Polymer-encapsulated phase-change fibers with a high thermal storage capacity and method for the preparation thereof

Abstract

The invention relates to a composite fiber comprising a core-shell structure, wherein the shell comprises a polymer and the core comprises a phase change material, and which is capable of achieving a high enthalpy of phase transition. The invention relates also to the method for producing such a fiber and to the use of said fiber in thermal energy storage.

Description

[0001] POLYMER-ENCAPSULATED PHASE-CHANGE FIBERS WITH A HIGH THERMAL STORAGE CAPACITY AND METHOD FOR THE PREPARATION THEREOF

[0002] FIELD OF THE INVENTION

[0003] This invention relates to the field of thermal energy storage, more precisely to the field of latent heat energy storage of phase-change composite materials.

[0004] BACKGROUND

[0005] Thermal energy storage (TES) technologies are valuable components in many energy systems and could be an important tool in achieving a low-carbon future. According to the storage principle, TES technologies can be divided into three categories: sensible heat storage, latent heat storage and thermochemical heat storage. Sensible heat storages are the most commonly deployed type of TES. However, latent heat storage technologies based on Phase-Change Materials (PCMs) are particularly attractive for applications where thermal energy has to be stored or delivered over a narrow temperature range or when compactness is a requirement. Indeed, PCMs are capable of absorbing or releasing great amounts of energy in the form of latent heat during phase transitions at nearly constant temperature. They enable compact TES systems with volumetric storage capacity five to ten times greater than that of sensible heat storage systems.

[0006] In recent years, there has been growing interest in developing polymeric phase change composites as TES materials. The principal aim in such composite materials is to encapsulate or disperse the PCM within the polymer so as to prevent the leakage of the phase change material especially after it transitions into a liquid or gaseous phase. However, achieving such purpose without sacrificing enthalpic power is not an easy task. Different strategies have been followed to produce polymeric fibers with encapsulated PCMs.

[0007] In some cases, prefabricated microencapsulated PCMs are added to polymeric solutions to produce fibers by melt or solution spinning. This allows to produce composite polymer- PCM fibers in a simple way, but the obtained PCM contents and enthalpy values of the final fibers are relatively low, not overpassing 40 J / g.

[0008] Other researchers have attempted the filling or impregnation of prefabricated polymeric fibers with liquid PCMs. In this case, first hollow or porous polymeric fibers are produced and these are then filled by injection, vacuum or impregnation in complex multistep processes. The obtained enthalpy values are in the range of 70-80 J / g. Some reports disclose the production of composite polymer-PCM fibers with high enthalpies. Such is the case of Li et al. (Phase Change Energy Storage Elastic Fiber: A Simple Route to Personal Thermal Management, Polymers, 2022, 14(1), 53; https: / / doi.org / 10.3390 / polym14010053), which managed to manufacture fibers with a PCM content of 77% and an enthalpy of up to 177 J / g, but which required the use of a complex multistep process (wet-spinning + freeze drying + vacuum absorbing + coating) also resulting in a highly complex fiber structure comprising a hierarchical pore matrix of a first polymer containing PCM particles within specific pores, said complex being coated by a second polymer.

[0009] Other reports describe the use of microfluidic systems to produce polymeric fibers and encapsulate PCMs into them in-situ, without the need of multistep processes. However, the enthalpies of the thus produced fibers are poorer than those manufactured with said multistep processes. For instance, CN 110016725 A describes the use of a microfluidic device to coaxially align i) an outer phase solution comprising a surfactant and a polymer curable by gelification, specifically a mixture of PVA, PEG600 and either sodium alginate or silk protein; and ii) an inner PCM material phase; wherein the outer phase solution and inner phase are combined at the exit of the inner phase channel of the microfluidic device to form droplets which are then gelled with a salt bath at the exit of the microfluidic device. Despite the relative simplicity of this microfluidic-gelification system, the maximum reported heat that 1 m of the fiber can absorb or release only reaches 6.31 J. Based on the above, it would be desirable to develop a method for the production of PCM fibers which both is simple to reduce to practice, thus making it attractive from an industrial reproducibility point of view; and yields fibers capable of competing with high- enthalpy PCM composites produced by more complex processes. Additionally, the structure of the fiber itself should not be complex, without the need of providing different means for preventing the leakage of the PCM (solid matrixes, coatings, etc.).

[0010] BRIEF DESCRIPTION OF THE INVENTION

[0011] The authors of the present invention have now surprisingly found that the above purpose is achievable by the method of the present invention.

[0012] Thus, in a first aspect, the invention relates to a method for preparing a composite fiber, the method comprising the steps of: i) Providing an inner channel, through which a stream comprising a phase change material is flowed; ii) Providing an outer channel, through which a stream comprising polymer / monomers is flowed; wherein the outer channel surrounds the inner channel, and wherein the outer and inner channel streams flow co-axially; iii) Combining the stream comprising a phase change material and the stream comprising polymer / monomers downstream of an exit end of the inner channel, thus yielding a fluid precursor composition; iv) Solidifying the fluid precursor composition, provided that said solidification is not curing by gelification.

[0013] In a second aspect, the invention relates to the composite fiber. Specifically, it is directed to a fiber comprising: a solid shell comprising a polymer that has not been solidified by curing by gelation, the shell coaxially surrounding a core comprising a phase change material, wherein said phase change material is not comprised in a porous polymer matrix.

[0014] Alternatively, the invention relates to a composite fiber obtainable by the method of the first aspect of the invention.

[0015] Lastly, the invention is directed to the use of a composite fiber according to the invention for thermal energy storage.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 shows the DSC-thermogram for a fiber prepared by the method of the invention wherein PVDF as polymer was dissolved in DMF; molten hexadecane was used as PCM; and water was the extraction fluid for DMF.

[0018] Figure 2 shows the DSC-thermogram for a fiber prepared by the method of the invention wherein PVDF as polymer was dissolved in DMF; molten eicosane was used as PCM; and water was the extraction fluid for DMF.

[0019] Figure 3 depicts the result of thermal gravimetric analysis (TGA) for a fiber prepared by the method of the invention wherein PVDF as polymer was dissolved in DMF; molten hexadecane was used as PCM; and water was the extraction fluid for DMF.

[0020] DETAILED DESCRIPTION OF THE INVENTION

[0021] The first aspect of the invention pertains to a method for preparing a composite fiber, the fiber comprising: a solid shell comprising a polymer that has not been cured by gelation, the shell coaxially surrounding a core comprising a phase change material (PCM), wherein said phase change material is not comprised in a porous polymer matrix; the method comprising the steps of: i) Providing an inner channel, through which a stream comprising a phase change material is flowed; ii) Providing an outer channel, through which a stream comprising a polymer dissolved in a solvent is flowed; wherein the outer channel surrounds the inner channel, and wherein the streams of the outer and inner channels flow co-axially; iii) Combining the stream comprising a phase change material and the stream comprising the polymer dissolved in a solvent downstream of an exit end of the inner channel, thus yielding a fluid precursor composition; iv) Solidifying the fluid precursor composition by extraction of said solvent into an extraction fluid.

[0022] The term “fiber” (aka fibre) has the common meaning attributed to it in the art. It refers to a product of an elongated shape, longer than it is thick or wide, in particular in a shape commonly referred to as a filament, thread, string or strand. Said fiber can be straight, or twisted, and may be combined with further fibers such as in a mat, coiled or braided fashion. However, in a preferred embodiment, the fiber is a single fiber which is not combined with further fibers, in particular with further fibers according to the present invention.

[0023] Steps i) and ii) of the method of the invention comprise providing a channel through which a stream is flowed.

[0024] The term “channel” refers to any means through which a stream can be flowed, preferably without leakage of said stream, such as tubes, pipes or capillaries.

