Thermally Reversible Light Scattering Films based on droplets of biphenyl/polystyrene solution dispersed in an epoxy matrix.

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

Brujas - Bélgica

Congreso; 8th European Symposium on Polymer Blends and Eurofillers.; 2005

UMH - Université de Mons-Hainaut (Prof. Philippe Dubois)

Films that can be thermally switched from opaque to transparent states, called thermally-reversible light scattering films (TRLS), have potential uses in privacy windows, displays, thermal sensors, active elements in thermal memory all-optical devices and other optical applications. Materials for these purposes consist of two phases that exhibit mismatch of refractive indices at a characteristic temperature, where one of the phases undergoes a disorder-order transition (i.e. crystallization or an isotropic-nematic transition). A typical TRLS film consists in a dispersion of domains of a relatively low molar mass that exhibit a high solubility in the polymer matrix that depends on temperature. This leads to a low activation energy for the variation of optical transmission with temperature which is one of the main drawbacks for practical applications. Besides, changes in morphologies in the course of successive cooling/heating cycles lead to corresponding variations in optical properties. It was shown that the mentioned drawbacks may be eliminated by adding a small amount of a thermoplastic polymer to a solution of a liquid crystal in thermoset precursors. The thermoplastic polymer must have a refractive index matching the one of the fully cured thermoset, and should exhibit a high compatibility with the crystal and a low compatibility with the thermoset precursors. This would lead to phase separation of thermoplastic/liquid crystal solutions at low conversions during the polymerization reaction. In this work, we studied an analogous system in which the liquid crystal is replaced for a crystalline solid in order to obtain a less expensive and more accessible thermoreversible material. A material with good thermorreversible optical properties was synthesized in one step by reaction-induced phase separation, starting from a solution of polystyrene and biphenyl in the precursors of an epoxy network (diglycidyl ether of bisphenol A (DGEBA)/meta-xilylenediamine (MXDA)).