pH-triggered reversible sol-gel transition in aqueous solutions of amphiphilic gradient copolymers

We demonstrate the possibility of reversible pH-controlled sol-gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly (styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized via nitroxide-mediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nano-domains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.