Controlling the swelling and rheological properties of hydrophobically modified polyacrylic acid nanoparticles: Role of pH, anionic surfactant and electrolyte

Abstract Nanosized particles of poly(acrylic acid), which is chemically cross-linked and hydrophobically modified, was investigated with the swelling behavior as triggered by ionization and surfactant binding. It was found that the viscosity, elasticity and transmittance of the solutions are dramatically affected when the pH increases above 6.5; viscosity changes can amount to more than 7 orders of magnitude. At low pH, where the polymer is weakly charged and the interaction with the solvent is poor, and the solution is liquid-like (G″ > G′) and turbid. At pHs above 6.5, the solution becomes a transparent gel with solid-like behavior (G′ > G″). Additionally, at low pH, the addition of anionic surfactant promotes the swelling of the polymer with consequent gelation leading to a solution behavior with essentially identical properties as when the polymer has a high charge density. Thus addition of moderate amounts of surfactant can increase the viscosity by 7 orders of magnitude. Moreover, an increase in the surfactant chain length is found to strengthen the surfactant binding to the polymer. The results are discussed in terms of the delicate balance between the osmotic swelling (counterion entropy effect) and restoring forces due to the hydrophobic interactions.