Polymer dynamics studied by NMR and light scattering methods : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in physics at Massey University

Abstract

Theoretical treatments of static and dynamic properties of polymer solutions are reviewed. Particular emphasis is placed on the discussion of diffusion in polymer solutions. The relationship between the mutual diffusion coefficient defined in non-equilibrium thermodynamics and the diffusion coefficient measured in a dynamic light scattering experiment is discussed. The blob model is applied to the calculation of the concentration and solvent quality dependence of the polymer self diffusion coefficient. An introduction to the theory of Pulsed Gradient Spin Echo (PGSE) NMR and Dynamic Light Scattering (DLS) is given. The possibility of directly measuring a nonlinear mean square displacement for a diffusing polymer molecule by Pulsed Gradient Spin Echo NMR is considered. Experimental techniques involved in PGSE NMR and DLS measurements are discussed. The variance of the normalized echo attenuation is derived and related to the weighting of least squares fits to PGSE NMR data. A run rejection scheme is proposed for DLS experiments. This scheme can be used to discriminate against data which may be distorted by the scattering from a small number of strongly scattering particulate contaminants in the sample. The concentration dependences of the polymer and solvent self diffusion coefficients in polystyrene-cyclohexane solutions have been measured. Cyclohexane is a theta solvent for polystyrene and the measurements were made at a temperature near the theta temperature. The polystyrene molar mass was 350,000 g mol-1. The exponent found for the concentration dependence was in good agreement with the theoretical prediction of the reptation model combined with scaling theory. The contribution of local friction effects, which often become apparent near the glass transition, is estimated from the solvent diffusion measurements. The preparation and characterization of poly(vinyl methyl ether) fractions is described. The fractionation was achieved by batch fractional precipitation from toluene solution with petroleum spirit. Gel permeation chromatography, dynamic light scattering and ultracentrifugation were used to characterize the fractions. The results of dynamic light scattering measurements on ternary polymer solutions are reported. Three sets of experiments are described. The first set of experiments was performed on the PS-PVME-toluene system. The PS and PVME molar masses were 929,000 g mol-1 and 102,000 g mol-1. PVME and toluene are very nearly isorefractive and the polystyrene was present only at trace concentrations. It has often been assumed in the literature that the self diffusion coefficient of the "visible" polymer is obtained in such measurements, but the range of the validity of this assumption has not been fully defined. Agreement with results in the literature is found, and the diffusion coefficient measured in these experiments is identified as the self diffusion coefficient of the polystyrene. However, in the second set of experiments, it is shown that the self diffusion coefficient of the polystyrene is not obtained from similar measurements when the solvent, toluene, is replaced by carbon tetrachloride. The difference is attributed to thermodynamic factors. The third set of experiments on ternary polymer solutions also investigated PS-PVME-toluene solutions. The polystyrene and PVME molar masses were 110,000 g mol-1. The effect of an increasing the polystyrene concentration was investigated. Both PGSE NMR and DLS experiments were performed on these samples. A direct comparison of the diffusion coefficients given by these two techniques showed that the self diffusion coefficient of the polystyrene was obtained from the DLS experiments, even when the polystyrene comprised 25% of the total polymer concentration in the samples. Although a cooperative mode of decay was expected to appear in the correlation functions measured in DLS experiments, it was not observed. These experiments are discussed using theoretical results from the literature.