Local dielectric spectroscopy of near-surface glassy polymer dynamics

A noncontact scanning probe microscopy method was used to probe local near-surface dielectric susceptibility and dielectric relaxation in polyvinyl acetate near the glass transition. Dielectric spectra were measured from 10(-4) to 10(2) Hz as a function of temperature. The measurements probed a 20 nm thick layer below the free surface of a bulk film. A significant change in the fragility index and moderate narrowing of the distribution of relaxation times were found in the near-surface layer. In contrast to results for ultrathin films confined on or between metallic electrodes, no reduction in the dielectric strength was found, inconsistent with the immobilization of slower modes.