MACROMOLECULAR CHEMISTRY AND POLYMERIC MATERIALS Interrelation between the Structural Parameters of Polyacrylonitrile-based Carbon Fiber

Interrelation between the structural parameters of polyacrylonitrile-based carbon fiber is studied. Correlation is established between the interplanar spacings d 002 and d 110 , as well as between the average dimensions of the coherent scattering regions L c , L a|| , and L a . Carbon fibers (CFs) are prepared by high-tempera- ture (1500 3100 C) treatment of carbon-containing polymer fibers (1, 2). The crystal structure of CFs can be characterized by five main parameters: periods of translation along the crystallographic axes a and c (or interplanar spacings d 002 = c /2 and d 110 = a /2) and the corresponding average dimensions of coherent scattering regions (CSRs), L c , L a|| , and L a (Fig. 1). The main parameter conventionally used for charac- terizing the structure of carbon materials is the easily measurable interplanar spacing d 002 (3, 4). In view of correlation between the structural parameters of car- bon materials it is possible to use other characteristics as the main parameter (3, 4). To choose among them, it is necessary to elucidate the interrelations between the structural characteristics. In this work, we study the interrelation between the structural characteristics of CFs based on polyacrylonitrile (PAN). EXPERIMENTAL Our studies were concerned with a CF whose struc- tural characteristics were studied earlier but were not analyzed for interrelations (5). Twenty-two samples of the carbon fiber were synthesized by high-temperature dynamic treatment of the VMN-4 fiber in a vacuum at 3000 3100 C; the bundle load was 1.5, 0.75, 0.375, and 0.125 kg. We also studied the initial fiber ob- tained by carbonization of a polymeric PAN fiber at 2500 C. The samples for X-ray diffraction analysis were prepared in the form of small joists of rectangular cross section by pressing fiber bundles arranged parallel to each other and impregnated with epoxy resin. The diffraction maximum profiles were re- corded on a DRON-3 X-ray diffractometer. The aver- age interplanar spacing d 002 and the CSR dimensions were determined using the 002 diffraction maxima recorded when irradiating the lateral surface of the fibers. The d 110 and L a|| parameters were determined using the 110 diffraction line recorded when irradiat- ing the end surfaces of the fibers. Because of the high degree of orientation of the crystallites and inter- atomic bonds in the CF obtained from PAN the 110 plane families are oriented predominantly at angles of 30 and 90 with respect to the fiber axis (Fig. 2).