Birefringence of multicomponent systems

If we assume additivity of polarisabilities it seems also reasonable to think of additivity of birefringences arising from difference phases, i.e. 

From eqn. (14), if one can specify the values of A°n. A/if and one of the orientation functions,/ then the other may be determined by measuring A it. This procedure has been used for semi-crystalline systems where X-ray diffraction is used to determine/r, of all crystalline axes. Thus one can determine/ . a quantity which, in general, is not easy to obtain from two-component systems. The results plotted in Fig. 7 are an example of the data obtained by combining both wide-angle X-ray diffraction and birefringence to uniaxially drawn low density polyethylene. It is clear 

Fifi- 7. Birefriruience as a fmction of percentage elongation for DuPont Alathon-2 Polyethylene. The amorphous and crystalline contributions to the total birefringence bare been determined by using both birefringence and X-ray methods. (From Ref. 28.) 

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