The low-temperature thermal conductivity of a semi-crystalline polymer, polyethylene terephthalate

The thermal conductivity, kappa , of seven samples of polyethylene terephthalate (PET) with volume fraction crystallinity varying from 0 to 0.51 has been measured between 1.5 and 70K. The amorphous sample exhibits the characteristic plateau region between 4 and 9K. Above 30K, kappa increases with crystallinity, but there is a cross-over near 20K so that kappa decreases with crystallinity at lower temperatures. This peculiar behaviour can be understood on the basis of the acoustic mismatch theory of Little (1959) by considering a semi-crystalline polymer as composed of crystalline regions dispersed in an amorphous matrix. As the temperature is lowered, the thermal boundary resistance due to acoustic mismatch at amorphous-crystalline interfaces increases and thus the thermal conductivity of the crystalline samples is reduced relative to that of amorphous PET.