Birefringence method

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

With the development of the non-Newtonian viscosity theories it is now possible to compare the rotary diffusion coefficient and thereby the calculated length (or diameter) of the rigid particles as obtained from this technique with that from the commonly used flow birefringence method. Since both measurements depend upon the same molecular distribution function (Section III) they should give an identical measure of the rotary diffusion coefficient. Differences, however, will arise if the system under study is heterogeneous. The mean intrinsic viscosity is calculated from Eq. (7) whereas the mean extinction angle, x, for flow birefringence is defined by the Sadron equation (1938) ... 

The sonic technique suffers from one of the same difficulties as the birefringence method in that only an average orientation of the total system is obtained. Thus, if one has a multicomponent system (e.g. semicrystalline, block copolymer. etc.X one cannot separate component orientation by this technique alone. It is noteworthy that the orientation measured by the sonic method can be correlated directly with birefringence data on the same material.This ease of correlation has been suggested to lead to values of the intrinsic birefringence by extrapolation of the sonic data to perfect orientation. 

Fig. 7.18. Steady-state visi ty t (jr) and the first normal stress coefiident Vi(sr). Sample 8% solution of polystyrene (Af 3.0x10 ) in chlorinated biphenyl. Open circles represent results from Weissenberg Rbeogoniometry and closed circles results from the birefringence method. Reproduced from rrf. 84.

Although nearly all shear normal stress data reported in the literature are measured by total thrust methods, some work has been done with the distribution of pressure across the plate with small flush transducers (Christiansen and Leppard, 1974 Magda et al., 1991 see Figure 5.4.2). Meissner and co-workers (1989) have used the total thrust on a central disk of smaller radius than the sample. Both techniques permit measurement of N2 as well as A i. Birefringence methods, discussed in the next chapter, can also give normal stress distributions. 

It is interesting to note that not only differences in refractive indexes, but also relaxation times, can be determined by the birefringence method [166]. The laser birefringence detection method is presented in Ref. [166]. Details of the necessary calculations and a comparison of the results obtained compared with rheological methods are also shown are shown in Ref. [166]. 

Hoffmann [283,288] conclude that electric birefringence measurements are useful for the determination of the dimensions of anisotropic surfactant aggregates. Although electric birefringence data alone are not sufficient to define the type of surfactant aggregates, the electric birefringence method complements other physical methods. 

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