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 lc , interlamellar amorphous thickness la , 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, la from SAXS increased slowly starting at about 100°C, suggesting melting of thin inserted lamellae, and at about 150°C, lc began to increase combined with a more rapid increase in la 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.