[0025] In a particular embodiment, the channel is comprised in a microfluidic device, hence it is a microfluidic channel. The term "microfluidic channel" refers to a channel having at least one dimension, such as channel width, that is less than 1 mm (1000 micrometers), such as between 1 and 1000 micrometers.

[0026] The channels of steps i) and ii) of the method of the invention are arranged in a coaxial manner. Specifically, the channel through which a stream comprising a phase change material is flowed is surrounded by the channel through which a stream comprising polymer / monomers is flowed, hence these channels are termed the inner and outer channel respectively. Said coaxial flow is in the same direction.

[0027] In a preferred embodiment, the wall of the inner channel serves as inner wall for the outer channel, i.e. it establishes the physical separation between the two streams.

[0028] The term “stream” refers to any body of flowing fluid, such as a liquid or a gas, preferably a liquid.

[0029] Step i) of the method of the invention refers to a phase change material.

[0030] In the context of the present invention, the term “phase change material” (aka PCM) has the common meaning attributed to it in the art. Specifically, it is a material which undergoes a first-order phase transition (e.g., solid-liquid, or crystal solid-solid) which includes a change in its atomic structure or state of aggregation and which is accompanied by a noticeable absorption or release of energy known as latent heat. When a PCM reaches its phase transition temperature or temperature range, its temperature is maintained constant during the phase transition since the external heat is no longer used to change the temperature of the PCM but to change the crystallographic structure of the material itself.

[0031] In an embodiment, the PCM material is a solid-liquid, solid-gas, liquid-gas or solid-solid PCM, i.e. a PCM capable of undergoing such a transition in either direction. Preferably, it is a solid-liquid PCM.

[0032] In an embodiment, the PCM phase-transitions at a temperature between -30 and 120 °C, preferably between -30 and 100 °C.

[0033] The PCM is flowed through the inner channel in the form of a stream. This means that the PCM must be provided in a fluid state. Fluidizing the PCM can be achieved by several means. The PCM itself may be in a molten or gaseous state, or solid PCM may be presented in the form of a solution or fluid dispersion. Preferably, the PCM is in a molten state.

[0034] Non-limiting examples of PCMs that may be employed in the present invention are paraffins such as eicosane, docosane, hexadecane or tetradecane; fatty acids or esters thereof such as caprylic acid, lauric acid or oleic acid; aliphatic alcohols such as sugar alcohols e.g. xylitol, mannitol, sorbitol, erythritol; metals or metal alloys, such as zinc or nickel-zinc; inorganic salts or eutectics thereof such as sodium nitrate, potassium nitrate, manganese chloride; salt hydrates, such as sodium acetate tri-hydrate.

[0035] In a preferred embodiment, the PCM is selected from paraffins such as eicosane, docosane, hexadecane or tetradecane; and fatty acids such as lauric acid or oleic acid. Preferably, the PCM is selected from paraffins such as eicosane, docosane, hexadecane.

[0036] In an embodiment, the stream comprising a PCM does not comprise said PCM in the form of an emulsion, more particularly in the form of an emulsion wherein said PCM is the dispersed phase.

[0037] Alternatively or additionally, the stream comprising a PCM does not comprise PAN, in particular it does not comprise said PAN in the form of an emulsion wherein said PAN is the continuous phase; more preferably the stream comprising a PCM does not comprise a polymer, in particular it does not comprise said polymer in the form of an emulsion wherein said polymer is the continuous phase.

[0038] In an embodiment, the stream comprising a PCM does not comprise poloxamer nor sorbitan monooleate, more preferably does not comprise a non-ionic surfactant, even more preferably does not comprise a surfactant. In these embodiments, the surfactant is preferably an emulsifier.

[0039] In an embodiment, the stream comprising a PCM does not comprise either one of a polymer or a surfactant as described in the immediately above embodiments.

[0040] Step ii) of the method of the invention refers to a polymer / monomers. The term polymer / monomers is meant as an alternative, i.e. polymer or monomers, but is expressed as polymer / monomers for simplicity. In any embodiment described herein, polymer / monomers preferably refers to polymer.

[0041] The term “polymer” has the common meaning attributed to it in the art. In an embodiment, it refers to a molecule comprising at least 10 monomeric (aka repeating) units, such as at least 100 or at least 1000 monomeric units.

[0042] The term “monomers” has the common meaning attributed to it in the art, and refers to the compounds which are polymerized into monomeric units in a polymer. In an embodiment, it refers to a mixture comprising at least 10 monomers, such as at least 100 or at least 1000 monomers.

[0043] The polymer / monomers is flowed through the outer channel in the form of a stream. This means that the polymer / monomers must be provided in a fluid state. Fluidizing the polymer / monomers can be achieved by several means. The polymer / monomers itself may be in a molten state, or polymer / monomers may be presented in the form of a solution or fluid dispersion. Preferably, the polymer / monomers is in the form of a solution, i.e. it is dissolved in a solvent.

[0044] Non-limiting examples of polymers that may be employed in the present invention are fluoropolymers such as polyvinylidene difluoride (PVDF), poly(vinyl alcohol) (PVA), acrylonitrile butadiene styrene (ABS), polyacrylates (including polymethacrylates), polyvinyl butyral (PVB), polyurethanes such as lineal polyurethanes or thermoplastic polyurethane (TPU), polystyrene (PS), polyacrylonitrile (PAN), poly(lactic-co-glycolic acid (PLGA), polyethylene (PET), polypropylene (PP), polycaprolactone (PCL), polyhydroxybutyrate (PHB).

[0045] In a preferred embodiment, the polymer is selected from fluoropolymers such as PVDF, PVB, PET or polyurethanes. Preferably, the polymer is a fluoropolymer, more preferably a fluoroplastic, such as tetrafluoroethylene hexafluoropropylene vinylidene fluoride copolymer (THV), fluorinated ethylene propylene copolymer (FEP), perfluoroalkoxy alkane copolymer (PFA), poly(ethene-co-tetrafluoroethene) (ETFE), ethylenechlorotrifluoroethylene copolymer (ECTFE), PVDF; even more preferably PVDF. The term “fluoropolymer” refers to a polymer comprising fluorocarbon repeating units. The term “fluorocarbon” refers to a straight or branched carbon chain in which one or more hydrogen atoms is replaced by a fluorine group. The term “fluoroplastic” refers to a plastic fluoropolymer.

[0046] Since the solidification of step iv) requires that it is not curing by gelification, in an embodiment, in any of the embodiments described herein referring to the nature of the polymer, the polymer is a polymer which can be solidified by other means different to curing by gelification, more preferably it is a polymer that cannot be solidified by curing by gelification. In further embodiments, said other means of solidification are those described further below.

[0047] In polymer chemistry, gelification or gelation or gel transition is the formation of a gel from a system with polymers. Branched polymers can form links between the chains, which lead to progressively larger polymers. As the linking continues, larger branched polymers are obtained and at a certain extent of the reaction, links between the polymer result in the formation of a single macroscopic molecule. At that point in the reaction, which is defined as gel point, a sudden increase in viscosity is observed.

[0048] Non-limiting examples of monomers are those that can be polymerized into the above polymers.

[0049] In an embodiment: the polymer is selected from fluoropolymers, PVA, ABS, polyacrylates, PVB, polyurethanes, PS, PAN, PLGA, PE, PP, PCL, PHB; preferably from fluoropolymers, PVB, PET or polyurethanes; more preferably from fluoropolymers, such as fluoroplastics, e.g. THV, FEP, PFA, ETFE, ECTFE, PVDF; and even more preferably PVDF; and

[0050] - the PCM is selected from paraffins such as eicosane, docosane, hexadecane or tetradecane; fatty acids or esters thereof such as caprylic acid, lauric acid or oleic acid; aliphatic alcohols such as alcohol sugars e.g. xylitol, mannitol, sorbitol, erythritol; metals or metal alloys, such as zinc or nickel-zinc; inorganic salts or eutectics thereof such as sodium nitrate, potassium nitrate, manganese chloride; salt hydrates, such as sodium acetate tri-hydrate; preferably from paraffins such as eicosane, docosane, hexadecane or tetradecane, and fatty acids such as lauric or oleic acid; more preferably the PCM is a paraffin; even more preferably it is eicosane, docosane or hexadecane.

