EP4689003A1

EP4689003A1 - Method

Info

Publication number: EP4689003A1
Application number: EP24721727.6A
Authority: EP
Inventors: Helen Frances Gleeson; Richard MANDLE; Thomas RAISTRICK; Matthew Reynolds; Stuart BERROW; Devesh Arvind MISTRY
Original assignee: University of Leeds; University of Leeds Innovations Ltd
Current assignee: University of Leeds; University of Leeds Innovations Ltd
Priority date: 2023-03-27
Filing date: 2024-03-25
Publication date: 2026-02-11
Also published as: WO2024201014A1; JP2026511548A; CN121079382A; KR20250166274A; GB202304439D0

Abstract

A method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer comprising chemically modifying the aligned nematic liquid crystal elastomer.

Description

METHOD

Field

[01] The present invention relates to a method of increasing or decreasing the auxetic response strain-threshold in an auxetic liquid crystal elastomer, in particular the present invention relates to a method of increasing or decreasing an auxetic response strain-threshold in an auxetic liquid crystal elastomer comprising increasing or decreasing the crosslinking density and/or the glass transition temperature (Tg) of a liquid crystal elastomer.

Background

[02] An auxetic material has a negative Poisson’s ratio, where the Poisson’s ratio is described as the negative ratio of the proportional decrease in a lateral measurement to the proportional increase in length in a sample of material that is elastically stretched or compressed. On stretching, auxetic materials become thicker in one or both of the directions perpendicular to the applied deformation.

[03] Auxetic materials are of particular interest because of their desirable and enhanced mechanical, and potentially meta-acoustic properties. Some auxetic materials exist in nature and other first synthetic auxetic materials, are carefully engineered structures that exhibit their auxetic behaviour because of their bulk structures such as re-entrant honeycomb or chiralbased structures. Such auxetic materials have been used in applications including sportswear due to their improved shock absorbance and shear-resistance performance.

[04] An aligned nematic liquid crystal elastomer having auxetic properties has recently been developed, as described in WO2019077361 A1 . The auxetic liquid crystal elastomers have significant advantage over previous synthetic auxetic structures including: transparency; no effective lower size limit on materials/devices; improved strength (no porosity); accessibility of a wide variety of manufacturing methods. The auxetic liquid crystal elastomers can also be chemically tuned, offering significant advantages over existing technology. Molecular liquid crystal elastomers are soft materials, with better potential for matching biological tissue and hence likely to give improved performance in biomedical applications.

[05] A factor that limits the potential applications of molecular auxetic liquid crystal elastomer materials is that the negative Poisson’s ratio is only observed at a specific threshold of deformation, hereinafter referred to as the auxetic response strain-threshold.

[06] Different applications for molecular auxetic materials will require the materials to have specific physical properties. It is therefore desirable to be able to fine tune the properties of molecular auxetic materials. Summary

[07] The applicant has surprisingly developed a method of increasing or decreasing the auxetic response strain-threshold in an aligned nematic liquid crystal elastomer. Being able to increase or decrease the auxetic response strain-threshold in such a manner may expand the application potential of auxetic liquid crystal elastomers. For example, it may be possible to finetune the properties of a liquid crystal elastomer according to the desired end use. In particular, it may be advantageous to reduce the auxetic response strain-threshold of a liquid crystal elastomer. For example, it may be advantageous to reduce the auxetic response strainthreshold so that the auxetic regime may be accessed at lower values of strain.

[08] According to a first aspect of the present invention there is provided a method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer comprising chemically modifying the aligned nematic liquid crystal elastomer.

[09] According to a second aspect of the present invention there is provided the use of a chemically modified aligned nematic liquid crystal elastomer in a method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer.

Detailed Description

[10] The method of the present invention comprises chemically modifying an aligned nematic liquid crystal elastomer.

[11] The auxetic response strain-threshold may be decreased (using one or more of the methods described herein or otherwise). The auxetic response strain-threshold may be increased (using one or more of the methods described herein or otherwise).

[12] It has advantageously been found by the present inventors that reducing the auxetic response strain-threshold means that the aligned nematic liquid crystal elastomers may be used in different applications because, for example, an auxetic response may be observed at a lower auxetic response strain-threshold. Applying strain/stress to an aligned nematic liquid crystal elastomer to reach the auxetic response strain-threshold can be very time consuming. Reducing the auxetic response strain-threshold may mean that less strain/stress has to be applied to the material in order to observe a response and/or that the stress/strain has to be applied over a shorter period of time. In the latter case, this may significantly reduce the time needed for the aligned nematic liquid crystal elastomer to enter the auxetic regime.

[13] It has also been advantageously found that being able to “fine-tune” the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer, i.e., by being able to increase or decrease the auxetic response strain-threshold thereof, means that the properties of the material may be adapted to its desired application (whereby a lower or higher auxetic response strain-threshold may be desirable). For example, and by way of example only, in applications where a damaged joint is being supported by the material, it could be advantageous to allow the joint some movement before the auxetic response resists the deformation.

[14] The chemical modification may be intended to alter a specific physical property of the aligned liquid crystal elastomer. For example, the chemical modification may be intended to increase or decrease the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer and/or to alter the crosslinking density of the aligned nematic liquid crystal elastomer. It has surprisingly been found by the present inventors that modifying the glass transition temperature (Tg) and/or the crosslinking density of an aligned nematic liquid crystal elastomer effects the auxetic response strain-threshold of the aligned nematic liquid crystal elastomer. Without wishing to be bound by theory, it is believed that increasing the conformational freedom of components of the aligned liquid crystal elastomer results in a reduction in the auxetic response strain-threshold and vice versa, i.e., decreasing the conformational freedom of the components of the aligned liquid crystal elastomer results in an increase in the auxetic response strain-threshold.

[15] Preferably, the method may comprise modifying the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer.

[16] For the avoidance of doubt, any reference herein to modification of an aligned nematic liquid crystal elastomer (by a method as described herein or otherwise) is by reference to an unmodified, or “starting”, aligned nematic liquid crystal elastomer, wherein the unmodified aligned nematic liquid crystal elastomer may be chemically modified in any way that effects the auxetic response strain-threshold. It will be appreciated that the modification may comprise one or more than one chemical modification(s).

[17] The aligned nematic liquid crystal elastomer may be modified by any suitable method. In certain embodiments, an aligned nematic liquid crystal elastomer may be modified by increasing or decreasing the crosslinking density of the aligned nematic liquid crystal elastomer. For example, an aligned nematic liquid crystal elastomer may be modified by increasing the crosslinking density of the aligned nematic liquid crystal elastomer. For example, an aligned nematic liquid crystal elastomer may be modified by decreasing the crosslinking density of the aligned nematic liquid crystal elastomer.

[18] The aligned nematic liquid crystal elastomer may be modified by increasing or decreasing the Tg of the aligned nematic liquid crystal elastomer. For example, an aligned nematic liquid crystal elastomer may be modified by increasing the Tg of the aligned nematic liquid crystal elastomer. For example, an aligned nematic liquid crystal elastomer may be modified by decreasing the Tg of the aligned nematic liquid crystal elastomer.

[19] It has advantageously been found by the present inventors that decreasing the crosslinking density and/or the glass transition temperature (Tg) of a liquid crystal elastomer reduces the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer. Conversely, it has been found that increasing the crosslinking density and/or the glass transition temperature (Tg) of a liquid crystal elastomer increases the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer. As such, it has advantageously been found that the properties, and in particular the auxetic response strain-threshold, of an aligned nematic elastomer can be fine-tuned according to the desired end use.

[20] The method may comprise increasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer. In such embodiments, the method may comprise increasing the crosslinking density and/or the glass transition temperature (Tg) of an aligned nematic liquid crystal elastomer. As such, the method may comprise increasing the auxetic response strainthreshold of an aligned nematic liquid crystal elastomer by increasing the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer.

[21] The method may comprise decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer. In such embodiments, the method may comprise decreasing the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer. As such, the method may comprise decreasing the auxetic response strainthreshold of an aligned nematic liquid crystal elastomer by decreasing the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer.

[22] The aligned nematic liquid crystal elastomer may comprise any suitable material. Examples of suitable materials are described in WO2019077361 A1 , the full contents of which is incorporated herein by reference.

