Molecular relaxations in polymer films studied by temperature-controlled friction-force microscopy

Temperature-controlled friction force microscopy (FFM), has been developed to determine the frictional (dissipative) character of thin polymer films. Using FFM variations in friction can be sampled over micron-scale regions and reduced to "friction histograms," yielding the distribution of frictional forces on the surface. The temperature dependence of the mean value of the frictional distribution was correlated to the known a-relaxation (glass-to-rubber transition) and/or secondary relaxation mechanisms of polymer films. The dominant contribution to friction, on polymer films, was attributed to viscoelastic mechanical loss. Using equivalent time scales, measured a-relaxations were found to occur at lower temperatures than bulk polymer values The frictional response of PMMA displayed time-temperature equivalency 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 was found to be lower than measured bulk energies.