[0051] In a particular embodiment, PVDF is used as polymer; and the PCM is a paraffin, such as hexadecane.

[0052] In another particular embodiment, PVDF is used as polymer; and the PCM is a paraffin, such as eicosane.

[0053] In another particular embodiment, PVB is used as polymer; and the PCM is a paraffin, such as docosane.

[0054] In another particular embodiment, PVB is used as polymer; and the PCM is a fatty acid, such as oleic acid.

[0055] In another particular embodiment, PET is used as polymer; and the PCM is a paraffin, such as eicosane.

[0056] In another particular embodiment, PET is used as polymer; and the PCM is a fatty acid, such as lauric acid.

[0057] In another particular embodiment, polyurethane is used as polymer; and the PCM is a paraffin, such as dodecane.

[0058] In a preferred particular embodiment, the PCM is flowed through the inner channel in a molten state, and the polymer / monomers is flowed through the outer channel in the form of a solution.

[0059] In a preferred embodiment:

[0060] - the polymer is selected from fluoropolymers, PVA, ABS, polyacrylates, PVB, polyurethanes, PS, PAN, PLGA, PE, PP, PCL, PHB; preferably from fluoropolymers, PVB, PET or polyurethanes; more preferably from fluoropolymers, such as fluoroplastics, e.g. THV, FEP, PFA, ETFE, ECTFE, PVDF; and even more preferably PVDF; and - the PCM is selected from paraffins such as eicosane, docosane, hexadecane or tetradecane; fatty acids or esters such as caprylic acid, lauric acid or oleic acid; aliphatic alcohols such as alcohol sugars e.g. xylitol, mannitol, sorbitol, erythritol; metals or metal alloys, such as zinc or nickel-zinc; inorganic salts or eutectics thereof such as sodium nitrate, potassium nitrate, manganese chloride; salt hydrates, such as sodium acetate tri-hydrate; preferably from paraffins such as eicosane, docosane, hexadecane or tetradecane, and fatty acids such as lauric or oleic acid; more preferably the PCM is a paraffin; even more preferably it is eicosane, docosane or hexadecane; and the PCM is flowed through the inner channel in a molten state, and the polymer is flowed through the outer channel in the form of a solution (polymer dissolved in a solvent). Which solvents dissolve a particular polymer is common general knowledge and can furthermore be determined according to Hansen parameters as described hereinbelow. By way of example, a fluoropolymer, such as PVDF (5 = 23.2), can be dissolved in dimethylformamide (DMF) (5 = 24.9); PVB (5 = 24.6) can be dissolved in ethanol (5 = 26.5); or PET (5 = 20.8) can be dissolved in dichloromethane (DCM) (5 = 20.2).

[0061] In a particular embodiment, PVDF dissolved in DMF is used as polymer; and the PCM is a paraffin, such as hexadecane.

[0062] In another particular embodiment, PVDF dissolved in DMF is used as polymer; and the PCM is a paraffin, such as eicosane.

[0063] In another particular embodiment, PVB dissolved in ethanol is used as polymer; and the PCM is a paraffin, such as docosane.

[0064] In another particular embodiment, PVB dissolved in ethanol is used as polymer; and the PCM is a fatty acid, such as oleic acid.

[0065] In another particular embodiment, PET dissolved in DCM is used as polymer; and the PCM is a paraffin, such as eicosane.

[0066] In another particular embodiment, PET dissolved in DCM is used as polymer; and the PCM is a fatty acid, such as lauric acid.

[0067] In another particular embodiment, polyurethane dissolved in THF is used as polymer; and the PCM is a paraffin, such as dodecane.

[0068] In a preferred embodiment, when the stream comprising polymer is a stream comprising a polymer dissolved in a solvent, then the PCM comprised in the stream comprising a PCM should not be miscible with, in particular soluble in, said solvent, as the streams will come into contact following exit from the channel(s) to form the fluid precursor composition. Which solvents mix with or dissolve a particular PCM or not is common general knowledge and can furthermore be determined according to Hansen solubility parameters as described hereinbelow.

[0069] In a preferred embodiment of the present invention, the stream of step ii), and therefore also the fluid precursor composition, and the fiber of the invention, does not comprise a surfactant.

[0070] In step iii) of the method of the invention, the stream comprising the phase change material and the stream comprising the polymer / monomers are combined. This combination entails removing any physical barrier between the stream of step i) and the stream of step ii), the physical barrier typically being the inner channel wall; or the inner channel wall and the inner wall of the outer channel, if the latter is present. In this case, the removal of the barrier typically corresponds to the end of the channel, specifically to the exit end of the channel, i.e. the end of the channel from which the stream exits. Therefore, the streams are combined downstream of the exit end of the inner channel, when the wall of the inner channel is the only physical barrier between the streams; or downstream of the exit end of the inner channel or outer channel inner wall, whichever ends latest, when both the inner channel wall and the inner wall of the outer channel serve as the physical barrier between the streams. Preferably, streams are combined immediately downstream of said exit end(s).

[0071] The combination of the streams leads to the formation of the fluid precursor composition. Therefore, the fluid precursor composition comprises PCM and polymer / monomers. More specifically, the fluid precursor composition comprises a shell comprising the polymer / monomers, the shell coaxially surrounding a core comprising the PCM. Although the composition is still fluid, its integrity, specifically its core-shell structure, is maintained by the laminar flow thereof, specifically of the stream comprising polymer / monomers with respect to the stream comprising the PCM. Attaining a laminar flow regime of the fluid precursor composition can be achieved based on common general knowledge, however, in an embodiment, the fluid precursor composition flows at a Reynolds number (Re) of 2000 or lower, preferably of 1500 or lower. In a particular embodiment, the method of invention is carried out in a microfluidic device, and the fluid precursor composition flows at a Reynolds number (Re) of 250 or lower, preferably 10 or lower.

[0072] The Reynolds number (Re) is a dimensionless quantity that accounts for characteristics of both the fluid and the environment in which the fluid is manipulated. It is determined by measuring the ratio between inertial and viscous forces, by the equation: Re = (inertial forces) / (viscous forces) = VDp / p where

[0073] V is the fluid precursor composition mean velocity (m / s);

[0074] D is the characteristic dimension (m), which preferably corresponds to the hydraulic diameter of the outer channel; p is the density of the fluid precursor composition (kg / m3); is the dynamic viscosity of the fluid precursor composition (Pa s), which can be calculated according to ISO 1628-1 :2021 (en) or with a rheometer such as is described in Fortunato et al., Polymers 2019, 11 (7), 1096.

[0075] By way of example, when employing PVDF dissolved in DMF as polymer; and the PCM is hexadecane; and an Re value of 0.1-250 is sought, the following can be established:

[0076] D = 0.001 m p = 984 kg / m3= 0.05 Pa s

[0077] By substituting these values into the above Reynolds equation, the required mean velocity for the fluid precursor composition is obtained:

[0078] V = 0.0005 - 1.2703 m / s.

[0079] Furthermore, discharge values to be applied, for instance with the aid of syringe pumps, can be obtained by applying the following simple discharge equation:

[0080] Q = V x A where

[0081] Q is the discharge (m3 / s);

[0082] A is the cross-sectional area of the portion of the channel occupied by the flow (m2); and

[0083] V is as indicated above.

