Topological constraints in interpenetrating polymer network hydrogels [electronic resource]
Keywords: Chemical Engineering

Interpenetrating polymer network (IPN) hydrogels consist of two independently crosslinked polymer networks, swollen in aqueous solution, that cannot separate due to physical entanglements. IPN hydrogels generally exhibit properties that are a combination of, or intermediate between, those of their component networks, which are often highly swollen and are therefore mechanically soft and weak. Certain combinations of neutral and ionized polymers, however, have been used to synthesize IPN hydrogels with both high water content (> 80%) and mechanical properties greatly exceeding the additive properties of the component networks. Inspired by these systems, we aimed to develop a more fundamental basis for understanding the complex interactions in IPN hydrogels by preparing sets of simple IPN hydrogels from poly(acrylamide) (PAAm) and poly(acrylic acid) (PAA) and examining the influence of specific types of constraint on structure, swelling, and mechanical properties. We began by comparing single network PAAm and PAA hydrogels to homo-IPNs, in which the first and second network were polymerized from the same monomer. This minimized the effects of chemical interactions between networks, allowing us to investigate physical constraints. Entanglements in the IPNs reduced the effective network chain length in these gels, contributing to a reduction in swelling. However, no mechanical enhancement was detected for these IPNs. Both the single networks and homo-IPNs followed classical scaling predictions for reduced modulus with increasing swelling ratio. Several forms of constraint were examined in IPN hydrogels prepared from contrasting networks of PAAm and PAA. At low pH, interpolymer complexation as a result of hydrogen bonding was found to produce an IPN swelling minimum at a molar ratio of PAAm:PAA = 0.7. At high pH, ionization of the PAA network increased the osmotic driving force for uptake of water by the IPN. This swelling forced the PAAm chains to become highly extended and led to an increase in modulus with water fraction. The composition and structure of these IPN hydrogels were dependent 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 IPN hydrogels exhibit a correlation peak in small-angle scattering, corresponding to an interdomain spacing of 15 nm, that 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. We confirmed this interpretation by forcing complete deswelling of these hydrogels and characterizing their collapsed structure. The range of structural and mechanical behavior attained by manipulating these materials serves as a framework for understanding the structure and properties of constrained IPN hydrogels and creates a basis for designing hydrogels with desired properties for specific applications.