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Ethylene-1-octene elastomers: Molecular structure characterization by advanced analytical methods
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Polyolefin elastomers are non-crystallising materials which are challenging to characterize with conventional crystallization-based techniques such as crystallization analysis fractionation (CRYSTAF) and temperature rising elution fractionation (TREF). However, interaction chromatography (IC) offers an alternative pathway for the chemical composition distribution definition of these materials. This work details the development of comprehensive characterization methods of linear low-density polyethylene (LLDPE) elastomers. In the first part of this work, four preparative fractionation methods namely, solution crystallization fraction (p-SCF), preparative temperature rising elution fractionation (p-TREF), modified p-TREF, and preparative molar mass fractionation (p-MMF) were compared for the analysis of an LLDPE elastomer with 12.8 mol% 1-octene content. While p-TREF showed the coelution effect at low elution temperatures, sufficient amount of fractions with narrow molar mass dispersity were obtained with the p-MMF method. Despite the limitations with the TREF method, it was demonstrated that both fractionation techniques provide detailed information on the complex nature of the LLDPE elastomer molecular structure after subsequent analysis with several other offline techniques such as solvent gradient interaction chromatography (SGIC). The modified p-TREF method also produced fractions with interestingly narrow dispersities and showed significant molar mass influence on its fractionation mechanism. The molar mass influence on the collected fractions was also observed with the p-SCF method, and both methods are a subject for future study. It was shown that when using a binary solvent mixture in which one of the solvents limits the solubility of the polyolefin chains, the separation is molar mass dependent even if column temperature is the active variable. In the second part of this work, four LLDPE copolymers with increasing amounts of 1-octene but similar molar masses were utilized to investigate the influence of chemical composition on the p-MMF technique. For all four LLDPE copolymers, eight fractions with decreasing molar masses were collected at different non-solvent/solvent ratios as the amount of non-solvent in the non-solvent/solvent mixture was increased. It was shown that chemical composition of these samples was independent of the fraction number, but was influenced by the average chemical composition of their respective bulk sample. Consequently, it was concluded that the molecular structure complexity of the four samples decreased with increasing 1-octene content. Finally, three commercial LLDPE elastomers with high 1-octene contents were comprehensively studied. Preparative molar mass fractionation was employed to obtain fractions which were further analysed using size exclusion chromatography (SEC) and differential scanning calorimetry (DSC). It was found that at bulk level, the samples appear to exhibit microstructural homogeneity, however, their respective p-MMF fractions exhibit multimodal chemical composition distributions. Furthermore, the p-MMF fractions showed an increase in their chemical composition heterogeneity as the molar mass decreased. Ultimately, it was concluded that the p-MMF method is the most suitable technique that provides in-depth insight on the molecular structure complexity of LLDPE elastomers.