Molecular structure and local dynamic in impact polypropylene copolymers studied by preparative TREF, solid state NMR spectroscopy, and SFM microscopy

Abstract

The phase morphology in impact polypropylene was studied by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and solid-state nuclear magnetic resonance (NMR). The characterizations of the raw copolymer materials indicate the presence of amorphous and crystalline phases. In order to gain more comprehensive insight in the microstructure of these materials, four different subsamples were prepared by Temperature Rising Elution Fractionation (TREF) and subsequently analyzed by DSC, SEC, analytical TREF and solid state NMR measurements. Fraction 1 consisted of non-crystalline ethylene–propylene random copolymer (EPR). Fraction 2 was composed of intermediate ethylene and propylene segments with different length and capabilities to crystallize. In fraction 3, long propylene segments were the dominating contribution, although some amorphous and crystalline ethylene sequences were detected, which are probably incorporated into the polypropylene main chains, since no soluble component was detected by analytical TREF. Finally, isotactic polypropylene (iPP) was determined to be the unique component of fraction 4. The local molecular mobility in the different phases was analyzed using 2D Wide Line Separation (WISE) NMR, while 13C detected spin diffusion experiments helped to elucidate local morphologies. Scanning Force Microscopy (SFM) studies of the impact polypropylene revealed a semicrystalline iPP matrix with dispersed softer regions, which involve not only soft amorphous elastomeric EPR components, but additional nano-domains with semi-crystalline properties different from the iPP matrix. Combining the SFM and solid state NMR results, a phase model for the EPR regions dispersed in the iPP matrix is proposed.