Surface characteristics of polyfluorene films studied by polarization-dependent NEXAFS spectroscopy

The chemical composition and the preferential orientation of polymer segments at the surface of thin polyfluorene films have been investigated by carbon K-edge soft X-ray absorption spectroscopy, resolving the near-edge X-ray absorption fine structure (NEXAPS). The outermost surface layer of about 10 A thickness exhibits a slight enrichment of alkyl side chains and a corresponding depletion of bipbenyl groups. Moreover, at the film surface the plane of biphenyl rings comprising the polymer backbone is strongly oriented parallel to the film surface. This equilibrium surface characteristics of polyfluorene films are found to be caused mainly by the polymer/air interface rather than by the polymer/solid (silicon) interface. A significant reorientation of polymer segments occurs at the film surface upon rubbing with velour cloth, which results in a preferential orientation of polymer molecular axis (biphenyl ring axis) along the rubbing direction and a decrease in the in-plane orientation of biphenyl planes. Upon annealing a rubbed polyfluorene film in its nematic melt state, the rubbing-induced reorientation of polymer molecules disappears completely for a film of about 600 Angstrom thickness. indicating that, the rubbing-induced reorientation does not propagate much further beyond the surface region of poly-fluorene films.