Interphase structure of oxidized carbon fiber reinforced polyamide 6 revealed by synchrotron microbeam x-ray diffraction and infrared-spectroscopy

Interphase molecular aggregation states of carbon fiber reinforced polyamide 6 (PA6) films were studied. DSC measurement of composites revealed that high modulus carbon fibers (HMCF(-)) and oxidized high modulus carbon fibers (HMCF(+)) effectively acted as nucleation sites in PA6 matrices during cooling from melt. Polarized optical microscopy (POM) observation revealed the presence of a ca. 2μm thick polyamide 6 transcrystalline layer with positive birefringence at the interphase region of PA6 and HMCF(-), while no clear crystal interphase layer was observed at that of PA6 and HMCF(+). The crystal form and orientation at the interphase region was further confirmed by microbeam wide angle X-ray diffraction (μWAXD) and microbeam infrared spectroscopy (μIR). The μWAXD and μIR results show that that the a axis of α crystals in the transcrystalline layer are perpendicular to the HMCF(-) fiber direction, while α-type crystals grows randomly in case of HMCF(+) interphase. One reason might be that during the oxidization process, the orientation of the HMCF surface graphite crystal may become disordered.