Shear rheology of anionic and zwitterionic modified polyacrylamides

Two groups of copolymers were synthesized from high molecular weight polyacrylamides. One group of copolymers consisted of sulfonated, anionic copolymers (PAM-S) of acrylamide with the sodium salt of 2-acryloamido-2-methyl-1-propane sulfonic acid, and the other consisted of zwitterionic copolymers (PAM-Z) of acrylamide with a sulfobetaine methacrylate monomer. The shear rheology of aqueous solutions of the copolymers and their mixtures was studied experimentally. Solutions of both copolymers exhibit shear thinning behavior in the range of concentrations explored. Solutions of mixtures of two copolymers (PAM-Z and PAM-S) exhibited a slight viscosity synergy at high relative contents of PAM-S. Addition of a relatively high concentration of an electrolyte (0.3 M NaCl) induces decreases in viscosity due to coil contraction and eliminates the synergy of the mixtures. Mixtures of the zwitterionic copolymer and a cationic surfactant, cetyl trimethylammonium p-toluene sulfonate (CTAT), were also studied. These solutions exhibit a strong synergistic effect at low-shear rates when the surfactant forms wormlike micelles. In addition, oscillatory shear measurements demonstrate that PAM-Z/CTAT mixtures are significantly more elastic than CTAT solutions, which indicates that PAM-Z is effective in promoting micelle entanglements, as reflected by the increase in relaxation time with PAM-Z content.