Abstract
We present a small-angle neutron scattering (SANS) structural characterization of n-alkyl-PEO polymer micelles in aqueous solution with special focus on the dependence of the micellar aggregation number on increasing concentration. The single micellar properties in the dilute region up to the overlap concentration ϕ* are determined by exploiting the well characterized unimer exchange kinetics of the model system in a freezing and diluting experiment. The micellar solutions are brought to thermodynamic equilibrium at high temperatures, where unimer exchange is fast, and are then cooled to low temperatures and diluted to concentrations in the limit of infinite dilution. At low temperatures the kinetics, and therefore the key mechanism for micellar rearrangement, is frozen on the experimental time scale, thus preserving the micellar structure in the dilution process. Information about the single micellar structure in the semidilute and concentrated region are extracted from structure factor analysis at high concentrations where the micelles order into fcc and bcc close packed lattices and the aggregation number can be calculated by geometrical arguments. This approach enables us to investigate the aggregation behavior in a wide concentration regime from dilute to 6·ϕ*, showing a constant aggregation number with concentration over a large concentration regime up to a critical concentration about three times ϕ*. When exceeding this critical concentration, the aggregation number was found to increase with increasing concentration. This behavior is compared to scaling theories for star-like polymer micelles.
Highlights
A classical feature of block copolymers is the spontaneous selfassembly in selective solvents into micellar aggregates
In this work we present a detailed study of the aggregation number of a micellar model system by small-angle neutron scattering (SANS) covering a very broad range of concentrations
In this paper we present a brief summary of the scaling theory and a detailed description of experiments and results including the freezing and diluting experiment
Summary
A classical feature of block copolymers is the spontaneous selfassembly in selective solvents into micellar aggregates. Good agreement between theory and experimental data have been achieved using amphiphilic diblock copolymers of the type poly(ethylene-alt-propylene)–poly(ethylene oxide) (PEP–PEO) in water or water–N,N-dimethylformamide mixtures.[16,17] crystalline lattices in these systems were found to be often suppressed by the formation of a glassy state It has been discussed and shown by Nicolai et al.[27,28,29] that for micellar systems a fast dynamic molecular exchange is necessary for the formation of ordered phases. In order to still measure form factors P(Q) and to determine Nagg at higher concentrations we have created a freezing and diluting experiment exploiting the known exchange kinetics[35] and the temperature independent aggregation behavior[19] of this particular micellar system. SANS measurements and data evaluation will be presented and the results are discussed in terms of the scaling prediction
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