INTRINSIC BIREFRINGENCE OF POLY(ETHYLENE-TEREPHTHALATE)

In the existing literature various values are given for the intrinsic birefringence of the crystalline and the amorphous phases in poly(ethylene terephthalate) (PET). These values have either been calculated theoretically or obtained from experimental data on the basis of certain models. In this investigation, using the Samuels two-phase model which correlates sonic modulus with structural parameters, intrinsic birefringence values for the crystalline ( DELTA n//c) and amorphous ( DELTA n//a) phases have been determined by studying 30 PET samples prepared by heat setting to have a wide range of structures; the results are DELTA n//c equals 0. 29 and DELTA n//a equals 0. 20. These values are discussed along with others in the literature and it is concluded that in the light of the present work, the values used by many authors need reexamination.