The structural features of three different graphite fibers were studied via small-and wide-angle xray techniques. The experimental evidence is consistent with a sheath/core fiber morphology. Graphitization, degree of orientation, crystallite size, and microporosity were analyzed. Samples included low (AS4) and high (HMS) modulus poly(acrylonitrile) (PAN) and melt-spun pitchbased (VSB-16) fibers. By wide-angle x-ray diffraction (W AXD) VSB-16 was found to have the highest degree of graphitization, the highest degree of orientation, and the largest crystallite regions, and AS4 the poorest graphitized structure. The void system in these graphite fibers was investigated by small-angle x-ray scattering (SAXS). SAXS from glycerin-soaked fibers indicates the scattering at very sman angles (2() < 10 mrad) is dominated by total reflection of x rays at the fiber surface. The pores in HMS and VSB-16 fibers are inaccessible to glycerin and the pores in AS4 fiber are partially accessible. The pores in PAN -based HMS and AS4 fibers are of needlelike shape and those in VSB-16 are ellipsoidal. The porosity is 12.6%, 8.4%, and 4.5% in HMS, AS4, and VSB-16 fibers, respectively. Deviations from Porod's law were observed at large angles and attributed to scattering from fractal aggregates of carbon atoms in the graphite crystallites and/or fractal boundaries of pores. The fractal dimension of the aggregates is 2.3 ± 0.1, 2.8 ± 0.2, and 3.0 ± 0.2 for AS4, HMS, and VSB-16 fibers, respectively. Speculations about the fractal nature of aggregation may stimulate some new insight to the graphitization process, paracrystaHinity, and the strength of graphite fibers.
The phenomenon of surface scattering of electromagnetic waves by single or multiple layers of films is reviewed and a special treatment for the total reflection of x rays is developed. This theory is applied to the analysis of the surface scattering observed in small‐angle x‐ray scattering (SAXS) studies of two‐phase matter in polymers having lamella stacks or a flat interfacial boundary structure. Important features of this vector theory are the ability to calculate the surface scattering invariant, the absolute scattering intensity, and the surface roughness, which gives rise to dispersion of specular reflection from perfectly smooth surfaces. By considering the interfacial surface roughness of polystyrene crazes, the surface scattering spectrum is calculated theoretically and compared with some experimental results. Also the theory is presented in such a way as to compare surface scattering with volume scattering; i.e., both two‐ and three‐dimensional scattering events can be simultaneously treated. This provides a new basis for quantitative analysis of crazes in polystyrene.
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