The structure and phase behavior of an ultra soft colloidal model system was studied by small angle neutron scattering. As a model system we have chosen polymeric micelles prepared from n-octacosanyl-poly(ethylene oxide) (C28-PEO) block copolymers in aqueous solution. The C28-PEO block copolymer forms micelles with starlike structure where the aggregation number, the size and the softness can be tuned by varying the PEO length of the polymer. Time-resolved SANS has shown that at low temperatures (15 ºC ) micelles are almost frozen, whereas at higher temperatures (60 ºC) the system is dynamic, i.e. an existing fast exchange of single chains. These properties allow to prepare thermodynamically stable micelles which were taken as model system for soft colloids. The system was systematically studied as a function of concentration and PEO chain length (5, 10 and 20 Kg/mol) by SANS. In the dilute limit we have determined the single micellar properties. At intermediate concentrations below the overlap concentration, c*, SANS curves develop a structure factor peak revealing a liquid-like organization in solution. In the gel state above c* higher order structure factor peaks were observed indicating crystalline order in micellar solutions. Using high resolution (HR)-SANS (wavelength spread ≈ 5 %) the crystalline structure of the C28-PEO5 at concentrations up two twice c* was identified as fcc. By further increasing c a transition to bcc (bco) occurs. Micelles prepared from C28-PEO10 and C28-PEO20 only show bcc structures in the studied concentration range. The sequence of crystalline structures were found to be in excellent agreement with a theoretical phase diagram derived for star polymers, based on an ultrasoft interaction potential. Details of this study will be presented.
l.willner@fz-juelich.de
