Interface and Surface Effects on the Glass Transition in Thin Polystyrene Films

Lifetime analysis of positronium annihilating in nanometer voids is used to study the thermal expansion behavior of thin, Si-supported polystyrene films near the glass transition temperature $T_g$. A reduction in void volume expansion is correlated with a reduction in the apparent $T_g$ as film thickness decreases. Our results can be fitted using a three-layer model incorporating a 50 Å constrained layer at the Si interface and a 20 Å surface region with reduced $T_g$.