Development of rigid amorphous fraction in bulk samples and confined films of semi-crystalline syndiotactic polystyrene: Studies by differential scanning calorimetry and ellipsometry

We investigated rigid amorphous fraction (RAF) in cold-crystallized, semi-crystalline syndiotactic polystyrene (sPS) as a function of crystallinity or crystalline fraction (CF). Characterization was by differential scanning calorimetry (DSC) for bulk samples with CF as high as 50% and ellipsometry for thin, confined films with CF as low as 1%. While DSC is commonly used to study RAF and its relationship to CF in bulk, semi-crystalline polymers, we developed for the first time a simple ellipsometry-based methodology for quantitative determination of CF and RAF in semi-crystalline films. Combining the results from bulk samples and thin films, with increasing CF during cold-crystallization we find that RAF in sPS increases from ~4% at 1% CF to ~16% at 10-25% CF and then decreases with increasing CF, achieving a RAF value of ~7% at the limiting CF of 50%. Importantly, there is a monotonic reduction in specific RAF, i.e., the ratio of RAF to CF, from 4.2 to 0.1 as CF increases from 1% to 50%, indicative of an increasing crystal perfection and extent of decoupling of amorphous and crystalline regions with cold-crystallization. We also observed a major dependence of maximum CF on film thickness, supporting a strong effect of near-nanoscale confinement on crystallization in sPS films.