High Crystallinity and Nature of Crystal−Crystal Phase Transformations in Regioregular Poly(3-hexylthiophene)

Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated polymers and are thus of pivotal relevance for the mobility 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 crystal−crystal 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−1H dipolar 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.