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 coreshell 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 ∼ γ 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 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.

Structural Observation and Kinetic Pathway in the Formation of Polymeric Micelles

“…Figure 2 shows that this theoretical model almost perfectly describes the growth behavior for the three concentrations with the same parameter set once allowing for slight variations within a few percent, reflecting stochastic minute variations in the experimental conditions. The fits yield a value of about 0 % 38:2 and a spread of less than 3% This corresponds to an effective microscopic interfacial tension % 19 mN=m that is of the same order as the measured macroscopic interfacial tension between PEP and this DMF-water mixture: % 12 mN=m [21]. For the other parameters we obtain % 1:35 AE 5%, c scal ¼ 2:777 Â 10 5 and f % 0:034 ms À1 yielding an asymptotic value for 1 of % 2 Â 10 À4 (0.02%) which is the critical micelle concentration (CMC).…”

“…The self-assembly process of a model amphiphilic block copolymer was triggered by an interfacial tension jump experiment by rapidly changing the solvent quality for one of the blocks. Using a detailed quantitative model, we further demonstrate that the kinetic pathway proceeds by unimer exchange where only single chains are added or removed at a time.As a model system, we employ a well-defined amphiphilic poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP1-PEO20, numbers indicate the approximate molecular weight in kg=mole) block copolymer [21]. Extensive previous studies have shown that these asymmetric PEP-PEO diblock copolymers form well-defined starlike micelles in water-dimethylformamide (DMF) solvent mixtures [7,8,21,22].…”

“…Extensive previous studies have shown that these asymmetric PEP-PEO diblock copolymers form well-defined starlike micelles in water-dimethylformamide (DMF) solvent mixtures [7,8,21,22]. This solvent pair provides a strong selectivity for PEO; however, since water and DMF have very different interfacial tensions towards the insoluble PEP block ( % 46 and 8 mN=m respectively) [21], the interfacial energy can be easily tuned by varying the composition. This tuning parameter intimately controls equilibrium unimer exchange rates [7,8].…”

“…As a model system, we employ a well-defined amphiphilic poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP1-PEO20, numbers indicate the approximate molecular weight in kg=mole) block copolymer [21]. Extensive previous studies have shown that these asymmetric PEP-PEO diblock copolymers form well-defined starlike micelles in water-dimethylformamide (DMF) solvent mixtures [7,8,21,22].…”

“…1(a), the induced self-assembly process causes a strong increase in intensity directly reflecting the growth of the micelles in real time. In order to extract the mean aggregation number P mean , the mean core radius, R c , the overall micellar radius R m , and the radial density distribution nðrÞ accurately (<10%) at any kinetic time, the scattering data were fitted over the whole Q range with a standard core-shell model [21]. This model describes the detailed structural features of the starlike micelles considering both the internal PEP core and the outer PEO shell.…”