Interpenetrating polymer network (IPN) hydrogels consist of two independently cross-linked polymer networks that cannot separate due to physical entanglements.  Certain combinations of neutral and ionized polymers have been used to synthesize aqueous IPN hydrogels with both high water content (>80%) and mechanical properties greatly exceeding the additive properties of the component systems.  Inspired by these results, we have systematically examined interpenetrating networks of polyacrylamide (PAAm) and poly(acrylic acid) (PAA) with our focus being on topological constraints and their influence on structure, swelling and mechanical properties.  Comparison of single network PAAm and PAA hydrogels with homo-IPNs in which the first and second network were polymerized from the same monomer showed no mechanical enhancement, with both the single networks and the homo-IPNs following classical scaling predictions for reduced modulus with increasing swelling ratio.  IPN hydrogels prepared from contrasting networks of PAAm and PAA exhibited composition and structure that depended on the sequence of network formation.  We identified critical IPN compositions above which an increase in water content improved the hydrogel modulus.  Finally, we examined the influence of constraint by the PAAm network on the calcium-induced contraction of PAA network chains.  In the presence of divalent cations, PAAm/PAA hydrogels exhibit a correlation peak in small-angle x-ray scattering that corresponds to an interdomain spacing of 15 nm, which is not seen for PAAm or PAA single network hydrogels.  We attribute the correlation peak to nanophase aggregation, with the coalescence of PAA-rich domains prevented by the presence of the more swollen PAAm network.

Topological constraints in interpenetrating polymer network hydrogels

247th National Meeting (2014)

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