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Description

The project aimed to design and establish a novel measurement technique for the glass transition phenomenon temperature (T_g) of polymeric materials. This has been achieved by examining the ability of carbon dioxide (CO₂) at high pressures to soften polymers and consequently monitoring the T_g of various polymers at extreme pressures encountered in high engineering applications. The study was conducted in two parts: first, the mechanical measurement of the process of ‘polymer softening’ in CO₂ at normal conditions, and second, the mechanical measurement of polymer softening in CO₂ at severe conditions.  In the first study, the softening of polymers as a function of applied CO₂ pressure and temperature was measured using Differential Scanning Calorimetry (DSC) and the designed facility. In initial experiments, the temperature was slowly ramped upwards and the nominal glass transition temperature was recorded corresponding to the temperature at which the central deflection suddenly began to increase. Significant reductions in the bending onset temperatures (typically 50 - 100^oC) were observed on applying CO₂ for polycarbonate, polypropylene, polyethylene and polystyrene over the range of pressures applied (24 to 120 bar). A model was developed to use onset and full-scale bending temperatures to estimate diffusion coefficients, assuming that full-scale bending corresponds to the centre of the strip being sufficiently plasticized by CO₂ to become rubbery. An equation for deflection versus time was derived from a diffusion analysis where it was assumed that the stiffness of the strip was only due to the glassy core region, which gradually reduces in thickness as the CO₂ penetrates the strip. The comparison of the experimental data obtained from this work with the literature data was found to be excellent.