SYNTHESIS AND IMPLEMENTATION OF PHASE CHANGE VITRIMERAL MATERIALS

Abstract

SYNTHESIS AND IMPLEMENTATION OF PHASE-CHANGE VITRIMERE MATERIALS The present invention relates to a phase-change vitrimer material and its preparation process. The process for preparing the vitrimer material comprises the following steps: copolymerization of a vinyl monomer bearing a crystallizable group and another vinyl monomer bearing a crosslinking function to give a copolymer bearing crystallizable groups and crosslinking functions; crosslinking of the copolymer obtained in step a) via the formation of crosslinking bridges having exchangeable functions by associative mechanisms, the exchangeable functions resulting from the reaction between the crosslinking functions of the copolymer and a hardening agent. Figure for the abstract: none

Description

Title of the invention: SYNTHESIS AND IMPLEMENTATION OF PHASE CHANGE VITRIMERAL MATERIALS technical field

[0001] The present invention relates to the field of phase change materials. More particularly, it relates to a solid-solid phase change material and its preparation process. STATE OF THE ART

[0002] Phase change materials (PCMs) are materials with crystalline phases that store heat during their melting and release heat during their crystallization. These materials can be used, in particular, for the thermal management of electronic systems because they reduce temperature variations in these components.

[0003] Conventional PCMs such as paraffin, fatty acids, or polyethylene glycol exhibit solid-liquid phase transitions during the melting of their crystalline phase. This liquefaction leads to durability issues for these cooling systems. The PCMs can leak and detach from their substrate, resulting in a loss of thermal contact and therefore a drop in performance. Their industrial applicability in thermal management is thus limited.

[0004] Several solutions exist to overcome this problem. For example, PCMs can be encapsulated in other materials that remain temperature-stable [Giro-Paloma, J.; Martinez, M.; Cabeza, LF; Fernandez, AI Types, Renewable and Sustainable Energy Reviews 2016, 53, 1059-1075]. However, this solution can lead to supercooling of the PCM, i.e., significant differences between the melting temperature and the crystallization temperature of the PCM. This phenomenon impairs the performance of the PCM because it reduces its ability to release stored energy. Alternatively, PCMs can be cross-linked. They then exhibit a solid-solid phase transition and are therefore temperature-stable [Xiao, C.; Zhang, G.; Li, Z.; Yang, XJ Mater. Chem. A 2020, 8 (29), 14624-14633]. However, once cross-linked, they can no longer be reshaped or recycled.

[0005] Vitrimeric PCMs make it possible to solve this latter problem because they exhibit a solid-solid phase transition but, thanks to their dynamic crosslinking network, they can be reshaped at temperature.

[0006] In general, vitrimeric materials are crosslinked polymers whose crosslinking bonds can undergo exchange reactions via associative mechanisms. These mechanisms ensure that the crosslinking density of the material remains constant during the exchange. These materials are therefore capable of relaxing at temperature while exhibiting the mechanical and chemical stability properties characteristic of cross-linked polymers. Thanks to these exchanges, vitrimeric materials can be shaped at temperature, repaired, or recycled.

[0007] The literature reports various syntheses of vitrimeric PCMs. Peng et al. [Peng, L.-M.; Xu, Z.; Wang, WY; Zhao, X.; Bao, R.-Y.; Bai, L.; Ke, K.; Liu, ZY; Yang, M.-B.; Yang, W. ACS Appl. Energy Mater. 2021, 4 (10), 11173-11182] functionalized polyethylene chains with dioxoborolane compounds. Partially crosslinked materials with free polyethylene microphases encapsulated in vitrimeric polyethylene were obtained. Such microencapsulated PCMs can exhibit significant supercooling phenomena as well as durability issues (leakage of the microencapsulated PCM).

[0008] Yang et al. [Yang, A.-S.; Huang, R.; Jiang, Z.; Li, Y.; He, F.; Zhang, Q.-P.; Zhou, Y.; Wang, P.; He, G.; Yang, W. ACS Appl. Polym. Mater. 2023, 5 (2), 1499-1508.] and Kong et al. [Kong, W.; Yang, Y.; Yuan, A.; Jiang, L.; Fu, X.; Wang, Y.; Xu, H.; Liu, Z.; Lei, J. Energy Storage. Energy 2021, 232, 121070.] developed vitrimeric PCMs by crosslinking polyethylene glycol with polyisocyanate compounds and disulfide compounds. These compounds, due to the use of polyisocyanate compounds, present toxicity problems, many of them are even classified as CMR (Carcinogenic, Mutagenic, Reprotoxic).

