Liquid-Like Transitions in Unsaturated Polyester Resin by thermally Stimulated Polarization and Depolarization Measurements

The polarization current measurements have been performed as a function of slowly decreasing temperature on liquid unsaturated polyester resins, diluted by various amounts of styrene. The lnI vs. T-1 curves exhibit several straight-line segments. From these the activation energies, and the glass and liquid-like transition temperatures have been evaluated for diluted UP resins. The absolute values of the polarization current decrease by dilution with styrene and by irradiation in the (pre) gel state. The liquid-like transition has been confirmed by thermally stimulated depolarization (TSD). Another liquid-liquid transition has been found at higher temperatures. This transition, which is masked by motion of space charges in TSD, has been verified by viscosity measurements. The liquid-like transitions correspond to the thermally activated disintegration of local ordered structures, which were well documented in the inherently heterogeneous UP resin system.