Rigid amorphous fraction and crystallinity in cold-crystallized syndiotactic polystyrene: Characterization by differential scanning calorimetry

The development of rigid amorphous fraction (RAF) in semi-crystalline syndiotactic polystyrene (sPS) with increasing crystallinity or crystalline fraction (CF) was investigated using standard differential scanning calorimetry (DSC). Bulk sPS samples with crystallinity levels as low as 11% were prepared via melt-quenching of compression-molded sPS; close-to-amorphous bulk sPS samples are difficult to obtain due to the intrinsic high crystallization rate of sPS. Cold-crystallization procedures were conducted on quenched sPS samples by isothermal annealing at different temperatures and timeframes. After cold-crystallization, the CF, RAF, and mobile amorphous fraction (MAF) of sPS samples were determined by DSC analysis based on heat capacity information. With increasing CF during cold-crystallization, RAF was maintained at ~16% in a CF range from 11 to ~25% and then gradually decreased to ~7% when the growth of CF ceased at ~50%. Specific RAF, i.e., the ratio of RAF to CF, exhibited a continuous reduction from 1.4 to 0.1 with increasing CF, indicating increasing crystal perfection and extent of decoupling of amorphous and crystalline regions with increasing cold-crystallization. Effects of sPS polymorphism (α vs. β forms of crystal cells) on RAF were also briefly considered. The α form, understood to be obtained to the exclusion of the β form by sPS cold-crystallization or at increasing levels with decreasing isothermal melt-crystallization temperature, leads to higher specific RAF than the β form at a given CF.