Arrhenius behavior shear viscosity temperature dependence

The shear viscosity /(r) measures the rate at which a liquid can rearrange in response to an applied shear stress. As explained in Section 1, the extent to which the temperature dependence of / (r) conforms to, or deviates from, Arrhenius behavior. 

Figure 13.2. Schematic illustration of general form of temperature dependence of zero-shear melt viscosities r o of amorphous polymers. It is seen that r o has a non-Arrhenius universal shape for Tg

There are five independent viscosities which describe the viscosity of the nematic phase depending on the position of the director with respect to the direction of flow, but only two are relevant to the flow in most nematic liquid crystal displays (Figure 2.8). In practice, it is difficult to determine the absolute value for any of them. The Meisowiscz viscosity q, which describes the shear viscosity along the director direction (Figure 2.8), can be determined approximately by a capillary flow method (Ostwald viscometer) and is useful in characterizing the decay time in TN displays. Typical values at 20°C are between 5 and 200 cSt (or mPa-s). As with isotropic liquids, the temperature dependence approximates to Arrhenius behavior. 

In this chapter, we have presented the rheological behavior of homopolymers, placing emphasis on the relationships between the molecular parameters and rheological behavior. We have presented a temperature-independent correlation for steady-state shear viscosity, namely, plots of log ri T, Y) r](jiT) versus log or log j.y, where Tq is a temperature-dependent empirical constant appearing in the Cross equation and a-Y is a shift factor that can be determined from the Arrhenius relation for crystalline polymers in the molten state or from the WLF relation for glassy polymers at temperatures between and + 100 °C. 

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