Significant enhancement in compressive strength of poly(p-phenylenebenzobisoxazole) fibers by incorporation of carbon nanofibers

Abstract

Poly(p-phenylenebenzobisoxazole) (PBO) composite fibers containing small amounts (~2 wt.%) of carbon nanofibers (CNFs) were successfully prepared by in-situ polymerization and dry-jet wet spinning. Polarized optical microscopy images displayed a lyotropic liquid-crystalline phase in the as-polymerized dopes. Solution viscosity measurements revealed that the molecular weights of the composite fibers were in the range from 1.81×104 gmol-1 to 2.2×104 gmol-1. Field emission scanning electron microscopy images of the composite fibers etched by UV irradiation showed that CNFs embedded in the PBO matrix were individually dispersed, and well oriented along the fiber direction. Tensile tests indicated that the tensile moduli of the PBO/CNF composite fibers were higher than those of the pristine PBO fibers, whereas the tensile strengths were lower or similar to those of the pristine PBO fibers. The elastic loop test was carried out to measure the compressive strength of the fibers. The compressive strength of the pristine PBO fiber was 0.3 GPa, whereas that of a PBO composite fiber containing 1.0 wt.% CNF increased to 0.92 GPa, which is about twice the compressive strength of Zylon (0.43 GPa). Moreover, a heat-treated PBO composite fiber containing 2.0 wt.% CNFs exhibited the high compressive strength of 1.17 GPa, which is the second highest value among all reported values for organic polymers.