Rigid amorphous fraction in isotactic polystyrene induced by cold crystallization: Characterization by differential scanning calorimetry and ellipsometry

Rigid amorphous fraction (RAF) in bulk and thin-film states of isotactic polystyrene (iPS) was studied as a function of crystalline fraction (CF). The CF increased with isothermal cold crystallization in initially amorphous, melt-quenched bulk iPS samples. Assessed by differential scanning calorimetry, RAF in bulk iPS increased and then decreased with increasing crystallization time and CF. Specific RAF, that is, RAF/CF, decreased with increasing CF. Using ellipsometry, similar behaviors of RAF and specific RAF as functions of CF were observed in initially amorphous iPS thin films. These results indicate that specific RAF in iPS exhibits a continuous decrease from slightly below 2 at very low CF to ~0.2 at the maximum CF, independent of sample type and characterization method. This trend of decreasing specific RAF is consistent with a progression of crystal perfection during cold crystallization which is associated with lamellae thickening and decoupling between crystalline and amorphous regions. Qualitatively similar behaviors of RAF and specific RAF have been reported in other semi-crystalline polymers and in our recent studies of syndiotactic polystyrene, indicating that this trend of decreasing specific RAF with increasing CF is general for many semi-crystalline polymers. Finally, maximum CF is strongly reduced by nanoscale confinement in thin iPS films.