Crystalline Structural Evolution in Polythiophene-Based Conjugated Polymer Blends During Rubbing

The ability to tailor the multilevel crystalline structures of conjugated polymers is highly desirable to establish the structure-property relationship. However, investigations into conjugated polymer blends are somewhat limited. Herein, we report crystalline structural evolution in a series of poly(3-alkylthiophene) (P3AT)-based P3AT/P3AT blends containing various alkyl side chains by mechanical rubbing, including cocrystalline and phase-separated structures, crystal orientations (i.e., edge-on and face-on), and large-scale anisotropic crystalline structures. The as-cast poly(3-hexylthiophene)/poly(3-octylthiophene) (P3HT/P3OT) and P3HT/poly(3-dodecylthiophene) (P3HT/P3DDT) blended films show phase-separated structures. Upon rubbing at increased temperatures, the P3HT/P3OT blend transforms into cocrystals at 60°C and 120°C, while P3HT/P3DDT blend becomes partially cocrystal at 60°C and changes back to phase-separated structures at 120°C. Simultaneously, both P3HT/P3OT and P3HT/P3DDT blends transition from the edge-on orientation in the as-cast state to mixed edge-on and face-on orientations after rubbing at 60°C, and then back to the edge-on orientation after rubbing at 120°C. On a larger scale, both blends become anisotropically aligned crystalline films along the rubbing direction. Formation mechanisms of these multilevel crystalline structures in P3AT/P3AT blends are elucidated.