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 earners in electro optical device applications Currently, poly(3-akylthiophenes) are discussed as one of the most promising classes of materials for photo voltaic applications Here, we use C-13 solid-state NMR to determine the crystallinity and details on crystal-crystal phase transformations in regioregular head to tall 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 Delta H-m(infinity) approximate to 37 J/g for use in DSC experiments Using 1D C-13 spectra and 2D experiments that measure the strength of C-13-H-1 dipolar couplings, we observe that the crystal crystal phase transition between a 3D- and a 2D ordered crystalline phase at around 60 degrees 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