Bivariate molecular structure distribution of randomly branched polyethylene by orthogonal preparative fractionation

Abstract

Randomly branched polyethylenes such as low density polyethylene (LDPE) exhibit complex molecular structures with broad distributions regarding branching and molar mass. Recently, a multiple preparative fractionation concept has been developed to reduce the molecular complexity of LDPE, whereby fraction libraries with narrow molar mass or branching distributions were generated. Depending on the type of preparative fractionation, library samples were obtained that had (A) narrow branching distributions but broad molar mass distributions or (B) narrow molar mass distributions but broad branching distributions. The proper analysis of such samples is still a challenge. In the present study, it is proposed to use an orthogonal preparative cross-fractionation approach to produce sample libraries with narrow molar mass and branching distributions. To this end, the majority fractions from a first step preparative temperature rising elution fractionation (pTREF) and preparative molar mass fractionation (pMMF) are subjected to a second preparative fractionation step. Accordingly, the (first step) majority pTREF fraction is subsequently fractionated by pMMF and the (first step) majority pMMF fraction is subsequently fractionated by pTREF. Using this preparative cross-fractionation approach (pTREF × pMMF or pMMF × pTREF) fraction libraries are obtained with significantly reduced molecular heterogeneities. The success of the dual preparative fractionation approach is confirmed by advanced analysis of the fractions using multidetector size exclusion chromatography, solvent gradient interaction chromatography, crystallization analysis fractionation, and thermal analysis.