Rheo-optical Studies of Concentrated Racemic Poly-γ-Benzyl Glutamate Solutions II.

Abstract

Rheo-optical experiments have been carried out to study the structural changes of concentrated solutions (20, 30, 40 wt%) of PBG (racemic poly-γ-benzyl glutamate) in m-cresol after cessation of steady shear. The transmitted light intensities (Ix, I″ and IE) of polarized light have been measured on a sample under the steady shear and after the cessation of the steady shear as functions of shear rate (2×10-2~3×102 s-1) at 25°C. The transmitted light intensities Ix (with crossed polarizers)and I″(with parallel polarizers) change in wave-like manner with time after the cessation of steady shear, indicating that the retardation changes with time, while the shear stress instantly decreases to zero for all solutions tested. The amplitude of wavy curves decreases at first and then increases. The transmitted light intensity IE (measured under extinction position of crossed polarizers) changes with time as if it envelopes the wavy curves of Ix and I″ and it reaches finally its equilibrium value close to 0(%), taking maximum at time tM. With the aid of quarter wave plate, it was ascertained that the retardation (Γ) decreases with time after the cessation of steady shear. The relaxation process of Γ is roughly represented by a single Maxwell model and the optical relaxation time τ0 can be determined from the inflection point of the Γ vs. log t curve.The monotoneous decrease in Γ is considered to arise from the relaxation in order parameter of the continuous phase. The appearance of the maximum in IE vs. log t curve is interpreted by considering temporary appearance of some defects in the continuous phase. The defects disappear in a course of time and the continuous phase becomes a relaxed nematic state, in which the system has lower order parameter than under steady shear.It was found that both tM and τ0 decrease with increasing shear rate. A straight line relationship between log tM and log (shear stress) is seen above 103dyne/cm3 for 20(%)solution, where the system has been transformed entirely to a monodomain continuous phase.