A neutron scattering study of triblock copolymer micelles

The thesis describes the neutron scattering experiments performed on poly(ethylene oxide)/poly (propylene oxide)/poly (ethylene oxide) triblock copolymer micelles in aqueous solution. The studies concern the non-ionic triblock copolymer P85 which consists of two outer segments of 25 monomers of ethylene oxide attached to a central part of 40 monomers of propylene oxide. The amphiphilic character of P85 leads to formation of various structures in aqueous solution such as spherical micelles, rod-like structures, and a BCC liquid-crystal mesophase of spherical micelles. The present investigations are centered around the micellar structures. In the first part of this thesis a model for the micelle is developed for which an analytical scattering form factor can be calculated. The micelle is modeled as a solid sphere with tethered Gaussian chains. Good agreement was found between small-angle neutron scattering experiments and the form factor of the spherical P85 micelles. Above 60° C some discrepancies were found between the model and the data which is possibly due to an elongation of the micelles. The second part focuses on the surface-induced ordering of the various micellar aggregates in the P85 concentration-temperature phase diagram. In the spherical micellar phase, neutron reflection measurements indicated a micellar ordering at the hydrophilic surface of quartz. Extensive modeling was performed based on a hard sphere description of the micellar interaction. By convolution of the distribution of hard spheres at a hard wall, obtained from Monte Carlo simulations, and the projected scattering length density of the micelle, a numerical expression was obtained which made it possible to fit the data. The hard-sphere-hard-wall model gave an excellent agreement in the bulk micellar phase. However, for higher concentrations (25 wt % P85) close to the transition from the micellar liquid into a micellar cubic phase, a discrepancy was found between the model and the reflection data. Free-form analysis hinted towards a precrystallization of the micelles at the surface at this concentration. For temperatures above 60° C a 5 wt % solution showed a high degree of surface association. The neutron reflection experiments indicated that the surface ordering of the triblock copolymers changed from spherical micelles to a lamellar phase with increasing temperature. Presumably, an intermediate region occurs with surface ordered rod-like micelles. It was not possible to further increase the surface ordering by applying a shear field to the solution. In the bulk micellar cubic phase, the surface ordering could not be determined unambiguously. However, the neutron reflection data is consistent with a periodic ordering of slightly anisotropic micelles.

This report is submitted in partial fulfilment of the requirements for a Ph.D. degree at the University of Copenhagen. The work has been carried out at Risø National Laboratory, Condensed Matter Physics and Chemistry Department (formerly Department of Solid State Physics). Supervisors have been Jan Skov Pedersen from Risø National Laboratory and Stig Steenstrup from the University of Copenhagen.