Ternary polymer solutions studied by dynamic light scattering and ultracentrifugation : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Physics at Massey University, New Zealand

Abstract

Dynamic light scattering has been used to measure the interpenetration diffusion coefficient of ternary polymer solutions formed with 233 000 dalton polystyrene and a trace amount of 330 000 dalton PMMA dissolved in thiophenol. These measurements have been compared with the diffusion coefficients obtained from ultracentrifuge studies of the same solutions by observing the change in shape of the sedimentation boundary curve with time. This novel use of ultra centrifugation to measure diffusion coefficients in ternary polymer solutions is possible because; (i) the solvent is isorefractive and isopycnic with polystyrene, and (ii) the PMMA was the minority polymer in the solutions studied (concentration 0.001 g/g). Hence only the PMMA was visible and it sedimented through an effective solvent of thiophenol and polystyrene. All measurements were made at 25°C. The diffusion coefficients obtained from the ultracentrifuge observations are in good agreement with the interpenetration diffusion coefficients obtained from dynamic light scattering studies on the same solutions. Solutions formed at polymer concentrations greater than 0.1 g/g suffered phase separation since PMMA and polystyrene are incompatible polymers. The solution with polymer concentration 0.0755 g/g was too unstable for ultra centrifugation studies, but was susceptible to investigation by dynamic light scattering. The interpenetration diffusion coefficient of this solution was found to be much smaller than those of the more dilute solutions. The temperature dependence of the interpenetration diffusion coefficient was also investigated on the solution with the largest polymer concentration. The interpenetration diffusion coefficient was found to decrease as the temperature was lowered. This behaviour is consistent with the prediction that the interpenetration diffusion coefficient approaches zero as phase separation is approached. A value for correlation length critical exponent, n, of this ternary solution system was found. This is the first time that dynamic light scattering has been used (successfully) to obtain a value for n. Dynamic light scattering and ultra centrifugation were also used to investigate the dynamical behaviour of ternary polymer solutions formed with 233 000 dalton polystyrene and a trace amount of 110 000 dalton poly(vinyl methyl ether) (PVME) dissolved in thiophenol. The preparation of this PVME fraction is described here. The fractionation was achieved by batch fractional precipitation from toluene solution with petroleum spirit All ultra centrifugation studies carried out on this system were unsuccessful because the solutions phase separated when subjected to strong centrifugal forces. The results of the light scattering measurements are reported and discussed.