Aligned nematic elastomer

Abstract

There is provided the use of an aligned nematic elastomer to form a material having auxetic properties wherein the aligned nematic material has a mechanical Fréedericksz transition. Also provided is a method of producing an aligned nematic elastomer for said use.

Claims

1. A synthetic aligned nematic elastomer having auxetic properties, wherein the aligned nematic elastomer has a mechanical Fréedericksz transition; wherein the synthetic aligned nematic elastomer comprises a monodomain liquid crystal elastomer; wherein the monodomain liquid crystal elastomer comprises: a polymeric component; a liquid crystal mesogen component; and a cross-linker component, wherein the liquid crystal mesogen component is physically linked to the polymeric component; and wherein the liquid crystal mesogen component comprises a liquid core component selected from one or more of the following systems: ##STR00003## wherein R and R′ are independently selected from the group consisting of alkyl, alkoxy, halide, NO.sub.2 and —CN, and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core component to the polymeric component; and X and Y are independently selected from the group consisting of —CH═CH—, —CH═N—, —N═N—, and —C(O)O—.

2. The synthetic aligned nematic elastomer according to claim 1, wherein the auxetic properties enable the material to be used in a medical device or in a biomedical application.

3. The synthetic aligned nematic elastomer according to claim 1, wherein the auxetic properties enable the material to be used in a piezoelectric sensor or actuator, or in a micro- or nano-mechanical or electromechanical device.

4. The synthetic aligned nematic elastomer according to claim 1, wherein the auxetic properties enable the material to be used in composite materials as reinforcements, or in personal protection clothing.

5. The synthetic aligned nematic elastomer according to claim 1, wherein the polymeric component is formed from both mesogenic and non-mesogenic components.

6. The synthetic aligned nematic elastomer according to claim 1, wherein the crosslinker component comprises a mesogenic component.

7. A method of using a synthetic aligned nematic elastomer having auxetic properties in an application requiring auxetic properties, wherein the aligned nematic elastomer has a mechanical Fréedericksz transition; wherein the monodomain liquid crystal elastomer comprises: a polymeric component a liquid crystal mesogen component and a cross-linker component, wherein the liquid crystal mesogen component is physically linked to the polymeric component; and wherein the liquid crystal mesogen component comprises a liquid core component selected from one or more of the following systems: ##STR00004## wherein R and R′ are independently selected from the group consisting of alkyl, alkoxy, halide, NO.sub.2 and —CN, and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core component to the polymeric component and X and Y are independently selected from the group consisting of —CH═CH—, —CH═N—, —N═N—, and —C(O)O—.

8. The method according to claim 7, wherein the auxetic properties enable the material to be used in a medical device or in a biomedical application.

9. The method according to claim 7, wherein the auxetic properties enable the material to be used in a piezoelectric sensor or actuator, or in a micro- or nano-mechanical or electromechanical device.

10. The method according to claim 7, wherein the auxetic properties enable the material to be used in composite materials as reinforcements, or in personal protection clothing.

11. A method of producing a synthetic aligned nematic elastomer having auxetic properties, said method comprising the steps of: a) applying an aligning means to a substrate b) applying liquid crystal elastomer components to the substrate and allowing them to form an aligned nematic phase c) curing the liquid crystal elastomer components to form an aligned nematic elastomer; wherein the aligned nematic elastomer has a mechanical Fréedericksz transition; wherein the synthetic aligned nematic elastomer comprises a monodomain liquid crystal elastomer; wherein the monodomain liquid crystal elastomer comprises: a polymeric component a liquid crystal mesogen component and a cross-linker component, wherein the liquid crystal mesogen component is physically linked to the polymeric component; and wherein the liquid crystal mesogen component comprises a liquid core component selected from one or more of the following systems: ##STR00005## wherein R and R′ are independently selected from the group consisting of alkyl, alkoxy, halide, NO.sub.2 and —CN, and wherein the alkyl and alkoxy groups may be bivalent when forming part of the linking group which connects the liquid crystal core component to the polymeric component and X and Y are independently selected from the group consisting of —CH═CH—, —CH═N—, —N═N—, and —C(O)O—.

