Crystal morphology in pristine and doped films of poly (p-phenylene vinylene)

Abstract

The crystal morphology of oriented films of poly (p-phenylene vinylene) (PPV) has been investigated using electron microscopy and X-ray diffraction. An X-ray diffraction rotation series confirmed the existence of fibre symmetry in bulk oriented films. Dark-field imaging by transmission electron microscopy (TEM) revealed small diffracting regions of the order of 7 nm in size with an aspect ratio near 1. These diffracting regions were shown by high resolution transmission electron microscopy (HREM) to be composed of small crystallites with an average size of 5 nm. Imaging of the lateral packing by HREM allowed the evaluation of local variations in crystallite orientation. This HREM method of orientation function determination compares well to bulk methods (e.g. wide-angle X-ray scattering, infrared dichroism) for PPV of similar draw ratio. A micellar model is presented to describe the crystalline morphology of oriented PPV. The model presents PPV as a highly connected network of small crystallites. The well-formed crystalline regions are thought to compose approximately 50% of the sample volume with the remainder of the volume being grain boundaries. Doping by AsF₅ led to the formation of an electron-dense overlayer, thought to be arsenic oxide, which prohibited darkfield imaging of the crystallites. After doping with H₂SO₄, crystallites of the electrically conductive phase were observed. The general morphological character is preserved in the conversion from insulating to conducting forms. For the conditions employed, the doped diffracting regions were 4 nm in size and retained the orientation initially present in the pristine film.