WO2002056105A1

WO2002056105A1 - Method for optically aligning and network-stabilizing ferroelectric liquid crystals

Info

Publication number: WO2002056105A1
Application number: PCT/CA2002/000040
Authority: WO
Inventors: Yue Zhao; Nadine Paiement
Original assignee: Universite De Sherbrooke
Priority date: 2001-01-12
Filing date: 2002-01-11
Publication date: 2002-07-18
Also published as: CA2330894A1

Abstract

The method induces and stabilizes a long-range molecular orientation in a chiral smectic-C phase of a ferroelectric liquid crystal host. The ferroelectric liquid crystal host defines a plurality of layers each having a director and a normal. Chirality in the smectic-C phase gives rise in each layer to spontaneous polarization normal to the layer's director and normal. Chirality in the smectic-C phase also leads to rotation of the directors of successive layers about the normal of the layers to form a helical structure whereby no resultant spontaneous polarization is observed. The method comprises dissolving an optically self-aligning monomer in the ferroelectric liquid crystal host to form a mixture, this monomer self-aligning in response to linearly polarized light. The method also includes suppressing the helical structure in the chiral smectic-C phase of the ferroelectric liquid crystal to obtain the long-range molecular orientation by irradiating the mixture with linearly polarized light and polymerizing the monomer. This produces an anisotropic polymer network which, in turn, induces and stabilizes the above mentioned long-range molecular orientation.

Description

METHOD FOR OPTICALLY ALIGNING AND NETWORK- STABILIZING FERROELECTRIC LIQUID CRYSTALS

BACKGROUND OF THE INVENTION

1. Field of the invention:

The present invention relates to a new optical technique for inducing long-range molecular orientation in ferroelectric liquid crystals (FLCs).

2. Brief description of the prior developments:

FLCs have a chiral smectic-C phase (S*_c). In this phase the chirality gives rise, for each layer of molecules, to a spontaneous polarization (formation of electric dipoles without the need for an electric field) which is normal to both the director (molecular orientation direction) and the layer normal. However, the chirality also leads to rotation of the directors of successive layers about the layers' normal, thus forming a helical, structure whose pitch is the distance needed for a complete 360° rotation of the director. In a bulk S*_c phase, the helical structure is free to develop and, consequently, no spontaneous polarization can be observed because the polarization vectors cancel each other over a pitch length. Therefore, the condition to obtain and to make use of the spontaneous polarization of FLCs is to suppress the helical structure by aligning the FLC molecules along a certain direction. Currently, the only successful method for reaching this condition, which resulted in commercial applications, is the surface-stabilized FLCs (SSFLCs) (N.A. Clark, S . Lagerwall Appl. Phys. Lett. 1980, 36, 899).

In SSFLCs, a FLC compound is filled in an ITO (indium-tin-oxide)- coated glass cell. This cell comprises binding plates, defining between them a gap and whose inner surfaces are parallely rubbed. When the cell gap is less than the helical pitch while the rubbed surfaces tend to align the molecules in the rubbing direction, no helix can develop, and a spontaneous polarization normal to the binding plates is established. As the polarity of an electric field applied across the cell changes, the polarization switches between two (up and down) states, which correspond to two stable orientation states of the FLC molecules around the layers' normal. This switching is the basis for many applications of FLCs such as displays and light modulators.

The FLC-based devices have a number of important advantages over other types of liquid crystal devices, such as a much faster switching speed and a viewing-angle independent contrast. However, some drawbacks limit wide application of SSFLCs. Among them is the difficulty to induce and sustain a uniform alignment, particularly over large areas, because of the very small cell gap (generally 2-4 μm). Also, similar to other liquid crystal devices that use surface orientation layers, the surface-rubbing process can generate dust and electrostatic charges which constitute serious problems upon manufacturing such devices. Despite those difficulties, the great potential of FLCs has been the driving force for considerable, and increasing, worldwide research efforts in developing FLCs since the discovery of SSFLCs in 1980. For the above reasons, the long-range molecular alignment, leading to the unwinding of the helix, is the key step for every FLC technology. Polymer-dispersed FLCs (H.Molsen, H.-S. Kitzerow, J. Appl. Phys. 1994, 15, 710), microphase-stabilized FLCs (G. Mao, J. Wang, C.K. Ober, M. Brehmer, M.J. O'Rourke, E.L. Thomas, Chem. Mater. 1998, 10, 1538) and ferroelectric elastomers (H. Poths, G. Andersson, K. Skarp, R. Zentel, Adv. Mater. 1992, 4, 12) are examples of the intensive research efforts that are underway. In these techniques, a mechanical shear is used to induce the alignment.

