Flow-induced orientation

Keller, A. and Kolnaar, H.W.H. (1997) Flow-induced orientation and structure formation, in Processing of Polymers, ed. Meijer, H.E.H. Materials Science and Technology, A Comprehensive Treatment, eds. Cahn, R.W., Haasen, P. and Kramer, E.J. (VCH, Weinheim) p. 189. 

The formation of a fibrillar structure in TLCP blends makes the mechanical properties of this kind of composites similar to those of conventional fiber reinforced thermoplastics [11,26]. However, because the molecular orientation and fibrillation of TLCPs are generally flow-induced, the formation, distribution, and alignment of these droplets and fibers are considerably more processing-dependent. We do not know ... 

Rotations around torsional barriers induce changes in chain conformation. For conjugated systems like polydiacetylenes, flow-induced changes in chain conformation can have a profound influence on the photon absorption and electronic conductivity properties of the material [73]. Flow-induced changes in molecular conformation form the basis for several technically important processes, the best known examples are the production of oriented fibers by gel spinning [74], the compatibility enhancement [75] and the shear-induced modification of polymer morphology [76]. 

Yoshida et al. reported that LB films of 48 and 92-97 showed piezochromism, observing a reversible UV spectroscopic change during monolayer compression on a water surface using the setup shown in Figure 33 which permitted both compression-induced and flow-induced orientation of the molecules.66 67 273... 

There has been much interest in flow and flow orientation effects with polymer melts containing anisometric particles which may be plate-like or fibrous. Flow-induced orientation of short reinforcing fibres is an area of considerable commercial importance, which is beyond the scope of the review [30]. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

Figure 9.10 Determination of flow-induced orientation angles in the presence of...

A. J. Salem and G. G. Fuller, Small angle light scattering as a probe of flow-induced particle orientation, J. Coll. Inter. Sci., 108,149 (1985). 

Fig. 3.3 A 9, 52-mm D aluminum rod rotating at 10 rps in a wide-diameter cylinder containing (a) Newtonian oil, and (b) polyisobutylene (PIB) solution, which exhibits the rod-climbing Weissenberg effect [from G. S. Beavers and D. D. Joseph, J. Fluid Mech., 69, 475 (1975)]. (c) Schematic representation of the flow direction flow-induced causing rod climbing. For Newtonian fluids, Tn = 0, since the small and simple Newtonian fluid molecules are incapable of being oriented by the flow.

Flow imparts both extension and rotation to fluid elements. Thus, polymer molecules will be oriented and stretched under these circumstances and this may result in flow-induced phenomena observed in polymer systems which include phase-changes, crystallization, gelation or fiber formation. More generally, the Gibbs free energy of polymer blends or solutions depends under non-equilibrium conditions not only on temperature, pressure and concentration but also on the conformation of the macromolecules (as an internal variable) and hence, it is sensitive to external forces. 

To manipulate and control ultimate properties of processed materials (for example, the strength, electrical, or optical properties of suspensions or polymeric liquids can be modified via flow-induced orientation, dispersion or crystallization) ... 

The compression-molded part, by definition, does not have flow-induced orientation. Comparison of compression-molded part properties with those of an injection-molded part can show the effect of melt temperature on properties. In the compression-molded article without flow-induced orientation, the impact strength remains constant until a certain melt temperature is surpassed and then decreases. This thermal degradation effect can be attributed to the polybutadiene component, which acts as an initiation site for oxidative degradation of the matrices. 

The competition between and flow-induced and boundary-induced orientation is quantified by an Ericksen number, defined by... 

In addition, measurements of orientation after cessation of shearing show that flow-induced orientation decreases after cessation of shearing toward an isotropic state of orientation, and the dynamic moduli increase with time (Hongladarom et al. 1994). This behavior is seen in PBLG solutions only under conditions where a pronounced Region I is seen for lower concentrations where only Region II is present, the orientation increases after cessation of shearing (Walker et al. 1995). 

U. S. Agarwal and D. V. Khakhar, Shear flow induced orientation development during homogeneous solution polymerization of rigid rodlike molecules. Macromolecules, 26 (1993) 3960-3965. 

D. K. 1998. Bias due to simple approximations for flow-induced mineral orientation distribution. EOS Transactions American Geophysical Union, 79, S212. 

The purpose of this paper is to explore various aspects of the rheological behaviour of lyotropic liquid crystalline systems. Lyotropics are often used as model systems for thermotropics because their viscoelastic behaviour seems to be quite similar (1) and solutions are much more easier to handle and can be studied more accurately than melts. The emphasis is on transient data as these are essential for verifying viscoelastic models but are hardly available in the literature. Transient experiments can also provide insight in the development of flow—induced orientation and structure. The reported experiments include relaxation of the shear stress and evolution of... 

Studies of the orientation phenomena in injection molding have shown that irrespective of the molding conditions and type of thermoplastic, molecular orientation is higher near the walls and decreases toward the interior. " " Low crystallinity and flow-induced structures have been observed near the walls for injection molded isotactic polypropylene parts. " High modulus materials have also been obtained with some injection molding techniques. " ... 

At high shear rates the chiral nematic structure changes to a flow-induced nematic phase. However, the shear-oriented phase is easy to disrupt after removing the shear force. This is because of the driving force for the liquid crystalline solution to form the more thermodynamically stable chiral nematic struc-ture. Relaxation in a pseudo-nematic lyotropic... 

The development of storage and loss moduli after flow cessation is a useful tool to analyze structural relaxations on LCPs. Upon flow cessation, the flow-induced orientation is lost. The evolution of the moduli with time is because of the reformation of a chiral nematic phase that had become nematic under flow. In Fig. 10, the relaxation in complex modulus with time for various AEC samples is shown. 

Crystallization can also be induced by the chain extension which occurs in an orientational flow. This is of obvious importance in rheology and processing, and it has been... 

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