A Theoretical Calculation of the Briefringence of Polypropylene Crystals

Abstract

In the course of some investigations on the effect of carbon-carbon and carbon-hydrogen bond polarizabilities on the optical properties of hydrocarbon polymers, a theoretical calculation of the polypropylene chain principal polarizabilities was performed. The polypropylene chain model given by Natta was used. Results were expressed in terms of carbon-carbon and carbon-hydrogen bond polarizabilities. Two sets of values of bond polarizabilities, one given by Denbigh, the other by Bunn and Daubeny, were substituted into the polarizability equation. Values of chain principal polarizabilities were then used to calculate the refractive index and birefringence using the Lorentz-Lorenz equation. Better agreement was found between experimental and theoretical birefringence values using Denbigh's data. Three explanations are suggested which might account for the differences between the two sets of bond polarizabilities.