Melt Temperature and Initial Polymorphs Dependencies of Polymorphs Selection during Subsequent Crystallization in Propylene-ethylene Random Copolymer

Subsequent crystallization behavior during cooling for two random propylene-ethylene copolymers with 6 mol % ethylene co-units but different molecular weights initially in a pure α-form or pure γ-form processed at different melt temperatures was investigated using differential scanning calorimetry and wide-angle X-ray diffraction techniques. The onset crystallization temperature (Tonset) and fraction of the two polymorphs after crystallization depended on the molecular weight, initial polymorph, and melt temperature (Tmelt). At high Tmelt (120–160 °C), Tonset and the fraction of the γ-form (fγ) increased with decreasing Tmelt. For the high molecular weight sample initially in α-form, a higher Tonset and fγ than the one in the γ-form was observed because a higher Tmelt was needed to fully homogenize the melt for samples initially in the α-form due to the different morphology between the samples of different polymorphs. For the low molecular weight sample, the initial polymorph did not affect this process.