Molecular Orientation of Poly(Ethylene Terephthalate) and Poly(Butylene Terephthalate) Probed by Polarized Raman Spectra: A Parallel Study

Abstract

Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) samples uniaxially drawn above Tg and beyond the yield point exhibit significant differences in their molecular orientation behavior as probed by polarized Raman spectra. The quasi-amorphous PET samples, drawn close to the Tg, manifest considerable molecular orientation development; however, when drawn above Tg + 30 °C, they exhibit significant molecular orientation relaxation. The semi-crystalline PBT samples maintain prominent molecular orientation even when drawn 110 °C above Tg. The drawing process, in PET samples, when resulting in molecular orientation, is accompanied by a gauche–trans transformation of the glycol linkage and a concurrent initiation of crystallinity development. In PBT specimens, it gives rise to a coexistence of α- and β-type crystalline phases. Phase a is predominant at high draw temperatures, i.e., Tg + 110 °C, while phase β dominates at low draw temperatures, i.e., Tg + 10 °C. PBT samples, with β-phase predominance, left at relevant draw temperatures without stress, exhibit a β–α phase change though no molecular orientation relaxation occurs. A note is made of the fact that complete molecular orientation analysis of PBT segments utilizing the depol method gives more reliable results than the simplified analysis assuming a cylindrical tensor for the 1614 cm−1 symmetric stretch of the para-disubstituted benzene ring of PBT. In this context, segments of PBT specimens rich in α-phase exhibit higher molecular orientation than those with β-phase predominance.