Crystallization and liquid crystallinity in heptadecanylcarbazole-dithienylbenzothiadiazole (PCDTBT) conjugated polymers*

Conjugated polymers exhibit rich phase behavior including liquid crystalline and crystalline phases. Conflicting reports of one such polymer, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), have claimed the material to be amorphous or semicrystalline. We use oscillatory shear rheology to demonstrate that PCDTBT exhibits liquid crystallinity by identifying the nematic to isotropic temperature. We also find a crystalline phase when annealed at high temperatures, which is likely due to the high T_g, near 120 ^oC, that is identified also through rheology. We corroborate the phase behavior using X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy. High resolution transmission electron microscopy reveals the consequences of annealing in the amorphous or liquid crystalline phase prior to crystallization, and yields images of the crystalline morphology that are produced under these different conditions.