Micellization of pH-stimulable poly(2-vinylpyridine)-b-poly(ethylene oxide) copolymers and their complexation with anionic surfactants

Abstract

The micellization behavior was examined for a series of 3 pH-stimulable poly(2-vinylpyridine)-b-poly(ethylene oxide) (P2VP-b-PEO) copolymers, with a constant composition of 67.5±1.5wt% PEO and increasing molecular weight. The micellar characteristics were determined by dynamic, static and electrophoretic light scattering, fluorescence spectroscopy, as well as by (1)H NMR in the pH range of 2-7 and in the presence or in the absence of the anionic surfactant sodium dodecylsulfate (SDS). In the absence of SDS, two pH regimes were investigated. For the pH range below 5, the micellar characteristics, such as C.M.C., particle size and zeta potential, were determined. For the first time, it could be shown that the micellization range could be extended to low pH values for relatively high copolymer concentrations. Above pH 5, correlations between the molecular characteristics and the aggregation number as well as the hydrodynamic diameter were established. A fair agreement could be demonstrated with the theoretical predictions for star-like micelles having a P2VP core and a PEO corona. Another original aspect of these double hydrophilic copolymers is their complex formation by electrostatic interaction between the protonated P2VP and SDS. At low pH and low copolymer concentrations, where only protonated unimers are present, the SDS induces the micellization at a given neutralization degree (DN). The process of complex formation was analyzed as a function of DN and pH by DLS, fluorescence spectroscopy, zeta potential measurements and by (1)H NMR, this last technique providing information on the mobility of the P2VP/SDS micellar core. A model for the micelle formation mechanism was suggested.