M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / There has been a genuine problem with plastic carrier bags (PCBs) pollution since the 1970’s. A literature review revealed that very few scientific studies have been undertaken globally on PCB. The South African Government promulgated regulations to reduce numbers, encourage reuse and recycling of plastic bags in 2003. The regulations introduced a charge for PCBs. This study looked at the handling and disposal of PCB after 2003 in Gauteng Province, South Africa; and looked at the movement of PCB from major retailers and informal traders to consumers and recycling and recovery. The study excluded the manufacturers and distributors of PCB. It began with the retailers and informal traders being the source of PCB and extended to consumers during their grocery shopping in large retail stores and purchases from informal traders. The study also looked at the individual waste collectors operating in landfill sites, residential and industrial areas to establish the level of recycling of PCB. Buyisa-e-Bag (B-e-B) was at the end of the collection of PCB pathway where it was supposed to provide leadership in the recovery of the bags. Semi-structured interviews were used to collate data on recycling approach with specific focus on PCB and understanding of the legislation. A total of one hundred consumers were interviewed in the shopping malls. Consumers did not know much about the regulations hence they could not explain the reason they have to pay for PCB. Ninety one percent of consumers did not reuse bags for shopping and 68% reused PCB at home to store waste before it is disposed of. Eighty informal traders were interviewed: all indicated that they did not charge for PCB. Forty chain supermarkets managers were interviewed from the shopping centres covered by the study. The retailers were affected by the PCB regulations; they reduced the number of grocery packers and increased security to prevent theft. Nevertheless, they complied with the regulations by selling only the thick bags. Twenty landfill and recycling facilities managers formed part of the study. All landfill managers encouraged general waste recycling to prolong lifespan of the site. Fifty individual recyclers were interviewed in the landfill sites, recycling facilities and on the road side while pushing their trolleys. They found it economically impractical to collect PCBs. Awareness of plastic litter has increased and less is visible though this was not measured. Legal compliance with regulations and specifications needed to be actively driven by all the relevant parties. PCBs are fully recyclable; hence more public awareness is required aimed at preventing the contamination of bags which deters re-claimers from collecting them. B-e-B has not met most of the objectives of their formation and has since been placed under administration by DEAT. Inadequate communication and collective bargaining between the key role parties resulted in the delays in getting the recycling projects off the ground. Major retailers complied with the Government regulations. Informal traders and consumers were generally unaware of regulations and consequences of PCB use. Consumers bought new PCB and in most cases, failed to reuse them for shopping. Recycling of PCBs is not carried out effectively as it is not economically worthwhile. This requires further research to explore the potential uses of PCBs at the end of their lifecycle

A combination of surface analytical techniques, colloid probe Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) was used to optimise the grafting density of covalently attached 5, 20 and 40 kDa methoxy-terminated PEO layers (under marginal solvation (cloud point) conditions for the PEO molecules). The combination of these techniques allowed us to relate the PEO layer density and molecular conformations to the range, magnitude and types of forces generated by coatings of various grafting densities. The key optimisation parameter was the grafting time with the concentration of PEO in solution having a weaker effect. Oxidation of the substrate occurred, but did not significantly limit the surface density of the functional groups used to chemically attach the PEO molecules. Interactions between the substrate and silica were electrostatic in origin and did not contribute to the interaction between silica and the PEO surfaces due to salt screening effects Surfaces with dense, highly stretched PEO layers (brushes) generated purely repulsive forces at all separation distances, arising from compression by the silica spherical probe used. The force profiles for lower density surfaces comprised long-ranged attractive and short-ranged repulsive forces. The attractive forces were most likely due to attractive bridging interactions between the PEO chains and the SiO2 surface. For low grafting densities, i.e. inter-chain grafting distances, s &gt ??RF, the PEO layers were not strongly stretched and free to adsorb onto the opposing silica surface. XPS analysis demonstrated that HSA and Fibrinogen adsorbed onto low density 20 kDa PEO coatings (s &gt ??RF), most likely via diffusion through the PEO layer. No protein adsorption was found (detection limit &gt 10 ng/cm2) on high density, ???strongly stretched brush??? coatings (s &lt ?? RF). Analysis of data from the more sensitive Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) techniques indicated that low amounts of adsorbed HSA, lactoferrin, lysozyme, and IgG were present on high density 20 and 40 kDa surfaces; the most likely explanation being attractive interactions between the proteins and the PEO layers during the protein adsorption experiments. ToF-SIMS data obtained for the strongly stretched (s &lt ?? RF) 5 kDa PEO surfaces suggested that no protein was adsorbed, in line with the XPS data for the same surfaces.

