Masatoshi Shioya

Organic and carbon aerogels derived from poly(vinyl chloride)

Organic aerogels were derived from dimethylformamide solution of poly(vinyl chloride) (PVC) via dehydrochlorination using a strong base, 1,8-diazabicyclo[5,4,0]undec-7-ene, and supercritical drying using carbon dioxide. From these organic aerogels, carbon aerogels were yielded via stabilization and carbonization. Changes in the porous structure of the aerogels during the preparation process and influences of the preparation conditions on the porous structure were investigated. The framework of the aerogels composed the walls of the meso- and macropores. The volume and the size of these pores were reduced during stabilization and carbonization due to the shrinkage of the framework caused by the release of decomposition gases and densification of the material. Simultaneously, the release of decomposition gases produced additional micropores. The extent of dehydrochlorination, the concentration of PVC in the starting solution and the molecular weight of PVC were the factors with which the porous structure of the aerogels could be controlled over a wide range. In addition, the stabilization conditions notably influenced the carbonization behavior of the organic aerogels and the porous structure of the carbon aerogels. The optimum stabilization conditions that minimized the loss of mass and maximized the pore volume of the carbon aerogels were determined.

X-ray measurements and the structure of polyacrylonitrile- and pitch-based carbon fibres

X-ray measurements were carried out on polyacrylonitrile- and pitch-based carbon fibres. The crystallites, disordered regions and microvoids in these carbon fibres were evaluated quantitatively by applying the methods previously proposed by the present authors. The structural parameters evaluated are the 1 1 plane spacing of the carbon layer, the average, standard deviation and asymmetry of the distribution of interlayer spacing, the stacking regularity parallel to the layer plane, the layer extents parallel and perpendicular to the fibre axis, the stacking height, the crystallite orientation, the volume fractions of crystallites, disordered regions and microvoids, the variation of the electron density in a microvoid, and the size parameters of the void cross-section perpendicular to the fibre axis, such as the area, radius of gyration, chord length and thickness. The mutual relationships of these structural parameters are presented, and parameters sensitive to the nature of the starting materials are pointed out.

Mechanical properties of woven laminates and felt composites using carbon fibers. Part 2: interlaminar properties

This paper compares interlaminar shear strength, interlaminar fracture toughness and compression-after-impact strength of epoxy resin matrix composites reinforced with carbon fiber woven fabrics and carbon fiber felts, and investigates the influences of weave pattern and needle-punching density. With regard to the weave pattern of fabric layers in woven laminates and felt/resin composites, plain weave with a smaller radius of curvature of yarns is preferable for obtaining higher damage tolerance of the composites. Felt/resin composites have improved interlaminar and damage tolerant properties as compared with woven laminates. These properties of the felt/resin composites improve with increasing needle-punching density. The compression-after-impact strength of the woven laminates and felt/resin composites can be uniquely related to the mode I interlaminar fracture toughness regardless of the types of composites, and the result has been interpreted by applying the equation derived by Ilic and Williams [Theoret. Appl. Fract. Mech. 6 (2) (1986) 121]. Damage tolerance of the felt-reinforced composites utilizing more brittle carbon matrix has also been investigated.

Non-hookean stress-strain response and changes in crystallite orientation of carbon fibres

The non-hookean stress-strain response of carbon fibres was investigated in relation to changes in crystallite orientation with tensile stress. Various one-dimensional array models and a mosaic model were examined. Amongst these models, only the mosaic model in which the stress of the crystallites can be transmitted in both the transverse and the axial directions showed any quantitative agreement with the measured increases in the tensile modulus and the crystallite orientation with tensile stress. This suggests that deformation of the crystallites is constrained with increasing tensile stress. It was also found that the ratio of the tensile stress of the fibre to that of the crystallites is close to the crystallite volume fraction rather than the ratio of the fibre density to the crystallite density.

Characterization of microvoids in carbon fibers by absolute small‐angle x‐ray measurements on a fiber bundle

A method has been developed to characterize the voids in a fibrous material by using the small‐angle x‐ray scattering from a unidirectionally aligned fiber bundle, and the parameters evaluating the sizes, volume fraction, and number of the voids have been deduced. The distribution of cross‐section size of voids has also been considered in relation to the size parameters deduced. The method developed has been applied to a commercial carbon fiber. The values of the size parameters of the carbon fiber, evaluated experimentally by the method developed here, are in good agreement with those calculated by assuming that the distribution of the cross‐section size of voids obeys a Maxwellian distribution. A convenient method for determining the primary beam power by comparing the observed and theoretically calculated scattering intensities of air has also been developed in order to measure the scattering intensity of a specimen in absolute units.

