Controlled Crystallization of Small Organic Molecules using Polymers: the Role of Hidden Liquid-Liquid Phase Domain

We investigated the crystallization behaviors of model organic compound pyrene mixed with polystyrene using differential scanning calorimetry. The thermal characterization of the pyrene and polystyrene mixtures provided extensive thermodynamic information regarding the phase states and transition temperatures. Using the glass transition temperatures, the chemical compositions of phase-separated phases are elucidated. Remarkably, the crystallization temperature curve of pyrene of the mixtures shows a clear signature of upper critical solution temperature (UCST) with the composition. We attribute the UCST signature to the hidden liquid-liquid phase domain of pyrene and polystyrene in which the nucleation barrier of crystallization is significantly depressed by the spinodal decomposition of liquid phases. This result suggests that polymer additives can be effectively utilized to control the crystallization behaviors.