Crystalline Structure and Thermal Transition of Regioregularity-Controlled Poly(3-dodecylthiophene)

We investigate the liquid crystalline (LC) phase transition of regioregularity (RR)-controlled poly(3-dodecylthiophene) (P3DDT). Controlling RR of the P3DDT from 95 to 65% provides significantly altered strength of the LC interactions while maintaining the chemical structures the same. As the RR decreases, the P3DDT exhibits progressively decreased phase transition temperatures under polarized microscopy due to weaker interchain interactions. In the X-ray scattering and thermal analysis, the melting of the side-chain and backbone lead to entering the nematic and isotropic phases, respectively. Furthermore, P3DDTs with high RR exhibit only the Form I crystal structures. However, P3DDTs with low RR adopt both the Form I and Form II crystal structures, where Form II crystals contain interdigitated alkyl side-chain. The amount of Form I crystals is significantly decreased as the RR decreases, and Form II crystals are dominantly observed for RR 60% P3DDT. These features explain the significant decreases in the phases transition temperatures of P3DDTs with lower RR. This study provides an understanding of the RR effect on crystalline structures and thermotropic LC behavior of P3DDTs, allowing for exquisite control over their self-assembly and optoelectrical properties.