Role of Interfacial Tension for the Structure of PEP−PEO Polymeric Micelles. A Combined SANS and Pendant Drop Tensiometry Investigation

Abstract

We investigated the influence of interfacial tension, γ, on the micellization properties of a highly asymmetric poly(ethylene-co-propylene)−poly(ethylene oxide) (PEP−PEO) block copolymer in mixed solvents consisting of water and dimethylformamide (DMF). Both are good solvents for PEO and nonsolvents for PEP but exhibit a large difference in γ with respect to the insoluble core block. Micellar characteristics were obtained by small-angle neutron scattering (SANS) and subsequent fitting of a core−shell form factor to the scattering patterns. The curves are perfectly described by a hyperbolic density profile for the shell, n(r) ∼ r-4/3, indicating a starlike structure of the micelles. The aggregation numbers of the micelles decrease with increasing DMF−water ratio from P = 120 in pure water to nonaggregated chains in pure DMF. Corresponding interfacial tensions were determined by pendant drop tensiometry using a PEP homopolymer of equal molar mass. A correlation of P with γ reveals a power law dependence, P ∼ γ6/5, in accordance with the scaling prediction of Halperin for starlike micelles. Additionally, it was found that the addition of DMF leads to a considerable decrease in the micelle radii, which cannot be explained by the decrease in P alone. Measurements of the second virial coefficients, A2, of a PEO homopolymer by SANS reveal clearly reduced values compared to A2 in pure water but still good solvent conditions for PEO in all water/DMF mixtures. However, a significant reduction in the radius of gyration was not found. Therefore, it was concluded that the reduced solvent quality has a more pronounced effect for the PEO chain dimensions in the confined geometry of a micellar corona.