[23] The aligned nematic liquid crystal elastomer suitably has a mechanical Freedericksz transition (MFT). A mechanical Freedericksz transition is defined as a deformation mode of an aligned elastomer wherein the director within the plane of the elastomer film appears to rotate sharply at a critical strain to reorient towards the direction parallel to the stress axis at a critical extension. Materials displaying this property were first described by Mitchell et al (Mitchell, G. R., Davis, F. J. and Guo, W., Phys. Rev. Lett., 1993, 71 (18), 2947) and Roberts et al (Roberts, P. M. S., Mitchell, G. R and Davis, F. J., J. Phys, II France, 1997, 7, 1337 and Roberts, P. M. S., Mitchell, G. R, Davis, F. J. and Pople, J. A., Mol. Cryst. Liq. Cryst., 1997, 299, 181). An MFT is often described in analogy to the well-known electric (or magnetic) field Freedericksz transitions (EFT) that occur in low molar mass nematic display devices. In the EFT, the director reorients sharply beyond a well-defined critical field (or voltage), becoming increasingly aligned with respect to the electric field as the amplitude of the field is increased. The EFT threshold is discontinuous in theory but is known to be softened if an ideal LC monodomain with alignment exactly parallel or perpendicular to the substrates is not achieved in practice. The threshold is nonetheless sharp and well-defined. The apparent sharp rotation of the director seen in an MFT is different from the director rotation response for an aligned elastomer which deforms via semi-soft elasticity (SSE), the alternative deformation mode. In the case of semi-soft elasticity, the director rotates comparatively gradually, and in the plane of the deformation, over a plateaulike region of the tensile load curve.

[24] The MFT for a given liquid crystal elastomer can be observed/measured by preparing a film of the material wherein the director lies in the plane of the film, applying stress in a direction perpendicular or close to perpendicular to the director, and tracking the orientation of the director, for example using polarising microscopy.

[25] The aligned nematic liquid crystal elastomer preferably comprises a monodomain liquid crystal elastomer. More preferably the aligned nematic elastomer is a monodomain liquid crystal elastomer.

[26] By “monodomain” herein is meant that the director orientation of the elastomer is macroscopically aligned in the sample. Monodomain alignment over the sample can be determined, for example, by polarising microscopy where it is characterised by uniform birefringence when the macroscopic sample is viewed between crossed polarisers.

[27] Preferably the aligned nematic elastomer comprises a monodomain liquid crystal elastomer comprising; a polymeric component; a liquid crystal mesogen component; and a crosslinker, wherein the liquid crystal mesogen component is physically linked to the polymeric component.

[28] Preferably the liquid crystal mesogen component is physically linked to the polymeric component via a flexible spacer.

[29] Preferably the flexible spacer comprises a C2-C10 alkylene group, preferably a linear C2- Cw alkylene group, more preferably a linear C3-C7 alkylene group, most preferably a linear Ce alkylene group. For example, the flexible spacer may comprise an ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene or decylene group.

[30] The liquid crystal mesogen component of the liquid crystal elastomer may comprise any suitable nematic mesogen.

[31] Preferably the liquid crystal mesogen component comprises a liquid crystal core component selected from the group consisting of aromatic rings, aliphatic rings, poly aromatic rings, poly aliphatic rings, phenyls, biphenyls, benzenes, and combinations thereof.

[32] Preferably the liquid crystal core component comprises a plurality of aromatic and/or aliphatic rings.

[33] Preferably the liquid crystal core component is selected from one or more of the following systems:

[34] wherein R and R’ are each independently selected from the group consisting of alkyl, alkoxy, halide, -NO2 or -CN and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core to the polymeric component; and X and Y are each independently selected from the group consisting of - CH=CH-, -C=C- -CH=N-, -N=N-, or -C(O)O-, preferably R and R’ are each independently selected from alkyl and -CN, preferably X and/or Y is -C(O)O-..

[35] In preferred embodiments the liquid crystal core component is selected from one or more of the following systems: wherein R and R’ are each independently selected from the group consisting of alkyl, alkoxy, halide, -NO2 or -CN and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core to the polymeric component; and X and Y are each independently selected from the group consisting of -CH=CH-, -CEC-, - CH=N-, -N=N-, or -C(O)O-, preferably R and R’ are each independently selected from alkyl and -CN, preferably X and/or Y is -C(O)O-

[36] In certain embodiments the liquid crystal mesogen component is present as part of the side chain of the polymeric component, i.e., the liquid crystal mesogen component is a pendant group extending from the backbone of the polymeric component. [37] In certain embodiments the liquid crystal mesogen component is present as part of the backbone of the polymeric component.

[38] The liquid crystal mesogen component may form part of both the side chain and backbone of the polymeric component.

[39] The crosslinker preferably comprises a bifunctional monomer having the same functionality as the polymeric component.

[40] Preferably the crosslinker also comprises a mesogenic component. Preferably the mesogenic component comprises a liquid crystal core component selected from one or more of the following systems:

[41] wherein R and R’ are each independently selected from the group consisting of alkyl, alkoxy, halide, -NO2 or CN and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core to the polymeric component; and X and Y are each independently selected from -CH=CH-, -CEC-, -CH=N-, - N=N-, or — C(O)O— , preferably R and R’ are each independently selected from alkyl and -CN, preferably X and/or Y is -C(O)O-..

[42] In preferred embodiments the liquid crystal core component is selected from one or more of the following systems: wherein R and R’ are each independently selected from the group consisting of alkyl, alkoxy, halide, -NO2 or -CN and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core to the polymeric component; and X and Y are each independently selected from the group consisting of -CH=CH-, -CEC-, - CH=N-, -N=N-, or -C(O)O-, preferably R and R’ are each independently selected from alkyl and -CN, preferably X and/or Y is -C(O)O-

[43] The polymeric component may be any suitable polymeric component. Preferably the polymeric component comprises an (alk)acrylate polymer, such as a (meth)acrylate polymer, such as an acrylate polymer or a methacrylate polymer, a vinyl polymer, a siloxane polymer, a thiol-based polymer, an amine based polymer or an epoxide based polymer or a combination thereof. The polymeric component may be an (alk)acrylate polymer (or poly(alk)acrylate). Preferably the polymeric component comprises a polyacrylate. It will be appreciated by a person skilled in the art that poly(alk)acrylates are typically formed from one or more (alk)acrylate monomer(s) (such as those (alk)acrylate monomers as defined hereinbelow). The terms “(alk)acrylate”, "(meth)acrylate", and like terms as used herein, are used conventionally and herein to refer to both alkacrylate and acrylate, such as methacrylate and acrylate.

[44] In certain embodiments the polymeric component is formed from both mesogenic and non-mesogenic components.

[45] Preferably the mesogenic components are formed from mesogenic monomers which comprise a monomer unit linked to a liquid crystal core component.

[46] In preferred embodiments of the invention, the polymeric component comprises a polyacrylate, the liquid crystal core component is a 4-cyano-biphenyl-4’yloxy component and the crosslinker comprises a bis-oxybenzoyloxy-2-methylbenzene comprising component.

[47] The elastomer is preferably formed by polymerising a reaction mixture comprising a mesogenic monomer, a crosslinker and an initiator. In other words, the elastomer may be the reaction product of a reaction mixture comprising a mesogenic monomer, a crosslinker and an initiator. The reaction mixture may further comprise a non-reactive mesogenic component to broaden the nematic phase range prior to polymerisation. In preferred embodiments the crosslinker also comprises a mesogenic component. [48] It will be appreciated that the amounts of each component present in the reaction mixture prior to polymerisation may suitably depend upon the desired end use. For example, as described hereinbelow, the amount of crosslinker may be varied in order to increase or decrease the crosslinking density of the aligned nematic liquid crystal elastomer. For example, as described hereinbelow, the amount of non-mesogenic monomer having one or more functional group(s) capable of crosslinking may be varied in order to increase or decrease the crosslinking density of the aligned nematic liquid crystal elastomer.

[49] Preferably the mesogenic monomer comprises from 5 to 50% by mol. of the reaction mixture prior to polymerisation, more preferably from 10 to 30% by mol., most preferably 15% by mol. In the final elastomer, the proportion of the material derived from the mesogenic monomer is preferably from 20 to 70% by mol., most preferably from 30 to 60% by mol. For the avoidance of doubt, the terms “mol%” and “% by mol.” are used interchangeably herein.

[50] Preferably the crosslinker comprises from 1 to 20% by mol. of the reaction mixture prior to polymerisation, such as from 3 to 20% by mol, such as from 3 to 10% by mol, such as from 3 to 8% by mol. Preferably the crosslinker comprises from 1 to 20% by mol. of the reaction mixture prior to polymerisation.