[0084] In step iv) of the method of the invention, the fluid precursor composition is solidified. More particularly, this step typically refers to solidifying the polymer / monomers comprised in said fluid precursor composition, said polymer / monomers corresponding to the polymer / monomers in step ii). As the polymer / monomers in step ii) was flowed through the outer channel, this polymer / monomers coaxially surrounds the PCM in the fluid precursor composition. Therefore, upon solidification of said polymer / monomers, the fluid precursor composition is transformed into a fiber comprising: a solid shell comprising, or in particular consisting of, the polymer, which coaxially surrounds a core comprising, in particular consisting of, the PCM.

[0085] The solidification of the polymer / monomers can be performed by several means known to the person skilled in the art that are different to gelation (aka curing by gelation), more specifically different to gelation by subjecting to a salt solution, such as a calcium chloride solution. Examples of solidification techniques suitable in the context of the present invention are provided below and include subjecting the fluid precursor composition to UV light, cooling, or solvent removal.

[0086] In an embodiment, the stream comprising polymer / monomers is a molten polymer stream, and the fluid precursor composition therefore comprises said molten polymer. In such a case, solidification of the molten polymer can be achieved by cooling the fluid precursor composition until the polymer solidifies (i.e. is no longer molten).

[0087] In an alternative embodiment, the stream comprising polymer / monomers is a stream comprising monomers, and the fluid precursor composition therefore comprises said monomers. In such a case, solidification of the monomers into a polymer can be achieved by polymerization of the monomers. In an embodiment, polymerization is thermal- or photo-polymerization, preferably it is photo-polymerization, more preferably it is photo-polymerization initiated by UV radiation.

[0088] In a preferred embodiment, the stream comprising polymer / monomers is a stream comprising a polymer solution, and the fluid precursor composition therefore comprises said polymer solution. In such a case, solidification preferably refers to removing said solvent from the fluid precursor composition, such as by evaporation of said solvent or by extraction of said solvent into an extraction fluid, more preferably by extraction of said solvent into an extraction fluid. The inventors have found that this method unexpectedly allows providing particularly high fiber production rates.

[0089] However, in an embodiment, solidification, in particular solidification by solvent removal, is not by electrospinning.

[0090] The extraction of said solvent into an extraction fluid can be achieved for instance by contacting the fluid precursor composition with the extraction fluid, such as by adding the precursor composition to a bath of the extraction fluid.

[0091] The term “extraction fluid” refers to a fluid, typically a liquid, which is miscible with the solvent comprised in the fluid precursor composition, but which does not dissolve the polymer comprised in the fluid precursor composition or it dissolves said polymer to a low extent, such as the extraction fluid and polymer possessing a total Hansen solubility parameter 5 which differs by at least 3, preferably at least 3.4. Alternatively, the extraction fluid does not dissolve the polymer comprised in the fluid precursor composition or it dissolves said polymer to a lesser extent than it dissolves the solvent, such as at least three Hansen solubility parameters selected from 5, bo, bp or 5H of the solvent are closer to the corresponding Hansen solubility parameter 5, bo, bp or 5H of the extraction fluid than the corresponding Hansen solubility parameters 5, bo, bp or bn of the polymer are.

[0092] In the present specification, the term "miscible" refers to the ability of two or more liquid materials to mix with each other into a single phase, or without separating into different phases; whereas two or more materials are termed “not miscible” when they form more than one phase when mixed.

[0093] Whether two or more materials are miscible or not can be determined based on common general knowledge, or by simple visual inspection, spectroscopy, viscometry, osmetry or calorimetry, following combining the materials, preferably by visual inspection. Alternatively, miscibility can be predicted as for solubility as described below.

[0094] Whether two or more materials are soluble or not, or whether a specific material is more or less soluble in one material than it is in another material, can be determined based on common general knowledge, such as by the Hansen parameters of the materials.

[0095] Hansen solubility parameters were developed by Charles M. Hansen in 1967 and are based on the idea that like dissolves like where one material is defined as being 'like' another if it bonds to itself in a similar way. Specifically, each material is given three Hansen parameters:

[0096] 5D refers to the Hansen dispersion solubility parameter, in MPa1 / 2(equivalent to joules / cubic centimeter; 2.0455 x (cal / cc)1 / 2) at 20-25°C. bp refers to the Hansen polar solubility parameter, in MPa1 / 2(equivalent to joules / cubic centimeter; 2.0455 x (cal / cc)1 / 2) at 20-25°C. H refers to the Hansen hydrogen bonding solubility parameter, in MPa1 / 2(equivalent to joules / cubic centimeter; 2.0455 x (cal / cc)1 / 2) at 20-25°C.

[0097] These three parameters can be treated as co-ordinates for a point in three dimensions also known as the Hansen space. The nearer two molecules are in this three- dimensional space, the more likely they are to dissolve or mix into each other.

[0098] The total Hansen solubility parameter b (square root of the sum of squares of the Hansen values for dispersion, polar, and hydrogen bonding components), in MPa1 / 2may also be employed when predicting solubility. The nearer the b value for two materials, the more likely they are to dissolve or mix into each other.

[0099] Hansen parameters of materials are well-known in the art (Hansen Solubility Parameters: A User's Handbook, C.M. Hansen, CRC Press LLC, 2000) and can furthermore be calculated using software specifically designed for said purpose, such as HSPiP, e.g. version 5.0.0.4 (www.hansen-solubility.com) developed by Dr Charles Hansen, Prof Steven Abbott and Dr Hiroshi Yamamoto; or by following the teachings of said Handbook, which describes the calculation method of each of the Hansen parameters. The dispersion component (bo) is obtained from the non-polar contribution of the vaporization energy of a fluid calculated by applying the procedures of Blanks and Prausnitz (Ind. Eng. Chem. Fundamen. 1964, 3, 1 , 1-8), which are based on Brown's concept of homomorphism, which relates the energy of vaporization of a polar fluid with that of a non-polar molecule with a very similar shape and size; the Polarity component (bp) is calculated by applying the Hansen and Beerbower equation, which takes into account the dipole moment and the molar volume of the fluid; the component associated with hydrogen bonds (bn) can be obtained by subtracting the polar and dispersion energies from the total vaporization energy, or by performing functional group contribution calculations provided in the Handbook.

[0100] Whilst a universal Hansen parameter value difference determining solubility or nonsolubility does not exist for any combination of materials, Hansen parameters represent excellent pointers when seeking soluble combinations.

[0101] In any case, in an embodiment, the polymer and the solvent in the polymer dissolved in a solvent do not possess: at least one Hansen solubility parameter b, bo, bp or bn which differs by 8 or more; or at least two Hansen solubility parameters b, bo, bp or bn which differ by 5 or more.

[0102] As is herein explained, the PCM should not be miscible with said solvent, and more particularly should not be soluble in said solvent. In an embodiment, alternatively or additionally, the PCM and the solvent possess: at least one Hansen solubility parameter b, bo, bp or bn which differs by 8 or more; or at least two Hansen solubility parameters b, bo, bp or bn which differ by 5 or more.

[0103] Alternatively, it is considered that a solvent does not dissolve a material when said material exhibits a solubility in said solvent of less than 1 mass percent, such as when 1 g of said material is immersed in 10 g of said solvent for 24 h at room temperature (23°C) and atmospheric pressure (1 atm). When the same solubility is 1 mass percent or greater, it is considered that the solvent dissolves the material.

[0104] In an embodiment, in any embodiment described herein, in the fluid precursor composition, the PCM is not miscible with the polymer / monomers. Additionally, when the polymer / monomers are dissolved in a solvent, said solvent is not miscible with, in particular does not dissolve, the PCM.

[0105] Preferably, in the method of the present invention, the stream comprising polymer / monomers is a stream comprising a polymer dissolved in a solvent; preferably the stream comprising a phase change material is a molten PCM stream; and in the fluid precursor composition, the PCM is not miscible with the polymer of the polymer dissolved in a solvent, and the PCM is not miscible with, in particular not soluble in, the solvent of the polymer dissolved in a solvent; in the fluid precursor composition, the polymer dissolved in a solvent is a polymer which is not miscible with the PCM; in the fluid precursor composition, the solvent of the polymer dissolved in a solvent is not miscible with, and in particular does not dissolve, the PCM.

