Flow Birefringence Applications

Sections 9.S. 1-9.S.9 briefly discuss applications of rheo-optics that have been selected to illustrate the main points made in the preceding text and to demonstrate the breadth and power of iheo-optical experiments. 

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In this respect, another insufficiency of Lodge s treatment is more serious, viz. the lack of specification of the relaxation times, which occur in his equations. In this connection, it is hoped that the present paper can contribute to a proper valuation of the ideas of Bueche (13), Ferry (14), and Peticolas (13). These authors adapted the dilute solution theory of Rouse (16) by introducing effective parameters, viz. an effective friction factor or an effective friction coefficient. The advantage of such a treatment is evident The set of relaxation times, explicitly given for the normal modes of motion of separate molecules in dilute solution, is also used for concentrated systems after the application of some modification. Experimental evidence for the validity of this procedure can, in principle, be obtained by comparing dynamic measurements, as obtained on dilute and concentrated systems. In the present report, flow birefringence measurements are used for the same purpose. 

A. Onuki and M. Doi, Flow birefringence and dichroism of polymers. I. General theory and application to the dilute case, J. Chem. Phys., 85,1190 (1986). 

J. L. S. Wales, The Application of Flow Birefringence to Rheological Studies of Polymer Melts, Delft University Press, Rotterdam (1976). 

F. H. Gortemaker, M. G. Hanssen, B. de Cindio, H. M. Laun, and H. Janeschitz-Kriegl, Flow birefringence of polymer melts application to the investigation of time-dependent rheological properdes, Rheol. Acta, 15, 256 (1976). 

Systematic application of hydrodynamic and light-scattering methods has been made in conjunction with optical rotatory studies upon poly-e-Y-car-bobenzoxy-L-lysine (Applequist and Doty, 1961 Daniel and Katchalski, 1960, 1961) and poly-L-lysine (Applequist and Doty, 1961). Poly-e-Y-car-bobenzoxy-L-lysine in dimethyl formamide displays light scattering, viscosity, and flow birefringence behavior that is consistent with a predomi-... 

A macroscopic, quantitative way to analyze birefringence is through Jones (33-40) and Mueller (41) calculus and the Poincare sphere (42). Those tools are widely used in the optical industry and references on them abound. Another active field of birefringence applications is the combination of the technique with polymer rheology, called flow-birefringence (43,44). 

Although flow birefringence has been used many times for determination of molecular weight of macromolecules, the method depends upon assumptions regarding the molecular geometry which Involves an appreciable degree of uncertainty. Moreover it requires evaluation of empirical constants. These serious drawbacks have limited the application of this method for molecular weight determination. 

The geometry used for flow birefringence measurements in an oscillatory shear (Miller and Schrag, 1975) consists of two flat, parallel surfaces, one of which is fixed the other is made to oscillate in the plane parallel to the fixed surface. The liquid is confined to the thin layer between the surfaces. The light beam propagates through the layer in a direction perpendicular to the flow direction and parallel to the surfaces. This experimental arrangement and its application are discussed in detail below. 

Lodge, T. P. Rheo-Optics Flow Birefringence, Chap. 9 of Macosko, C. W. Rheology Principles, Measurements and Applications. (1994) VCH Publishers, NY, Weinheim... 

Finally, we mention some other applications of rheo-optics. It has been used to study liquid crystals, has utilized IR spectroscopy, and heat flow birefringence has been observed. Three-dimensional flow birefringence is discussed by Funatsu et al Multicomponent-multiphase materials have been studied. " There has also been considerable theoretical activity. ... 

There are a number of applications where the orientation angle is known a priori. This is the case whenever uniaxial fields such as electric and magnetic fields are applied, or when purely extensional flows are used. In that case the experiments are normally performed by aligning the sample so that its principal orientation direction is at 45° with respect to the polarizers. If the sample is weakly birefringent, so that 8 1, and if dichroism is negligible, then (8.4) predicts the intensity to be... 

The final chapter on applications of optical rheometric methods brings together examples of their use to solve a wide variety of physical problems. A partial list includes the use of birefringence to measure spatially resolved stress fields in non-Newtonian flows, the isolation of component dynamics in polymer/polymer blends using spectroscopic methods, the measurement of the structure factor in systems subject to field-induced phase separation, the measurement of structure in dense colloidal dispersions, and the dynamics of liquid crystals under flow. 

Birefringence and dichroism represent two optical methods which can be applied to materials under flow conditions, forming the basis of Optical Rheometry [3,4]. The aim of these two techniques is to measure the anisotropy of the complex refractive index tensor n = n - i n". Birefringence is related to the anisotropy of the real part, whereas dichroism deals with the imaginary part. Recent applications of birefringence measurements to polymer melts can be foimd in Chapter III.l of the present book. 

Validity of the stress optical law and application of birefringence to polymer complex flows... 

We presented some applications of the bireftingence techniques for the characterization of orientation and stress field in both simple and complex deformations. The validity of the stress optical law in these particular flow situations was checked and assessed. Birefringence techniques appear to be very powerful for the study of polymer melt processing and for the understanding of polymer flow behaviour. 

An independent method of determining stress, especially useful for materials under flow, is from the optical birefringence. Of course, it is limited to transparent materials, which precludes application to filled polymers. Because of the need for transparency, stress birefringence has been used more often for plastics than rubber. Residual anisotropy due to unrelaxed orientation can also be assessed using birefringence this is commonly known as the photoelastic effect. Generally the birefringence is directly proportional to the tme stress... 

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. 

Extensional flow and structure-property relationships may also be studied by birefiringence. A particularly interesting application from transient flow is presented in Figure 2, where phenomena such as overshoot and ringing are observed (3). In this example we see results for 1.5% polyethylene oxide solution for three different velocity gradients. Birefringence detects the presence of an overshoot,... 

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