Wiener birefringence

However, when applying results of the type (57), it must be borne in mind that such equations apply to the amorphous constituent only, and must fail at advanced degrees of stretch where crystalliisation sets in since the influence of crystallisation on the double refraction of rubber-like polymers is considerable In addition to the intrinsic double refraction of the gel frame, the swollen gel may show structural double refraction and adsorption double refraction. The structural, or textural , birefringence is caused by the difference between the refractive indices of polymer substance and solvent. It was estimated by Wiener for rod-shaped particles and for platelets. Wiener s theory is not applicable to rods of molecular thickness, and the structual double refraction caused by oriented long-chain molecules has not yet been accessible to a reliable theoretical treatment. 

Fig. 64. Wiener component of birefringence in dependence on the degree of swelling g, calculated for the values of Hi indicated and for = 1.3333.

The intrinsic double refraction of cellulose nitrate being negative, the curves exhibit a maximum. It is seen that both the birefringence in the maximum and the Wiener component increase with orientation. Kratky and Platzek, as well as the other authors just cited, have endeavoured to interpret the results of their measurements using the equation... 

Besides intrinsic dichroism, structural dichroism may occur too. The latter will be e, g. observed if coloured isotropic parallelized rodlets are imbedded in a matrix with a different light absorption. The theory has been treated also by Wiener The results are much more complicated than in the case of structural double refraction of colourless objects. The birefringence itself is also affected if one of the components of the composite body is coloured, and will for instance in general not be zero at equal refractive power of rodlets and matrix. We shall not enter into further details here. 

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