KINETICS OF THE CROSS-LINKING OF POLY(ACRYLAMIDE) WITH CR(III) .3. RHEOLOGICAL MEASUREMENTS OF THE GELATION

Rheological measurements have been performed on a system of a partiall y hydrolyzed polyacrylamide (HPAM) with Cr(III) as a cross-linking age nt. A sol/gel phase diagram of the system has been established The sca ling properties of the gel have been investigated varying the cross-li nk concentration at a fixed polymer concentration, measuring the dynam ic storage modulus, G', after the reaction was completed. The variatio n of final storage modulus versus Cr(III) concentration is described b y the percolation model, giving a scaling exponent for G' of 1.9. Comb ining the percolation model description with a traditional kinetic app roach for first-order reactions, the kinetics of gelation were determi ned from measured time dependence of G'. Comparison has been made to m easurements by UV-vis spectroscopy on the same systems. The measuremen t of G' provides additional mechanistic information about the gelation process. It indicates that the observed biphasic natures of the first -order plots from theology and spectroscopy measurements are due to a side reaction. The cross-linking is believed to proceed through a rate -determining introduction of the first HPAM to Cr(III).