Monitoring Molecular Ordering of a Conjugated Polymer: PBTTT during Conformational Evolution

The conformational evolution of a conjugated polymerfrom amorphousto (poly)crystalline is crucial for achieving charge transport inflexible electronic devices. However, the molecular ordering evolutionprocess 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 scanningcalorimetry (FSC) in real time to investigate morphology-dependentmain-chain/side-chain aggregates and thermotropic mesophase formationbehavior in PBTTT. Our findings reveal that the conformational transitionof 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 causeda decrease in the packed side chains, which was associated with thegrowth of the thermotropic mesophase. At 353 and 363 K, the competitionbetween the growth of main-chain crystallization and the thermotropicmesophase formation influences the molecular ordering of the sample.Furthermore, during crystallization at 413 K, the increased mobilityof the main and side chains causes more perfect crystallization ofthe main chain without thermotropic mesophase formation, resultingin a more ordered structure. These findings help us better understandthe conformational transition in the variations of the molecular orderingof conjugated polymers with the ongoing crystallization and can beemployed to fabricate conjugated polymer flexible devices.