NEUTRON REFLECTIVITY OF AN ADSORBED WATER-SOLUBLE BLOCK-COPOLYMER - ASURFACE TRANSITION TO MICELLE-LIKE AGGREGATES AT THE AIR WATER INTERFACE/

We describe neutron reflectivity measurements on layers of poly(2-(dim ethylamino)ethyl methacrylate-block-methyl methacrylate) copolymer (po ly(DMAEMA-b-MMA)) (70 mol % DMAEMA, M-n = 10k) adsorbed at the air/wat er interface. Measurements of the partially deuterated copolymer on nu ll reflecting water give the adsorption isotherm and show that there i s a surface phase transition from a layer about 20 Angstrom thick to o ne about 40 Angstrom thick. The concentration of polymer at which the phase transition occurs is close to that at which micellar aggregation in the bulk solution also occurs. Using two other isotopic compositio ns, the structure of the layer above and below the phase transition ha s been determined. At low concentration, the two blocks of the copolym er are approximately uniformly distributed in the direction normal to the interface and the layer is partially immersed in water, At high co ncentrations, the adsorbed layer has a cross-sectional structure resem bling that expected for a micelle with the majority of the more hydrop hobic MMA residues forming the core. The outer layers, comprising pred ominantly DMAEMA residues, are not equivalent, being much more highly extended on the aqueous side of the interface.