Composite material

Abstract

A composite material includes a plurality of particles interspersed within an elastomer, where the elastomer is an auxetic oriented nematic liquid crystal elastomer. Another composite material includes first and second outer layers and an interlayer between the first and second outer layers, wherein the interlayer includes an auxetic oriented nematic liquid crystal elastomer.

Description

[0001] The present invention relates to a composite material comprising an aligned nematic liquid crystal elastomer.

[0002] Composite materials can be layered, comprising multiple layers. Another type of composite material is a particulate composite, where particles of a first material are disposed within a second material. Composite materials are widely used in various industries, such as aerospace structures, automotive construction, and micro-electromechanical systems.

[0003] Debonding can be a significant problem in composite systems. Layers in layered composites can separate, and particles in particulate composites can separate from the surrounding material. This separation can be caused by a variety of factors, including free edge effects, structural discontinuities, and localized perturbations caused by shock, humidity, and temperature fluctuations. Thermomechanical reliability of microelectronic packaging is a major concern in the electronics industry. Delamination is also a significant issue in the glass industry.

[0004] Some composite systems can secrete self-healing materials when debonding occurs, thereby preventing it. Fiber composite systems can utilize Z-pin bridging, which provides through-thickness reinforcement. However, this approach to addressing debonding may only be effective for certain composite systems.

[0005] According to the present invention, there is provided an apparatus and a method as described in the accompanying claims. Further features of the invention will be apparent from the dependent claims and the subsequent description.

[0006] According to a first aspect, there is provided a composite material comprising an auxetic oriented liquid crystal elastomer.

[0007] The composite material may be a particulate composite material and may comprise a plurality of particles dispersed within an elastomer. The composite material may be a layered composite material and the elastomer may form a sandwich between the first outer layer and the second outer layer.

[0008] An aligned nematic liquid crystal elastomer with auxetic properties has recently been developed, as described in WO2019077361A1. Auxetic materials exhibit a negative Poisson's ratio, which is described as the negative ratio of the proportional decrease in transverse measurements to the proportional increase in length when a sample of the material is stretched. Upon stretching, the auxetic material thickens in one or two directions perpendicular to the applied deformation.

[0009] Surprisingly, composite materials comprising auxetic nematic liquid crystal elastomers have been found to provide debonding resistance. In composite materials comprising particles dispersed within the elastomer, improved debonding resistance between the elastomer and the particles has been observed. The debonding resistance of the composite materials can be used in applications such as body armor or semiconductor encapsulation. In composite materials comprising a liquid crystal elastomer as an interlayer between a first outer layer and a second outer layer, delamination between the layers can be reduced. Such composite materials can be used in applications such as automotive glazing and photovoltaic device encapsulation, where an auxetic nematic liquid crystal elastomer interlayer can replace existing interlayer materials, such as polyvinyl butyral layers.

[0010] The particle composite material may be a thin film. The thickness of the thin film may be ≤ 100 μm. In other examples, the thickness of the thin film may be greater than 100 μm.

[0011] The particles can have any shape. Preferably, the particles can be spherical. In other examples, the particles can be elongated, such as rod-shaped.

[0012] The size of the particles can be smaller than the thickness of the film. The diameter of the spherical particles can be ≤ 100 μm. Preferably, the diameter of the spherical particles can be 10 μm. In other examples, the diameter of the particles can be greater than 100 μm.

[0013] The liquid crystal elastomer can be a monodomain liquid crystal elastomer. "Monodomain" as used herein means that the director orientation of the elastomer is macroscopically uniform within the sample. For example, monodomain orientation can be determined in a sample using polarizing microscopy, where it is characterized by uniform birefringence when the macroscopic sample is viewed between crossed polarizers.

[0014] The liquid crystal elastomer may be oriented along the longitudinal axis of the film.

[0015] In the layered composite material, the auxetic liquid crystal elastomer can be oriented in a predetermined direction. The predetermined direction can be substantially parallel to the outer layer or perpendicular to the outer layer. In other examples, the predetermined direction can be at an oblique angle relative to the outer layer. The layered composite material can include an orientation layer positioned between at least one of the first and second outer layers, wherein the orientation layer is configured to promote alignment of the liquid crystal elastomer in the predetermined direction. In the layered composite material, at least one of the outer layers can be glass.

