Lower and upper limits to the vibrational thermal conductivity of amorphous polymers and polymer salts

The low thermal conductivity of glassy polymers is advantageous for thermal insulation and problematic for the thermal management of systems that use polymeric materials as dielectrics, adhesives, and for environmental protection. Our work experimentally explores the lowest and highest possible thermal conductivities that can be achieved in amorphous polymers. The pressure dependence (P < 10 GPa) of the thermal conductivity of PMMA provides insights on how the thermal conductivity is controlled by the strength of molecular bonds. Using the high throughput capability of time-domain thermoreflectance (TDTR), we studied the thermal conductivity of eight amorphous polymers and ten polymer salts to advance fundamental understanding of the lower and upper limits to heat conduction in this class of materials. We prepare film of polymers with a thickness on the order of 100 nm on Si substrates and measure their thermal conductivities and heat capacities using TDTR. Varying the modulation frequency allows us to span from thermally thick to thermally thin and change the relative sensitivities of the TDTR measurement to thermal conductivity and heat capacity. The thermal conductivities vary by an order of magnitude, from 0.06 W m^-1 K^-1 for functionalized fullerenes to 0.7 W m^-1 K^-1 for poly(vinylphosphonic calcium salt). We measure the longitudinal modulus by picosecond acoustics and the shear modulus using an elastomeric phase-shift mask that enables us to use pump-probe methods to determine surface acoustic wave velocities for acoustic wavelengths of 700 nm and frequencies on the order of a few GHz. Overall, the thermal conductivities are well correlated with the scaling of the model of the minimum thermal conductivity with heat capacity and sound velocities.