WO2015030572A1

WO2015030572A1 - Liquid crystal network

Info

Publication number: WO2015030572A1
Application number: PCT/NL2014/000025
Authority: WO
Inventors: Danqing Liu; Dirk J. Broer
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2013-08-28
Filing date: 2014-08-27
Publication date: 2015-03-05
Also published as: NL1040357C2

Abstract

The invention relates to a process for preparing a layer of a liquid crystal network, the layer having an image comprising random shapes and being capable of developing the image to a surface profile under the influence of an external stimulus, the process comprising 1) applying a layer of a mixture comprising a photopolymerizable liquid crystal monomer on a support having an orientation layer on which the monomers self-organize to form cholesteric liquid crystals comprising a helix having a helical axis that is perpendicular to the longitudinal axes of the monomers, wherein the helical axes are positioned substantially parallel to the support, and 2) photopolymerizing the monomers to form the layer of a liquid crystal network.

Description

Liquid crystal network

The invention relates to a process for preparing a layer of a liquid crystal network, to a liquid crystal network obtainable by such method and to a process for forming or amplifying a surface relief on a layer of a liquid crystal network.

In order to move products through a supply chain and to exclude counterfeiting once sold, one should be able to identify the products and then match the physical product to the related transaction information. This means that every item that is to be transported or sold must have a unique product identification code.

Conventional methods for identification comprise bar codes

(including QR codes), electronic tags such as RFID codes, magnetic stripes, holograms, etc. However, providing each product with a unique code that is also difficult to reproduce, is not realized by these methods.

From ancient history, we know that identification on an individual basis was performed by cutting a stone in two pieces. In this way, two unique parts are formed that complement each other. By "tagging" a person with one of the parts, this person can be identified by another person having the complementary part. Although this method may very effective because the surface resulting from the fracture of a stone can be considered as truly unique, it would be insufficient in the present days of mass production, wherein a fast identification with high turnovers is required, preferably performed on a distance.

It is therefore an object of the invention to provide a material that can be used for identification of persons or goods on an individual basis. One particular object is that the identification can be performed quickly, for example by scanning with electromagnetic radiation. Another particular object is that the material is robust, or at least that the identification means (for example a unique surface structure) is not lost when the material is exposed to abrasive conditions. Yet another particular object is that the identification means can easily be analyzed by an image analyzer and stored digitally. It has now been found that one or more of these objectives can be reached by using a material comprising a particular liquid crystalline network.

Accordingly, the present invention relates to a process for preparing a layer of a liquid crystal network, comprising

- applying a layer of a mixture comprising at least

1) a chiral monofunctional photopolymerizable liquid crystal monomer and a difunctional photopolymerizable liquid crystal monomer; or a chiral difunctional photopolymerizable liquid crystal monomer;

2) a photopolymerizable mono- or difunctional momomer that is capable of changing its conformation under the influence of electromagnetic radiation;

3) a photoinitiator;

on a support having an orientation layer on which the monomers self- organize to form cholesteric liquid crystals comprising a helix having a helical axis that is perpendicular to the longitudinal axes of the monomers, wherein the helical axes are positioned substantially parallel to the support; - photopolymerizing the monomers to form the layer of a liquid crystal network.

The process of the invention is usually a process for preparing a layer of a liquid crystal network, wherein the layer has an image comprising random shapes and is capable of developing the image to a surface profile under the influence of an external stimulus. An external stimulus is for example a change in temperature or exposure to electromagnetic radiation (usually of a particular wavelength). In particular, an external stimulus is exposure to electromagnetic radiation of a wavelength that effects a change of the conformation of a photopolymerizable mono- or difunctional momomer in a mixture of a process of the invention.

By random is meant that the shapes are not in a definite pattern, and that each time the process of the invention is carried out, a layer with different shapes (and thus with a different image) is formed.