[0016] In a particle composite, the bulk of the liquid crystal elastomer can have a single domain orientation, while the liquid crystal elastomer can have a different orientation near the particle. The nematic liquid crystal elastomer molecules can be oriented parallel or perpendicular to the particle surface. The orientation of the liquid crystal molecules around the particle may depend on the particle material.

[0017] The particle composite material may include an alignment surface disposed on a surface of the particle, wherein the alignment surface is configured to promote alignment of the liquid crystal elastomer in a direction parallel to or perpendicular to the particle surface. The particles may be pretreated to select the perpendicular or parallel alignment. For example, a solution may be applied to the particles to promote alignment.

[0018] While certain preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.

[0019] For a better understanding of the present invention, and in order to show how embodiments thereof may be implemented, reference will now be made, by way of example only, to the accompanying schematic diagrams, in which:

[0020] Figure 1 A first embodiment of a composite material is shown;

[0021] Figure 2 A second embodiment of the composite material is shown;

[0022] Figure 3A and Figure 3B shows a graph representing the auxetic threshold of an aligned nematic liquid crystal elastomer;

[0023] Figures 4A to 4E An image showing an aligned nematic liquid crystal elastomer with embedded particles; and

[0024] Figures 5A to 5F An image of an isotropic elastomer with embedded particles is shown.

[0025] like Figure 1 As shown, exemplary composite material 10 includes a first outer layer 12, a second outer layer 14, and an intermediate layer 16 positioned between first and second outer layers 10, 12. Intermediate layer 16 is formed from an auxetic oriented nematic liquid crystal elastomer, such as the liquid crystal elastomer described in Example 1 below.

[0026] The nematic liquid crystal elastomer is oriented in the Y direction, parallel to the outer layer. When strain is applied in the X direction, the liquid crystal elastomer undergoes a mechanical Friedericksz transition (MFT), where the orientation effectively rotates to the Z direction.

[0027] The mechanical Fredericks transition is defined as a mode of deformation in an oriented elastomer in which the director in the plane of the elastomer film appears to rotate sharply at a critical strain and reorient parallel to the stress axis at a critical extension. Materials with this property were first described by Mitchell et al. (Mitchell, GR, Davis, FJ and Guo, W., Phys. Rev. Lett., 1993, 71(18), 2947) and Roberts et al. (Roberts, PMS, Mitchell, G. R and Davis, FJ, J. Phys, II France, 1997, 7, 1337 and Roberts, PMS, Mitchell, G. R, Davis, FJ and Pople, JA, Mol. Cryst. Liq. Cryst., 1997, 299, 181). The MFT is often described as analogous to the well-known electric (or magnetic) field Fredericks transition (EFT) that occurs in low molar mass nematic display devices. In EFT, the director abruptly reorients beyond a well-defined critical field (or voltage), gradually aligning with the electric field as the field amplitude increases. While the EFT threshold is theoretically discontinuous, it is known to soften in practice if an ideal LC single domain, perfectly aligned parallel or perpendicular to the substrate, is not achieved. Nevertheless, the threshold remains rigid and well-defined. The apparent abrupt rotation of the director observed in MFT differs from the director rotation response of oriented elastomers deformed by semisoft elasticity (SSE), an alternative deformation mode. In the SSE case, the director rotation is relatively gradual and occurs within the deformation plane, on a plateau-like region of the tensile loading curve.

