Viscosity, apparent temperature dependence

Fig. 6 Apparent viscosity and temperature dependence of carbonateous materiai-filied ER fiuid.

With most homopolymers and copolymers the apparent viscosity is less dependent on temperature and shear stress (up to 10 dyn/cm ) than that of the polyolefins, thus simplifying die design. On the other hand the melt has a low elasticity and strength and this requires that extruded sections be... 

The solvent composition affects not only the hysteresis or history dependence of the viscosity, but also its magnitude and temperature dependence. The viscosity was 10% higher using pure MeOH as the solvent than when a 1 1 MIBK/MeOH mixture was used. However, the 9 1 solvent mixture produces the highest solution viscosity by more than a factor of four. (A solution using a 19 1 MIBK/MeOH solvent mixture was so viscous it would barely flow in the flask in which it was prepared.) The apparent activation energy for flow... 

Flow does the occurrence of two fluorescing states for MK fit into the dynamic picture developed in Section IV The observed temperature dependence of the fluorescence quantum yield of MK in ethanol206 yields direct evidence that in this case, also, EBA < Ev. Recent time-resolved measurements at the Berlin Electron Storage Ring for Synchrotron Radiation (BESSY)207 support this argument The viscosity dependence of the decay of the short-wavelength fluorescence band in ethanol is consistent with an apparent value BA — 0.5Ev. Moreover, the decay is nonexponential, as would be expected for a barrierless relaxation. The lifetime of the TICT state (exponential decay) is 0.65 ns in acetonitrile at room temperature, that is, it is unusually short. 

The results of the calculations shown in Fig. 2.32 represent a complete quantitative solution of the problem, because they show the decrease in the induction period in non-isothermal curing when there is a temperature increase due to heat dissipation in the flow of the reactive mass. The case where = 0 is of particular interest. It is related to the experimental observation that shear stress is almost constant in the range t < t. In this situation the temperature dependence of the viscosity of the reactive mass can be neglected because of low values of the apparent activation energy of viscous flow E, and Eq. (2.73) leads to a linear time dependence of temperature ... 

Extensional viscosity is obviously dependent on average molecular weight, temperature and rate of extension. Apparently, also the tensile strain (degree of extension) is important. 

Shear viscosity (rj) is the most important intrinsic property determining extrudability. Since the apparent viscosity is highly dependent on temperature and shear stress (hence on pressure gradient), these variables, together with the extruder geometry, determine the output of the extruder. 

Self-diffusivity, cooperatively with ionic conductivity, provides a coherent account of ionicity of ionic liquids. The PGSE-NMR method has been found to be a convenient means to independently measure the self-diffusion coefficients of the anions and the cations in the ionic liquids. Temperature dependencies of the self-diffusion coefficient, viscosity and ionic conductivity for the ionic liquids, cannot be explained simply by Arrhenius equation rather, they follow the VFT equation. There is a simple correlation of the summation of the cationic and the anionic diffusion coefficients for each ionic liquid with the inverse of the viscosity. The apparent cationic transference number in ionic liquids has also been found to have dependence on the... 

Now let us show that in the nonisothermal motion of fluid in tubes and channels some critical phenomena may occur related to the existence of a maximum admissible pressure gradient. Once this value is exceeded, the steady-state flow pattern is violated. This is accompanied by an accelerated decrease in the apparent viscosity and increase in the fluid velocity. This phenomenon is known as the hydrodynamic thermal explosion [52] and is caused by the nonlinear dependence of the apparent viscosity on temperature. Specifically, under certain... 

So far we have considered nonisothermal flows of non-Newtonian fluids with allowance for dissipative heating and the dependence of the apparent viscosity on temperature. It has been assumed that the wall temperature is constant and convective heat transfer is absent. 

Figtire 8 Temperature dependent apparent viscosity changes of 20wt.% EOEOVE200-6-MOVE400- Solution appearances (a) at 30°C and (b) at 40°C are also indicated. (From Ref. 60.)... 

For rubbers such as NR (or synthetic 1,4-polyisoprene) (Santangelo and Roland, 1998) and polybutadiene (both 1,2- and 1,4-isomers) (CareUa et al., 1986), LCB increases the temperature sensitivity of the viscosity and terminal relaxation time. Thus, by comparing apparent activation energies, or, in the more usual case where the behavior is non-Arrhenius, the temperature dependence of the ratio of relaxation times for an unknown and a linear sample of the same polymer, inferences can be drawn concerning LCB (Figure 3.9). For polymers such as 1,4-polyisoprene, which have a dipole moment parallel to the chain, dielectric measurements of the normal mode can be used to measure the temperature dependence and thus assess the presence of LCB. 

FIGURE 10.25 Temperature dependence of capillary apparent viscosity, -pa of a 24% cellulose-NNMO solution (7.2% water content) at different shear rates. (From Navard, P. and Haudin, J.P., Br. Polym. /., 12(4), 174, 1980. Reprinted with permission from John Wiley Sons on behalf of SCI.)... 

Of all the physico-chemical properties, it is the rheology which shows the strongest temperature dependence. For instance, the decrease in apparent viscosity at a fixed shear rate is well represented by the Arrhenius-type exponential expression the pre-exponential factor and the activation energy are then both fimctions of shear rate. It is thus customary to denote the temperature dependence using rheological constants such as the power-law consistency coefficient and flow behaviour index. It is now reasonably well established that the flow behaviour index, n, of suspensions, polymer melts and solutions is nearly independent of temperature, at least over a range of 40-50°C, whereas the consistency coefficient exhibits an exponential dependence on temperature, i.e. 

Thus a momentum and a thermal boimdaiy layer will develop simultaneously whenever the fluid stream and the inunersed surface are at different temperatures (Figure 7.3). The momentum and energy equations are coupled, because the physical properties of non-Newtonian fluids are normally temperature-dependent. The resulting governing equations for momentum and heat transfer require numerical solutions. However, if the physical properties of the fluid do not vary significantly over the relevant temperature interval, there is little interaction between the two boimdaiy layers and they may both be assumed to develop independently of one another. As seen in Chapter 6, the physieal properties other than apparent viscosity may be taken as constant for commonly encountered non-Newtonian fluids. 

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