Photo‐induced association behavior of poly(sodium acrylate) bearing a small amount of malachite green

The photo‐induced association and dissociation of poly(sodium acrylate) containing a small amount of photoresponsive malachite green (MG) in aqueous solution were studied. It is known that MG dissociates into ion pairs under UV irradiation to produce the green triphenylmethyl cation. The cation thermally recombines with its counter anion to regenerate the colorless neutral compound. The random copolymer of acrylic acid with 0.05 mol % of MG monomer P(A/MG0.05)] was soluble in aqueous 0.01 M NaCl at pH 12 as a unimer due to electrostatic repulsion between carboxylate pendent groups when the MG moieties were in the neutral form. On the other hand, these MG groups were converted to the cationic form on UV irradiation, leading to polymer aggregation driven by electrostatic interactions between the cationic MG and anionic carboxylate pendent groups. These aggregates could be dissociated by heating in the dark, as the cationic MG reverted to its neutral form, eliminating these attractive electrostatic interactions. The association and dissociation of the copolymer was monitored by dynamic light scattering (DLS). When the salt concentration in aqueous solutions of P(A/MG0.05) was increased from 0.01 to 0.5 M at pH 12, no aggregation was observed on UV irradiation because of ionic screening of the aforementioned electrostatic interactions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011