Effect of temperature and inorganic salts concentration on syneresis rate of AM/DAC hydrogel

Abstract

Compared with ordinary polyacrylamide, acrylamide copolymer of acryloyloxyethyl trimethylammonium chloride (AM/DAC) hydrogel is easier to reside in large pores and fractures in sandstone formation through electrostatic adsorption. However, the syneresis mechanism of AM/DAC hydrogel caused by high temperature and inorganic salts has not been clear. In this paper, the type of water in gelling solutions and hydrogels was analyzed by the DSC method, the results showed that the bound water in the hydrogel was transformed into free water and then the free water was separated from the hydrogel during the syneresis of AM/DAC hydrogel. Experimental investigations, including dynamic light scattering (DLS), differential scanning calorimetry (DSC), high-performance liquid chromatography (HPLC), phase-analysis light-scattering (PALS), and scanning electron microscope (SEM), have been conducted to elucidate the mechanism of temperature and inorganic salts on syneresis rate of AM/DAC hydrogel. The result showed that with the increase of temperature, the polymer in the AM/DAC hydrogel degraded and the molecular chain broken, which destroyed the grid of hydrogel and reduced the water holding capacity. At the same time, the formation of the hydrophobic group in the molecular chain led to the decrease of the hydrophilicity so that the AM/DAC hydrogel was easy to syneresis. In addition, the increase of temperature will promote the formation of crosslinking intermediate (di/trimethylol), improve the crosslinking rate between crosslinking intermediate and polymer, which makes the AM/DAC hydrogel easy to over crosslink. Inorganic salts diffused the electric double layer by compressing the polymer and competed with the hydrophilic groups on the polymer to absorb water, which will reduce the elongation and hydrophilicity of the polymer molecular chain. This will lead to the decrease of the hydrodynamic radius of polymer molecules in AM/DAC hydrogel, which is the main reason why inorganic salts promote hydrogel dehydration. Besides, inorganic salts had little effect on the formation rate and crosslinking rate of di/trimethylol. Therefore, reducing the molecular hydrodynamic radius of AM/DAC is the main reason why inorganic salts promote the syneresis of AM/DAC hydrogel. The research demonstrated the mechanism of high temperature and inorganic salts on syneresis rate of AM/DAC hydrogel. This study also provides technical guidelines for inhibiting hydrogel syneresis.