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-methy1-2-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 ≤ [NaNO ₃] ≤ 279 mM, a temperature increase for 25 ≤ T ≤ 80°C, a variation of the fraction of EO monomers in the corona, f_EO, at a fixed corona block length, N_corona, for 0 ≤ f_EO ≤ 1, a decrease in the PEO block length, N_PEO, at a fixed PAAm block length, N_PAAm, for 200 ≤ N_PEO ≤ 450, and finally, upon a decrease in corona block length at N_PAAm = N_PEO for 100 ≤ N_corona ≤ 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.