Rheo-NMR of Semidilute Polyacrylamide in Water

Rheo-NMR 1 H spectroscopy has been performed on semidilute solutions of polyacrylamide (PAA) in water using a cylindrical Couette cell in which the solution in the annulus may be imaged in order to investigate spatially dependent NMR parameters. A number of interesting properties emerge. Shear banded flow is observed, in contrast to rheo-NMR studies of other semidilute polymers in which simple power-law shear thinning is found. Furthermore, we find that the PAA chain protons exhibit a marked T 2 reduction under shear and that the recovery on shear cessation is indicative of slow reorganizational dynamics. The observed T 2 recovery is multiexponential, the fast molecular relaxation times correlate well with the terminal times measured from the steady-state flow curve, and we interpret our findings in terms of shear-induced deformation of the Doi−Edwards tube. We suggest that slower relaxation processes may be due to hydrogen bond formation.