Abstracts Chemistry

Multidimensional analytical approach for the characterization of complex ethylene-propylene copolymers

by Justice Mohau Phiri

ENGLISH ABSTRACT : Impact polypropylene copolymers (IPC) are complex multiphase materials having ethylenepropylene rubber particles incorporated in a semi-crystalline isotactic polypropylene (iPP) matrix. The ethylene-propylene rubber (EPR) particles form the most critical part of IPC. They are continuously dispersed in the iPP matrix and are supposed to prevent crack propagation under mechanical stress. The EPR phase decreases the crystal size of the crystalline iPP phase, and as a result an improvement in the impact strength of IPC is usually observed. IPC materials have much improved impact resistance as compared to conventional iPP and, thus, they can be used in a wide temperature range. IPCs usually have low melt flow rates (MFR) that limit their application in a number of moulding applications. One way to increase the MFR is the vis-breaking process that decreases the molar mass of the polymer. Details regarding the molecular structures of the vis-broken IPC and the molecular background of the vis-breaking process are not completely available up to date. Therefore, it was the main aim of the present study to investigate the compositional heterogeneities of EPR and vis-broken IPC materials using multidimensional analytical approaches. A set of EPR copolymers prepared by Ziegler-Natta and metallocene catalyst systems were used in the present study as model systems to mimic the rubber phase in IPC. The EPR samples had high comonomer contents in the range of 30 – 70 mol %. For another part of the work, a set of vis-broken IPC samples with different MFR values were prepared by degradation with an organic peroxide. Bulk samples from the two sets of samples were analysed by SEC, DSC, FT-IR, 13C NMR, CRYSTAF and HT-HPLC. The bulk samples were fractionated by preparative TREF procedures to obtain fractions at different TREF elution temperatures which could be further analysed to obtain comprehensive information on the molecular compositions of the samples. For the EPR copolymer samples, HT-SEC showed that the bulk samples and their TREF fractions have broader MMDs as compared to similar metallocene-based samples. The bulk sample analyses showed that the EPR samples consist mainly of random EP copolymers making about 65 % of the bulk samples. DSC and CRYSTAF were able to analyse only the semi- and highly crystalline TREF fractions. Both techniques did not fully characterise the 30 °C TREF fractions since these fractions contained random EP copolymers which were completely amorphous. 13C NMR was used to determine the comonomer contents of the fractions as well as their monomer sequence distributions. NMR analyses of comparable metallocene-based copolymer fractions showed that these had remarkably high chemical heterogeneities. The separation of the samples and their TREF fractions by HT-HPLC was found to be sensitive to the ethylene content of the samples. The exact chemical compositions of the fractions were determined by coupling HT-HPLC to FT-IR spectroscopy. This hyphenated technique… Advisors/Committee Members: Pasch, Harald, Stellenbosch University. Faculty of Science. Department of Chemistry and Polymer Science.