Study of polyethylene solution fractionation and resulting fractional crystallization behavior

Solution fractionation for four different polyethylenes including high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and very low-density polyethylene (VLDPE) are conducted by stepwise controlling both the temperature and the amount of precipitant. The size exclusion Chromatograph (SEC) measurements indicate that solution fractionation technique can successfully separate all the polyethylene samples in accordance with their molecular weight and molecular-weight distributions. In addition, infrared spectroscopy analysis shows that the degree of short-chain branching for each fraction of each polyethylene varies with the fraction's molecular weight. The effect of the molecular weight with different short-chain branching on each fraction's crystallinity represents the characteristics of chain components for different polyethylenes. The crystallinities of HOPE, LLDPE, and LDPE decrease with the increase in their molecular weights; however, for VLDPE, its crystallinity increases with the increase in the molecular weight. The research revealed that the degree of short-chain branching, together with the molecular weight, can greatly affect the crystallinity of polyethylene.