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Direct measurement of molecular motion in freestanding and supported polymer thin films

Mark Ediger Department of Chemistry, University of Wisconsin-Madison

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding and supported polymer films as thin as 11 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with mobilities that differ by up to 4 orders of magnitude. These data are consistent with the presence of a high mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and is 10 nm or less at Tg for all the systems investigated. Additional experiments will be described showing that surface mobility for low molecular weight glassformers near Tg can be 7 orders of magnitude larger than bulk mobility.