An investigation of heat transfer effects in isothermal crystallization studies of low-density polyethylene

Isothermal crystallization kinetics of low-density polyethylene (LDPE) were measured from 96 degreesC-103 degreesC using a power-compensating differential scanning calorimetry (DSC). Crystallization kinetics were measured using different sample thicknesses and on samples compounded with nickel, a filler with high thermal conductivity. Far the unfilled material, sample thickness and temperature had a significant effect on the rate of crystallization as measured by the Avrami rate constant K, but had no effect on the nucleation mechanism and dimensionality of growth, as measured by the Avrami constant rr. The crystallization growth rate as expressed by K-1/n scaled approximately with the thickness of the sample. For the filled material, R was much higher and independent of nickel content, suggesting a limiting growth rate for polyethylene at a given temperature in this equipment. The dependence of crystallization rate on sample thickness indicates that barriers to heat transfer can be important, This work shows that for most crystallization rates, thermal conductivity, rather than interfacial resistance between sample and pan, limits heat transfer. Even though thermal conductivity typically dominates heat-transfer resistance, sample-pan thermal contact is still important, and some guidelines are given to determine whether good contact is being made.