Fragility Nanoconfinement Effect in Thin Polymer Films: Novel Characterization by Ellipsometry

A novel ellipsometry-based method was introduced to determine kinetic fragility in polymer films and to investigate the effect of nanoscale confinement on polymer fragility. Three systems were studied: polystyrene (PS), polycarbonate (PC), and PS doped with small molecule diluents of 1,10-bis-(1-pyrene)decane (BPD). In bulk-like films, fragility index measured by ellipsometry agreed very well with that by differential scanning calorimetry. With confinement, a dramatic decrease in fragility was observed in highly fragile PS and PC. The fragility decreased by 58{\%} from 166 to 69 in PS and by 65{\%} from 214 to 75 in PC as film thickness decreased from bulk to 27-28 nm; a substantially muted response was observed in the strongest of the three: PS $+$ 2 wt{\%} BPD, where the fragility decreased only 21{\%} from 134 to 106 from a bulk film to a 27-nm-thick film. The larger fragility-confinement effect in more fragile polymers strongly correlates with a previous discovery of the $T_{\mathrm{g}}$-confinement effect: the strength of the $T_{\mathrm{g}}$-confinement effect increases with increasing fragility of bulk polymers. It indicates that bulk fragility is associated with the susceptibility of polymers to effects of nanoscale confinement.