Interfacial mobility of polymers on inorganic solids

The segmental mobility of a typical amorphous polymer, polystyrene, at the interfaces with solid substrates was noninvasively examined by fluorescence lifetime measurements using evanescent wave excitation in conjunction with coarse-grained molecular dynamics simulation. The glass transition temperature (T_g) was discernibly higher at the interface than in the internal bulk region. Measurements at different incident angles of excitation pulses revealed that T_g became higher closer to the interface. The gradient became more marked with an increasing difference in the free energy at the interface between the polymer and solid substrate. The T_g value at the interface decreased with decreasing molecular weight. However, the decrement for the interfacial T_g was not as much as that for the bulk Tg, due to the restriction of chain end portions by the substrate. Finally, it was observed that when a film became thinner than 50 nm, the depressed mobility at the interface coupled with the enhanced mobility induced by the presence of the surface. The experimental and simulation results were in good accord with each other.