THE STRUCTURE OF POLYMERIC ALKYLPYRIDINIUM MONOLAYERS SPREAD ON WATERSTUDIED BY SPECULAR X-RAY AND NEUTRON REFLECTION

Grazing incidence X-ray reflection has been used to examine the struct ure of spread monolayers of poly(N-alkyl-4-vinylpyridinium bromide)s, denoted by PC(n) where n is the length of the alkyl side chain, on wat er. Members of the series with even n from 8 to 22 were investigated, and the results for PC18 are compared with those of neutron reflection on the same sample. The comparison of neutron and X-ray results on PC 18 makes it possible to identify those contributions to the X-ray refl ectivity coming from the polymer backbone and alkyl chains for this pa rticular polymer. On the basis of this comparison the thickness of the polymer layer has been estimated for each member of the series. The r atio of the observed thickness of the monolayer to its maximum possibl e value (delta/l) varies from 1.3 for n = 8 through a minimum of 0.7 a t n = 16 to 0.8 at n = 22. Even when allowance is made for the contrib ution of capillary waves to the thickness of the layer, delta/l remain s above unity for the shortest chain compounds. This is interpreted as arising from disorder in the layer. As the side chain length increase s, it imposes order on the chain orientations. For small n an increase of n therefore decreases the thickness of the layer, but once the sid e chains are reasonably ordered the increase in chain length causes th e thickness to increase once more. The X-ray reflectivity shows, indep endently of the neutron result, that part of the polymer layer protrud es out of the water. For small n this projection is about one-third of the thickness of the total monolayer thickness, but this proportion i ncreases with n until eventually it has reached about the total thickn ess of the layer. Comparison with similar dimensions in surfactant lay ers and the PC(n) Langmuir-Blodgett films indicates similar behavior a t small n. The comparison between LB films and the precursor monolayer of the PC(n) shows that disorder in the latter carries over into the LB film.