For the past thirty years, the author has been conducting researches on the mechanics of rubber elasticity and fibrous materials. This article presents a historical review of the development of these researches. The author's first research project was an investigation of thickness irregularity in fiber bundles. This irregularity is caused by the random arrangement of the short fibers that form the bundle, and increases during the drafting of the bundle in the spinning process. The author presented a theory that made it possible to predict the power spectrum of this irregularity. This research stimulated the author's interest in the mechanics of fiber assembly bodies, also known as ”fibrous materials”.
This interest remains strong today. After completing his first research project, the author spent a year at the California Institute of Technology, in Pasadena, where he initiated research on rubber elasticity as a basis for research on the failure behavior of solid rocket propellant. He has developed a bi-axial extension experiment for rubbers, and found a simple form of the strain energy density function for these rubbers. Parallel with this research on rubber elasticity, the author continued research on the deformation mechanism of fibrous materials such as yarns and woven and knitted fabric structures. On the basis of this research, the author developed an objective evaluation of fabric hand; evaluations had previously been made by a subjective method called ”hand judgment”. There is a similarity between research on the mechanics of rubber and research on the mechanics of fibrous materials. These researches deal with assembly body of molecular chain or fiber and have been carried out with an interactive concept. The author has also introduced his recent research on the measurement of anisotropy in the mechanical property of single fibers. This measurement was conducted with a micro-measurement instrument developed by the author. Some of the results of these measurements are also presented here.

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Relationship between rheological behavior and structures of styrene-butadiene (PS-PB) diblock copolymers was examined in a PB-selective solvent, n-tetradecane (C14). At low temperatures the behavior of concentrated PS-PB/C14 micellar solutions was dominated by plasticity due to a macrolattice of micelles. Thermodynamic confinement (balance of elastic and osmotic requirements) on corona PB blocks led to macrolattice formation. Differing from the PS-PB/C14 solutions, blends of PS-PB and homo-PB chains were not suffered from the confinement and exhibited linear viscoelastic relaxation that was retarded by entanglements of PB blocks. In further investigation of entanglement dynamics, constraint release (CR) relaxation in PS/PS binary blends was found to be reasonably but not completely described by a Rouse-type CR model. This model was further examined for dielectric relaxation of polyisoprene (PI) chains having dipoles parallel to the chain contour. Specifically, eigenfunctions associated with a local correlation function were evaluated from dielectric data of dipole-inverted PI chains. Those eigenfunctions were non-sinusoidal, most clearly exhibiting non-Rouse nature of actual CR processes.

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Studies on nonlinear viscoelastic properties of well entangled polymers and change in entanglement structure by the author and his coworkers are summarized. In concentrated solutions and melts of polymers with high molecular weights, the polymers are entangled with each other, forming a quasi-network structure. Each molecule is repeating the process of leaving old constraint and entering a new one. In the range of linear viscoelasticity, the dissolution and re-formation of entanglements are balanced. Hence the density of entanglement remains constant. Outside the range of linear viscoelasticity, however, the entanglement structure may be changed by external stimuli such as shear strain and shear rate. Such an idea was supported by the observations of transient viscoelastic phenomena in various types of shear rate jump experiments. The relationship between nonlinear viscoelasticity and change in entanglement structure was manifested from simultaneous measurements of stress growth and/or stress relaxation, and differential dynamic modulus by small oscillatory deformations superposed on a large deformation. The entanglement structure does not change from the equilibrium structure in the linear region, whereas it deviates from the equilibrium one in the nonlinear region. Such the change in entanglement structure arises from a decrease in entanglement density and an anisotropic configuration of the chain molecule.

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Measurments of pressure at representative points in a converging-diverging channel were carried out for viscoelastic liquids of aqueous solutions of polyethyleneoxide(PEO)and polyacrylamide(PAA)as well as for Newtonian liquids. The pressure drop at the section of converging channel and the pressure rise at the section of diverging channel were simultaneously measured for various flow rates. The experimental results indicate that;
i) magnitudes of both pressure drop and rise for the PEO solution increase with increasing PEO concentration, and the pressure characteristics are similar to those of Newtonian liquids.
ii) the pressure characteristics of the PAA solution are significantly different from those of Newtonian liquids when the PAA concentration is increased. The differneces seem to be due to elasticity of the PAA solution.

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