[0028] like Figure 2 As shown, another exemplary composite material 20 includes a thin film formed of an auxetic nematic liquid crystal elastomer 22 and a plurality of particles 24 dispersed therein. In composite material 20, the bulk of the liquid crystal elastomer is oriented in the Y direction, while near particles 24, the liquid crystal elastomer is aligned around the particles. When strain is applied in the X direction, the liquid crystal elastomer undergoes MFT, in which the orientation direction rotates to the Z direction. Example

[0029] The auxetic nematic liquid crystal elastomer used in accordance with the present invention was synthesized as follows using the following materials:

[0030] 2-Ethylhexyl acrylate (EHA),

[0031] 6-(4-cyanobiphenyl-4'-yloxy)hexyl acrylate (A6OCB),

[0032] 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (RM82),

[0033] 4-Cyano-4'-hexyloxybiphenyl (6OCB) and

[0034] Methyl benzoylformate (MBF).

[0035] The elastomers were prepared using the following starting monomer mixture:

[0036]

[0037] Silica microspheres (1% wt) with a diameter of 10 µm and no surface treatment were immersed in a liquid crystal monomer mixture. In other examples, particles of other materials and diameters can be immersed in the liquid crystal monomer mixture.

[0038] Liquid crystal elastomer films are produced by polymerizing a monomer mixture within a film mold having a thickness of approximately 100 µm, a width of approximately 15 mm, and a length of approximately 60 mm. According to the present invention, a first film is cured at room temperature in a nematic phase with a planar orientation, wherein the mold surface is coated with a thin layer of a PVA orientation layer and the rubbing direction is antiparallel. In other examples, other orientation layers, such as polyimide, can be used. As a comparative example, a second film is cured at 50°C in an isotropic phase without an orientation layer. As a comparative example, a third film is cured at room temperature in a nematic phase with a planar orientation, but without the presence of microsphere particles. The first, second, and third films are each polymerized for two hours. After polymerization, the films are removed from the mold and kept in a methanol / DCM solvent mixture overnight to wash out the 6OCB.

[0039] Figure 3A and Figure 3B The first film exhibits auxetic behavior. When strain is applied in the x-direction, the strains in the x- and y-directions are measured using particle tracking as the elastomer deforms in the x-direction. The selected particle is located near the center of the elastomer. Assuming the volume of the liquid crystal elastomer remains constant, the strain in the z-direction (auxetic response) is calculated from the strains measured in the x- and y-directions using Equation 1:

[0040] (Formula 1)

[0041] Determine Poisson's ratio using Equation 2 , where the true strain is determined from the engineering strain using Equation 3.

[0042] (Formula 2)

[0043] (Formula 3)

[0044] Figure 3A shows the relationship between instantaneous Poisson's ratio and strain, Figure 3B The relationship between the strain in the z direction and the strain in the x direction is shown. Figure 3A It is shown that as the x-strain (axis 30) increases, the Poisson's ratio (axis 32) decreases and becomes negative, indicating the presence of auxetic behavior. Figure 3A It is shown that the first film ( Figure 3A The Poisson's ratio of the third film (marked as 34) is lower than that of the third film ( Figure 3A The strain becomes negative at the strain marked as 36 in the figure. Figure 3B The auxetic threshold is shown at the point where the z strain (axis 38 ) increases as the x strain (axis 40 ) increases. Figure 3B It is shown that compared with the third film without particles ( Figure 3B Compared with the first film with particles ( Figure 3B 42 in the figure) has an auxetic threshold at reduced x strain.

[0045] Figures 4A to 4E The first film exhibits resistance to debonding. Figures 4A to 4E The images in the figure were captured with a 50x objective during the straining of the first film, which was placed between crossed polarizers. The film was strained in steps of 0.2 mm and allowed to relax for 120 seconds before the next strain step. Figures 4A to 4E The axis indicated by the arrow X is provided. Figures 4A to 4E In FIG. 5 , the image labeled (i) shows the appearance of the first film through crossed polarizers, and the image labeled (ii) shows the appearance of the first film through crossed polarizers with wave plates. The position of the crossed polarizers is indicated by the marker 52.

[0046] Figure 4A shows the appearance of the first film before straining; Figure 4B shows the appearance of the first film at a strain of 0.25; Figure 4C shows the appearance of the first film at a strain of 0.5; Figure 4D shows the appearance of the first film at a strain of 0.75; Figure 4E The appearance of the first film at a strain of 1 is shown. Figures 4A to 4E The spherical particles show no signs of separation of the liquid crystal elastomer. The fringes in the image at high strain indicate changes in birefringence ( Figure 4D and Figure 4E ), which can be attributed to the strain interaction between particles and the result of MFT.

