Poly(styrene-co-acrylic acid) (St/AA) and poly(styrene-co-methacrylic acid) (St/MA) nanolatexes with different acid contents were prepared by emulsion copolymerization and were analyzed by capillary electrophoresis (CE) and by laser doppler velocimetry (LDV). Due to the intrinsic differences in the methodologies, CE (separative technique) and LDV (zetametry, nonseparative technique) lead to very different electrophoretic mobility distributions. Beyond these differences, the variation of the electrophoretic mobility is a complex and nonlinear function of the hydrodynamic radius, the ionic strength, and the zeta potential. To gain better insight on the influence of the ionic strength and the acid content on the electrophoretic behavior of the nanolatexes, the electrophoretic mobility data were changed into surface charge densities using the O'Brien, White, and Ohshima modeling. This approach leads to the conclusion that the surface charge density is mainly controlled at high ionic strength (∼50 mM) by the adsorption of anionic surfactants coming from the sample. On the contrary, at low ionic strength, and/or in the presence of neutral surfactant in the electrolyte, the acid content was the main parameter controlling the surface charge density of the nanolatexes.
Aqueous solutions of poly(vinyl acetate)-b-sodium polyacrylate (PVAc-b-NaPAA) block
copolymers were characterized by capillary electrophoresis (CE). CE experiments reveal the presence of
NaPAA dead chains, acetate ions (which are a byproduct of the hydrolysis of PVAc), and nonassociated
copolymers (unimers). This fraction of unimers is dependent on the chemical composition of the copolymers
and on the purification procedure. It remains constant when increasing copolymer concentration. The
fraction of unimers was too high to correspond to a critical micellar concentration. Instead, this high
content of unimers is likely due to the polydispersity in composition of the copolymers, sodium polyacrylate-rich copolymers being excluded from the micellization process. This interpretation is consistent with the
study of the effect of temperature on the free copolymer fraction. Above a temperature threshold, the
unimer population increases due to the destabilization of part of the micelles. Addition of a neutral
surfactant leads to the formation of mixed micelles, the electrophoretic mobility of which can be modeled
using recent theoretical developments on electrokinetic migration of composite objects.
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