The molecular weight distributions of two high molecular weight (M[subscript w]>>1 million) polyethylene polymers, WSR-308 and WSR-301, were characterized with gel permeation chromatography (GPC) coupled with a multi-angle laser light scattering detector (MALLS). The M[subscript w] of the WSR-308 was found to be 5.10x10⁶ g/mol with a molecular weight range from about 1 million g/mol to as high as 10 million g/mol. The M[subscript w] of the WSR-301 was found to be 3.16x10⁶ g/mol with the lowest molecular weight about 400,000 g/mol while the highest molecular weight component may have been as high as 8 million g/mol. Attempts to measure the M[subscript w] of the two polymers using static light scattering (SLS) techniques proved to be difficult. In conjunction with these studies, drag reduction and shear degradation studies of the two polymers in water were also conducted. Solutions of the two polymers, ranging from 1 to 10 ppm including mixtures of the two, were tested in a pipe-flow apparatus to obtain friction factor and %DR data. In every case, the greater the concentration and/or the molecular weight of the polymer, the greater the drag reduction effects. Additionally, the higher molecular weight polymer and mixtures with a greater weight percentage of the higher molecular weight polymer were found to shear degrade less quickly than otherwise. A unique point along the maximum drag reduction asymptote (MDA) termed the "divergence point" was a focus of this study and an energy model based on frictional losses correlates well to the data. The correlation developed here relates the difference in frictional losses between the solvent by itself and the polymer solution directly to the mass concentration and molecular weight of the polymer. This frictional difference was proportional to the product of the mass concentration and molecular weight where both quantities were to approximately the first power. / Graduation date: 2002

The rheological and structural properties of polymeric liquids cannot be condensed within a single numerical model. They should be described within hierarchical, multi-level numerical models in which each sub-model is responsible for different time and length scales; atomistic, mesoscopic, and continuum. In this study, the rheological and structural properties of linear, short-chain polyethylene liquids were investigated from the classical atomistic level to the mesoscopic and continuum levels of description. At the atomistic level of description, nonequilibrium molecular dynamics (NEMD) simulations of linear, short-chain polyethylene liquids spanning from C16H24 to C128H256 were performed to advance our knowledge of fundamental characteristic of chain molecules under shear and planar elongational flow. Furthermore, entanglement characteristics, such as the shortest primitive path length, and the network configurations, were investigated as functions of strain rate in both vastly different flow fields using the topological Z-code. At the mesoscopic level of description, Brownian dynamics (BD) simulations of a freely-jointed chain with equivalent contour length to C78H158 were carried out to compare single-chain dynamics in dense liquids (NEMD) and dilute solutions (BD) under shear flow. In addition, the macromolecular configurational diversity of individual chains in dense liquids and dilute solutions was explored using a brightness distribution method inspired by the rheo-optical investigation of DNA solutions. Based on these observations, a simple coarse-grained mesoscopic model for unentangled polymeric liquids and semi-dilute solutions was proposed and compared with NEMD simulation data and experiments of semi-dilute DNA solutions under shear flow in terms of the rheological and structural properties, such as viscosity, normal stress coefficients, conformation tensor, and so on. Moreover, this model was further coarse-grained to the continuum level through pre-averaging and compared with NEMD simulation data to examine the relationships between different levels of description on the rheological and structural properties of unentangled polymeric materials under shear flow.