[51] In embodiments where the aligned nematic liquid crystal elastomer is modified so as to increase the crosslinking density thereof, the crosslinker may comprise from 5 to 20% by mol., such as from 5 to 15% by mol., such as from 5 to 10% by mol., such as from 5 to 8% by mol., or even 8% by mol. of the reaction mixture prior to polymerisation. In embodiments where the aligned nematic liquid crystal elastomer is modified so as to decrease the crosslinking density thereof, the crosslinker may comprise from 1 to 5% by mol., such as from 2 to 5% by mol., such as from 3 to 5% by mol., such as from 4 to 5% by mol., or even 4 to 4.5% by mol. of the reaction mixture prior to polymerisation. In the final elastomer, the proportion of the material derived from the crosslinker may be from 2 to 20% by mol., such as from 3 to 17% by mol, such as from 3 to 15% by mol., such as from 3 to 10% by mol. In embodiments where the aligned nematic liquid crystal elastomer is modified so as to increase the crosslinking density thereof, the proportion of material derived from the crosslinker may be from 2 to 20% by mol., such as from 2 to 15% by mol., such as from 2 to 10% by mol., such as from 2 to 8% by mol., such as from 3 to 5% by mol., such as from 3 to 4% by mol., or even 3.5% by mol. In embodiments where the aligned nematic liquid crystal elastomer is modified so as to decrease the crosslinking density thereof, the proportion of material derived from the crosslinker may be from 0.5 to 5% by mol., such as from 1 to 3% by mol., such as from 1 to 2.5% by mol., such as from 1 .5 to 2% by mol., or even 1 .8% by mol.

[52] The initiator chosen will suitably be dependent on the polymer used and may be any suitable initiator. However, when the polymer is a polyacrylate the initiator is preferably a photoinitiator. Possible photoinitiators are well known to those skilled in the art and include benzoin ethers, benzyl ketals, alpha-dialkoxyacetophenones, alpha-hydroxyalkylphenones, acylphosphine oxides, benzophenones and thioxanthones. Preferably the photoinitiator is methyl benzoylformate. Preferably the initiator is present in an amount of 1 to 2% by mol., such as 1 .5% by mol. based on the total moles of the reaction mixture.

[53] When a non-mesogenic monomer is present in the reaction mixture, the non-mesogenic mixture preferably comprises from 10 to 40% by mol. of the reaction mixture, more preferably from 15 to 30% by mol., most preferably from 15 to 20% by mol. In the final elastomer, the proportion of the material derived from the non-mesogenic monomer, if present, is preferably from 20 to 60% by mol., most preferably from 35 to 50% by mol.

[54] When a non-reactive mesogen is present in the reaction mixture, the non-reactive mesogen preferably comprises from 10 to 70% by mol. of the reaction mixture, more preferably from 20 to 60% by mol., or from 30 to 60% by mol., most preferably 55% by mol. In preferred embodiments the non-reactive mesogen is 4-cyano-4’-hexyloxybiphenyl.

[55] In certain embodiments, the mesogenic monomer is 6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate, 4-methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy)phenyl ester or 4-{6- (acryloyloxy)hexyloxy}phenyl 4-(trans-4-propylcyclohexyl)benzoate, the crosslinker is 1 ,4-bis-[4- (6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene or 1 ,4-bis-[4-(3- acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene, the non-mesogenic monomer is 2- ethylhexyl acrylate and, if present, the non-reactive mesogen is 4-cyano-4’-hexyloxybiphenyl.

[56] The choice of mesogenic monomer may modify the Tg of the resulting aligned nematic liquid crystal elastomer.

[57] In certain embodiments, the mesogenic monomer is 6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate, the crosslinker is 1 ,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene, the non-mesogenic monomer is 2-ethylhexyl acrylate and, if present, the non-reactive mesogen is 4-cyano-4’-hexyloxybiphenyl; or the mesogenic monomers are 4-methoxybenzoic acid 4-(6- acryloyloxy-hexyloxy)phenyl ester and 4-{6-(acryloyloxy)hexyloxy}phenyl 4-(trans-4- propylcyclohexyl)benzoate, the crosslinker is 1 ,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2- methylbenzene the non-mesogenic monomer is 2-ethylhexyl acrylate and, if present, the non- reactive mesogen is 4-cyano-4’-hexyloxybipheny; or the mesogenic monomers are 4- methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy)phenyl ester and 4-{6- (acryloyloxy)hexyloxy}phenyl 4-(trans-4-propylcyclohexyl)benzoate, the crosslinker is ,4-bis-[4- (3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene, the non-mesogenic monomer is 2- ethylhexyl acrylate and, if present, the non-reactive mesogen is 4-cyano-4’-hexyloxybiphenyl.

[58] In certain embodiments, the mesogenic monomer is 6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate, the crosslinker component is 1 ,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2- methylbenzene, the non-mesogenic monomer is 2-ethylhexyl acrylate and, if present, the non- reactive mesogen is 4-cyano-4’-hexyloxybiphenyl. [59] When the crosslinker comprises a mesogenic component and therefore may also be considered a mesogenic monomer, the ratio of mesogenic monomers to non-mesogenic monomers in the final elastomer is preferably between 2:1 and 1 :1.

[60] The Tg of the aligned nematic liquid crystal elastomer may be any suitable Tg. It will be appreciated that the Tg of the aligned nematic liquid crystal elastomer may suitably depend upon the desired end use. For example, as described hereinbelow, the Tg may be modified (by one or more of the methods described herein or otherwise) in order to increase or decrease the Tg of the aligned nematic liquid crystal elastomer.

[61] The Tg of the aligned nematic liquid crystal elastomer may be from -150 to 50°C, such as from -100 to 50°C, such as from -80 to 50°C, such as from -60 to 50°C, such as from -40 to 50°C, such as from -20 to 50°C, such as from -10 to 40°C, such as from -5 to 30°C, such as from 0 to 25°C, such as from 1 to 23°C, or even from 2 to 20°C.

[62] Suitable methods to measure the Tg will be known to a person skilled in the art. As reported herein, the Tg was measured according to ASTM E1356-08(2014) (“Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry”. Heat-flux differential scanning calorimetry (DSC), sample pans: aluminium, reference: blank, calibration: indium and adamantane (solid-solid transition), heating rate: 10°C/min). The Tg is reported as the midpoint of the transition. All values for Tg reported herein were measured in this way unless specified otherwise.

[63] As described herein, the method may comprise modifying the crosslinking density of the aligned nematic liquid crystal elastomer.

[64] The crosslinking density may be modified by any suitable method. Examples of suitable methods to modify the crosslinking density of an aligned nematic liquid crystal elastomer will be known to a person skilled in the art. For example, the crosslinking density may be modified by varying the amount of crosslinker, by varying the type of crosslinker used and/or by varying the amount of monomer(s) having one or more functional group(s) capable of crosslinking, such as acrylate monomer(s) having one or more functional group(s) capable of crosslinking.

[65] The crosslinking density may be modified by varying the amount of crosslinker (i.e., in the modified compared to unmodified aligned nematic liquid crystal elastomer). In certain embodiments, the crosslinking density may be decreased by decreasing the amount of crosslinker used, i.e., by decreasing the mol% of crosslinker present in the reaction mixture prior to polymerisation (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In certain embodiments, the crosslinking density may be increased by increasing the amount of crosslinker used, i.e., by increasing the mol% of crosslinker present in the reaction mixture prior to polymerisation (in the modified compared to unmodified aligned nematic liquid crystal elastomer). [66] When the crosslinking density of the aligned nematic liquid crystal elastomer is decreased by decreasing the amount of crosslinker used, the mol% of crosslinker present in the reaction mixture prior to polymerisation may be reduced by any suitable amount. For example, the mol% of crosslinker present in the reaction mixture prior to polymerisation may at least 1 mol%, such as by at least 1 .5 mol%, such as by at least 2 mol%, such as by at least 2.5 mol%, such as by at least 3 mol%, such as by at least 3.5 mol%, such as by at least 4 mol%, such as by at least 4.5 mol%, such as by at least 5 mol%, such as by at least 5.5 mol%, such as by at least 6 mol%, such as by at least 7 mol%, such as by at least 7.5 mol%, such as by at least 8 mol%, such as at least 8.5 mol%, such as at least 9 mol%, such as at least 9.5 mol%, or even at least 10 mol% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer.

[67] When the crosslinking density of the aligned nematic liquid crystal elastomer is decreased by decreasing the amount of crosslinker used, the amount of crosslinker present in the reaction mixture prior to polymerisation may be at least 10%, such as at least 15%, such as at least 20%, such as at least 25%, such as at least 30%, such as at least 35%, such as at least 40%, such as at least 45%, such as at least 50% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[68] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be at least 10% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[69] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be at least 20% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[70] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 30% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[71] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 40% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[72] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 50% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[73] When the crosslinking density of the aligned nematic liquid crystal elastomer is increased by increasing the amount of crosslinker used, the mol% of crosslinker present in the reaction mixture prior to polymerisation may be increased by any suitable amount. For example, the mol% of crosslinker present in the reaction mixture prior to polymerisation may at least 1 mol%, such as by at least 1 .5 mol%, such as by at least 2 mol%, such as by at least 2.5 mol%, such as by at least 3 mol%, such as by at least 3.5 mol%, such as by at least 4 mol%, such as by at least 4.5 mol%, such as by at least 5 mol%, such as by at least 5.5 mol%, such as by at least 6 mol%, such as by at least 7 mol%, such as by at least 7.5 mol%, such as by at least 8 mol%, such as at least 8.5 mol%, such as at least 9 mol%, such as at least 9.5 mol%, or even at least 10 mol% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer.

