On the transition between a heterogeneous and homogeneous corona in mixed polymeric micelles.

Two-dimensional NMR and small-angle neutron scattering experiments were performed on comicelles of poly(N-methyl-2-vinyl pyridinium iodide)-block-poly(ethylene oxide), P2MVP-b-PEO, and poly(acrylic acid)-block-poly(acryl amide), PAA-b-PAAm, in aqueous solutions to study whether a transition between a heterogeneous (Janus-type) and homogeneous corona can be observed upon a variation of parameters that are anticipated to affect the miscibility of the PEO and PAAm coronal blocks. Investigated were the effect of a salt-induced decrease in micellar aggregation number, P agg for 1<or=[NaNO3]<or=279 mM, a temperature increase for 25<or=T<or=80 degrees C, a variation of the fraction of EO monomers in the corona, fEO, at a fixed corona block length, N corona, for 0<or=fEO<or=1, a decrease in the PEO block length, N PEO, at a fixed PAAm block length, NPAAm, for 200<or=NPEO<or=450, and finally, upon a decrease in corona block length at NPAAm=NPEO for 100<or=N corona<or=400. These parameters should affect the mixing/demixing transition via their effect on the PEO/PAAm interfacial area (e.g., as in the case of f EO and P agg) or the relevant Flory-Huggins interaction parameters (e.g., as in the case of temperature). None of the above parameters was shown to yield a transition toward a homogeneous corona wherein the polymer chains are randomly mixed; i.e., the segregation of PAAm and PEO chains within the micellar corona of comicelles of PAA-b-PAAm and P2MVP-b-PEO appears to be rather robust.