Quantification of Preferential Orientation in Conjugated Polymers Using X-ray Diffraction

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