Effect of Salt on the Micellization Kinetics of pH-Responsive ABC Triblock Copolymers

Abstract

The pH-induced micellization kinetics of poly(glycerol monomethacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (PGMA-b-PDMA-b-PDEA) in the presence of salt (NaCl) was investigated by stopped-flow light scattering and fluorescence, and the results were compared to those obtained with salt-free solutions. Upon jumping from pH 4 to 12, this triblock copolymer forms micelles that consist of a central PDEA core, shielded by a PDMA inner shell and a PGMA outer corona. For micelle formation in the presence or absence of salt, all relaxation curves recorded by stopped-flow light scattering can be well fitted with a double-exponential function, leading to a fast relaxation time constant (τ1) and a slow relaxation time constant (τ2). The fast process (τ1) is associated with the formation of quasi-equilibrium micelles, while the slow process (τ2) is associated with micelle formation−breakup, approaching the final equilibrium state. Both processes occur much more slowly on initial addition of NaCl and then level off at higher salt concentrations (>0.5 M NaCl). The concentration dependence of τ2 revealed that the mechanism of micelle formation/breakup process transforms from unimer insertion/expulsion in the absence of salt to micelle fusion/fission in the presence of high NaCl concentrations. This is because the PGMA corona and PDMA inner are less highly hydrated and the PDEA core is more compact in the presence of salt, thus favoring micelle fusion/fission instead of unimer insertion/expulsion. Relaxation curves obtained with stopped-flow fluorescence using pyrene as a probe can be well-fitted with a single-exponential function, and the relaxation time (τpy) was in agreement with τf, the relaxation time of the overall micellization process as detected by stopped-flow light scattering. Compared to the kinetics obtained with salt-free solutions, the micelle dissociation kinetics in the presence of salt resulting from a pH jump from 12 to 4 reveals a drastically different two-stage process with the two stages separated by a delay time of a few seconds.