Probing Polymer Viscoelastic Relaxations
with Temperature Controlled Friction Force Microscopy


Jon A. Hammerschmidt and Wayne L. Gladfelter*
Department of Chemistry
University of Minnesota, Minneapolis, Minnesota 55455

Greg Haugstad
Center for Interfacial Engineering
University of Minnesota, Minneapolis, Minnesota 55455


Abstract

A quantitative method, using temperature-controlled friction force microscopy (FFM), has been developed to determine the frictional (dissipative) character of thin polymer films. With this method variations in friction are sampled over micron-scale regions and are reduced to "friction histograms," yielding the distribution of frictional forces on the surface. The temperature dependence of the mean value of the frictional distribution is correlated to the known glass-to-rubber transition (Tg) and/or secondary relaxation mechanisms in films of polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polystyrene (PS). The dominant contribution to friction, on polymer films, was attributed to viscoelastic mechanical loss. Using equivalent time scales, measured Tg’s were lower than bulk polymer values. The frictional response of PMMA displayed time-temperature equivalence upon variation of scan-velocity and temperature. The rate dependence of the hindered rotation of the -COOCH3 group (b relaxation) in PMMA was consistent with Arrhenius type behavior, allowing calculation of an activation energy. The activation energy of the thin film was found to be lower than measured bulk energies.