Reduced Glass Transition Temperatures of Thin Polymer Films - Confinement Effect or Artifact?

For two decades there have been reports of measurements of reduced glass transition temperatures ($T_{\mathrm{g}}$) in polymer, and in particular poly-styrene, films. These results have motivated theoretical models and a variety of sophisticated experiments probing interfacial polymer properties. While the much larger reductions in $T_{\mathrm{g}}$ for free standing films have suggested the importance of the free surface, a significant concern has been raised about a possible correlation between anomalous dynamics and incomplete equilibration of the sample. Here, we present new ellipsometry measurements which unambiguously address this concern. The glass transition in free standing and supported films can be changed by many 10's of degrees by manipulating the interfacial properties. Taken together with previous work the results clearly reveal the importance of free interfaces as we transition from two, to one, to zero free interfaces.