[74] When the crosslinking density of the aligned nematic liquid crystal elastomer is increased by increasing the amount of crosslinker used, the amount of crosslinker present in the reaction mixture prior to polymerisation may be at least 10%, such as at least 15%, such as at least 20%, such as at least 25%, such as at least 30%, such as at least 35%, such as at least 40%, such as at least 45%, such as at least 50% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[75] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 10% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[76] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 20% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[77] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 30% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[78] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 40% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[79] In certain embodiments, the amount of crosslinker present in the reaction mixture prior to polymerisation may be least 50% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer (based on the number of moles, or mol%, of crosslinker present in the reaction mixture prior to polymerisation).

[80] The crosslinking density may be modified by varying the amount of monomer(s) having one or more functional group(s) capable of crosslinking (i.e., in the modified compared to unmodified aligned nematic liquid crystal elastomer). In certain embodiments, the crosslinking density may be decreased by decreasing the amount of monomer(s) having one or more functional group(s) capable of crosslinking present in the reaction mixture prior to polymerisation (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In certain embodiments, the crosslinking density may be increased by increasing the amount of monomer(s) having one or more functional group(s) capable of crosslinking present in the reaction mixture prior to polymerisation (in the modified compared to unmodified aligned nematic liquid crystal elastomer).

[81] Monomers having one or more functional group(s) capable of crosslinking will be known to a person skilled in the art. Preferably, the monomers having one or more functional group(s) capable of crosslinking are mesogenic, more preferably mesogenic acrylate monomers. Suitable examples of monomers having one or more functional group(s) capable of crosslinking include, but are not limited to, those having one or more acid, hydroxyl oxirane and/or unsaturated functional group(s).

[82] The crosslinking density may be modified by varying the type of crosslinker used. For example, a different crosslinker or combination of crosslinkers may be used in the modified compared to unmodified aligned nematic liquid crystal elastomer. For example, in embodiments where a bis-(acryloyloxyalkyloxy)benzoyloxy-2methylbenzene crosslinker is used, the crosslinking density may be modified by varying the length of the alkyl chain. In such embodiments, the crosslinking density of the aligned nematic liquid crystal elastomer may be decreased by increasing the length of the alkyl chain in the bis-(acryloyloxyalkyloxy)benzoyloxy- 2-methylbenzene crosslinker (in the unmodified compared to modified aligned nematic liquid crystal elastomer). In such embodiments, the crosslinking density of the aligned nematic liquid crystal elastomer may be increased by decreasing the length of the alkyl chain in the bis- (acryloyloxyalkyloxy)benzoyloxy-2-methylbenzene crosslinker (in the unmodified compared to modified aligned nematic liquid crystal elastomer). For example, and by way of example only, when a bis-(acryloyloxyalkyloxy)benzoyloxy-2-methylbenzene crosslinker is used, the crosslinking density of the aligned nematic liquid crystal elastomer may be decreased by using a bis-(acryloyloxynonyloxy)benzoyloxy-2methylbenzene crosslinker in the modified aligned nematic liquid crystal elastomer compared to using bis-(acryloyloxyhexyloxy)benzoyloxy- 2methylbenzene crosslinker in the unmodified aligned nematic liquid crystal elastomer. It will be appreciated by a person skilled in the art that the reverse is true when looking to increase the crosslinking density of the aligned nematic liquid crystal elastomer, i.e., when a bis- (acryloyloxyalkyloxy)benzoyloxy-2methylbenzene crosslinker is used, the crosslinking density of the aligned nematic liquid crystal elastomer may be increased by using a bis- (acryloyloxyhexyloxy)benzoyloxy-2methylbenzene crosslinker in the modified aligned nematic liquid crystal elastomer compared to using bis-(acryloyloxynonyloxy)benzoyloxy- 2methylbenzene crosslinker in the unmodified aligned nematic liquid crystal elastomer, for example.

[83] Preferably, the crosslinking density may be decreased. More preferably, the crosslinking density may be decreased by decreasing the amount of crosslinker used, i.e., by decreasing the mol% of crosslinker present in the reaction mixture prior to polymerisation (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the auxetic response strain-threshold is suitably decreased (in the modified compared to unmodified aligned nematic liquid crystal elastomer).

[84] As described herein, the method may comprise modifying the Tg of the aligned nematic liquid crystal elastomer.

[85] The Tg may be modified by any suitable method. Examples of suitable methods to modify the Tg of an aligned nematic liquid crystal elastomer will be known to a person skilled in the art. For example, the Tg of the aligned nematic liquid crystal elastomer may be modified by including, excluding and/or varying, such as increasing or decreasing, the amount of one or more Tg modifying components present in the aligned nematic liquid crystal elastomer and/or by controlling the polymerisation process. [86] Examples of suitable Tg modifying components include, but are not limited to, plasticisers, non-mesogenic monomers, mesogenic monomers, the flexible spacers, solvents and/or combinations thereof.

[87] The Tg modifying component may comprise a plasticiser. Suitable plasticisers will be known to a person skilled in the art. Typically, plasticisers are substances that are added to a material to render the material softer and more flexible. As such, but without wishing to be bound by theory, it is believed that the inclusion of higher levels of one or more plasticiser makes the aligned nematic liquid crystal elastomer more flexible, for example by increasing the conformational freedom of the material, thus reducing the Tg and auxetic response strainthreshold. Examples of suitable plasticisers include, but are not limited to: poly(ethylhexyl acrylate), such as poly(2-ethylhexyl acrylate), such as poly(ethylhexyl acrylate) and/or poly(2- ethylhexyl acrylate) comprising at least 2, 3, 4, 5, 6, 7, 8, 9 or 10 (2-)ethylhexyl acrylate units; esters, such as adipates, azelates, citrates, benzoates, phthalates, including ortho-phthalates and terephthalates, sebacates and trimellitates; and combinations thereof.

[88] In certain embodiments, the Tg of the aligned nematic liquid crystal elastomer may be decreased by including or increasing the amount of one or more plasticise r(s) present in the aligned nematic liquid crystal elastomer (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the unmodified aligned nematic liquid crystal elastomer may comprise a lower amount of one or more plasticiser(s) compared to the modified aligned nematic liquid crystal elastomer. Preferably, when one or more plasticiser(s) are used to decrease the Tg of the aligned nematic liquid crystal elastomer, the unmodified aligned nematic liquid crystal elastomer may comprise substantially no plasticiser and the modified aligned nematic liquid crystal elastomer may comprise at least one plasticiser.

[89] In certain embodiments, the Tg of the aligned nematic liquid crystal elastomer may be increased by excluding or decreasing the amount of one or more plasticise r(s) present in the aligned nematic liquid crystal elastomer (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the unmodified aligned nematic liquid crystal elastomer may comprise a higher amount of one or more plasticise r(s) compared to the modified aligned nematic liquid crystal elastomer. Preferably, when one or more plasticiser(s) are used to increase the Tg of the aligned nematic liquid crystal elastomer, the unmodified aligned nematic liquid crystal elastomer may comprise at least one plasticiser and the modified aligned nematic liquid crystal elastomer may comprise substantially no plasticiser.

[90] When the Tg of the aligned nematic liquid crystal elastomer is decreased, the Tg may be reduced by any suitable amount. For example, the Tg of the modified aligned nematic liquid crystal elastomer may be at least 1 °C such as by at least 2°C, such as by at least 3°C, such as at least 4°C, such as at least 5°C, such as at least 6°C, such as at least 7°C, such as at least 8°C, such as at least 9°C, such as at least 10°C, such as at least 11 °C, such as at least 12°C, such as at least 13°C, such as at least 14°C, such as at least 15°C, such as at least 16°C, such as at least 17°C, such as at least 18°C, such as at least 19°C, or even at least 20°C lower than the Tg of the unmodified aligned nematic liquid crystal elastomer.

[91] When the Tg of the aligned nematic liquid crystal elastomer is increased, the Tg may be increased by any suitable amount. For example, the Tg of the modified aligned nematic liquid crystal elastomer may be at least 1 °C such as by at least 2°C, such as by at least 3°C, such as at least 4°C, such as at least 5°C, such as at least 6°C, such as at least 7°C, such as at least 8°C, such as at least 9°C, such as at least 10°C, such as at least 11 °C, such as at least 12°C, such as at least 13°C, such as at least 14°C, such as at least 15°C, such as at least 16°C, such as at least 17°C, such as at least 18°C, such as at least 19°C, or even at least 20°C higher than the Tg of the unmodified aligned nematic liquid crystal elastomer.