[0106] More preferably, the solidification of this fluid precursor composition is carried out by removing therefrom the solvent of the polymer dissolved in a solvent by extraction of said solvent into an extraction fluid, and the extraction fluid does not dissolve the polymer of the polymer dissolved in a solvent comprised in the fluid precursor composition, or it dissolves said polymer to a low extent, such as the extraction fluid and polymer possessing a total Hansen solubility parameter 5 which differs by at least 3, preferably at least 3.4; and the extraction fluid is miscible with the solvent of the polymer dissolved in a solvent comprised in the fluid precursor composition. Preferably, the extraction fluid does not dissolve the polymer of the polymer dissolved in a solvent comprised in the fluid precursor composition, or it dissolves said polymer to a lesser extent than it dissolves the solvent, such as at least three Hansen solubility parameters selected from 5, bo, bp or 5H of the solvent are closer to the corresponding Hansen solubility parameters 5, 5D, 5P or H of the extraction fluid than the corresponding Hansen solubility parameters b, bo, bp or bn of the polymer are. As the PCM is comprised at the core of the fluid precursor composition, the extraction fluid neither dissolves nor mixes with the PCM, as no contact takes place.

[0107] Table 1 below summarizes the above conditions:

[0108] In an embodiment, in any embodiment herein referring to a material being less soluble than another in a given solvent (or inversely a solvent dissolving a material less than another material), said lesser solubility refers to at least 10 times, preferably at least 100 times, more preferably at least 1000 times less soluble.

[0109] In an embodiment, in the method of the invention, the solvent of the polymer dissolved in a solvent, and therefore also the solvent comprised in the fluid precursor composition, is a polar solvent. A polar solvent preferably refers to a solvent with a dielectric constant of 5 or higher, preferably measured at 20°C. Non-limiting examples of the polar solvent are water, methanol, ethanol, ammonia, DMSO, DMF, acetone, THF.

[0110] In an embodiment, in the method of the invention, the polymer of the polymer dissolved in a solvent, and therefore also the polymer comprised in the fluid precursor composition, is a polymer that can be dissolved by a polar solvent as defined above. Non-limiting examples of polymers that can be dissolved by a polar solvent as defined above are polyvinylidene difluoride (PVDF), poly(vinyl alcohol) (PVA), acrylonitrile butadiene styrene (ABS), polyacrylates (including polymethacrylates), polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), polystyrene (PS), Polyacrylonitrile (PAN).

[0111] In another embodiment, the solvent of the polymer dissolved in a solvent, and therefore also the solvent comprised in the fluid precursor composition, is a non-polar solvent. A non-polar solvent preferably refers to a solvent with a dielectric constant of lower than 5, preferably measured at 20°C. Non-limiting examples of the non-polar solvent are chloroform, diethylether, toluene, benzene, hexane, pentane. In an embodiment, in the method of the invention, the polymer of the polymer dissolved in a solvent is a polymer that can be dissolved by a non-polar solvent as defined above. Non-limiting examples of polymers that can be dissolved by a non-polar solvent as defined above are polystyrene (PS), lineal polyurethanes, poly(lactic-co-glycolic acid (PLGA), polyethylene (PET), polypropylene (PP), poly(vinyl alcohol) (PVA), polycaprolactone (PCL), polyhydroxybutyrate (PHB).

[0112] In a very particular embodiment, PVDF dissolved in DMF is used as polymer; and the PCM is a paraffin, such as hexadecane; and solidification may be by extraction of DMF into water.

[0113] In another very particular embodiment, PVDF dissolved in DMF is used as polymer; and the PCM is a paraffin, such as eicosane; and solidification may by be extraction of DMF into water.

[0114] In another very particular embodiment, PVB dissolved in ethanol is used as polymer; and the PCM is a paraffin, such as docosane; and solidification may by extraction of ethanol into water.

[0115] In another very particular embodiment, PVB dissolved in ethanol is used as polymer; and the PCM is a fatty acid, such as oleic acid; and solidification may be by extraction of ethanol into water.

[0116] In another very particular embodiment, PET dissolved in DCM is used as polymer; and the PCM is a paraffin, such as eicosane; and solidification may be by extraction of DCM into hexane.

[0117] In another very particular embodiment, PET dissolved in DCM is used as polymer; and the PCM is a fatty acid, such as lauric acid; and solidification may be by extraction of DCM into hexane.

[0118] In another very particular embodiment, polyurethane dissolved in THF is used as polymer; and the PCM is a paraffin, such as dodecane; and solidification may be by extraction of THF into ethanol.

[0119] The method of the invention can be carried out using any device comprising an outer channel coaxially surrounding an inner channel. Since streams are to be flowed through these channels, each channel must comprise an entry end for the fluid to enter the channel, as well as an exit end for the fluid to exit the channel. Furthermore, since a stream implies flow, the device preferably comprises means for feeding a fluid into and through each channel, preferably at a controllable rate. This may be achieved by placing a syringe pump loaded with its corresponding fluid in fluid communication with each channel. Actuating the syringe pump will cause the loaded fluid to flow through the channel. The use of independent means for feeding fluid respectively into each channel (i.e. the inner and outer channels) furthermore allows adjusting the flow rate of the stream comprising a phase change material independently from the flow rate of the stream comprising polymer / monomers. The control of each flow rate can advantageously be used to tailor the properties of the fluid precursor composition and in turn of the corresponding fiber following solidification of the fluid precursor composition, such as the amounts of the shell and core with respect to the total weight of the fiber, or the continuity of the core material along the fluid precursor composition and corresponding fiber length. With respect to the latter, under the combined action of the surface tension and viscous force, as the flow rate of the inner channel stream is reduced, a threshold will be reached at which the outer channel stream cuts inward into the inner channel stream (this typically occurs at the point in which the two streams come into physical contact), resulting in a node-type structure of the fluid precursor composition and in turn of the corresponding fiber wherein areas of only shell (areas in between nodes) and areas of core microdroplets wrapped in shell (nodes) are distinguishable along the fluid precursor composition and fiber length. By reducing the flow rate of the inner channel stream further, the frequency of said nodes or microdroplets is decreased (i.e. the distance between nodes is increased). At a flow rate of the inner channel stream above said threshold, the core will be present in a continuous manner along the fluid precursor composition and along the corresponding fiber length.

[0120] Such devices are well known in the art and are described for instance in CN 110016725 A (albeit related to different solidification means), incorporated herein by reference.

[0121] Lastly, the device should comprise means for solidification of the fluid precursor composition. Said means for solidification will depend on the specific type of solidification sought. For instance, when solidification is by extraction of the solvent comprised in the fluid precursor composition into an extraction fluid, the means for solidification may be a vessel comprising the extraction fluid, such as a bath of extraction fluid. The means for solidification are arranged with respect to the exit ends of the channels such that the fluid precursor composition can reach the means for solidification. This may be achieved by placing the exit end or ends of the channels in fluid communication with the means for solidification, or by positioning said end or ends such that the fluid precursor composition reaches, such as falls into (e.g. is ejected or drops into, one or the other depending on the flow rate of the streams and in turn of the fluid precursor composition) the means for solidification. In an embodiment, the device may comprise a means for controlling the temperature of the device, such as a heating mat or an incubation chamber, or of specific parts of the device, such as the channels, e.g. a resistive heating wire or a thermoelectric heating element.

[0122] The method of the invention is a method for preparing a composite fiber. The fiber prepared by said method is described in the second aspect of the invention.

[0123] Once the fiber is obtained, it may be subjected to conventional processes such as collecting, drying, molding, cutting or combining with further fibers, which may be the same or different fibers, such as in a mat, coiled or braided fashion, depending on the desired use of the fiber.

