Heterogeneity of spiral wear patterns produced by local heating on amorphous polymers

Abstract

We report on spiral wear patterns produced at constant angular velocity by hot tip atomic force microscopy (HT-AFM) on surfaces of two common amorphous polymers: polystyrene (PS) and polymethylmethacrylate (PMMA). Topography of these patterns is obtained with regular AFM cantilevers. Topography cross-sections taken from a center of each spiral at a given azimuthal angle Θ relate changes of surface corrugation hcorr with tangential velocity v of a thermal cantilever. Polymer wear is characterized by a power law hcorr(v) = α(v/vmax)−β, which yields a pre-factor α and an exponent β. Below the glass transition temperature Tg, α is polymer specific and β varies weakly between similar conditions and samples. Variations of β are hypothesized to reflect polymer relaxation processes, which are expected to vary only weakly between amorphous polymers. At and above Tg, α approaches initial thermal tip indentation depth within a polymer, β plummets, and a power law relation of hcorr with v diverges. These results are explained by heterogeneous wear around Tg due to a local nature of glass transition. At all studied temperatures, additional wear heterogeneities are found as due to position on the polymer and Θ. Variations of α and β with position on the polymer are found to be only marginally larger then uncertainties of the thermal tip–polymer interface temperature. Variations of α and β with Θ are found to be largely influenced by buckling of thermal cantilevers traveling in a spiral pattern.