It is essential that the applied mixture comprises a chiral

photopolymerizable liquid crystal monomer, since this ensures the self- organization of the monomers in the form of a helix. In addition, at least part of the monomers needs to be difunctional so that a polymer network is formed. These two properties may be united in one monomer so that in principle a chiral difunctional photopolymerizable liquid crystal monomer may be the only liquid crystal monomer in a mixture of the invention. Usually, however, the mixture comprises a plurality of different liquid crystal

monomers. It may for example also comprise a plurality of difunctional photopolymerizable liquid crystal monomers (chiral and/or achiral). And it may also comprise (in addition to the one or more difunctional photopolymerizable liquid crystal monomers) one or more monofunctional photopolymerizable liquid crystal monomers (chiral and/or achiral).

The liquid crystal monomers comprise one or more functional groups. For the purpose of the invention, by such functional group is meant a functional group that is capable of participating in the polymerization reaction. A functional group of a liquid crystal monomer of the invention is preferably an acrylate ester or methacrylate ester, but may also be an epoxide group. Thus, a monofunctional monomer of the invention may be selected from the group of (mono)acrylate esters, (mono)methacrylate esters and (mono)epoxides. A difunctional monomer of the invention may be selected from the group of diacrylate ester, dimethacrylate ester and diepoxide. The polymerization reaction with such functional groups would yield a poly(acrylate),

poly(methacrylate) or a poly(ethylene oxide), respectively. In liquid crystal monomers of the invention, the functional group(s) is (are) usually terminal functional group(s), having an ethylene group or an epoxide group essentially at the end(s) of the molecule.

A network of the invention can also be obtained by using a

combination of one or more thiols with one or more alkenes to perform the polymerization, wherein at least part of the thiols and/or alkenes are

polyfunctional, in particular difunctional. This reaction is the so-called thiol- ene reaction. It is also possible to use in this reaction a compound containing both an SH group and a -CH=CH₂ group.

In a mixture of the invention, an achiral photopolymerizable liquid crystal monomer may be of the formula

where X is selected from the group of hydrogen, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an alkoxy carbonyl group having 1 to 20 carbon atoms, a formyl group, an alkyl carbonyl group having 1 to 20 carbon atoms, an alkyl carbonyloxy group having 1 to 20 carbon atoms, a halogen such as chloro or fluoro group, a nitrile group and a nitro group.

In the case of a difunctional monomer, Ri and R₂ are independently selected from the group of

and

where x is in the range of 1-20, and where R₃ is selected from the group of hydrogen, fluoro, chloro and methyl.

In the case of a monofunctional monomer, R-i is selected from the group as defined hereinabove for a difunctionai monomer, and R2 is selected from the group of where x is 0 - 17,

where x is 0 - 17

where x and y are independently of each other 1 - 6 and Z is selected from the group of methyl, chloro, fluoro, nitro and nitrile, and

where x and y are independently of each other 1 - 6 and Z is selected from the group of methyl, chloro, fluoro, nitro and nitrile. In a mixture of the invention, a chiral photopolymerizable liquid crystal monomer may be monofunctional or difunctional. In principle, it may also be polyfunctional, for example trifunctional or tetrafunctional. Usually, a chiral monomer comprises one chiral moiety that is substituted with two R-groups. In principle, however, it is also possible that a chiral moiety comprises more than two R-groups, for example three or four.

The chiral moiety is for example a bicyclic ether diol, wherein two R- groups, usually chains, are attached to the two diol functionalities via an ester bond. The R-groups in a chiral monomer usually comprise chains such that the layer of a liquid crystal network obtained by a process of the invention is chiral-nematic. The R-groups in a chiral photopolymerizable liquid crystal monomer may comprise groups defined hereinabove for the monofunctional and difunctional achiral monomers. In particular, when the chiral moiety comprises two R-groups, it comprises an Ri group and an R₂ group, wherein the R group and R₂-group are independently selected from the groups as defined hereinabove for the R groups and R2-groups of monofunctional and difunctional achiral monomers.

