Effect of copolymerized ethylene unit on the crystallization behavior of poly(propylene-co-ethylene)s

The crystallization behavior of two kinds of commercial poly(propylene-co-ethylene)s (PPE1, PPE2) with similar average molecular weight and molecular weight distribution, isotacticity and copolymerized ethylene unit content and their fractions was investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and polarized optical microscopy (POM) techniques. The results indicate that the PPE1 isothermally crystallized films possess thicker and less cross-hatched lamellar structure than those of the PPE2. As for the fractionated samples, the thin films of low temperature (less than or equal to 90 degreesC) fractions (PPE1-80, PPE2-80) of both PPE1 and PPE2 exhibit similar crystallization behavior, while for the high temperature ( greater than or equal to 95 degreesC) fractions (PPE1-108, PPE2-108), the crystalline morphology has marked differences. Compared with PPE2-108, the PPE1-108 isothermally crystallized thin films possess thicker lamellae and less crosshatched lamellar structure, while for the fibrous crystal number, the former is less than that of the latter. The main reason to create the crystallization behavior differences between the two PPEs and their fractions is due to the effect of molecular chain structure, i.e. the different distribution of copolymerized ethylene unit in polypropylene chains.