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Thermoplastic elastomers have been found to have unsual properties, a consequence of composition and structure. The molecular composition comprises hard thermoplastic blocks which aggregate into domains, and flexible elastomer blocks in a linear or inter-penetrating structure. The mechanical properties such as stress relaxation, tensile strength, elongation, recovery and hardness of some thermopastic elatomers have been studied in some detail. The stress relaxation studies have been made possible with the development of the stress relaxation measuring equipment at the Institute of Polymer Technology. Highly accurate and reproducible results were obtained from the "ideal curve" measurements taken with the equipment, which permits continuous measurements of residual force and instantaneous modulus. It was noted that stress relaxation, while not only dependent on thermoplastic type and/or formulation (as expected) but also depended on the measuring technique (e.g. the strain rate, continuous loading, interrupted loading etc.) which have significant effects on subsequent stress relaxation. Temperature and environment also affect the results. The effect of thermal treatment, lubrication of surfaces and interrupted loading were investigated. An attempt was made to relate the modulus enhancement factor "MEF" to Hysteresis. An attempt was also made to relate the change in "MEF" to the continuous structural re-organization in the material and finally to stress relaxation.The commercial significance of stress relaxation and "MEF" in the performance of seals and gaskets is also explored. Some of the material supplied by industry for this project was prepared by "dynamic vulcanization". Attempt has been made using peroxide cross-linking agent to prepare EPDM/PP blends by this technique to explore structure-property relationship. As expected, the cured samples out-performed the uncured samples. Long term stress relaxation measurement (up to 10,000 hours) revealed the low premanent set and modest stress relaxation associated with thermoplastic elastomers in general.

Existing methods of designing load-bearing thermoplastics, products are based upon the application of Strength of Materials formulae in conjunction with material data obtained from simple specimens in standard tests. In injection moulding the melt flow causes the material to be oriented and consequently to become anisotropic. In this case, the application of current design methods may not predict with reasonable accuracy the behaviour of components subjected to complex states of stress.

FEM has been applied to analyse the influence of interlayers on the stress distribution around cylindrical fibres in an elastic matrix. This being part of a research programme aiming to enhance the energy absorption characteristics of composites by means of interlayers from materials exhibiting high ductility under plane strain conditions, e. g. UHMWPE. The theoretical model is based on the Galerkin weighted residual finite element in conjunction with a "penalty approach". The advantage of this method for polymer composites is in its ability to cope very effectively with non-linear systems. Glass microscope slides were initially used to develop a technique for bonding the ductile layers on to glass and polyamides respectively, as well as to provide simple verifications of the applicability of the aforementioned model. Further experiments were carried out on glass fibres coated with UHMWPE from a xylene solution, in order to evaluate the applicability of the above technique and of the theoretical model in actual composite systems. Although great difficulties were encountered in achieving a well bonded uniform coating on the glass fibres the results have confirmed the viability of the approach: The impact strength of compression moulded glass reinforced Nylon plaques, measured with an instrumented falling weight apparatus was increased up to 400% with a corresponding loss in flexural modulus of only 10-15%. The ductile nature of the interfacial failure between fibres and matrix was also confirmed by SEM examination of fractured specimens. Thermal analysis results, especially from DMA indicate that an UHMWPE interlayer substantially increases the tan 8 of the short fibre composite over a wide temperature range, albeit with some reduction in modulus. There was good agreement between the flexural modulus results obtained from DMA tests and those obtained using 3 point bending at room temperature.

The project involved the synthesis, and subsequent characterisation of structured latex particles, namely materials of core- shell morphology, produced by emulsion polymerisation techniques. The polymeric cores were polystyrene which has a high glass transition temperature, and the outer shell was prepared from a copolymer of vinyl acetate and nbutyl acrylate which has a low glass transition temperature. Five polystyrene core latices were prepared, in which the amount of the crosslinker agent, tetraethylene glycol dimethacrylate, used varied from zero to fifteen mole percent. Experimental work was undertaken in order to evaluate what the effect of varying the crosslink density had on the morphology of the structured latex particles. All of the materials were found to film-form at room temperature, but the particle morphology varied as the crosslinking agent concentration increased. The particle morphology was also dependent on the mode of polymerisation. In general, as the degree of crosslinking increased within the polystyrene cores, the glass transition temperature of this component was raised. At very high levels of crosslinking the particle morphology was found to be less like the idealised core-shell morphology. Overall, raising the crosslink density within the cores led to an increase in mixing of the two components within the particles. Analytical characterisation techniques employed in this project have included dynamic mechanical thermal analysis, differential scanning calorimetry, modulated-temperature differential scanning calorimetry, formation microscopy thermogravimetric analysis, minimum film temperature, transmission electron microscopy and particle sizing using correlation spectroscopy.

High performance aromatic polymers such as PEEK and PEK are widely used in composite and related applications. However, their high rate thermomechanical properties are not well understood. This thesis describes a series of investigations into their mechanical behaviour over a large range of strain rates (10-3 -103 s-1 ) and temperatures (20-200° C) which were carried out in order to more fully understand their properties and to assess the applicability of standard polymer property models to their behaviour. The experiments involved the design and construction of two novel sets of high rate test apparatus. These were a cross-bow based system which enabled high strains to be obtained at strain rates of 103 s-1 and a drop-weight system based around a high speed camera which enabled direct measurements of radial strain and observation of sample behaviour to take place. The cross-bow apparatus incorporated a laser-photodiode system to enable direct strain measurements to be made and thus had an advantage over conventional Hopkinson bars of direct, rather than derived, strain measurements with a sufficiently energetic projectile to produce large deformations. These systems were used in addition to standard hydraulic ram and dropweight equipment. A heater unit to enable tests to be carried out over the desired temperature range was also designed and constructed and used with all the above systems. A comprehensive set of Differential Scanning Calorimetry and X-ray tests were carried out on samples before and after mechanical testing in order to provide structural data to aid the interpretation of the mechanical test results