Abstract Chapter 1 provides an overview of this study and a literature review. Emphasis is put on the materials used, the different processes available to synthesize polymer-clay nanocomposites, analytical methods to characterize nanophase materials and on the impact of the nanophase on the final physical properties of polymer-clay nanocomposites. Chapter 2 discusses PE-clay nanocomposites which were synthesized using metallocene and Ni-diimine catalysts through in-situ polymerization. Morphological studies were carried out by XRD, SEM, EDX, and TEM to investigate the intercalation and exfoliation mechanism. Prior to its injection into the polymerization reactor, montmorillonite (MMT) was treated with triisobutyl aluminum and undecylenyl alcohol (UOH). Triisobutyl aluminum (TIBA) can react with hydroxyl groups on the surface of MMT and UOH is able to react with TIBA on the MMT surface. An alkoxy bond is generated by the reaction of the hydroxyl groups of UOH with the TIBA on the surface of MMT. A single site catalyst was then supported on the MMT/TIBA/UOH support, generating a MMT/TIBA/UOH/CAT system. The free vinyl groups of the surface UOH molecules can be copolymerized with ethylene, leading to the formation of chemical bonds between the MMT surface and polyethylene (PE). Ethylene polymerizations with the MMT/TIBA/UOH/CAT system were compared with ethylene polymerization with unsupported catalysts. The resulting PE-clay nanocomposites were analyzed with electronic and optical microscopes to confirm the nanophase distribution of MMT platelets in the polymer matrix. TEM images showed that the exfoliated MMT layers appeared as single layers or aggregated layers in the polyethylene matrix. After Soxhlet extraction with boiling 1,2,4-trichlorobenzene, the morphology of the residue particles remaining the thimble showed polymer fibrils stemming from the MMT surface, providing direct evidence of the chemical bonds between MMT surfaces and polymer matrix. Some residue particles also show PE-clay hybrid fibers between the particles. Through SEM/EDX analysis, it was confirmed that the fiber’s composition possessed silicone atoms together with carbon atoms. Chapter 3 discusses the results of in-situ polymerizations in gas-phase. The same catalyst systems and polymerization conditions discussed in Chapter 2 for slurry polymerization were applied to the gas-phase polymerization in order to investigate the particle fragmentation mechanism. After gas-phase polymerization at atmospheric pressure, the surface morphologies were investigated by SEM and TEM. In the case of the MMT/TIBA/UOH/Cp2ZrCl2 system, small particles (< 10m) were shattered from the larger particles (> 100 m) in the early stages of polymerization. After 24-hours of continuous polymerization, polymer fibrils growing from the inside of the MMT particles were observed by SEM. After further investigation with TEM, the cross-section profile of the particles showed curved bundles of MMT platelets, which illustrates exfoliation starting from the edges of the MMT particles. The MMT/TIBA/UOH/Ni-diimine system shows a different surface morphology after polymerization. In the early stages of the polymerization, polymer films were generated from the inside of the particles. After further polymerization, the MMT particles shattered and formed aggregates of PE-clay nanocomposites, similar to the ones proposed in the multigrain model. Chapter 4 discusses the copolymerization of ethylene and acrylonitrile. Ethylene/acrylonitrile copolymers were produced in the presence of a Ni-diimine/EASC catalyst system without the use of supports. Polymerizations of ethylene and acrylonitrile showed comparable activities in low concentrations of acrylonitrile. However, in higher concentrations, acrylonitrile induced a reductive elimination of the alkyl groups in the activated nickel-diimine catalyst. Conclusively, GPC analyses showed that acrylonitrile behaves as a chain transfer agent, showing reductive elimination of alkyl groups in the catalytic active center. The polymerization product morphology was analyzed by SEM and TEM. Polyacrylonitrile domains were observed in the polyethylene matrix and confirmed its nanosize distribution in the polyethylene matrix. DSC analysis of ethylene/acrylonitrile copolymers shows that an exothermic reaction takes place from 300 C to 370 C. This exotherm band detected by DSC can be related to the cyclization and aromatization of the nitrile groups of polyacrylonitrile. Through IR analysis of the ethylene and acrylonitrile polymer under high temperatures, this cyclization and aromatization was confirmed to be the cause of the decrease of the nitrile band (at 2244 cm-1) and increase of the vinyl bands (at 1640 cm-1). In addition, thermal treatment in DSC and successive XRD analysis showed the formation of the lamellar structures in the polyethylene matrix, reported as lamellar formation of polyacrylonitrile due to cyclization and aromatization of nitrile groups. The decomposition temperatures measured by TGA increased up to 50 C due to the presence of the nitrile groups in the polymer matrix. Tensile testing showed that the modulus increased, together with the yield strength and elongation. This phenomenon supports that strong interfacial interactions exist between the polyethylene matrix and polyacrylonitrile domains, as confirmed by TEM and IR analysis. Chapter 5 introduces the idea of acrylonitrile as a clay surface modifier. MMT was treated with acrylonitrile, using the same modification method of MMT that was applied in the MMT/TIBA/UOH/CAT system in Chapter 2. The nitrile groups in PE-MMT/TIBA/AN/CAT composites were confirmed at 2244 cm-1 by IR analysis. DSC analysis of PE-MMT/TIBA/AN/CAT showed that an exothermic reaction takes place from 300 C to 375 C. Successive DSC analysis with the same sample showed a new glass transition temperature band, induced by the reduction of polymer chain mobility. The basal diffraction band disappeared due to the exfoliation of MMT. Tensile tests showed an increase in modulus, without sacrificing the yield strength and elongation of PE-clay hybrid composites. Through these analyses, it was confirmed that strong interfacial forces exist between the polyethylene matrix and MMT layers in these PE-clay nanocomposites.