Effect of hydrolysis on spatial inhomogeneity in poly(acrylamide) gels of various crosslink densities

The spatial inhomogeneity in poly(acrylamide) (PAAm) gels of various crosslink densities has been investigated with the static light scattering measurements. The gels were prepared using N,N′-methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed initial monomer concentration but at various crosslink densities. Ammonium persulfate-N,N,N′,N′-tetramethylethylenediamine (TEMED) redox initiator system was used to initiate the polymerization reactions as well as to create charged groups in the aged gels. The gels were characterized by elasticity tests and by light scattering measurements at a gel state just after their preparation. Elasticity measurements show that 91-94% of the crosslinker molecules used in the hydrogel preparation were wasted in ineffective crosslinks. Debye-Bueche analysis of the light scattering data indicates frozen concentration fluctuations within the gel samples, which increase continuously with increasing crosslink density of the hydrogels. This phenomenon was explained with the multiple crosslinking reactions leading to the formation of highly crosslinked regions in the final hydrogel. The extent of concentration fluctuations was found to decrease drastically with increasing time of aging of gels in the synthesis reactor, indicating that the hydrolysis of amide groups into carboxylate anions facilitates the homogenization of the gel samples. A thermodynamic model was developed to explain the experimental observations in terms of the osmotic pressure of counterions in the aged gels.