Lamellar formation and relaxation in simple sheared poly(ethylene terephthalate) by small-angle x-ray scattering

Semicrystalline poly(ethylene terephthalate) (PET) was oriented with an equal channel angular extrusion (ECAE) process at room temperature, which applied simple shear deformation and generated unique asymmetric lamellar texture. In-situ small-angle X-ray scattering (SAXS) measurements were utilized to study the changes of lamellar structure during heating in samples with and without experiencing the simple shear deformation. The total scattered intensity, fractions of isotropic and anisotropic scattered intensities, orientation factors, and long periods along different shear directions were determined from 2-dimensional SAXS patterns. Orientation-stable α-lamellae with their normal perpendicular to the shear plane exhibited a larger long period after shear and a rapid decrease in long period upon heating, while less orientation-stable β-lamellae with their normal perpendicular to the flow direction exhibited a smaller long period after shear but only a slight decrease in long period upon heating. The thermally induced lamellar misorientaton could be attributed to the processes of lamellar relaxation and recrystallization of the amorphous phase. The possible molecular mechanisms for lamellar formation during simple shear are also discussed.