Abstract
The melting behavior of two non‐oriented poly(3‐alkylthiophene)s (P3ATs) containing 12 and 14 carbon atoms, respectively, in the alkyl side chain was investigated by means of differential scanning calorimetry. We show that the two P3ATs display three melting endotherms after an isothermal annealing from the melt. Two of the melting peaks arise from the ordered structures which are completed in the course of the quenching of the sample from the melt, and judging from the essentially constant melting temperatures and heats of fusion, they are not influenced by the subsequent isothermal annealing conditions. On the other hand, a much slower crystallization induced by the annealing, which occurs over a wide range of temperatures, is responsible for the appearance of a third melting endotherm which is strongly dependent on the crystallization condition. We show that this secondary crystallization is not a perfection of the existing ordered structures, rather it results in distinct crystals which melt at a temperature T~m~ slightly above the crystallization temperature T~c~. Moreover, it is shown that the plot of T~m~ versus T~c~ parallels the line of T~m~ = T~c~, and that the difference T~m~ − T~c~ increases linearly as the number of carbon atoms in the side chain decreases. On the basis of the calorimetric and X‐ray diffraction measurements, it is suggested that the secondary crystallization originates from the alkyl side chains.