Absorbent hydrogels: Design and synthesis of degradable and macroporous polymer networks

This thesis presents two new types of hydrogels that address limitations of current solution-polymerized polyacrylate gels used in superabsorbent applications. The lack of degradability of superabsorbent gels is addressed by synthesizing hydrogels incorporating hydrolyzable crosslinkers. Slow swelling kinetics are addressed by synthesizing porous gels using surfactant cubic liquid crystals as polymerization templates. An appendix presents a theoretical model that explains the dependence of network structure of superabsorbent gels on monomer ionization.; Gels with high swelling capacities that degrade under mild conditions without liberating toxic monomeric species were obtained by designing novel hydrolyzable crosslinkers. A phosphazene crosslinker, containing polymerizable allylamine groups and imidazole groups to introduce degradability, was used to form degradable poly(acrylic acid) (PAA) gels. Zirconium propoxides and butoxides, derivatized with acetoacetoxy-ethylmethacrylate, were used to crosslink poly(N-isopropylacrylamide) gels. Hydrogels crosslinked with phosphazenes or alkoxides degraded over periods ranging from days to weeks.; Porous gels were produced from surfactant cubic liquid crystals. Two cubic systems, consisting of Brij{dollar}circler{dollar}78 (C{dollar}sb{lcub}18{rcub}{dollar}E{dollar}sb{lcub}20{rcub}{dollar})-toluene-10% aqueous acrylic acid and didodecyldimethylammonium bromide (DDAB)-decane-10% aqueous acrylic acid, were studied. Interactions of the acrylic acid with Brij{dollar}circler{dollar}78 due to hydrogen bonding and with cationic DDAB due to electrostatic interactions substantially modified the cubic phase behavior and polymerization. SAXS indicated that the cubic structure was retained upon polymerization. Only the Brij{dollar}circler{dollar}78 system resulted in crosslinked PAA gels. Polymerized cubic gels of the Brij{dollar}circler{dollar}78 system swelled with a cooperative diffusion coefficient of about {dollar}2 times 10sp{lcub}-7{rcub}{dollar} cm{dollar}sp2{dollar}/s, equal to that of a homogeneous PAA gel.; The effect of monomer neutralization during polymerization on superabsorbent gel properties was examined by developing a model of network formation. Based on the Macosko-Miller cascade approach and incorporating effective monomer concentrations accounting for electrostatic repulsion of ionized species, this model indicated that polyelectrolyte networks exhibited a 40% reduction in effective strand density as the monomer preneutralization was raised from 0 to 60%. The model is compared to existing experimental data.