Glass Transition Temperature of Conjugated Polymers by Oscillatory Shear Rheometry

The stiff backbones of conjugated polymers can lead to a rich phase behavior that includes both crystalline and liquid crystalline phases, making measurements of the glass transition challenging. In this work, the glass transitions of regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT), regiorandom (RRa) P3HT, and poly((9,9-bis(2-octyl)-fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5',5 ''-diyl) (PFTBT) are probed by linear viscoelastic measurements as a function of molecular weight. We find two glass transition temperatures (T-g's) for both RR and RRa P3HT and one for PFTBT. The higher T-g, T-alpha, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory-Fox model yields T-alpha = 22 and 6 degrees C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of Ta is seen below M-n = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower T-g (T-alpha PE 100 degrees C) is associated with the side chains and is independent of molecular weight. RRa P3HT exhibits a lower Ta and higher Taps than RR P3HT, possibly due to a different degree of nanophase separation between the side chains and the backbones. In contrast, PFTBT only exhibits one Tg above 120 degrees C, at 144 degrees C in the long chain limit.