A mixture of the invention may comprise a solvent, for example dichloromethane and/or tetrahydrofuran (THF). For the purpose of the invention, the concentration of the solutes in a solvent is defined as the "solids content" of the solution. For example, when the solvent is

dichloromethane, the solids content is preferably 33 wt%. In the case of THF, the solids content is preferably 20 wt%.

In addition to the one or more liquid crystal monomers mentioned above, the applied mixture comprises a photopolymerizable polyfunctional {e.g. difunctional, trifunctional or tetrafunctional) monomer that is capable of changing its conformation under the influence of electromagnetic radiation. The change of conformation in such monomer usually entails a reversible conversion from a more or less straight, rod-like shape to a more or less hooked shape. Upon irradiation, the rod-like shape, which usually has a lower chemical potential, is converted into the hooked shape. After irradiation has ceased, the molecule reverts to the initial rod-like shape. Therefore, under normal circumstances (in the absence of the appropriate radiation), the monomer that is capable of changing its conformation under the influence of electromagnetic radiation can have a rod-like shape and will tend to align along the director as the other liquid crystal monomer also do. It is in principle also possible that the hooked shape has a lower chemical potential than the rod-like shape, so that the rod-like shape is formed upon irradiation.

Examples of monomers capable of changing their conformation under the influence of electromagnetic radiation are monomers comprising a stilbene or a diazobenzene moiety. In a photopolymerizable difunctional momomer of the invention, both phenyl rings of a stilbene or an azobenzene are usually substituted (at e.g. the para-position) with a linear chain having a (terminal) functional group, so that a rod-like difunctional monomer is formed. The linear chain may be a chain of 2-16 atoms, comprising carbon atoms and oxygen atoms. The linear chain may be an alkyl chain having an oxygen atom on one or both ends, or a poiyether chain. The functional group is in particular selected from the group of a terminal acrylate ester group, a methacrylate ester group and an epoxy group.

A photopolymerizable difunctional momomer of the invention may in particular be of the formula

where x is in the range of 2-12.

A photopolymerizable difunctional momomer of the invention may also be a spiropyran compound, in particular a spiropyran of the formula

where Ri is a methacryloyl group or an omega-(methacryloyloxy)alkyl group, for example 5-(methacryloyloxy)pentyl; and wherein R₂ is an

omega-(methacryloyloxy)alkyl group, for example 5-(methacryloyloxy)pentyl.

The support on which a mixture of the invention is applied may in principle be any suitable support. Preferably, the support is a glass support or a plastic support.

The photoinitiator in a mixture of the invention may in principle be any photoinitiator. Preferably, the photoinitiator is a dibenzoyl(phenyl)phosphine oxide, in particular bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide. The photoinitiator is usually not of a rod-like nature. Thus, in principle all molecules in a mixture of the invention, except for the photoinitiator and an eventual solvent, have a rod-like nature and tend to align along the director.

A mixture of the invention may contain a polymerization inhibitor, which serves to prevent early polymerization during storage or processing. An inhibitor may be present in the mixture in an amount of 100 - 200 ppm based on the total amount of photopolymerizable liquid crystal monomers. An inhibitor is for example p-methoxyphenol.

A mixture of the invention may further contain a surfactant. Such surfactant usually comprises a fluorinated tail. In particular, it is a

monoacrylate wherein the alkoxy group of the ester group comprises a fluorinated tail, e.g. a tail comprising 2 or more atoms. It is contemplated that such surfactant provides anchoring at the interface of the mixture with air.