[92] The aligned nematic liquid crystal elastomer may comprise any suitable amount of plasticiser. For example, in the final elastomer the proportion of the material derived from the plasticiser may be from 1 to 50% by mass, such as from 1 to 40% by mass, such as from 1 to 30% by mass, such as from 1 to 20% by mass, such as from 1 to 10% by mass. It will be appreciated by a person skilled in the art that the amount of plasticiser present will suitably depend on the desired Tg. For example, if it is desired to reduce the Tg of an aligned nematic liquid crystal elastomer, the amount of plasticiser present in the modified aligned nematic liquid crystal elastomer will typically be higher than the amount of plasticiser present in the unmodified aligned nematic liquid crystal elastomer (and vice versa if it is desired to increase the Tg of an aligned nematic liquid crystal elastomer).

[93] The one or more plasticiser(s) may be added to the aligned nematic liquid crystal elastomer by any suitable method and at any suitable time. For example, the plasticiser may be added before, during and/or after the aligned liquid crystal elastomer is formed. For example, the plasticiser may be added to the reaction mixture used to form the aligned nematic liquid crystal elastomer or may be used to modify the aligned nematic liquid crystal elastomer once formed.

[94] The one or more plasticiser(s) may be added to the aligned nematic liquid crystal elastomer before, during or after polymerisation of the reaction mixture used to form the aligned nematic liquid crystal elastomer. In certain embodiments, the one or more plasticiser(s) may be added to the reaction mixture used to form the aligned nematic liquid crystal elastomer before polymerisation thereof. In certain embodiments, the one or more plasticiser(s) may be added to the reaction mixture used to form the aligned nematic liquid crystal elastomer during polymerisation thereof. When the one or more plasticiser(s) are added to the reaction mixture used to form the aligned nematic liquid crystal elastomer before or during polymerisation, the plasticiser may be co-polymerised in the polymer chain. A person skilled in the art will appreciate that only certain plasticisers may be co-polymerised into the polymer chain, for example, those having unsaturated bonds that are capable of participating in a free-radical polymerisation reaction.

[95] In certain embodiments, the one or more plasticiser(s) may be added to the aligned nematic liquid crystal elastomer after polymerisation thereof, i.e., to the aligned nematic liquid crystal elastomer that is the reaction product of the reaction mixture. When the one or more plasticise r(s) are added to the aligned nematic liquid crystal elastomer after polymerisation, the one or more plasticiser(s) may be added thereto by any suitable method. For example, the one or more plasticiser(s) may be applied directly to the aligned nematic liquid crystal elastomer either alone or dissolved, dispersed or suspended in a suitable solvent or mixture of solvents. Suitable solvents, when used, include, but are not limited to, cyclohexane, dichloromethane (DCM), hexane, pentane, heptane, xylene, benzene, petroleum ether, toluene, toluene/methanol, DCM/methanol, and/or combinations thereof.

[96] The Tg modifying component may comprise a non-mesogenic monomer. The Tg may be modified by altering the conformational freedom within the non-mesogenic monomer. Suitable non-mesogenic monomers will be known to a person skilled in the art. Examples of suitable non-mesogenic monomers include, but are not limited to, monomers having pendant C2-C12 alkyl, alkenyl or alkynyl groups, for example acrylate monomers having pendant C2-C12 alkyl, alkenyl or alkynyl groups. For the avoidance of doubt, the monomers having pendant C2-C12 alkyl, alkenyl or alkynyl groups may have one or more than one pendant C2-C12 alkyl, alkenyl or alkynyl group. The pendant C2-C12 alkyl, alkenyl or alkynyl groups may be straight chain or branched. The pendant C2-C12 alkyl, alkenyl or alkynyl groups may be substituted or unsubstituted. Typically, non-mesogenic monomers having pendant C2-C12 alkyl, alkenyl or alkynyl groups may be included or the amount present may be increased in order to reduce the Tg of the aligned nematic liquid crystal elastomer. Without wishing to be bound by theory, it is believed that including or increasing the amount of monomer in the modified aligned nematic liquid crystal elastomer, wherein the poly(monomer) of said monomer has a lower Tg than the polymeric component in the unmodified aligned nematic liquid crystal elastomer, may reduce the Tg of the material.

[97] In certain embodiments, the Tg of the aligned nematic liquid crystal elastomer may be decreased by including or increasing the amount of non-mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups present in the aligned nematic liquid crystal elastomer (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the unmodified aligned nematic liquid crystal elastomer may comprise a lower amount of non-mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups compared to the modified aligned nematic liquid crystal elastomer.

[98] The aligned nematic liquid crystal elastomer may comprise any suitable amount of non- mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups. For example, the reaction mixture may comprise from 0.5 to 60% by mol, such as from 1 to 50% by mol. non- mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups prior to polymerisation. It will be appreciated by a person skilled in the art that the amount of non- mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups present will suitably depend on the desired Tg. For example, if it is desired to reduce the Tg of an aligned nematic liquid crystal elastomer, the amount of non-mesogenic monomer having pendant C2- C12 alkyl, alkenyl or alkynyl groups present in the modified aligned nematic liquid crystal elastomer will typically be higher than the amount of non-mesogenic monomer having pendant C2-C12 alkyl, alkenyl or alkynyl groups present in the unmodified aligned nematic liquid crystal elastomer (and vice versa if it is desired to increase the Tg of an aligned nematic liquid crystal elastomer).

[99] The non-mesogenic monomer may suitably be added to reaction mixture used to form the aligned nematic liquid crystal elastomer.

[100] The Tg modifying component may comprise a mesogenic monomer. Suitable mesogenic monomers are as defined herein. The Tg may be modified by altering the conformational freedom within the mesogenic monomer. Without wishing to be bound by theory, by increasing the conformational freedom within the mesogenic monomer, the Tg may be reduced. For example, the mesogenic monomer may comprise an alkyl chain and the Tg may be modified by varying the length of the alkyl chain. For example, in embodiments where a 6-(4-cyano- biphenyl-4'-yloxy)alkyl acrylate mesogenic monomer is used, the Tg may be modified by varying the length of the alkyl chain. In such embodiments, the Tg of the aligned nematic liquid crystal elastomer may be decreased by increasing the length of the alkyl chain in the 6-(4-cyano- biphenyl-4'-yloxy)alkyl acrylate monomer (in the unmodified compared to modified aligned nematic liquid crystal elastomer). In such embodiments, the Tg of the aligned nematic liquid crystal elastomer may be increased by decreasing the length of the alkyl chain in the 6-(4- cyano-biphenyl-4'-yloxy)alkyl acrylate monomer (in the unmodified compared to modified aligned nematic liquid crystal elastomer). For example, and by way of example only, when a 6- (4-cyano-biphenyl-4'-yloxy)alkyl acrylate monomer is used, the Tg of the aligned nematic liquid crystal elastomer may be decreased by using 6-(4-cyano-biphenyl-4'-yloxy)nonyl acrylate in the modified aligned nematic liquid crystal elastomer compared to using 6-(4-cyano-biphenyl-4'- yloxy)hexyl acrylate in the unmodified aligned nematic liquid crystal elastomer. It will be appreciated by a person skilled in the art that the reverse is true when looking to increase the Tg of the aligned nematic liquid crystal elastomer, i.e., when a 6-(4-cyano-biphenyl-4'-yloxy)alkyl acrylate monomer is used, the Tg of the aligned nematic liquid crystal elastomer may be increased by using 6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate in the modified aligned nematic liquid crystal elastomer compared to using 6-(4-cyano-biphenyl-4'-yloxy)nonyl acrylate in the unmodified aligned nematic liquid crystal elastomer, for example. The Tg may be modified by utilising an alternative mesogenic monomer or by replacing a portion of the mesogenic monomer with an alternative mesogenic monomer. Suitable alternative mesogenic monomers are as defined herein.

[101] The Tg modifying component may comprise a flexible spacer. Suitable flexible spacers are as defined herein. In certain embodiments, the Tg of the aligned nematic liquid crystal elastomer may be decreased by including or increasing the amount of flexible spacer present in the aligned nematic liquid crystal elastomer (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the unmodified aligned nematic liquid crystal elastomer may comprise a lower amount of one or more flexible spacers compared to the modified aligned nematic liquid crystal elastomer. In certain embodiments, the Tg of the aligned nematic liquid crystal elastomer may be increased by excluding or decreasing the amount of one or more flexible spacers present in the aligned nematic liquid crystal elastomer (in the modified compared to unmodified aligned nematic liquid crystal elastomer). In such embodiments, the unmodified aligned nematic liquid crystal elastomer may comprise a higher amount of one or more flexible spacers compared to the modified aligned nematic liquid crystal elastomer.

