Estimation of intrinsic birefringence of poly(ethylene terephthalate) crystal

The intrinsic birefringence of poly(ethylene terephthalate) crystal has been estimated by means of the Orito method using experimental data and using theoretical calculation. The experimental estimation was performed on the basis of the assumption of a two-phase model consisting of crystalline and amorphous phases and the additivity of birefringence contributions — that total birefringence is equal to the sum of the orientation contributions from crystalline and amorphous phases, and of form birefringence. The value experimentally extrapolated by the Orito method is approx. 0.24. On the other hand, the theoretical calculation was carried out in the usual way using the values of bond polarizabilities according to Bunn and the atomic arrangements within the crystal according to Daubeny et al. The calculated value is 0.251. The difference between two results is also discussed.