In general, liquid crystal molecules self-organize by orienting their longitudinal axes along a common direction, usually named "director". In the event that the liquid crystal film is chiral-nematic (also known as cholesteric liquid crystals), the director describes a helix having a helical axis that is perpendicular to the longitudinal axes of the monomers. Cholesteric liquid crystals in a layer usually have their helical axes parallel to the normal of the layer's surface. Under the appropriate conditions, however, the helical axes can be positioned parallel to the layer's surface (and thus also parallel to the support of the layer). In a process of the invention, the conditions are chosen such that the helical axes are positioned substantially parallel to the support. Such a parallel positioning is schematically shown in Figure 1. A parallel positioning in a process of the invention is promoted by the orientation layer. Due to this layer, liquid crystal monomers can be anchored to the support. Preferably, the orientation layer is a homeotropic orientation layer, which effects an anchoring of a part of the monomers such that their longitudinal axes are parallel to the normal of the support. The drive of the molecules to form a helix is in a balance with the anchoring force of the orientation layer. Due to the presence of a helix, only part of the monomers is capable of directing their longitudinal axes substantially parallel to the normal of the support (homeotropic orientation). As a result, another part of the monomers then has an orientation substantially parallel to the surface of the support (planar orientation). The planar orientation and homeotropic orientation of monomers in a helix are schematically shown in Figure 1. The helical axis of the helix shown in Figure 1 is positioned substantially parallel to the support. The pitch P_f of such a helix may be different from the pitch P_p it would have under conditions without the anchoring forces (the so-called

"natural pitch"), for example when the helical axis is substantially parallel to the normal of the support. It can generally be stated that the larger the anchoring forces are, the larger the difference between P_f and P_p is. This means that when the perpendicular anchoring forces are small, P_f equals P_p. If they are large, P_f is larger than P_p, which means that the helix usually unwinds to a certain extent when anchoring forces play a role.

The conditions that allow a self-organization wherein the helical axes are positioned substantially parallel to the support are known to the skilled person, or can be determined by the skilled person by routine

experimentation.

Appropriate conditions are for example reached by a homeotropic orientation layer comprising a compound with an alkyl tail, for example an alkyl tail of at least 3 carbon atoms. A compound that may be used as an orientation layer is in particular selected from the group of a surfactant, a polyimide modified with alkyl tails and a silane compound having one or more alkyl tails. A surfactant for example comprises lecithine. A modified polyimide is for example obtainable from Nissan Chemical (Japan) under the brand name Sunever 7511L. A silane compound is for example an (alkyl)trialkoxysilane, in particular it is

[3-(methacryloyloxy)propyl]trimethoxysilane (brand name Silane A174).

Appropriate conditions can also be reached when 0.3 < d I P_p < 0.5, where d is the thickness of the layer of a liquid crystal network according to the invention and P_p is the natural pitch of the helix (i.e. the pitch it would have under conditions without forces that anchor the helix to the support).

In the absence of any other anchoring forces, the planar oriented monomers and homeotropic oriented monomers form random worm-like figures. By using difunctional liquid crystal monomers (i.e. at least part of the liquid crystal monomers would then be difunctional), these worm-like figures can be frozen-in into a solid liquid crystalline network (LCN) by in situ photopolymerization. This results in a LCN comprising an image of random shapes that have a worm-like appearance. Such a LCN is shown in Figure 2. This is an optical microscopy image, recorded with the LCN between crossed polarizers. The bright areas correspond to monomers with a more or less planar orientation and the black areas to monomers with a more or less homeotropic orientation.

When the mixture comprising the liquid crystal monomer(s) is photopolymerized so that the fingerprints are frozen-in, the LCN usually has an almost flat surface with only a minor relief of < 100 nm. Figure 3a shows an example of such a surface profile as measured by confocal microscopy. The surface profile corresponds to microscope pictures and the minor relief is attributed to the Maragoni effect related to small differences in surface energy between the planar and the homeotropic orientation. The photopolymerization of the layer comprising the liquid crystal monomer(s) is optionally performed with a cover layer on top of it. This means that the surface of the layer adopts the shape of the surface of the cover layer. The cover layer may in particular have a flat shape, so that after photopolymerization the LCN will also have a flat shape. The relief may then be less than that obtained without the cover layer. It may for example be 50 nm or less, 20 nm or less, or 5 nm or less.

