Rheopectic behavior manifested by suspensions of titanate fibers in polyacrylamide solutions has been measured to investigate the process of breaking and formation of internal structure. The rheopexy was measured by dropping shear rate stepwise from high to low γf through γs (γi>>γs>γf), or directly from γi to γf (γi>γf). Rheopectic behavior observed depends upon the combination of shear rates. When the shear rate is dropped from γi to γs or from γs to γf, shearing stress increases exponentially with time. On the other hand, when the shear rate is dropped from γi directly to γf, shearing stress increases sigmoidally with time. These results suggest that two mechanisms operate with respect to the structure formation process of suspended particles. A new kinetic model is proposed to interpret the rheopectic behavior of the suspensions. The model consists of two processes of formation and breaking of structure; in the first process a linear (not branched) chain structure is formed by suspended particles, and in the second the linear structure develops into more complex structure such as three-dimensional network or skeleton structure. The model is found to explain successfully the general behavior of rheopexy, when the parameter which characterizes the final structure is assumed to be proportional to the viscosity of suspension.
A hierarchy of approximations is developed for describing the large deformation of the elastic bodies. This is made by re-examining the concept of approximations so far used in the theory of finite deformations from a purely mathematical point of view, and by organizing the possible approximations of the strain energy function in a hierarchy according to the order of the Cauchy-Green deformation invariants. Many features are clarified in the theory of finite deformations. These includes theorems on the quadratic surface of the strain energy, a law stating the sum of coefficients of each approximate constitutive equation to be constant and to be equal to the rigidity, and a correspondence between the hierarchy of approximate constitutive equations and that of molecular theory, the gaussian and non?gaussian statistical theory of rubber elasticity. The above findings correct various confusions and errors which have been often involved in the study of large deformation of rubber. Some widely spread concepts, such that the Mooney equation of the strain energy function W is derivable on the firm theoretical basis are criticized, and a three term equation, involving a term of (I1-3)2 in addition to those of (I1-3) and (I2-3) is proposed as an adequate first approximation for W.
The dynamic properties of powder?polymer solution suspensions at large amplitudes were investigated using a coaxial cylinder rheometer. The results are as follows: (1) The time-dependence of stress amplitude σ0, divided by strain amplitude γ0 disappears with increasing γ0, (2) the elastic behavior becomes indistinct, while the plasto-viscous behavior becomes distinct under strain amplitudes larger than 2.0, and (3) the time?temperature superposition principle is applicable to the σ0/γ0 vs. ω relation under strain amplitudes larger than 2.0. The steady flow properties were compared with the results of dynamic measurements. The time?temperature superposition principle is also applicable to ηa vs. γ relations, but ηa does not agree with |η*|. Here ηa is the apparent viscosity, η* is the complex viscosity, and γ is the rate of shear.