Photorefractive properties in homogeneous and heterogeneous polymer / liquid crystal composites ∗

The photorefractivity of a photorefractive composite consisting of poly(N-vinylcarbazole) (PVK) doped with a non-room-temperature liquid crystal, 4-butyloxy4’-cyanobiphenyl (4OCB), and C60 was studied by means of measuring the two-beam coupling coefficients. The results show that the photorefractivity is enhanced by phase separation. PACS: 42.65; 61.30; 61.40 Interest in materials that exhibit photorefractivity has been intense. More than 10 years ago, studies of the photorefractive effect were focused on inorganic crystals, such as LiNbO3, BaTiO3 and several others [1]. Since the first photorefractive effect in polymers was reported in 1991 [2], rapid progress has been made in the exploration of new materials and investigation of the mechanisms accounting for photorefractivity in this class of materials. In 1994, Meerholz et al. studied photorefractive composites with low glass transition temperature (Tg) and found that the two-beam coupling coefficient (Γ ) was over 220 cm−1 [3]. Later investigations proved that the high two-beam coupling coefficient was attributed to the orientational enhancement effect [4]. From then on, highly anisotropic non-linear chromophores have become important components in the photorefractive polymer composites [5, 6]. Liquid crystals are long rod-shaped molecules that can produce large birefringence, which would maximize the orientational enhancement effect. Thus, liquid crystals became other chromophores for photorefractive materials. Khoo et al. found that low-molar-mass nematic liquid crystals doped with fullerene show orientational photorefractive effects and presented a detailed discussion of the mechanism [7, 8]. In order to improve the resolution of the photorefractive lowmolar-mass liquid crystal, polymer dispersed liquid crystals (PDLCs) and highand low-molar-mass liquid-crystal mixtures were also investigated [9–13]. Zhang and Singer reported a homogeneous photorefractive composite consisting ∗This work was supported by the National Natural Science Foundation of China (59873001) ∗∗Corresponding author. (E-mail: [email protected]) of poly(N-vinylcarbazole) (PVK) doped with a low-molarmass liquid crystal and C60 [14]. Gain coefficients over 100 cm−1 were observed at high electric field (150 V/μm). However, the liquid crystals used as photorefractive chromophores up to now have been room-temperature liquid crystals. In fact, the number of non-room-temperature liquid crystals is much larger than that of room-temperature liquid crystals and many of the non-room-temperature liquid crystals are highly anisotropic. Thus, it is possible for the non-room-temperature liquid crystals to become another kind of photorefractive chromophores. On the other hand, phase separation is a common problem in low-Tg photorefractive polymeric composites [15] and should be avoided. But for a polymer/liquid crystal system, such as a polymer-dispersed liquid crystal, the phase separation is induced purposely. As far as we know, there is no report on effects of phase separation on photorefractive properties in a polymer/liquid crystal composite. In this study, we have chosen a non-room-temperature liquid crystal, 4-butyloxy-4’cyanobiphenyl (4OCB), as the non-linear chromophore, PVK as the photoconductive agent and matrix and C60 as a sensitizer. By controlling the sample-making process, two samples were obtained: one heterogeneous and one homogeneous. The effect of phase separation on photorefractive properties is discussed.