Micellization and demicellization of amphiphilic poly(vinyl acetate)-graft-poly(N-vinyl-pyrrolidone) graft copolymers in the presence of sodium dodecyl sulfate

Abstract

The micellization of amphiphilic biocompatible poly(vinyl acetate)-graft-poly(N-vinyl-2-pyrrolidone) (PVAc-graft-PNVP) graft copolymers, characterized by a constant PVAc backbone and PNVP side chains of low grafting density as well as of variable chain lengths, reveals the formation of star-like micelles with, at 25°C, a partially “frozen-in” PVAc core and a PNVP corona. The interaction of these graft copolymers with sodium dodecyl sulfate (SDS) was examined on the one side by the so-called “in situ micellization” process consisting in the solubilization of the copolymer in SDS solutions of fixed concentrations and on the other side by the demicellization of preformed copolymer micelles by adding increasing amounts of SDS. In both cases the SDS concentration was kept below its critical micellar concentration (CMC) and the system was investigated by dynamic light scattering (DLS), nuclear magnetic spectroscopy (NMR) and cryogenic-temperature transmission electron microscopy (cryo-TEM). When increasing amounts of SDS are added to a micellar solution of graft copolymer, three regions are observed. At very low surfactant concentrations (region I), a slight decrease in the hydrodynamic radius is observed as a consequence of the copolymer-rich/surfactant complex formation. This complex, which is preferentially formed between SDS hydrocarbon chains and PVAc sequences by hydrophobic interactions, leads to a higher content of the “mobile PVAc”. By a further increase of the SDS concentration (region II), the copolymer micelles start to disintegrate and an abrupt particle size decrease was noticed. A complete demicellization occurs at even higher SDS concentrations where small surfactant-rich/copolymer complexes, as unimers, are predominately present in the system (region III).