Orientation and structure of drawn poly(ethylene terephthalate)

Abstract

Drawing of poly(ethylene terephthalate) (PET) films was performed from the amorphous state at different drawing rates and at 80°C. Crystallinity of the films was determined by thermal analysis and orientation of the different phases was determined by birefringence, Fourier transform infra-red ( FTi.r.) specular reflection and wide angle X-ray diffraction. The mechanical properties of the oriented films were studied by dynamic mechanical analysis. FTi.r. and X-ray results showed large orientation of the different phases ( trans conformers and crystalline phase), in contrast with refractive index measurements which showed unusually low birefringence results. The observed orientation was particularly high for draw ratios (λ) higher than 3. The orientation contribution of the non-crystalline phase was determined by combining FTi.r. and X-ray results. The trans-conformer contribution to the non-crystalline phase is shown to be very high for the high λ for which crystalline orientation was observed. A structural model consisting of a crystalline phase, a mesomorphic amorphous phase (constituted of trans conformers) and a purely amorphous phase (constituted solely of gauche conformers) is a possible representation of the observed results. Finally, the tensile modulus increased continuously with the overall orientation.