A simultaneous small- and wide-angle X-ray scattering study of the early stages of melt crystallization in polyethylene

Abstract

The early stages of isothermal melt crystallization in linear polyethylene (PE) were investigated via simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. During these stages, noticeable short-range density fluctuations with periodic spacing from 40 to 80nm were detected (by SAXS) before the identification of three-dimensional crystal ordering (by WAXD). These results are consistent with the recent findings in several other polymers (polypropylene, poly(aryletherketone), poly(ethylene terephthalate) and poly(butylene terephthalate) by our and other laboratories. Some groups have proposed that the spinodal decomposition due to chain conformation in the molten state may act as a precursor to crystallization based on these findings. Detailed examination of the SAXS and WAXD data, however, indicated that the early stages of crystallization also follow the classical nucleation and growth behavior with a simple Avrami expression. The earlier detection of density fluctuations can be attributed to the lower detection limit of crystallinity in SAXS (0.1%) than in WAXD (1%). In addition, we found that the long spacing associated with the SAXS peak decreased slightly with time, which opposed the behavior of the spinodal decomposition.