Surface Induced Ordering of Triblock Copolymer Micelles at the Solid−Liquid Interface. 1. Experimental Results

Neutron reflectometry has been employed to measure the surface induced ordering of a triblock copolymer comprised of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic, P85) in aqueous solution far above the critical micelle concentration at the hydrophilic surface of quartz. In the bulk micellar liquid phase, the data are consistent with a micellar layering at the solid-liquid interface, as indicated by free-form analysis and subsequent detailed modeling which includes Monte Carlo simulations of hard spheres at a hard wall. Treating the micelles as either solid spheres or solid spheres with tethered Gaussian chains interacting as hard spheres, important parameters can be obtained from fitting the reflectivity data such as the interaction and core radius of the micelles, the volume fraction, and the surface polymer density at the solid-solution interface as a function of temperature and concentration. The dependence of these variables was found to be similar to the bulk solution behavior, signifying that the micelles behave like a hard-sphere liquid (as in the bulk) at a hard structureless wall. The behavior is mainly a result of the hydrophilic nature of the poly(ethylene oxide) chains surrounding the micelles as well as the hydrophilic surface. Further support for the bulklike behavior was seen by the appearance of Bragg-like peaks occurring in the reflectivity as the prolate ellipsoidal phase and the body-centered cubic micellar crystal phase were entered.