Role of solvent protic character on the aggregation behavior of polybenzimidazole in solution

The aggregation behavior of poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) in polar aprotic (dimethyl acetamide, DMAc) and protic (formic acid, FA) solvents is studied as a function of the polymer concentration and solution temperature. The effects of solvent protic character on the aggregation behavior of OPBI are elucidated. The photophysical studies suggest that the OPBI chains form aggregated structures in both DMAc and FA solutions when the OPBI concentration is increased. The dependences of the emission spectra on the polymer concentrations in two solvents are not similar in nature, indicating that in both of the solvents the aggregations are intermolecular processes, though their mechanisms are different owing to the polyelectrolytic nature of OPBI in FA medium. The triexponential decay profiles obtained from the time-resolved fluorescence study for the concentrated solutions (both in DMAc and FA) display a negative fractional coefficient and longer excited state lifetime, providing support for the aggregations at higher concentration. The temperature dependence emission spectra suggest that the aggregations in both of the solvents destabilize with increasing temperature. The higher activation energy of aggregation (E(A)) in DMAc (5.62 KJ/mol) compared with that in FA (3.07 kJ/mol) reveals that the aggregation formation pathways are different in two solvents and stronger aggregates are formed in the former solvent. The dilute solution viscometry (DSV) studies demonstrate that the OPBI chains adapt a bigger extended conformation in FA compared with DMAc owing to the stronger intramolecular chain repulsion in FA arising due to the polyelectrolyte nature of OPBI in this solvent. A conformation transition of OPBI chains from compact collapsed to extended conformer is observed in DMAc solvent with increasing concentration, whereas any such transition is absent in FA medium. Transmission electron microscope (TEM) images and circular dichroism (CD) spectra are also in agreement with the presence of a conformational transition in DMAc and the absence of it in FA. The temperature dependent DSV studies further support the disruption of aggregated structure with increasing temperature in both of the solvents. DSV studies exhibit that the deaggregation is driven by a conformation transition (extended to compact collapsed) in DMAc, whereas in FA the disruption happens without conformational transition.