COMPARISON AMONG THERMALLY REVERSIBLE FILMS OBTAINED FROM DIFERENT ORGANIC SOLIDS.

ILEANA A. ZUCCHI; TATHIANA RESNIK; PATRICIA OYANGUREN; MARÍA J. GALANTE; ROBERTO JJ WILLIAMS

Los Cocos - Argentina

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

Thermally reversible light scattering (TRLS) films are materials that can be reversibly switched from opaque to transparent states by varying temperature. They have potential applications in 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. 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 opaque. In recent papers we showed that TRLS films consisting in dispersions of a crystalline solid (diphenyl, DP) / polystyrene (PS) domains or a liquid crystal (N-4-ethoxybenzylidene-4’-n-butylaniline, EBBA) / PS domains in epoxy matrices, could be synthesized in one step by polymerization-induced phase separation (Hoppe et al., 2004a, b; Zucchi et al., 2005). As PS is slightly compatible with the epoxy precursors but miscible in all proportions with the organic crystal (DP) or the liquid crystal (EBBA), above their melting or nematic-isotropic transition temperatures, a small amount of PS added to the initial solution produced phase separation of DP/PS or EBBA/PS domains during the polymerization reaction. This enabled to generate dispersions with primary morphologies that remained stable in the course of successive heating-cooling cycles. The order-disorder transition was confined to the interior of the dispersed domains that act as light-scattering centers of the TRLS films. The refractive indices of DP and EBBA in the amorphous state were the same as those of PS and the cured epoxy matrix. Therefore, in spite of being optically heterogeneous TRLS films were transparent above the melting temperature of DP or the nematic-isotropic transition temperature of EBBA. The films became opaque when the DP crystallized or EBBA was transformed to the nematic state in the interior of dispersed domains, due to the mismatching of refractive indices. Aims of this work are to analyze optical properties of materials based on naphthalene (NP) as organic crystal and to compare them with those of TRLS films based on EBBA.