• O. F. Pascui, R. Lohwasser, M. Sommer, M. Thelakkat, T. Thurn-Albrecht, K. Saalwächter.
  High Crystallinity and Nature of Crystal-Crystal Phase Transformations in Regioregular Poly(3-hexylthiophene).
  Macromolecules 43, 9401–9410 (2010). DOI 10.1021/ma102205t.

  Abstract

  Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated polymers and are thus of pivotal relevance for themobility of charge carriers in electro-optical device applications. Currently, poly(3-akylthiophenes) are discussed as one of the most promising classes of materials for photovoltaic applications. Here, we use 13C solid-state NMR to determine the crystallinity and details on crystalcrystal phase transformations in regioregular head-to-tail poly(3-hexylthiophene) of well-defined molecular weight and demonstrate that the crystallinity was previously severely underestimated. Typical crystallinities are at least around 37% for the lowest molecular weights and as high as about 64% upon increasing MW, corresponding to a corrected maximum value for the reference melting enthalpy of ΔH_m^∞ ≈ 37 J/g for use in DSC experiments. Using 1D ¹³C spectra and 2D experiments that measure the strength of ¹³C-¹Hdipolar couplings, we observe that the crystal-crystal phase transition between a 3D- and a 2D-ordered crystalline phase at around 60°C entails a structural disordering process of the alkyl side chains, while not affecting rigidity and conformation of the backbones. This phase transition is suppressed at higher molecular weights due to a kinetic suppression of the formation of the alkyl-ordered low-temperature phase.

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