Fragility-Confinement Effects: Apparent Universality as a Function of Scaled Thickness in Films of Freely Deposited, Linear Polymer and Its Absence in Densely Grafted Brushes

Ellipsometry measurements as a function of cooling rate are used to study nanoscale confinement effects on dynamic fragility (kinetic fragility), m, in supported films of freely deposited, linear polymer. Polymers include neat polystyrene (PS), neat polycarbonate (PC), and PS + 2 wt % 1,10-bis(1-pyrene)decane (BPD) as small-molecule diluent; in each case, the substrate/polymer interface lacks significant attractive interactions. In terms of both the length scale at which confinement effects become evident and the percentage reduction in m from its bulk value, the magnitude of the m-confinement effect increases with increasing bulk polymer system m. Additionally, for films of linear polymer lacking significant attractive interactions with the substrate surface, m-confinement effects are evident at larger onset thicknesses than those commonly reported in the literature for the glass transition temperature (T_g)-confinement effect. Evans et al. [ Macromolecules 2013, 46, 6091 ] found that the T_g-confinement effect in related films exhibits a universal nature as a function of scaled thickness. Fragility-confinement effects of films of freely deposited, linear polymer chains exhibit a similar universal nature as a function of scaled thickness using shift factors consistent with those used by Evans et al. However, when PS is confined in a dense brush with one end of each chain covalently attached to the substrate surface, both m and T_g are independent of brush thickness. The strong correlation of fragility-confinement and T_g-confinement effects has important implications for understanding the fundamental natures of both the T_g-confinement effect and the glass transition itself.