Comparison Of Chain Microstructure Between Two Propylene-Ethylene Copolymer Resins With Bimodal Melting Temperature Distribution

Polypropylene (PP) has been commonly used as packaging materials due to the favorable mechanical and optical properties related to its diverse and complex chain microstructure which can be better understood through effective fractionation. In this work, two commercial PP resins (A and B) with similar bimodal melting temperature distribution in the range of 100-160 degrees C but different ethylene content (9.1-12.3 mol%) are fractionated by preparative temperature rising elution fractionation (P-TREF) and relevant fractions are characterized by a variety of techniques such as high temperature-gel permeation chromatography (HT-GPC), differential scanning calorimetry (DSC), C-13 nuclear magnetic resonance (NMR) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, and wide-angle X-ray diffraction (WAXD). The chain microstructures of the two resins used as food packaging film exhibit heterogeneity and some differences, which are mainly caused by broad and different distribution of ethylene comonomers. Resin B contains more ethylene units than resin A, and the ethylene units are incorporated into the PP chains in the forms of individual unit or very short segments, resulting in large amount of fractions with relatively low isotacticity and crystallinity that eluted at the temperature range of 35-90 degrees C. These fractions account for 84.07 wt% of resin B, whereas 59.66 wt% of resin A. The propylene sequences of different isotacticity and crystallinity generated by the insertion of ethylene comonomers in resin B are mainly responsible for its biomodal melting behavior. However, ethylene comonomers tend to insert into the PP chains of resin A in the forms of long polyethylene (PE) blocks and generate propylene-ethylene block copolymers with high crystallizability which can be eluted at high temperatures of 120-140 degrees C. These fractions constitute 3.16 wt% of resin A, but barely exist in resin B. Moreover, 35.83 wt% of resin A were eluted at 110 degrees C, whereas only 13.22 wt% of resin B can be eluted at this temperature. These fractions with high isotacticity and molecular weight eluted at high temperatures contribute to the higher flexural modulus of resin A. Both long ethylene blocks and propylene sequences of different lengths contribute to the biomodal melting peaks on the thermogram of resin A.