Monitoring Molecular Ordering of a Conjugated Polymer: PBTTT during Conformational Evolution
Tengfei Qu, Yukun Li, Linling Li, Chen Zhang, Xiaoliang Wang, Wei Chen, Gi Xue, Evgeny Zhuravlev, Shaochuan Luo, Dongshan Zhou
Abstract
The conformational evolution of a conjugated polymer from amorphous to (poly)crystalline is crucial for achieving charge transport in flexible electronic devices. However, the molecular ordering evolution process between these two states is not yet well understood in poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2- b ]thiophene] (PBTTT). To gain insight into this process, we used a combination of resonance Raman spectroscopy and fast scanning calorimetry (FSC) in real time to investigate morphology-dependent main-chain/side-chain aggregates and thermotropic mesophase formation behavior in PBTTT. Our findings reveal that the conformational transition of the molecular ordering strongly depends on the isothermal temperatures. Specifically, when crystallized at 333 K from an amorphous state, the twist motion of the thienothiophene and thiophene rings caused a decrease in the packed side chains, which was associated with the growth of the thermotropic mesophase. At 353 and 363 K, the competition between the growth of main-chain crystallization and the thermotropic mesophase formation influences the molecular ordering of the sample. Furthermore, during crystallization at 413 K, the increased mobility of the main and side chains causes more perfect crystallization of the main chain without thermotropic mesophase formation, resulting in a more ordered structure. These findings help us better understand the conformational transition in the variations of the molecular ordering of conjugated polymers with the ongoing crystallization and can be employed to fabricate conjugated polymer flexible devices.