Tuning the Light Response of Strong Polyelectrolyte Brushes with Counterions

In this work, a negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brush has been employed as a model system to demonstrate the tuning of the light response of strong polyelectrolyte brushes (SPBs) with counterions. The substitution of K⁺ counterions by azobenzene-containing counterions (Azo-N⁺) renders the PSPMA brush light-responsive in aqueous solutions. Nevertheless, the strength of the light response of the PSPMA brush is weak due to the inefficient disassembly of the micelle-like aggregates in the brush upon irradiation with ultraviolet light. Counterion mixtures of Azo-N⁺ and K⁺ are employed to realize a strong light response of the PSPMA brush by incorporating a reasonable amount of Azo-N⁺ counterions into the brush. The strength of the light response of the PSPMA brush can be tuned by the mole ratio of Azo-N⁺ to K⁺. Furthermore, properties including the hydration and conformation of the PSPMA brush can be reversibly switched via alternating ultraviolet and visible light irradiation. This work opens up the opportunities available for the use of counterions to tune the light response of SPBs.