Physical ageing, which is the spontaneous increase of mechanical relaxation times after quenching from above the glass transition temperature, was the subject of this study. The quench at the beginning of physical ageing serves to 'rejuvenate' the material. This means it reproducibly creates a frozen-in non-equilibrium material structure with an increased mobility which then relaxes towards equilibrium. The creep behaviour is used to represent the molecular mobility, and was measured with small strain torsional creep tests. As the mobility decreases the retardation time increases. A quantitative evaluation of the changes in the creep behaviour as a result of physical ageing was undertaken. The creep data were combined with measurements that tested other thermodynamic quantities also known to exhibit recovery. These are the specific enthalpy measured with DSC (diferential scanning calorimetry) and specific volume measured using a density gradient column. Physical ageing experiments on polystyrene (an amorphous polymer) were intended to choose between the wide variety of existing theories. No appreciable change in the relaxation spectrum was found; howver, a closer look at the common superposition procedures revealed that such change might easily remain undetected. The experiments measuring volume relaxation and enthalpy relaxation show that physical ageing cannot be related in a straightforward sense to enthalpy relaxation or volume relaxation. In polypropylene, a semicrystalline polymer, various recovery effects were observed at room temperature. This is above the material's glass transition temperature and can therefore not be explained in the same terms as physical ageing in polystyrene. The phenomena are however strikingly similar. Not only does the relaxation spectrum of polypropylene shift to longer relaxation times without much change in the spectrum, but also volume relaxation and enthalpy relaxation are observed in magnitudes comparable to polystyrene. The enthalpy relaxation as observed using DSC shows complexities which help to understand the mechanism of structural relaxation in polypropylene.

The carbocationic polymerisation of polyisobutenes initiated by zinc-based Lewis acids has been investigated. Polyisobutenes are important materials that find their applications depending on the molecular weight. Thus, the production of butyl rubber ( Mw ~500,000) by the copolymerisation of isobutene (ill) with isoprene (IP) is an important industrial process. Commercially, polymerisation is initiated by protons generated using an AICh / H20 slurry in chloromethane solvent at low temperatures (ca. -100°C). Incorporation of IP is typically of the order of 1-2%. However, under such conditions isoprene acts as a powerful retardant, and both polymer molecular weight and polymer yield decrease steeply with increasing [IP] in the monomer feed. A system based on Zn(C~5h and an alkyl halide that produces copolymers with high molecular weights and increased isoprene incorporation have been found. Hence with increased main chain unsaturation, which is desirable to achieve more efficient crosslinking and improved compatibility with other unsaturated polymer materials. Studying the reactivity of this Lewis acid we could isolated a non-nucleophilic zincate and proved the capability of generating carbocations. On the other hand, the low reaction rate in covulcanisation found for Butyl rubber could be circumvented by polymerising isobutene with a small fraction of a comonomer different to IP that led to a search for new materials. Hence, polymers containing functionalities such as unsaturations of potentially higher reactivity than internal double bonds, phenylchlorinated units or pendant trimethylsilyl groups have been incorporated into the polymer chain. Finally, a system based on alkylzinc halides and alkyl halides provides polyisobutenes of a wide range of molecular weights ( Mn = 10,000- 150,000) at temperatures between -78 °C and +35 °C. These systems are the first that generates polyisobutenes with high ratios of teoninal double bonds at high temperatures that do not rely on transition metal components or complex noncoordinating anions.

