(253f) Understanding Polymer Properties That Impact Thin Film Behavior

The physical properties of polymers when confined to nanoscale dimensions are a source of major disagreement.  Depending upon the techniques and processing conditions, apparent contradictions in the effect of confinement have been reported for the same polymer system.  In particular, the glass transition temperature (T_g) in polymer thin films has been the focus of many researchers.  Moreover, polystyrene is the most examined system, but there is still no consensus on its thin film T_g despite nearly 20 years of experiments.  Here we take an alternative approach by utilizing two techniques (wrinkling and ellipsometry) to examine the elastic moduli and T_g of systematic series of glassy polymers.  We find several simple relationships between bulk polymer properties and deviations in thin film T_g and moduli.  There appear to be two important contributions: the free surface of the polymer and the relative flexibility of the polymer chain.  From this proposed mechanism, we can reconcile some of the apparent contradictions in the literature.