[0047] Figures 5A to 5GA comparative example is shown where the second film is strained in the X direction, where the second film is placed between crossed polarizers. Figures 5A to 5G The image in was captured with a 50x objective during the straining of the second film, which was placed between crossed polarizers. The second film was strained in steps of 0.2 mm and allowed to relax for 120 seconds before the next strain step. Figure 5A In each of the figures through FIG4G , the image labeled (i) shows the appearance of the second film through crossed polarizers; the image labeled (ii) shows the appearance of the second film through crossed polarizers with a wave plate; the image labeled (iii) shows the appearance of individual particles of the second film through crossed polarizers; and the image labeled (iv) shows the appearance of individual particles of the second film through crossed polarizers with a wave plate. The location of the crossed polarizers is indicated by reference numeral 52.

[0048] Figure 5A shows the appearance of the second film before straining; Figure 5B shows the appearance of the second film at a strain of 0.13; Figure 5C shows the appearance of the second film at a strain of 0.32; Figure 5D shows the appearance of the second film at a strain of 0.5; Figure 5E shows the appearance of the second film at a strain of 0.65; Figure 5F shows the appearance of the second film at a strain of 0.84; Figure 5G The appearance of the second film at a strain of 0.97 is shown. Figure 5F and Figure 5G It is shown that at higher strains there is separation of the liquid crystal elastomer from the spherical particle. This separation is represented by the black triangles on the left and right sides of the particle. Figure 5F Some particles in the Figure 5G Almost all particles in the sample exhibit this defect. Local strain between particles induces very small birefringence. Figures 5A to 5G The black background in the images marked (i) is due to the fact that the elastomer in these regions is isotropic.

[0049] Attention is directed to all documents and files related to this application that are filed concurrently with or previous to this specification, which are open to public inspection with this specification, and the contents of all such documents and files are incorporated herein by reference.

[0050] All features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all steps of any disclosed method or process, may be combined in any combination, except combinations in which at least some of the features and / or steps are mutually exclusive.

[0051] Unless expressly stated otherwise, 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. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0052] The invention is not limited to the details of the foregoing embodiments. Rather, the invention extends to any new one or any new combination of features disclosed in this specification (including any accompanying claims, abstract and drawings), or any new one or any new combination of steps in any disclosed method or process.

Claims

1. A composite material comprising a plurality of particles dispersed in an elastomer, wherein the elastomer is an auxetic nematic liquid crystal elastomer.

2. The composite material according to claim 1, wherein The composite material is a thin film.

3. A composite material according to any one of the preceding claims, wherein The particles are spherical particles.

4. The composite material according to claim 3, wherein The diameter of the spherical particles is ≤100 μm, preferably 10 μm.

5. The composite material according to claim 1 or 2, wherein The particles are elongated.

6. A composite material according to any one of the preceding claims, wherein The liquid crystal elastomer is a monodomain liquid crystal elastomer.

7. A composite material according to any one of the preceding claims, comprising an orientation surface disposed on the surface of the particle, wherein the orientation surface is configured to promote orientation of the liquid crystal elastomer in a direction parallel to or perpendicular to the surface of the particle.

8. A composite material comprising a first outer layer and a second outer layer and an interlayer between the first outer layer and the second outer layer, wherein the interlayer comprises an auxetic nematic liquid crystal elastomer.

9. The composite material according to claim 8, wherein The liquid crystal elastomer is a monodomain liquid crystal elastomer having a predetermined alignment direction substantially parallel to the first outer layer and the second outer layer.

10. The composite material of claim 9, further comprising an alignment layer positioned between at least one of the first outer layer and the second outer layer, wherein the alignment layer is configured to promote alignment of the liquid crystal elastomer in the predetermined direction.

11. The composite material according to any one of claims 8 to 10, wherein At least one of the first outer layer and the second outer layer is glass.

Images