[0124] However, in an embodiment, the method of the invention does not comprise a freeze- drying step after solidifying the fluid precursor composition.

[0125] In the second aspect of the invention, the invention relates to a fiber comprising: a solid shell comprising a polymer, preferably a polymer that has not been cured by gelation, more preferably a polymer that has been solidified as described in the first aspect of the invention; the shell coaxially surrounding a core comprising a phase change material, wherein said phase change material is not comprised in a porous polymer matrix (or simply polymer matrix).

[0126] Advantageously, the fiber of the invention possesses an enthalpy of phase transition of at least 140 J / g, such as of from 140 J / g to 240 J / g.

[0127] The enthalpy of phase transition preferably refers to a solid-liquid or liquid-solid transition of the fiber, more particularly of the PCM comprised in the fiber. In the context of the present invention, the enthalpy of phase transition can be calculated by Differential Scanning Calorimetry (DSC), more specifically by subjecting the fiber to heating or cooling at a respective heating or cooling rate of 5 °C / min followed by integration of the resulting DSC-thermogram phase transition peak. More specifically, the enthalpy of phase transition can be calculated as described in the Examples hereinbelow (see section DSC Analysis).

[0128] Alternatively, the enthalpy of phase transition can be measured according to ASTM E793 - 06(2018).

[0129] In a preferred embodiment, in any embodiment described herein, at least part, preferably at least 50%, more preferably at least 70%, even more preferably at least 90%, in particular all, of the length of the fiber possesses a diameter of at least 30 pm, preferably at least 50 m, more preferably at least 100 pm; such as from any of these values up to 4000 pm, preferably up to 2000 pm, more preferably up to 1000 pm. In an embodiment, the diameter refers to the average diameter. Preferably, it refers to the minimum diameter.

[0130] In an embodiment, if the diameter is not constant at a specific point of the fiber length (i.e. the periphery is not circular), these diameters refer to the average diameter at said specific point of the fiber length; preferably these diameters refer to the shortest diameter at said specific point of the fiber length.

[0131] The diameter of the fiber can be calculated by scanning electron microscopy (SEM), for instance as described in the Examples, at the Fiber diameter section.

[0132] The polymer comprised in the fiber is a polymer as was described in the first aspect of the invention for the stream of step ii) and the fluid precursor composition.

[0133] In an embodiment, the shell is comprised in the fiber in an amount of 50% or less, preferably 30% or less, by weight of the fiber. In a more particular embodiment, the shell is comprised in the fiber in an amount of from 10 to 50%, more particularly 20 to 30%, by weight of the fiber.

[0134] As was already described, the shell need not comprise a surfactant. This is advantageous as the absence of a surfactant at the shell allows maximizing the amount of polymer at said shell. In an embodiment, the shell comprises the polymer in an amount of at least 80%, preferably at least 90%, by weight of the shell, more preferably it consists of the polymer.

[0135] In any embodiment described herein, the fiber does not comprise a second shell, in particular a second solid shell, more particularly a second solid shell comprising polymer. In particular, said second shell surrounds the solid shell of the fiber.

[0136] The PCM comprised in the fiber is a PCM as was described in the first aspect of the invention for the stream of step i) and the fluid precursor composition.

[0137] In an embodiment, the core is comprised in the fiber in an amount of 50% or more, preferably 70% or more, by weight of the fiber. In a more particular embodiment, the core is comprised in the fiber in an amount of from 50 to 90%, more particularly 70 to 80%, by weight of the fiber.

[0138] In an embodiment, the PCM comprised in the core is not comprised in a (porous) polymer matrix. This is advantageous as the absence of a polymer matrix at the core allows maximizing the amount of PCM at said core. Thus, in an embodiment, the core comprises the PCM in an amount of at least 80%, preferably at least 90%, more preferably at least 95%, such as at least 97% or at least 98% or at least 99%, by weight of the core, more preferably it consists of the PCM.

[0139] In an embodiment, the core does not comprise PAN, in particular it does not comprise said PAN encapsulating PCM; more preferably it does not comprise a polymer, in particular it does not comprise said polymer encapsulating PCM.

[0140] In an embodiment, the core does not comprise poloxamer nor sorbitan monooleate, more preferably does not comprise a non-ionic surfactant, even more preferably does not comprise a surfactant. In these embodiments, the surfactant is preferably an emulsifier. In an embodiment, the core does not comprise either one of a polymer or a surfactant as described in the immediately above embodiments.

[0141] Throughout the present disclosure, amounts of the different components comprised in the fiber are chosen so as to never exceed 100% by weight with respect to the total weight of the fiber. In another embodiment, they are chosen so as to reach 100% by weight with respect to the total weight of the fiber.

[0142] In an embodiment, the fiber comprises the core in a continuous manner throughout the fiber length. More particularly, core is found throughout the fiber length in a continuous manner, except at the ends of the fiber, which act as caps to seal the fiber. This means that, except at the ends of the fiber, there is no point throughout the fiber length where no core is present. In a more particular embodiment, the relative amount by weight of core to shell throughout the fiber length is maintained within an at most 20%, preferably at most 10%, more preferably at most 5% variation, between any two points along the fiber length. More particularly, the relative amount by weight of core to shell throughout the fiber length is maintained within an at most 20%, preferably at most 10%, more preferably at most 5% variation, between any two points along the fiber length, excluding the ends of the fiber, which act as caps to seal the fiber, such as between any two points along fiber length excluding the 10%, particularly 5%, of the fiber length at each longitudinal end of the fiber.

[0143] Alternatively, and as was explained elsewhere herein, the fiber may adopt a node-type structure. In such a case, the shell and core weight percentages provided herein to define the fiber apply to the node center, i.e. the point of the node length of greatest core diameter.

[0144] In a preferred embodiment, the fiber of the invention is obtained by a method as described in any one of the embodiments of the first aspect of the invention.

[0145] In another aspect, the invention also relates to the fluid precursor composition, more particularly to a fluid composition comprising: a shell comprising a polymer or monomers that cannot be cured by gelation, the shell coaxially surrounding a core comprising a phase change material.

[0146] The polymer or monomers comprised in the fluid composition is a polymer or monomers as was described in the first aspect of the invention for the stream of step ii).

[0147] The PCM comprised in the fluid composition is a PCM as was described in the first aspect of the invention for the stream of step i).

[0148] In a preferred embodiment, the fluid composition is obtainable by executing steps i) to iii) of the method of the invention.

[0149] In another aspect, the invention is directed to the use of a fiber according to the invention for thermal energy storage, such as in electronics, solar energy, batteries, buildings, waste heat recovery, air-conditioning, temperature-adaptable greenhouses and textiles. Alternatively, the invention is directed to a product of manufacture, an electronic device, a solar energy system device, an energy storage device, a computer, a medical device, a storage unit, a building or building material, a container, an insulation or construction material, an automotive material, a vehicle, a boat, an airplane, a weapon or weapon system, industrial machinery, a pharmaceutical or a drug or a food package or storage device or container, a textile, a clothing or an apparel, footwear, a bedding or bedding system, a flame retardant material, comprising the fiber of the invention.

[0150] The following represent highlighted embodiments of the invention.

[0151] 1. Method for preparing a composite fiber, the method comprising the steps of: i) Providing an inner channel, through which a stream comprising a phase change material is flowed; ii) Providing an outer channel, through which a stream comprising polymer or monomers is flowed; wherein the outer channel surrounds the inner channel, and wherein the streams of the outer and inner channels flow co-axially; iii) Combining the stream comprising a phase change material and the stream comprising polymer or monomers downstream of an exit end of the inner channel, thus yielding a fluid precursor composition; iv) Solidifying the fluid precursor composition, provided that said solidification is not curing by gelification.

[0152] 2. Method according to embodiment 1 , wherein the PCM is a solid-liquid PCM.

