Morphological changes during crystallization and melting of polyoxymethylene studied by synchrotron X-ray scattering and modulated differential scanning calorimetry

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 and 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 degrees C, suggesting melting of thin inserted lamellae, and at about 150 degrees 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 degrees 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 degrees C) isothermally crystallized POM sample with a higher degree of crystal perfection.