[102] The Tg of the aligned nematic liquid crystal elastomer may be modified by varying the nature of the flexible spacer. For example, the Tg of the aligned nematic liquid crystal elastomer may be modified by varying the length of the flexible spacer, such as by varying the length of an alkyl chain present in the flexible spacer. In such embodiments, the Tg may suitably be reduced by increasing the length of an alkyl chain present in the flexible spacer. In such embodiments, the Tg may suitably be increased by decreasing the length of an alkyl chain present in the flexible spacer.

[103] The Tg of the aligned nematic liquid crystal elastomer may be modified by controlling the polymerisation process. Suitable methods to control the polymerisation process will be known to a person skilled in the art. Examples of suitable methods to control the polymerisation process include, but are not limited to, using techniques such as photo-induced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerisation. PETRAFT is a form of free radical polymerisation that provides spatial and temporal control over the polymerisation reaction meaning that polymer size, composition and architecture can be accurately controlled. Without wishing to be bound by theory, it is believed that spatiotemporal control over polymerisation may be improved using PET-RAFT polymerisation and, as such, that desired glass transition temperatures, i.e., an increased or decreased Tg, can be achieved. Without wishing to be bound by theory, it is also believed that desired glass transition temperatures maybe achieved using PET-RAFT by stopping/starting the polymerisation to vary the degree of cure, by changing quantities of photocatalyst and RAFT agent in order to tune molecular weight/molecular weight distribution, by specific control of the number distribution of monomers between crosslinking groups, by specific control over monomer sequence and/or by controlling the degree of branching. [104] It will be appreciated by a person skilled in the art that the Tg of the aligned nematic liquid crystal elastomer may be affected by modifying the crosslinking density of the aligned nematic liquid crystal elastomer. However, it will also be appreciated that there is not necessarily a linear relationship between crosslinking density and Tg. For example, decreasing the crosslinking density of an aligned nematic liquid crystal elastomer does not always reduce the Tg of the material. Also, the relationship between concentration of crosslinking groups may have a non-linear form.

[105] Preferably, the Tg may be decreased. More preferably, the Tg may be decreased by including or increasing the amount of plasticiser present (in the unmodified compared to modified aligned nematic liquid crystal elastomer). In such embodiments, the auxetic response strain-threshold is suitably decreased (in the modified compared to unmodified aligned nematic liquid crystal elastomer).

[106] The aligned nematic liquid crystal elastomers may be produced by any suitable method. Preferably, the aligned nematic liquid crystal elastomers may be produced by a method comprising the steps of: a) applying an aligning means to a substrate; b) applying the liquid crystal elastomer components to the substrate and allowing them to form an aligned nematic phase; and c) curing the liquid crystal elastomer components to form an aligned nematic liquid crystal elastomer.

[107] Various techniques for aligning mesogenic compositions exist. For example, techniques exist to create a monodomain during synthesis, including applying a magnetic field, mechanical brushing, mechanical shear alignment, flow, applying an electric field, applying a thermal gradient, or providing an alignment layer or layers. The monomeric solution may also be heated, cooled or exposed to other environmental factors to influence synthesis of the monomer mixture into an aligned state.

[108] Preferably the aligning means is an aligning force which is applied by m the substrate, preferably to impart a static force to the substrate, and/or mechanical shear alignment.

[109] In a second aspect, the invention extends to the use of a chemically modified aligned nematic liquid crystal elastomer in a method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer.

[110] Suitable features of the second aspect of the present invention are as defined in relation to the first aspect of the present invention.

[111] The term "alk” or “alkyl", as used herein unless otherwise defined, relates to saturated hydrocarbon radicals being straight, branched, cyclic or polycyclic moieties or combinations thereof and contain 1 to 20 carbon atoms, such as 1 to 10 carbon atoms, such as 1 to 8 carbon atoms, such as 1 to 6 carbon atoms, or even 1 to 4 carbon atoms. These radicals may be optionally substituted with a chloro, bromo, iodo, cyano, nitro, OR¹⁹, OC(O)R²⁰, C(O)R²¹, C(O)OR²², NR²³R²⁴, C(O)NR²⁵R²⁶, SR²⁷, C(O)SR²⁷, C(S)NR²⁵R²⁶, aryl or Het, wherein R¹⁹ to R²⁷ each independently represent hydrogen, aryl or alkyl, and/or be interrupted by oxygen or sulphur atoms, or by silano or dialkylsiloxane groups. Examples of such radicals may be independently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tertbutyl, 2-methylbutyl, pentyl, iso-amyl, hexyl, cyclohexyl, 3-methylpentyl, octyl and the like. The term “alkylene”, as used herein, relates to a bivalent radical alkyl group as defined above. For example, an alkyl group such as methyl which would be represented as -CH3, becomes methylene, -CH2-, when represented as an alkylene. Other alkylene groups should be understood accordingly.

[1 12] The term “alkenyl”, as used herein, relates to hydrocarbon radicals having, such as up to 4, double bonds, being straight, branched, cyclic or polycyclic moieties or combinations thereof and containing from 2 to 18 carbon atoms, such as 2 to 10 carbon atoms, such as from 2 to 8 carbon atoms, such as 2 to 6 carbon atoms, or even 2 to 4 carbon atoms. These radicals may be optionally substituted with a hydroxyl, chloro, bromo, iodo, cyano, nitro, OR¹⁹, OC(O)R²⁰, C(O)R²¹, C(O)OR²², NR²³R²⁴, C(O)NR²⁵R²⁶, SR²⁷, C(O)SR²⁷, C(S)NR²⁵R²⁶, or aryl, wherein R¹⁹ to R²⁷ each independently represent hydrogen, aryl or alkyl, and/or be interrupted by oxygen or sulphur atoms, or by silano or dialkylsiloxane groups. Examples of such radicals may be independently selected from alkenyl groups include vinyl, allyl, isopropenyl, pentenyl, hexenyl, heptenyl, cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, 1 -propenyl, 2-butenyl, 2- methyl-2-butenyl, isoprenyl, farnesyl, geranyl, geranylgeranyl and the like. The term “alkenylene”, as used herein, relates to a bivalent radical alkenyl group as defined above. For example, an alkenyl group such as ethenyl which would be represented as -CH=CH2, becomes ethenylene, -CH=CH-, when represented as an alkenylene. Other alkenylene groups should be understood accordingly.

[1 13] The term "alkynyl", as used herein, relates to hydrocarbon radicals having, such as up to 4, triple bonds, being straight, branched, cyclic or polycyclic moieties or combinations thereof and having from 2 to 18 carbon atoms, such as 2 to 10 carbon atoms, such as from 2 to 8 carbon atoms, such as from 2 to 6 carbon atoms, or even from 2 to 4 carbon atoms. These radicals may be optionally substituted with a hydroxy, chloro, bromo, iodo, cyano, nitro, OR¹⁹, OC(O)R²⁰, C(O)R²¹, C(O)OR²², NR²³R²⁴, C(O)NR²⁵R²⁶, SR²⁷, C(O)SR²⁷, C(S)NR²⁵R²⁶, or aryl, wherein R¹⁹ to R²⁷ each independently represent hydrogen, aryl or lower alkyl, and/or be interrupted by oxygen or sulphur atoms, or by silano or dialkylsiloxane groups. Examples of such radicals may be independently selected from alkynyl radicals include ethynyl, propynyl, propargyl, butynyl, pentynyl, hexynyl and the like. The term “alkynylene”, as used herein, relates to a bivalent radical alkynyl group as defined above. For example, an alkynyl group such as ethynyl which would be represented as -CECH , becomes ethynylene, -CEC-, when represented as an alkynylene. Other alkynylene groups should be understood accordingly.

