Stephen Pickup

Solvent Self-diffusion in Polymer Solutions

Abstract We compare the solvent self-diffusion coefficients in a variety of aqueous and nonaqueous solutions, including polystyrene (PS) in toluene or cyclohexane, poly(ethylene oxide) (PEO) in water, and various biological materials. Remarkably similar changes are observed in the solvent self-diffusion properties relative to those of the neat solvent for all systems; no chemically specific effects can be resolved. Taken together, the transport data indicate substantial changes in the motional properties of the solvent due to interactions with the polymer. The reduction in transport might be determined by nonspecific kinetic effects associated with the presence of relatively immobile solute molecules.

Application of One-Dimensional NMR Imaging to Solvent Diffusion Measurements in Polymeric Samples

Small-scale, low-resolution NMR imaging has been performed on a standard NMR spectrometer equipped with a homogeneity spoiling unit without instrument modification. This technique uses the homogeneity spoiling unit to generate the magnetic field gradient needed in the imaging experiment. We demonstrate how this imaging technique can be used to observe a solvent diffuse into a polymer gel. By fitting experimental images to calculated ones it is possible to extract a diffusion coefficient from the imaging data. This experiment has been used to measure the diffusion of deuterium oxide in aqueous carboxymethyl-cellulose solutions. Two different samples were studied having concentrations of 4 and 6% cellulose by weight in water. Diffusion coefficients of 1.67×10−9 and 1.36×10−9 m2/s were determined using the imaging experiment for the 4 and 6% samples, respectively. In samples of such low polymer concentration gradient the value of the solvent mutual-diffusion coefficient is nearly equal to that of the self-diffusion coefficient. Thus, for this set of samples, a comparison of self-diffusion measurements to the imaging results would serve as a check of the accuracy of the measurement. The pulsed-gradient spin-echo (PGSE) technique was used to determine the self-diffusion coefficients in the same set of samples. The results of the two diffusion measurements are found to be within experimental error of each other. These experiments demonstrate the feasibility of measuring mutual-diffusion coefficients for other systems in this way.