[0009] J. Lessard et al. [J. Lessard et al., Block copolymer vitrimers, J. Am. Chem. Soc. 2020, 142, 283-289.] described a promising synthetic route for vitrimer materials by copolymerizing n-alkyl methacrylate compounds with (2-acetoacetoxy)ethyl methacrylate. The resulting copolymer thus exhibits acetoxyacetate groups that can crosslink in a second step with polyamine compounds to obtain exchangeable vinylogous urethane-type groups. However, the n-alkyl methacrylate compounds used have short alkyl chain lengths (number of carbons less than or equal to 4) and therefore cannot crystallize.

[0010] Thus, the vitrimer PCM materials known to date exhibit low performance in terms of durability, repeatability or require the implementation of toxic isocyanate compounds.

[0011] A need therefore remains for the provision of new solid-solid phase transition PCM materials that are durable, thermoformable, repairable, recyclable and that can be formed without the use of isocyanate precursors. Summary of the invention

[0012] The present invention relates to a process for preparing a phase-change vitrimer material comprising the following steps: a. copolymerization of a vinyl monomer bearing a crystallizable group and another vinyl monomer bearing a crosslinkable function to give a copolymer bearing crystallizable groups and crosslinkable functions; b. crosslinking of the copolymer obtained in step a) via the formation of crosslinking bridges comprising exchangeable functions by associative mechanisms, the crosslinking bridges resulting from the reaction between the crosslinkable functions of the copolymer and a hardening agent.

[0013] The present invention also relates to the phase-change vitrimer material which is the product of the reaction between: a. a copolymer based on vinyl monomers bearing a crystallizable group and vinyl monomers bearing a crosslinkable function; and b. a hardener;

[0014] the material comprising crosslinking bridges comprising functions exchangeable by associative mechanisms, the crosslinking bridges resulting from the reaction between the crosslinkable functions carried by the copolymer and the hardener.

[0015] Finally, the present invention relates to the use of the material of the present invention for the thermal management of electronic systems.

[0016] Other aspects of the invention are as described below. FIGURES

[0017] [Fig.1] represents the thermogram obtained during the differential scanning calorimetric analysis of the vitrimer PCM of example 1 (10°C / min).

[0018] [Fig.2] represents the thermogram obtained during the calorimetric analysis differential scanning of the PCM vitrimer composite of example 2 (10°C / min). DETAILED DESCRIPTION OF THE INVENTION

[0019] The inventors have developed a new process for preparing vitrimers leading to materials that meet the stated requirements. Thus, the new solid-solid phase-transition PCM materials prepared are durable, can be thermoformed, repaired or recycled, and advantageously their preparation does not require the use of isocyanate precursors.

[0020] Thus, the present invention relates to a process for preparing a phase-change vitrimer material comprising the following steps: a. copolymerization of a vinyl monomer bearing a crystallizable group and another vinyl monomer bearing a crosslinkable function to give a copolymer bearing crystallizable groups and crosslinkable functions; b. crosslinking of the copolymer obtained in step a) via the formation of crosslinking bridges comprising functions exchangeable by associative mechanisms, the functions exchangeable by associative mechanisms resulting from the reaction between the crosslinkable functions of the copolymer and a hardening agent.

[0021] The proposed process allows for precise control of the macromolecular structure of the vitrimer formed, thus obtaining repeatable properties. The resulting material can be reshaped at temperature while meeting the durability requirements specific to thermal management systems.

[0022] The material obtained is therefore particularly useful for the thermal management of electronic systems. Step a)

[0023] The first step consists of the copolymerization of vinyl monomers bearing a crystallizable group and vinyl monomers bearing a crosslinkable function.

