1985 Volume 13 Issue 1 Pages 25-36
Dynamic measurements are made with cone and plate geometry on the aqueous solutions of hydroxypropyl cellulose (HPC) in the frequency region from 10-1 to 102 radian·sec-1. The torque resulting from sinusoidally alternating strain imposed on the cholesteric phase of 60 wt% HPC solution exhibits undeformed sinusoidal wave which is out of phase with the input strain, as is the case with linear viscoelastic materials. Distinct difference in dynamic properties is recognized between optically isotropic and optically anisotropic solutions of HPC. The critical concentration which marks the transition from isotropic to anisotropic state is about 42 wt% at 25°C at which most of the experiments are made. The logarithmic plots of dynamic storage modulus (G′) and loss modulus (G″) against angular frequency for the optically anisotropic solutions of HPC tend to form a plateau in the low frequency region, while the plots for the optically isotropic solutions decline steeply with decreasing frequency, as is usual for the ordinary polymer solutions and melts. The order of G′ and G″ of liquid crystalline solutions expressed in terms of concentration is 60>45>48>50 wt%. A minimum value is found at a concentration of about 50 wt% at which the solution first exhibits the cholesteric texture and iridescence. The effect of strain amplitude upon G′ and G″ of cholesteric phase is minor, as in the case of linear viscoelastic materials. The real component η′ of complex viscosities of the optically anisotropic solutions tends to increase with decreasing angular frequency, while the values of η′ of the optically isotropic solutions are constant in the low frequency region. The plot of η′ against concentration shows a sharp peak at about 42 wt% at which the transition takes place and a minimum point at about 50 wt% at which the cholesteric structure is formed. The difference between η′ and the steady-shear viscosity η is interpreted in terms of the susceptibility of cholesteric phase to the prolonged effect of shear. The HPC solution containing TiO2 particles shows the dynamic properties similar to those of liquid crystalline solutions, but the resemblance seems to be casual. The stress relaxation taking place after the cessation of steady flow is measured. The later stage of relaxation of 60 wt% solution at 23°C is explained in terms of a Maxwell model with a relaxation time of 42 sec. Most of the relaxation takes place within 5 sec. This early stage of relaxation is the origin of production of zigzag-shaped fibrillar structure in the fibers and films made from liquid crystalline solutions. The dynamic properties of cholesteric liquid crystals of HPC are not permanent but variable with the history of mechanical treatments.