Evaluation of molecular mass and tacticity of polyvinyl alcohol by non-equilibrium capillary electrophoresis of equilibrium mixtures of a polymer and a dye

Non-equilibrium capillary electrophoresis of equilibrium mixtures (NECEEM) has been used to characterize polyvinyl alcohol (PVA). Commercial PVA samples with different molecular masses, from Mw = 15 up to 205 kDa, were used. According to the 13C NMR spectra, the samples also differed in tacticity (stereoregularity). Mixtures of PVA and the anionic azo-dye Congo Red (CR) were injected in the presence of a borate buffer. The electropherograms gave a band and a peak due to the residual PVA–CR complex and the excess dye, respectively, plus a superimposed exponential decay due to the partial dissociation of the complex during migration. The stoichiometry of the PVA–CR complex, q = [monomer]/[dye], reached a maximum, qsat, which depended on both Mw and tacticity of PVA. Thus, qsat decreased from a molar ratio of ca. 4.9 to 3.6 at increasing Mw values, this variation also being largely dependent on tacticity. A similar dependence of the electrophoretic mobility of the complex on both Mw and tacticity was also observed. A possible explanation, based on the formation of a stack of CR ions inside the PVA–CR complex, was proposed and discussed. Finally, at increasing Mw values, the stability constant of the complex increased slightly, and the pseudo-first order dissociation rate of the complex decreased, this later parameter also showing a dependence on both Mw and tacticity.