Prominent viscoelasticity emerges in entangled polymeric liquids composed of long, flexible polymer chains exhibiting active thermal motion. In attempt of establishing a molecular picture of the entanglement dynamics, extensive theoretical and experimental efforts have been made over several decades. This article summarizes the current picture of the equilibrium dynamics of the entangled chains and addresses some remaining problems.
Particles dispersed in a liquid tend to form flocs due to attractive forces between particles. The rheological properties of suspensions depend on the dynamic structures of flocs in shear fields. Main task of suspension rheology is to establish the quantitative relation among the mechanical properties of particle bond, geometrical structures of flocs, and macroscopic rheology. For understanding of current status of suspension rheology, the fundamental properties of ordinary flocculated suspensions are summarized. The most important aspect is that the particle bonds are not broken down in a quiescent state. Hence, the gross structure of flocs may be statistically invariant. The relation between rheology and floc structure is discussed on the basis of fractal and percolation concepts. The suspensions flocculated by polymers show interesting rheological behavior. For example, the suspensions flocculated by reversible bridging are Newtonian at low shear rates and shear-thickening at high shear rates, whereas the ordinary flocculated suspensions are shear-thinning in a wide range of shear rates due to the progressive breakdown of flocs. Since the particle-particle interactions are strongly influenced by adsorption affinity for surfaces, the flocculation induced by polymer has great potential as a new technique in rheology control of suspensions. The unique rheological behavior can be directly connected with the mechanical properties of the bonds between two primary particles. The physical quantities which determine the floc structures and rheological properties are the transverse and central components of attractive force between particles. The importance of vector nature of particle bonds is demonstrated, with a perspective of quantitative understanding of suspension rheology.
A cationic surfactant, cetyltrimethylammonium p-toluenesufonate (CTA pTS), forms long threadlike micelles in aqueous solution. The threadlike micelles make concentrated entanglement networks to show pronounced viscoelastic behavior as well as concentrated polymer systems. However, a responsible mechanism for the longest relaxation mode of the threadlike micellar system is different from that of the polymer system. The threadlike micellar system exhibits unique viscoelasticity described well by a Maxwell model. The longest relaxation time of the threadlike micellar system is not a function of the concentration of CTApTS, but of p−toluenesufonate (pTS−) ions in the bulk aqueous phase supplied by added sodium p−toluenesulfonate (NapTS). The rates of molecular motions in the threadlike micelles are not influenced by the concentration of pTS − anions, therefore, molecular motions in the threadlike micelles (micro-dynamics) are independent of the longest relaxation mode (macro-dynamics). A nonionic surfactant, oleyldimethylamineoxide (ODAO), forms long threadlike micelles in aqueous solution without any additives. The aqueous threadlike micellar system of ODAO also shows Maxwell type viscoelastic behavior. However, the relaxation mechanism for the longest relaxation process in the system should be different from that in the threadlike micellar systems of CTApTS, since the system of ODAO does not contain additive anions. Because increase in the average degree of protonation of ODAO head groups in micelles by adding hydrogen bromide lengthens the relaxation time remarkably, changes in micro-structure and micro-dynamics in the threadlike micelle are closely related to macro−dynamics in contrast with the threadlike micellar system of CTApTS.
Surface rheological analyses of polymeric solids were presented on the basis of scanning viscoelasticity microscopy (SVM) and lateral force microscopy (LFM). SVM observation for phase-separated polymer blend films revealed two-dimensional distribution of elasticity. In order to study surface thermal molecular motion of monodisperse polystyrene (PS), SVM and LFM measurements were made at various temperatures. Glass transition temperature, Tg, at the surface was discerned to be markedly lower than the corresponding bulk Tg, and the discrepancy of Tg between surface and bulk became larger with decreasing molecular weight. Such an intensive activation of thermal molecular motion at the PS surface can be explained in terms of an excess free volume induced by the segregated chain ends and a reduced cooperativity at the surface. Bulk Tg of miscible binary blends can be well expressed by the Gordon-Taylor equation. Extending this notion to surface, the surface composition in blends would be obtained by measuring surface Tg of each constituent as well as their blend. The surface composition in blend films of two PSs with different molecular weights was experimentally and systematically elucidated. The surface enrichment of a smaller molecular weight component became more remarkable with increasing molecular weight disparity between the two components due probably to an entropic effect.
The role of rheology in food and nutrition science and technology is discussed. Since the mouthfeel or texture is one of the most important attributes of foods, rheological studies have been carried out extensively. Rheology is important in the understanding textural properties of foods and food processing, and mastication and deglutition in eating process. These problems are becoming more important with the advent of aged society where dysphagia is becoming a serious problem. Large deformation and fracture of food gels is discussed. Recent advances in the understanding of sol-gel transition of food macromolecules, sensory evaluation of viscosity in mouth, and flavour release are described.
Computer simulation has received attention as "computational rheology" since about ten years ago. Flow analysis with rheological properties is recognized to be a useful and necessary tool for mold and die design in polymer processing CAE (Computer Aided Engineering). Moreover, dynamics simulation has been scientifically applied for studying the polymer dynamics and predicting the rheological properties of polymer melts and particle-dispersed suspensions. Here, numerical methods and recent topics related to rheology are reviewed.
As some examples for Biorheology, especially in hemorheology that relates to blood circulation, experimental and theoretical studies are reviewed on the basis of our works. First erythrocyte sedimentation and mobility are quantitatively considered as rigid oblate ellipsoids against the axial ratio of red blood cell (RBC) at different osmotic pressures in non-aggregative RBC suspensions, while RBCs may be most deformable near physiological osmolality at which the mobility is maximum. Second dynamic viscoelasticities in non-aggregative RBC suspensions are predicted by an elastic shell model and compared with complex viscosities measured with MLR (multi-lumped resonator). Third it is discussed also in aggregative RBC suspensions. Next we turn to blood vessels. Fourth nonlinear elastic stress-strain relations are represented by a single- or double-exponential equation for arterial vessels in both directions and for veinous vessels in longitudinal or in circumferential direction respectively. Fifth pressure pulse wave propagation is analyzed by the soliton model with the above exponential relation as well as experimental observations.