M C. Paina et al. / Carbon 41(2003)1399-1409 (temperature and intensity of UV radiation). It is empha- sized, however, that the UV irradiation step was successful in crosslinking the polymer to an extent that further thermal stabilization was feasible Because of the limited depth of penetration of UV radiation in majority lymers, the directional, asymmetric extent of crosslinking in the PAN-based fibers was examined by IR microscopy. Specifically, the larger VT fibers(diameter <50 um) were ideally suited because these could be embedded in a resin and thin cross sections (4+1 um) could be obtained by microtoming. These sections were spread on a ZnSe crystal and analyzed after reduced to the lower limit of the equipment by using an lav 132 m 23 G SEL 202 1424. aperture of approximately 20X20 um". This way, the fiber cross-section could be divided in five different regions, as shown in Fig. 7. Measurements taken from several cross sections of two different fibers show a nitrile ratio of 0.55+0. 04 for the fiber center area. For the external areas values varying from 0.39 to 0.58 where measured, showing regions where the nitrile consumption was reasonably higher relative to the sample center, and regio results similar to the center area. These results suggest a variation in nitrile ratio not only in the radial direction but the decrease of UV intensity accessible to the fiber along its diameter. The results suggest that the depth of penetra- tion of UV for these PAN-based fiber precursors is -15 m. Therefore, in a scaled-up process the precursor fiber diameter should be held below 15 m, a value not different from that used in current commercial processes 4.3. Evolution of properties Scanning electron microscopy (SEM) images of the fracture surfaces(after tensile testing) of the VT fibers that re displayed in Fig. 8. The as-spun fibers display a ductile esponse in Fig. &a. With crosslinking and thermal stabili- zation, the fiber ductility decreases, as illustrated by the o ontrast, the rather brittle nature of carbonized fibers is Fig. 8. FESEM micrographs of VT fibers (a) as spun(b)UV confirmed by micrographs presented in Fig. 9 for both VT- irradiated for 2.5 h, and (c)Uv irradiated and heat stabilized and M-based carbon fibers. It is also noted that the microstructure of the PAN-based fibers is fairly featureless This is in contrast to the radially oriented graphene-layer and VT fibers is displayed in Fig. 10a-d. The effect of UV arrangement observed in mesophase pitch-based carbon radiation on properties is similar for both types of fibers, fibers at similar (or even lower) carbonization tempera- although in absolute terms the M fibers possess better tures. The lateral surfaces of the two fibers display a properties at all stages. M and VT fibers show a consider ignificant difference. The carbon fibers produced from M able reduction in strain-to-failure after UV irradiation. This fibers by continuous spinning process display far fewer reduction likely results from crosslinking of the polymer, aws than do carbon fibers produced from batch VT- which was observed earlier by solubility tests and enthalpy measurements. The effect of thermal oxidation is also Single filament tensile tests were performed on as-spun, similar for both types of fibers, although the yield strength M, M2, VT, and VT, fibers. The effect of UV irradiation slightly decreased for the UV irradiated fibers, and slightly and thermal oxidation on the tensile properties of the M increased for the thermally oxidized fibers. The decrease in1406 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 (temperature and intensity of UV radiation). It is empha￾sized, however, that the UV irradiation step was successful in crosslinking the polymer to an extent that further thermal stabilization was feasible. Because of the limited depth of penetration of UV radiation in majority of polymers, the directional, asymmetric extent of crosslinking in the PAN-based fibers was examined by IR microscopy. Specifically, the larger VT fibers (diameter |50 mm) were ideally suited because these could be embedded in a resin and thin cross sections (461 mm) could be obtained by microtoming. These sections were spread on a ZnSe crystal and analyzed after drying. The area of fiber to be analyzed by FT-IR was reduced to the lower limit of the equipment by using an 2 aperture of approximately 20320 mm . This way, the fiber cross-section could be divided in five different regions, as shown in Fig. 7. Measurements taken from several cross sections of two different fibers show a nitrile ratio of 0.5560.04 for the fiber center area. For the external areas, values varying from 0.39 to 0.58 where measured, showing regions where the nitrile consumption was reasonably higher relative to the sample center, and regions with results similar to the center area. These results suggest a variation in nitrile ratio not only in the radial direction but also across the fiber diameter, possibly as a consequence of the decrease of UV intensity accessible to the fiber along its diameter. The results suggest that the depth of penetra￾tion of UV for these PAN-based fiber precursors is |15 mm. Therefore, in a scaled-up process the precursor fiber diameter should be held below 15 mm, a value not different from that used in current commercial processes. 4 .3. Evolution of properties Scanning electron microscopy (SEM) images of the fracture surfaces (after tensile testing) of the VT fibers that are displayed in Fig. 8. The as-spun fibers display a ductile response in Fig. 8a. With crosslinking and thermal stabili￾zation, the fiber ductility decreases, as illustrated by the relatively smoother fractured surfaces of Fig. 8b and c. In contrast, the rather brittle nature of carbonized fibers is Fig. 8. FESEM micrographs of VT fibers (a) as spun (b) UV confirmed by micrographs presented in Fig. 9 irradiated for 2.5 h, and (c) UV irradiated and heat stabilized. for both VT￾and M-based carbon fibers. It is also noted that the microstructure of the PAN-based fibers is fairly featureless. This is in contrast to the radially oriented graphene-layer and VT fibers is displayed in Fig. 10a–d. The effect of UV arrangement observed in mesophase pitch-based carbon irradiation on properties is similar for both types of fibers, fibers at similar (or even lower) carbonization tempera- although in absolute terms the M fibers possess better tures. The lateral surfaces of the two fibers display a properties at all stages. M and VT fibers show a consider￾significant difference. The carbon fibers produced from M able reduction in strain-to-failure after UV irradiation. This fibers by continuous spinning process display far fewer reduction likely results from crosslinking of the polymer, flaws than do carbon fibers produced from batch VT- which was observed earlier by solubility tests and enthalpy polymer. measurements. The effect of thermal oxidation is also Single filament tensile tests were performed on as-spun, similar for both types of fibers, although the yield strength M , M , VT and VT fibers. The effect of UV irradiation slightly decreased for the UV irradiated fibers, and slightly 12 3 4 and thermal oxidation on the tensile properties of the M increased for the thermally oxidized fibers. The decrease in