Elongational flow orientation

Although as-spun fibers of Cl-HQ/BB/TA with m/n = 70/30 exhibited a modulus of 95 GPa, as-spun fibers of Cl-HQ/BB/TA with m/n = 80/20 exhibited a lower modulus of only 11 GPa in spite of the more rigid chemical structure due to the decreased elongational flow orientation, as shown in Figure 19.3.1 Tensile-fractured as-spun fibers of Cl-HQ/BB/TA with m/n = 70/30 exhibited many fibrils when compared to those of Cl-HQ/BB/TA with m/n = 80/20. 

The lower modulus seemed to be referred to the lower elongational flow orientation. Tensile-fractured as-spun fiber of Me-HQ/BB/NDA with m/n = 92.5/7.5 exhibited many fibrils when compared to Me-HQ/BB/NDA as-spun fiber with m/n = 95/5, as shown in Figure 19.5. 

The moduli of as-spun fibers of polyarylates depend highly upon the stability of the liquid crystalline state, the rigidity of the polymer chain, and the degree of elongational flow orientation (F-value). On the other hand, the moduli of injection molded specimens of polyarylates depend upon the stability of the liquid crystalline state, and the rigidity and packing density of the polymer chain. 

In order to obtain high modulus as-spun fibers,the stability of liquid crystalline state and the rigidity of polymer chain are influential factors.Therefore,we expected that the polyarylates derived from BB would show higher modulus than those from PEC or Cl-PEC.As-spun fiber of the polyarylate derived from Cl-HQ and BB modified with TA(Cl-HQ/BB/TA) showed the modulus of 95 GPa(m/n= 7/3) which was higher modulus compared with that of Me-HQ/Cl-PEC. However,as-spun fiber of Cl-HQ/BB/TA showed the modulus of only 11 GPa(m/n=8/2) in spite of the rigid chemical structure,and this is referred to the decreased elongational flow orientation as shown in Fig. 4(3). 

The elongational flow orientation could be obsereved as the orientation of fibrils on the cross section of the tensile fractured fibers as shown in Fig. 2,3 and 5(1,3). 

Keywords blends, alloys, compatibilizer, rheology, flow, viscosity, extru-date swell, shear modulus, shear flow, elongational flow, orientation in flow, sizing, yield stress, fillers, talc, mica, glass fibers, reinforced system, interlayer slip, log-additivity rule, concentration dependence, Trouton rule. 

C. Characteristically, these nematic melts show the persistence of orientational order under the influence of elongational flow fields which result in low melt viscosities under typical fiber formation conditions even at high molecular weights. 

The degree of deformation and whether or not a drop breaks is completely determined by Ca, p, the flow type, and the initial drop shape and orientation. If Ca is less than a critical value, Cacri the initially spherical drop is deformed into a stable ellipsoid. If Ca is greater than Cacrit, a stable drop shape does not exist, so the drop will be continually stretched until it breaks. For linear, steady flows, the critical capillary number, Cacrit, is a function of the flow type and p. Figure 14 shows the dependence of CaCTi, on p for flows between elongational flow and simple shear flow. Bentley and Leal (1986) have shown that for flows with vorticity between simple shear flow and planar elongational flow, Caen, lies between the two curves in Fig. 14. The important points to be noted from Fig. 14 are these ... 

If the mixing device generates a simple shear flow, as shown in Fig. 3.23, the maximum separation forces that act on the particles as they travel on their streamline occur when they are oriented in a 45° position as they continuously rotate during flow. However, if the flow field generated by the mixing device is a pure elongational flow, such as shown in Fig. 3.24, the particles will always be oriented at 0° the position of maximum force. 

In 4.3 we have already seen that polymers, in the rubber or fluid condition, crystallize much more rapidly when their chains are oriented. Therefore a stretched rubber, if stereospecific in its molecular structure, is able to crystallize at a temperature considerably above its equilibrium thermodynamic melting point. Also a thermoplast such as polyethylene, when in the molten state or in solution, can crystallize spontaneously when the chains are being orientated in elongational flow. The latter case is utilized when polyethylene is spun from a diluted solution (gel spinning process), resulting in fibres of super-high strength and stiffness ( Dyneema fibres). 

---