Ultra-high molecular weight polyethylene (UHMWPE) is an engineering polymer that is utilized in many applications. Due to the presence of a transient physical network of entanglement, the processing of UHMWPE through conventional melt-extrusion techniques is extremely challenging. Reducing entanglement may provide a way to produce UHMWPE with superior mechanical properties by increasing the possibility of chain extension and chain orientation. A novel solvent-free route has been shown to produce disentangled UHMWPE by controlling the polymer synthesis. The polymerization conditions play a pivotal role in determining the entanglement density of UHMWPE. Homogeneous and heterogenous synthesis routes for the production of disentangled UHMWPE are viable. This Master Thesis performed sensitivity analysis on the controlled synthesis of disentangled UHMWPE based on MAO-activated bis(phenoxy imine) titanium dichloride catalytic system. Both Homogenous and heterogeneous synthesis routes were investigated and compared in terms of polymerization activity as well as entanglement density utilizing thermal, rheological, and morphological analysis techniques. Under the studied polymerization conditions, the catalytic system is able to produce disentangled UHMWPE with a minimum weight-average molecular weight of 4 million grams per mole via homogeneous as well as heterogeneous synthesis routes.

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This thesis is concerned with the development of a continuous melt extrusion process utilizing CO₂ for the production of materials that cannot be typically melt processed. The first goal of this study is to determine under what conditions it is possible to use CO₂ to plasticize and, thereby, reduce the viscosity of an acrylonitrile (AN) copolymer in an extrusion process and render it melt processable. In order to assess whether it was possible to absorb adequate amounts of CO₂ in short residence times by injection into a single screw extruder, a slit-die rheometer was attached to the end of the extrusion system for the purpose of directly assessing the viscosity reduction. A chemorheological analysis was performed on 65 and 85% AN copolymers to establish the temperature at which the 85% material would be stable for melt processing. This, coupled with studies correlating the degree of Tg and viscosity reduction with the amount of absorbed CO₂, allowed one to establish conditions for melt extrusion of the 85% AN. It was determined that the 85% AN material should absorb at least 5 weight percent CO₂ for a processing temperature reduction of 26°C in the extrusion process. The second goal of this study is to determine to what extent CO₂ can be used as a processing aid to melt process polyethylenes of higher molecular weight than can be typically melt processed. To assess the ability to melt process high molecular weight polyethylenes with CO₂, the viscosity of a 460,000 g/mol HDPE plasticized with various amounts of absorbed CO₂ as determined with the slit-die rheometer. A relationship was developed to determine the maximum molecular weight polyethylene that could be processed at a given viscosity reduction due to absorbed CO₂. The viscosity of a blend of 40 weight percent UHMWPE with the 460,000 g/mol HDPE with 12 weight percent CO₂ was reduced to that of the pure 460,000 g/mol HDPE as predicted by the relationship. Preliminary studies using a pressurized chamber attached to the exit of the die allowed one to assess the conditions under which suppression of foaming is possible. / Ph. D.

Blown films of high molecular weight high density polyethylene (HMW-HDPE) were produced from two resins of differing molecular weight (MW) and molecular weight distribution (MWD) using a high stalk bubble configuration. The processing conditions were varied such that three film gauges, each fabricated at three frost line heights (FLH), were produced. Crystalline orientation and tear resistance properties of the films were measured. Under appropriate conditions, the formation of two populations of lamellar stacks with their surface normals orthogonal to one another were observed. Increasing the FLH increased the amount of transverse direction (TD) stacked lamellae. This finding was related to bubble shape and relaxation behavior. Balanced in plane crystalline orientation was noted to give the best dart impact performance. Interestingly, for the lower Mw resin in the study, this could be achieved by down gauging. In a second project, structure-property-processing relationships were investigated in a series of high density polyethylene (HDPE) blown films. The use of metallocene and chromium oxide based resins allowed the effects of MW and MWD on orientation behavior to be studied. All films possessed Keller-Machin low stress morphologies oriented along the film MD. Under identical processing conditions, the narrower MWD resins produced films with greater orientation than the broader MWD resins of equivalent weight average MW. Greater processing stresses and shorter quench times were noted to produce higher levels of orientation. Moisture vapor transmission rate (MVTR) performance of these films was also measured. Orientation effects were seen to influence MVTR as permeation behavior did not scale directly with the crystalline content in the films. Additional studies investigated the relationship between comonomer content and the thermal and structural properties of novel poly(acrylonitrile-co-methyl acrylate) materials. Five polymers were studied with methyl acrylate (MA) content varying between 0 and 15 mol%. The MA decreased both the glass transition and melting temperatures. Melting point depression was sufficient in the two highest MA content copolymers to allow for complete melting prior to the onset of thermal degradation using modest heating rates (20 ºC/min). Insight into the heterogeneous structure of poly(acrylonitrile) homopolymer was gained through both conventional and modulated differential scanning calorimetry. / Ph. D.