[0153] 3. Method according to any one of embodiments 1 to 2, wherein the PCM is flowed through the inner channel in a molten state.

[0154] 4. Method according to any one of the previous embodiments, wherein the PCM is selected from paraffins such as eicosane, docosane, hexadecane or tetradecane; fatty acids or esters thereof such as caprylic acid, lauric acid or oleic acid; aliphatic alcohols such as alcohol sugars e.g. xylitol, mannitol, sorbitol, erythritol; metals or metal alloys, such as zinc or nickel-zinc; inorganic salts or eutectics thereof such as sodium nitrate, potassium nitrate, manganese chloride; salt hydrates, such as sodium acetate tri-hydrate; preferably from paraffins such as eicosane, docosane, hexadecane or tetradecane; and fatty acids such as lauric acid or oleic acid; more preferably, the PCM is selected from paraffins such as eicosane, docosane, hexadecane.

[0155] 5. Method according to any one of the previous embodiments, wherein the stream comprising polymer or monomer is a stream comprising polymer.

[0156] 6. Method according to any one of the previous embodiments, wherein the polymer or monomers is dissolved in a solvent.

[0157] 7. Method according to any one of the previous embodiments, wherein the polymer is selected from a polymer which can be solidified by means different to curing by gelification, more preferably it is a polymer that cannot be solidified by curing by gelification.

[0158] 8. Method according to any one of the previous embodiments, wherein the polymer is selected from fluoropolymers such as polyvinylidene difluoride (PVDF), poly(vinyl alcohol) (PVA), acrylonitrile butadiene styrene (ABS), polyacrylates (including polymethacrylates), polyvinyl butyral (PVB), polyurethanes such as lineal polyurethanes or thermoplastic polyurethane (TPU), polystyrene (PS), Polyacrylonitrile (PAN), poly(lactic-co-glycolic acid (PLGA), polyethylene (PET), polypropylene (PP), polycaprolactone (PCL), Polyhydroxybutyrate (PHB); preferably the polymer is a fluoropolymer, more preferably a fluoroplastic, such as THV, FEP, PFA, ETFE, ECTFE, PVDF; even more preferably it is PVDF.

[0159] 9. Method according to any one of the previous embodiments, wherein the stream comprising polymer or monomers does not comprise a polymer, in particular it does not comprise said polymer in the form of an emulsion wherein said polymer is the continuous phase.

[0160] 10. Method according to any one of the previous embodiments, wherein the stream comprising polymer or monomers does not comprise a non-ionic surfactant, preferably a surfactant. . Method according to any one of the previous embodiments, wherein the fluid precursor composition comprises a shell comprising the polymer or monomers, the shell coaxially surrounding a core comprising the PCM. . Method according to any one of the previous embodiments, wherein in the fluid precursor composition, the PCM is not miscible with the polymer or monomers.. Method according to any one of the previous embodiments, wherein the stream comprising polymer or monomers is a stream comprising a polymer dissolved in a solvent, and solidification refers to removing said solvent from the fluid precursor composition. . Method according to the previous embodiment, wherein in the fluid precursor composition, the PCM is not miscible with the polymer of the polymer dissolved in a solvent, and the PCM is not miscible with the solvent of the polymer dissolved in a solvent. . Method according to any one of the previous embodiments, wherein the stream comprising polymer or monomers is a stream comprising a polymer dissolved in a solvent, and solidification refers to removing said solvent from the fluid precursor composition by a method which is not electrospinning, preferably the solvent is removed by extraction of said solvent into an extraction fluid. . Method according to the previous embodiment, wherein the extraction fluid and the polymer dissolved in the solvent comprised in the fluid precursor composition possess a total Hansen solubility parameter 5 which differs by at least 3; and the extraction fluid is miscible with the solvent of the polymer dissolved in a solvent comprised in the fluid precursor composition. . Method according to any one of the previous embodiments, not comprising a freeze-drying step after solidifying the fluid precursor composition. . Fluid composition, comprising: a shell comprising a polymer that cannot be cured by gelation, the shell coaxially surrounding a core comprising a phase change material. . Fluid composition according to embodiment 18, not comprising a surfactant.. Fluid composition according to any one of embodiments 18 or 19, obtainable by executing steps i) to iii) of the method of any one of embodiments 1 to 17. . Fiber comprising: a solid shell comprising a polymer, preferably that has not been cured by gelation, the shell coaxially surrounding a core comprising a phase change material, wherein said phase change material is not comprised in a porous polymer matrix.

[0161] 22. Fiber according to embodiment 21 , wherein at least part of the length of the fiber possesses a diameter of at least 30 pm, preferably an average diameter of at least 30 pm, more preferably a minimum diameter of at least 30 pm.

[0162] 23. Fiber according to any one of embodiments 21 or 22, wherein the shell is comprised in the fiber in an amount of 50% or less, in particular of from 10 to 50%, preferably of 30% or less, in particular of from 20 to 30%, by weight of the fiber.

[0163] 24. Fiber according to any one of embodiments 21 to 23, wherein the shell comprises the polymer in an amount of at least 80%, preferably at least 90%, by weight of the shell.

[0164] 25. Fiber according to any one of embodiments 21 to 24 wherein the core is comprised in the fiber in an amount of 50% or more, in particular of from 50 to 90%, preferably of 70% or more, in particular of from 70 to 80%, by weight of the fiber.

[0165] 26. Fiber according to any one of embodiments 21 to 25, wherein the core comprises the PCM in an amount of at least 80%, preferably at least 90%, more preferably at least 95%, by weight of the core.

[0166] 27. Fiber according to any one of embodiments 21 to 26, the core not comprising a polymer, in particular not comprising a polymer encapsulating PCM.

[0167] 28. Fiber according to any one of embodiments 21 to 27, not comprising a surfactant.

[0168] 29. Fiber according to any one of embodiments 21 to 28, not comprising a second shell.

[0169] 30. Fiber according to any one of embodiments 21 to 29, obtainable by the method of any one of embodiments 1 to 15.

[0170] 31 . Fiber according to any one of embodiments 21 to 30, which is not combined with further fibers.

[0171] 32. Use of a fiber as described in any one of embodiments 21 to 31 , for thermal energy storage.

[0172] In the present disclosure, any one instance of the term “comprising” may be replaced by the term “consisting”.

[0173] EXAMPLES

[0174] Specific examples of the present invention are described herein and are intended to further illustrate the present invention, but not to limit the scope thereof. Materials

[0175] Commercial PVDF Solef® 5130 from SOLVAY was dissolved in N,N-Dimethylformamide (DMF) (99.5% Pure) from EMPARTA®. This solution was used as a precursor for fiber shell production. Hexadecane ReagentPlus® (99% Pure) or Eicosane ReagentPlus® (99% Pure) were supplied by Sigma-Aldrich and used as core material of the fibers. All chemicals were used as received with no further purification. Tap water was utilized as extraction fluid for the DMF extraction process.

[0176] Experimental Procedure

[0177] The different materials were chosen based on their Hansen solubility parameters:

[0178] The fibers were produced by injecting two liquid streams, consisting of a PVDF / DMF solution and molten PCM respectively. These streams were pumped by two syringe pumps (Pump 11 - Pico Plus Elite from HA Harvard Apparatus) through silicon tubing into a microfluidic device in which they were aligned coaxially, with the PCM stream being located inside the PVDF / DMF solution stream. The microfluidic device is based on coaxial channels.

[0179] Once the streams were aligned in the microfluidic device, they were ejected into a glass beaker filled with tap water. At this point, the solvent extraction process began, extracting DMF from the PVDF / DMF solution into the tap water. In this way, the formation of the PVDF shell took place, trapping the PCM inside the PVDF fiber.

