Morphological Changes During Crystallization and Melting of Polyoxymethylene Studied by Synchrotron X-Ray Scattering and Modulated Differential Scanning Calorimetry

Abstract

Temperature scanning techniques, including synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction(WAXD), and temperature-modulated differential scanning calorimetry (TMDSC), were used to study melting recrystallization in semicrystalline polyoxymethylene (POM). The isothermal crystallization of POM was also studied by time-resolved SAXS. From SAXS profiles, several morphological variables were calculated, including the long period L, lamellar thickness l_c , interlamellar amorphous thickness l_a , and scattering invariant Q. The lamellar parameters were also obtained using tapping atomic force microscopy (AFM) for two thermal histories, and some lamellar-scale and larger-scale morphological changes were characterized before and after partial melting. These real-space images also provided support to interpretation of SAXS analysis in the interpretation of first- and second-order intensity maxima. During melting at constant heating rates, l_a from SAXS increased slowly starting at about 100°C, suggesting melting of thin inserted lamellae, and at about 150°C, l_c began to increase combined with a more rapid increase in l_a due to further melting of inserted lamellae and some recrystallization into separate stacks of lamellae. The end of melting was about 182°C. TMDSC data also characterized the level of melting and recrystallization starting at low temperatures for a quenched sample. The DSC data provided the total extent of melting, and this was contrasted with the drop in SAXS and WAXD intensities. SAXS and TMDSC temperature scans on a quenched, but still highly crystalline, POM sample were compared with the data from a high-temperature (145°C) isothermally crystallized POM sample with a higher degree of crystal perfection.

ACKNOWLEDGMENTS

We thank Dr. E. Flexman of DuPont for his important contributions. We also acknowledge discussions and critical reviewing of the manuscript by Professor P. Geil of the University of Illinois. B. H. acknowledges the financial support of this work by an NSF grant (DMR 11732653) and a DuPont Young Faculty grant.