Polyacrylamide gels formed with Cr⁺³ ion and Cr(acetate)₃: Thermodynamically and kinetically controlled crosslinking reactions

Abstract

Abstract The gelation of buffered aqueous Cr +3 /polyacrylamide (PAAm) solutions has been studied over the pH range 2‐11. With Cr +3 supplied as Cr(NO 3 ) 3 and a 7.5% hydrolyzed PAAm the upper limit for gelation was about pH 7, but Cr(acetate) 3 /PAAm solutions gelled as high as pH 9. Evidence is presented that the formation of unreactive colloidal Cr(OH) 3 (H 2 O) 3 , rather than polymer crosslinking, is favored thermodynamically for Cr +3 /PAAm solutions at pH > 7. The crosslinks formed in Cr(acetate) 3 /PAAm solutions at pH 7–9 are deduced to be kinetically controlled products. Independent evidence for the kinetic stability of Cr +3 /PAAm gels above pH 7 has also been obtained. Other observations relevant to the mechanism of crosslinking of PAAm with Cr(acetate) 3 are described.