Elongational flow field

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. 

There are a few instances where a macromolecule in solution can be degraded even in the absence of an elongational flow field. In these circumstances, the chain scission was believed to happen in a partly extended state this situation is sufficiently related to the degradation in transient elongational flow to be worth... 

Elongation flow fields are likely to be present whenever streamlines cease to be parallel and in particular converging flow typical of that at the entrance of, for example, a capillary flow device is known to develop strong stretching flow The strength of elongational flow at the entrance of a capillary can be determined by one of several equations [106]. In the case of capillary flow, an estimate of the normal (tensile) stress at the entrance can be obtained from ... 

Whilst the physical and chemical nature of the filler will determine its effectiveness in a functional role, the presence of sohd additives in thermoplastics melts inevitably influence their processability. The extent to which this occurs depends on many factors including the amount of filler present, possible interactive effects between the filler and polymer, or between the filler particles themselves, together with the conditions experienced during melt processing, in particular the shear and/or elongational flow fields developed. 

If the duration and intensity of the elongational stress are sufficiently large, the macromolecule can be stretched to a greater extent. Keller was able to prove this using a cross-slot device producing a planar elongational flow field with the aid of flow diffraction. 

The elongational flow field is determined from equation ... 

The microrheology makes it possible to expect that (i) The drop size is influenced by the following variables viscosity and elasticity ratios, dynamic interfacial tension coefficient, critical capillarity number, composition, flow field type, and flow field intensity (ii) In Newtonian liquid systems subjected to a simple shear field, the drop breaks the easiest when the viscosity ratio falls within the range 0.3 < A- < 1.5, while drops having A- > 3.8 can not be broken in shear (iii) The droplet breakup is easier in elongational flow fields than in shear flow fields the relative efficiency of the elongational field dramatically increases for large values of A, > 1 (iv) Drop deformation and breakup in viscoelastic systems seems to be more difficult than that observed for Newtonian systems (v) When the concentration of the minor phase exceeds a critical value, ( ) >( ) = 0.005, the effect of coalescence must be taken into account (vi) Even when the theoretical predictions of droplet deformation and breakup... 

Better mixing is obtained in the extensional than in the shear flow. The elongational flow field exists anywhere where the streamlines are not parallel. 

The wall inherently introduces shear deformation containing a rotation component to the deformation gradient, whereas a pure elongational flow field exists only in a small region. 

Fig. 33 Schematic drawing of probable conformations of star-shaped pol5oners in the elongational flow field (horizontal), (a) Conformation disfavors the building up of force at the core, (b) Interarm entanglement leads to stress concentration at the entanglement point, (c) Arm individualism. Each arm may have independent conformations (1) folded, (2) coiled, (3) kinked,...

Theoretical description of dendritic structures is an active but difficult topic because the length between branched points is close to or even smaller than the Kuhn length, above which the segments can be thought of as if they are freely jointed with each other. Most modeling or simulation work has focused on the quasi-static and rheological properties of hyperbranched polymers and dendrimers in shear flow [240, 244, 255-263]. Limited amounts of data were published for dendrimers and hyperbranched polymers within elongational flow field. 

Thermotropic polyesters are melt-spun from the nematic phase and orient easily in an elongational flow field (moderate drawdowns/forces are sufficient). In the fiber case, highly oriented fibers form easily with an initial modulus close to theory—typical values range from about 70 to 150 GPa. Ward [46] has shown that the tensile modulus may be described by an aggregate model, i.e., the modulus is a function of the inherent chain modulus, the molecular chain orientation, and the shear modulus (which described the stress transfer between chains). The tensile strength of LCP fibers follows the prediction of the lag-shear model [47]. Both the aggregate model and the lag shear model treat the LCP as though it... 

These materials exhibit mechanical properties (e.g., modulus, tensile) which are significantly greater than those exhibited by conventional thermoplastic resins. In fabricated articles these properties are highly anisotropic. This anisotropy is caused by the sensitivity of these stiff molecules to the imposed elongational flow fields which cause molecular orientation in the longitudinal direction. 

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