The structure of block copolymers at the fluid/fluid interface

We have developed a method of measuring the distribution of polymer segments at a liquid/liquid interface using neutron reflection. The method consists of forming a stable (1-4 mu m) hexane layer on top of an aqueous sub-phase so that two interfaces contribute to the measured reflectivity. This has been done by condensing vapour onto the surface in an accurately temperature-controlled environment. The neutron reflectivity was measured at four angles of incidence on the SURF reflectometer. The attenuation of the reflectivity from the lowest angle is used to determine the thickness of the hexane layer. A series of poly(ethylene oxide) poly(propylene oxide) ABA block copolymers adsorbed at the hexane/water interface have been investigated. Since the polymers were hydrogenous, measurements were made with D2O as the aqueous phase and with either null reflecting hexane or 60% D-hexane as the oil phase in order to obtain adequate contrast. The polymer volume fraction profile has been determined by simultaneous fits to the data from these combinations of solvent contrasts. It was found that there was a good correlation between the molar mass of the poly(ethylene oxide) block and the thickness of the polymer layer in the aqueous phase and that the chains were stretched beyond their radii of gyration expected for bulk solution. (C) 1998 Elsevier Science B.V. All rights reserved.