Surface Characteristics of Polyfluorene Films Studied by Polarization-Dependent NEXAFS Spectroscopy

Abstract

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 (NEXAFS). The outermost surface layer of about 10 Å thickness exhibits a slight enrichment of alkyl side chains and a corresponding depletion of biphenyl 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 Å thickness, indicating that the rubbing-induced reorientation does not propagate much further beyond the surface region of polyfluorene films.

Vertical confinement and interface effects on the microstructure and charge transport of P3HT thin films

“…A number of studies have documented the trend of better crystalline texture resulting in improved electronic performance. 16,23,[46][47][48] Our data confirms and expands that structural details of semicrystalline polymers have an important role in controlling charge transport.…”

Structural Characterization of Conjugated Polyelectrolyte Electron Transport Layers by NEXAFS Spectroscopy

“…A plane-on orientation has been observed before for nonionic fluorene oligomers; the orientation appears to be most prominent at the top of the film. [24] A plane-on preference for MEH-PPV has not been previously reported. The observation of any surface-relative orientation to the PF-X conjugated plane is unexpected given that polyfluorenes confined to films typically adopt a helical long-axis twist in a hexagonally packed structure that should exhibit no preferential surface-relative conjugated plane orientation.…”

“…The orientations are also similar to those observed in nonionic oligofluorenes. [24] These considerations suggest that the BAr orientation preference is caused by the PF backbone. We speculate that even very weak cofacial p-p interactions between a rigidly oriented backbone and the aromatic rings on BAr molecular orientation required to account for the observed p* orientation.…”

Interrogation of “surface,” “skin,” and “core” orientation in thermotropic liquid‐crystalline copolyester moldings by near‐edge X‐ray absorption fine structure and wide‐angle X‐ray scattering

“…8 This convention is based upon a configuration in which the perpendicular relationship for the coordinates of the analysis is taken relative to the normal to the sample surface. This convention has been used by several authors, [15][16][17]20 including previous work on the surface orientation in TLCPs. 7 Under this convention, an average uniaxial orientation parameter, S a , is defined as:…”