Small-angle X-ray scattering study of the pore structure of carbon fibers prepared from a polymer blend of phenolic resin and polystyrene

Abstract

A novel technique to make porous carbon fibers without small-angle X-ray scattering intensities in the present activation was developed, where the fibers are prepared measurable region. from the polymer blend of carbon precursor resin that is SAXS intensities were measured by using the apparatus forming carbon matrix by heat treatment and pore forming that some of the present authors (Y. K. and K. N.) had resin that disappears by pyrolysis [1–3]. By the method, constructed for the purpose of convenient use in a laborawe can produce different types of fibers depending on the tory with overall efficiency [4]. A point-focused X-ray combination of polymers. For example, fibers with closed beam was adopted in the diffractometer. To prevent pores are made from the blend of phenol–formaldehyde scattering and absorption of X-rays by air, all X-ray paths 21 resin as carbon precursor and polystyrene as pore forming were kept in vacuum of about 10 Pa. The wavelength of resin [3]. Moreover, the pore size and pore concentration incident X-rays was 1.5406 A (CuKa ). A position-sensi1 can be designed by choosing conditions of the pore tive proportional counter (PSPC) was used as the detector. forming resin. By use of small-angle X-ray scattering The distance from sample to the detector was 1170.5 mm (SAXS), we have investigated the structure of closed pores and the accumulation time for the intensity measurement in the carbon fibers mentioned above and determined the was 1800 s. Fiber samples were powdered with an agate shape and size of the pores with reliable accuracy. pestle. It was difficult to crush the fibers in parallel to the The polymer blend of the present study was made from elongated axis of them. ‘‘The powdering’’ means here the a mixture of Novolac type phenol–formaldehyde resin cutting of the fibers in perpendicular to the fiber axis. The (Gun-ei Chemical Co. Ltd.) and polystyrene (JSR Corporafinal mesh was about 0.1 mm, which was the almost the tion). The content of polystyrene was 10% by weight and same order as the diameter of the fibers. It is possible to say that most of the closed pores were not crushed or the grain size was 300A. This polymer blend was spun at deformed by the powdering. The powder was filled in the 135|1408C, stabilized in an acidic solution and carbonized sample cell with mylar films for the window. Judging from in a nitrogen atmosphere. Details of the preparation of the the size of powdered samples, the filling was made fibers are reported elsewhere [3]. Two kinds of samples uniformly in orientation. The thickness of the sample was heat-treated at 500 and 10008C were made and used for the 1.5 mm. measurements. Hereafter, they are named sample A The SAXS patterns are shown in Fig. 1, where s is a (5008C) and sample B (10008C). The diameter of the scattering parameter defined by 4p sin u /l (2u : scattering fibers is about 0.1 mm; this size does not affect the angle, l: wavelength of X-ray). The intensity, I(s), is in arbitrary units but is normalized to the scattering intensity from the same number of carbon atoms. The scattering *Corresponding author. Fax.: 181-43-290-3939. intensity of sample B is stronger than that of sample A on E-mail address: nisikawa@cuphd.nd.chiba-u.ac.jp (K. Nishikawa). the whole, but these scattering patterns almost overlap with