M C. Paina et al. / Carbon 41(2003)1399-1409 factor was smaller for the VT polymer, which would tend enthalpy of cyclization as a result of crosslinking from to reduce the overall reaction rate. The net result was that prior UV irradiation, similar to the one observed for M overall reaction rate was similar for both polymers fibers. The measured enthalpy approaches an asymptotic value as the UV exposure time approaches 2 h. 42. UVthermal stabilization To determine if cyclization reactions took place, FT-IR spectra were obtained in the atR mode for fibers stabi Initial testing verified that the Vt precursor fibers melted lized at various stages, as displayed in Fig. 5. In these before they could be thermally stabilized. However, after tests, the intensity of the CH,(1450 cm )and CN (2240 they had been exposed to UV irradiation, these same fibers cm )bands were measured. The cyclization reaction ould be thermally stabilized with little or no melting. This would involve reaction of the nitrile unit, but not dehydro- indicated that UV-induced crosslinking reactions were genation. Therefore, the nitrile ratio, I(CN)/I(CH,), should taking place. To determine the proper stabilization pro- provide a good estimate of the degree of cyclization since cedure for the VT precursor, the melt-spun fibers were UV nitrile groups are consumed and the CH, groups are and/or thermally stabilized under the conditions listed in unaffected [12]. Fig. 6a displays the evolution of the nitrile Table 1. Then, DSC analyses were used to assess the ratio for M fibers stabilized at constant load and constant esulting crosslinking, and FT-IR analysis to estimate the length conditions, and Fig. 6b the evolution of this same extent of cyclization reactions atio for the VT fibers stabilized at constant length. From ince the heat released by the exothermic cyclization Fig. 6a, it can be inferred that the nitrile ratio for the reaction decreases as the prior degree of crosslinking as-spun M fibers is not significantly different (at 95% increases, DSC measurements of enthalpy may be used to confidence interval) from that of the UV irradiated fibers detect the evolution, and eventually saturation, of cross- indicating that UV irradiation induces little or no cycliza- linking. This approach was used to estimate the degree of tion in the solution spun precursor(M). However, the osslinking in the as-spun M fibers, and for the same nitrile ratio decreases significantly after thermal/oxidative ers after prior exposure to UV for periods of I and 2.5h tabilization and continues to decrease with increasi (at constant length). The heat for cyclization reactions for stabilization time. This indicates that cyclization does the as-spun fiber was the highest(600 J/g)and the value occur during thermal stabilization and longer times en- decreased with increasing prior exposure to UV radiation, hance the degree of cyclization achieved reaching a value of 477 J/g for an exposure time I h, and Similar trends were observed for the vT fibers(as those 463 J/g after 2.5 h of exposur observed for M fibers ). However, this copolymer precursor Similar DSC measurements were performed on the VT, had a lower acrylonitrile content than the mitsubishi VT,, and VT, fibers before and after exposure to UV copolymer precursor, and the measured nitrile ratio for VT radiation. The measured heat of reaction for the as-spun samples, reported in Fig. 6b was correspondingly lower fibers(prior to UV exposure)averaged 470 J/g. After UV after each step. Cyclization was not observed during the exposure for I and 2.5 h, the enthalpy values had reduced UV irradiation for either of the two precursors, indicating to 434 and 304 J/g, respectively. Although the small size potential differences from the reactions suggested by of the dsC sample(less than I mg) resulted in about 15% Ranby and rabek [20]. The differences may arise from variation in measured values, there was a clear decrease in differences in resin composition and reaction conditions 0.5 0.1 center Fig. 7. Optical micrograph of a fiber cross-section, the squares represent the approximate area where FT-IR spectra where obtained, and the results are presented in the graphM.C. Paiva et al. / Carbon 41 (2003) 1399–1409 1405 factor was smaller for the VT polymer, which would tend enthalpy of cyclization as a result of crosslinking from to reduce the overall reaction rate. The net result was that prior UV irradiation, similar to the one observed for M overall reaction rate was similar for both polymers. fibers. The measured enthalpy approaches an asymptotic value as the UV exposure time approaches 2 h. 4 .2. UV/thermal stabilization To determine if cyclization reactions took place, FT-IR spectra were obtained in the ATR mode for fibers stabi￾Initial testing verified that the VT precursor fibers melted lized at various stages, as displayed in Fig. 5. In these 21 before they could be thermally stabilized. However, after tests, the intensity of the CH (1450 cm ) and CN (2240 2 21 they had been exposed to UV irradiation, these same fibers cm ) bands were measured. The cyclization reaction could be thermally stabilized with little or no melting. This would involve reaction of the nitrile unit, but not dehydro￾indicated that UV-induced crosslinking reactions were genation. Therefore, the nitrile ratio, I(CN)/I(CH ), should 2 taking place. To determine the proper stabilization pro- provide a good estimate of the degree of cyclization since cedure for the VT precursor, the melt-spun fibers were UV nitrile groups are consumed and the CH groups are 2 and/or thermally stabilized under the conditions listed in unaffected [12]. Fig. 6a displays the evolution of the nitrile Table 1. Then, DSC analyses were used to assess the ratio for M fibers stabilized at constant load and constant resulting crosslinking, and FT-IR analysis to estimate the length conditions, and Fig. 6b the evolution of this same extent of cyclization reactions. ratio for the VT fibers stabilized at constant length. From Since the heat released by the exothermic cyclization Fig. 6a, it can be inferred that the nitrile ratio for the reaction decreases as the prior degree of crosslinking as-spun M fibers is not significantly different (at 95% increases, DSC measurements of enthalpy may be used to confidence interval) from that of the UV irradiated fibers, detect the evolution, and eventually saturation, of cross- indicating that UV irradiation induces little or no cycliza￾linking. This approach was used to estimate the degree of tion in the solution spun precursor (M). However, the crosslinking in the as-spun M fibers, and for the same nitrile ratio decreases significantly after thermal/oxidative fibers after prior exposure to UV for periods of 1 and 2.5 h stabilization and continues to decrease with increasing (at constant length). The heat for cyclization reactions for stabilization time. This indicates that cyclization does the as-spun fiber was the highest (600 J/g) and the value occur during thermal stabilization and longer times en￾decreased with increasing prior exposure to UV radiation, hance the degree of cyclization achieved. reaching a value of 477 J/g for an exposure time 1 h, and Similar trends were observed for the VT fibers (as those 463 J/g after 2.5 h of exposure. observed for M fibers). However, this copolymer precursor Similar DSC measurements were performed on the VT , had a lower acrylonitrile content than the Mitsubishi 1 VT , and VT fibers before and after exposure to UV copolymer precursor, and the measured nitrile ratio for VT 2 3 radiation. The measured heat of reaction for the as-spun samples, reported in Fig. 6b was correspondingly lower fibers (prior to UV exposure) averaged 470 J/g. After UV after each step. Cyclization was not observed during the exposure for 1 and 2.5 h, the enthalpy values had reduced UV irradiation for either of the two precursors, indicating to 434 and 304 J/g, respectively. Although the small size potential differences from the reactions suggested by of the DSC sample (less than 1 mg) resulted in about 15% Ranby and Rabek [20]. The differences may arise from variation in measured values, there was a clear decrease in differences in resin composition and reaction conditions Fig. 7. Optical micrograph of a fiber cross-section, the squares represent the approximate area where FT-IR spectra where obtained, and the results are presented in the graph