General relationships and usefulness of birefringence measurements

Flow birefringence is due to the optical anisotropy created by the orientation of the macromolecules within the flow field. In a plane flow, if we denote by I and II the principal axes of the refractive index tensor (principal axes are those for which the tensor is diagonal they are defined by the eigenvectors of the tensor), the local birefringence A is defined as  

After passage through a birefringent material of thickness e, the two components of linearly polarized light show a phase difference 6 equal to  

From Eq. (4), it may be deduced that extinction bands will be observed for two different conditions  

If one assumes fibre symmetry of the sample (like in uniaxial deformation), transverse isotropy for the molecular units (for instance ai U2 = 03), and additivity of polarizabilities, it is easily shown [9] that the difference in macroscopic polarizabilities along and perpendicular to the fibre axis of the sample is simply  

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