Creeping friction on amorphous polymers: Dissipation through molecular relaxation

The dissipation mechanisms of nanoscale friction between a scanning force microscopy (SFM) tip and amorphous polystyrene are found to reside solely within the polymer's intrinsic molecular mobility, and are discussed with respect to the glass transition temperature. In both the glassy and the rubbery states, lateral force microscopy friction results revealed the dissipative behavior as activated relaxation processes with potential barrier heights of 7 kcal/mol and 83 kcal/mol, respectively. These values correspond to hindered phenyl (side chain) rotation and to the a-relaxation, respectively. The velocity relationship with friction, F(v), was found to satisfy simple fluctuation surface potential models with F proportional to const - ln(nu) and F proportional to const - ln(nu)(2/3). Within similar to27 K above the glass transition temperature, friction displayed a shear thinning type behavior, also found in materials that exhibit multiple phases.