolymerization-based curing, as described above, were observed to show improved long-term stability.

[0124] The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably.

[0125] The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

[0126] The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’.

[0127] The term “and / or” especially relates to one or more of the items mentioned before and after “and / or”. For instance, a phrase “item 1 and / or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of' but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

[0128] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0129] The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

[0130] The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

[0131] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

[0132] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0133] Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

[0134] The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

[0135] The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0136] The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system. The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

[0137] The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

CLAIMS:

1. A solid-state thermochromic hydrogel (10), wherein the thermochromic (10) hydrogel comprises:10-50 wt.% of water (1); and50-90 wt.% of a copolymer (2), wherein the copolymer comprises an N,N- dimethylaminoethyl methacrylate moiety (21), a fluorinated monomer moiety (22), and a crosslinker moiety (23); wherein the thermochromic hydrogel (10) has a transition temperature (TT) selected from the range of 24-40°, wherein the thermochromic hydrogel (10) is configured to be (i) transparent below the transition temperature (TT) and (ii) turbid at or above the transition temperature (TT).

2. The solid-state thermochromic hydrogel (10) according to claim 1, wherein the copolymer (2) comprises a mole ratio Ri of the N,N-dimethylaminoethyl methacrylate moiety (21) to the fluorinated monomer moiety (22), wherein the mole ratio Ri is selected from the range of 7:3 to 27:3.

3. The solid-state thermochromic hydrogel (10) according to claim 2, wherein the fluorinated monomer moiety (22) comprises 2,2,2-trifluoroethyl methacrylate.

4. The solid-state thermochromic hydrogel (10) according to claim 3, wherein the fluorinated monomer moiety (22) consists of 2,2,2-trifluoroethyl methacrylate, and wherein the mole ratio Ri is selected from the range of 8: 1 to 10: 1.

5. The solid-state thermochromic hydrogel (10) according to any one of the preceding claims, wherein the thermochromic hydrogel (10) comprises 10-30 wt.% water (1) and 70-90 wt.% copolymer (2).

6. The solid-state thermochromic hydrogel (10) according to any one of the preceding claims 1-4, wherein the thermochromic hydrogel (10) comprises 30-50 wt.% water (1) and 50-70 wt.% copolymer (2).

7. The solid-state thermochromic hydrogel (10) according to any one of the preceding claims, wherein the transition temperature (TT) is selected from the range of 26-358. The solid-state thermochromic hydrogel (10) according to any one of the preceding claims, wherein the thermochromic hydrogel (10) further comprises an additive (24) selected from the group comprising: (i) a dye, (ii) light-reflective nanoparticles such as gold nanoparticles, silver nanoparticles, titania (TiCh) nanoparticles, and aluminum nanoparticles, (iii) thermochromic nanoparticles, (iv) ionic liquid, and (v) hydroxypropyl cellulose.

9. A thermochromic device (100) comprising the solid-state thermochromic hydrogel (10) according to any one of the preceding claims 1-8 and an encapsulant (90), wherein the encapsulant (90) comprises a transparent material (95); and wherein the thermochromic device (100) has a sheet-like shape.

10. The thermochromic device (100) according to claim 9, wherein the encapsulant (90) comprises a transparent material (95) selected from the group comprising: a glass, a polymeric material, a cellophane, and a quartz.

11. The thermochromic device (100) according to any one of the preceding claims 9-10, wherein the thermochromic hydrogel (10) has a thickness (t) defined as the smallest dimension of the thermochromic hydrogel (10), wherein the thickness (t) is selected from the range of 0.4-2 mm.

12. A system (1000) for regulating temperature in an indoor space (50), the system comprising the thermochromic device (100) according to any one of the preceding claims 9- 11, wherein the thermochromic device (100) is arranged at an interface (1500) between the indoor space (50) and an outdoor space (60).

13. A method of providing a thermochromic hydrogel (10), the method comprising: a preparation stage comprising providing a precursor solution (11), wherein the precursor solution (11) comprises N,N-dimethylaminoethyl methacrylate (21), a fluorinated monomer (22), a crosslinker (23), and 10-50 wt.% of water (1), wherein the precursor solution(11) comprises the N,N-dimethylaminoethyl methacrylate (21) and the fluorinated monomer (22) in a mole ratio Ri selected from the range of 7:3 to 27:3; and a curing stage comprising curing the precursor solution (11) to obtain the thermochromic hydrogel (10).

14. The method according to claim 13, wherein the precursor solution (11) comprises an initiator (25), wherein the initiator (25) comprises a photoinitiator, wherein the curing stage comprises: a pre-curing step comprising pre-curing the precursor solution (11) under UV light (400) to obtain a viscous precursor solution (15); arranging the viscous precursor solution (15) in a mold (9); and a curing step comprising curing the viscous precursor solution (15) under UV light (400) to obtain the thermochromic hydrogel (10).

15. The method according to any one of the preceding claims 13, wherein the precursor solution (11) comprises an initiator (25), wherein the initiator (25) comprises 2- hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, and wherein the precursor solution (11) comprises 0.03-0.12 wt.% of the initiator (25), and wherein the curing stage comprises curing the precursor solution (11) under UV light (400).

16. The method according to any one of the preceding claims 13-15, wherein the crosslinker (23) comprises one or more of N,N' -methylenebisacrylamide, and polyethylene glycol diacrylate, and wherein the precursor solution (11) comprises 0.03-0.12 wt.% of the crosslinker (23).

17. A temperature management method for regulating temperature in an indoor space (50), the temperature management method comprising configuring the thermochromic device (100) of any one of the preceding claims 9-11 at an interface (1500) between the indoor space (50) and an outdoor space (60).

18. Use of the solid-state thermochromic hydrogel (10) according to any one of the preceding claims 1-8 in smart window applications.

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