Matrix molecular orientation in fiber-reinforced polypropylene composites

A distinctive crystalline morphology which develops in certain fiber-reinfo rced thermoplastics, termed "transcrystallinity", occurs as the result of d ense nucleation of polymer crystals at the surface of reinforcing fibers. A s these fiber-sponsored nuclei grow, they impinge upon one another, such th at crystal growth occurs essentially perpendicular to the fiber axis. Previ ous 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 i n quantifying the fraction of transcrystallized matrix, where present. In t he present work, we develop a wide-angle X-ray scattering (WAXS) technique to investigate composites formed from an isotactic polypropylene (PP) matri x with practical loading levels of unidirectional pitch-based carbon, polya crylonitrile (PAN)-based carbon, or aramid fibers. The transcrystalline fra ction of the crystalline matrix approaches 0.95 in pitch-based carbon compo sites and 0.50 in the aramid composites near fiber loadings of 30 vol %. In addition, a previously-unreported mode of matrix orientation is observed i n composites containing the non-transcrystallizing PAN-based carbon fibers, arising from restrictions on the isotropic growth of PP crystallites by th e unidirectional fibers. This "constrained growth" orientation can coexist with the transcrystallized matrix at lower fiber loadings. (C) 1998 Kluwer Academic Publishers.