Chapter 12
Enhancement in Response Speed of Bistable Switching for Composites of Liquid-Crystalline Polymers and Low-Molecular-Weight Liquid Crystals Tisato Kajiyama, Hirokazu Yamane, Hirotsugu Kikuchi, and Jenn Chiu Hwang 1
Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan The ternary composite was prepared from a miscible mixture of nematic liquid crystalline polymer (LCP) with a weak polar terminal unit in the mesogenic side group, nematic low molecular weight liquid crystal (LC) with a strong polar cyano end group (LC1) and LC with the similar chemical structure to the mesogenic side group of LCP (LC2). The reversible and bistable turbid-transparent states were successfully realized for the binary (LCP/LC1) and ternary (LCP/LC2/LC1) composites in an induced smectic phase upon the application of a.c. electricfieldswith low and highfrequencies,respectively. The response speed for reversible light switching could be remarkably improved by an addition of LC2 to the binary composite at room temperature. Theriseresponse time and decay one were a few seconds and several hundred ms, respectively. The molecular weight dependence of the electro-optical effect for the (LCP/LC) composite system was investigated by using LCPs with 12 or 40 of n. The magnitude of τ exhibited minimum for LCP with n=12, indicating that an appropriate length of polymeric main chain is required to realize an optimum condition for the balance between an electric field effect and an electric current effect D
The preparation methods and the novel permselective characteristics of (polymer/LC) composite films have been extensively studied (1-3). Also, various types of (polymer/LC) composite systems have been reported as large area and flexible lightintensity controllable films (4-11). Since thermotropic liquid crystalline polymers (LCPs) with mesogenic side chain groups exhibit both inherent mesomorphic properties of LC and excellent mechanical characteristics of polymeric materials, LCPs 1
Current address: Department of Chemical Engineering, Yuan-ze Institute of Technology, 135 Yuan-tung Road, Nei-li Chung-li 320, Taiwan
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12. KAJIYAMA ET AL.
Response Speed ofBistable Switching for Composites
have attracted a major attention due to their promising applications as electro-optical devices (72). However, since LCPs in a mesophase state are more viscous than LCs, the response time of LCPs to an external stimulation such as an electric or a magnetic field is fairly longer than that of LCs. Therefore, (LCP/LC) mixtures in which LC takes a role of solvent or diluent to LCP has been studied in order to reduce the magnitude of viscosity of LCP, in other words, to reduce the magnitude of response time for the electro-optical devices of LCP (13-16). It was reported that the binary mixtures of nematic LCs with both a strong polar terminal group (cyano or nitro) and a weak polar one formed an induced smectic phase (ISP) (17-20). Then, this concept was applied to the binary mixture composed of nematic LCP with a weak polar end group in the mesogenic side chain and nematic LC with a strong polar end group, resulting in the formation of an induced smectic phase (21-24). Then, the (LCP/LC) mixture in an induced smectic state is expected as a novel type of "light valve" which exhibits bistable and reversible light switching characteristics, that is, a memory effect. In this paper, the phase transition behavior , aggregation states, and also, a bistable and reversible light switching behavior based on light scattering have been investigated for the ternary (LCP/LC2/LC1) composite system in an induced smectic phase. Experimental Materials. The chemical structures of LCPs and LCs are shown in Figure 1. PS3EM is polymethylsiloxane with a weak polar terminal group in the mesogenic side group. The molecular weight of PS3EM is l.lxlO . 50CB and HPPB were used as LCs, that is, LCI and LC2, respectively. 50CB has a strong polar cyano terminal group and HPPB does the similar chemical structure to the mesogenic side chain group of PS3EM. The mesomorphic phase of the (PS3EM/HPPB) composite is nematic. The mixture of PS3EM/HPPB/50CB was dissolved in acetone and the ternary composite film was prepared by a solvent-casting method. To investigate the role of LCP in the composite on the electro-optical properties, PS[4BC/DM]s with the degree of polymerization of 12 and 40 were used as LCP. In order to investigate the role of LCD in the composite on the electrooptical effect, low molecular weight smectic LC, S2 was employed as a sample of composite in which any LCP was not included. S2 is a binary smectic mixture of low molecular weight liquid crystals as shown in Figure 1. The ionic impurities (Pt catalyst) of about 1000 ppm was added to S2 to unify the condition of electric current effect with the (PS[4BC/DM](n=12, 40)/E7) composites which contain about 1000 ppm of Pt catalyst. 4
Characterization of the composite system. The phase transition behaviors and the aggregation states of the binary and ternary (LCP/LC(s)) composites were investigated on the basis of differential scanning calorimetry (DSC), polarizing optical microscopic (POM) observation and X-ray diffraction studies. The heating and cooling rates for DSC study and POM observation were 5 and 1 K/min, respectively. X-ray diffraction
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