[1 14] The term “aryl” as used herein, relates to an organic radical derived from an aromatic hydrocarbon by removal of one hydrogen, and includes any monocyclic, bicyclic or polycyclic carbon ring of up to 7 members in each ring, wherein at least one ring is aromatic. These radicals may be optionally substituted with a hydroxy, chloro, bromo, iodo, cyano, nitro, OR¹⁹, OC(O)R²⁰, C(O)R²¹, C(O)OR²², NR²³R²⁴, C(O)NR²⁵R²⁶, SR²⁷, C(O)SR²⁷, C(S)NR²⁵R²⁶, or aryl, wherein R¹⁹ to R²⁷ each independently represent hydrogen, aryl or lower alkyl, and/or be interrupted by oxygen or sulphur atoms, or by silano or dialkylsilicon groups. Examples of such radicals may be independently selected from phenyl, p-tolyl, 4-methoxyphenyl, 4-(tert- butoxy)phenyl, 3-methyl-4-methoxyphenyl, 4-fluorophenyl, 4-chlorophenyl, 3-nitrophenyl, 3- aminophenyl, 3-acetamidophenyl, 4-acetamidophenyl, 2-methyl-3-acetamidophenyl, 2-methyl-3- aminophenyl, 3-methyl-4-aminophenyl, 2-amino-3-methylphenyl, 2,4-dimethyl-3-aminophenyl, 4-hydroxyphenyl, 3-methyl-4-hydroxyphenyl, 1 -naphthyl, 2-naphthyl, 3-amino-1 -naphthyl, 2- methyl-3-amino-1 -naphthyl, 6-amino-2-naphthyl, 4,6-dimethoxy-2-naphthyl, tetrahydronaphthyl, indanyl, biphenyl, phenanthryl, anthryl or acenaphthyl and the like. The term “arylene”, as used herein, relates to a bivalent radical aryl group as defined above. For example, an aryl group such as phenyl which would be represented as -Ph, becomes phenylene, -Ph-, when represented as an arylene. Other arylene groups should be understood accordingly.

[1 15] For the avoidance of doubt, the reference to alkyl, alkenyl, alkynyl, aryl or aralkyl in composite groups herein should be interpreted accordingly, for example the reference to alkyl in aminoalkyl or alk in alkoxyl should be interpreted as alk or alkyl above etc.

[1 16] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. Also, the recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g., 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all subranges subsumed therein.

[1 17] Singular encompasses plural and vice versa. For example, although reference is made herein to “a” crosslinker, “an” acrylate polymer, “an” elastomer, and the like, one or more of each of these and any other components can be used. As used herein, the term "polymer" refers to oligomers and both homopolymers and copolymers, and the prefix "poly" refers to two or more. [118] The terms "comprising", "comprises" and "comprised of’ as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. Additionally, although the present invention has been described in terms of “comprising”, the coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’.

[119] As used herein, the term "and/or," when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a list is described as comprising group A, B, and/or C, the list can comprise A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination.

[120] All of the features contained herein may be combined with any of the above aspects in any combination.

BRIEF DESCRIPTION OF DRAWINGS

[121] Embodiments of the invention will now be described with reference to the following examples and by reference to the drawings in which:-

[122] Figure 1A is a graph of z-strain versus x-strain (10 min between 5% strain steps at 23°C) for each of examples 1 A and 1 B.

[123] Figure 1 B is a graph of the instantaneous Poisson’s ratio vs x-strain calculated from polynomial fits of data in Figure 1A for each of examples 1A and 1 B.

[124] Figure 2A is a graph of z-strain versus x-strain (10 min between 5% strain steps at 23°C) for each of examples 2A and 2B.

[125] Figure 2B is a graph of the instantaneous Poisson’s ratio vs x-strain calculated from polynomial fits of data in Figure 1 A for each of examples 2A and 2B.

[126] Figure 3A is a graph of z-strain versus x-strain (10 min between 5% strain steps at 23°C) for each of examples 3B and 3C.

[127] Figure 3B is a graph of the instantaneous Poisson’s ratio vs x-strain calculated from polynomial fits of data in Figure 1A for each of examples 3B and 3C.

[128] Figure 4A is a graph of z-strain versus x-strain (10 min between 5% strain steps at 23°C) for example 4A.

[129] Figure 4B is a graph of the instantaneous Poisson’s ratio vs x-strain calculated from polynomial fits of data in Figure 1 A for example 4A.

EXAMPLES

[130] Aligned nematic liquid crystal elastomer films were synthesised according to the methods below using the following materials: 2-ethylhexyl acrylate (EHA),

6-(4-cyano-biphenyl-4'-yloxy)hexyl acrylate (A6OCB),

1 ,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82),

4-cyano-4’-hexyloxybiphenyl (6OCB) and methyl benzoylformate (MBF).

Example 1 - Effect of crosslinking density

[131] In this example, aligned nematic liquid crystal elastomer films (examples 1A and 1 B) having different crosslinking densities were prepared using the following starter monomer mixtures:

[132] Films of liquid crystal elastomer were produced by polymerising the monomer mixture inside thin film moulds of approximately 100 pm thickness, a width of approximately 25 mm and a length of approximately 70 mm. The films were cured in the nematic phase at room temperature, with planar alignment, where the surface of the mould was coated with a thin layer of PVA alignment layer and the rubbing direction was anti-parallel. The films were polymerised for two hours. After polymerisation, the films were removed from the moulds and kept in a methanol/DCM solvent mixture overnight in order to wash out the 6OCB.

[133] The resultant liquid crystal elastomer films, i.e., after removal of 6OCB, contained 8.0 mol% and 4.2 mol% crosslinker for example 1A and 1 B, respectively.

[134] The liquid crystal elastomer films were tested in accordance with the following methods:

Test methods

[135] The films were strained in steps of 0.05 (5%) and the films allowed to equilibrate between each strain step for 10 minutes. Local strains in the x- and y- direction, EX and sy respectively, were tracked and the strain in the z-direction (the auxetic response) was calculated based on the measured strains in the x- and y- direction, using Equation 1 , by assuming that the volume of the liquid crystal elastomer remains constant:

£_z = - (Equation 1)

(l+e_x) -( -l+e_y) 1

[136] Below the auxetic response strain-threshold, the strain in the z-direction decreases as the applied strain in the x-direction increases. Above the auxetic response strain-threshold, the strain in the z-direction increases as the applied strain in the x-direction increases. The auxetic response strain-threshold can thus be determined based on the calculated strain in the z- direction.

[137] All measurements were taken with the liquid crystal elastomer film at room temperature (23°C).

[138] The results are shown in Figures 1A and 1 B.

Figure 1A shows a graph of strain in the z-direction as the applied strain in the x-direction is increased for each of examples 1A and 1 B. Figure 1 B shows a graph of the Poisson’s ratio for each of examples 1A and 1 B. The Poisson’s ratio, Vxz, is determined using Equation 2, wherein the true strain is determined based on the engineering strain using Equation 3. v_xz de_z t_rue/d£_x t_rue (Equation 2)

Etrue = ln(Eeng + 1) (Equation 3)

[139] As shown in figures 1A and 1 B, the auxetic response strain-threshold (at which the strain in the z-direction begins to increase in figure 1A and at which the Poisson’s ratio becomes negative in figure 1 B) of Example 1A is higher than the auxetic response strain-threshold of Example 1 B (79 ± 5% and 32 ± 5%, respectively). Figures 1A and 1 B also show that the auxetic behaviour of the materials of examples 1A and 1 B are altered under these experimental conditions, as indicated by the general shape of the strain response and the Poisson’s ratio.

[140] This example therefore shows that decreasing the crosslinking density of a liquid crystal elastomer results in a reduction in the auxetic response strain-threshold of the liquid crystal elastomer. Conversely, this example shows that the auxetic response strain-threshold of a liquid crystal elastomer can be increased by increasing the crosslinking density of a liquid crystal elastomer.

Example 2 - Effect of Tq

[141] Aligned nematic liquid crystal elastomer films were prepared in accordance with example 1A above. After polymerisation, one film was tested without further modification (example 2A). To another film, 50% by mass of poly(2-ethylhexyl acrylate) - a plasticiser having approximately 10 monomer units - was added directly to the film at 60°C (example 2B).

[142] As reported herein, the Tg was measured according to ASTM E1356-08(2014) (“Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry”. Heat-flux differential scanning calorimetry (DSC), sample pans: aluminium, reference: blank, calibration: indium and adamantane (solid-solid transition), heating rate: 10°C/min) using the standard “heat-cool-heat” method. The samples were equilibrated at -60°C then heated to 80°C at a heating rate of 10°C/min. The samples were then cooled to -60°C and heated to 80°C at a heating rate of 10°C/min. The cool/heat cycle - i.e., -60°C to 80°C - was then repeated a further time (such that three cool/heat cycles were performed). The glass transition was measured as the inflection point of the transition of the second cooling cycle. The Tg of each of examples 2A and 2B is provided below.

[143] The films were tested in accordance with the test methods described above.

[144] Figure 2A shows a graph of strain in the z-direction as the applied strain in the x-direction is increased for each of examples 2A and 2B. Figure 2B shows a graph of the Poisson’s ratio for each of examples 2A and 2B.

[145] As shown in figures 2A and 2B, the auxetic response strain-threshold (at which the strain in the z-direction begins to increase in figure 2A and at which the Poisson’s ratio becomes negative in figure 2B) of Example 2A is higher than the auxetic response strain-threshold of Example 1 B (79 ± 5% and 37 ± 5%, respectively). Figures 2A and 2B also show that the auxetic behaviour of the materials of examples 2A and 2B are altered under these experimental conditions, as indicated by the general shape of the strain response and the Poisson’s ratio.