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Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Linear low density polyethylene (LLDPE), one of the fastest growing types of polyethylene, is made from the copolymerisation of ethylene and higher 1-olefin comonomers. 1-octene is the comonomer of choice as it gives mechanically better LLDPEs as compared to other 1-olefins. Recently, a shortage of 1-octene has been observed in the global market. Considering the fact that ethylene/1-heptene (EH) copolymers may have properties that are very similar to those of ethylene/1-octene (EO), replacing 1-octene with 1-heptene as the comonomer in the manufacture of commercial linear low density polyethylene (LLDPE) is a viable option. In order to do so, evaluation of microstructural and mechanical properties of both types of resins and their comparison were carried out first. Several LLDPE resins were synthesised using Ziegler-Natta (ZN) and metallocene type catalysts. The LLDPE resins were made using varying amounts of the comonomer to obtain copolymers of different compositions. Ten of the ZN-LLDPE resins became the core focus of the present study. Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) showed the differences in the compositions of both the EH and EOresins. Crystallisation analysis fractionation (CRYSTAF), differential scanning calorimetry (DSC) and high temperature high performance liquid chromatography (HT-HPLC) revealed the presence of at least two fractions within the EH and EO copolymers which varied in quantity and chemical composition as the comonomer content was increased. The fractions were identified as being the copolymer (of ethylene and the comonomer) and polyethylene. Comparisons of the EH and EO CRYSTAF and HPLC data showed similarities in the microstructures of the resins. Preparative-temperature rising elution fractionation (prep-TREF) was used to obtain several fractions from each resin for quantification and analyses. DSC, HT-HPLC, CRYSTAF, and 13CNMR revealed close similarities in the fractions of EH and EO copolymers with comparable comonomer contents. It also was revealed that TREF fractionations are influenced by the bulk resin comonomer content. EH and EO copolymers demonstrated high similarities in tensile strength and Young’s modulus at comonomer contents of < 3 mol %. Minor differences in the mentioned properties at comonomer content of > 3 mol % were attributed to the slightly better ability of 1-octene at reducing crystallinity as compared to 1-heptene as well as small differences in the comonomer contents of the test samples. The results of the study suggest that 1-heptene can be used in the place of 1-octene in the commercial manufacture of LLDPE. / AFRIKAANSE OPSOMMING: Lineêre lae digtheid poliëtileen (LLDPE), een van die vinnigste groeiende poliëtileen tipes, word produseer deur die ko-polimerisasie van etileen en ‘n hoër 1-olefien ko-monomeer. 1-okteen is die ko-monomeer wat die meeste gebruik word aangesien dit LLDPE met die beste meganiese eienskappe produseer. Daar is egter ‘n tekort aan 1-okteen in die globale mark. Aangesien etileen/1-hepteen (EH) kopolimere moontlik soortgelyke eienskappe het as etileen/1-okteen (EO), kan 1-okteen moontlik vervang word deur 1-hepteen as ‘n komonomeer in die produksie van LLDPE. Om dit te doen is die meganiese en mikrostrukturele eienskappe van beide polimere geëvalueer. Verskeie LLDPE polimere is gesintetiseer met behulp van Ziegler-Natta (ZN) en metalloseen kataliste. Die komonomeer inhoud is gevarieer om LLDPE polimere te produseer met verskillende komposisie. Tien van die gesintetiseerde ZN-LLDPE polimere is gekies en is die kernfokus van die huidige studie. 13-Koolstof kern magnetiese resonans spektroskopie (13C KMR) het die variasie in ko-monomeer inhoud bevestig van beide die EH en EO polimere. Kristallisasie analise fraksioneering (CRYSTAF), differensiële skandeer kalorimetrie (DSC) en 'n hoë temperatuur hoë verrigting vloeistof chromatografie (HT-HPLC) het die teenwoordigheid van ten minste twee fraksies binne die EH en EO ko-polimeer bevestig wat ‘n variasie in hoeveelheid en chemise samestelling getoon het met ‘n toename van die ko-monomeer inhoud in die ko-polimeer. CRYSTAF en HT-HPLC data het getoon dat hierdie fraksies in EH en EO ooreenkomstige mikrostrukturele gedrag getoon het. Preparatiewe temperatuur styging elueering fraksioneering (prep-TREF) is gebruik om die polimere te fraksioneer om sodoende kwantitief die poliëtileen fraksies te verky en te analiseer. Verdere analise van die fraksies deur DSC, HT-HPLC, CRYSTAF en 13C KMR het getoon dat die fraksies, bekom van die EH en EO kopolimere met vergelykbare ko-monomeer inhoud, baie dieselfde eienskappe toon. Die analises het ook getoon dat die TREF fraksionering beinvloed word deur die ko-monomeer inhoud van die oorspronklike ko-polimeer.

published_or_final_version / Dentistry / Master / Master of Philosophy

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