Branching and molar mass analysis of low density polyethylene using the multiple preparative fractionation concept

Despite being a homopolymer, low density polyethylene (LDPE) exhibits a complex molecular structure that is determined by multiple molar mass and branching distributions. For a comprehensive microstructural analysis, preparative fractionation methods are combined with multiple advanced analysis techniques. Preparative temperature rising elution fractionation (pTREF) and preparative molar mass fractionation (pMMF) are used to fractionate three LDPE resins with different melt flow indexes into fractions with narrower branching and molar mass distributions, respectively. The chain structures of the bulk resins and their corresponding pTREF and pMMF fractions are further analyzed using size exclusion chromatography, crystallization analysis fractionation, differential scanning calorimetry, high-temperature solvent gradient interactive chromatography and SEC coupled to molar mass sensitive detectors to provide detailed information particularly on long chain branching and its correlation to molar mass. It is shown that the multiple fractionation concept is a powerful approach to generate LDPE sample libraries that may constitute fractions (samples) of comparable molar masses and different branching structures or alternatively have comparable branching but different molar masses. Cross-fractionation of these library samples with advanced analytical techniques provides in-depth information on the molecular heterogeneity of these samples as compared to bulk sample analysis.