12. The method according to claim 11, wherein the aligning means is an aligning force which is applied by brushing the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described with reference to the accompanying examples and by reference to the drawings in which:

(2) FIG. 1 shows a theoretical plot of extension ratio vs stress for a material showing an SSE transition;

(3) FIGS. 2a, 2b, 2c and 2d show plots of the fractional thickness vs the extension ratio and the Poisson's ratio vs the extension ratio of the materials of examples 1 to 4 respectively. Sub-zero values of the Poisson's ratio indicate the auxetic behaviour;

(4) FIGS. 3a, 3b, 3c and 3d show plots of the tensile load curves and director angle response vs extension ratio for materials of examples 1 to 4 respectively; and

(5) FIG. 4 shows a plot of the fractional change vs strain for the material of example 1 at varying temperature and varying extension rate.

EXAMPLES

Elastomer Synthesis

(6) Aligned nematic elastomers for use according to the invention were synthesised as follows using the following materials:

(7) 2-ethylhexyl acrylate (EHA),

(8) 6-(4-cyano-biphenyl-4′-yloxy)hexyl acrylate (A6OCB),

(9) 4-methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy)phenyl ester (M1)

(10) 4-{6-(acryloyloxy)hexyloxy}phenyl 4-(trans-4-propylcyclohexyl)benzoate (M2)

(11) 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82),

(12) 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257)

(13) 4-cyano-4′-hexyloxybiphenyl (6OCB) and

(14) methyl benzoylformate (MBF).

(15) The elastomers for use according to the invention were prepared using the following starting compositions:

(16) TABLE-US-00001 Material % by mol Component Example 1 Example 2 Example 3 Example 4 A60CB 14.6 24.4 0 0 M1 0 0 15.5 15.3 M2 0 0 5.6 5.5 60CB 55.9 54.6 50.0 50.7 RM82 7.1 3.5 5.6 0 RM257 0 0 0 5.3 EHA 20.9 16.0 22.4 22.3 MBF 1.6 1.5 0.8 0.8

(17) Using a balance with an accuracy of 0.3 mg, dry materials were measured into a 4m1 sample vial. The mixture was then heated to 100° C. and stirred at 60 rpm for 5 minutes. The liquid materials were added and the vial was placed on a separate stirring plate held at 40° C. and stirred at 60 rpm for a further 5 minutes.

(18) The mixtures were then filled in the isotopic phase at 40° C. into the cells previously prepared via capillary action and left for approximately half an hour to cool to ambient temperature allowing the nematic phase to form via alignment with the rubbing direction. Once aligned, the cells were placed under a low intensity UV fluorescence light source (intensity of 2.5 mW cm.sup.−2) for two hours to cure. Once separated from the cells, the film was washed in dicholoromethane (DCM) by slowly adding DCM stepwise to about 30% concentration. Solvents were exchanged several times to ensure all waste materials were removed before deswelling the LCE films by adding methanol stepwise. The films were left to dry fully overnight before testing.

(19) The auxetic properties of the four materials are demonstrated in FIGS. 2a, 2b, 2c and 2d respectively which show the materials to have a negative Poisson's ratio. Beyond an extension ratio of approximately 1.8 in FIG. 2a, approximately 1.5 in FIG. 2b, approximately 1.6 in FIG. 2c and approximately 1.6 in FIG. 2d, the fractional thickness of the materials increases with increasing extension ratio. FIGS. 3a, 3b, 3c and 3d demonstrate that the materials each possess an MFT. In FIG. 3a a sharp change in director angle is seen at a strain of approximately 2.1. In FIG. 3b a sharp change in director angle is seen at an x deformation of approximately 1.9. In FIG. 3c a sharp change in director angle is seen at an x deformation of approximately 1.9. In FIG. 3d a sharp change in director angle is seen at an x deformation of approximately 1.9.