Rheo-NMR of semidilute polyacrylamide in water

Rheo-NMR H-1 spectroscopy has been performed on semidilute solutions of pol yacrylamide (PAA) in water using a cylindrical Couette cell in which the so lution in the annulus may be imaged in order to investigate spatially depen dent NMR parameters. A number of interesting properties emerge. Shear bande d flow is observed, in contrast to rheo-NMR studies of other semidilute pol ymers in which simple power-law shear thinning is found. Furthermore, we fi nd that the PAA chain protons exhibit a marked T-2 reduction under shear an d that the recovery on shear cessation is indicative of slow reorganization al dynamics. The observed T-2 recovery is multiexponential, the fast molecu lar relaxation times correlate well with the terminal times measured from t he steady-state flow curve, and we interpret our findings in terms of shear -induced deformation of the Doi-Edwards tube. We suggest that slower relaxa tion processes may be due to hydrogen bond formation.