The viscosity of concentrated solutions of rigid rodlike molecules (poly-γ-benzyl- l-glutamate in m-cresol)

Abstract

Because of the agreement between the theory for the intrinsic viscosity of suspensions of rigid rods and the values obtained experimentally with solutions of poly-γ-benzyl- l-glutamate in m-cresol, the same system was investigated at higher concentrations. Four different molecular weight fractions were studied at concentrations up to 18% at shear stresses from 50 to 4 × 10 4 dynes/cm. 2. The initial slope of the log η sp / c versus c curves is found to be proportional to the intrinsic viscosity (both are determined at zero shear stress), as predicted theoretically, the experimental proportionality constant differing little from the theoretical one. Furthermore, the solutions contain an anisotropic phase when the concentration is above some value c ∗ . Below c ∗ the viscosity at low shear stress increases with c, above c ∗ η decreases with c. The value of c ∗ can thus be determined, and, for each molecular weight sample, agrees well with that calculated by Flory. All aspects of the flow behavior can qualitatively be explained ( 1) by assuming that the anisotropic solutions are fully oriented at the lowest shear stresses attained, at which the individual particles in the isotropic solutions are completely unoriented; ( 2) by allowing for considerable disorientation of the individual particles in the anisotropic solutions.