3D structure of nano-oriented crystals of poly(ethylene terephthalate) formed by elongational crystallization from the melt

We studied the elongational crystallization of poly(ethylene terephthalate) (PET) from the melt using polarizing optical microscope and X-ray observation. We verified that the structure and morphology discontinuously changed from conventional stacked lamellae of folded chain crystals (FCCs) to nano-oriented crystals (NOCs) when the elongational strain rate ( $$\dot \varepsilon $$ e ° ) exceeded a critical value of $$(\dot \varepsilon ^*) \cong 10^2\,{\mathrm{s}}^{ - 1}$$ ( e ° * ) ≅ 1 0 2 s - 1 . Therefore, the universality of NOC formation was verified. We found that the NOCs of PET show a novel three-dimensional (3D) structure and morphology: (i) nanocrystals (NCs) were arranged in a monoclinic lattice, which is a specific morphology for NOCs of PET, compared to iPP, and (ii) the unit cell structure of NOCs was a triclinic system with biaxial orientation. We showed the important role of the primary structure of the plate, such as a benzene ring, in the formation of a novel 3D structure and the morphology of the NOCs of PET. We also clarified that the NOCs of PET showed high performance, such as a high heat resistance temperature (Th) $$\ \cong\ $$ ≅ 281 °C, a high melting temperature (Tm) $$\ \cong\ $$ ≅ 285 °C, high maximum tensile stresses for the machine direction (MD) and transverse direction (TD) $$\ \cong\ $$ ≅ 2.8 × 102 and 74 MPa, respectively, and high Young’s moduli for MD and TD $$\ \cong\ $$ ≅ 5.4 and 1.7 GPa, respectively.