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 differen tial scanning calorimetry (DSC). Crystallization kinetics were measured usi ng different sample thicknesses and on samples compounded with nickel, a fi ller with high thermal conductivity. Far the unfilled material, sample thic kness and temperature had a significant effect on the rate of crystallizati on as measured by the Avrami rate constant K, but had no effect on the nucl eation mechanism and dimensionality of growth, as measured by the Avrami co nstant rr. The crystallization growth rate as expressed by K-1/n scaled app roximately with the thickness of the sample. For the filled material, R was much higher and independent of nickel content, suggesting a limiting growt h rate for polyethylene at a given temperature in this equipment. The depen dence of crystallization rate on sample thickness indicates that barriers t o heat transfer can be important, This work shows that for most crystalliza tion rates, thermal conductivity, rather than interfacial resistance betwee n sample and pan, limits heat transfer. Even though thermal conductivity ty pically dominates heat-transfer resistance, sample-pan thermal contact is s till important, and some guidelines are given to determine whether good con tact is being made.