Thermal transitions and crystalline structures of regioregularity-controlled poly(3-alkylthiophene)s

Abstract

i) Regioregularity Dependent Thermal Transitions and Crystalline Structures in Poly(3-dodecylthiophene)s: The temperature-dependent LC phases of regioregularity (RR)-controlled poly(3-dodecylthiophenes) (P3DDTs) are systematically investigated. Controlling the RR of P3DDTs from 94% to 60% significantly modifies the strengths of their LC interactions, whereas their chemical structures remain the same. As the RR decreases, P3DDT exhibits progressively decreased phase transition temperatures, i.e., the nematic-to-isotropic transition temperature, as measured by using polarized optical microscopy (POM), UV-vis spectroscopy, differential scanning calorimetry (DSC), and X-ray scattering. The amount of Form I crystals observed is significantly decreased as the RR decreases. While P3DDTs with a high RR (83% and 94 %) exhibit only Form I crystal structures, P3DDTs with a low RR (60% and 76%) allow for the formation of both Form I and Form II crystal structures, with Form II crystals being dominantly observed for RR 60% P3DDT polymers. These features explain the significant decreases in the transition temperatures between the phases for P3DDTs with lower RR. This study provides an understanding of the RR effect in determining the crystalline structures and thermotropic LC behavior of P3DDTs, allowing for exquisite control over their self-assembly and optoelectrical properties. ii) Crystallinity and Thickness Dependent Thermo-Mechanical Behavior of RR-Controlled P3HT Thin films: The thermo-mechanical properties of poly(3-hexylthiophene) (P3HTs) thin films are investigated with controlled RR from 95 to 60 % to access systemically different crystallinity. In addition, using novel thermal strain (εT) measurements where the P3HT thin films are floated on ionic liquid under heating/cooling, the intrinsic thermo-mechanical properties of thin films can be obtained in a wide range of temperature, even above melting temperature (Tm). All P3HT thin films exhibit linear thermal expansions (e.g., constant coefficient of thermal expansion, CTE), and then a rapid increase of CTE from chain melting. Interestingly, lower RR P3HT has higher CTE and lower Tm of thin film (T$_{m, thin-film}$), respectively. This is furtherly investigated by X-ray scattering, indicating that the small amounts of crystallites surrounded by large amorphous domains require less energy for expansions and disruptions of chain orderings. Additionally, it is noted that CTE and T$_{m, thin-film}$ are highly influenced by the thickness of thin film (D$_{film}$), which can apply the confinement effects on polymer chains. With decreasing D$_{film}$, a lower CTE, and a higher T$_m$, thin-film can be obtained because the polymer chains have stress to suppress the in-plane expansions and spherulite crystal growths in thin films, respectively. Then, CTE or T$_m$, thin-film is converged from a certain thickness (D$_{eff}$, CTE, and D$_{eff}$, T$_m$, respectively), where the confinement effects are not available. More interestingly, D$_{eff}$, Tm of P3HT thin film at RR 72 %, (161 nm) is ~ 70 nm lower than that at RR 82 % (229 nm), because more flexible chains can release more stress and effectively narrow the regime for the confinements. However, D$_{eff}$, CTE of P3HT at RR 73 and 82 % are very similar to ~ 110 nm, suggesting that both P3HT films have a similar strength of film hardening by confinements. Thus, our study provides important correlations between thermo-mechanical properties of CPs thin films with thickness and crystallinity.