Probing polymer viscoelastic relaxations with temperature-controlled friction force microscopy

A quantitative method, using temperature-controlled friction force microsco py (FFM), has been developed to determine the frictional (dissipative) char acter of thin polymer films. With this method variations in friction are sa mpled over micrometer-scale regions and are reduced to "friction histograms , yielding the distribution of frictional forces on the surface. The temper ature dependence of the mean value of the frictional distribution is correl ated to the known glass-to-rubber transition (T-g) and/or secondary relaxat ion mechanisms in films of poly(methyl methacrylate) (PMMA), polyethylene t erephthalate) (PET), and polystyrene (PS). The dominant contribution to fri ction, on polymer films, was attributed to viscoelastic mechanical loss. Us ing equivalent time scales, measured T-g's were lower than bulk polymer val ues. The frictional response of PMMA displayed time-temperature equivalence upon variation of scan-velocity and temperature. The rate dependence of th e hindered rotation of the - COOCH3 group (beta relaxation) in PMMA was con sistent 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.