Role of Glass Fiber in the Flow-Induced Crystallization of Poly(ether ether ketone)

This study investigates the effects of specific work on flow-induced crystallization and orientation in neat poly(ether ether ketone) (PEEK) and its glass fiber-reinforced composites (PEEK-15GF). Utilizing rheometry, it was found that increased specific work significantly accelerates crystallization kinetics, with the effect being more pronounced in the PEEK composites. Simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements reveal that PEEK composites exhibit stronger orientation effects compared to neat PEEK, especially notable at lamellar and unit cell length scales. Furthermore, X-ray computed tomography (X-ray CT) analyses of fiber properties show that unlike resin orientation that is a function of specific work, fiber orientation depends on the distance from the origin of the sheared disk. To explain the mechanisms behind the flow-induced acceleration of crystallization kinetics and the enhancement of resin orientation, this study uses an energy conversion factor alongside factors such as the specific work and critical nucleus size. The incorporation of glass fibers was found to have a negligible effect on the critical nucleus volume. However, the microscopic flow field generated by the fibers within the composite matrix significantly amplifies the conversion factor, increasing it by a factor of 5. This enhancement arises because the fibers result in localized shear and extensional flows and promote more effective alignment of resin molecules in addition to impeding the relaxation of the PEEK resin once rotational shear is stopped. The conversion factor, together with other material-related parameters, was used to develop models that describe flow-induced kinetic acceleration and orientation in neat PEEK and its glass fiber-reinforced composites.