Poly(acrylic acid) (PAA) is an anionic polyelectrolyte that sees a number of commercial uses, chiefly in those that exploit lightly crosslinked PAA’s superabsorbance. These materials tend to be soft and brittle, with low elasticity. In contrast, we have found that PAA hydrogels synthesized with high concentrations of salt exhibit properties markedly different from both PAA hydrogels synthesized without salt and those incubated in the same amount of salt post-synthesis. Examples of these changes include reduced equilibrium swelling, substantially increased elongation, increased modulus, and near-complete recovery after strain. Investigated salts include chloride salts of lithium, sodium, cesium, calcium, and zinc. The greatest enhancement in viscoelastic behavior comes from multivalent salts such as zinc chloride and calcium chloride, but is also evident in lithium chloride-loaded samples. The enhanced mechanical properties of these salt-loaded gels significantly diminish upon washing the materials with water to remove the salt, and cannot be restored upon incubation in solutions containing equivalent concentrations of salt. This indicates that the presence and organization of the salt during synthesis is critical to the exhibited property changes. Furthermore, the inability of the materials to regain these mechanical properties by simply soaking them in salt solutions preclude simple explanations for enhancement, such as the chelation of multivalent ions leading to ionic crosslink formation. To fully understand this phenomenon, we have utilized x-ray absorption fine structure (XAFS) experiments to identify local metal structure and chemistry, which indicates how the materials are organized in relation to the polymer matrix. Comparison of the organization of the metals in the material under various processing conditions has provided a fundamental knowledgebase needed to develop metal-derived structure/function relationships for hydrogel materials. Additionally, small-angle neutron scattering has allowed the investigation of polymer chain behavior in the presence of salts, allowing for construction of a complete picture of the high-salt hydrogel system. The primary end-use envisioned for these gels is as a mimic for biological materials such as nerve and muscle tissue, which are essentially complex cation exchange membranes. Understanding the role and organization of the metal in these high-salt PAA gels will aid in the design said tissue mimics.

Enhanced elasticity in poly(acrylic acid) hydrogels: Understanding mechanisms and exploring applications

254th National Meeting (2017)

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