This research has investigated nanocomposite based masterbatches as routes to improve the CO2 retention properties of PET bottles. Masterbatches of different types of polyamide/clay, PET/clay, PET/nano-silica flakes and PET/divalent layered metal phosphonates (DLMP) were produced by melt compounding and evaluated. In the case of polyamide based nanocomposites PA6 was found to produce the best dispersed nanocomposites followed by PA-MXD6, PA-6I/6T and PA-6-3-T. It was concluded from the results that surfactant/polymer compatibility and thermal stability play some role, but the most significant factor in effecting good dispersion was the polarity of the polymer and its ability to directly interact with the clay surface. The CO2 retention of PET/PA blends showed MXD6 to offer by far the greatest improvement (100% increase) but the use of PA-MXD6 nanocomposite did not result in further improvement. It was concluded that transfer of exfoliated clay platelets from the PA phase to the PET phase had not occurred. In order to address this issue and disperse the filler effectively through both polymer matrices several novel new processes were developed and the use of a catalyst was investigated. Overall, the novel PET/MXD6/clay blends had reduced CO2 retention compared to the direct PET/MXD6 blend due to significant degradation of the polymers in the extrusion stage prior to bottle manufacture. Nanocomposites produced by direct melt mixing of PET and organoclay were always intercalated in nature (with the exception of C30B and hexadecyl pyridinium surfactant where the layered structure collapsed due to degradation of the surfactant). A consistent interlayer spacing of ~3.15-3.25nm was observed for all these materials and it was concluded that a stable PET crystal structure had formed as the distance between layers corresponds to three repeats of the c dimension of the crystal unit cell. It is proposed that the stable equilibrium forms due to insufficient direct interaction between the polymer and the clay surface. Despite relatively poor dispersion modest improvements in CO2 barrier were achieved (up to 25%). The use of novel nano-silica flakes resulted in improved CO2 retention, particularly with the 100nm thickness grade (30% improvement) despite considerable breakage of the nano-silica flakes during melt compounding. In the case of DLMP the dispersion of the fillers was found to be poor and no improvement in CO2 barrier was obtained. It was also observed that all the fillers applied acted as nucleating agents for polymer crystallisation in the polymer systems to which they were applied.

Polymer ageing models predict the working lifetime of polymeric insulation in terms of the electrical and thermal stresses to which it is subjected. Two such models are investigated in this thesis and are found to be mathematically similar even though they are based on different mechanisms for the way in which an electric field accelerates the ageing process. It is shown that both models can successfully fit characteristic time-to-failure data from ageing experiments involving thin films. A new method is developed to allow the ageing models to be applied to cable insulation, where the field and temperature are not spatially constant. This method is used to apply one of the models to characteristic lifetime data from experiments involving cables. The fits to data are found to be good, and resulting parameter values are used as the basis for a discussion of the possible effects of specimen volume on ageing. The distribution of failure times observed when thin films and cable insulation are aged at a given experimental condition has also been investigated. This has been carried out using distributions of the activation free energy of ageing within one of the ageing models. It is established that small changes in the minimum activation energy from specimen to specimen could be responsible for the observed failure statistics. Changes in the activation energy distributions with ageing condition suggest that ageing may involve conformational re-arrangements of chain segments in the crystalline-amorphous interface. This is in broad agreement with the conclusions of other workers.

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The effects of material composition on the post-ageing mechanical response of elastomers are examined in this thesis. A series of experimental tests was carried out on two types of elastomer, namely Hydrogenated Nitrile Butadiene Rubber (HNBR) and the fluorocarbon (FKM). Both HNBR and FKM are widely used for various sealing applications in the automobile and the oil and gas industries. The experimental tests included uniaxial tension and compression; Shore A hardness; compression set and tear. These tests were conducted on the elastomers at room temperature after ageing for 3, 14 and 28 days at 50°, 100° and 150°, and at the elevated test temperatures of 50°C, 100° and 150°. Additional specimens were also tested as received. HNBR and FKM compound formulations differed in terms of elastomer base type, filler type (carbon black grade; FEF or SRF) and filler loading (10 to 50 parts of filler per hundred parts rubber (phr) by weight). This allowed the investigation into the effects of the composition on the mechanical properties of HNBR reinforced with SRF carbon black, HNBR reinforced with FEF carbon black, FKM reinforced with SRF carbon black and FKM reinforced with FEF carbon black. The results indicate that the amount of filler and type, the type of base polymer and the test conditions greatly influence the material properties of these elastomers. In some cases, the increase in filler loading not only results in an increase in the elastomer's overall cost and poor processing properties, but also impairs some of the mechanical properties. E.g. a maximum tensile strength is noted at filler loading of 40 phr for HNBR and at 30 phr for FKM irrespective of carbon black grade. The main observations are conveniently summarised in a tabular form. The implications of the experimental results for the design of oil well packers and bridge plugs are also discussed. Design charts are presented to aid the selection of elastomeric materials for various high temperature applications. The charts are shown to work for a simplified sealing packer problem.

