Crystallization behavior of isotactic propylene-1-hexene random copolymer revealed by time-resolved SAXS/WAXD techniques

The crystallization behavior of isotactic propylene-1-hexene (PH) random copolymer having 5.7% mole fraction of hexene content was investigated using simultaneous time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. For this copolymer, the hexene component cannot be incorporated into the unit cell structure of isotactic polypropylene (iPP). Only α-phase crystal form of iPP was observed when samples were melt crystallized at temperatures of 40 °C, 60 °C, 80 °C, and 100 °C. Comprehensive analysis of SAXS and WAXD profiles indicated that the crystalline morphology is correlated with crystallization temperature. At high temperatures (e.g., 100 °C) the dominant morphology is the lamellar structure; while at low temperatures (e.g., 40 °C) only highly disordered small crystal blocks can be formed. These morphologies are kinetically controlled. Under a small degree of supercooling (the corresponding iPP crystallization rate is slow), a segmental segregation between iPP and hexene components probably takes place, leading to the formation of iPP lamellar crystals with a higher degree of order. In contrast, under a large degree of supercooling (the corresponding iPP crystallization rate is fast), defective small crystal blocks are favored due to the large thermodynamic driving force and low chain mobility.