Flow-induced crystallization of poly(ether ether ketone): Universal aspects of specific work revealed by corroborative rheology and x-ray scattering studies

When a semicrystalline polymer melt is processed in intense flow, the nucleation rate can be accelerated and the resultant morphology is transformed to anisotropic structures. These cumulative changes to the crystallization process are referred to as flow-induced crystallization (FIC). In this study, shear flow-induced crystal formations of poly(ether ether ketone) (PEEK) are investigated after applying a short-term shear (γ' = 20 s^-1 and t_s < 230 s) via rheology and ex situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Using rheology, three types of dynamic response are monitored during FIC: no flow effect, nucleation acceleration, and instant crystal growth without crystallization induction time. Ex situ SAXS is employed with sheared PEEK disks to evaluate the flow-induced lamellar structure and orientation. The short-term shear changes the fraction and degree of lamellar stack orientation, whereas the lamellar structure is barely affected by shear, in terms of the long spacing (L* = 14.6 nm), linear crystallinity (χ_c = 0.34), and crystalline and amorphous layer thicknesses (L_c = 5.0 nm and L_a = 9.6 nm). Ex situ WAXS patterns indicate that PEEK chains (c-axis) are aligned in the shear direction within crystalline domains.