When the LCN obtained by a process of the invention is exposed to electromagnetic radiation of the appropriate wavelength, its surface undergoes an enormous change in its topography. The image recorded in the LCN expands into the third out-of-plane dimension. This expansion is termed as the development of the image, by which is meant that a surface relief is formed or that a surface relief is amplified. The initial state and the final state of this process are displayed Figure 3a and 3b, respectively. The minor surface relief of the LCN of Figure 3a is amplified to form the surface relief of the LCN of Figure 3b. In the event that the image in the initial state does not have a surface relief (for example because the LCN was prepared with a cover layer on top of it), the surface relief of Figure 3b is also formed. As soon as the electromagnetic radiation is switched off, the surface relief decreases until the surface has adopted its (flat or flatter) initial state again.

The aspect ratio of a surface relief obtained by exposure to

electromagnetic radiation of the appropriate wavelength may be up to 0.5. For the purpose of the invention, the aspect ratio is defined as the height of the relief divided by half the pitch.

It has surprisingly been found that the presence of inhibitor (at a concentration higher than needed for monomer stabilization) may lead to permanent deformation of the surface, i.e. the surface relief structures do not revert to the initial flat or flatter state after the (first) exposure to

electromagnetic radiation has been terminated. This effect is in particular observed when the inhibitor is present in an amount of at least 2 wt% based on the LCN. It is also observed when the polymerization is performed under an oxygen atmosphere instead of a nitrogen atmosphere.

The appropriate wavelength of the electromagnetic radiation is the wavelength at which the photo-induced isomerization takes place of the monomer built in into the LCN that has such photoinducible properties. It is contemplated that the isomerization induces dimensional changes to the LCN wherein at the location of planar orientation hills are being formed leaving valleys at the locations of homeotropic orientation. This principle is visualized by the arrows in Figure 1. Usually, the moiety that actually undergoes photo- inducible isomerization is chosen such that UV light or visible light is required for the isomerization. In the case of UV light, the photo-induced isomerization usually occurs at a wavelength in the range of 200-400 nm, in particular in the range of 280-380 nm. In the case of visible light, the wavelength is usually in the range of 400-700 nm.

The invention further relates to a layer of a liquid crystal network (LCN) obtainable by a process of the invention. This layer has an image comprising random shapes. These shapes are usually worm-like, and are reminiscent of the shape of human fingerprints (see Figure 2). Each image in a LCN of the invention is a unique composition of shapes, usually worm-like shapes. The image in a LCN of the invention can be analyzed by an image analyzer and stored digitally like is done for human fingerprint. This property makes the material suitable for applications requiring identification on an individual basis. Such applications can be identification of products, for example components in a production line, retail items in a supply chain, valuable equipment belonging to a person (notebook, phone, watch), or pieces of art and fashionable items that have been reproduced in a limited edition. Another type of application is identification of persons, for example for building entrances, customs, computer activation or paycards.

A special property of a liquid crystal network (LCN) obtainable by a process of the invention is that is substantially flat in its normal state but becomes topographically shaped (like real fingerprints) when exposed to the appropriate electromagnetic radiation, in particular UV light. The flat state is essentially invisible under normal conditions (except, for example, when it is analyzed with polarized light between crossed polarizers yielding the image shown in Figure 2). Also, it is then less sensitive for deterioration under abrasive conditions. When the system is exposed to (UV) light, it forms the surface relief (or increased surface relief), that can more easily be read, for example by a sensor or a solid-state camera system. The applications mentioned above (identification of products and/or persons) can make use of the LCN under normal conditions (having a substantially flat surface), but it is usually more convenient to perform the identification together with the appropriate radiation so that use can be made of the increased surface relief, because the increased surface relief is in many applications easier to detect than the substantially flat image. Therefore, the invention also relates to a process for developing the image of a layer of a liquid crystal network according to the invention to form a surface profile, comprising exposing the layer to the appropriate

electromagnetic radiation. By developing is meant that a surface relief is formed or that a surface relief is amplified. The invention therefore also relates to a process for forming or amplifying a surface relief in a layer of a liquid crystal network according to the invention. The invention further relates to a layer of a liquid crystal network with a surface relief obtained by such process.

