Local dielectric spectroscopy of near-surface glassy polymer dynamics

TL;DR

This study uses non-contact scanning-probe microscopy to investigate local dielectric properties of near-surface PVAc, revealing subtle changes in glass transition behavior without immobilizing slow relaxation modes.

Contribution

It introduces a non-contact method to measure local dielectric susceptibility and relaxation in polymer surfaces, providing new insights into near-surface glassy dynamics.

Findings

20 nm near-surface layer shows a 4 K reduction in Tg

Moderate narrowing of relaxation time distribution observed

No reduction in dielectric strength, unlike ultra-thin films

Abstract

A non-contact scanning-probe-microscopy method was used to probe local near-surface dielectric susceptibility and dielectric relaxation in poly-vinyl-acetate (PVAc) near the glass transition. Dielectric spectra were measured from 10-4 Hz to 102 Hz as a function of temperature. The measurements probed a 20 nm thick layer below the free-surface of a bulk film. A small (4 K) reduction in glass transition temperature and moderate narrowing of the distribution of relaxation times was found. In contrast to results for ultra-thin-films confined on or between metallic electrodes, no reduction in the dielectric strength was found, inconsistent with the immobilization of slower modes.