Fiber-Induced Crystallization in Elongational Flows

Morphology development at the fiber/matrix interphase in fiber-reinforced isotactic polypropylene composites is a widely studied topic. While the application of shear flow may strongly enhance the nucleation density around the fiber, little is known about the influence of fibers on the crystallization of polypropylene subjected to an extensional flow. In this work, the flow around a single glass fiber upon uniaxial elongation of the melt is examined using X-ray scattering and diffraction techniques and compared to the response measured for the neat matrix. A comparison between a neat and compatibilized matrix is made given the strong influence of the addition of an adhesion modifier on the bulk crystallization kinetics of polypropylene. The flow is applied using an in-house-built filament stretching extensional rheometer, which, due to its midfilament control scheme, allows for in situ X-ray experiments. Combined small-angle X-ray scattering/wide-angle X-ray diffraction patterns are acquired during the flow and subsequent crystallization step. Postcrystallization area scans of the filament show that the introduction of a single glass fiber gives rise to the development of beta-phase crystals, particularly in the area around the fiber ends, and in contrast to what is observed for the matrix materials alone, where solely alpha-phase is found. Surprisingly enough, the addition of a single fiber (0.00045 vol %) alters the crystallizing polymorph in almost the entire filament. However, the addition of the adhesion modifier hinders the formation of beta-phase crystals around the fiber due to an acceleration of the bulk crystallization kinetics. Finite element simulations provide insight into the flow field around the fiber during stretching and demonstrate that the flow is no longer uniaxial extension, but dominated by shear, even though the volumetric amount of fiber as compared to the matrix is negligible. These findings explain the experimental observation of substantial beta-phase formation after the introduction of a single fiber, while this is not observed in the matrix material. Worth noting, the formation of beta-phase polypropylene depends not only on the presence and the strength of the flow but predominantly on the type of flow, i.e., shear as opposed to elongation.