Effects of molecular mass of polymer and composition on the compressive properties of hydrogels composed of Laponite and sodium polyacrylate

• Mechanical properties of clay/sodium polyacrylate hydrogels were investigated.
• Effects of composition on the mechanical properties were investigated.
• The mechanical properties were affected by the molecular mass of the polymers.
• High-molecular-mass polymers were suitable to produce mechanically tough gels.

Recent studies have shown that hydrogels prepared by simple mixing of a polyelectrolyte (sodium polyacrylate (PAAS)) and clay (Laponite) with a dispersant (tetrasodium pyrophosphate) are mechanically tough. In this study, the mechanical and structural properties of such hydrogels were investigated in detail as a function of both the molecular mass of the polymers and the hydrogel composition by means of compression measurements and synchrotron small-angle X-ray scattering analysis to clarify the origin of the mechanical toughness. The hydrogels prepared under optimum conditions were not broken, even after 90% compression. The elastic moduli of the hydrogels gradually decreased with increasing concentrations of PAAS, whereas they reached a maximum with respect to the concentration variation of the dispersant. The gels prepared at the compositions where the modulus was maximal were mechanically tough. These composition dependencies of the mechanical properties were related to structural inhomogeneity in the gel; thus, the results of this study demonstrate that inhomogeneity substantially lowers the mechanical properties of these hydrogels. At high clay concentrations, the elastic moduli of the hydrogels were not affected by the molecular mass of PAAS; in contrast, at low clay concentrations, they substantially decreased with decreasing the molecular mass of PAAS. The results of this study reveal that PAAS with a molecular mass greater than ~ 106 is necessary for fabricating the mechanically tough hydrogels.