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