Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method-high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method-HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene-propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.