Browsing by Author "Ndlovu, Petronella Zabesuthu"

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    Long chain branching analysis of polyethylene using advanced fractionation methods
    (Stellenbosch : Stellenbosch University, 2021-12) Ndlovu, Petronella Zabesuthu; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: olyolefins (POs) have contributed immensely to the quality of life since their commercialization in the mid-1950s and they are modified continuously to suit new applications. PO homopolymer properties for materials such as polyethylene (PE) are strongly influenced by molar mass (MM), molar mass distribution (MMD), as well as branching types and branching distributions. These distributions influence the processability as well as the physical and mechanical properties of the PO; in turn affecting its end-use properties. In the first part of this study, three commercial low density polyethylenes (LDPES) and four long chain branched PEs (LCBPEs) are comprehensively analyzed using various advanced analytical techniques to elucidate their MM and branching structures. Fourier-transform infrared spectroscopy (FTIR) and high-resolution carbon-thirteen nuclear magnetic resonance spectroscopy (13C-NMR) were used for average chemical composition determination. 13C- NMR enabled the identification and quantification of the diverse short chain branches (SCB) e.g., methyl, ethyl, butyl amyl groups as well as long chain branches (LCB). High-temperature quadruple-detector size exclusion chromatography (HT-SEC-d4) revealed the branching differences via specific conformation plots. The differences in the LCB contents of the LCBPEs was readily identified in HT-SEC-d4 using Mark-Houwink-Sakurada (MHS) plots. These LCBs are estimated to be longer than C50 or C60 and this information cannot be readily obtained from 13C-NMR. LDPEs also showed deviation, although less significant, from linear behaviour and influences of both SCB and LCB could be readily identified. It was shown that in the absence of SCB, LCBs encourage formation of compact structures with low chain entanglement. High melting (Tm) and crystallization temperatures (Tc) as well as crystallinities (Xc) for LCBPEs which were different to LDPEs (where SCB was dominant) were obtained using differential scanning calorimetry (DSC). Interaction chromatography (HT-IC) was used to separate differently branched molecules on two different stationary phases using two interaction modes. Owing to the strong adsorptive force on porous graphitic carbon (PGC), the components in the LDPEs and LCBPEs could not be separated efficiently using temperature gradient interaction chromatography (TGIC). However, a solvent gradient (SGIC) could resolve the copolymer and homopolymer components. TGIC was more efficient in separating the differently branched chains when silica was used as the stationary phase due to the absence of a strong adsorptive force. The differences in SCB of the LDPEs could be readily recognised using TGIC wherein silica was used as the stationary phase. These differences were linked to tensile strength and Young’s moduli of the samples. Hyphenation of HT-IC in the first dimension to HT-SEC in the second dimension as in high-temperature two-dimensional liquid chromatography (HT-2D-LC) confirmed the molar mass of the eluting components. In the second part of the study the bulk samples are fractionated using preparative molar mass fractionation (p-MMF) to obtain fractions with distinctly different molar masses. HT-SEC-d4 used for molar mass and branching analyses confirmed 13C-NMR findings that SCB is inherent across the MM fractions of the LDPEs. On the other hand, LCB content was shown to increase with decrease in the fraction molar mass of LCBPE fractions. Chemical composition analyses using HT-TGIC showed that multiple branching distributions were present in the LCBPE fractions as seen in the multimodal elution patterns. Further fractionation of fractions and bulk samples exhibiting multimodal elution behaviours could possibly be carried out to investigate the underlying complex compositions.