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Strain-induced crystallization (SIC) of poly(ethylene terephthalate) (PET) slightly above glass transition temperature ( T g ) is investigated by adopting a sequential biaxial stretching mode in this work. Small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) are employed to study variation of orientation parameter and crystallinity of stretch films in machine and transverse draw ratio space. Our results reveal that improving chain mobility by stretching is necessary before crystallization occurs slightly above T g , during which the total external work of sequential biaxial stretching is not dissipated but “stored” in polymer chains with high mobility. The total crystallinity is precisely correlated with the total external work, which does not relate to stretch direction and demonstrate that enhanced chain mobility have no directionality. Considering the structures that restrict the polymer chains in the initial sample, a molecular model is proposed to interpret the structure changes during SIC of PET above T g . [ABSTRACT FROM AUTHOR]

Combining a homemade uniaxial stretching device and the simultaneous in-situ synchrotron radiation WAXS measurement, the structural evolution during transverse direction (TD) stretching of machine direction (MD) pre-stretched Poly(ethylene terephthalate) (PET) film has been investigated. Current work reveals that increasing the MD pre-stretch before the occurrence of stretch induced crystallization (SIC) can enhance the crystallization and orientation in TD stretching. The concept of parallel chains structures (PCS) cross-linked network is proposed to interpret the rubber-like mechanical behavior. An appropriate stretch ratio window in stretching above the glass transition temperature (T g) is existed, where the PCS cross-linked network before crystallization can be induced by stretching above T g. With the help of the PCS cross-linked network, the chain orientation is retained more effectively under stretching, promoting the formation of subsequent crystallization and crystal cross-linking network. The strain hardening in different pre-stretched samples occurs accompanied by the same crystallinity and various chains orientation. This demonstrates that the strain hardening is determined by the crystal cross-linked network rather than the orientation. Current work supplies a guidance for getting the improved film products of PET with enhanced comprehensive mechanical properties, process stability and higher chain orientation. • The structural evolution in subsequent stretching of pre-stretched Poly(ethylene terephthalate) (PET) film is investigated by the in-situ synchrotron radiation WAXS measurement. • The origin of the novel rubber-like mechanical behavior under the complicated stretching field is better understood by the combination of the in-situ chains orientation, crystallization behavior and mechanical behavior. • The concept of parallel chains structures (PCS) cross-linked network is proposed to interpret the rubber-like mechanical behavior. • The onset of strain hardening corresponds to the formation of the crystal cross-linked network. [ABSTRACT FROM AUTHOR]