Higher order liquid crystalline structure in low-polydispersity DEH-PPV

Abstract

Monodisperse, low molecular weight poly(2,5-di(2'-ethylhexyloxy)-1,4-phenylenevinylene) (DEH-PPV) demonstrates significantly better structural order than polydisperse PPVs. Since optical and electrical properties of polymer electronics are closely related to the structure and morphology of the active layer, morphological control is important for the fabrication of PPV-based devices. Soluble, monodisperse DEH-PPVs with a range of molecular weights showed a sequence of transitions through crystalline, smectic, nematic, and isotropic phases upon heating, and the transition temperatures increased sharply with increasing molecular weight. The layer spacing of the smectic phase is in good agreement with the length of a PPV molecule. The Maier-Saupe parameter for this polymeric system is estimated, and both energetic and entropic contributions are found to be important in describing the liquid crystalline interaction. Thin films revealed that the PPV molecules ordered into thermally stable layers a single molecule thick. The layers arranged into grains that were aligned by shearing. The orientation of molecules within the aligned layers was characterized by polarization-dependent X-ray absorption spectroscopy, and the PPV molecular axis was found to align parallel to the shear direction while the layers aligned perpendicular to the shear direction. Low polydispersity is critical to forming these types of highly ordered structures, and an analogous PPV sample with polydispersity greater than 5 does not form smectic layers.