A different application of a layer of a liquid crystal network according to the invention is in the field of robotics.

(Micro-) robots are widely integrated in our daily life. They perform heavy or monotonous tasks like placing components in production lines and working under extreme conditions where direct human intervention is impossible, but also delicate tasks like surgery or replacing human body parts of disabled or incapacitated persons. Moreover there is a current trend in miniaturizing robots and have them functioning autonomously, e.g. in healthcare and aerospace. A recurring and often essential action of robots is grasping and releasing of objects. Thereto various grippers have been developed often inspired by the human hand. They are operated by e.g. vacuum, electric power, magnetic attraction, hydraulic mechanics and magneto-rheological fluids.

Despite all these mechanisms for grabbing, we are still far from mimicking the dexterous grip carried out by the human hand with the subtle friction regulation at the finger tips. For this purpose, the materials used to construct grippers also play an important role. Materials known in the art range from hard metal to soft elastomers, but such materials have the disadvantage that particular surface properties such as friction and the tendency to stick to objects on contact cannot easily be changed at will. This means that their interaction with the object is constant and cannot be varied. For example, what often happens when a gripper (of for example a silicon rubber) is opened with the intention to release the object, is that the object sticks to the surface and is not released upon opening of the gripper. The invention now provides a gripper material that can be used in gripping equipment such as robots, which is a new concept of implementing grasping via a reversible and controlled formation of fingerprints. The artificial fingerprints resemble those of humans in which the ridge-shaped topography is used to increase grip and prevent objects from slipping. Inactivated fingerprints of the invention are not visible and the layer of LCN is smooth and flat. When addressed with for example UV light, the three-dimensional fingerprints appear in the layer of LCN and consequently change its surface friction, promoting firm grasping. In the event that an object sticks to the surface after the gripper has been opened, the object can actively be released through modifying the surface of the gripper by changing the exposure to the electromagnetic radiation. For example, the fingerprints can be introduced by exposure to the appropriate electromagnetic radiation, or can be erased by terminating the exposure. In this way, the friction and the smoothness of the surface can be modified by applying an external stimulus. The exposure of the surface to electromagnetic radiation can occur from the back of the material, when the exposure through air is blocked by the presence of the object at the surface of the gripper. In that case, the material needs to transparent, which is a usual property of many polymers.

EXAMPLE

In a preferred method to make the polymer network coatings with the fingerprint texture polyimide 7511 L (Sunever, Nissan Chemical, Japan) was coated on a glass support. This liquid crystal alignment layer has the property to align nematic liquid crystal in a homeotropic with their long axes on average perpendicular to the support. In the case of chiral-nematic liquid crystals the same coating stimulates the formation of the fingerprint texture. The polyimide coating is spin coated (at 500 rpm during 20 seconds) on cleaned glass, followed by baking. A liquid crystal mixture consisting of liquid crystal acrylates, diacrylates, at least the chiral component and a monomer that contains the photoresponsive group is spin coated spin coated on the treated glass plates. The mixture comprises the following monomers:

The mixture comprises from 20 wt% of monomer 1), 44.57 wt% of monomer 2), 32 wt% of monomer 3), 0.43 wt% of monomer 4) and 2 wt% of monomer 5). The photointiator bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide was present in the mixture in an amount of 2 wt%. The solvent used is dichloromethane.

