Flow-induced orientation and stress-induced reorientation of semicrystalline PBT-based vitrimers

The microstructure and deformation mechanisms of polybutylene terephthalate (PBT)-based vitrimer samples produced by injection molding were characterized in a 120–180 °C temperature range through tensile tests calibrated by image correlation, coupled with in situ synchrotron x-ray scattering. Vitrimerization was achieved by an 8 min melt-mixing step followed by injection. Flow-induced orientation of polymer chains after injection strongly depends on the cross-linker content [diglycidyl ether of bisphenol A (DGEBA)]. Up to 0.5 wt. %, the crystallized polymers exhibit almost no orientation. Above 1.0 wt. %, a pronounced shish-kebab morphology emerges, with little further variation at higher DGEBA contents. Lamellae (kebabs) are initially perpendicular to the injection direction, while within the lamellae, the chain axis (c axis) is tilted, as shown by an ∼28° angle between the c* axis and the injection direction. Under tension, polymer chains realign along the stretching direction (35° angle between c* and tensile direction), leading to fibrillar morphology. Increasing cross-linking slows down chain relaxation, as reflected in a Weissenberg number greater than unity during injection—a condition favorable for retaining orientation in the crystalline phase. Unlike typical injection-molded polymers, orientation is observed throughout the entire 4 mm specimen thickness, not only in the skin. Tensile properties strongly correlate with flow-induced orientation. Temperature rise modifies the mechanism of lamellar destruction during deformation: At lower temperatures, crystal fragmentation and reorientation dominate, while at higher temperatures, crystal melting and rapid recrystallization occur. PBT-based vitrimers exhibit the same polymorphic transitions as pristine PBT, specifically the stress-induced α → β transformation and the emergence of a smectic phase during fibrillar morphology development.