Also, a number of studies were reported utilizing a polymer network in FLCs. Among the most representative and significant works, in one case (R.A.M. Kikmet, M. Michielsen Adv. Mater. 1995, 7, 300) the stabilization of FLC alignment by the network was demonstrated for thick films (cells), while in another case the polymer network was used to improve the mechanical properties such as the shock resistance of SSFLCs (C.A. Guymon, L.A. Dougan, P.J. Martens, N.A. Clark, D.M. Walba, ON. Bowman Chem. Mater. 1998, 10, 2378). In all cases, however, rubbed surfaces were used to induce the FLC orientation.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method for inducing and stabilizing a long-range molecular orientation in a chiral smectic-C phase of a ferroelectric liquid crystal host, wherein the ferroelectric liquid crystal host defines a plurality of layers each having a director and a normal, and wherein chirality in the smectic-C phase (a) gives rise in each layer to spontaneous polarization normal to the layer's director and normal and (b) leads to rotation of the directors of successive layers of the ferroelectric liquid crystal host about the normal of the layers to form a helical structure whereby no resultant spontaneous polarization is observed. This method comprises: dissolving an optically self-aligning monomer in the ferroelectric liquid crystal host to form a mixture, this monomer self-aligning in response to linearly polarized light; and suppressing the helical structure in the chiral smectic-C phase of the ferroelectric liquid crystal to obtain the long-range molecular orientation by:

- irradiating the mixture with linearly polarized light; and

- polymerizing the monomer; to produce an anisotropic polymer network which, in turn, induces and stabilizes the long-range molecular orientation.

The present invention also relates to a ferroelectric liquid crystal system, comprising a ferroelectric liquid crystal host having a given stabilized molecular orientation, and an optically induced anisotropic polymer network within this ferroelectric liquid crystal host for stabilizing the molecular orientation.

The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non restrictive description of an embodiment thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the appended drawings, in order to schematically illustrate the induction and stabilization of a long-range molecular orientation of FLCs by an oriented azobenzene polymer network formed by irradiation and polymerization in the isotropic phase of the FLC host:

Figure 1a schematically shows a mixture of azobenzene- containing diacrylate monomer and FLC host in the isotropic phase of this FLC host;

Figure 1b schematically shows the mixture during irradiation and polymerization of the azobenzene-containing diacrylate monomer in the isotropic phase of the FLC host;

Figure 1c schematically shows the mixture cooled to a chiral nematic phase (N*) of the FLC host under irradiation;

Figure 1d schematically shows the mixture cooled to a smectic-A (S_A) phase of the FLC host under irradiation; and

Figure 1e illustrates the mixture cooled to the S*_c phase of the

FLC host under irradiation.

Also in the appended drawings:

Figure 2a is a polarized optical micrograph showing the texture of the mixture of FLC host and azobenzene-containing diacrylate monomer in the S*_c phase at 25 °C prior to irradiation and polymerization;

Figure 2b is a polarized optical micrograph showing the bulk alignment in the N^* phase of the mixture of FLC host and azobenzene- containing diacrylate monomer at 95 °C, where P indicates the direction of polarization of the irradiation light;

Figure 2c is a polarized optical micrograph showing the bulk alignment in the S_A phase of the mixture of FLC host and azobenzene- containing diacrylate monomer at 75 °C;

Figure 2d is a polarized optical micrograph showing the bulk alignment in the S*_c phase of the mixture of FLC host and azobenzene- containing diacrylate monomer at 55 °C;

Figure 3 is a graph showing the absorbance of the infrared band at 1430 cm^"1, mainly arising from the phenyl groups of the FLC molecules, as a function of the angle between the polarization of the infrared beam used for the polarized infrared spectroscopy and the normal of the polarization of the irradiation light;

Figure 4a is a polarized optical micrograph showing bulk alignment of the S*_c phase observed in a light-irradiated area, this optical micrograph also showing a stable morphology of the aligned S*_c phase characterized by the formation of parallel lines of the azobenzene- containing polymer network in the molecular orientation direction of the FLC host; and

Figure 4b is a polarized optical micrograph showing the absence of bulk alignment of the S*_c phase in a non irradiated area.

