EFFECT OF SURFACTANT ON THE SOLUTION PROPERTIES OF HYDROPHOBICALLY MODIFIED POLYACRYLAMIDE

The interactions, in aqueous solution, of sodium dodecyl sulfate (SDS) with copolymers of acrylamide (AM) and N-(4-ethylphenyl)acrylamide (e-psi-AM; [e-psi-AM]/([AM] + [e-psi-AM]) between 1 and 1.3 mol %) have been examined in the semidilute concentration range by rheology, fluorescence, and conductometry. Data are discussed with regard to changes in the hydrophobe content, the molecular weight, and the microstructure of the copolymers. The hydrophobic groups of such copolymer chains, in pure water solution, are associated by either inter- or intrachain liaisons. The presence of interchain liaisons leads to apparent viscosities much greater than those seen for corresponding polyacrylamides. Addition of SDS to these aqueous copolymer solutions was seen to cause dramatic increases in the viscosity at concentrations below that of the critical micelle concentration (cmc). Examination of the viscosity both as a function of shear rate and as a function of time (fixed shear rate) showed these solutions to have shear thickening, rheopexic, and thixotropic behavior. This complex behavior is explained in terms of the balance between inter- and intrachain liaisons and their effects on chain dimensions. Fluorescence and conductometric data have shown that the SDS associates with the hydrophobic regions of the copolymers in a noncooperative continuous binding process.