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

Polyethylene has been studied from many different perspectives; a final application property perspective, in which the response of the material to loads is the topic; a micromechanical point of view, in which the macroscopic state of the material is related to its microstructure, e.g., Alvarado (2007), and a chemical point of view in which the molecular structure and the processes that create polyethylene are investigated. This thesis focuses on the mechanical behavior of polyethylene observed from testing and relates the mechanical behavior to the molecular structure of the material. High density polyethylene is a material used in civil engineering applications such as pipes and containers. There are two general modes of failure for polyethylene: ductile failure that happens at relatively large stresses (up to 200MPa) and in short amount of time, and brittle failure that occurs when a much lower stress is sustained over a long period of time (Cheng 2008). Other than these two modes of failure, excessive deformation of the material that is usually caused by creep is also to be avoided. This thesis studies the relationship between short-term and long-term creep of polyethylene and its molecular structure. In this work three types of mechanical tests were performed on six samples of polyethylene. The existing models that prescribe the constitutive behavior of the material were then critically evaluated against the observed data. Furthermore the molecular properties of the samples that had been obtained from previous research by Cheng (2008) were compared against the mechanical behavior observed from testing in order to assess what molecular properties are important in determining the mechanical behavior of polyethylene. This information can also help polyethylene designers to produce longer lasting material, or a material that has high stiffness, by knowing what molecular properties to control and optimize.

Abstract Chapter 1 provides an overview of this study and a literature review. Emphasis is put on the materials used, the different processes available to synthesize polymer-clay nanocomposites, analytical methods to characterize nanophase materials and on the impact of the nanophase on the final physical properties of polymer-clay nanocomposites. Chapter 2 discusses PE-clay nanocomposites which were synthesized using metallocene and Ni-diimine catalysts through in-situ polymerization. Morphological studies were carried out by XRD, SEM, EDX, and TEM to investigate the intercalation and exfoliation mechanism. Prior to its injection into the polymerization reactor, montmorillonite (MMT) was treated with triisobutyl aluminum and undecylenyl alcohol (UOH). Triisobutyl aluminum (TIBA) can react with hydroxyl groups on the surface of MMT and UOH is able to react with TIBA on the MMT surface. An alkoxy bond is generated by the reaction of the hydroxyl groups of UOH with the TIBA on the surface of MMT. A single site catalyst was then supported on the MMT/TIBA/UOH support, generating a MMT/TIBA/UOH/CAT system. The free vinyl groups of the surface UOH molecules can be copolymerized with ethylene, leading to the formation of chemical bonds between the MMT surface and polyethylene (PE). Ethylene polymerizations with the MMT/TIBA/UOH/CAT system were compared with ethylene polymerization with unsupported catalysts. The resulting PE-clay nanocomposites were analyzed with electronic and optical microscopes to confirm the nanophase distribution of MMT platelets in the polymer matrix. TEM images showed that the exfoliated MMT layers appeared as single layers or aggregated layers in the polyethylene matrix. After Soxhlet extraction with boiling 1,2,4-trichlorobenzene, the morphology of the residue particles remaining the thimble showed polymer fibrils stemming from the MMT surface, providing direct evidence of the chemical bonds between MMT surfaces and polymer matrix. Some residue particles also show PE-clay hybrid fibers between the particles. Through SEM/EDX analysis, it was confirmed that the fiber’s composition possessed silicone atoms together with carbon atoms. Chapter 3 discusses the results of in-situ polymerizations in gas-phase. The same catalyst systems and polymerization conditions discussed in Chapter 2 for slurry polymerization were applied to the gas-phase polymerization in order to investigate the particle fragmentation mechanism. After gas-phase polymerization at atmospheric pressure, the surface morphologies were investigated by SEM and TEM. In the case of the MMT/TIBA/UOH/Cp2ZrCl2 system, small particles (< 10m) were shattered from the larger particles (> 100 m) in the early stages of polymerization. After 24-hours of