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Recent theories have suggested that mutual diffusion between miscible polymers may be strongly influenced by the unusual thermodynamics of mixing of high polymers; in addition the mobility properties of polymer blends are not, in general well understood. This dissertation describes experiments to investigate how these factors influence mutual diffusion in miscible polymer blends. After a general introduction and a review of some recent theories of mutual diffusion in polymer blends, experiments are described in one miscible blend system, Polyvinyl Chloride (PVC)/Polycaprolactone (PCL);x-ray microanalysis in a scanning electron microscope was used to measure the concentration dependence of the mutual diffusion coefficient. To explain this concentration dependence we need to invoke not only the thermodynamics of mixing but also the dependence on composition of the monometric friction coefficients in the system. This dependence was investigated using an ESR spin probe technique. The final section of the dissertation deals with an attempt to use the potentially powerful ion beam analysis techniques of Rutherford Backscattering (RBS) and Forward Recoil scattering (FReS) to measure mutual diffusion coefficients, as well as intradiffusion coefficients (whose concentration dependence should not be influenced by thermodynamic effects). Results obtained by these techniques are presented for three blend systems, including PVC/PCL; the results by RBS for the latter system are consistent with the results obtained by x-ray microanalysis.

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The work contained in this thesis is concerned with the flow behaviour of polymer solutions and melts. It is a theoretical study, though it refers extensively to experimental work by others. Calculations are analytic where possible, though some numerical work is required. The principal source is the theoretical model of linear polymer melts developed by Doi and Edwards. The presentation is as follows: Chapter 1 reviews the experimental properties of polymeric liquids and some of the phenonenological schemes used to describe them. Review material extends into chapter 2 which covers molecular theory, the Doi-Edwards model, and developments in the last few years. Chapter 3 applies the model to the 'spurt effect', a well known unstable flow of linear polymer melts. We show that this may be explained quantitatively by the multivalued constitutive behaviour of the model. Chapter 4 examines the stability of the flow field associated with the spurt effect and shows how certain pulsing periodicities in the flow may be related to the 'normal stress' behaviour of the fluid. Chapter 5 is a case study of a more complex branched polymer: the 'H-polymer'. It is shown that both the linear and non-linear viscoelastic response may be calculated in a straightforward extension of the Doi-Edwards model, comparing favourably with experiments to date.

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