Localized low-energy excitations in two amorphous polymers

Abstract

The low-temperature specific heat, the thermal conductivity, and the temperature dependences of ultrasonic velocities have been measured for two epoxies differing primarily in crosslink density. The difference between the two epoxies is explained by a factor of ≈ 1.5 difference in density of localized excitations. Below ≈ 1 K all data are accounted for quantitatively by the tunneling-states model. Above ≈ 1 K, however, the model does not explain both the "plateau" in the thermal conductivities and the temperature dependences of the ultrasonic velocities without additional assumptions.