Region I Shear Thinning

Region I shear thinning has not been observed for all LCPs in fact, it is perhaps absent as often as it is present in experimentally reported flow curves for LCPs. However, since all rheometers have a shear stress below which they cannot measure stresses reliably, the existence of a Region I at lower shear rates than those accessed experimentally is always at least a theoretical possibility. 

In lyotropic LCPs, the shear-rate range over which Region I exists is sensitive to polymer concentration (Walker and Wagner 1994). In one batch of PBLG, Region I appears suddenly as the polymer concentration is increased above about 37% (see Fig. 11-9). In other PBLG solutions. Region I behavior has been reported at much lower concentrations 

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Finally, we come to one of the most interesting, and perplexing, phenomena in LCPs. This is the existence in some LCPs of Region I shear thinning at low shear rates (see Fig. 11-6 and the discussion in Section 11.3.2). Region I cannot be interpreted as classical yield (as described in Section 1.5.3), since the scaling of viscosity with shear rate y is a power law rj [Pg.544]

Ugaz, V.M. Cinader, D.K. Burghardt, W.R. Origins of region I shear thinning in model lyotropic liquid crystalline polymers. Macromolecules 1997, 30 (5), 1527-1530. 

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...

Experimentally, melts of low polydispersity that do not overtly spurt, slip, or succumb to other material instabilities will typically show steeply decreasing values of the viscosity and first normal stress coefficient in the shear-thinning region. Menezes and Graessley (1980) reported that t] oc y ° y i- at large y. These dependencies... 

Fig. 2.10 Relative intrinsic viscosity as a function of the shear rate poly(a-methylstyrene) in toluene with molecular weight is (1) 690k, (2) 1240k, (3) 1460k (4) 1820k, (5) 7500k, polystyrene with a molecular of weight 13 000 k in toluene, (6) and in decalin (7) The viscosity exhibits shear thinning phenomena. The Newtonian plateau region depends on the molecular weight. (Reprinted with permission from Noda, I. Yamada, Y Nagasawa, M., /. Phys. Chem. 72, 2890 (1968).)...

Figure 2.10 shows an example of the viscosity of a polymer solution measured as a function of the shear rate. The relative intrinsic viscosity [ i](k)/[ i]()> =0) is plotted against the reduced shear rate r y, where r is the characteristic relaxation time. Crossover from the Newtonian region to the thinning region can be seen. 

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