Fractionation and Characterization for a Propylene–Ethylene Random Copolymer

A random copolymer of propylene with 3.5 mol% ethylene comonomer is firstly fractionated by temperature rising elution fractionation (TREF). Techniques including 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), gel-permeation chromatography (GPC), crystallization analysis fractionation (Crystaf) and differential scanning calorimetry (DSC) are used to characterize the obtained fraction polymers. The results show that each fraction polymer is composed mainly of isotactic propylene sequence plus a small amount of ethylene comonomer and has uniform molecular weight and ethylene content. The PPP, PPE, and PEP are main part of triad sequence unit. As the elution temperature increasing, ethylene content of the fraction polymers decreases, number average molecular weight increases, and meanwhile number average sequence length of propylene, nP, increases, while that of ethylene, nE, decreases, close to 1. The results show that ethylene content affects linearly the melting temperature (Tm) in a range of ethylene content being as low as less than 10.23 mol%; there is a linear relationship between the reciprocal melting temperature (1000/Tm, K-1) and reciprocal number average molecular weight (Mn-1) in a range of number average molecular weight as low as less than 1.7 × 105.