Concentration-Dependent Main-Chain Dynamics of Sodium Polyacrylate As Probed by NMR in the Semidilute Regime

Article abstract of DOI:10.1021/j100285a043

Sodium polyacrylate chains exhibit reduced reorientational mobility in the semidilute regime as probed by frequency-dependent nuclear magnetic relaxation experiments.Nuclear magnetic relaxation rates of deuterons have been measured in aqueous solutions of methylene-deuteriated poly(acrylic acid) (CD2-PAA).Upon diluting a neutralized solution of CD2-PAA (sodium polyacrylate), a sharp increase of the transverse relaxation rate R₂ was observed, whereas the longitudinal relaxation rate R₁ remained unaffected.It is shown that in the semidilute regime slow motions with correlation times > 1E-8 s show up irrespective of the fast (internal) motions with correlations times ca. 1E-9 s.The origin of these slow motions is indicated by polyelectrolyte theory; the large correlation times are probably due to the enhanced (electrostatic) stiffness of the CD2-PAA chain in the semidilute regime.In concordance with this idea is the fact that the large correlation times decrease when a simple salt (NaCl) is added to the solution.An implication for the interpretation of counterion relaxation is discussed.