A NEUTRON REFLECTIVITY INVESTIGATION OF SURFACE AND INTERFACE SEGREGATION OF POLYMER FUNCTIONAL END-GROUPS

The distribution of polymer terminal groups at surfaces and interfaces is assessed by neutron reflectometry (NR) experiments on end-functional polystyrenes. Mono-terminated polystyrenes (PS) are synthesized anionically to include a short perdeuteriostyrene sequence adjacent to the end groups for the purpose of selective contrast labeling of the end groups for NR. The location of deuterium serves as a marker to indicate the location of the adjacent end group. Three cases of end group surface segregation are examined: a ''neutral'' control specimen prepared by proton termination, a ''repulsive'' end group system terminated with high surface energy carboxylic acid end groups, and an ''attractive'' end group system containing low surface energy fluorocarbon chain ends. All three systems exhibit damped oscillatory end group concentration depth profiles at both the air and substrate interfaces. The periods of these oscillations correspond approximately to the polymer chain dimensions. The surface structure of the ''control'' sample is dominated by the sec-butyl initiator fragment located at one end of the chain. This end group has a lower surface energy than that of the PS backbone and segregates preferentially to both the air and substrate interfaces. In the fluorosilane-terminated material, the low energy fluorinated end groups are depleted from the substrate interface but are found in excess at the air interface. In the carboxy-terminated material, the high energy carboxyl end group segregates preferentially to the silicon oxide overlayer on the substrate and is depleted at the air surface. X-ray photoelectron spectroscopy (XPS) is utilized to provide complementary characterization of the atomic surface compositions for the three systems.