[0024] Vinyl monomers bearing a crystallizable group

[0025] The term "crystallizable group" refers to a group of atoms capable of reversibly transitioning from an amorphous to a crystalline state depending on whether the temperature is below or above the melting point. These are preferably -(CH2)n- and / or -((CH2)mO)n- groups in which n varies from 10 to 20 and m varies from 10 to 100.

[0026] More particularly, vinyl monomers bearing a crystallizable group useful in the context of the invention can be vinyl monomers bearing an alkyl group comprising 10 to 20, preferably 14 to 18, carbon atoms.

[0027] Examples of vinyl monomers bearing a crystallizable group include, but are not limited to, (meth)acrylic monomers (e.g., alkyl acrylates or alkyl methacrylates), acrylamide monomers (e.g., alkyl acrylamides), and vinyl ester monomers (e.g., alkyl vinyl esters). The alkyl group in these monomers generally comprises 10 to 20, preferably 14 to 18, carbon atoms.

[0028] Thus, preferably, vinyl monomers bearing a crystallizable group are selected from the group consisting of monomers having the following formulas a, b, c or d and their mixtures: abcd

[0029] in which n varies from 10 to 20, preferably from 14 to 18.

[0030] Vinyl monomers bearing a crosslinkable function

[0031] The term "crosslinkable function" as used herein refers to a function capable of reacting with a similar or different function and leading to the formation of covalent bonds. In the context of the present invention, crosslinkable functions are capable of reacting with a hardener and forming exchangeable functions through associative mechanisms, such as vinylogous urethane, disulfide, sillyl ether, trialkyl sulfonium, diketoenamine, alkene, amide, dioxoborolane, carbonate, and ester functions.

[0032] In particular, the crosslinking function may be an epoxy, acetoacetate, alcohol, aldehyde, allyl, vinyl, acid, amine, silane, anhydride, maleimide, imide, furfuril, cyanate ester, benzoxazine, or alkyne. The crosslinking function may be part of a group attached to the monomer.

[0033] Examples of vinyl monomers bearing a crosslinkable function include, but are not limited to, glycidyl methacrylate (GMA) and acetoacetoxyethyl methacrylate (AAEMA).

[0034] The polymerization step can be carried out according to free radical polymerization mechanisms or controlled by thermal or photochemical means. It can be carried out under conditions well known to those skilled in the art.

[0035] The resulting copolymers bear crystallizable groups and crosslinkable functions. In other words, they generally comprise crystallizable pendant chains (corresponding to the crystallizable groups of the monomers) and crosslinkable pendant chains (corresponding to the groups bearing a crosslinkable function of the monomers).

[0036] The copolymers obtained can be block, alternating, gradient or statistical.

[0037] In some embodiments, step a) consists of the copolymerization of vinyl monomers bearing a crystallizable group of formula (I) and vinyl monomers bearing a crosslinkable function of formula (II): R, / ¾ B

[0038] (I) (II)

[0039] in which:

[0040] A designates a crystallizable group;

[0041] B designates a group bearing a crosslinkable function;

[0042] Ri and R2 designate, independently of each other, a hydrogen atom or a methyl group.

[0043] The crystallizable group and the group bearing a crosslinkable function can be as previously described. Step b)

[0044] The second step consists of crosslinking the copolymer obtained at the end of step a) by reaction between the crosslinkable functions carried by the copolymer and a hardener.

[0045] The reaction between the crosslinking functions carried by the copolymer and the hardener leads to the formation of crosslinking bridges comprising exchangeable functions through associative mechanisms, such as the functions described above, in particular vinylogous urethane or disulfide functions. These functions can undergo exchange reactions and confer to the resulting material its thermoformability, repairability, and recyclability properties despite its crosslinked nature.

[0046] The term “hardener” refers to a chemical compound capable of reacting with crosslinkable functions to obtain a crosslinked polymer network.

[0047] The hardener is preferably selected from the group consisting of 4-aminophenyl disulfide and tris(2-aminoethyl)amine.

[0048] It will be readily understood by those skilled in the art that the hardener is chosen according to the nature of the crosslinking functions present on the copolymer. The hardener is chosen so that the reaction between the crosslinking functions of the copolymer and the hardener forms crosslinking bridges containing exchangeable functions through associative mechanisms, such as vinylogous urethane or disulfide functions.

