Rheological behaviour steady flow

Analysis of flow curves of these polymers has shown that for a nematic polymer XII in a LC state steady flow is observed in a broad temperature interval up to the glass transition temperature. A smectic polymer XI flows only in a very narrow temperature interval (118-121 °C) close to the Tcl. The difference in rheological behaviour of these polymers is most nearly disclosed when considering temperature dependences of their melt viscosities at various shear rates (Fig. 20). 

Transient shear flows involve examining the shear stress and viscosity response to a time-dependent shear. The stress build up at the start of steady flow (<7+) and at the cessation of steady flow (a ) and the stress decay (ff(0) after a dynamic instantaneous impulse of deformation strain (y) can be used to characterize transient rheological behaviour. 

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. Viscoelastic behaviour can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behaviour [54] assumes the presence of a surface yield stress, as, i.e. 

Another peculiar property of LCPs is shown in Fig. 15.47, where the transient behaviour of the shear stress after start up of steady shear flow is shown for Vectra A900 at 290 °C at two shear rates. We will come back to this behaviour in Chap. 16 for lyotropic systems where this behaviour is quite common and in contradistinction to the transient behaviour of conventional polymers, as presented in Fig. 15.9. This damped oscillatory behaviour is also found for simple rheological models as the Jeffreys model (Te Nijenhuis 2005) and according to Burghardt and Fuller, it is explicable by the classic Leslie-Ericksen theory for the flow of liquid crystals, which tumble, rather than align, in shear flow. Moreover, it is extra complicated due to the interaction between the tumbling of the molecules and the evolving defect density (polynomial structure) of the LCP, which become finer, at start up, or coarser, after cessation of flow. 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

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