Molecular Weight Optimum in the Mesoscopic Order of Chiral Fluorene (Co)polymer Films

A chiral fluorene homopolymer (PF) and two new chiral alternating polyfluorenes, poly(fluorene-alt-benzothiadiazole) (PFBT) and poly(fluorene-alt-dithinenylbenzothiadiazole) (PFDTBT), have been synthesized by palladium-catalyzed Suzuki polycondensations. In these polymers, chirality was introduced by attaching (S)-3,7-dimethyloctyl substituents to the 9-positions of the fluorene monomers. Upon thermal annealing, PF and PFBT undergo an irreversible phase transition to a liquid crystalline state that upon cooling to room temperature is vitrified and exhibits considerable circular dichroic (CD) effects. By contrast, PFDTBT forms an amorphous glass with hardly any CD activity. After fractionation by preparative size exclusion chromatography (SEC), the phase behavior of the low polydispersity polymers was studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and CD spectroscopy. Remarkably, after fractionation also for PFDTBT a CD effect was observed and for all three materials the maximum CD effects were recorded at intermediate polymer length, indicating the presence of an optimal molecular weight for the chiral organization. In addition to the influence of molecular weight on the supermolecular organization, also annealing temperature and time, as well as film thickness were investigated. The three polymers showed very different ordering kinetics while the temperature dependence was much more uniform. A nonlinear increase of CD values with film thickness was observed, giving rise to higher g-values in thicker films.