[0049] For example, when the copolymer includes an epoxide function (e.g., the epoxide function of GMA), this can react with amine functions, such as, for example, the amine functions of 4-aminophenyl disulfide. Crosslinking bridges The resulting copolymers thus contain disulfide functions. When the copolymer includes an acetoacetate function (e.g., the acetoxyacetate function of AAEMA), this can react with polyamine compounds such as tris(2-aminoethyl)amine to form vinylogous urethane functions.

[0050] Crosslinking is carried out under suitable operating conditions.

[0051] Crosslinking can be carried out in the presence of conductive charges. The addition of Conductive fillers improve the thermal conduction properties of the resulting material and thus optimize its efficiency in thermal management.

[0052] Examples of conductive fillers include, but are not limited to, carbon nanotubes, graphene, graphite, expanded graphite and boron nitride. Vitrimeric material

[0053] The present invention also relates to the phase-change vitrimer material obtained or capable of being obtained by the process of the present invention.

[0054] Thus, the present invention relates to a phase-change vitrimer material which is the product of the reaction between: a. a copolymer based on vinyl monomers bearing a crystallizable group and vinyl monomers bearing a crosslinkable function; And b. a hardener;

[0055] the material comprising crosslinking bridges comprising functions exchangeable by associative mechanisms, the functions exchangeable by associative mechanisms resulting from the reaction between the crosslinkable functions carried by the copolymer and the hardener.

[0056] The phase-change vitrimer material is particularly useful for the thermal management of electronic systems. EXAMPLES Example 1#: Preparation of a vitrimer PCM Step 1 - Copolymer Synthesis#:

[0057] The synthesis of the AAEMA-OA copolymer is carried out according to the following reaction scheme:

[0058] Octadecyl acrylate (m=8.114 g, n=25 mmol), (2-Acetoacetoxy)ethyl methacrylate (m=0.536 g, n=2.5 mmol), and 2,2'-azobis(2-methylpropionitrile) (m=0.226 g, n=1.38 mmol) are dissolved in 15 mL of toluene at 50°C. The solution is then transferred to a sealable funnel fitted with a magnetic stir bar and sealed under vacuum after three vacuum cycles in liquid nitrogen. The funnel is then incubated at 60°C for 24 hours. Following the reaction, the polymer is purified three times by precipitation in 500 mL of methanol at room temperature. It is then dried under vacuum at 80°C for three hours. Proton NMR characterization indicates an acetoxyacetate function concentration of 0.28 mmol / g. Step 2 - Vitrimer PCM Synthesis:

[0059] The synthesis of the vitrimer PCM is carried out according to the following reaction scheme: o 'o

[0060] The copolymer (m=2g, nacetoxyacetate=0.56 mmol) is dissolved in 12 mL of tetrahydrofuran at 50°C. Tris(2-aminoethyl)amine (nm=40.9 mg, n=0.28 mmol) is then added, and the solution is poured into an aluminum dish. The mixture is left to react overnight at room temperature and then placed in an oven for 3 hours at 80°C. The vitrimer PCM is obtained as a light yellow solid.

[0061] Hot pressing tests (160°C, 1h30, 2 kg) were carried out using powdered PCM vitrimer material. The material was successfully pressed into pellets. and then ground into powder three times consecutively. This result confirms the thermoformability, repairability, and recyclability properties inherent to vitrimeric materials. Characterization of vitrimer PCM by IR

[0062] Characterization by infrared spectroscopy highlights two signals characteristic of the vinylogous urethane functions at 1604 cm1 and 1654 cm'. The material obtained clearly exhibits a vitrimeric chemical structure.

[0063] Characterization by differential scanning calorimetry analysis:

[0064] The material was characterized by differential scanning calorimetry ([Fig. 1]). It exhibits a melting point (Tonset) of 40.4°C and an enthalpy of fusion of 73 J / g. These characteristics remain stable throughout the melting and crystallization cycles. This analysis confirms the PCM functionality of this vitrimer material.

