Effects of NaCl on steady rheological behaviour in aqueous solutions of hydrophobically modified polyacrylamide and its partially hydrolyzed analogues prepared by post-modification

The influences of NaCl on the steady rheological behaviour of aqueous solution were investigated for a hydrophobically modified polyacrylamide (HMPAM) and its hydrolyzed derivatives (HHMPAM) prepared by post-modification. The apparent viscosity (η app ) and reduced viscosity (η red ) were measured as a function of NaCl content. As salinity increased, η red decreased for both HMPAM and HHMPAM in dilute regime; in the semidilute solution, a continuous increase of η red was observed for HMPAM while η red passed through a minimum in the case of HHMPAM. At a given polymer concentration in semidilute regime, both shear thickening and anti-thixotropic properties were observed as a function of salinity and hydrolysis degree, as well as ageing time. Dilatancy was observed over a range of shear rates beyond which solutions behaved as classical polymer systems. When salinity exceeded a critical value, dilatancy and anti-thixotropy appeared as increasing functions of ionic strength and decreasing functions of hydrolysis degree. Prolongation of ageing time strongly enhanced both shear thickening and anti-thixotropic behaviors. These results are interpreted by the changes of the relative numbers of intra- and intermolecular associations, and the participation of free hydrophobes in the formation of clusters under shear.