Comprehensive Analysis of Polyethylene Graft Copolymers by Preparative Fractionation, Interaction Chromatography, and Thermal Analysis

The comprehensive analysis of complex copolymers is a challenge and requires the combination of results from different methods including size exclusion chromatography for molar mass and molecular spectroscopy for chemical composition analysis. In true copolymers, comonomers with different chemical structures are combined, resulting in various molecular compositions and topologies which can readily be identified. Different analytical challenges arise when only one monomer is used to produce complex molecular topologies. In this work, polyethylene architectures are designed by using the grafting of low density polyethylene (LDPE) onto a reactive backbone of high density linear polyethylene (HDPE). These graft copolymers consist of only ethylene repeat units. The graft copolymers are prepared by using a dual reactor setup where in the first reactor HDPE is catalytically produced. HDPE is then used as the feed for the free radical polymerization in the tube reactor. At different tube temperatures, polymers with different compositions are produced. To comprehensively analyze the molecular structure of the reaction products, they are subjected to preparative molar mass fractionation and preparative temperature rising elution fractionation. The analysis of the fractions by gradient (solvent and temperature) interaction chromatography, successive self-nucleation and annealing, and triple detection size exclusion chromatography reveals the presence of the true graft copolymer HDPE-g-LDPE along with HDPE and LDPE fractions and some HDPE-b-LDPE copolymer chains.