Combination of preparative and two-dimensional chromatographic fractionation with thermal analysis for the branching analysis of polyethylene

Abstract

Low density polyethylene (LDPE) or, more generally, branched polyethylene exhibits excellent processing properties thanks to its complex molecular structure. The branches on the polymer backbone are distributed by location and size and these molecular properties determine the crystalline/amorphous morphology of the crystalline, semi-crystalline and amorphous components. In the present study, the molecular complexity of branched PE is correlated to its thermal properties and morphology. The study follows the multiple fractionation protocol established on a representative LDPE resin, whereby, narrowly dispersed branched fractions having broad molar mass distributions are generated using preparative temperature rising elution fractionation (pTREF). Alternatively, broadly branched fractions having narrow molar mass dispersities are obtained via preparative molar mass fractionation (pMMF). The molecular structure of the fractions and the bulk resin are investigated using solution crystallization analysis by laser light scattering (SCALLS), differential scanning calorimetry (DSC) and successive self-nucleation and annealing fractionation (SSA) to describe the thermal properties and the morphology of the different molecular species. Since these techniques are biased towards the analysis of the crystallisable components, solvent gradient high performance liquid chromatography (HPLC) is used as a complementary technique to account for both the crystalline and amorphous components. A comprehensive two-dimensional liquid chromatography technique coupling HPLC and size exclusion chromatography (SEC) is used to investigate the bivariate distributions of branching and molar mass. SEC-MALLS and SEC-IR5 are used to determine long chain branching and total branching of the fractions and the bulk resin.