Surface relaxations of poly(methyl methacrylate) assessed by friction force microscopy on the nanoscale

Abstract

A variation of the spatial distribution of segments has been anticipated for polymer chains at free surfaces. This effect, which should alter surface viscoelastic properties in comparison with the bulk, remained hitherto a controversial issue in the literature. We here present the first comprehensive, quantitative AFM study of surface relaxations of poly(methyl methacrylate) (PMMA) to address the experimental shortcomings experienced so far in addressing the top nanoscale layer. The broad range of scanning velocities accessible through the use of a high velocity accessory, temperature control, and in particular tips with significantly differing radii, allowed us to cover a frequency range from 1 to 10⁷ Hz. Friction data acquired at various temperatures and velocities were successfully shifted to yield a single mastercurve in which the onset of the α- and the β-relaxation processes of PMMA were identified. The substantially reduced activation energies (E_a^α ∼ 110 kJ/mol and E_a^β = 35 kJ/mol) and the significantly higher relaxation frequencies as compared to the bulk support the notion of a significantly higher mobility of the macromolecules present at the free surface of glassy polymers.