Polymorphism, Preferred Orientation and Morphology of Propylene-Based Random Copolymer Subjected to External Force Fields
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Crystallization of propylene-based random copolymers was investigated by using time-resolved wide- and small-angle X-ray scattering (WAXS/SAXS) techniques. With low content of 1-butylene as comonomer, both gamma- and alpha-phase of polypropylene crystals co-existed in the system with the gamma-phase being the dominant modification. When a step shear was applied, the c-axis of the gamma-phase oriented horizontally, while that of the alpha-phase pointed to the vertical direction (i.e., the shear direction). Crystallization was greatly enhanced by the shear. Uniaxial stretching experiments were also performed on the same sample at different temperatures. At high temperature (373K), the gamma-phase adopted a distinctive tilted `cross-beta' configuration. During stretching, the gamma-phase was gradually transformed into the alpha-phase with parallel chain packing. Lamellar branching was also observed at this temperature, but was not observed at lower temperatures. The stress associated with the mid-temperature (333K) deformation was found to be much higher than those at other temperatures, where this observation could be explained by the fragmentation of lamellar crystals as evident from the profile analysis of the WAXS data. At low temperate (e.g. 298K), the WAXS pattern was featured with prominent equatorial diffuse scattering due to the mesomorphic phase. The nature of the mesomorphic phase could be modeled with tiny beta-phase crystals with about 50% oriented amorphous phase. The thermal stability and reversibility of the gamma-to-alpha phase transition were investigated by heating of the stretched sample to a temperature close to the nominal melting temperature and subsequent cooling to crystallization temperature of 373K, where the in-situ re-crystallization process was monitored. The heating process was found to remove residues of the gamma-phase with tilted `cross-beta' configuration, whereby the newly grown gamma-phase adopted a parallel orientation mode.