Nanoscale thermal mechanical probe determination of softening transitions in thin polymer films
Jing N. Zhou, Jack F. Douglas, Alamgir Karim, Chad R. Snyder, Christopher L. Soles, Brian Berry
We report a quantitative study of softening behavior of glassy polystyrene (PS) films at the 100 nm length scales using nano-thermomechanometry (nano-TM), an emerging scanning probe technique where a highly-doped silicon AFM tip is resistively heated on the surface of the film. The apparent softening transition temperature, Ttranst, follows a logarithmic relation with the square root of thermal ramping rate, R, from which a quasi-equilibrium or zero rate thermal transition temperature, T0, can be obtained. We observe marked shifts of T0 with decreasing film thickness indicative of finite size effects. However, the nature of these shifts, even their sense in terms of positive or negative deviations, depends strongly on the thermal and mechanical properties of the supporting substrate. Finite element methods suggest that thin PS films on high modulus substrates with large thermal conductivities lead to increasing transition temperatures whereas softer, less thermally conductive substrates promote reductions in the apparent T0. Experimental observations on a range of substrates confirm this behavior and indicate a complicated interplay between thermal and mechanical properties of the thin PS film and the substrate. This study directly points to the relevant considerations those are needed for quantitative measurements of materials thermophysical properties at the nanoscale using this nano-TM based method.