There is increasing demand for micro components in large quantities for medical as well as non-medical applications. To a large extent, the required component are hollowed shaped made from polymers such as micro-needles for drug infusion, microfluidic systems for the life-sciences, micro-pumps and fluidic connectors in micro-heat exchanger. However the manufacture of polymeric micro hollow (tubular) components requires innovative and economically competitive micro manufacturing processes. Blow forming processes are based on internal pressurization of an extrusion shaped polymer micro tube could provide an economic and reliable micro manufacturing process to the mass production of precise complex-shaped hollow micro parts. Despite the advantages of blow forming process in conventional polymeric tubes expansion, the application to micro manufacturing has not been investigated to date. This work represents therefore a systematic investigation of the polymeric micro tube blow forming process aimed at providing fundamentals for micro manufacturing applications. Based on reviews of micro forming, conventional stretch blow forming of polymers and other forming processes such as hydroforming, process configuration and machine and, tools design consideration were defined. Investigations to miniaturize the design consideration concepts were conducted, which focused on the influences of tube dimensions (handling, flow stress, materials forming limit, instabilities, and friction), quality of extruded polymer micro-tubes, complexity of polymeric material properties, heat distribution and heat transfer in the mold insert, new sealing, and pressurization technique and process cycle-time variations. Practical experiments supported by numerical analysis formed the basis of the investigations. Experimental investigations were conducted with a flexible-custom designed pneumatic-electric hybrid device, while numerical analyses were FE program based. For systematic blow forming processing tests, process parameters were varied systematically to analyze their influence. The parameters were: temperature (cavity temperature), pressure, elongation compensation force (axial tension), and cycle time. Dimensions of micro tubes used were: initial outer diameter dα₀ between 630 μm and 1300 μm and initial inner diameter di₀ between 500 μm and 1000 μm. Customized software was developed in-house, to implement the expansion process configuration defined. The process software was written in LabVIEW, although data acquisition and control were possible via appropriate hardware interface. In the experiments, the mould was first heated to a polymer processing temperature and other parameters were varied systematically to analyze their influences and interrelations in the process. The research objectives of a functional demonstrator as shown in the Figure on page II, were achieved through application of precise control of the process parameters. The manufactured machine specifications are: maximum internal pressure of 50 bar, sealing force 120 N, closing force 132 N and local heating temperature up to 600 °C (heating cartridge 100 Wcm-²) and a cycle times variation from 30 to 40 ms (without handling time). Machine design specifications supports processing of polymeric micro tubes with dα₀ below 1340 μm and di₀ below 1000 μm and a tube length of 60 mm. Operation on industrial automation control environment such as programmable logic controllers (PLC) was also supported, to enable integration on micro manufacturing platform for volume manufacture of hollow polymer components Experimental results conducted with the developed machine showed significant process temperature deviation for a failure free process and part compared to results from the literature for PA 6 and PC micro-tube materials. PET showed less deviation. Achieved suitable process parameters for PET were a maximum internal pressure of 18 bar and working temperature Tf of 90 °C, while optimal parameters to form PA 6 are maximum internal pressure, 15 bars and process temperature of 190 °C. Tube outer diameter of 1300 μm and inner diameter of 1000 μm was used in both cases to achieve maximum formed hoop ratio of 2. Cooling and handling were conducted manually and will require more investigations and refinement to enable integration in a full automated system.

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