Influence of molecular weight and film thickness on the glass transition temperature and coefficient of thermal expansion of supported ultrathin polymer films

Abstract The influence of film thickness and molecular weight on the glass transition temperature and coefficient of thermal expansion for polystyrene thin films supported on silicon native oxide surfaces has been studied in detail using variable angle spectroscopic ellipsometry. It was observed that Tg exhibits a dependence on both film thickness and polymer molecular weight. The Tg of polystyrene was found to decrease with decreasing film thickness below a critical film thickness of approximately 10 times the polymer radius of gyration. The coefficient of thermal expansion was also found to increase sharply as film thickness is decreased below approximately 10 times the polymer radius of gyration. It was found that the Tg of polystyrene thin films supported on silicon native oxide surfaces can be modeled using a ‘master’ curve based on reduced thickness (i.e. thickness relative to the polymer radius of gyration) and reduced glass transition temperature (i.e. Tg relative to the polymer bulk Tg). Using this concept, a single equation has been generated that describes the dependence of Tg on molecular weight and film thickness for the polystyrene on silicon native oxide system.