Shear flow-induced birefringence

Fig. 5. Shear-flow induced birefringence as a function of temperature and shear-rate for an isotropic PpPTA 9% (w/w) solution in sulfuric acid...

Shear flow-induced birefringence measurements of an isotropic solution of PpPTA in concentrated sulfuric acid with a clearing point of 45 °C were also performed by Picken [77]. As shown in Fig. 5 the flow-induced birefringence increases strongly when the isotropic-nematic transition is approached. The results demonstrate that the application of a relatively small shear rate already leads to a degree of orientational order, in the initially isotropic solution, that is comparable with the order in the nematic phase. This points to a strong coupling between the orientation and the external flow field, and to the occurrence of a shear-induced phase transition. 

D. S. Pearson, A. D. Kiss, L. J. Fetters, andM. Doi, Flow-induced birefringence of concentrated polyisoprene solutions, J. Rheol., 33, 517 (1989) D. S. Pearson, E. Herbolzheimer, N. Grizzuti, and G. Marrucci, Transient behavior of entangled polymers at high shear rates, J. Polym. Sci., Part B, 29,1589 (1991). 

Lee JY, Fuller GG, Hudson NE, Yuan XF (2005) Investigation of shear-banding structure in wormlike micellar solution by point-wise flow-induced birefringence measurements. J Rheol... 

The viscous heating was taken into account using the energy dissipation function [12], The polymer density change due to temperature and pressure variations was calculated using the Tait equation [13]. The flow-induced birefringence was calculated from the normal and shear stresses using the stress-optical rule [8]. 

At the same time, to our surprise, P8 M appears visually transparent and nonbirefringent, both in bulk samples and in thin films. This is illustrated by Figure 5.3, where the spot of chiral polymer can hardly be recognized between two glasses in contrast to its racemic isomer, P8 M. We should stress here that it is impossible to induce any birefringence in P8 M using shear flow, electric field up to 15 V/pm, or magnetic field up to 2.5 T. 

Now consider shear flow along x with a velocity gradient du/dz. The flow induces a birefringence proportional to the velocity gradient with the principal axes of the index ellipsoid inclined at 45° to the x, z axes. In the steady state = dr/dz. Therefore... 

In regime III, the flow field is very strong and shear-induced molecular orientation becomes important. According to birefringence measurements for anisotropic HPC/H2O solutions and HPC/ m-cresol solutions, the molecular orientation is a monotoni-cally increasing function of the steady state shear rate. 

We have seen that it can be difficult to reach the critical concentration required to observe an isotropic-anisotropic transition because concentrated suspensions of colloids are not always stable. However, orientation of flexible polymers as well as of anisotropic particles in suspension can be induced by flow, a phenomenon that has long been observed, reported, and studied. This phenomenon is especially strong when a pretransitional effect exists, which can be easily observed by the naked eye on a sample that is shaken between crossed polarizers (see for example the section on clays). In these systems, birefringence is induced via mechanical forces, like the shear stresses in a laminar flow ( Maxwell-dy-namo-optic effect ). 

In the shear-induced state, the solutions are birefringent and exhibit a strongly anisotropic scattering in light and neutron experiments. This anisotropy is compatible with a strong alignment of the shear-induced structures in the flow. 

A remarkable property of the shear-induced phase is its flow birefringence. Flow birefringence experiments on shear-thickening surfactant solutions were introduced... 

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