Comparison of optical properties of thermally reversible light scattering films consisting in dispersions of polystyrene/naphthalene domains or polystyrene/liquid crystal (EBBA) domains in epoxy matrices

Abstract

Thermally reversible light scattering (TRLS) films are materials that can be reversibly switched from opaque to transparent states by varying temperature. TRLS films containing different types of dispersed domains in an epoxy matrix: a) naphthalene/polystyrene (NP/PS), and b) liquid crystal (EBBA/PS), were synthesized by polymerization-induced phase separation. The intensity of transmitted light was measured as a function of temperature in successive heating/cooling cycles. Transparent states were observed during heating cycles at temperatures close to the melting point of NP (81 °C) or the nematic-isotropic transition of EBBA (at about 70 °C). The NP crystallization from their polystyrene solutions in the course of cooling cycles required large undercooling (about 30 °C). In the temperature range comprised between melting and crystallization, optical properties depend on the cooling rate and the thermal history of the material. The situation was quite different for TRLS films based on EBBA. In this case, the generation of a nematic phase from the polystyrene solution required a small undercooling (about 2-3 °C), leading to a relatively sharp transition between opaque and transparent states, independent of the thermal history. This constitutes a significant advantage of the use of liquid crystals compared to organic crystals in the manufacture of TRLS films.