Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 less than or equal to MW less than or equal to 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (Delta T). On the contrary, at the same crystallization temperature (T-c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing Delta T for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T-c increases. For the low MW PE's, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE's, the transitions occur at an almost constant Delta T of 17.5+/-1 degrees C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low Delta T's. Typical banded spherulites and axialites, which are MW and Delta T dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.