The phenomena and mechanism in wood rheology on changing moisture content are reviewed. Creep and stress relaxation of wood are greatly influenced by changes in moisture content. The creep rate and total creep of wood are found to be considerably greater when the moisture content of loaded specimen decreases than when it remains constant at either green or dry states. During subsequent cycles of moisture changes, the loaded specimen shows partial recovery during each absorption followed by increased deformation during desorption. After extensive moisture cycling, such a deformation may reach more than 20 times the initial one. In unloading the dry specimen, little instantaneous elastic recovery takes place, but on putting the specimen in saturated water vapor, a large recovery quickly occurs. Similarly both the rate of relaxation and the residual deformation are accelerated by reduction of moisture content, and the residual deformation can be largely removed by a subsequent increase in moisture content. The rates of creep and stress relaxation caused by changing moisture content depend on the rate of moisture content change and are little affected by the duration of the process. Several interpretations have been reported for the behavior. In practice, deflection of wooden beam in service, drying stress and set in seasoning of wood, and bent wood process are related to the above-mentioned behavior.
The compressibility of polymer solutions was measured by the ultrasonic interferometer in MHz region and the partial specific compressibility of the solution (κ20) was determined. In good solvent systems, the partial specific compressibility increased with the intrinsic viscosity [η]. Near the θ-temperature, on the other hand, κ20 was independent of [η]. The behavior of compressibility in good solvent systems can be well interpreted by considering the compressibilities associated with the segment-segment, segment-solvent, and solvent-solvent interactions. But this is not the case for the polymer solution near θ-state. The polymer solution near the θ-state was considered to be a liquid mixture composed of the segmental liquid (a liquid of the hypothetical mixing units) and solvent molecules.
Linear and nonlinear viscoelastic properties of disperse systems of photosensitive particles were studied in relation to the degree of aggregation of dispersing particles. The disperse system has the characteristic long-time relaxation process attributed to the aggregate structure of dispersing particles. In that relaxation process, the system in which the dispersing particles aggregate in some degree shows markedly nonlinear viscoelasticity. The characteristic relaxation time in the long-time relaxation process can be closely related with the rotational diffusion of the aggregate. It is also shown that the characteristic relaxation time is independent of the dynamic strain amplitude.
Flow behavior of sterically stabilized suspensions was studied by the use of silica suspensions in polyacrylamide (PAAm) solutions. Adsorption of PAAm on the silica particles causes depletion of the polymer concentration in the liquid phase and volume augmentation of the dispersed phase. The former effect is predominant at low particle concentrations and hence the zero shear viscosity of suspension becomes lower than that of the medium. At intermediate concentrations, pseudoplastic behavior appears at shear rates corresponding to Newtonian range of the medium. At high concentrations, suspensions exhibit pseudoplastic behavior, followed by dilatancy progressing to viscosity instability. The viscosity instability may be attributed to the abrupt break up of ordered arrays of particles discovered by Hoffman for systems including particles by more than 50%. The total effective volume of our dispersed phase is likely to be increased to about 50%. The flow behavior is discussed in relation to a potential energy diagram which combines van der Waals attraction and steric repulsion due to the interaction of adsorbed polymer layers.
Viscoelastic properties of a lyotropic liquid crystal were determined with a capillary rheometer. So-called Bagley end correction was taken into consideration by using three dies which have the same diameter but different lengths. Measurements included the shear rate (γ) and concentration (c) dependences of shear viscosity (η) and three elastic parameters: Bagley end correction (ν), entrance pressure loss (ΔPent), and die swell (B). Shear viscosity behavior typical of lyotropic liquid crystal was observed. The shear rate dependence of ν varied greatly with the change of concentration: ν increased with γ at some concentrations, but decreased at other concentrations. η and ν showed the maximum and minimum with respect to concentration. However, the concentration at the minimum was not identical with that for viscosity. ΔPent increased with shear rate at all concentrations. The concentration dependence of ΔPent was the same as that of viscosity. B increased with shear stress only for isotropic solutions and decreased for anisotropic ones. The following relations have been found: ν∝Bn ΔPent∝Bn′ where n and n′ are constants which depend on concentration.
Weight average molecular weight (Mw) of sodium carboxymethylcellulose (NaCMC) has been determined by the light scattering in the solution of triethylenediamine cadmium hydroxide (cadoxene). The relation between Mw and the limiting viscosity number [η] in 0.1N NaCl aqueous solution has been obtained and it has been found that the Staudinger's relationship [η]=KMw can be applied for these systems. Dynamic viscoelastic properties of aqueous disperse systems of NaCMC have been measured by means of a cone-and-plate type rheometer. Using three series of NaCMC in the range of substitution (carboxymethyl residue/glucose unit) 0.5 to 1 with various degree of polymerization, the effects of the degree of substitution (DS), the degree of polymerization and the concentration upon rheological properties have been investigated. The influence of DS is marked in the long time-scale region. NaCMC of low DS remarkably shows a second plateau which is a characteristic property of the heterogeneous system. On the other hand, NaCMC of high DS behaves like homogeneous polymeric liquid.
The dielectric relaxation spectrum g(τ) for undiluted cis-polyisoprene (cis-PI) was compared with the shear stress relaxation spectrum H(τ). The peaks of g(τ) due to the normal mode and the segmental mode processes corresponded well to the peaks of H(τ) located in the terminal and transition regions, respectively. Thus, the mechanical relaxation process around the maximum of the wedge shaped spectrum was assigned to the local segmental motions. The longest relaxation time, τR(Mc), for the polymer chains having the characteristic molecular weight Mc was compared with the relaxation time, τs, for the dielectric segmental mode process. The ratio, τR(Mc)/τs, was found to be approximately 2×104 independently of the chemical structure of polymers. From this relation, the frictional coefficient, ζ, for a monomer unit was expressed as ζ=3400mTρτs/Mc where m is the number of backbone atoms in the monomeric unit and ρ the density.