Detection of Spatial Inhomogeneity in Poly(acrylic acid) Gels by Measuring Time-Dependent Diffusion Coefficients of a Probe in NMR Experiments: Effect of the Degree of Cross-Linking and Degree of Swelling

Abstract

Inhomogeneities in poly(acrylic acid) (PAA) gels differing in their degree of cross-linking were detected as a function of the degree of swelling, Q, defined as the mass of the swollen gel divided by the mass of the dry polymer, Q = Mswollen/Mdry. Q was in the range 2.8−10.0 for the gels studied. The networks were prepared at 70 °C by simultaneous polymerization and cross-linking of a mixture of acrylic acid (AA), sodium carbonate, cross-linker (1,4-butanediol diacrylate), and the redox couple sodium persulfate/sodium isoascorbate as the initiator. Two types of networks were prepared by using the same monomer concentration (30 wt %) and the same amount of sodium carbonate but different amounts of the cross-linker, 1.1 and 0.5 wt %, respectively, in the monomer mixture. The corresponding notation is GEL1 and GEL2. Detection of heterogeneities was based on measuring the diffusion coefficients of the probe molecule poly(ethylene glycol) (PEG) as a function of the degree of swelling and diffusing time, Δ, by means of field-gradient 1H NMR spectroscopy. The inhomogeneities emerged as the degree of swelling of the gels was reduced. For the highly swollen gels (Q = 10.0 and 5.2 for GEL1 and Q = 10.0 and 5.1 and 4.5 for GEL2) only one diffusion coefficient was detected, independent of the diffusing time, Δ, in the range 30−500 ms. For less swollen gels (Q = 2.9−4.5 for GEL1 and 2.8−3.9 for GEL2) two diffusion coefficients were detected, Dfast and Dslow, with values that depended on Δ; for these less swollen gels the diffusion distances dfast and dslow, and the relative fractions of the fast and slow diffusion components, ffast and fslow, were calculated. We defined a specific degree of swelling, Qs, above which the diffusion of the probe in the two gel systems changed from one to two components. A larger value of Qs in GEL1 was taken as an indicator of a more inhomogeneous gel. Analysis of the effect of Δ on the diffusion coefficients, diffusion distances, and fractions of slow and fast diffusion components indicated that the gels form a highly cross-linked region in a narrow Q range. In this Q range, the polymer chains interact and form a highly restricted diffusion region. The extent and distribution of the cross-links form different restricted diffusion regions in GEL1 and GEL2 systems.