Defining the distribution of ethylene-propylene copolymer phases in heterophasic ethylene-propylene copolymers by a sequential xylene extraction method: Chemical and morphological analysis

Abstract

Sequential xylene extraction (SXE) in combination with scanning electron microscopy (SEM) and high temperature solvent gradient interaction chromatography (HT-SGIC) were employed for the detailed visualisation and understanding of the evolution of phase morphology in heterophasic ethylene-propylene copolymer (HEPC) particles. The study focused on sequentially extracting the soluble EP rubber phase and EP block or segmented copolymers by SXE at various temperatures. Major changes in the particle morphology (significant increase in both size and number of voids in the particles) after SXE were revealed using SEM imaging. These void structures are believed to result from the amorphous phase (EPR) being removed during the xylene extraction which was further confirmed by differential scanning calorimetry (DSC), high temperature 13C NMR spectroscopy and HT-SGIC. The extractables obtained at 100 °C were found to be a mixture of EP random copolymers, semi-crystalline EP (block or segmented) copolymers, as well as iPP and PE homopolymers. Upon complete dissolution of the particles by SXE at 100 °C, significant amounts of the EP rubber fractions were obtained. Our results show a very heterogeneous distribution of EPR components with varying chemical compositions in HEPC particles produced by dual-reactor processes. For the layered structure observed from this study, truly amorphous EP rubber was found in the outer most regions followed by continuous regions of EP random copolymers having increasing ethylene contents. Propylene-rich EPR, extracted at temperatures of 100 and 130 °C, was observed as the inner EPR phase. Semi-crystalline EP (segmented or block copolymer) and PE homopolymer were detected as intermediate structures between these two EPR regions, inside and outside the pores of the iPP particles. Based on these findings a modified multi-layered core–shell structure was proposed. The results obtained by the proposed SXE fractionation method and its combination with various analytical approaches are found to be very effective for the investigation of the phase composition present in HEPC particles. The present approach is general and can also be used for other multiphase semi-crystalline polyolefins.