The nucleation process and the crystalline structure of poly(3-alkylthiophenes) precipitated from marginal solvents

Abstract

Soluble poly(3-alkylthiophenes) with alkyl side chains longer than butyl precipitate from solution to suspension during cooling from solvents with strong temperature dependent solubility (marginal solvents). The precipitation is accompanied by a colour change from yellow to red, which is in accordance with the thermochromic- and solvachromic shifts previously reported. Optical absorption measurements were used to monitor the precipitation process. The measurements showed an energy shift for the absorption edge and a growth of some sharper (fine structure) peaks. These peaks can be interpreted as a vibronic combination of the π - π ∗ electronic transition and the 1460 cm −1 vibrational mode, the latter known to occur in well ordered poly(3-alkylthiophenes). The fine structure peaks were used to monitor the crystallization process, and indicated a high degree of polymer ordering in the precipitants. Mats of the precipitated materials were studied by x-ray diffraction. The amount of crystallinity were high compared to ordinary solution cast samples, whereas the crystal structure itself was the same, i.e. straight polymer chains with an alternating “up-down” conformation of the thiophene rings. The polymer mats were found to be anisotropic with the alkyl side chains oriented perpendicular to the mat surface.