The formation of crystal cross-linked network in sequential biaxial stretching of poly(ethylene terephthalate): The essential role of MD pre-stretch

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.

Automated summary

This study investigated the structural evolution of PET film during TD stretching with a homemade device and in-situ synchrotron radiation WAXS measurement. It was found that increasing MD pre-stretch can enhance crystallization and orientation in TD stretching. The concept of PCS cross-linked network was proposed to explain the rubber-like mechanical behavior, and an appropriate stretch ratio window above T-g was identified for inducing the PCS cross-linked network.