This article summarizes our recent studies on structures and viscoelastic properties of polymer solutions and multicomponent-multiphase polymer systems, particularly polyelectrolyte solutions and polymer blends. Viscoelastic properties of polyelectrolyte solutions can be understood by classifying the solutions at least into dilute, semidilute and concentrated regions almost similarly to non-ionic polymer solutions, and they can be explained by a reptation model with electrostatic interactions in the semidilute region. Viscoelastic properties of polymer blends are different in miscible and immiscible regions, and regions close to phase separation temperature. In the miscible region polymer blends behave like polymer solutions when one of two components is not entangled. In the immiscible region first normal stress difference N1 as well as shear stress are proportional to shear rate in accordance with the theory of textured materials presented by Doi and Ohta. In the two-phase region close to phase separation temperature, the shear rate dependence of N1 changes with shear rate owing to shear-induced homogenization. The similar shear rate dependence of N1 is also observed for block copolymers in the ordered region close to order-disorder transition, but it is due to the orientation of lamellar domains.
Mechanical and viscoelastic properties of connective tissue were investigated on the basis of the hierarchical structure of collagen which is the major constituent of the connective tissue. Tension-induced structural changes in bovine Achilles tendon collagen were investigated by means of the X-ray diffraction method. The major contribution to the observed strain in the characteristic 67 nm D-period, εD, was revealed to be originated from the molecular elongation but the intensity analysis of the D-period reflection indicated the contribution also by the molecular rearrangement in collagen fibril. As the viscoelasticity of collagen in bone is expected to change with mineral content, the relaxation Young′s modulus of bovine femoral bone was measured as a function of mineral content. A well-specified master curve of stress relaxation was obtained from a set of relaxation curves for samples with different mineral contents. The time-mineral content superposition principle was explained by the consideration that the reinforcement of the collagen matrix around the mineral increases the apparent modulus of the matrix and lengthens the relaxation time. The shape function of the relaxation modulus of collagen and bone was determined to be a linear combination of a Kohlrausch-Williams-Watts (KWW) function and a simple exponential decay (Debye) function. By the comparison with the result of time resolved small angle X-ray scattering measurement of collagen during creep, the elementary process for the KWW-type relaxation would be a rearrangement process of local disorders in collagen molecular array. The Debye-type relaxation was well described by the concept of non-elasticity of a solid by the nucleation of microcracks at the area of stress concentration in the system.
Studies on mechanical properties of hydrophilic polymer gels were summarized. Mechanical properties of poly (vinyl alcohol) (PVA) hydrogels and their precursor gels are shown in the first part of this paper. Structure of PVA hydrogels as well as the precursor gels estimated on the basis of the experimental results is also presented. Divergent behavior of specific viscosity as well as intrinsic viscosity for PVA sols, and convergent behavior of modulus for PVA gels near the gelation point are described in the second part. The experimental results for the quantities are compared with the theoretical predictions. In the final part, theory and experimental results on coupling between diffusion and stress relaxation are shown. The comparison between theoretical predictions and experimental results for poly (acrylamide) (PAAm) gels are made.
The contact angle between solid surface and molten polymer was measured by a sessile drop method. The results of contact angle measurements were compared with the wall slip velocity determined previously by the present authors. It was proved that a good correlation was observed between the wall slip velocity and the work of adhesion which was calculated by the measured contact angle. Since the larger wall slip velocity showed previously a significant improvement of the efficiency of color and/or material change-over in a blow molding process, it is concluded that we should select a surface condition which show a smaller value in work of adhesion. The conditions for smaller work of adhesion was obtained experimentally in this study.
The transient flow behavior of viscoelastic fluids is studied. In the present paper, a start-up flow which is a typical unsteady flow is examined and the flow through a slit is numerically solved with FEM. The Leonov model is applied to describe the rheological properties of the viscoelastic fluids. The numerical simulations are carried out using two Weissenberg numbers in order to study the effect of elastic property of the fluid. The present simulation predicts viscoelastic phenomena such as the stress relaxation, the Barus effect and the growth of flow field with time. The difference of the flow patterns on the upstream and downstream sides of the slit becomes apparent with time because the stress is not perfectly relaxed in the slit region. This phenomenon appears more apparently at the higher Weissenberg number.
The transient flow behavior of viscoelastic fluids is studied. In the present paper, the start-up flow through a slit is numerically solved by using FEM. The simulation is carried out for the three channels with a slit of different length. The Leonov model is applied to de-scribe the rheological properties of the viscoelastic fluid. The simulation shows transient viscoelastic flow has complex behavior and the following results are obtained: (1) The difference between the flow patterns on the upstream and downstream sides of the slit becomes apparent with time. (2) The Barus effect is observed and it is stronger in the channel with the shorter length slit. (3) When the slit length is long, the velocity field develops in the slit region and then the velocity gradient in the flow direction comes to be nearly equal to zero. In this region the stress relaxation is indicated to occur.