Photoreversible viscosity changes and gelation in mixtures of hydrophobically modified polyelectrolytes and photosensitive surfactants

The viscosity and gelation of mixtures of hydrophobically modified poly(acrylic acid) (HMPAA) and a cationic photosensitive surfactant can be controlled reversibly by changing between UV and visible light irradiation of the sample. At the critical aggregation concentration (cac) of the surfactant, micellar aggregates form on the polymer and solubilize the alkyl side chains grafted on the HM-PAA, leading to physical cross-linking and gelation. The hydrophobic trans (visible light) form, with a planar azobenzene group in the surfactant tail, has a lower cac than the more polar cis (UV light) form, resulting in gelation. at lower surfactant concentrations under visible light. Reversible viscosity changes of up to 2 orders of magnitude are observed upon exposure to UV or visible light. Observed viscosity maxima of 5.6 x 10(4) cPa for the trans form and 2.2 x 10(3) cPa for the cis form of the surfactant suggest that the transform surfactant micelles are more effective at solubilizing the alkyl side chains than are the more hydrophilic cis-form micelles. Steady-state fluorescence studies of the cationic probe crystal violet reveal an enhancement in binding of the crystal violet to sites adjacent to bound surfactant molecules as the surfactant concentration is initially increased prior to the cac, followed by a decrease in binding of crystal violet at concentrations above the cac due to a decrease in the number of available binding sites as the anionic polyelectrolyte wraps around the newly formed cationic micelles. The nearest-neighbor binding is greater in the trans form as opposed to the cis. Surface tension values decrease slowly with increasing surfactant concentration below the cac due to strong binding of the surfactant to the polyelectrolyte, followed by a large drop in the surface tension at the cac that results from release of bound surfactant upon micelle formation and subsequent wrapping of the surfactant aggregates by the polyelectrolyte. Dynamic viscoelastic measurements are typical of gel systems with G' > G above the cac and also indicate that approximately 25% of the polymer chains are elastically effective in the presence of the trans form, while only 7.5% of polymer chains participate in cross-linking in the presence of the cis form of the surfactant.