Photoinduced Alignment of Ferroelectric Liquid Crystals Using Azobenzene Polymer Networks of Polyethers and Polyepoxides

Abstract

Several divinyl ether and diepoxide monomers bearing an azobenzene moiety were synthesized and used to investigate the photoalignment of a ferroelectric liquid crystal (FLC) with no use of surface orientation layers. Azobenzene polymer networks, obtained by cationic polymerization of the monomers dissolved in the FLC host and exposed to linearly polarized irradiation, were found to be able to induce and stabilize a bulk alignment of FLC. Photoinduced reorientation of the FLC can also be achieved by changing the polarization of irradiation light, but in contrast with the use of chiral azobenzene poly(meth)acrylates, the results suggest a mechanism based on the formation of an anisotropic azobenzene polyether or polyepoxide network for the commanding effect on the FLC alignment.

Azobenzene polymer stabilized ferroelectric liquid crystals: simultaneous photopolymerization and photoalignment

“…The synthesis of the azobenzene-containing diepoxide monomer used, whose chemical structure is shown below, has already been reported [9]. To perform photoinduced cationic polymerization of the monomer, the initiator system was composed of 50/50 (w/w) of diphenyliodonium hexafluorophosphate (DiPhIF 6 P), which is a cationic initiator, and the photoinitiator Irgacure 784 (Ciba).…”

“…1% of each initiator component. Similarly to AIBN used in thermal cationic polymerization [9], photoinduced radicals of Irgacure 784 act as electron donors to the iodonium salt to generate cationic reactive centres that initiate the polymerization [12]. In addition to the major absorption in the UV region, Irgacure 784 has significant absorption in the 400-500 nm region (spectrum not shown), which is some distance from the absorption maximum of the azobenzene monomer in the UV region, centred at about 375 nm.…”

“…In the case of FLCs, another benefit of having a polymer network is to improve the shock resistance of FLC cells [4]. In recent years, we have explored the use of azobenzene-containing polymer networks in PSLCs [5][6][7][8][9]. The trans-cis photoisomerization and the related photoalignment of the azobenzene chromophore can impart new features to these materials.…”

“…The trans-cis photoisomerization and the related photoalignment of the azobenzene chromophore can impart new features to these materials. In particular, we showed that the photo-orientation of azobenzene groups on the polymer network upon irradiation with linearly polarized light could be used to induce bulk alignment of a nematic LC [5,6] and FLC [7][8][9] with no need for surface orientation layers. The alignment is a bulk effect given by a 3-dimensional azobenzene polymer network dispersed in the LC host, which differs from the socalled commanding surfaces coated with azobenzene polymers as developed by Ichimura and co-workers [10].…”

“…The alignment is a bulk effect given by a 3-dimensional azobenzene polymer network dispersed in the LC host, which differs from the socalled commanding surfaces coated with azobenzene polymers as developed by Ichimura and co-workers [10]. We have conducted a systematic investigation on FLC photoalignment using various synthesized azobenzene-containing, network-forming monomers (diacrylates, dimethacrylates, divinyl ethers and diepoxides) and different commercially available FLC mixtures [7][8][9]. In all previous studies, polymerization of the azobenzene monomer dissolved in the FLC host was thermally initiated.…”