Effect of internal field on the birefringence of polyethylene crystals and on the anisotropy of polymer chains. I. Birefringence effects

Values of the anisotropy of polarizability of methylene groups deduced from the refractive indexes of n ‐paraffin crystals differ appreciably from those obtained from gas‐ or liquid‐phase measurements. The differences is attributed to the erroneous application of the Lorenz—Lorentz internal field to the anisotropic n ‐paraffin crystal. A more detailed calculation of the internal field is carried out by detailed summation of the dipolar field over an idealized n ‐paraffin crystal in which the molecular chains are replaced by anisotropic rods. With reasonable values of parameters and assumed crystal shape, the discrepancy in polarizability anisotropy is resolved.