Browsing by Author "Eselem Bungu, Paul Severin"
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- ItemDevelopment of a multiple fractionation protocol for the comprehensive analysis of low density polyethylene(Stellenbosch : Stellenbosch University, 2018-12) Eselem Bungu, Paul Severin; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Low density polyethylene (LDPE) is a branched synthetic thermoplastic with a complex microstructure. In addition to the broad molar mass distribution (MMD), LDPE exhibits long chain branching (LCB) and short chain branching (SCB). Both LCB and SCB are statistically distributed across different polymer chains of varying molar masses, thus providing broad distributions in molar mass and branching. This interrelated MMD and branching distribution (BD) influences the end-use properties and, therefore, the applications. To be able to design new materials, comprehensive structure-property relationships must be established. To this aim, the microstructural parameters (MMD and BD) must be measured quantitatively and related to the thermophysical properties. In the current study, the microstructure of different branched polyethylenes was investigated. The quantification and determination of MMD was achieved by multidetector size exclusion chromatography (SEC). The branch types and branching contents were quantitatively measured using carbon-13 nuclear magnetic resonance spectroscopy (13C NMR). As an alternative and complementary method, SEC was coupled to an infrared detector to measure total branching and BD as a function of molar mass. LCB distributions were measured as a function of molar mass by SEC coupled to multiangle laser light scattering (MALLS). Branching information as a function of crystallizability was obtained by crystallization analysis fractionation (CRYSTAF), solution crystallization analysis by laser light scattering (SCALLS) and differential scanning calorimetry (DSC). Chain branching was also measured by high-temperature solvent gradient interactive chromatography (HT-SGIC). The effects of BD and MMD on the thermophysical behaviour were investigated by observing the crystallization and melting behaviour using DSC. Thermal properties to a large extent determine the processing properties of a given material. In the first part of this work, a multiple preparative fractionation concept was developed and used for the comprehensive characterization of LDPE. Narrowly dispersed molar mass and branching fractions were obtained using preparative molar mass fractionation (pMMF) and preparative temperature rising elution fractionation (pTREF) techniques, respectively. The molar mass and branching information were obtained by analysing the separated fractions using advanced analytical techniques. Cross-correlation of molar mass and branching was obtained by combining pTREF and pMMF results with SEC and CRYSTAF to construct 2D images of molar mass vs. branching. In the second part of the work, the multiple fractionation concept was used to investigate the microstructural differences between different LDPE samples. With the help of pTREF and pMMF, fractions with narrow molar mass and branching were generated. The fractions were analysed for branching and molar mass and cross-fractionation plots highlighted the microstructural differences between the samples. From the preparative fractions having broad molar mass and branching ranges, libraries were obtained with samples (1) having similar molar masses but different degrees of branching and, alternatively, (2) having different molar masses but similar degrees of branching. These library samples were analysed by CRYSTAF, SGIC, and multidetector SEC to investigate the effects of branching and molar masses on thermal properties. In the third part of this study, the multiple preparative fractionation concept was used to generate samples with similar molar mass/varying branching (pTREF) and similar branching/varying molar mass (pMMF). The library samples and bulk resins were analysed by DSC and thermal fractionation by successive self-nucleation and annealing (SSA) to provide information regarding crystal sizes and crystal size distributions. From the SSA results, methylene sequence length distribution (MSLD) plots were constructed providing information that was directly related to the branching and branching heterogeneity of these samples. In the last part of this study, the molecular structure of novel grafted polymers HDPE-g-LDPE and their linear and branched PE references was investigated. Fractionations were conducted by pTREF to generate fractions with varying degrees of branching and/or grafting. The cross-correlation techniques (TREF-SEC and TREF-CRYSTAF) were used to compare the grafting products. In addition, 2D- LC experiments were conducted to correlate branching/grafting to molar mass.