[0180] The glass beaker and the air surrounding the experimental setup were heated up to 30 °C by an electric fan-heater to ensure homogeneous temperature of the setup. After the injection process, the fibers were collected and stored in tap water for 24h for the maximum extraction of DMF and then were dried at room temperature in a fume hood.

[0181] Charcterization Techniques Hansen solubility parameters

[0182] Hansen solubility parameters were taken from Hansen Solubility Parameters: A User's Handbook, C.M. Hansen, CRC Press LLC, 2000.

[0183] DSC Analysis

[0184] DSC was used to determine phase transition temperatures and their corresponding latent heats (enthalpies). A power-compensation DSC Q2500 from TA Instruments was employed with sealed aluminium crucibles. The mass of the samples was 9 mg. Argon (50 ml / min) was employed as the purge gas. Each sample was subjected to three heating and cooling cycles, with a heating / cooling rate of 5 °C / min. The transition temperatures were determined by the onset temperature of the corresponding endothermic peaks in the DSC-thermograms, whereas the enthalpy was calculated by integrating the latter, assuming a linear baseline. The DSC was calibrated for heat flow and temperature using high purity (>99.99%) reference materials indium and sapphire. The accuracy in determining melting temperatures is ±0.5 °C, whereas the enthalpy of phase transition and specific heat is ±5%.

[0185] Thermal stability and PCM content of the fibers

[0186] Thermogravimetric analysis of the fibers and raw materials was performed in a TG209F1 Libra® Thermogravimetric Analyzer from NETZSCH. The analysis was based on a heating ramp from room temperature to 600 °C with a heating rate of 10 °C / min under nitrogen atmosphere. The PCM content of the fibers was determined as the total mass loss suffered by the fiber samples at 215 °C, which is the mass loss associated to the pure PCM.

[0187] Fiber diameter

[0188] To analyze the diameter of the fibers, samples were imaged using a Quanta 200 FEG scanning electron microscope operated in low vacuum mode at 10 kV, equipped with a backscattered electron detector (BSED), avoiding paraffin melting during the study.

[0189] Samples for diameter analysis were prepared as follows: fiber were fractured with tweezers under liquid nitrogen to preserve their original shape and prevent fiber flattening. A single fiber was positioned in the SEM capture area to facilitate diameter evaluation.

[0190] The SEM image used for diameter assessment includes a scale bar, serving as a reference for determining the fiber diameter.

[0191] Fiber diameter analysis is conducted using Imaged software (version 1.53, developed by Wayne Rasband and contributors, available from the National Institutes of Health, USA, at http: / / imagej.nih.gov / ij). The SEM image described above is used as the working image. To initiate the analysis, a reference measurement is performed using the 'Straight line' tool on the scale bar. A straight line is drawn over the scale bar, and Ctrl+M is pressed to obtain the measurement. This action provides, among other information, the Length in pixels of that straight line. Using the length in pixels of the created line and the actual length displayed on the scale bar, we can obtain the conversion factor used for subsequent measurements:

[0192] Once the conversion factor is obtained, effective measurements of fiber diameter are conducted. Using the 'Straight line' tool, draw a line across the SEM image, perpendicular to the longitudinal axis of the fiber, covering the entire cross-sectional diameter of the fiber. To record each measurement, 'Ctrl+M' is pressed after each line is drawn.

[0193] The measurements, obtained in pixels, are converted to microns using the previously described conversion factor:

[0194] Fiber diameter (gm) = A ■ Fiber diameter (pixels)

[0195] From these values, the average, minimum, and maximum diameter of the fibers can be determined. For instance, where the average diameter of the fiber is to be determined, at least 5 lines covering the entire cross-sectional diameter of the fiber, parallel to each other, are drawn to evenly cover the length of the fiber, and an average from said at least 5 measurements is calculated.

[0196] Where the diameter is not constant at a specific point of the fiber length (i.e. the periphery is not circular), the fiber can be rotated around its longitudinal axis and the above measurement can be repeated at the same specific point of the fiber length. For instance, the fiber can be rotated evenly at least 5 times spanning a 360° rotation in total. This allows determining the average, minimum, and maximum diameter of the fiber at a specific point of its length.

[0197] Results

[0198] As shown in Figures 1 , 2 and 3 the fibers obtained by the method of the invention achieve an outstanding PCM mass content of about 70%, and an excellent high thermal storage capacity, with an enthalpy of about 160 J / g.

Claims

CLAIMS1 . Method for preparing a composite fiber, the fiber comprising: a solid shell comprising a polymer that has not been cured by gelation, the shell coaxially surrounding a core comprising a phase change material (PCM), wherein said phase change material is not comprised in a porous polymer matrix; the method comprising the steps of: i) Providing an inner channel, through which a stream comprising a phase change material is flowed; ii) Providing an outer channel, through which a stream comprising a polymer dissolved in a solvent is flowed; wherein the outer channel surrounds the inner channel, and wherein the streams of the outer and inner channels flow coaxially; iii) Combining the stream comprising a phase change material and the stream comprising the polymer dissolved in a solvent downstream of an exit end of the inner channel, thus yielding a fluid precursor composition; iv) Solidifying the fluid precursor composition by extraction of said solvent into an extraction fluid.

2. Method according to claim 1 , wherein the PCM is flowed through the inner channel in a molten state.

3. Method according to any one of the previous claims, wherein the PCM is selected from paraffins; fatty acids or esters thereof; aliphatic alcohols; metals or metal alloys; inorganic salts or eutectics thereof; and salt hydrates.

4. Method according to any one of the previous claims, wherein the stream comprising polymer does not comprise surfactant.

5. Method according to any one of the previous claims, wherein the stream comprising the PCM does not comprise surfactant.

6. Method according to any one of the previous claims, wherein the stream comprising the PCM does not comprise a polymer.

7. Method according to any one of the previous claims, wherein the stream comprising the PCM is not in the form of an emulsion.

8. Method according to any one of the previous claims, wherein the polymer is selected from fluoropolymers, poly(vinyl alcohol) (PVA), acrylonitrile butadiene styrene (ABS), polyacrylates, polyvinyl butyral (PVB), polyurethanes, polystyrene (PS), Polyacrylonitrile (PAN), poly(lactic-co-glycolic acid (PLGA), polyethylene (PET),polypropylene (PP), polycaprolactone (PCL), Polyhydroxybutyrate (PHB).

9. Method according to any one of the previous claims, wherein: in the fluid precursor composition, the PCM is not miscible with the polymer of the polymer dissolved in a solvent, and the PCM is not miscible with the solvent of the polymer dissolved in a solvent; the extraction fluid and the polymer of the polymer dissolved in the solvent comprised in the fluid precursor composition possess a total Hansen solubility parameter 5 which differs by at least 3; and the extraction fluid is miscible with the solvent of the polymer dissolved in a solvent comprised in the fluid precursor composition.

10. Method according to any one of the previous claims, not comprising a freeze-drying step after solidifying the fluid precursor composition.

11. Fiber comprising: a solid shell comprising a polymer that has not been solidified by curing by gelation, the shell coaxially surrounding a core comprising a phase change material, wherein said phase change material is not comprised in a porous polymer matrix; and wherein at least part of the length of the fiber possesses a diameter of at least 30 pm.

12. Fiber according to claim 11 , wherein the shell is comprised in the fiber in an amount of from 10 to 50% by weight of the fiber, and wherein the shell comprises the polymer in an amount of at least 80% by weight of the shell.

13. Fiber according to any one of claims 11 to 12, wherein the core is comprised in the fiber in an amount of from 50 to 90% by weight of the fiber, and wherein the core comprises the PCM in an amount of at least 95% by weight of the core.

14. Fiber according to any one of claims 11 to 13, wherein the shell does not comprise a surfactant.

15. Fiber according to any one of claims 11 to 14, wherein the core does not comprise a surfactant.

16. Use of a fiber as defined in any one of claims 11 to 15, for thermal energy storage.

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