CA2330894A1 - Optically aligned and network-stabilized ferroelectric liquid crystals using azobenzene-containing diacrylate monomers - Google Patents

Abstract

The invention is a technique that uses reactive diacrylate monomers carrying azobenzene groups to accomplish both the induction and the stabilization of a long-range, uniaxial molecular orientation of ferroelectric liquid crystals (FLCs) in the absence of rubbed surfaces. The process consists in dissolving 5-10 wt% of an azobenzene diacrylate monomer in a FLC host, and polymerizing the monomer while irradiating the mixture with linearly polarized light. The mechanism involves the formation of an optically induced anisotropic azobenzene polymer network that, in turn, induces and locks in the FLC orientation. Both thermal and photopolymerization can be used. In the former case, the polymerization can be carried out in either the isotropic or the nematic phase of the FLC host; while in the latter case, it can be proceeded in all phases. The irradiation is generally turned off after polymerization, but if the polymerization is performed in the isotropic phase, it should be turned off after cooling the sample into one of the liquid crystalline phases of the FLC host. The stabilization of the aligned chiral smectic-C phase is characterized by the recoverable long-range molecular orientation when cooled from the isotropic phase.

Description

Optically Aligned and Network-Stabilized Ferroelectric Liquid Crystals Using Azobenzene-containing Diacrylate monomers Yue Zhao~, Nadine paiement Dpartement de chimie, Universit de Sherbrooke, Sherbrooke, Qubec, Canada J1K
2R.1 (yzhao@courrier.usherb. ca) Background Ferroelectric liquid crystals (I'LCs) have a chiral smectic-C phase (S~~). In this phase, the chirality gives rise to a spontaneous polarization (electric dipole without the need for an electric field) for each layer, which is normal to the director 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 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 it averages to zero upon a pitch. 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 uniaxially along a certain direction. Currently, the only successful method to do that, which results in commercialized applications, is the surface-stabilized FLCs (SSFLCs)~.

