Structure evolution of PET under step-wise and continuous deformation modes: the effect of stress relaxation on the strain-induced morphology

This study reports about an in-situ deformation investigation under X-ray synchrotron source (Wide-Angle X-ray Scattering, WAXS) of polyethylene terephthalate (PET) following step-wise and continuous stretching modes. In this way the effect of stress relaxation on the strain-induced phase and structure evolution is investigated for PET with different initial structures: quasi-amorphous and semi-crystalline. These PET initial morphologies were developed from fully amorphous PET by uniaxial stretching above the glass transition, T-g, and rapidly quenched. The in-situ experiments reveal significant influence of stress relaxation on the strain-induced phase and structure evolution for both PET morphologies. A relative delay in structure evolution is presented for continuous stretching because of reduced macromolecular chain relaxation during deformation. A constant increasing trend of amorphous orientation with deformation is assessed by both deformation protocols. The strain-induced phase results disclose that relaxation phenomenon contributes for: i) crystalline phase preserving during deformation for initially semi-crystalline PET; and ii) greater amorphous mass fraction for the quasi-amorphous PET.