Deformation Induced Phase Transition and Superstructure Formation in Poly(ethylene Terephthalate) (Hsiao):
Deformation induced phase transitions and superstructure formation in poly(ethylene terephthalate) (PET) was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) methods as well as Raman spectroscopy. The deformation conditions involved uniaxial stretching of PET films at a temperature just below Tg, where a ‘plastic deformation’ stage was observed. WAXD results indicated that the initial quenched sample contained an amorphous/nematic ‘slush’ structure, whereby deformation sequentially induced oriented nematic, smectic (C and quasi-A) and stable triclinic crystalline phases. SAXS results indicated that the fibrillar superstructure was formed upon the formation of the oriented slush structure (amorphous + nematic). In-situ Raman spectroscopic results revealed the orientation information of ethylene glycol and benzene ring as well as the gauche-trans transition in deformation PET chains, which are in good agreements with X-ray results. A mechanism for deformation-induced phase transitions and for hierarchical structure formation has been proposed to correlate with the measured mechanical responses.