Large deformation behavior and effective network chain density of swollen poly(N-isopropylacrylamide)–Laponite nanocomposite hydrogels

Abstract

Hydrogels were often used in aqueous and/or physiological saline media, thus, the mechanical properties of the swollen gels were of particular importance. Until now, these data were lacking due to the most swollen hydrogels being too weak and brittle to undergo the deformation test at all. The mechanical behavior of swollen poly(N-isopropylacrylamide) (PNIPAm)–Laponite nanocomposite hydrogels (NC gels) were studied with compression and elongation under large strains causing deformation. The swollen NC gels were intact after compression and sustained elongation of up to 500%. Strain hardening was observed in the swollen NC gels for the first time, which was considered to be the extensional limitation of the polymer network chains due to the orientation of the Laponite platelets. Therefore, the strain hardening was enhanced by increasing the Laponite content in the NC gels. When the deformation was low, the compression and elongation stress-deformation curves of the as-prepared and swollen NC gels were described quantitatively with the Mooney–Rivlin model. Whilst for high deformation, Creton's model considering the finite extensibility enabled the prediction of these curves almost quantitatively. Stress-deformation hysteresis was found in the as-prepared and swollen NC gels, which was the result of the deorientation of the clay platelets and relaxation of the polymer chains. The orientation process of the clay platelets dissipated energy during deformation, resulting in a high toughness of the NC gels even in the swollen state. The effective network chain density in the swollen NC gels manifested that the swelling in water only expanded the gel volume and did not damage the cross-linking points. The number of effective network chains attached to one Laponite XLS platelet was almost independent of the clay content, so the addition of clay platelets in the NC gel formed new cross-linking junctions.