Flow birefringence and conformational characteristics of molecules of para-and meta-isomers of polyoxyphenyl-benzoxazoleterephthalamides

Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Δa = (330 ± 30) 10^?25 cm^?3 and the length of the Kuhn segment $\text{A = (330 ± 30)}\text{A}\text{?}$ and $\text{A = (115 ± 20)}\text{A}\text{?}$ for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A. confirming the validity of the molecular models used.