Towards Connecting Fundamental Properties of Conjugated Polymers with Performance in Electronic Devices

Predicting the performance of conjugated polymers in electronic devices requires knowledge of fundamental properties, such as tie chain density, entanglement molecular weight (M_e), glass transition temperature (T_g) and liquid-crystal-to-isotropic transition temperature (T_c). Oscillatory shear rheometry can unambiguously yield the glass transition temperature of conjugated polymers, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), to settle a long-standing debate full of conflicting reports in the literature. Furthermore, the molecular weight dependence of T_g is studied with regioregular (RR) and regiorandom (RRa) P3HT, such that Flory−Fox model yields T_g = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a very different molecular weight dependence of T_g is seen below M_n = 14 kg/mol, suggesting this is the typical molecular weight of tie chains. Linear viscoelastic studies in molten and semicrystalline states also reveal that the presence of liquid crystalline phase can affect the effective M_e and the tie chain contribution to the semicrystalline modulus. These microstructural parameters connect to both charge transport and mechanical flexibility of conjugated polymers, thereby establishing design rules for flexible electronics.