DETAILED DESCRIPTION OF EMBODIMENTS The present invention originates from the discovery that the photoisomerization-induced alignment of azobenzene molecules when exposed to linearly polarized light (K. Ichimura, Chem. Rev. 2000, 100, 1847; and A. Natansohn, P. Rochon, J. Gosselin, S. Xie, Macromolecules 1992, 25, 2268) can be used for aligning FLCs and maintaining the molecular orientation by a polymer network. For example, one can use a diacrylate monomer having a structure bearing azobenzene group(s) to accomplish both the induction and the stabilization of a long-range molecular orientation of FLC in the absence of rubbed surfaces. More specifically, the process comprises dissolving 5-10 wt% of an azobenzene-containing diacrylate monomer in a FLC host, irradiating the mixture with linearly polarized light, and polymerizing the monomer.

The basic mechanism is as follows: when an azobenzene- containing diacrylate monomer is mixed with a FLC host, irradiation of the mixture with linearly polarized light results in preferential orientation of the azobenzene-containing diacrylate monomer in the direction normal to the polarization of the irradiation light. Subsequent polymerization of the monomer under irradiation leads to the formation of an optically induced anisotropic azobenzene polymer, network that, in turn, induces and stabilizes the FLC molecular orientation. This molecular orientation of the FLCs can be induced and stablized in various liquid crystalline phases including the chiral smectic-C phase.

Both thermal polymerization and photopolymerization can be used. In both cases, polymerization carried out in the isotropic phase of the FLC host has resulted in a good alignment of the FLC host when cooled under irradiation into the chiral smectic-C phase. The irradiation is turned off after polymerization and cooling into the chiral smectic-C phase. Stabilization of the aligned chiral smectic-C phase is characterized by the recoverable long-range molecular orientation when cooled repeatedly from the isotropic phase afterward.

Figures 1a-1e schematically illustrate the induction and stabilization of a long-range molecular orientation of FLCs by an oriented azobenzene polymer network formed by irradiation and polymerization in the isotropic phase of the FLC host. More specifically:

- Figure 1a illustrates a mixture of azobenzene-containing diacrylate monomer and FLC host in the isotropic phase of this FLC host;

- Figure 1b illustrates the mixture during irradiation and polymerization of the azobenzene-containing diacrylate monomer in the isotropic phase of the FLC host;

- Figure 1c illustrates the mixture cooled to the chiral nematic phase (N*) of the FLC host under irradiation;

- Figure 1d illustrates the mixture cooled to the smectic-A (S_A) phase of the FLC host under irradiation; and

- Figure 1e illustrates the mixture cooled to the S*_c phase of the FLC host under irradiation.

When the mixture is cooled under irradiation, the anisotropy of the network induces a long-range molecular orientation of the FLC in the chiral nematic phase. On further cooling, this orientation is maintained as the FLC host goes through phase transitions into the S_A and S*_G phases. The following two examples describe how optically aligned and network-stabilized FLCs can be prepared without the use of rubbed surfaces.

Example 1 : Thermal polymerization

An azobenzene-containing diacrylate monomer having the following chemical structure was synthesized:

This azobenzene-containing diacrylate monomer is nonmesogenic and has a crystal melting temperature of about 80 °C and a maximum absorption around 442 nm.

The FLC host used, CS-1031 , was purchased from CHISSO Corporation (Japan). This FLC host is a ferroelectric smectic mixture having the following phase transition temperatures: Cr (crystalline phase) - 12°C S^* _c 60°C S_A 85°C N* 97°C Iso (Isotropic). The helical pitch is 37 μm in the N* phase (close to the N*-S_A transition) and 3 μm in the S*_c phase.

Typically, to prepare a mixture of CS-1031 and azobenzene- containing diacrylate monomer, weighted amounts of both components were dissolved in a common solvent, THF (tetrahydrofuran), together with an initiator (∞ 2 wt%) for polymerization. Once a homogeneous solution was formed, the solvent was evaporated, and the mixture was dried under vacuum. For thermal polymerization, the initiator was azobisisobutyronitrile (AIBN) purchased from the Aldrich company.

