(415a) Local Chain Alignment Via Nematic Ordering Reduces Chain Entanglement in Conjugated Polymers

Chain entanglements govern the dynamics of polymers and will therefore affect the processability and kinetics of ordering; it follows that through these parameters chain dynamics can also affect charge transport in conjugated polymers. We probed the effect of nematic coupling on chain entanglements using linear viscoelastic measurements on various conjugated polymers, such as poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) and poly-((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5’,5”-diyl) (PFTBT) with varying molecular weights. We first verify the existence of nematic phases in both PFTBT and PCDTBT and identify nematic−isotropic transition temperatures, T_IN, between 260 and 300 °C through a combination of differential scanning calorimetry, polarized optical microscopy, temperature-dependent X-ray scattering, and rheology. In addition, both PCDTBT and PFTBT show a glass transition temperature (Tg) and TIN, whereas only PFTBT has a melting temperature Tm of 260 °C. We find that the entanglement molecular weight (Me) in the isotropic phase is 11 ± 1 kg/mol for PFTBT and 22 ± 2 kg/mol for PCDTBT by modeling the linear viscoelastic response. Entanglements are significantly reduced through the isotropic-to-nematic transition, leading to a 10-fold increase in Me for PFTBT and a 15-fold increase for PCDTBT in the nematic phase. Thus, we show that liquid crystalline order can significantly impact mechanical and conductive properties of conjugated polymers.