[0065] Example 2: Preparation of a composite vitrimer PCM

[0066] Expanded graphite (m=0.857 g) is suspended in 50 mL of tetrahydrofuran and stirred for 24 h. The copolymer prepared according to Example 1 (step 1) (m=1.98 g, nacetoxyacetate=0.248 mmol) is dissolved in 12 mL of tetrahydrofuran. Tris(2-aminoethyl)amine (m=18.2 mg, n=0.124 mmol) is then added to the copolymer solution. This solution is added to the expanded graphite suspension, and the mixture is poured into an aluminum capsule. The mixture is reacted at room temperature overnight and then at 80°C for 3 h. The powder is then collected and pressed into a 13 mm diameter pellet at a pressure of 10 t.

[0067] Characterization by differential scanning calorimetry analysis:

[0068] The composite material is characterized by differential scanning calorimetry ([Fig. 2]). It exhibits a melting point (Tonset) of 43.5°C and an enthalpy of fusion of 50 J / g. These characteristics remain stable throughout the melting and crystallization cycles. This analysis confirms the PCM functionality of this composite vitrimer material. Thermal conductivity

[0069] The composite material is also characterized with a Latima thermal analyzer (Nanotest). The thermal conductivity in the plane of the pellet is 28.5 ±2.1 W / (mK) at 37°C. The pure vitrimer PCM material exhibits a thermal conductivity of less than 0.4 W / (mK). This result confirms the significant gain in thermal conductivity provided by the conductive fillers.

Claims

Demands

1. A process for preparing a phase-change vitrimer material comprising the following steps: a. copolymerization of a vinyl monomer bearing a crystallizable group and another vinyl monomer bearing a crosslinkable function to give a copolymer bearing crystallizable groups and crosslinkable functions; b. crosslinking of the copolymer obtained in step a) via the formation of crosslinking bridges having exchangeable functions by associative mechanisms, the exchangeable functions by associative mechanisms resulting from the reaction between the crosslinkable functions of the copolymer and a hardening agent.

2. A process according to claim 1 wherein the crystallizable group is a -(CH2)n- and / or -((CH2)mO)n- group in which n varies from 10 to 20 and m varies from 10 to 100.

3. A process according to claim 1 or 2 wherein vinyl monomers bearing a crystallizable group are selected from the group consisting of monomers having the following formulas a, b, c or d and their mixtures: / pd - O « HR _ „ h" hhhabcd in which n varies from 10 to 20.

4. A method according to any one of claims 1 to 3 wherein the crosslinkable function is an epoxy function or an acetoacetate function.

5. A method according to any one of claims 1 to 4 wherein the functions exchangeable by associative mechanisms are vinylogous urethane, disulfide, sillyl ether, trialkyl sulfonium, diketoenamine, alkene, amide, dioxoborolane, carbonate or ester functions.

6. A method according to any one of claims 1 to 5 wherein the vinyl monomer bearing a crosslinkable function is selected from the group consisting of glycidyl methacrylate (GMA) and acetoacetoxyethyl methacrylate (AAEMA).

7. A process according to any one of claims 1 to 6 wherein step a) consists of the copolymerization of a vinyl monomer bearing a crystallizable group of formula (I) and a vinyl monomer bearing a crosslinkable function of formula (II): y =< “"8 A (I) (II) in which: A denotes a crystallizable group; B denotes a group bearing a crosslinkable function; Ri and R2 denote, independently of each other, a hydrogen atom or a methyl group.

8. A method according to any one of claims 1 to 7 wherein the hardening agent is selected from the group consisting of 4-aminophenyl disulfide and tris(2-aminoethyl)amine.

9. A method according to any one of claims 1 to 8 wherein step b) is carried out in the presence of conductive fillers, preferably in the presence of carbon nanotubes, graphene, graphite, expanded graphite and / or boron nitride.

10. Phase-change vitrimer material that is the product of the reaction between: a. a copolymer based on vinyl monomers bearing a crystallizable group and vinyl monomers bearing a crosslinkable function; and b. a hardener; the material comprising crosslinking bridges comprising functions exchangeable by associative mechanisms, the functions exchangeable by associative mechanisms resulting from the reaction between the crosslinkable functions borne by the copolymer and the hardener.

11. Use of the material according to claim 10 for the thermal management of electronic systems.

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