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Investigation of Trans Content in Amorphous Phase of stretched Polyethylene Terephthalate by Solid-state High resolution 13C NMR Spectroscopy

Crystalline, constrained noncrystalline, and amorphous phase structures for polyethylene terephthalate (PET) stretched at 75, 80, and 90°C were investigated by solid-state high resolution 13 C NMR. Saturation recovery pulse sequence is a very powerful tool for investigating not only the molecular structure but also the molecular mobility. Spin-lattice relaxation behavior of trans methylene carbon was decomposed by three components, which are assignable to crystalline, constrained noncrystalline, and amorphous phases. The weight content of each component can be estimated by the relaxation behavior. The stretching process induced the transformation from gauche to trans conformation. The trans content with the longest T 1c corresponds to the crystallinity, which was in good agreement with DSC measurement. The trans content in the amorphous phase was also estimated by the ratio between the gauche content assignable to the amorphous phase and the amorphous component of the trans conformer decomposed by relaxation measurement. The trans content of unstretched PET was as low as 21%, whereas that for five-times stretched PET was about 60%.