Determination of the molecular parameters and studies of the aggregation behavior of polybenzimidazole in solution.

Static light scattering using a seven angle BI-MwA instrument has been used for the first time to obtain the molecular parameters as well as study the aggregation behavior of previously synthesized $m$-PBI samples in DMAc/LiCl solvent mixtures. Depolarization ratios for the samples were obtained using a BI-200SM goniometer system. Molecular parameters obtained were $m-$PBI(118) M$_{w}$: 179,300 g/mol; R$_{g}$: 36.4 nm; A$_{2}$: 0.0016 cm$^{3}$mol/g$^{2}$ and $m-$PBI(159) M$_{w}$: 206,500 g/mol; R$_{g}$: 46.6 nm; A$_{2}$: 0.0020 cm$^{3}$mol/g$^{2}$. The aggregation behavior of $m$-PBI(159) was studied as a function of salt concentration (0 to 7 {\%}) at a fixed polymer concentration of 0.3 mg/mL. With increasing salt concentration, R$_{g}$ decreases initially and goes through three maxima with a minimum R$_{g}$ of 31.0 nm at 4 {\%} LiCl. This result parallels the previously observed aggregation behavior of corn amylase in DMAc/LiCl. Thus our results suggest a general mechanism for the aggregation of semiflexible polymers which can be controlled by varying polymer or salt concentration.