NEUTRON REFLECTION STUDIES OF COPOLYMERS AT THE HEXANE WATER INTERFACE

The conformations of three copolymers at the hexane/water interface have been studied using neutron reflection. The experiment is made possible by using a specially-designed apparatus in which a thin (approximately 10 mum) layer of hexane is spread on a water surface, thus allowing acceptable transmission of a neutron beam incident at a grazing angle to the surface. Since none of the polymers used was deuterated, sensitivity to the polymer structure was achieved using contrast variation of the two solvents. A triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (F127 of the Pluronic series) is shown to adopt a conformation that extends beyond the micellar radius in water and, thus, appears quite stretched. A random copolymer of poly(vinyl alcohol-co-acetate) is shown to adopt a very flat conformation at the interface, forming a dense layer no more than 20 angstrom thick. While both of these polymers have hydrophobic moieties, hexane is not a good solvent for either. In contrast, the hydrophobic block of a diblock copolymer of poly(dimethylsiloxane)-poly(2-vinylpyridine N-oxide) is very soluble in hexane, and in this case the hexane appears to have a much greater influence over the polymer structure. A substantial proportion of the polymer is found on the hexane side of the interface.