Conformation changes in stiff‐chain poly‐n‐butyl isocyanate. The dilute regime

Abstract

A change of conformation of the rod‐like poly‐n‐butyl isocyanate (PBIC), in the dilute regime, was induced by temperature and observed via an abrupt change of index of refraction increment, Δn, and of viscosity, η. PBIC was dissolved in carbon tetrachloride where it exhibits its most extended conformation, with persistence length of 15 nm. The index of refraction increment Δn and viscosity η were measured as a function of temperature in the dilute regime, that is, the concentration range below the threshold concentration c* where intermolecular interactions are negligible. Strikingly, increasing the temperature an abrupt change of Δn was observed, at a given solution concentration. The change of Δn is associated to a change in macromolecular size per the Clausius‐Mossotti relation. At the same solution concentration and temperature range, the shear viscosity also exhibited an abrupt increase Δη. This investigation showed that the conformation transition temperature Ttr is independent of the wavelength utilized to measure Δn, and it is an increasing function of the solution concentration and of PBIC molecular weight. Static light scattering confirmed that the sudden changes of Δn and η are not associated to macromolecular aggregation. Finally, the magnitude of the change of Δn and Δη was gradually reduced when increasing the solution concentration c, that is, when c > c*, therefore suggesting that the conformation transition was screened by intermolecular interactions.