Thickness dependence of the glass transition temperature in thin polymer films

In this paper the glass transition temperature (T-g) in thin polymer films has been studied. Ellipsometry has been used to measure T-g of poly(alpha - methylstyrene) (PAMS) thin films as a function of the film thickness for tw o molecular weights. When the films were thinner than a few hundred angstro ms, substantial. reduction in T-g was apparent. The T-g depression pattern did not show the difference between molecular weights. The Michaelis-Menten equation was adapted and used to fit the experimentally obtained Tg data. From this analogized Michaelis-Menten function fitting, the fitting paramet ers T-g,T-infinity and xi could be obtained. The obtained T-g,T-infinity co rresponded with the bulk T-g, and the parameter xi was correlated with the statistical segment length. A continuous multilayer model was proposed and derived to describe the effect of surface on the observed T-g reduction in thin films, and the depth-dependent T-g profile was obtained. These results showed that T-g at the top surface was much lower than the bulk T-g and gr adually approached the bulk Tg With increasing distance from the edge of th e film. The model and equation were modified to apply for the polymer coate d on the strongly favorable substrate and the freely standing film.