Chemorheology is the best means to elucidate the degradation mechanism of cross-linked polymer at its initial stage. Most of chemorheological research which have been carried out all this while are those related to the scission mechanism of main-chains. The paper deals with the scission mechanism of crosslink site of crosslinked polymers which case is very rare. Polyurethane vulcanizates are exposed in N2 gas at high temperatures, 383, 393 and 403K, and in the solution of n-BuNH2/DMSO at 298, 303 and 308K. Chemical relaxation curves thus obtained were found to be consistent with theoretical prediction by Tobolsky. The scission mechanism of cross-link sites of polyurethane vulcanizates was also studied by the swelling, GPC, and IR measurements.
1, 3; 2, 4-p, p′-ditoluyliden sorbitol (PDTS) is an excellent gelling agent. A great variety of organic solvents and oligomers can be gelled by addition of small amount of PDTS. We studied thermoreversible sol-gel transitions in PDTS/organic solvent systems and PDTS/oligomer systems by means of a falling-ball (FB) method. Sol-gel transition temperature (Tfg) was obtained as a function of PDTS concentration (C) for several PDTS/organic compound systems by the FB method. We found a linear relationship between lnC and l/Tfg and that Ferry-Eldridge equation for sol-gel transition of polymeric gelling agent systems is applicable to the PDTS/organic compound systems. The heat evolved in the sol-gel transition (ΔH) was found to be in the range of 10.3-26.2 kcal/mol depending upon the solubility parameter (SP) of the organic compounds. |ΔH| decreases with increasing SP due to the solvation effect, if the SP for the organic compounds is smaller than that for PDTS, SP for PDTS being 12.5. We also found that the gel formation by PDTS was stopped by vitrification of the matrix organic compound and that the gel structure preexisting in the system was not destroyed by crystallization or vitrification of the matrix.
Some rheological properties were investigated for poly (vinyl alcohols) in dimethyl-sulfoxide solutions over a wide range of molecular weight and concentration. Basic properties of the storage modulus G′(ω) and the loss modulus G″(ω) were similar to the previous results of other polymer solutions. The frequency-temperature and and the frequency-concentration superposition methods were applied to the data and the concentration dependence of the characteristic relaxation time,τ, and the steady shear compliance, Je, were determined: τ∝C3, Je-1∝C0.6(C<Cc : Cc is critical concentration), and Je-1∝C3.4 (C>Cc) were obtained. The viscosity varied as η0∝C6.4 (C>Cc) and η0C3.6 (C<Cc). The critical molecular weight Me was evaluated from dependences of the zero-shear viscosity on moleculare weight and concentration. The results were in approximate agreement with that of PVA aqueous solutions in previous papers.
Recent development of rheology of hydrogels is reviewed. Rheological studies of clotting process of a fibrinogen-thrombin system and of a rennet-milk system are described. The temperature dependence of elastic modulus of thermo-reversible gels is explained by a simple model consisting of junction zones which are connected by long flexible chains. The entropic and energetic contributions to the elastic modulusare separated for agarose gels with different molecular weights. The concentration dependence of elastic modulus of gels is explained by Clark and Ross-Murphy's cascade treatment, and by Oakenfull's model. Percolation theory was used for analyzing the cascade treatment, and by Oakenfull's mcdel. Percolationwas theory used for analyzing the temperature and concentration dependence of elastic modulus by Tokita for agarose vels is briefly described.
Emulsion systems have potential applications for the formulation and processing of the various products appearing in a variety of the rheological properties. This article reviews a series of works on the flow properties of emulsions integrating advances made in the area of rheology so as to obtain further insights into the dispersion state of emulsions. The contents are roughly divided into five sections; concept and type of emulsion state, viscosity equations, relative and intrinsic viscosities, effect of the dispersed globule size on the viscosity, and non-Newtonian flow.
Colloidal particles show some extraordinary behavior in their structural properties when they are in deionized suspension. In this article, some typical properties are reviewed. (1) Microscopic observation of crystal-like structures of spherical, cylindrical and bowl-like particles in sedimentation equilibrium gives information about the microstructures and Young's elastic modulus (0.1-16 Pa) of the colloidal crystals. (2) The microstructures of crystal-like macroions are clarified by reflection and transmitted-light spectrum measurements. Stability of the face-centered cubic and body-centered cubic lattice structures is studied as a function of macroion and foreign salt-concentrations, temperature, electric field, hydrodynamic pressure, shear rate, etc. The interparticle distances correspond to the effective sizes of macroions which include the Debye-screening length. (3) The structural relaxation times, translational-and rotational- diffusion constants of spherical and ellipsoidal colloids are discussed in gas-like, liquid-like and crystal-like solutions. These dynamical properties are reasonably explained by the Einstein-Stokes and Perrin theories, if we take account of the contribution of the Debye length. (4) Evidence of the important role of the Debye length is obtained from the unusual concentration dependencies of viscosity for deionized colloids: reduced viscosities of liquid-like colloids are much higher than would be expected by Einstein's prediction and decrease sharply with concentration. Furthermore, a sharp peak is observed in the reduced viscosity vs. concentration curves, which corresponds to the transition between liquid-like and crystal-like structures. All these properties of charged colloidal suspensions are adequately described by the electrostatic intermacroion repulsion and the simple idea of the elongated Debye-screening length around macroions.
The rheological properties of white sauce (based on wheat starch) and several starch pastes were investegated. Calculation of the rigidity of white sauce was made by using Lin's equation. The yield stress of white sauce was investigated by three methods. It was shown that the rheological properties of white sauce was plastic flow with rigidity and yield stress, and also was shear thinning flow. By cooking, temperature of the white sauce changes from 60°C to 20°C, and the yield stress was increased remarkably. The apparent activation energy for flow of white sauce was estimated as 4.52 kcal/mol. The flow hysteresis curves for potato starch pastes were classified into three patterns, and the respective thixotropic parameters Th were calculated from the hysteresis loops. Values of the consistency index K, the flow behavior index n and the thixotropic coefficient λ were able to be determined from the curves of shear stress versus time under constant shear rate. The potato starch paste heated at 80°C exhibited thixotropy (softening for shear), and that heated at 98°C for 2 min showed negative thixotropy (hardening for shear). The spinnability of potato starch pastes was found to be related to the relaxation time τ for the samples if the samples showed similar values of the dynamic viscosity η′. Consequently, a linear relationship could be obtained between the spinnability and the product of η′ and τ for the potato starch pastes. Changes in the dynamic properties of starch pastes were followed during a short span aging (0-2 hr) at 25°C. Based upon the relationship between the dynamic modulus and the mechanical loss tangent of the samples paste, it was interpreted that sol-gel transition of the system occurs at a region where dynamic modulus and mechanical loss tangent are about 1.75×102(dyn/cm2) and about 0.45, respectively.