Chain structure comparison of two low density polyethylene resins fractionated by temperature rising elution fractionation and thermal fractionation

Abstract

Two low-density polyethylene resins were fractionated into 11 fractions using the temperature rising elution fractionation technique. The resins were of injection molding grade with similar densities but different melt indexes and mechanical properties. High-temperature gel permeation chromatography (HT-GPC), ¹³C-nuclear magnetic resonance (¹³C-NMR) spectroscopy, differential scanning calorimetry (DSC), and successive self-nucleation/annealing (SSA) thermal fractionation were employed to characterize the original resins and their fractions. Resin A had a higher molecular weight and broader molecular weight distribution than resin B did. The highest contents were found for the 80 °C and 75 °C fractions of resins A and B, respectively. In addition, the highest molecular weights for resins A and B at 2.14 × 10⁵ and 1.42 × 10⁵ g/mol, respectively, were attained for the fractions eluted at 80 °C. According to the results of ¹³C-NMR, the total branching contents of resins A and B, including short- and long-chain branching, were 3.63 mol% and 4.89 mol%, respectively. DSC analysis revealed that the crystallinity of fractions 3–10 of resin A was lower than that of resin B. We noted the presence of approximately eight multiple melting peaks in the original sample and fractions from the results of SSA. These peaks indicated the inhomogeneity of the molecular chain. L_w and L_n of resin A were longer than those of resin B. Lastly, the relationship between chain structure and property was also discussed.

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