Experimental research of syneresis mechanism of HPAM/Cr3+ gel

Experimental investigations have been conducted to elucidate the syneresis mechanism of HPAM (partly hydrolyzed polyacrylamide)/Cr3+ gel. The gel prepared with HPAM of high hydrolysis degree is apt to result in the syneresis, since HPAM with more carboxylate group is easier to be over-crosslinked. The over-crosslinking, which contributes a lot to the syneresis, occurs between C-O bond in -COO- of HPAM and Cr3+. The decreasing hydrophilicity of HPAM molecule, the increase of crosslinking density, and the complexation of the carboxylate group with Ca2+ are the main syneresis mechanisms of the gel influenced by the inorganic salt. The Ca2+ reacts with the C-O bond in -COO-, and the reaction produces a tabular structure when the concentration of Ca2+ is high, whereby the water in the initial gel is extruded. The produced water resulted from the syneresis mainly originates from the water bounded to the carboxylate group of HPAM, and this confirms that the decreasing hydrophilicity of the carboxylate group in HPAM molecule is one of the important reasons to the gel syneresis. Sodium D-isoascorbate inhibits the generation and growth of the polynuclear olation complex by coordinating with Cr3+, whereby the crosslinking speed is decreased and the syneresis is suppressed. (C) 2015 Elsevier B.V. All rights reserved.