A Phenomenological Model for Predicting Thermochromism of Regioregular and Nonregioregular Poly(3-alkylthiophenes)

Thermochromism of a series of cast films of poly(3-alkylthiophenes) (P3ATs, A = hexyl, octyl, dodecyl, and hexadecyl) with different head-to-tail (HT) diad content (70−100%) was studied. The thermochromic behavior is found to be controlled by the HT diad content and the side chain length. P3ATs with moderate HT diad content give rise to a clear thermochromic isosbestic point irrespective of the length of the alkyl side chain. Regioregular polymers possessing a >90% HT diad content exhibit either a continuous thermochromic blue shift or an isosbestic point, depending on the alkyl side chain length. The different thermochromic characteristics are attributed to subtle morphological differences between the polymers. DSC and X-ray diffraction studies indicate that P3AT samples with moderate HT regularity are formally amorphous with a quasi-ordered phase dispersed in continuous disordered domains. P3ATs with high HT regularity consist of crystalline, quasi-ordered, and disordered phases. For polymers with short...