Glass transition in ultrathin polymer films: a thermal expansion study

Abstract

The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T_g) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T_g was found to be very small. The existence of reversible negative and positive thermal expansion below and above T_g increased the sensitivity of our thickness measurements performed using energy-dispersive X-ray reflectivity. A simple model of the T_g variation as a function of depth expected from sliding motion could explain the results.