After warming a freshly prepared mixture, in a small bottle, containing 10 wt% of the azobenzene-containing diacrylate monomer at 50 °C for 10 minutes, a spatula was used to deposit a small amount of the mixture uniformly on a CaF₂ window having a diameter of 12 mm. A second CaF₂ window was then superposed to the mixture to sandwich that mixture. A slight pressure was required to obtain a uniform film. No spacers were used and the amount of mixture was chosen to give rise to a film having a thickness of about 4 μm.

Afterward, the two windows were fixed inside a temperature- controlled optical oven, for example a microscope hot stage purchased from the Instee company. This temperature-controlled optical oven was placed in front of a 1000-W Hg (Xe) lamp (Oriel). The distance separating the lamp and the sample (including the mixture) was about 50 cm. The lamp was used with a polarizer producing linearly polarized light and a filter allowing for the passage of light at 440 nm; this filter had a spectral resolution of 80 nm and a transmittance of 45%. Figure 2a is a polarized optical micrograph showing the texture of the mixture of FLC host and azobenzene-containing diacrylate monomer in the S*_c phase at 25 °C prior to irradiation and polymerization.

The lamp was turned on at room temperature to irradiate the sample (including the mixture) while this sample was heated to 120 °C for polymerization in the isotropic phase. It took about five (5) minutes for the sample (including the mixture) to reach 120 °C and the polymerization at that temperature lasts 10 minutes.

Then the sample was cooled to room temperature under irradiation. This completed the preparation process.

To make sure that any obtained alignment in the sample was held by the azobenzene-containing polymer network, following the preparation process the sample was re-heated, in the absence of irradiation, to 120 °C for 10 minutes for equilibrium. It was then re-cooled successively to the N*, S_A and S*_c phases; the polarized optical micrographs of Figures 2b-2d were taken during this re-cooling of the sample. Figure 2b is a polarized optical micrograph showing the bulk alignment in the N* phase of the mixture of FLC host and azobenzene-containing diacrylate monomer at 95 °C where P of Figure 2 indicates the direction of polarization of the irradiation light, Figure 2c is a polarized optical micrograph showing the bulk alignment in the S_A phase of the mixture of FLC host and azobenzene-containing diacrylate monomer at 75 °C, and Figure 2d is a polarized optical micrograph showing the bulk alignment in the S*_c phase of the mixture of FLC host and azobenzene-containing diacrylate monomer at 55 °C.

A number of techniques were employed to characterize the alignment. A first technique was the optical micrographs of Figures 2a-2d, taken under crossed polarizers of the sample of concern. As indicated in the foregoing description:

- Figure 2a is a polarized optical micrograph showing the texture of the mixture of FLC host and azobenzene-containing diacrylate monomer in the S^* _c phase at 25 °C prior to irradiation and polymerization;

- Figure 2b is a polarized optical micrograph showing the bulk alignment in the N* phase of the mixture of FLC host and azobenzene- containing diacrylate monomer at 95 °C;

- Figure 2c is a polarized optical micrograph showing the bulk alignment in the S_A phase of the mixture of FLC host and azobenzene-containing diacrylate monomer at 75 °C; and

- Figure 2d is a polarized optical micrograph showing the bulk alignment in the S*_c phase of the mixture of FLC host and azobenzene- containing diacrylate monomer at 55 °C.

Bulk alignment is visible in both the S_A and S*_c phases (Figures 2c and 2d), in the expected direction, i.e., normal to the polarization P of the irradiation light.

According to a second technique, the average orientation of the FLC molecules was characterized by polarized infrared spectroscopy. The spectra were recorded in the various phases during the cooling, with the infrared beam polarized parallel and perpendicular to the direction of polarization of the irradiation light. The sampling area covered almost the entire film having a diameter of about 10 mm. Figure 3 is a graph showing the absorbance of the infrared band at 1430 cm^"1, mainly arising from the phenyl groups of the FLC molecules, as a function of the angle between the polarization of the infrared beam used for the polarized infrared spectroscopy and the normal of the polarization of the irradiation light. The graph of Figure 3 reveals no infrared dichroism in the isotropic phase, which indicates the absence of a molecular orientation. Once cooled in the N* nematic phase, the strong dichroism indicates the induction of a long-range molecular orientation. Upon further cooling this orientation is retained in the S_A and S*_c phases due to the produced polymer network. Repeated heating-cooling cycles resulted in no change in the orientation. Moreover, polarized ultraviolet (UV) spectra (not shown) confirmed that the azobenzene moieties in the polymer network are oriented in the expected direction in all the phases of the FLC host.

