Studies on rheological behaviour of hydrophobically associating polyacrylamide with strong positive salinity sensitivity

Abstract

Hydrophobically associating polymer has been obtained by homogeneous copolymerization of acrylamide and an anionic surface-active monomers (surfmer) of acrylamide-type, sodium 2-acrylamido-tetradecane sulfonate (NaAMC₁₄S) in aqueous solution, and under different concentration conditions of NaAMC₁₄S above its critical micellar concentration (CMC) and below CMC, two structure types of copolymers P (AM–NaAMC₁₄S) were prepared in which main chains NaAMC₁₄S were distributed in microblock and in random manner, respectively. The hydrophobically associating property and rheological behaviour of the copolymers were studied with viscosimetry and then by fluorescence probe method. It was found that the apparent viscosity of the aqueous solution of the copolymers with microblock structures exhibits strong positive salinity sensitivity, whereas the positive salinity sensitivity of the apparent viscosity of the aqueous solution of the copolymers with random structure only appears generally and the apparent viscosity was diminutively increased because of salinity-promoting hydrophobic association. For those copolymers with microblock structures, the apparent viscosities of their brine solution are much higher than that of their pure water solution. In CaCl₂ solution (2 wt%), as the copolymer concentration attains 0.2 wt%, the apparent viscosity enhances dramatically with the increase in concentrations, and as the copolymer concentration was 0.3 wt%, the apparent viscosity can enhance by almost two orders of magnitude (from 32 to 1,000 mPa·s) as compared with in pure water solution. Thus, for copolymer P (AM–NaAMC₁₄S) with microblock structure, the peculiar positive salinity sensitivity, i.e. salinity thickening, is displayed distinctly. The unusual positive salinity sensitivity of copolymers P (AM–NaAMC₁₄S) with microblock structure was also revealed by fluorescence probe method, and the value I ₁/I ₃ of pyrene in the brine solution of the copolymers was smaller than that in pure water solution of the copolymers. This unique salinity sensitivity is a reflection of the unusual microstructure of the copolymers. The experiment results are interpreted in terms of the effects of the surfmer distribution manner in the copolymer main chain and bonding way of the ionic surfmer to the copolymer main chain on the forming intermolecular association.