(744d) Anomalous Behavior of Ultrathin Polymer Films

There is considerable interest in the behavior of ultrathin polymer films. This has been especially true since the discovery in the mid-1990s that freely standing films of polystyrene can show glass transition temperature reductions of greater than 50 K when thicknesses were reduced below 50 nm. Here we report novel findings from our laboratories in which we see non-universal behavior in freely standing polymer films. The first behavior is that the glass transition does not decrease dramatically for poly(vinyl acetate)(PVAc) while it does for polystyrene(PS) in freely standing films. The second is that the rubbery plateau response in both PVAc and PS shows enormous stiffening (>1000-fold) in the "nanobubble" inflation measurement pioneered in our labs and for the thinnest films. We also find that the stiffening is much less when the experiments are performed on poly(n-butyl methacrylate) (PBMA). In the nanobubble inflation experiments we also have been able to characterize films of polycarbonate (PC) as thin as three nm. In this case, not only do we see dramatic plateau stiffening, but we also see a reduction in the glass transition temperature in excess of 120 K. We also show, for the first time, the terminal flow response of ultrathin polymer films below the bulk glass transition temperature.

Finally, in experiments following the liquid dewetting approach pioneered by Bodiguel and Fretigny, we show that polystyrene films floating on glycerol exhibit neither rubbery stiffening nor reductions in the glass transition temperature. Possible reasons for the discrepancies between the film dewetting and bubble inflation experiments will be discussed.