Glass Transition of Thin Star Polymer Films

The thickness dependence of the glass transition, $T_{g}$, of thin film polystyrene (PS) star molecules, supported by SiO$_{x}$ substrates, has been examined using spectroscopic ellipsometry and compared to the behavior of linear PS chains. Linear PS chains exhibit a film thickness dependence on SiO$_{x}$ substrates, decreasing with decreasing film thickness, for thicknesses $h$ less than approximately 45 nm. This thickness dependence, when normalized by the bulk $T_{g}$, is observed for chains with a wide range of degrees of polymerization $N$, from $N < \quad N_{e}$ (molecular weight between entanglements) to very large values of $N$. The $T_{g}$s of long chain star molecules, of low functionalities, $f$, exhibit the same thickness dependence. However, as the degree of polymerization of an arm length, \textit{Na}, decreases the thickness dependence undergoes a transition, wherein $T_{g}$ increases with decreasing $h$. These effects are discussed in terms of the role of architecture and entropic effects on the structure of the system. Implications on the chain dynamics will also be discussed.