Experimental studies by the author and coworkers on rheological properties of concentrated polymer liquids are reviewed. The molecular weight, molecular weight distribution and concentration dependences of viscoelastic functions, relaxation spectra, steady-state viscosity, steady-state compliance, entanglement compliance and others have been investigated with relation to the entanglement structure and the relaxation mechanism of polymer chains. The rheological properties of branched polymers, star-shaped and randomly branched systems have also been discussed in terms of the length and number of the branches. Emphasis is placed on that the employment of monodisperse polymers is essentially significant to obtain a definite conclusion on the relationship between molecular characteristics and rheological properties.
A brief account is given for the development of studies in nonlinear viscoelasticity of polymer melts and concentrated solutions. The first stage of rapid progress starting around 1950 was led by the discovery of the normal stress effect. In this period, the rheological properties in steady shear flow were extensively studied and the concept of entanglement network was introduced as the origin of stress in polymeric liquids. The second stage which was partly induced by the development of rheological apparatuses in steady shear, was concerned with the nonsteady as well as nonlinear stress-strain relations. About the middle of 1970s, the strain-dependent constitutive equation was recognized as an appropriate equation to describe the stress for a wide group of flow histories. At the third stage of progress the tube model theory of Doi and Edwards was published in 1978 and it has been proved capable of describing most of the nonlinear viscoelastic behavior so far investigated. In the present paper a special emphasis is put on the birefringence induced by the strain. This phenomenon, if studied in appropriate nonsteady flows, may well give a clue to prove the molecular motion assumed in the tube model theory.
The entanglement effect in the nonlinear viscoelasticity of concentrated polymer solutions is now understood by the tube model. The basic idea underlying this model is reviewed with some historical background. A brief discussion is also made on the direction of the future development of this model, and on the problems which should be sorted out for such purposes.
Polymers have large bulk compressibility in the molten state, and their rheological properties are largely affected by pressure applied in polymer processing. The volumetric strain induced by the pressure consists of an instantaneous and a retarded elastic strains, both of which are proportional to the pressure, and recover reversibly on removal of the pressure. In many crystalline polymers, as observed by Maxwell for polyethylene, the retarded elastic strain is large, and it is mostly due to pressure crystallization. This paper describes results of experimental studies relating pressure effects on rheological properties of melt polymers with polymer processing and bulk properties of the products. Following items are discussed; pressure-induced shear stress, analysis of local deformation pattern, critical shear stress for melt fracture, relation between power-law index and bulk compressibity, effects of hydrostatic pressure on melt flow behavior, pressure efficiency of injection molding, jetting phenomena, shrinkage in injection molding, residual strain, and high pressure injection molding precess.
Rheological behavior after yielding to steady flow of concentrated suspensions, Bentonite, Kaolin (pH=3.9, rapidly flocculated and pH=5.0, weakly flocculated), and Kanto-Loam, was studied using a cone-and-plate rheometer. The plate was rotated at a constant acceleration velocity, θp, from rest up to a certain steady-state velocity, θp. In addition to stress measurements, deformation profiles on the side surface of the suspension were visually observed. For all the samples except for Kaolin of pH=5.0, the average shear rate in actually flowing zone, γ, which was estimated from the visualization, was almost equal to the applied shear rate, γap, until a certain critical rotation angle, Nc, was reached. During this period, however, the shear stress, τ, increased drastically with shear rate. Thus the thixotropic nature of the suspensions was apparent. At rotation angles larger than Nc, γ was always larger than γap and attained the steady value depending on θp. This behavior is due to the existence of non-flowing layers and their growth with time. This also gives a time dependence of τ different from that in the period of smaller rotation angles, and the apparent viscosity evaluated by using showed an abrupt decrease with time. The Kaolin suspension of pH=5.0 did not show such behavior; γ was always larger than γap, and the τ-γ relation in the accelaration period can be well estimated from the flow properties in the steady state.
Viscoelastic properties of aqueous disperse system of clay-starch and calcium carbonate-starch, so-called coating colors, were studied in the frequency range 10-2-103 rad/sec using two types of rheometers. Flow properties were also measured using a coaxial cylinder rheometer. Coating colors having different average sizes and distributions were used. In the low frequency region, values of G′ and G″ at small strains increase with decreasing particle diameter and become independent of frequency. The result suggests that the network structure, due to the aggregation of dispersed particles, becomes denser with decreasing size of dispersed particles. Under steady shear flow, the yield values as well as the shear stresses at finite rates of shear increase with decreasing particle size. The phenomena may be explained in terms of the of fective volume of dispersed particles including the solvating phase, which is considered to depend mostly on the interaction between particles or strength of the interparticle bond. In the disperse systems which consist of various sizes of dispersed particles, the discrepancy is observed between the viscoelastic properties for small sinusoidal deformation and those for steady shear flow at the corresponding time scale.
By using a tensile testing instrument (UTM-4L, Toyo Baldwin), load-elongation curves were obtained with six types of surgical sutures, i. e. absorbable sutures (plain catgut, chromic catgut, and Dexon) and nonabsorbable sutures (Silk, Nylon, and Nespolen), of sizes No.3-0, No.2-0, and No.1-0. Load-elongation curves of all absorbable sutures are nearly linear over a wide load range. Silk and Nespolen give convex curves when the load is taken on the vertical axis while Nylon gives concave curves. For all sutures the breaking strength increases with increasing diameter, and the values are sufficiently higher than the minimum standards specified by U. S. Pharmacopeia. The load-elongation behavior is discussed in relation to the structure of the sutures observed with scanning electron microscope.