Insights on the interaction between sodium dodecyl sulfate and partially hydrolyzed microblock hydrophobically associating polyacrylamides in different polymer concentration regimes

The partially hydrolyzed microblock hydrophobically associating polyacrylamides (HMBHAPs) with different microblock lengths (NH) and some positive charges were prepared by free radical copolymerization in aqueous solution and the HMBHAPs were characterized by FTIR, ¹H-NMR, ion titration, elemental analysis and thermal gravimetric analysis. Moreover, the hydrodynamic size and microtopography of HMBHAPs were also characterized by dynamic light scattering and environment scanning electron microscope. The self-association effect of HMBHAPs and the interaction between HMBHAPs and oppositely charged sodium dodecyl sulfate (SDS) in different polymer concentration regimes were investigated through the measurements involving rheology, fluorescence, conductivity and zeta potential. The results show that HMBHAPs presented different viscosity behaviors in the concentration range concerned that could be divided into three different concentration regimes, where the viscosity contribution made by association effect was significantly discrepant and enhanced with increasing microblock length. Moreover, rheological measurements indicate that, with the addition of SDS, zero-shear viscosity (η₀) of HMBHAPs presented different variation trends in three polymer concentration regimes: at lower HMBHAPs concentration, the η₀ of HMBHAPs/SDS system was smaller than that in absence of SDS; at moderate HMBHAPs concentration, the system viscosity increased slightly; whereas, the viscosity distinctly increased with increasing SDS concentration at higher HMBHAPs concentration. The I₃/I₁ and Iₑ/Iₘ also demonstrated that the aforementioned viscosity behaviors were closely related to the efficiency and number of the associations, as well as the micro-block longer the association effect stronger. In addition, η₀ and Iₑ/Iₘ exhibited a very interesting “double peaks” phenomenon with increasing SDS, which may be interpreted by the simultaneous influence of electrostatic interaction and the association effect between cationic hydrophobic groups and anionic surfactant. The SDS amounts binded by HMBHAPs showed a slightly increasing trend as increasing polymer concentration and decreasing NH. Zeta potentials at n-octane-water interface showed a “charge reversal” phenomenon with increasing HMBHAPs concentration, which appeared earlier for longer microblock. With the addition of SDS, Zeta potential rapidly varied from an initial smaller positive charge to a greater negative charge, and the variation presented earlier with varied polymer concentration regimes and increasing NH. This result may be ascribed to the adsorption of HMBHAPs macromolecules at the oil-water interface and different inter-molecular conformations after interacting with SDS at the interface.