[146] This example therefore shows that decreasing the Tg of a liquid crystal elastomer results in a reduction in the auxetic response strain-threshold of the liquid crystal elastomer. Conversely, this example shows that the auxetic response strain-threshold of a liquid crystal elastomer can be increased by increasing the Tg of a liquid crystal elastomer.

Example 3 - Effect of Chain Length

[147] Aligned nematic liquid crystal elastomer films were prepared in accordance with Example 1A above but in Examples 3B and 3C the monofunctional mesogenic monomer 4’-(6- hydroxyhexyloxy)-[1 ,1 ’-biphenyl]-4-carbonitrile (A6OCB) was replaced with the following monomers: 3B: 4-(4-Cyano-biphenyl-4'-yloxy)butyl acrylate (A4OCB)

3C: 5-(4-Cyano-biphenyl-4'-yloxy)pentyl acrylate (A5OCB)

[148] All of the other components of the LCE were consistent with the original unmodified auxetic LCE, as is the mol% of each component.

[149] The variation in monomer resulted in a change in the glass transition temperature (Tg) (recorded as the onset value on heating samples at 10 °C/min) of the LCE. The Tg was measured as set out above in Example 2.

[150] The films were tested in accordance with the test methods described above. Decreasing the chain length in the monofunctional mesogenic monomer results in an increased Tg.

[151] Figure 3A shows a graph of strain in the z-direction as the applied strain in the x-direction is increased for each of examples 1A, 3B and 3C. Figure 3B shows a graph of the Poisson’s ratio for each of examples 1 A, 3B and 3C.

[152] As shown in Figures 3A and 3B, the auxetic response strain-threshold (at which the strain in the z-direction begins to increase in Figure 3A and at which the Poisson’s ratio becomes negative in Figure 3B) of Example 3B is higher than the auxetic response strain-threshold of Example 3C which in turn is higher than that of Example 1A (81 ± 5%, 65 ± 5% and 57 ± 5%, respectively). Figures 3A and 3B also show that the auxetic behaviour of the materials of examples 3B and 3C are altered under these experimental conditions, as indicated by the general shape of the strain response and the Poisson’s ratio.

[153] This example therefore shows that chain length of the monomers can be used to alter Tg and further shows that decreasing the Tg of a liquid crystal elastomer results in a reduction in the auxetic response strain-threshold of the liquid crystal elastomer. Conversely, this example also further shows that the auxetic response strain-threshold of a liquid crystal elastomer can be increased by increasing the Tg of a liquid crystal elastomer.

Example 4 - Alternative Functionality in Mesogenic Monomer

[154] Aligned nematic liquid crystal elastomer films were prepared in accordance with Example 1A above but in Example 4B the monofunctional mesogenic monomer 4’-(6-hydroxyhexyloxy)- [1 ,1 ’-biphenyl]-4-carbonitrile (A6OCB) was replaced with the following monomer: 4B: 4-Methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy)phenyl ester

[155] All of the other components of the LCE were consistent with the original unmodified auxetic LCE, as is the mol% of each component.

[156] The variation in monomer resulted in a change in the glass transition temperature (Tg) (recorded as the onset value on heating samples at 10 °C/min) of the LCE. The Tg was measured as set out above in Example 2.

[157] The film was tested in accordance with the test methods described above.

[158] Figure 4A shows a graph of strain in the z-direction as the applied strain in the x-direction is increased for each of examples 1A, and 4B. Figure 4B shows a graph of the Poisson’s ratio for examples 1 A, and 4B.

[159] As shown in Figures 4A and 4B, the auxetic response strain-threshold (at which the strain in the z-direction begins to increase in Figure 4A and at which the Poisson’s ratio becomes negative in Figure 4B) of Example 4B is lower than the auxetic response strain-threshold of Example 1A (35 ± 5% and 57 ± 5%, respectively). Figures 4A and 4B also show that the auxetic behaviour of the material of example 4B is altered under these experimental conditions versus that of Example 1A, as indicated by the general shape of the strain response and the Poisson’s ratio.

[160] This example therefore shows that altering the functionality of the monomers can be used to tune the Tg and further shows that decreasing the Tg of a liquid crystal elastomer results in a reduction in the auxetic response strain-threshold of the liquid crystal elastomer. Conversely, this example also further shows that the auxetic response strain-threshold of a liquid crystal elastomer can be increased by increasing the Tg of a liquid crystal elastomer.

[161] In Example 4C, an LCE was prepared using the following starter monomer mixtures: wherein SRB-CHEM-023 is (4‘ ‘-Acryloyloxybutyl) 2,5-Di(4‘-butyloxybenzoyloxy)benzoate.

[162] The variation in monomer resulted in a change in the glass transition temperature (Tg) (recorded as the onset value on heating samples at 10 °C/min) of the LCE. The Tg was measured as set out above in Example 2.

[163] The film was tested in accordance with the test methods described above.

[164] The auxetic response strain-threshold of Example 4C is higher than the auxetic response strain-threshold of Example 1A (76 ± 5% and 57 ± 5%, respectively).

[165] This example therefore shows again that altering the functionality of the monomers can be used to tune the Tg.

[166] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[167] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[168] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[169] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1 . A method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer comprising chemically modifying the aligned nematic liquid crystal elastomer.

2. The method according to claim 1 , wherein the method comprises modifying the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer.

3. The method according to claim 2, wherein the method comprises decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer by decreasing the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer; or wherein the method comprises increasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer by increasing the crosslinking density and/or the glass transition temperature (Tg) of the aligned nematic liquid crystal elastomer.

4. The method according to any one of claims 1 to 3, wherein the aligned nematic liquid crystal elastomer is the reaction product of a reaction mixture comprising a mesogenic monomer, a crosslinker, an initiator and, optionally, a non-mesogenic monomer.

5. The method according to claim 4, wherein the crosslinking density is modified by varying the amount of crosslinker, by varying the type of crosslinker used and/or by varying the amount of monomer(s) having one or more functional group(s) capable of crosslinking present in the reaction mixture.

6. The method according to any one of claims 4 or 5, wherein the crosslinking density is decreased by decreasing the amount of crosslinker present in the reaction mixture.

7. The method according to claim 6, wherein the amount of crosslinker present in the reaction mixture is at least 10%, such as at least 20%, such as at least 30%, such as at least 50%, such as at least 50% lower in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer.

8. The method according to any one of claims 4 or 5, wherein the crosslinking density is increased by increasing the amount of crosslinker present in the reaction mixture.

9. The method according to claim 8, wherein the amount of crosslinker present in the reaction mixture is at least 10%, such as at least 20%, such as at least 30%, such as at least 50%, such as at least 50% higher in the reaction mixture used to form the modified aligned nematic liquid crystal elastomer compared to the reaction mixture used to form the unmodified aligned nematic liquid crystal elastomer.

10. The method according to any one of claims 2 to 9, wherein the glass transition temperature (Tg) is modified by including, excluding and/or varying the amount of one or more Tg modifying components present in the aligned nematic liquid crystal elastomer and/or by controlling the polymerisation process used to form the aligned nematic liquid crystal elastomer.

11 . The method according to claim 10, wherein the Tg modifying component is selected from the group consisting of: plasticisers, non-mesogenic monomers, mesogenic monomers, flexible spacers, solvents and/or combinations thereof.

12. The method according to claim 11 , wherein the glass transition temperature (Tg) is decreased by including or increasing the amount of one or more plasticiser(s) present in the aligned nematic liquid crystal elastomer.

13. The method according to claim 12, wherein the unmodified aligned nematic liquid crystal elastomer comprises substantially no plasticiser and the modified aligned nematic liquid crystal elastomer comprises at least one plasticiser, for example in an amount of 1 to 50% by mass.

14. The method according to claim 11 , wherein the glass transition temperature (Tg) is increased by excluding or decreasing the amount of one or more plasticise r(s) present in the aligned nematic liquid crystal elastomer.

15. The method according to claim 14, wherein the unmodified aligned nematic liquid crystal elastomer comprises at least one plasticiser, for example in an amount of 1 to 50% by mass., and the modified aligned nematic liquid crystal elastomer comprises substantially no plasticiser.

16. The method according to claim 11 , wherein the glass transition temperature (Tg) is increased by decreasing the conformational freedom within the mesogenic and/or non- mesogenic monomer or wherein the glass transition temperature (Tg) is decreased by increasing the conformational freedom within the mesogenic and/or non-mesogenic monomer.

17. Use of a chemically modified aligned nematic liquid crystal elastomer in a method of increasing or decreasing the auxetic response strain-threshold of an aligned nematic liquid crystal elastomer.