Quantification of preferential orientation in conjugated polymers using X-ray diffraction

Abstract

The varying degree of preferential orientation observed in films of poly(alkylthiophene)s obtained by solution casting from different solvents has been studied quantitatively with X-ray diffraction. The (uniaxial) orientational distribution function is obtained after some important instrumental corrections that are described in detail. Orientational distribution functions of an exponential type are found to give an adequate description with only a few fitted parameters, unlike Legendre series fits, which require longer series. An attempt has been made to relate the anisotropy to the chemical and physical properties of the solvents used. The anisotropy seems to be higher for samples cast from solvents with low boiling points and, correspondingly, high vapor pressures. Also, the average crystallite size is solvent-dependent. The characteristic dimension of the crystalline in the a axis direction appears to be longer for crystallites with the a axis parallel to the film normal. Along the b axis, that is, the π-stacking direction, the crystalline dimension is shorter than along the a axis. Also, the amorphous regions of the films exhibit anisotropy.