Investigation on Morphological and Structural Evolution during Stretching Process of Polyethylene Terephthalate Glycol Films with Different Molecular Structures

Three polyethylene terephthalate glycol (PETG) resins with different molecular structures were used, and their molecular structures (comonomer types, comonomer content), fluidity, and crystallinity were comparatively studied. They were further molded into cast films and subjected to tensile tests at different temperatures, and a series of samples stretched to different ratios were prepared. The tensile behavior and morphological evolution mechanism of PETG cast films during stretching process were revealed by means of differential scanning calorimetry (DSC), two-dimensional small angle X-ray scattering (2D-SAXS), X-ray diffraction (XRD), polarized IR spectra, and tensile performance test. The results showed that the tensile stress-strain curves of the three PETG cast films are quite different, and PETG-A and PETG-C undergo more obvious stretch-induced crystallization behavior during the stretching process. With the increase of the stretching ratio, PETG-B is most likely to be oriented under the external force; while PETG-C is less likely to be highly oriented. Although PETG-A will also be oriented under stretching, its degree of orientation is significantly lower than that of PETG-B. After pre-stretching, the three PETG cast films were stretched again in different directions respectively, and the stress-strain curves of the secondary stretching was investigated. It was found that the secondary stretching behavior was closely related to the orientation and crystallization behavior during the initial pre-stretching. The related mechanism was discussed.