Effect of Composition on The Morphology and Electro-Optical Properties of Physically Crosslinked Liquid Crystals
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Mat. Res. Soc. Symp. Proc. Vol. 559 ©1999 Materials Research Society
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Figure 1. Schematic diagram illustrating the electric responses of (a) a polymer-dispersed liquid crystal (PDLC) and (b) a physically crosslinked liquid crystal (PCLC). mechanical properties of a physical gel [ 19,20]. Analogous microstructures form in systems composed of a diblock copolymer and an isotropic solvent [2 1]. In LC solvents, the isotropic blocks of liquid crystalline diblock copolymers form thread-like microtubules [22] due to the elasticity of the solvent matrix. In this study, we examine the phase behavior and electro-optical properties of a trtiblock cc-polymer with a side-chain liquid crystal midblock in the presence of a nemnatic LC solvent. Due to the anticipated (but not yet confirmed) formation of a copolymer-stabilized network in blends with sufficiently high copolymer concentrations, we refer to such systems as physically crosslinked liquid crystal (PCLCs) to distinguish them from covalently-crosslinked liquid crystals. EXPERIMENT Materials The tritblock. copolymer employed in this work, designated SBS 1005b, was described in detail elsewhere [1231. It consisted of polystyrene endblocks and a polybutadiene midblock modified with mesogenic side chains, as shown in Scheme I. The number-average molecular weight of the copolymer was 84 kg/mol, and its composition was 12 wt% styrene. The anisotropic solvent used here was E7, a nemnatic cyanobiphenyl mixture manufactured and kindly supplied by Merck. Figure1. SChemaicdigam ilstro ating the elec05btriblcresoneof()apolymer -dimpersed iqsuidy lnt Methods The phase behavior and electro-optical response of mixtures of SBS 1005b (or homopolystyrene) with E7 were investigated by optical microscopy. Several optical microscopes equipped with heating stages were used to discern the phase stability of mixtures mounted between glass slides. Electro-optical responses were monitored by imbibing copolymer/E7 mixtures between commercially available ITO-coated glass slides and applying a potential across the slide.
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Mass Fraction Polystyrene Figure 2. TEM image of the SBS1005b liquid crystal triblock copolymer employed here [23]. Polystyrene cylinders are stained with RuO4.
Figure 3. Polystyrene/E7 phase diagram showing the n+i biphasic region required in the design of the SBS 1005b/E7 PCLC.
RESULTS Shown in Fig. 2 is a TEM image of the SBS1005b at ambient temperature. Both TEM and small-angle x-ray scattering (SAXS) have previously demonstrated [23] that this copolymer forms spherical micelles at temperatures above the nematic-+isotropic transition temperature. Below this temperature, the isotropic polystyrene blocks order into cylinders, which are evident in the image presented in Fig. 2, due to the elasticity of the liquid crystalline matrix. Upon spin-casting this copolymer into an ultrathin film (data not shown), the polystyrene cylinders appe
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