Investigation on gelation process and microstructure for partially hydrolyzed polyacrylic amide (HPAm)–Cr(III) acetate–methanal compound crosslinked weak gel

Compound crosslinked hydrolyzed polyacrylic amide (HPAm) weak gels were synthesized by an in situ crosslinking method using Cr(III) acetate and methanal as crosslinkers. The weak gels were crosslinked by both covalent bonds and coordination bonds. A Haake MARS III rotational rheometer was used to measure viscosities to monitor gelation process and evaluate the sensitive of six main factors including polymer concentration etc. An orthogonal experiment (OA₂₅ (5⁶) matrix) was conducted to help to evaluate the importance of the factors, and determine the optimal level and combination of factors. ESEM was employed to image the morphologies of gels to assist to analyze crosslinking mechanism. We found that the six factors have significant influences on gelation properties. The importance of factors for viscosity of 40 days followed by polymer concentration, weight ratio of polymer to crosslinker, NaCl concentration, thiourea concentration, resorcin concentration and methanal concentration. The reasons why these factors had such effect were analyzed. On the basis of the analysis of gelation process and ESEM morphology, we preliminarily concluded that crosslinking mainly happened among different polymer molecules and crosslinking by coordination bonds prevails in early stages.