Development of rigid amorphous fraction in cold-crystallized syndiotactic polystyrene films confined near the nanoscale: Novel analysis via ellipsometry

Rigid amorphous fraction (RAF) in syndiotactic polystyrene (sPS) thin films was studied as a function of crystalline fraction (CF). In contrast to bulk sPS, confined films yield close-to-amorphous sPS, thus allowing for a complete description of RAF development. To study thin-film samples, we developed an ellipsometry-based method to quantify CF and RAF. Maximum crystallinity was thickness dependent, indicating a strong effect of near-nanoscale confinement on CF. ~360-nm-thick spin-coated films were isothermally cold crystallized to tune CF from 1% to a maximum of 12%. Combining RAF characterized at low CF here by ellipsometry with results from an earlier differential scanning calorimetry (DSC)-based study on bulk sPS with higher CF, a complete picture was obtained of RAF development as a function of CF in cold-crystallized sPS. RAF increases from ~4% at 1% CF to ~16% for 10% < CF < 25%; above 25% CF, RAF decreases monotonically with increasing CF, reaching ~7% at a maximum 50% CF. Specific RAF (=RAF/CF) decreases with increasing CF, consistent with increasing crystal perfection with isothermal cold crystallization. This ellipsometry method can be an important tool for evaluating RAF, mobile amorphous fraction (MAF), and CF in semi-crystalline polymer films and for studying confinement effects on RAF, MAF, and CF.