It is subsequently heated to 80° for 1 minute just above its clearing temperature to the isotropic phase and cooled to its chiral-nematic phase at 45 °C and cured by UV exposure at 45 °C for 5 minutes under N₂ using a mercury lamp (EXPR Omnicure S2000) equipped with a cut-off filter transmitting light > 400 nm (Newport FSQ-GG400 filter). The samples were post-baked at 120 °C under the N₂ condition to ensure full cure of the acrylate monomers.

In a specific embodiment the formation of the fingerprint is further stimulated by the addition of a small amount of a surfactant (0.6 w%) to the reactive monomer mixture.

Claims

Process for preparing a layer of a liquid crystal network, the layer having an image comprising random shapes and being capable of developing the image to a surface profile under the influence of an external stimulus, the process comprising

- applying a layer of a mixture comprising at least

3) a photoinitiator;

on a support having an orientation layer on which the monomers self- organize to form cholesteric liquid crystals comprising a helix having a helical axis that is perpendicular to the longitudinal axes of the monomers, wherein the helical axes are positioned substantially parallel to the support;

- photopolymerizing the monomers to form the layer of a liquid crystal network.

Process according to claim 1 , wherein the monofunctional monomer is a mono-acrylate and the difunctional monomer is a diacrylate.

Process according to claim 1 or 2, wherein the mixture comprises one or more monomers of the formula

where X is selected from the group of hydrogen, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an alkoxy carbonyl group having 1 to 20 carbon atoms, a formyl group, an alkyl carbonyl group having 1 to 20 carbon atoms, an alkyl carbonyloxy group having 1 to 20 carbon atoms, a halogen, a nitrile group and a nitro group;

where Ri is selected from the group of

where x is in the range of 1-20, and where R₃ is selected from the group of hydrogen, fluoro, chloro and methyl; and

where R₂ is selected from the group of chloro, fluoro, nitro, nitrile,

where x is in the range of 1-20 and where R3 is selected from the group of hydrogen, fluoro, chloro and methyl; where x is 0 - 17,

where x is 0 - 17,

where x and y are independently of each other 1 - 6 and Z is selected from the group of methyl, chloro, fluoro, nitro and nitrile.

4. Process according to any of claims 1 - 3, wherein the chiral

photopolymerizable liquid crystal monomer comprises one chiral moiety that is substituted with an R-group comprising an Regroup as defined in claim 3.

5. Process according to any of claims 1 - 4, wherein the support comprises glass and/or plastic.

6. Process according to any of claims 1 - 5, wherein the photoinitiator is a

dibenzoyl(phenyl)phosphine oxide, in particular bis(2,4,6- trimethylbenzoyl)phenylphosphine oxide.

7. Process according to any of claims 1 - 6, wherein the mixture contains a

polymerization inhibitor.

8. Process according to any of claims 1 - 7, wherein the orientation layer

comprises a compound selected from the group of a surfactant, a polyimide modified with alkyi tails and a silane compound having one or more alkyi tails.

9. Process according to any of claims 1 - 8 wherein the orientation layer is a homeotropic orientation layer.

10. Process according to any of claims 1 - 9, wherein the conditions under which the helical axes are positioned substantially parallel to the support are such that 0.1 < d l P_p < 0.8, in particular 0.3 < d I P_p < 0.5, where d is the thickness of the helix and P_p is the pitch of the helix.

11. A layer of a liquid crystal network obtainable by a process of any of claims 1 - 10.

12. A process for forming or amplifying a surface relief in a layer of a liquid crystal network according to claim 11 , comprising exposing the layer to

electromagnetic radiation of a wavelength at which the isomerization occurs in the compound capable of changing its conformation.

13. A layer of a liquid crystal network obtainable by the process of claim 12.

14. A layer according to claim 13, wherein the hills of the formed or amplified

surface profile are more than 600 nm higher than the valleys of the formed or amplified surface profile.

15. Use of a layer of a liquid crystal network according to claim 11 , 13 or 14 as an identification means and/or as a means for improving the grip of robotic equipment.