Matrix molecular orientation in fiber-reinforced polypropylene composites

A distinctive crystalline morphology which develops in certain fiber-reinforced thermoplastics, termed "transcrystallinity", occurs as the result of dense nucleation of polymer crystals at the surface of reinforcing fibers. As these fiber-sponsored nuclei grow, they impinge upon one another, such that crystal growth occurs essentially perpendicular to the fiber axis. Previous studies concerning transcrystallized composites have generally focused on single-fiber composites or model systems. Our interest is in elucidating the crystal orientation in conventional fiber-reinforced composites, and in quantifying the fraction of transcrystallized matrix, where present. In the present work, we develop a wide-angle X-ray scattering (WAXS) technique to investigate composites formed from an isotactic polypropylene (PP) matrix with practical loading levels of unidirectional pitch-based carbon, polyacrylonitrile (PAN)-based carbon, or aramid fibers. The transcrystalline fraction of the crystalline matrix approaches 0.95 in pitch-based carbon composites and 0.50 in the aramid composites near fiber loadings of 30 vol %. In addition, a previously-unreported mode of matrix orientation is observed in composites containing the non-transcrystallizing PAN-based carbon fibers, arising from restrictions on the isotropic growth of PP crystallites by the unidirectional fibers. This "constrained growth" orientation can coexist with the transcrystallized matrix at lower fiber loadings.