Flow birefringence and the equilibrium rigidity of the molecules of some polyamide benzimidazoles in sulphuric acid

The authors have studied the birefringence of solutions of samples of four polymers of the polymide benzimidazole class in H2SO4. From the value of reduced birefringence they evaluate the equilibrium rigidity A of the chains falling five times on replacing in the repeat polymer unit two phenyl rings in the para-position (A-25 nm) by phenyl rings in the meta-position. (A-5·7 nm). The equilibrium rigidities of the chains on the assumption of complete freedom of rotation and the degrees of braking of rotation in the chains of the polymers studied are calculated.