Viscoelastic behaviour of partly hydrolysed polyacrylamide/chromium(III) gels

This paper reports an experimental investigation of the crosslinking process of high molecular weight partly hydrolysed polyacrylamides (HPAAm) in aqueous brine solution by trivalent chromium ions (Cr(III)). Crosslinking took place in the presence of a retardant agent (sodium citrate). First, sol-gel phase diagrams (in the polymer and concentration space) were established using the tilting tube method. Then, for a fixed composition, the gelation process was monitored systematically using dynamic viscoelastic measurements, varying the main parameters (pH, time, temperature and retardant concentration). Network formation proceeds rather slowly and an equilibrium state was not reached within 12 h. The gel was formed only at pHs between 5 and 9 and thus two gel points (i.e. at two pHs) were determined with the Winter-Chambon method. This is in agreement with the chemistry of aqueous chromium and of acrylic acid groups along the polymer backbone. Kinetics of network formation depends strongly on retardant concentration. Temperature plays an important role: network formation proceeds much faster at high temperature, in agreement with chemical kinetics.