Flow curve, three region

FIG. 15.44 Schematic representation of the bulk structure of an LCP during shear flow in the respective regions of the three-region flow curve, as proposed by Onogi and Asada (1980) and by Beekmans (1997). The lower cartoon in region II is in agreement with newer ideas of the structure obtained in the 1990 s by Burghardt. Fuller, 1991, Vermant et al., 1994, Walker et al., 1995. 

Generally large yield stress effects were dominant in the nematic melts, but they were strongly pre-history dependent. A three region flow curve for 15 mol % modified poly(pheny1-1,4-phenylene terephthalate) was probably due to a not completely molten system. Dynamic viscosity measurements showed strong pseudoplastic behaviour. Strain and time dependence phenomena were not observed. 

Figure 11.1 Schematics of three-region flow curve.

This effect has mostly been observed for lyotropic LCPs, sometimes also for thermotropic ones. The existence of region I in Figure 11.1 is explained by the formation of texture, a domain structure observed in many, mostly lyotropic LCPs. The texture occurs during relaxation when the stress levels are very low, that is, when approaching the rest state. Such a three-region flow curve was first observed in Ref. [50] and explained theoretically for lyotropic LCPs in Refs [51, 52] (see also Refs [4, 5, 53]). These theoretical descriptions are typically complementary to the more fundamental monodomain nematodynamic theories of both the molecular and the continuous types. 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

Shear thickening materials show an increase in viscosity with increasing shear strain rate. An idealized flow curve is presented in Fig. 6, and the viscosity as a function of shear strain rate is depicted in Fig. 7. The shear thinning region usually extends only over about one decade of shear rate (power law index n > 1) in contrast to shear thinning, which usually covers at least two or three decades. Also, in many cases, shear thickening is preceded by a short phase of shear thinning at low shear strain rates. ° ... 

The change in biofilm accumulation on a surface with time, under flowing water conditions, would be expected to follow the idealized curve in Fig. 3. Three regions can be seen in the diagram ... 

We will touch upon one of the thorniest problems in the rheology of LC polymers here the presence of a creep limit and the mechanism of flow. From an experimental point of view, it has not yet been clearly determined whether there is a creep limit for LC systems. This is due to the nonreproducibility of the data on the viscosity in the region of low stresses and the large errors which are unavoidable when x < 0.1 Pa. The flow curve which contains three segments (Fig. 9.13) (this was first described in articles by Soviet investigators [19, 70, 71] and then in the publications of Asada et al. [42, 72-73]) is now almost universally accepted ... 

As discussed above, polymer melts and solutions can be treated as shear-thinning fluids. However, this is trae only over a certain shear rate range. Figure 8.10 shows the typical logarithmic flow curves of polymer melts and solutions over a very wide range of shearing. Three different regions can be found in the flow curves ... 

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