Shear birefringence

Another observation made by Larson and Mead [103] is that in addition to a lower critical shear rate for band formation after cessation of shear, there is also an upper critical shear rate. Tor this sample at lower shear rates, 0.2 dy/dt 30 s, after cessation of shearing birefringent bands form that are orthogonal to the previous flow direction but for dy/dt = 100 s these bands usually do not form. This is a rather moderate shear rate and it is odd that such an interesting observation had escaped the notice of other investigators. The reason might be found in the following comment ... 

Figure 2 (4,5) summarizes some key points on what was the first reported and wel 1-characterized thermotropic polymer. The Eastman workers melt copolymerized p-acetoxybenzoic acid with poly(ethylene terephthalate) (PET) to form a series of thermotropic aliphatic-aromatic polyesters (X7G polymers). Some of these compositions showed the characteristic melt optical anisotropy typical of nematic small molecules, that is, the melt between crossed polaroids on a hot stage microscope transmits light. This nematic order was persistent. Of course, shear birefringence is also shown by isotropic polymer melts but light transmission decay is quite rapid. 

In contrast, the phase diagram of the polymeric surfactant shown in Fig. 5.15 displays three well-defined liquid crystal regions, namely lamellar (L j), hexagonal (Hi) and cubic (Vi). The two former phases are optically anisotropic and may therefore be easily identified by their characteristic textures. The narrow region of cubic phase, however, is optically isotropic and was identified by virtue of its high viscosity and shear birefringence. 

L ) and a shear birefringent isotropic phase (D ) before the OAV emulsion is formed. 

FIG. 5 Phase behavior of water/Brij 30/decane system at 25°C. 0 , isotropic liquid phase L , lamellar liquid crystalline phase D, shear birefringent liquid phase W , bluish liquid phase (O/W microemulsion) W, aqueous liquid phase O, oil liquid phase Mlc, multiphase region including lamellar liquid crystal. (From Ref. 16 by permission of Langmuir, Copyright 2001, American Chemical Society.)... 

The above description refers to a Lagrangian frame of reference in which the movement of the particle is followed along its trajectory. Instead of having a steady flow, it is possible to modulate the flow, for example sinusoidally as a function of time. At sufficiently high frequency, the molecular coil deformation will be dephased from the strain rate and the flow becomes transient even with a stagnant flow geometry. Oscillatory flow birefringence has been measured in simple shear and corresponds to some kind of frequency analysis of the flow... 

Banding effects have also been seen in these wormlike micelle materials via optical birefringence [31, 32], although it is not clear that birefringence banding necessarily corresponds to shear banding [33], Of course, the anisotropy of bi-... 

Birefringence can also be used to analyze polymer samples after melt processing. As we will see in the next chapter, the shear produced in certain molding techniques, such as injection molding, can orient polymer chains in certain parts of the mold, especially near the mold walls, whereas the chains in low-shear regions, such as in the middle of the mold, are not as oriented. Figure 6.104 shows the variation in birefringence, as... 

Tumbling regime At very low shear rates, the birefringence axis (or the director) of the nematic solution tumbles continuously up to a reduced shear rate T < 9.5. While the time for complete rotation stays approximately equal to that calculated from Eq. (85), the scalar order parameter S,dy) oscillates around its equilibrium value S. Maximum positive departures of S(dy) from S occur at 0 n/4 and — 3n/4, and maximum negative departures at 0 x — k/4 and — 5it/4, while the amplitude of oscillation increases with increasing T. 

This article builds upon an earlier review on the same subject by Porter and Johnson in 1966 (14), and on the recent treatise on viscoelasticity in polymers by Ferry (15). We have generally tried to maintain the same nomenclature as the latter. Recent reviews on the relation between the zero-shear viscosity and molecular structure (16), crosslinked networks (17), and flow birefringence (18) in this same journal cover portions of the subject. We have tried to minimise redundancy with these works while at the same time making the review reasonably self-contained. 

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