NEUTRON REFLECTIVITY STUDIES OF SPREAD MONOLAYERS OF DERIVATIVES OF STYRENE MALEIC-ANHYDRIDE COPOLYMERS AT THE AIR-WATER-INTERFACE

Reaction of alternating styrene-maleic anhydride copolymers with various alcohols gave half ester derivatives. Neutron reflectivity measurements have been used to study monolayers of these derivatives at the air/water interface. Isotopic substitution has been used to label different parts of the copolymer and to locate different groups within the surface region. For the derivatives prepared using long chain alcohols (dodecyl or undecyl alcohol) the alkyl chains are completely out of the water subphase at high surface pressures (pi = 38 mN m-1) at the expense of the phenyl groups, which are almost completely submerged. At lower surface pressures (pi = 30 mN m-1) the phenyl groups are about 50% out of the water and approximately 30% of the alkyl chains are immersed. For the derivatives prepared using ethyl alcohol there is no detectable segregation of ethyl and phenyl groups. The thicknesses of the dodecyl chain layer at 11 angstrom and the undecyl chain layer also at 11 angstrom are substantially less than the fully extended chain lengths (16.7 and 15.5 angstrom, respectively). This is probably associated both with large random tilts of the chains away from the surface normal and with a significant incidence of cis conformations in the alkyl chains. The backbone and phenyl group layer, immersed in the water, is thicker (15-19 angstrom) than expected from the dimensions of the individual groups. This results from the disorder imposed on it by the requirements of the alkyl chains to be out of the water. As the layer is compressed, the disorder increases and the layer is observed to thicken.