Determination of the Molecular Parameters and Studies of the Chain Conformation of Polybenzimidazole in DMAc/LiCl

Static light scattering has been used for the first time to obtain the molecular parameters and to study the chain conformation of a previously synthesized m-polybenzimidazole (m-PBI) sample in N,N-dimethylacetamide (DMAc) with 4% (g/mL) LiCl. A weight-average molecular weight Mw = 199 200 g/mol; radius of gyration Rg = 44.0 nm and A2 = 0.0022 cm3 mol/g2 are reported. The chain conformation of m-PBI was studied as a function of (a) polymer concentration (0.1−20 mg/mL) at a fixed salt concentration of 4% LiCl and (b) salt concentration (0.05−7%) at a fixed polymer concentration of 0.3 mg/mL. As polymer or salt concentration was increased, the chains collapsed initially, and then underwent fluctuations in size without a significant decrease in their average size. The collapse was associated with a conformational transition from random coil to an "extended wormlike" chain at increasing polymer concentration. At polymer concentrations greater than 9 mg/mL, the average size of the polymer chain was relatively constant at 31.2 ± 1.7 nm. Reduced viscosity measurements indicated that m-PBI behaved like a polyelectrolyte in the presence of DMAc/LiCl. Circular dichroism measurements confirmed the conformational transitions observed from light scattering by indicating the presence of optical activity at polymer concentrations above 2 mg/mL. Birefringence measurements at polymer concentrations between 4 and 25 mg/mL indicated the presence of possible relaxation caused both by the degree of chain flexibility and by deformation in polymer chains due to the influence of the laser intensity. These results contribute to our overall understanding of chain stiffness and conformational transitions in PBI polymers, which may play an important role in the preparation of polymer electrolyte membranes (PEMs) via sol−gel processing.