Research involving supercritical fluids has received much interest in the scientific community over the last decade. With the ever increasing call for scientists, particularly chemists, to adapt to 'greener' technology, the use of supercritical fluids represents a way of decreasing the amount of more toxic solvents utilised in polymer synthesis and characterisation. For this reason, the fundamental interactions that occur upon exposure of polymers to supercritical carbon dioxide (supercritical C02) were investigated using high-pressure nuclear magnetic resonance (NMR) spectroscopy. An innovative, high-pressure NMR cell was designed that allowed in situ imaging and diffusion measurements of polymers under supercritical C02. Numerous polymer systems were investigated, including melts, fully amorphous and semi-crystalline polymers. The swelling of polymers in supercritical C02 was characterised, using NMR imaging techniques for the first time. This novel approach afforded simultaneous measurement of polymer swelling and diffusion along with measurement of NMR relaxation times. Indeed, the macromolecular properties of the swelling polymer - predominantly measured in the literature by gravimetry and optical viewing - were measured using high-pressure NMR, in addition to measurement of the intermolecular interactions that occur on a microscopic scale.
The swelling of numerous polymers upon exposure to supercritical C02 was investigated in order to gain an understanding of both the kinetics of C02 diffusion and also polymer chain dynamics. Poly (dimethyl siloxane) (PDMS) was shown to swell considerably in supercritical C02. The equilibrium degree of swelling increased with increasing pressure, but decreased with molecular weight of the PDMS.
In order to model the kinetics of C02 diffusion into solid substrates, PDMS matrices were synthesised with varying crosslink densities. These matrices also showed considerable swelling in supercritical C02, however the PDMS-C02 interaction parameter under the conditions of this experiment was, at best, 0.62. Hence, C02 is classed as a poor solvent for PDMS. Nonetheless, the increases in dimensions of the PDMS matrices were impressive, with volume increases of up to 200 %. On the other hand, the semicrystalline polymer, linear low density poly (ethylene) (LLDPE), exhibited minimal swelling in the solvent under the conditions studied. The effect of supercritical CO2, on the chain dynamics of LLDPE was investigated by NMR relaxometry.The information obtained from the polymer swelling and NMR relaxation experiments was used to describe the kinetics of styrene monomer diffusion into poly (ethylene). Highly miscible polymer blends were formed using a unique method that utilised the exceptional mass transport and swelling properties of supercritical C02. Indeed, various unique characterisation techniques were used to show that the poly (styrene) domain sizes were up to three orders of magnitude smaller when the blend was formed in supercritical C02, compared to blends formed in the absence of solvent. Characterisation techniques included Raman microprobe spectroscopy, modelling of relaxation times measured by solid-state NMR with spin diffusion measurements, differential scanning calorimetry and small angle x-ray scattering.The formation of micelles in supercritical CO2 using a novel, perfluoropoly( ether) surfactant, was investigated in situ by high-pressure NMR spectroscopy and by optical observation in a high-pressure view cell. Spontaneous microemulsion formation, in the absence of mechanical shear, was observed for water in supercritical CO, emulsions under modest temperatures and pressures. This was the first report of self-forming microemulsions with this surfactant and one of only a few using supercritical CO, as the bulk phase. Microemulsion droplet sizes were characterised using self-diffusion coefficients measured by NMR and restricted-diffusion models. The mean micelle radius was predicted to be 4 nm. However, subsequent attempts to polymerise acrylamide within the droplets led to the collapse of the microemulsion.
In summary, this work reports a comprehensive study of the behaviour of polymers in this unique solvent, using innovative methodology and novel spectroscopic techniques.