367767 Rheology at the Nano-Scale and at Surfaces
There is considerable discussion in the literature concerning the dynamics of glass-forming materials, especially polymers, at the nanometer size scale. Of particular interest is the wide-spread view that increases in the mobility, especially as seen in reduced glass transition temperatures Tg, occur due to enhanced surface dynamics. One way of addressing the problem is to perform rheological measurements both on ultrathin films and on the film surface itself. Here we describe work in which ultrathin films are examined using both a novel nanobubble inflation test and a liquid dewetting experiment. We further look at surface dynamics using a spontaneous particle embedment method to extract surface rheological properties. Examination of the results shows while the thin films may show highly accelerated dynamics, their behaviors are surprisingly non-universal and, in some instances, contrary to what is observed for the surface dynamics. Furthermore, the surface dynamics themselves do not exhibit the typical Vogel-Fulcher-Tammann (VFT) or Williams, Landel and Ferry (WLF) behaviors expected for the temperature dependence of the dynamics of glass-forming materials, hence, suggesting that the surface either does not exhibit cooperative dynamics or it does not exhibit a glass transition at all.
In either case the surface, then, cannot be easily identified as the cause of observed reductions in Tg. We further remark that a great stiffening of the rubbery plateau in ultrathin films is observed and this may be consistent with observations that surface dynamics of polymers above the macroscopic glass transition seem to be slower than the macroscopic dynamics.
The authors thank the National Science Foundation under grants DMR-1207070 and CHE-1112416, the Office of Naval Research under project N00014-11-1-0424, and the John R. Bradford endowment at Texas Tech, each for partial support of this work.