Form II Poly(3-butylthiophene): Crystal Structure and Preferred Orientation in Spherulitic Thin Films

Poly(3-n-butylthiophene) (P3BT) samples with different average molecular weights and head-to-tail regioregularity were crystallized in the form II crystal polymorph either from solution or by annealing with CS2 vapors. With a combined approach, making also use of literature electron diffraction data, we show that form II is well described by a limit-ordered monoclinic model in space group P2(1)/c with lattice parameters a = 10.76(1) angstrom, b = 7.77(1) angstrom (chain axis), c = 9.44(1) angstrom, and beta = 64.66, yielding a calculated density of 1.29 g/cm(3) in qualitative agreement with proposals by Winokur et al. for the form II structures of regioregular poly(3-octylthiophene) and poly(3-dodecylthiophene) (P3DDT). Our structural model was refined by Rietveld analysis and has been confirmed by molecular mechanics (MM) and molecular dynamics (MD) calculations adopting a thiophene-specific force field developed in our group. Consistent with its higher density and with thermal data, form II shows lower potential energy than the form I' crystalline polymorph of P3BT. Both the main-chain and the side-chain conformations closely correspond to those found in form I' polymorph. The form II P3BT refined structural model presents an antidirectional looser stacking and tightly interdigitated layering, different from those observed in the form 1 family of poly(3-alkythiophenes) (P3ATs) and crystallite dimension of 20-30 angstrom along the chain axis. This feature and the lamellar structure implied by the spherulitic morphology are consistent with substantial chain-folding for high molecular weight samples. Oriented X-ray diffraction patterns from thin films of form II P3BT are explained assuming that the stacking axis c, corresponding to the radial, fast growth direction of the bidimensional form II spherulites, is preferentially in the plane of the film, while the layer axis a and the chain axis b approach random orientation around c, at variance with recent literature suggestions. The small crystal dimensions along the chain axis, the looser stacking, the relevance of chain folding and the spherulitic morphology implying film discontinuity suggest that the form II structural family of P3AT's are less viable than the form I polymorphs for molecular electronics applications.