Rheology of Cross-Linked Poly(Sodium Acrylate)/Sodium Silicate Hydrogels

Transparent hydrogels consisting of poly(sodium acrylate) and sodium silicate were synthesized by free-radical polymerization of sodium acrylate (ANa) in an aqueous solution of sodium silicate (i.e. water glass) with the silicate modulus (M) 2.50 and the Midafen R-102 polymer filler, in the presence of sodium thiosulphate/potassium persulphate (NTS/KPS) as the redox initiators and N, N′-methylenebisacrylamide as the cross-linking monomer. The hydrogels, obtained in this way, were rheologically tested, and also the gelling point, dependence of shear stress on shear rate, and oscillation constants were determined. The results indicated that during the cross-linking reaction, which is associated with a gradual building of the three-dimensional gel structure, the reaction mixture changes its rheological behaviour from pseudo-thixotropic at the beginning of the reaction to pseudo- anti-thixotropic in the end. Both elasticity modulus G′ and viscosity modulus G′′ during the reaction and for the cast samples after 24 h have practically the same values and the phase shift angle δ is below 20° which means that we obtained a highly elastic material. It was also found that the elasticity modulus G′ values increase with the content of sodium silicate in the sample.