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

摘要

We investigated the influence of interfacial tension,y,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 y 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)approxr~(-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 y reveals a power law dependence,P approx gamma~(6/5),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,A_2,of a PEO homopolymer by SANS reveal clearly reduced values compared to A_2 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.