In SSFLCs, a FLC compound is filled in an ITO (indium-tin-oxide)-coated glass cell 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 be developed, and a spontaneous polarization normal to the binding plates is obtained. 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 FCL-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 the wide applications of SSFLCs. Among them, it is difficult to make and sustain a uniform alignment, particularly over large areas, because of the very small cell gap (generally 2-4 um). Also, similar to other liquid crystal devices that use surface orientation layers, surface-rubbing process can generate dust and electrostatic charges that are serious problems for manufacturing of the 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 reasons explained above, the uniaxial alignment, leading to the unwinding of the helix, is the key step for every FL,C technology. Polymer-dispersed FLCs2, microphase-stabilized FLCs3 and ferroelectric elastomers4 are examples of the intensive research efforts that are underway. In these techniques, a mechanical shear is used to induce the alignment.
Recently, making use of the well-known photoisomerization-induced alignment of azobenzene molecules when exposed to linearly polarized light~'g, we have succeeded in aligning FLCs by light and fixing the molecular orientation by a polymer network, without the use of rubbed surfaces. This novel technique has been developed in our laboratory, and some results for nematic liquid crystals have been reported5'6 . The idea is to use a diacrylate monomer bearing azobenzene group in its structure to accomplish both the induction and the stabilization of the FLC orientation. The basic mechanism is as follows: when an azobenzene-containing monomer is mixed with a FLC host, irradiation of the mixture with linearly polarized light results in preferential orientation of the azobenzene monomer in the direction normal to the polarization of the irradiation light, and a subsequent polymerization of the monomer under irradiation leads to the formation of an oriented azobenzene polymer network that, in turn, is able to align and stabilize the FLC molecular orientation in various liquid crystalline phases including the chiral smectic-C phase. An example is given in Figure 1 that schematically illustrates the action of an oriented azobenzene network formed by irradiation and polymerization in the isotropic phase of the FLC host. When the mixture is cooled under irradiation, the anisotropy of the network induces a uniaxial molecular orientation of the FLC
in the chiral nematic phase; once the orientation is built up, the irradiation can be turned ofd; on further cooling, this orientation is retained as the FL,C host goes through phase transitions into the smectic-A (Sri) and the S~~ phase.
As compared to SSFLCs, the optically aligned and network-stabilized FLCs may have a number of advantages. First, the photoalignrnent of the azobenzene monomer or network is a bulk effect and can be uniform along the thickness direction;
consequently, the preparation of thicker samples is feasible. Second, as no rubbed surfaces are needed, the rubbing-related problems are eliminated, and the cost of fabrication may also be reducf:d.
The main disadvantage is the presence of a polymer network that can reduce the spontaneous polarization and increase the switching time to some extent. This optical technique is completely original as it uses azobenzene-containing diacrylate monomers to induce uniaxial orientation in FLCs and to stabilize this orientation by the azobenzene network; no rubbed surfaces are required. To the best of our knowledge, a number of studies were reported utilizing a polymer network in FLCs. Among the most representative and significant works, in one cases, 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 SSFLCs9. In all cases, however, the orientation mechanism was different;
rubbed surfaces were used to induce the FLC orientation, and the polymer networks contain no azobenzene moieties.
Experimental Details for the Invention The azobenzene-containing diacrylate monomer was synthesized in our laboratorys and.
has the chemical structure shown below =~~~0(C!-~)z~N/(C~)20~~==
,.
( ~~ ~:~ l _, N
N
[y No~
It is nonmesogenic and has a crystal meting temperature at 80 °C and the maximum absorption around 442 nm. The FLC host used, CS-1031, was purchased from CI~ISSO
Corporation (Japan). It is a ferroelectric smectic mixture having the following phase transition temperatures: Cr -12 °C Sc' 60 °C SA 85 °C N' 97 °C Iso. The helical pitch is 37 ~tm in the chiral nematic phase and 3 p,m in the Sc' phase. Typically, to prepare a mixture of CS-1031 with the azobenzene monomer, weighted amounts of both components were dissolved in a common solvent, T'HF, together with an initiator (~ 2 wt%) for polymerization; once a homogeneous solution was formed, the solvent was evaporated, and the mixture was dried in vacuum. For thermal polymerizations, the initiator was azobisisobutyronitrile (AIBN) purchased from Aldrich, while for photopolymerization it was Irgacure 907 provided by CIBA. The concentration of the azobenzene monomer that results in better alignment was found to be between 5 and 10 wt%. While warmed at about 50 °C, a freshly prepared mixture can then be cast betwef;n CaF2 windows to form thin films of about 4-5 Itm thick. For mixtures containing the photoinitiator, they can also be warmed at higher temperatures and flow-filled in 5-pm electrooptic cells. The cells used, purchased from E.H.C (Japan), have ITO-coated but non-rubbed surfaces. Various conditions for irradiation and polymerization were investigated. For thermal polymerization, it can be carried out either in the isotropic or the nematic phase of the FLC host; while temperatures in the SA and S'~ phases are too low to initiate thermal polymerization with AIBN. For photopolymerization, it can be performed in all phases of the FLC host. Using both polymerization methods, better alignment and homogeneity of the samples were obtained when the irradiation was applied at room temperature while heating the mixture to a specific phase for polymerization. The two examples below describe the experimental details on how to prepare optically aligned and network-stabilized FLCs without the use of rubbed surfaces.
Example I Thermal polymerization After warming a freshly prepared mixture, in a small bottle, containing 10 wt%
