Search

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]

Structural evolutions of cellulose triacetate (CTA) film during uniaxial stretching process at different temperatures were studied by in-situ synchrotron radiation wide-angle X-ray scattering (WAXS) combined with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The results show that four temperature regions can be divided for the CTA film stretching, in which different structural evolutions and deformation mechanisms will be observed. The structural evolutions of CTA film at temperatures between 60 and 125 °C (region I) are seriously influenced by the absorbed moisture, resulting in a large variation of the crystallinity ( χ c ). At temperatures around T g (195 °C), stretch-induced orientations for crystal can be achieved more easily although χ c and crystal size have no large change compared with the flexible-chain semi-crystalline polymers like polyethylene. Interestingly, the CTA film presents various deformation mechanisms including the elongation of amorphous and slipping, formation, destruction and recrystallization of crystals during stretching at different temperatures, which are often observed for the flexible-chain semi-crystalline polymers. These results can provide valuable guidance for the hot-stretching processing and application of CTA retardation film. Image 1 • The structural evolutions of CTA film during stretching were studied by in-situ WAXS. • Four temperature regions with different structural evolutions were observed. • The structural evolution is affected by absorbed moisture at low temperature. • Stretch-induced orientation can be achieved more easily at temperatures around T g. [ABSTRACT FROM AUTHOR]

Abstract The structural evolution of poly (ethylene terephthalate) (PET) with distinct water absorption during uniaxial deformation slightly above glass transition temperature (T g) were investigated by in-situ synchrotron radiation wide angle X-ray scattering (WAXS). Results show that the locally ordered structures are not conducive to stretch-induced crystallization (SIC) although the corresponding entropy is more close to crystals compared to random chains, which is due to the limited chain mobility therein. The "frozen orders" should be destroyed to achieve the critical chain mobility required by SIC even with the presence of water as plasticizer. Unexpectedly, absorption of water does not enhance but suppresses SIC though it does promote the chain mobility, originating from lower stress level with higher water contents. Stress and chain mobility directly correlate with chain stretch and orientation in positive and negative manners, respectively, demonstrating that chain stretch and orientation also play important role in SIC around T g. Graphical abstract Image 10457 Highlights • The structural evolutions of PET samples with different contents of water are investigated with in-situ WAXS. • It is necessary to accumulate sufficient chain mobility for SIC by destroying the frozen orders. • The SIC in PET can be inhibited by water above glass transition temperature. [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]