Ellipsometric study of the glass transition and thermal expansion coefficients of thin polymer films

The glass transition (T_g) of thin polystyrene films (ca. 3000 A?) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. T_g was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in T_g, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley & Sons, Inc.