Hydrophobic interactions in associative polymer/nonionic surfactant systems: Effects of surfactant architecture and system parameters

The rheological behavior of a comb-like associative polymer with pendant hydrophobes in the presence of nonylphenol polyethoxylate (NPe) nonionic surfactants is examined. In particular, the effects of surfactant concentration, polymer concentration, and surfactant’s hydrophilic–lipophilic balance (HLB) are investigated with the latter being varied by using surfactants with different number of ethoxylate units (e=6, 8, 12, and 15). Steady shear and dynamic measurements are performed and the zero/low shear viscosity (η0), the number and average life time of the hydrophobic junctions, and the numbers of polymer hydrophobes and NP8 surfactant molecules per hydrophobic junction are determined. The trend of zero/low shear viscosity with increasing NP8 surfactant concentration exhibits four distinct regions, each of them related to molecular-level changes: (1) surfactant addition to the hydrophobic junctions (no change in η0 from that of pure hydrophobically modified alkali–soluble emulsion polymer solution); (2) formation of mixed micelles containing surfactants and several polymer hydrophobes (η0 increases); (3) saturation of each polymer hydrophobe by individual surfactant micelles (η0 decreases); and (4) formation of free surfactant micelles (η0 is constant). The viscosity trend of the polymer/NP8 surfactant systems with increasing surfactant concentration does not change upon increasing polymer concentration from 0.5 to 1wt%. Moreover, the number of surfactant molecules bound to polymer hydrophobes at the viscosity maximum is the same for both polymer concentrations. In terms of the effects of surfactant HLB on polymer/NPe surfactants systems, we find essentially similar trends of zero/low shear viscosity with increasing surfactant concentration for systems containing surfactants of higher HLB (NP12 and NP15). However, the viscosity maximum is lower, and viscosity in the fourth region gradually decreases for systems containing higher HLB surfactant. In contrast, systems containing a surfactant with lower HLB (NP6) exhibit a different behavior with a continuous increase in zero/low shear viscosity with increasing surfactant concentration.