Fibrillar Morphology of Derivatives of Poly(3-alkylthiophene)s by Solvent Vapor Annealing: Effects of Conformational Transition and Conjugate Length

A fibrillar morphology was obtained, compared to the featherless pristine films, via solvent annealing the films of a series of derivatives of poly(3-alkylthiophene)s (P3ATs): poly(3-dodecylthiophene) (P3DDT), poly(3,3'''-didodecyl-quaterthiophene) (PQT12), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT12). Among the solvents used, including dichloromethane, chloroform, tetrahydrofuran, and carbon disulfide (CS2), CS2 was the best to induce fibril formation because its solubility parameter is closest to those of the P3AT derivatives. It was found that higher critical CS2 vapor pressures were needed to form crystal nuclei with increasing conjugation length and molecular weight of the P3AT derivatives; i.e., the critical vapor pressures for P3DDT 13.9k and PQT12 15.5k were 59.0% and 80.7%, respectively, and there were no nuclei of fibrils for pBTTT12 15.6k with the highest conjugation length, even at a CS2 vapor pressure of 98.3%. Meanwhile, at the highest vapor pressure, the fibril density decreased with increasing conjugation length and molecular weight of the P3AT derivatives. This is attributed to the rod-like conformation prevailing for polymers with larger conjugation length and higher molecular weight during solvent annealing, making the conformational transition toward coils more difficult and hindering diffusion of molecules. The results presented here are expected to be helpful for the design and processing of conjugated semiconductor polymers.