Effect of Initial Total Monomer Concentration on the Swelling Behavior of Cationic Acrylamide-Based Hydrogels
Hydrogels were synthesized by copolymerizing acrylamide with the cationic monomer [(methacrylamido)propyl]trimethylammonium chloride (MAPTAC) and the bifunctional crosslinking monomer N,N' -methylenebisacrylamide (BIS).A series of seven hydrogels was prepared in which the initial total monomer concentration %T (w/v) was varied from 10 to 40%, while the mole percents (solvent-free basis) of MAPTAC and BIS were held constant at 3.0 and 0.2, respectively.Compressive stress-strain measurements were performed on water-equilibrated hydrogel samples; the measurements are well described by I .the phantom theory of network elasticity.Donnan exclusion of sodium ions from the hydrogels was used to estimate the effective fixed-charge content of the hydrogels.While the nominal fixed-charge content of the hydrogels was 3.0 mole percent, the results of the sodium-exclusion experiments showed on average an effective fixed-charge Content o( 1.9 mole percent.The hydrolysis of pendant amide groups to anionic carboxylate functionalities may mask some of the fixed-cationic charge in the hydrogels.Swelling equilibria were measured for the %T-varying hydrogels in pure water andin aqueous NaCl solutions.Swelling was observed to be a strong decreasing function of %T.Good agreement is obtained between measured swelling equilibria in aqueous NaCl and that calculated from a Flory-type swelling model.The good agreement is facilitated by fitting an adjustable parameter in the model to swelling equilibria for the hydrogels in pure water.The experimental results presented here are all consistent with the concept of increasing chain interpenetration with rising %T.