Ultrahigh Deformability and Transparence of Hectorite Clay Nanocomposite Hydrogels with Nimble pH Response

Abstract

Nimble pH response nanocomposite hydrogels (NC gel) with ultrahigh tensibility and high transparency were synthesized via in situ copolymerization of acrylamide and sodium acrylate (SA) in the aqueous suspension of hectorite clay Laponite RDS with a minute amount of N,N′-methylenebisacrylamide (BIS). The stability of the Laponite suspension containing ionic monomers and the tensile properties of the NC gels were investigated. The addition of ionic monomer SA was found to reduce the ζ potential and stability of the suspension. The tensile strength and elongation at break of these ionic NC gels obviously decreased when SA was greater than 10 mol % in the monomers. Interestingly, the addition of a minute amount of BIS (≤0.05 mol %) enhanced the homogeneity of the ionic NC gels and thus improved their transparency (transmittance >90%), tensile strength (>100 kPa), and elongation (>2000%). The relaxation modulus of the ionic NC gels was fit with G(t) = Ge[1 + (t/λ0)−n], where Ge was the equilibrium modulus and λ0 was a material-dependent time constant. The relaxation exponent, n, for the ionic NC gels was 0.10 to 0.18, which is similar to that of the lightly cross-linked nature rubber. This moderate relaxation observed was considered to be the origin of the ultrahigh tensibility of the ionic NC gels. Fortunately, the nimble pH response still remained in the present NC gels containing carboxyl groups. The oscillatory swelling−shrinking circles switched by pH at 7.4 and 3.0 were observed from the present NC gels.