| Atomic force and scanning tunneling microscopies (AFM & STM) have been used to investigate surfaces of various PAN-based carbon fibers in the range of microns down to atomic dimensions. The objectives are to gain a better understanding of the fibers' interfacial interactions with PMR-15 polyimide and to provide the basis for interface tailoring to improve the performance of the fiber/polyimide composites. The AFM and STM results yield surface topographic information together with surface parameters, such as, average roughness (Ra), surface area (Ac), and fractal dimension (Fd). It has been found that there is a nice correlation between the roughness of some fibers (G- and T-types) with their surface dipole components represented as % WDM (% weighted dipole moment) and % polar energy. The fiber roughness also correlates well with the ILSS (interlaminar shear strength) representing the interfacial bonding between the fiber and the polymer matrix. The correlations between Ra, Ac, % WDM, % polar energy, and ILSS are shown, but are not as satisfactory when extended to additional types of fibers (A-fibers). The results suggest that better correlations of surface morphology with ILSS might be obtained by examining the surface at nano-scales. The STM study of the fiber surfaces at atomic scales reveals some interesting features, such as, pleats, graphitic regions, and basal planes on the fiber surfaces. It is concluded that interfacial adhesion can be enhanced by increasing the fiber roughness to provide not only greater mechanical interlocking, but also greater dipolar bonding with the matrix through polar moieties attached to the basal plane edges exposed by roughness. |
