Characterization of the microstructure of impact polypropylene alloys by preparative temperature rising elution fractionation

Two polypropylene alloys (Samples A and B), as impact polypropylene (PP) with similar ethylene contents and melt indices but different impact properties at low temperatures, are fractionated into eight fractions using preparative temperature rising elution fractionation. The microstructure of the original samples and their fractions are studied using high-temperature gel permeation chromatography, Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. The results indicate that the two alloys are mainly composed of four portions: ethylene–propylene random copolymer (EPR), ethylene–propylene segmented copolymer, ethylene–propylene block copolymer, and propylene homopolymer. Sample A contains more EPR and more fractions with higher isotacticity eluted at 120 and 140 °C than Sample B. The difference in the microstructure distributions of both PP alloys results in observable differences in their mechanical properties: Sample A has better impact toughness and possesses higher rigidity than Sample B. Sample A also exhibits better balance between toughness and stiffness.

Three-dimensional surface plots can be achieved from cross-fractionation results obtained by combining TREF and GPC data, as shown in figures. Each region on the 3D surface gives the relative amount of each species with a given molecular weight and composition. The 3D surfaces show clear structural features of both samples and the minimal difference in their microstructure distributions.Figure optionsDownload as PowerPoint slide