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_α, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory-Fox model yields T_α = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of T_α is seen below M_n = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower T_g (T_αPE ≈ -100 °C) is associated with the side chains and is independent of molecular weight. RRa P3HT exhibits a lower T_α and higher T_αPE 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 T_g above -120 °C, at 144 °C in the long chain limit.