Low, high, and switchable thermal conductivity in soft materials

A century of experiment and theory have produced a thorough understanding of heat conduction by phonons in simple inorganic crystals. By contrast, basic understanding of heat conduction by molecular vibrations in soft materials (amorphous and crystalline polymers, small molecule solids, biological materials) is much less mature. Complex, non-periodic structures spanning multiple length scales are difficult to characterize and model. Low thermal conductivity, fiber morphologies, poor control of defects, and anisotropy created by molecular order create daunting challenges for experiment. I will discuss our past work on the thermal conductivity and elastic constants of a wide variety of polymeric materials in the form of thin films and fibers that span a factor of 300 in thermal conductivity, 0.06 to 20 W/m-K. Time-domain thermoreflectance (TDTR) provides a common experimental platform for these studies; varying the thickness and modulation frequency changes the relative sensitivities of the TDTR measurement to thermal conductivity and heat capacity. Our recent work has employed light-activated changes in the morphology azo-polymers to switch by a factor of 3 between a low conductivity amorphous form and higher thermal conductivity crystalline form. We are developing frequency-domain probe beam-deflection and optical-fiber-based TDTR measurements to provide new capabilities for measurements of the thermal conductivity, effusivity and diffusivity of small volumes of soft materials.