Swelling and mechanical behavior of ionizable hydrogels containing highly branched polyamine macromonomers

Linear alpha,o diepoxide-terminated poly(ethylene glycol)s (PEG di-epoxide) of molar mass 4,000 and 20,000 g mol-1 were end-linked with amine-terminated poly(amidoamine) (PAMAM) dendrimers of Generations 0, 2, and 4 in a good solvent (water) to prepare architecturally well-defined copolymer hydrogels. PEG di-epoxide was also randomly crosslinked with branched poly(ethylenimine) (PEI) in water to prepare hydrogels with high concentrations of amine functional groups, much like the dendrimer gels, but with an ill-defined architecture. The gelation and equilibrium swelling of these gels in water were characterized while systematically varying the polymer concentration at preparation, dendrimer generation, mole ratio of crosslinker endgroups to PEG endgroups, and PEG precursor molar mass. The Ahmad-Rolfes-Stepto (ARS) theory of non-linear polymerization was applied to predict conditions favoring gelation, and the model predictions captured many features of the experimental results. Some hydrogels exhibited "superabsorbent" behavior upon extraction and equilibrium swelling in pure water. The end-linking reaction stoichiometry was identified as the most important factor governing the swelling behavior, whereas the polymer volume fraction at crosslinking, precursor chain molar mass, and dendrimer generation played less significant roles. The superabsorbency arised in part from ionic effects related to protonation of the amine groups at external pH=7. Upon air-drying and re-swelling of the extracted networks, a substantial decrease in equilibrium swelling was noted, which may be attributed to a secondary crosslinking reaction that occured during drying. For dried and re-swollen networks, the scaling of storage modulus with equilibrium swelling was not much different from scaling predictions for neutral polymer networks in good solvents.;The pH-dependent swelling behavior of these two hydrogel systems containing high concentrations of amine-functional macromonomers was also examined. After extraction in neutral water, the hydrogels were swollen in aqueous solutions of HCl or NH4OH to vary the external pH. Equilibrium volume swelling ratios (Qs) passed through a maximum value ( Qmax) at an external pH (denoted pH*) which was approximately 4 to 5 for the gels studied. The swelling behavior was modeled using Donnan equilibrium theory to describe the ion swelling pressure, with the Flory-Rehner phantom network expression representing the elastic and mixing contributions to the free energy. The model accurately predicted the maximum in swelling near pH=4 to 5, but overestimated Qmax for several of the gels due to neglecting the finite extensibility of the short linear PEG chains.;Two possible applications of polyamine hydrogels were examined. First, the PAMAM/PEG hydrogel system was used as an immobilization matrix for nitrogen-fixing bacteria, Rhizobium leguminosarum, with a focus on identifying structural factors that maximize cell density. Second, the effect of crosslink density on DC ionic conductivity of a gel electrolyte was examined, in order to determine whether PEI-based gels might be useful as electrolytes for Lithium ion batteries.