Activated carbon fibers and films derived from poly(vinylidene fluoride)

Abstract Activated carbons having different ranges of pore sizes from those derived by the conventional heat-treatment of organic compounds have been produced by carrying out a part of the conversion process through a liquid phase chemical treatment. Poly(vinylidene fluoride) (PVDF) was chosen as a starting polymer by taking into account the processability of this polymer into various geometries. The PVDF fibers and films were converted to activated carbons by using the combination of chemical dehydrofluorination with a strong base, high-temperature heat-treatment and activation in a carbon dioxide gas. The resulting activated carbon film exhibited superior methylene blue adsorption of 538 mg g−1. Formation of a rigid skeleton during dehydrofluorination accounted for the ability of dehydrofluorinated PVDF to maintain its macroscopic precursor geometry during high-temperature heat-treatment and the formation of pores with large sizes.

Axial compressive fracture of carbon fibers

The axial compressive strength was measured on single-filaments of polyacrylonitrile and pitch-based carbon fibers. The compressive fracture mechanism of carbon fibers was investigated based on the crystallite and the microvoid structures characterized with X-ray scattering. The distribution of the compressive strength at a fixed fiber length, and the length dependence of the average compressive strength were much smaller than those of the tensile strength. The compressive strength is proposed to be limited by the buckling stress of individual carbon layers in the transversely unsupported regions of the crystallites, the length of the unsupported regions being determined by the axial length of the microvoids. This model represents the variation in the compressive strength of carbon fibers with different microstructures very well.

Estimation of fibre and interfacial shear strength by using a single-fibre composite

The fibre/matrix interfacial shear strength is often estimated from the fragmentation process of a fibre in a single-fibre composite loaded in tension. The interfacial shear strength is calculated from a knowledge of the critical length and the tensile strength of the fibre. Unfortunately, if the tensile strength of fibres distributes widely and increases with decreasing fibre length, then problems arise as to the value used for the tensile strength of the fibre. In this paper an alternative approach has been given to analyze the fragmentation process of the fibres. From the number of fractures of the fibre at various strain levels of the composite, the tensile strength of the fibre, the interfacial shear stress at each strain level, and the interfacial shear strength have been estimated. Glass/epoxy single-fibre composites have been analyzed with this method, and the influence of the exposure of the composites to hydrothermal conditions has been shown.

Compressive strengths of single carbon fibres and composite strands

Abstract Micro-compression and recoil tests have been carried out on single filaments of pitch- and polyacrylonitrile-based carbon fibres. Axial compression and bending tests were also carried out on unidirectional composite strands containing these fibres and a reduced compressive strength was calculated by dividing the fracture load of the composite strand by the cross-sectional area of the fibres. The fracture surfaces produced by different test methods were compared and a correlation between the compressive strength values determined by these test methods was investigated. The fracture surfaces of the fibres and composite strands showed different features depending on the type of fibre and matrix resin. The compressive strength of the composite strands increased with increasing matrix modulus. The compressive strengths of the fibres determined by the recoil test and from the axial compression test on the composite strand with a stiff matrix resin were almost in proportion to the strength determined with the micro-compression test.

Influence of swelling of noncrystalline regions in silk fibers on modification with methacrylamide

The degrees of swelling of noncrystalline regions of domestic and tussah silk fibers were investigated by measuring the small-angle X-ray scattering intensity of the fibers in wet conditions and analyzing the scattering intensity based on a two-phase model, i.e., crystalline regions and water-swollen noncrystalline regions. The influence of the degree of swelling of noncrystalline regions on the graft treatment of these fibers with methacrylamide was investigated. The changes in the structure caused by the graft treatment were also analyzed using the wide-angle X-ray diffraction measurements. As compared with the tussah silk fibers, the domestic silk fibers showed a larger degree of swelling of the noncrystalline regions, and gained a larger amount of resin by the graft treatment. The crystallites with smaller sizes in the tussah silk fibers were destroyed preferentially by the graft treatment. For the domestic silk fibers, the crystallites were destroyed more seriously and rather homogeneously independent of the crystallite sizes.