Example 2: Photopolymerization

Again, an azobenzene-containing diacrylate monomer having the following chemical structure was synthesized:

The FLC host used, CS-1031 , was purchased from CHISSO Corporation (Japan). This FLC host is a ferroelectric smectic mixture having the following phase transition temperatures: Cr (crystalline phase) - 12°C S^* _c 60°C S_A 85°C N* 97°C Iso (Isotropic). The helical pitch is 37 μm in the N* phase (close to the N^*-S_A transition) and 3 μm in the S*_c phase. Typically, to prepare a mixture of CS-1031 and azobenzene- containing diacrylate monomer, weighted amounts of both components were dissolved in a common solvent, THF (tetrahydrofuran), together with an initiator (« 2 wt%) for polymerization. Once a homogeneous solution was formed, the solvent was evaporated, and the mixture was dried under vacuum. For photopolymerization, the initiator was Irgacure 907 commercialized by the CIBA company. A freshly prepared mixture containing 10 wt% of the azobenzene- containing diacrylate monomer was heated to 80 °C and, then, flow-filled in a 5-μm electrooptic cell. The cell used, purchased from E.H.C (Japan), had ITO-coated but non rubbed surfaces.

The cell was then fixed inside the optical oven and placed in front of the lamp.

The irradiation was turned on at room temperature, using the same polarizer and filter as indicated above.

For photopolymerization, the mixture was heated to 120 °C for 10 minutes under linearly polarized irradiation at 440 nm. Afterward, both the polarizer and the filter were removed for 20 seconds, during which the non-polarized broadband light performed the photopolymerization.

After these 20 seconds, the initial linearly polarized irradiation was turned on again and the sample was cooled to room temperature.

In the absence of irradiation, the cell was reheated to 120 °C, in the isotropic phase, for 10 minutes for equilibrium. During cooling, similar to samples prepared by thermal polymerization, a long-range molecular orientation of the FLC host was observed in the N*, S_A and S*_c phases.

Figures 4a and 4b show an example of the optical micrographs taken under crossed polarizers for the cell. At room temperature, bulk alignment of the S*_c phase is observed in an irradiated area (Figure 4a) but is absent in a non irradiated area (Figure 4b). Figure 4a also shows a stable morphology of the aligned S*_c phase; this stable morphology is characterized by the formation of parallel lines of the azobenzene- containing polymer network, in the molecular orientation direction of the FLC host. Generally, these parallel lines appear several hours after the preparation process. These lines are absent from the S_A and N* phases. Fewer lines were observed for mixtures containing 5 wt% of azobenzene.

In Example 2, the exposure to linearly polarized light as well as the photopolymerization of the mixture was carried out using the same irradiation source (lamp). It appears that the very short time required for the photopolymerization with non-polarized broadband light (e.g. 20 seconds) was insufficient to disturb the azobenzene orientation so that an anisotropic polymer network could be formed. The use of two different irradiation sources was also explored, one for the alignment and one for the photopolymerization. The monomer could then be photopolymerized under irradiation of the sample with linearly polarized light. It was found that alignment of FLCs could still be achieved in this manner.

A concentration of azobenzene-containing diacrylate monomer ranging between 5 and 10 wt% was found to result in better alignment of the FLC host.

As compared to SSFLCs, the optically aligned and network- stabilized FLCs present, amongst others, the following advantages:

- the photoalignment of the azobenzene monomer or network is a bulk effect and can be uniform along the thickness direction whereby the preparation of thicker samples is feasible; and

- as no rubbed surfaces are needed, the rubbing-related problems are eliminated, and the cost of fabrication may also be reduced. The main disadvantage is the presence of a polymer network that can reduce the spontaneous polarization and increase the switching time to some extent.

