ESR IMAGING OF TRACER TRANSLATIONAL DIFFUSION IN POLYSTYRENE SOLUTIONS AND SWOLLEN NETWORKS

The translational diffusion coefficients of a small paramagnetic molec ule (the deuteriated nitroxide PDTEMPONE) as tracer were measured in s olutions of polystyrene (PS) in toluene and dimethylformamide (DMF) an d in cross-linked polystyrene (cPS) networks swollen by the same solve nts, using two-dimensional spatial-spectral electron spin resonance im aging (2D ESRI). PS solutions containing 30, 40, and 50 w/w % polymer were examined in toluene at 300, 305, 310, and 315 K and in DMF at 300 K. PS networks prepared with 0.5, 1.0, 2.5, and 5.0 mol % divinylbenz ene as cross-linker were swollen in toluene or DMF and studied at 300 K. Two-dimensional images describing the distribution of the diffusant in the sample were reconstructed from a complete set of projections o btained as a function of the magnetic field gradient applied along the symmetry axis of the sample. The concentration profile of the diffusa nt at a given time was obtained by integrating absorption ESR spectra in slices perpendicular to the spatial axis, which was divided into 25 6 points. These profiles were obtained as a function of time, and the diffusion coefficients were determined by simulating the experimental diffusion profiles using Fick's model of diffusion. The diffusion coef ficients were found to depend on the solvent, temperature, and PS conc entration in the solutions and are significantly reduced by cross-link ing in the swollen networks. The temperature dependence of the diffusi on coefficients shows an Arrhenius behavior, and their dependence on t he polymer concentration in the solutions is consistent with the free volume theory. Results from this study were compared with those obtain ed for solvent selfdiffusion and diffusion of other tracers in PS solu tions and swollen networks.