Chain Conformation and Liquid-Crystalline Structures of a Poly(thieno)thiophene

The chain conformation and solid-state structure of conjugated polymers are important for understanding the structure origin of electronic properties. In this work, a high-performanceconjugated polymer, poly(2,5-bis(2-thienyl)-3,6-ditetradecylthieno[3,2-b]thiophene) (PBTTT_C14), is chosen. The chain conformation ofPBTTT_C14 in a good solvent (1,2-dichlorobenzene) in the dilutecondition is characterized by small-angle neutron scattering (SANS).The persistence length of PBTTT_C14 is estimated as 4.3 +/- 0.3 nm,indicating a more rigid backbone compared with poly(3-hexylthio-phene-2,5-diyl) (P3HT). In addition, the structural evolution ofPBTTT_C14 in bulk was studied in detail. Three transitions aredetected during heating, located at 57 degrees C(Tm1), 133 degrees C (peak A), and223 degrees C(Tm2). Fourier transform infrared spectroscopy (FTIR) resultsshow that a transition from thetrans-togauche-conformation of the side chains occurs atTm1, corresponding to the melting of theregular packing of the side chains. After that, the system enters liquid crystal states. Orientedfilms are obtained by shear within theliquid-crystalline phases, in which the backbones align along the shear direction. After assignment of the FTIR bands, it is found thatthe twist motion of thienothiophene rings in the backbone is activated at peak A. Two liquid-crystalline (LC) phases with differentstructures at the temperature range ofTm1 similar to peak A (LC-1) and peak A similar to Tm2(LC-2) are determined for PBTTT_C14: the twistmotion of backbones leads to an enhanced lamellar order, weaker interchain pi-pi interaction and vanishes the positional order along the chain axis (disappearance of (003)) for LC-2