The present work was concentrated on exploring and developing a toughening material system to be used in engineering products made of high crystalline PET [poly(ethylene terephthalate)]. The work addressed some fundamental objectives in the technology. of crystallised PET (CPET) related to toughness and thermoforming. PET is widely used in fibres and oriented films but is also an attractive material for moulded products, owing to its high melting point and solvent resistance. Crystalline PET, however, is brittle and susceptible to hydrolytic degradation through water absorption. The polymeric toughening system was based on suitably modified UHMWPE (ultra high molecular weight polyethylene) a crystalline polymer with very high fracture toughness under plane strain conditions, low water absorption, but with a relatively low melting point. Being a thermoplastic it has a modulus about 1,000 times higher than the equivalent hydrocarbon elastomer usually used to toughen PET and polyamides. Hence, deterioration of strength and stiffness is minimized. Blending PET with UHMWPE produces coarse microstructures and poor mechanical properties due to lack of compatibility of the two polymers. Compatibility is expected to be improved by reducing the interfacial tension and increasing the adhesion between the two phases; the first will reduce the size of the dispersed phase, while the latter would improve the mechanical properties. Compatibilisation of a pair of immiscible polymers, like in blend of PET with UHMWPE, can be achieved by incorporation of reactive units along the main chains of the UHMWPE, which are capable of strong interactions with the matrix component (PET). The methods used in this investigation involved smface grafting and irradiation treatment to produce chemically active groups on the UHMWPE polymer chain to achieve the required level of interfacial adhesion assisting the agglomerate particle breakdown into small size and to develop strong bonding to the PET when the two polymers are blended in a high shear mixing. The results obtained with several modified UHMWPE powders incorporated into PET by melt mixing showed a clear formation of small primary particles of UHMWPE strongly bonded to the matrix, thereby achieving a vast improvement in toughness while preserving other important fundamental properties related to both the end product and the material processing.

Advanced composite materials based mainly on epoxy resins are being used in increasing numbers of components in the aerospace industry. Such components have to survive in a range of moisture and temperature environments in different parts of the world at varying altitudes. It is important therefore to have sufficient information about the behaviour of composite components to predict what effect these environments will have on their properties. The aim of the work reported in this thesis was to provide such information not only for epoxy based systems but to make comparisons with polyester and vinyl ester based materials. Five glass fibre reinforced resin systems were used. Two vinyl esters, one polyester, one straight epoxy and one epoxy prepreg mixture. The effect of immersion in distilled water and exposure to humid air at 60% and 95% relative humidity was investigated at temperatures ranging from 25°c to 70°c, for periods of sixteen and forty days. During these periods moisture uptake for both unidirectional and bidirectional materials was recorded on a daily basis, and variations in glass transition temperature were determined. At the end of each period the ultimate tensile stress, tensile modulus, tensile strain to failure, interlaminar shear strength and interplanar shear strength of each material was determined. The water absorption results for the vinyl ester resins, polyester and straight epoxy resin initially showed Fickian diffusion characteristics. In the epoxy prepreg material a two stage diffusion process was observed. No equilibrium water absorption plateau was obtained over forty days at 60% relative humidity for any of the materials, at any temperature. All the mechanical properties dropped under these conditions and this was shown to be as a result of degradation at the glass-resin interface. At 95% relative humidity the fall in mechanical properties was greater and not recoverable. Under these conditions plasticization of the matrix had occurred. All the material samples which were subject to hot water under- went pronounced degradation. The degradation process was shown to be due to penetration of water at the glass-resin interface, followed by attack on the coupling agent and glass fibre surface. This degradation process was confirmed.by micro-observations of the fracture surfaces.

There has been a growing trend in recent years for polymer product manufacturers to use natural polymer and additive masterbatches instead of premixed compounds. For both polymer converters and polymer manufacturers, masterbatching makes economic sense. For the converter, the advantage is in the ability to buy and store in bulk a small number of base polymers which may be modified according to the dictates of the order book. This prevents the need to maintain an inventory of a large number of special compounds. Masterbatch base is very often low molecular weight polyethylene or some suitable low molecular weight compound. They are usually used with a wide range of polymer compounds. Manufacturers assume good additive dispersion in the products with use of masterbatch. This study investigates the quality of dispersion in masterbatches and the extent they can be used with varying polyolefin polymers. Also investigated is how additive particles are transferred from the masterbatch to another polymer during mixing and any morphological features that might relate to the degree of dispersion. A quantitative dispersion procedure in polyolefin products is also sought. X-ray microradiography, light microscopy and ultraviolet microscopy have enabled pigment and ultraviolet absorber dispersion in masterbatches and products to be studied. Pigment dispersion in low density polyethylene masterbatch is almost invariably bad. Iron oxide particularly was found to be the most poorly dispersed compared to other inorganics such as zinc sulphide, titanium dioxide and cadmium sulphide. On the other hand, the distribution of Cyasorb 531 in LDPE masterbatch is uniform. The degree of dispersion of UV absorber in polyethylene products depends on the difference between the melt flow index between the masterbatch base and the base polymer. Simulated sunlight exposure experiments have shown that increased absorber distribution significantly increases photostability of a high MFI HDPE/LDPE UV masterbatch blend. A semi-automatic procedure for quantifying pigment dispersion in polyolefin products has been developed. It involves a motorised stage scanning of a microtomed section of a polyolefin product with measurements being made with a photometer operating in a dark-field illumination and interfaced to a microcomputer. The procedure has enabled the point of significant agglomeration as well as the effect of shear rate and temperature on degree of dispersion in extruded products to be determined.