of the azobenzene monomer at 50 °C for 10 min, a spatula was used to deposit a small amount of the mixture uniformly on a CaF2 window (12-mrn diameter), and a second CaF2 window was then placed to sandwich the mixture (a slight pressure was necessary to obtain a uniform film). No spacers were used in this case; the amount of the mixture was chosen to give rise to a film having a thickness of about 4 arm. Afterward, the two windows were fixed inside a temperature-controlled optical oven (microscope hot stage:
purchased from lnstec) that was placed in front of a 1000-W Hg (Xe) lamp (Oriel). The distance separating the lamp and the sample was about 50 cm. The lamp was used with a polarizes for linearly polarized light and a filter allawing for the passage of light at 440 nm (spectral resolution: 80 nm; transmittance: 45%). The irradiation was turned on at room temperature while the sample was heated to 120 °C for polymerization in the isotropic phase. It took about 5 min for the sample to reach 120 ° C
and the polymerization at that temperature last 10 min. Finally the sample was cooled, under irradiation, to 90 °C (N' phase); then the irradiation was turned off and the sample coolf:d to room temperature.
To make sure that any obtained alignment in the sample was held by the azobenzene polymer network, following the preparation process the sample was reheated, in the absence of irradiation, to 120 °C for 10 min for equilibrium. It was then cooled to the N', SA and S'~ phases, and a number of techniques were employed to characterize the alignment. The example in Figure 2 shows a set of optical micrographs taken under crossed polarizers for this sample. Picture a) shows the texture of the mixture before the:
irradiation and polymerization process; while pictures b), c) and d) reveal the aligned FLC host in its N', S~1 and S'c phases, respectively, when cooled from the isotropic phase after the preparation process. Bulk alignment is visible in both the S~ and S'c phases, in the expected direction, i.e., normal to the polarization of the irradiation light. 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 polarization direction of the irradiations light; the sampling area covered almost the entire film (about 10-mm diameter). Figure 3 show the absorbance of the infrared band at 1430-crri 1, mainly arising from the phenyl groups of the FLC molecules, as a function of the angle between the polarization of the infrared beam and the normal of the polarization of the irradiation light. No infrared dichroism was observed in the isotropic phase, indicating the absence of a molecular orientation; once cooled into the Ns phase, the strong dichroism indicates the induction of a long-range molecular orientation; on further cooling this orientation is retained in the SA and S~c phases due to the network. Repeated heating-cooling cycles resulted in no change in the orientation. Moreover, polarized ultraviolet (UV) spectra (not shown) confirm that the azobenzene moieties on the network are oriented in the expected direction in all the phases.
Example 2 Photopolymerization A freshly prepared mixture containing 10 wt% of the azobenzene monomer was heated to 80 °C, and flow-filled in a S-trm, non-rubbed cell. 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. Before photopolymerization, the mixture was heated to 120 °C for 5 min, and then cooled to 90 °C for 10 min and to 50 °C for 30 min. As the linearly polarized irradiation was set at 440 nm, which was far from the absorption maximum of the photoinitiator, no polymerization was initiated. After the 30-min-stay at 50 °C, both the polarizer and the filter were removed for 20 seconds, during which the non-polarized broadband light led to the photopolymerization. After the 20 seconds, the irradiation was turned off 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 min for equilibrium. On cooling, similar to samples prepared by thermal polymerization, a long-range molecular orientation of the FLC host was observed in the N', SA and S~~ phases. Figure 4 shows an example of the optical micrographs taken under crossed polarizers for the cell. At room temperature, bulk alignment of the S~~ phase is obtained in the irradiated area while it is absent in non-irradiated areas.
Figure 4 also shows another feature for the aligned S'~ phase. Its stable morphology is characterized by the formation of parallel lines of the azobenzene polymer network, in the molecular orientation direction of the FLC host. After the preparation process, it generally takes several hours to have those lines appeared, which are absent in the SA and N' phases.
Fewer lines were observed for mixtures containing 5 wt% of the azobenzene network.
In this example, the exposure with linearly polarized light as well as the photopolymerization of the mixture was carried out using the same irradiation source. It seems that the very short time of photopolymerization using non-polarized broadband light (e.g. 20 seconds) could not randomize the aligned azobenzene orientation so that an anisotropic polymer network could be formed. We have also examined the use of two different irradiation sources, one for the alignment and one for the photopolymerization;
and found that alignment of FLCs could be achieved.

Claims (9)

1) A new optical technique for inducing a uniaxial molecular orientation in the chiral smectic-C phase of ferroelectric liquid crystals (FLCs), thus suppressing the helical structure, without the use of rubbed surfaces (surface orientation layers) has been invented. The technique consists in dissolving an optically active monomer, which can be photoaligned, as well as an initiator for polymerization in a FLC host, irradiating with linearly polarized light and polymerizing the monomer to obtain an anisotropic (oriented) polymer network that, in turn, is able to induce and stabilize the molecular orientation of the FLC host in the absence of rubbed surfaces.

2) The monomer in claim 1) is a diacrylate monomer that bears an azobenzene group.

3) The polymerization in claim 1) can be both thermal polymerization, performed in either the isotropic or the nematic phase of the FLC host, and photopolymerization carried out in all the phases of the FLC host.

4) The liquid crystals in claim 1) also include nematic liquid crystals.

5) For photopolymerization in claim 3), the photoalignment of the azobenzene monomer (or the azobenzene network) and the polymerization can be accomplished by using either a single light source or two different light sources (one for alignment and one for polymerization).

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