Thermal Conductivity of Polyurethane Thin Films

Heat transport by solid conduction of polyurethane (PU) is a significant fraction of the total thermal conductivity of rigid PU foam. Despite its significance, the intrinsic thermal conductivity of solid PU materials is not well characterized, a circumstance largely attributable to the challenges of preparing fully dense materials that are free of voids due to CO₂ gas formation and entrapment caused by the fast reaction between a trace amount of water in a polyol and isocyanate during curing. We developed a protocol to prepare dense homogeneous PU thin films with thicknesses on the order of 100 nm, prepared eight formulations of PU films from a series of polyols and isocyanates with different functionalities, and measured their thermal conductivity, heat capacity, longitudinal speed of sound, and density by time-domain thermoreflectance, picosecond acoustics, and Rutherford backscattering spectrometry, respectively. The thermal conductivity of these PU materials spans a narrow range of 0.19 ± 0.03 W/(m K). The glass transition temperature of polymer thin films as thin as 100 nm was measured based on a photothermal displacement method. We used data for the longitudinal speed of sound and atomic density to predict the thermal conductivity using the minimum thermal conductivity model (MTCM). While the MTCM agrees with the measured thermal conductivity to within 30%, the variations in the thermal conductivity that we observe experimentally cannot be explained by the small variations in sound velocity and density. Our findings provide new insights into thermal conduction properties of PU solids in rigid PU foam and provide a fundamental basis for the construction of models of the thermal performance and the development of better insulation materials.