A PHENOMENOLOGICAL MODEL FOR PREDICTING THERMOCHROMISM OF REGIOREGULAR AND NONREGIOREGULAR POLY(3-ALKYLTHIOPHENES)

Thermochromism of a series of cast films of poly(3-alkylthiophenes) (P 3ATs, A = hexyl, octyl, dodecyl, and hexadecyl) with different head-to -tail(HT) diad content (70-100%) was studied. The thermochromic behavi or 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 ther mochromic isosbestic point irrespective of the length of the alkyl sid e chain. Regioregular polymers possessing a >90% HT diad content exhib it either a continuous thermochromic blue shift or an isosbestic point , depending on the alkyl side chain length. The different thermochromi c characteristics are attributed to subtle morphological differences b etween the polymers. DSC and X-ray diffraction studies indicate that P 3AT 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 dis ordered phases. For polymers with short alkyl side chains, these phase s equilibrate with each other and destroy the thermochromic isosbestic point. Polymers with long alkyl side chains melt at much lower temper ature and are converted directly between the crystalline and disordere d phases.