Melt crystallization and crystal morphology of two low molecular weight linear polyethylene fractions

摘要

Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G(b)) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17 degrees C (at 117 degrees C and 120 degrees C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122 degrees C and 124 degrees C), at which the cusps of the G(b) data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments, it is found that within a 2 degrees C temperature region, the G(b) change is accompanied by a sharp long period increase and a drastic change in single-crystal morphology from a truncated lozenge with curved (200) and (110) planes to a lenticular-shape crystal. The morphological change may result from a sudden increase in the G(b) coupled with a smaller change in the growth rate along the a axis with undercooling. This implies that, within this temperature region (2 degrees C), the crystals may undergo substantial changes in the geometry of the (110) and (200) crystal growth fronts and chain folding behavior.