Solution viscosity measurements have been used to define the optimum concentration for the ultradrawn films of high molecular weight polyethylene produced by gelation/crystallization from solution. For a molecular weight of 4×106 the optimum concentration was 0.5g/100ml. Dry gel films prepared from the solution of this concentration could be elongated at 135°C to a remarkably high draw ratio λ=300. The Young's modulus and the tensile strength, 202 and 6.2 GPa respectively, of the drawn film are among the highest reported for polyethylene ; the observed Young's modulus is consistent with the modulus of polyethylene in the chain axis direction. The origin of the high drawability was discussed in terms of the morphology of the dry gel film as studied by wide-angle X-ray diffraction, small angle X-ray scattering, and scanning electron microscopy. The facile drawability is interpreted as due to the transformation from a folded to a fibrous crystal. A suitable number of entanglements in the chains connecting crystal lamellas is necessary to induce the transformation. Temperature-dependence of complex dynamic modulus functions was measured in the range of frequency from 0.1 to 100 Hz in order to study crystal dispersions of polyethylene. At temperatures lower than 70°C, master curves of loss modulus are obtained through only horizontal shift of the modulus function along the logarithmic frequency axis. By contrast, the temperature-frequency superposition above 70°C requires the vertical shift as well as the horizontal shift of the function. The Arrhenius plots indicate that there exist two mechanical dispersions, one at higher and the other at lower temperature sides of the reference temperature of 70°C. The activation energies of these dispersions range from 98.3 to 104.2 kJ/mol (from 23.5 to 24.9 kcal/mol) and from 78.2 to 80.8 kJ/mol (from 18.7 to 19.3 kcal/mol), respectively. These results indicate that the dispersions belong to α1 and α2 mechanisms, respectively, which have been reported by a number of authors.
A dispersion parameter is a quantity to estimate the quality of dispersed materials, e. g., particle size and homogeneity, in terms of the mixing condition such as the shear rate t and the mixing time t. Several types of dispersion parameters were evaluated for mixing processes, employing two types of mixers, of low density polyethylene and carbon black particles. The conventional type of parameter, E01=K01γα1t, did not correspond to the final dispersion level with sufficient precision for an optimum dispersion parameter. The observation of three definite dispersion levels depending on the mixing time led us to propose a new type of parameter, E02=K02γα1tα2. This showed better correspondence in both the laminar and the melt-fracture (turbulent) flow regions. At each of the three levels of dispersion, corresponding to three sets of α1, and α2, the parameter enabled us to estimate the final dispersion quality to within 10%. In the laminar flow region, the ratio of α1, and α2 was 1.0: the parameter was expressed as a function of total shear strain. In the melt fracture flow region, the ratio was 2.9, which is equal to 2(1+n) where n is the power-law index for the shear stress-shear rate relation. The latter result indicates that the dispersion process depends mostly on the shear rate, but little on time, and that the energy of ficiency in the melt-fracture region is much higher than that in the laminar flow region. Application of the optimum dispersion parameter to actual mixer systems was made in two cases: one was the case of the coexistence of laminar, melt-fracture, and folding flows, and the other was that of the existence of air with agglomerated carbon black particles.
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.