Elongational flow studies of hinged rodlike molecules

Poly(amino acid) in an intermediate state of its helix‐coil transition is known to be in a hinged rodlike conformation. In this work, the responses of poly(amino acids) in the hinged rodlike conformation against an elongational flow field were investigated by monitoring their flow‐induced birefringence. Poly(L‐glutamic acids) (PGA) and poly(γ‐benzyl‐L‐glutamate) (PBLG) were examined as polyelectrolyte and noncharged poly(amino acids), respectively, and the results were compared. In the plots of flow‐induced birefringence, Δn, against strain rate, \documentclass{article}\pagestyle{empty}\begin{document}$ {\dot \varepsilon } $\end{document} , for hinged rodlike PBLG, there was a critical strain rate, \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon_0 $\end{document} , below which Δ n was not observed. Over \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon_0 $\end{document} , the birefringence pattern observed was identical with that of rodlike molecules. The Δ n vs. \documentclass{article}\pagestyle{empty}\begin{document}$ {\dot \varepsilon } $\end{document} plot for hinged rodlike PGA had characteristics of a rigid rod at any strain rate and there was no \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon_0$\end{document} observed. The rotational diffusion coefficient, D r , of PBLG in the hinged rodlike conformation was larger than that for its helical conformation, while D r , for the hinged‐rodlike PGA was smaller than that for its helical conformation. It is concluded that the hinged‐rodlike PGA molecule is in an extended form and that the hinged‐rodlike PBLG is hydrodynamically more compact and rigid than that in its quiescent state. It is deduced that at \documentclass{article}\pagestyle{empty}\begin{document}$\dot \varepsilon_0$\end{document} hinged rodlike PBLG molecules collapse to a conformation optically anisotropic and mechanically rigid. © 1996 John Wiley & Sons, Inc.