Structural anisotropy in unstretched and uniaxially stretched poly(p-phenylene vinylene)

Poly(p-phenylene vinylene) or PPV is gaining increasing importance because of its superior electroluminescent efficiency and electrical conductivity. The most widely followed synthetic route for PPV involves synthesis of a pr ecursor polymer using a sulfonium monomer salt. Previous studies have prove n that the monomer salt which contains cyclic sulfonium groups yields bette r quality PPV than when the monomer containing dialkyl sulfonium groups is used. The structure of PPV synthesized using cyclic sulfonium precursor has not been as widely reported as that synthesized using dialkyl sulfonium mo nomer. In the current work, the structure of PPV, synthesized using a cycli c viz, tetrahydrothiophenium monomer salt, has been studied in detail using the wide angle X-ray diffraction (WAXD) technique. The study reveals that even in the cast (unstretched) form, PPV shows considerable biaxial orienta tion in the plane of the film. This preferred orientation is found to occur during the casting process and is independent of the solvent used and cast ing substrate. On stretching these films to a final draw ratio of 7 : 1, th is biaxial orientation is transformed into uniaxial orientation with nemati c ordering of PPV chains along the stretch direction and PPV chains assumin g three preferred orientations in the plane of the film. (C) 1999 John Wile y & Sons, Inc.