INFLUENCE OF THE ADDITION OF PS ON THERMALLY REVERSIBLE LIGHT SCATTERING MATERIALS BASED ON DP/EPOXY BLENDS

ILEANA A. ZUCCHI; MARÍA J. GALANTE; ROBERTO JJ WILLIAMS

Los Cocos - Argentina

Congreso; III Simposio Binacional de Polímeros Argentino-Chileno (Archipol III).; 2005

Thermally reversible light scattering films (TRLS) are materials that can be repeatedly switched from transparent to opaque states by varying temperature. They have potential applications in privacy windows, thermal sensors, optical devices, recording media and several other applications (Dabisch et al., 1980; Hotta et al., 1995; Mucha, 2003). In most cases these materials are based on multiphase blends with one of the phases undergoing an order-disorder transition at a particular temperature (e.g. nematic-isotropic, crystalline-isotropic). In the isotropic (disordered) state every phase has the same refractive index and the material is transparent. In the ordered state there is a mismatching of refractive indices and the material is translucent or opaque. Polymer-dispersed liquid crystals (PDLC) can be employed as thermoreversible materials (Mucha, 2003; Hoppe et al., 2004a,b). However, the expensive liquid crystal can be replaced by a low-molar-mass organic molecule that crystallizes and melts at convenient temperatures. But there is also the need to generate reproducible morphologies in successive synthesis. And this is a main problem because of the several parameters affecting the crystallization process (impurities, cooling rate, presence of crystal seeds, etc.). For example, TRLS films based on dispersions of behenic acid in different polymer matrices showed that crystalline-phase morphologies and corresponding optical properties depended on the selected thermal cycles (Hotta et al., 1995).          In this work we show that it is possible to synthesize TRLS films exhibiting invariant morphologies and repeatable optical properties in the course of successive thermal cycles based on solutions of an organic molecule (diphenyl, DP) in epoxy precursors (diglycidyl ether of bisphenol A, DGEBA and m-xylylenediamine, MXDA). This was achieved using a similar principle that we developed for PDLC films (Hoppe et al., 2004a), consisting in adding a small amount of a linear polymer with a similar refractive index than the two other constituents to the initial solution (polystyrene, PS). Polymerization-induced phase separation led to a dispersion of PS/DP domains in the epoxy matrix. In this way melting and crystallization processes were confined to the interior of dispersed domains leading to an excellent reproducibility of the optical properties of TRLS films during successive heating-cooling cycles.