Probing polymer viscoelastic relaxations with temperature-controlled friction force microscopy

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 micrometer-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 (T_g) and/or secondary relaxation mechanisms in films of poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate) (PET), and polystyrene (PS). The dominant contribution to friction, on polymer films, was attributed to viscoelastic mechanical loss. Using equivalent time scales, measured T_g'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 (β 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.