Enhanced melt-recrystallization process of propylene-ethylene copolymer during the uniaxial stretching with the aid of isotactic polypropylene

The structural evolution of propylene-ethylene copolymer (coPP) and propylene-ethylene copolymer/polypropylene (coPP/iPP) films has been studied by in-situ WAXD/SAXS in a tensile process. Both coPP and coPP/iPP films show obvious melt-recrystallization during the deformation process. In the initial stage of stretching, the crystallinities of films decrease due to the stress-induced crystal fragmentation. After that, a large number of oriented crystals form in the late stage of the tensile process. Compared with that of coPP, the initial strain of recrystallization for coPP/iPP is smaller, and recrystallization-induced crystallinity (from 4.9% for coPP sample to 10.2% for coPP/iPP sample) and crystal orientation (from 0.75 for coPP to 0.93 for coPP/iPP) are improved. Structural evolution occurs in close relation to tensile rates. At low strain rate, molecular chains are easier to pull from the lattice, resulting in a significant decrease in crystallinity. At low strain rate, samples display stronger recrystallization in the late stage of stretching. Interestingly, there is a difference in the growth process for lamellae between coPP and coPP/iPP samples. For coPP samples, crystals grow on existing lamellae, leading to an increase in the average thickness of crystals. In contrast, recrystallization between lamellae tends to occur in coPP/iPP samples, leading to a decrease in average lamellar thickness and long period.