Viscoelasticity in nanoscale friction on thin polymer films

Friction force microscopy is employed to investigate sliding friction at nanometer-scale contacts on thin polymer films, as a function of relative humidity, scan velocity and temperature. Strong humidity and rate dependence is observed on amorphous regions of two water-soluble polymers, polyvinyl alcohol (PVOH) and gelatin. Crystalline regions of the same polymers exhibit relatively flat frictional response with respect to these variables. Friction on polymethyl methacrylate (PMMA), polystyrene (PS) and polyethylene terephthalate (PET) varies strongly with temperature, similar to the variations in tanδ observed in bulk dynamic mechanical measurements. Together these findings reveal the inherently viscoelastic nature of friction on amorphous polymers. The positions of peaks in friction data, associated with the glass transition, indicate enhanced molecular freedom relative to the bulk polymers. A time-temperature analysis of friction on PMMA related to secondary (β) relaxations produces a measurement of the activation energy of the hindered rotation of the -COOCH₃ group. This energy also is reduced relative to values tabulated for bulk polymers. A model of energy dissipation throughout the deformed volume of polymer is developed to explain the observed dependence of friction force on tanδ. Dissipation due to interfacial "bond" shearing apparently is of minor importance, as is the related role of contact area.