Stratification in monolayers of a bidisperse melt polymer brush as revealed by neutron reflectivity

The volume fraction profile of long chains within a monolayer of a dense bi disperse polymer brush is investigated using neutron reflectivity. Monolaye rs of polymer brushes are prepared by spreading equimolar mixtures of short nondeuterated and long perdeuterated polyisoprenes with quaternary ammoniu m headgroups onto the surface of a Langmuir trough filled with H2O or H2O/D 2O mixtures. The chain lengths of the shorter and longer chains are approxi mately 485 and 970 repeat units, respectively. The neutron reflectivity of the monolayer-covered water surface is recorded at two different mean areas per polymer chain (220 and 310 Angstrom(2)). The analysis of the reflectiv ity data yields a total brush thickness that is proportional to the surface concentration and is in accordance with the bulk density of the polymer. T he inner structure can be described by a stratified model: The bottom regio n is formed by a mixture of long and short chains. On top of this region is a second part composed only of segments of the longer chains. The content of long chains in the bottom region is higher than that predicted by analyt ical self-consistent field (ASCF) theories. Thus, the parts of the long cha ins within the mixed region are less stretched than predicted by ASCF theor ies. The structure approximately scales with the total brush height, althou gh small deviations occur. The width of the transition zone between the two regions seems to be independent of the brush height.