A study of self-diffusion of molecules in polymer gel by pulsed-gradient spin-echo H-1 NMR

Pulsed-gradient spin-echo (PGSE) H-1 NMR experiments were carried out on swollen poly(N,N-dimethylacrylamide) (PDMAA) gels with various degrees of swelling (Q) under a state of equilibrium with deuterated water (D2O) as solvent and poly(ethylene glycol) (PEG) as a probe polymer, of which the molecular weight ranged from 4250 to 20000. Self-diffusion coefficients of PEG and HDO in D2O solvent (containing a small amount of HDO) in the gels were determined. From these-results, the diffusion coefficient for HDO in the gel (D-HDO) was found to increase with an increase in Q and asymptotically approach the diffusion coefficient for HDO in neat D2O. This behavior was reasonably explained by the modified free volume theory. Further, it was found that the diffusion coefficient for PEG (D-PEG) in the gel is followed by D-PEG/D-PEG(0) = exp(-kappa R), where D-PEG(0) is the diffusion coefficient for an isolated PEG, kappa(-1) the dynamical screening length, and R the hydrodynamic radius of PEG, and that kappa(-1) is proportional to c(-0.71), where c is the polymer concentration.