Low and High 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, and biological materials) is much less mature. The lack of basic understanding of heat conduction in soft materials hinders applied research on the discovery of low and high thermal conductivity materials for thermal insulation and thermal management.

I will discuss our on-going work on the thermal conductivity of polymers that examines the changes in thermal conductivity that are produced by systematic variations in molecular structure and crystallinity. Our interests are in exploring both the lower and upper limits of the isotropic thermal conductivity in polymers that are relatively simple to synthesize and process into desired shapes. Our recent work emphasizes low and high thermal conductivity in a series of polyester linear polymers and networks than spans a factor of 10 in thermal conductivities from 0.10 to nearly 1.0 W/(m K); and the lower limit to thermal conductivity that we can achieve in polyurethane chemistries, i.e., reactions of polyols and isocyanates. In each of our studies of soft materials, we strive to fully characterize the thermal conductivity, heat capacity, density, coefficient of thermal expansion, longitudinal modulus, vibrational spectra by Raman and infrared spectroscopy, and microstructure via x-ray scattering and diffraction.

Our work is facilitated by a recent development in thermal conductivity metrology that we refer to as “thermo-optic phase spectroscopy” (TOPS). Displacement-TOPS (D-TOPS) and immersion-TOPS (I-TOPS) are variations on traditional photothermal displacement and mirage techniques, respectively. In comparison to time-domain thermoreflectance (TDTR), TOPS has higher signal-to-noise, is less sensitive to surface roughness, provides a larger thermal penetration depth, and uses simpler and less costly hardware. D-TOPS is well-suited to the measurement of the in-plane thermal conductivity of small regions (10x10 μm²) of soft materials that are thicker than 10 μm.