(122h) High Charge Density Interpenetrating Hydrogels of Hydrolyzed Networks of Poly(N-Vinyl Formamide) and Polyacrylamide

The primary goal of this work was to make tough hydrogels from high charge density interpenetrated networks (IPNs) of hydrolyzed networks of poly(N-vinyl formamide) PNVF and polyacrylamide (PAAm). Inspired by previous research on IPN hydrogels exhibiting the “double network” effect the focus of this work was to create a similar high toughness hydrogel that could withstand repeated loading. First described by JP Gong in 2003 using a hydrogel of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS) and PAAm, the “double network effect allows a hydrogel to achieve a high toughness by combining a stiff brittle network with a soft ductile one. As the gel is strained the stiff brittle network breaks down dissipating energy while the soft ductile network holds the hydrogel together, thus giving rise to its high toughness. However, since this is achieved by breaking permanent covalent bonds in the stiff brittle network the hydrogels mechanical properties degrade under repeated loading. Overcoming this limitation is the main focus of this work, instead of breaking permanent covalent bonds to dissipate energy this work examines a IPN that achieves a high toughness by utilizing ionic bonds that can be broken and reformed with out permanent damage to either network. In order to achieve this a formulation utilizing two similar neutral networks was developed, thus ensuring minimum phase separation between the networks. These two networks could then be hydrolyzed into oppositely charged networks that would form ionic bonds between the two networks. To achieve this it was hypothesized that IPNs of PNVF and PAAm, as isomers of each other, will have intimate molecular mixing of the two networks to help maximize charge-charge interactions of the networks after hydrolysis to polyvinylamine (PVAm) and polyacrylic acid (PAAc). Therefore, the same charge density of the two networks would lead to maximizing charge complexation. Earlier work on SN PAAm and SN PNVF show that they have similar c parameters and similar structures being isomers of each either; suggesting IPNs will have minimal molecular phase separation. IPNs of both PNVF/PAAm and PAAm/PNVF were synthesized and were optically transparent, indicating homogeneity at submicron length scales. Both IPNs were successfully hydrolyzed to PVAm/PAAc and PAAc/PVAm as noted in a ~5-fold or greater decrease in swelling at intermediate pH values (3-6), consistent with the hypothesis of charge complexation. Finally, tough networks were achieved as shown in roughly 10-fold increase in toughness of the hydrolyzed IPNs when compared to their unhydrolyzed states at intermediate pH values.