continuous polymerization, polymer fibrils growing from the inside of the MMT particles were observed by SEM. After further investigation with TEM, the cross-section profile of the particles showed curved bundles of MMT platelets, which illustrates exfoliation starting from the edges of the MMT particles. The MMT/TIBA/UOH/Ni-diimine system shows a different surface morphology after polymerization. In the early stages of the polymerization, polymer films were generated from the inside of the particles. After further polymerization, the MMT particles shattered and formed aggregates of PE-clay nanocomposites, similar to the ones proposed in the multigrain model. Chapter 4 discusses the copolymerization of ethylene and acrylonitrile. Ethylene/acrylonitrile copolymers were produced in the presence of a Ni-diimine/EASC catalyst system without the use of supports. Polymerizations of ethylene and acrylonitrile showed comparable activities in low concentrations of acrylonitrile. However, in higher concentrations, acrylonitrile induced a reductive elimination of the alkyl groups in the activated nickel-diimine catalyst. Conclusively, GPC analyses showed that acrylonitrile behaves as a chain transfer agent, showing reductive elimination of alkyl groups in the catalytic active center. The polymerization product morphology was analyzed by SEM and TEM. Polyacrylonitrile domains were observed in the polyethylene matrix and confirmed its nanosize distribution in the polyethylene matrix. DSC analysis of ethylene/acrylonitrile copolymers shows that an exothermic reaction takes place from 300 C to 370 C. This exotherm band detected by DSC can be related to the cyclization and aromatization of the nitrile groups of polyacrylonitrile. Through IR analysis of the ethylene and acrylonitrile polymer under high temperatures, this cyclization and aromatization was confirmed to be the cause of the decrease of the nitrile band (at 2244 cm-1) and increase of the vinyl bands (at 1640 cm-1). In addition, thermal treatment in DSC and successive XRD analysis showed the formation of the lamellar structures in the polyethylene matrix, reported as lamellar formation of polyacrylonitrile due to cyclization and aromatization of nitrile groups. The decomposition temperatures measured by TGA increased up to 50 C due to the presence of the nitrile groups in the polymer matrix. Tensile testing showed that the modulus increased, together with the yield strength and elongation. This phenomenon supports that strong interfacial interactions exist between the polyethylene matrix and polyacrylonitrile domains, as confirmed by TEM and IR analysis. Chapter 5 introduces the idea of acrylonitrile as a clay surface modifier. MMT was treated with acrylonitrile, using the same modification method of MMT that was applied in the MMT/TIBA/UOH/CAT system in Chapter 2. The nitrile groups in PE-MMT/TIBA/AN/CAT composites were confirmed at 2244 cm-1 by IR analysis. DSC analysis of PE-MMT/TIBA/AN/CAT showed that an exothermic reaction takes place from 300 C to 375 C. Successive DSC analysis with the same sample showed a new glass transition temperature band, induced by the reduction of polymer chain mobility. The basal diffraction band disappeared due to the exfoliation of MMT. Tensile tests showed an increase in modulus, without sacrificing the yield strength and elongation of PE-clay hybrid composites. Through these analyses, it was confirmed that strong interfacial forces exist between the polyethylene matrix and MMT layers in these PE-clay nanocomposites.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

Polyethylene has been studied from many different perspectives; a final application property perspective, in which the response of the material to loads is the topic; a micromechanical point of view, in which the macroscopic state of the material is related to its microstructure, e.g., Alvarado (2007), and a chemical point of view in which the molecular structure and the processes that create polyethylene are investigated. This thesis focuses on the mechanical behavior of polyethylene observed from testing and relates the mechanical behavior to the molecular structure of the material. High density polyethylene is a material used in civil engineering applications such as pipes and containers. There are two general modes of failure for polyethylene: ductile failure that happens at relatively large stresses (up to 200MPa) and in short amount of time, and brittle failure that occurs when a much lower stress is sustained over a long period of time (Cheng 2008). Other than these two modes of failure, excessive deformation of the material that is usually caused by creep is also to be avoided. This thesis studies the relationship between short-term and long-term creep of polyethylene and its molecular structure. In this work three types of mechanical tests were performed on six samples of polyethylene. The existing models that prescribe the constitutive behavior of the material were then critically evaluated against the observed data. Furthermore the molecular properties of the samples that had been obtained from previous research by Cheng (2008) were compared against the mechanical behavior observed from testing in order to assess what molecular properties are important in determining the mechanical behavior of polyethylene. This information can also help polyethylene designers to produce longer lasting material, or a material that has high stiffness, by knowing what molecular properties to control and optimize.