Internal field of polymer crystals: Polarizability tensor of the methylene group from the birefringence of paraffin crystal

Using the concept of a point dipole lattice, it is shown that the internal field of induced dipoles can be calculated for crystals comprised of simple chain molecules. The only structure which must be taken into account accurately is that of the chain molecule itself. From the calculations, reliable values of the polarizability tensor of the CH 2 unit are deduced from the birefringence of the paraffin crystal. In addition, it is shown that birefringence measurements provide a method for demonstrating the consistency of polarizability data so that no detailed structural information is needed. For the CH 2 unit, it is found by both methods that α ∥ − α ⟂ = − 0.63 with respect to the chain direction [the units of polarizability α are 10 −24 cm 3 (cgs)]. The most probable anisotropies for the bond polarizabilities are α − α = 0.30, α − α = − 0.62.