Formation of colloidal dispersion gels from aqueous polyacrylamide solutions

The formation of colloidal dispersion gels (CDGs) in aqueous solution by cross-linking partially hydrolyzed polyacrylamide (HPAM) with aluminum citrate has been studied as a function of time, temperature, and pH by using rheology and electrophoretic measurements. At low to moderate concentrations the viscosity of the HPAM solutions decrease rapidly upon addition of cross-linker. However, after this initial rapid decrease, a further slow decrease in viscosity is observed over several days. This has been interpreted as a slow rearrangement of cross-linked bonds, from inter- to intra-molecular cross-linker bonding. The CDG solution in contrast to the polymer solution show nearly Newtonian behaviour. A reduction of the solution pH provided both lower viscosities and lower electrophoretic mobilities for CDG solutions. The initial cross-linking reaction is fast, and it appears that the CDG units once formed are stable, as suggested by measured particle sizes after dialysis when all free ions, including aluminum were removed. (C) 2007 Elsevier B.V. All rights reserved.