Laponite-Doped Poly(acrylic acid-co-acrylamide) Hydrogels

Abstract

Electroacoustic characterization of soft nanocomposites has provided unique insights into the microstructure of soft nanocomposites, including nanoparticle (NP)-doped hydrogels and polyelectrolyte hydrogels without NPs. An outstanding problem is how to interpret the electrokinetic sonic amplitude (ESA) of charged hydrogels bearing charged NPs because both components generate an acoustic response to the electrical forcing. To this end, we study a series of Laponite XLG-doped, neutralized poly(acrylic acid-co-acrylamide) hydrogels, drawing principally on the ESA, electrical conductivity, and linear viscoelastic rheology. The hydrogel charge density was varied by the fraction of acrylic acid monomer fAAc = 0–1 while maintaining the total monomer concentration ≈8 wt % with Laponite concentration ≈0.85 wt %. Upon comparison of data from this study to those in a recent benchmark study of charged hydrogels without NPs, Laponite doping increased the electroacoustic signal and ionic conductivity but decreased the hydrogel storage modulus. Mechanistic theoretical models predicting how the real part of the ESA (at low frequency) and ionic conductivity of polyelectrolyte hydrogels depend on fAAc were extended to Laponite-doped hydrogels, together furnishing an estimate of the partial molar volume of acrylamide (in polymer form) that is close to the value for pure acrylamide (based on its density and molecular weight). The generally lower storage modulus with Laponite doping contrasts with previous studies of Laponite-doped polyacrylamide and poly(acrylic acid) hydrogels and solutions. This seems to reflect the high degree of neutralization, which transforms an attraction between protonated carboxyl moieties and Laponite to an electrostatic repulsion. The hindering effects of polymerization and cross-linking on acrylic acid-co-acrylamide networks were also investigated by comparing the ESA and conductivity of hydrogels with their monomer solution counterparts. Systematically varying the ratio of charged to uncharged monomers, with and without chemical cross-linking, provides insights to benefit a broad range of technological applications for hydrogel nanocomposites.