Studying the concentration dependence of the aggregation number of a micellar model system by SANS

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.

Studying Soft-matter and Biological Systems over a Wide Length-scale from Nanometer and Micrometer Sizes at the Small-angle Neutron Diffractometer KWS-2

“…When the system is investigated in the conventional pinhole mode using the wavelength spread of Δλ/λ= 20%, as provided by the velocity selector, and continuous (static) data acquisition, three broad peaks are observed in the scattering patterns at L D = 4 m. In the tunable resolution mode, by using the chopper and the TOF data acquisition in combination with the velocity selector, the wavelength spread can be improved so that it can be checked whether these features are real or if a fine structure of them will eventually appear. The first and second peaks observed at Δλ/λ= 20% reveal a splitting when they are measured with Δλ/λ= 5%, which enabled the clear identification of the ordering of micelles into face-centered cubic (fcc) crystals 5,13 .…”

Spherical Micelles with Nonspherical Cores: Effect of Chain Packing on the Micellar Shape

“…In all our previous works on the C n -PEO x system, we used a spherical core–shell model for starlike micelles which yielded very good agreement with experimental small-angle scattering data (see for instance Figure 2 a). 15 , 16 , 19 , 20 , 23 , 29 However, it is not obvious how (partially) crystalline n -alkyl chains could be arranged in a spherical core. Vilgis and Halperin predicted that diblock copolymers with crystallizing solvophobic blocks would form disklike cores, where the core chains align along their axis and the solvophilic chains stick out from the basal planes.…”

On the Colloidal Stability of Spherical Copolymeric Micelles

“…The decrease of the aggregation number of the micelles as they get closer (see Figure 3) contrasts with regular scaling models, where the aggregation number is assumed to remain constant up to the limit where micelles overlap and increases beyond overlap of the micelles. 38 We note that the models presented here concern dilute suspensions of micelles, as we do not study the micellar changes for r < 2 R h o . Upon approaching overlap of micelles, a small decrease of the aggregation number (of the order of what we find here) has been found experimentally.…”