Chapter 3: NMR Diffusion Measurements

The basic theory of and diffusion measurements by pulsed-field gradient (PFG) NMR are described. Experimental results on solvent and polymer diffusivity are presented, elucidating the dynamics of gels. The diffusivity of water solvent in poly(N,N-dimethylacrylamide) (PDMAA) and poly(N-isopropylacrylamide) (PNIPAM) gels and of benzene–cyclohexane solvent in a polyisoprene gel was shown as a function of the degree of swelling, Q. Solvent behaviors were further analyzed using modified free volume theory. Diffusion of poly(ethylene glycol) (PEG) in PDMAA was restrained by its hydrodynamic interaction with the polymer. A hydrodynamic interaction was also observed from the dendrimer in agar gels. The mesh size ξ was used to quantify the evolving structure of the agar gel network during gelation. PEG in poly(acrylic acid) (PAA) showed lower diffusivity than in the PDMAA gel, suggesting that a “loose” complex between the PEG and PAA network is formed by intermolecular hydrogen bond interactions. Furthermore, the diffusion of PEG in N,N-dimethylacrylamide (DMAA)–acrylic acid (AA) copolymer gels is strongly restrained by complex formation stabilized by hydrogen bonds between PEG and PAA. Diffusion of PEG-oleyl ether (PEG-Ole) in a stearyl itaconamide (SIA)–DMAA copolymer gel showed that comicellization of PEG-Ole and SIA occurred via a hydrophobic interaction between the oleyl group in PEG-Ole and the stearyl group in SIA.