Nanoscale thermal-mechanical probe determination of 'softening transitions' in thin polymer films

We report a quantitative study of the softening behavior of glassy polystyrene (PS) films at length scales on the order of 100 nm using nano-thermomechanometry (nano-TM), an emerging scanning probe technique in which a highly doped silicon atomic force microscopy (AFM) tip is resistively heated on the surface of a polymer film. The apparent 'softening temperature' Ts of the film is found to depend on the logarithm of the square root of the thermal ramping rate R. This relation allows us to estimate a quasi-equilibrium (or zero rate) softening transition temperature Ts0 by extrapolation. We observe marked shifts of Ts0 with decreasing film thickness, but the nature of these shifts, and even their sign, depend strongly on both the thermal and mechanical properties of the supporting substrate. Finite element simulations suggest that thin PS films on rigid substrates with large thermal conductivities lead to increasing Ts0 with decreasing film thickness, whereas softer, less thermally conductive substrates promote reductions in Ts0. Experimental observations on a range of substrates confirm this behavior and indicate a complicated interplay between the thermal and mechanical properties of the thin PS film and the substrate. This study directly points to relevant factors for quantitative measurements of thermophysical properties of materials at the nanoscale using this nano-TM based method.