Electrical conductivity is greatly dependent on the quality and degree of crystallinity in semiconjugated polymers. We have investigated the order formation in electrically conducting poly(3alkylthiophenes) by combining the uses of thermal analysis and optical microscopy. Initially, we were able to observe a bimodal endothermic melting point using differential scanning calorimetry (DSC). Then, using a slow cooling cycle from the melt state with immediate reheating, it was shown that the two melting points are separate phenomena and depend on the thermal history. A single melting point was observed when the sample was cooled to 120 °C and annealed for 1 h. Optical microscopy was then applied for the same conditions. A thermochromic change was observed immediately. This change has been related to changes in chain conformation as the chain planarity increases with the decrease of thermal energy. With the annealing at 120 °C a formation of red dot-like domains could be seen. The formation of these domains was related to the above mentioned single high-temperature melting point. This observation is the first visual observation of the order formation in poly(alkylthiophenes) and also the first evidence of thermochromism taking place by an interchain mechanism such as crystallization. The reason for our observation is the very narrow thermal condition used in the experiments. In further experiments we notice further thermochromic change in the background, indicating a further ordering process that is related to the second endothermic melting point observed in the thermal analysis experiments. Our experiments support the earlier findings of two different ordered phases in poly-(alkylthiophenes).

Molecular mechanics minimizations were performed on alkyl-substituted biphenyls to explore the effect of the position and length of side chains on the planarity of the phenyl rings. This approach was extended to poly(p-2,5-di-n-alkylphenylene)s. The structural behavior was investigated by systematically modifying the length of the main chain and the side chains. Studies of substituted biphenyls showed that the methyl substituent at 2, 2‘, 6, and 6‘ positions much affected the torsion angle between the phenyls. The torsion angle decreased and converged to an asymptotic value of ∼70° from trans as the length of side chains was increased in 2,2‘,5,5‘-tetraalkylbiphenyls. The results of a single poly(p-2,5-di-n-alkylphenylene) chain clearly showed that the main chain and side chains cooperate on the molecular level to make a stable conformation. Poly(p-2,5-di-n-alkylphenylene)s with long side chains showed partial ordering of side chains with a plain plate structure, whereas those with short side chains did not show side-chain ordering and had cylindrical, or hairy-rod structure. The onset of side-chain ordering for this single-chain system was found to occur for a sexiphenylene unit with eight carbons in the side chains. The average torsion angles of poly(p-2,5-di-n-alkylphenylene)s decreased from 89° to 61° relative to trans as the length of side chains was increased from methyl to dodecyl.

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