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.

The self-diffusion coefficient of a polymer in solution may be expanded in the concentration of the polymer, as seen in equation 1. The linear term would represent a perturbation due to the presence of another polymer; the c^{2} term would represent a perturbation due to interactions of trios of polymers. Phillies determined the c^{2} term of a virial expansion of the self-diffusion coefficient for trios of polymers interacting via a ring. Here I determine a correction to the c^{2} term due to trios of polymers interacting via a figure-eight scattering diagram: the equivalent of four polymers interacting in a ring where the second polymer and the fourth polymer are the same. D_{s}(c) = D_{0}(1+ alpha D_{0} c + beta D_{0}^{2}c^{2}+...) 1 or, D_{s}(c) = D_{0}(1+ alpha D_{s}(c)c). 2 A D_{0} may be replaced by D_{s}(c) in equation 1 to arrive at equation 2. The left-hand-side of equation 2 is the final self-diffusion coefficient, and the D_{s}(c) on the right-hand-side of this equation is that due to the question of self-similarity. If the D_{s}(c) on the right-hand-side is given by equation 1, resulting in beta=alpha^{2}, it may be said that the system exhibits self-similarity. I demonstrate self-similarity quantitatively for a polymer solution using a generalized Kirkwood-Riseman model of polymer dynamics. The major physical assumption of the model I utilize to derive equation 2 is that, in solution, polymer motions are dominantly governed by hydrodynamic interactions between the chains. First, I review the Kirkwood-Riseman model for intrachain hydrodynamic interactions. I then discuss Phillies' extension of this model to interchain interactions for duos or trios of polymers in a ring. I analytically calculate the hydrodynamic interaction tensor from a multiple scattering picture T_{54321}, for five polymers in solution and verify this tensor by numerical differentiation. Finally, I perform the ensemble average of the self-interaction tensor b_{1232} appropriate to the figure-eight scattering diagram both analytically and with a Monte Carlo routine, thereby verifying equation 2 to second order in concentration.