To unwind the helical structure and allow for the macroscopic electric dipole to form without the need for an electric field (spontaneous polarization), the foregoing specification has described the addition of an azobenzene-containing diacrylate monomer in a FLC host, followed by irradiation and polymerization. It is also within the scope of the present invention to use other suitable monomers such as azobenzene-containing chiral and non-chiral diacrylates, dimethacrylates, divinylethers and diepoxides.

Although the present invention has been described hereinabove by way of embodiments thereof, these embodiments can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.

Claims

WHAT IS CLAIMED IS:

1. A method for inducing and stabilizing a long-range molecular orientation in a chiral smectic-C phase of a ferroelectric liquid crystal host, wherein the ferroelectric liquid crystal host defines a plurality of layers each having a director and a normal, and wherein chirality in said smectic-C phase ( a) gives rise in each layer to spontaneous polarization normal to the layer's director and normal and ( b) leads to rotation of the directors of successive layers of the ferroelectric liquid crystal host about the normal of said layers to form a helical structure whereby no resultant spontaneous polarization is observed, said method comprising: dissolving an optically self-aligning monomer in the ferroelectric liquid crystal host to form a mixture, said monomer self-aligning in response to linearly polarized light; and suppressing the helical structure in said chiral smectic-C phase of the ferroelectric liquid crystal to obtain said long-range molecular orientation by:

- irradiating the mixture with linearly polarized light; and

- polymerizing the monomer; to produce an anisotropic polymer network which, in turn, induces and stabilizes said long-range molecular orientation.

2. A method as recited in claim 1 , further comprising, prior to irradiating the mixture and polymerizing the monomer, dissolving an initiator for polymerization in the ferrroelectric liquid crystal host.

3. A method as recited in claim 1 , wherein the optically self- aligning monomer comprises an azobenzene-containing diacrylate monomer.

4. A method as recited in claim 1 , wherein polymerizing the monomer comprises thermally polymerizing the monomer in an isotropic phase of the ferroelectric liquid crystal host.

5. A method as recited in claim 1 , wherein polymerizing the monomer comprises polymerizing the monomer under irradiation of the mixture with the linearly polarized light.

6. A method as recited in claim 1, comprising turning off irradiation of the mixture with linearly polarized light after polymerization of the monomer.

7. A method as recited in claim 1 , comprising thermally polymerizing the monomer in an isotropic phase of the ferroelectric liquid crystal host, and turning off irradiation of the mixture with linearly polarized light after cooling of the ferroelectric liquid crystal host to room temperature.

8. A method as recited in claim 2, wherein polymerizing the monomer comprises thermally polymerizing the monomer, and wherein the initiator for polymerization comprises azobisisobutyronitrile.

9. A method as recited in claim 1 , wherein polymerizing the monomer comprises thermally polymerizing the monomer, and wherein said method comprises:

- turning on irradation of the mixture with linearly polarized light at room temperature;

- heating said mixture for polymerization in an isotropic phase of the ferroelectric liquid crystal; and - cooling said heated mixture under irradiation to room temperature.

10. A method as recited in claim 1 , wherein polymerizing the monomer comprises photopolymerizing the monomer.

11. A method as recited in claim 1 , wherein polymerizing the monomer comprises photopolymerizing the monomer in an isotropic phase of the ferroelectric liquid crystal host.

12. A method as recited in claim 2, wherein polymerizing the monomer comprises photopolymerizing the monomer, and wherein the initiator for photopolymerization comprises Irgacure 907.

13. A method as recited in claim 10, comprising turning off irradiation of the mixture with linearly polarized light during photopolymerization of the monomer.

14. A method as recited in claim 10, comprising photopolymerizing the monomer with non-polarized broadband light under irradation of the mixture with linearly polarized light.

15. A method as recited in claim 10, wherein irradiating the mixture and polymerizing the monomer comprise using a single light source.

16. A method as recited in claim 10, wherein irradiating the mixture comprises using a first light source, and photopolymerizing the monomer comprises using a second light source.

17. A ferroelectric liquid crystal system, comprising: a ferroelectric liquid crystal host having a given stabilized molecular orientation; and an optically induced anisotropic polymer network within said ferroelectric liquid crystal host for stabilizing said molecular orientation.

18. A ferroelectric liquid crystal system as defined in claim 17, wherein said optically induced anisotropic polymer network is an optically induced anisotropic azobenzene polymer network.