M C. Paina et al. / Carbon 41(2003)1399-1409 1401 o-monomer reduces the length of the acrylonitrile se- between the sample and the light source was approximate quences in the copolymer, therefore limiting the extent of ly 100 mm cyclization that can occur during stabilization. Crosslink The fibers were thermally stabilized at different con- ing can also affect the extent of cyclization at the structural ditions, as summarized in Table 1. After UV irradiation. level by"freezing"the spatial distribution, thus inhibiting the industrial M fibers were heated to 230C in air for molecular mobility periods of 45 min, I h and 2 h. One set of fibers were stabilized by thermal oxidation conducted under constant weight condition of 0.03 g/denier(approximately 4 MPa stress level). The second set was thermally stabilized under 3. Experimental constant length condition by wrapping a continuous fila ment around a grafoil sheet, exposing the sample to UV The materials used in the current work were:(a) radiation, and subsequently subjecting the fibers to thermal commercial fibers produced from a Mitsubishi copolymer oxidation. The final degree of stabilization was compared by wet spinning, hereafter designated as M fibers and, (b) to that obtained for the fibers heated to 230C for 2 h acrylonitrile/ methyl acrylate copolymer, produced at Vir- without UV irradiation(M.) ginia Tech by solution polymerization and stabilized with The melt-spun VT fibers were heat stabilized in air after 1% of boric acid [10], hereafter designated as VT fibers. UV irradiation (Table 1). After several trials, a heating The Mitsubishi copolymer had a nominal AN/MA ratio of program was developed that rendered the fibers infusible 94: 6 and an intrinsic viscosity (V), obtained by dilute during the final carbonization step. The present work solution viscometry, of 1.98 dl/g. The VT copolymer had a reports results obtained from fibers heat stabilized follow comonomer ratio of 88: 12, and an Iv of 0.49 dl/g ing this four-step heating program: 2 h at 180C, 2 h at The VT copolymer was melt spun into fibers using a 200C, 2 h at 210C, and finally I h at 220C. No load rate-controlled capillary rheometer Instron 3211 a was applied during the stabilization of the VT fibers. Recall capillary die with a diameter of 150 um diameter and an that these were melt-spun fibers. Like all melt-spun LID of 3. The results reported in this paper were all materials, they are soft and tend to break easily when even obtained for single-filaments. The extrusion temperature a small weight is applied at a temperature close to T for all tests was 225C and the nominal shear rate was 500 Therefore, the Vt fibers were stabilized only at constant s. The fibers solidified as they exited the capillary and length. As noted earlier, M fibers were also stabilized at were collected on a winder for a nominal draw-down ratio constant length for comparison. After stabilization, both sets of fibers (M and vr)were carbonized in an Astro The fibers were placed inside a temperature-controlled furnace, at 1500C, under a constant fiow of He oven equipped with a window that allowed exposure to The thermal stability and reactivity of the precursors, UV radiation(100 W Hg arc lamp, Oriel). The lamp was as-spun and UV irradiated fibers were studied by dsC, mounted in a Series Q housing equipped with a rear using a Pyris 1 DSC (Perkin-Elmer). Isothermal experi- reflector and a condenser, to concentrate the radiation on a ments were performed in which the polymer was heated to circle of approximately 60 mm of diameter. The distance a given temperature and was held at that temperature for Conditions for UV and heat stabilization of the m and vt fibers studied at constant load and constant length UV irradiation(h)(T=130C) Heat oxidation Stabilization performed at constant load 2h(230°C) MMMMM 45min(230°C) 2222 lh(230°C) 2h(230°C) Stabilization performed at constant length** 44Tm 221222 2h(230°C) VT 2h(180°C),2h(200°C) 2h(210°C),1h(220°) UV irradiation performed at T=150CM.C. Paiva et al. / Carbon 41 (2003) 1399–1409 1401 co-monomer reduces the length of the acrylonitrile se- between the sample and the light source was approximate￾quences in the copolymer, therefore limiting the extent of ly 100 mm. cyclization that can occur during stabilization. Crosslink- The fibers were thermally stabilized at different con￾ing can also affect the extent of cyclization at the structural ditions, as summarized in Table 1. After UV irradiation, level by ‘‘freezing’’ the spatial distribution, thus inhibiting the industrial M fibers were heated to 230 8C in air for molecular mobility. periods of 45 min, 1 h and 2 h. One set of fibers were stabilized by thermal oxidation conducted under constant weight condition of 0.03 g/denier (approximately 4 MPa stress level). The second set was thermally stabilized under 3. Experimental constant length condition by wrapping a continuous fila-  ment around a Grafoil sheet, exposing the sample to UV The materials used in the current work were: (a) radiation, and subsequently subjecting the fibers to thermal commercial fibers produced from a Mitsubishi copolymer oxidation. The final degree of stabilization was compared by wet spinning, hereafter designated as M fibers and, (b) to that obtained for the fibers heated to 230 8C for 2 h acrylonitrile/methyl acrylate copolymer, produced at Vir- without UV irradiation (M ). a ginia Tech by solution polymerization and stabilized with The melt-spun VT fibers were heat stabilized in air after 1% of boric acid [10], hereafter designated as VT fibers. UV irradiation (Table 1). After several trials, a heating The Mitsubishi copolymer had a nominal AN/MA ratio of program was developed that rendered the fibers infusible 94:6 and an intrinsic viscosity (IV), obtained by dilute during the final carbonization step. The present work solution viscometry, of 1.98 dl/g. The VT copolymer had a reports results obtained from fibers heat stabilized follow￾comonomer ratio of 88:12, and an IV of 0.49 dl/g. ing this four-step heating program: 2 h at 180 8C, 2 h at The VT copolymer was melt spun into fibers using a 200 8C, 2 h at 210 8C, and finally 1 h at 220 8C. No load rate-controlled capillary rheometer Instron 3211 and a was applied during the stabilization of the VT fibers. Recall capillary die with a diameter of 150 mm diameter and an that these were melt-spun fibers. Like all melt-spun L/D of 3. The results reported in this paper were all materials, they are soft and tend to break easily when even obtained for single-filaments. The extrusion temperature a small weight is applied at a temperature close to T . g for all tests was 225 8C and the nominal shear rate was 500 Therefore, the VT fibers were stabilized only at constant 21 s . The fibers solidified as they exited the capillary and length. As noted earlier, M fibers were also stabilized at were collected on a winder for a nominal draw-down ratio constant length for comparison. After stabilization, both of 4. sets of fibers (M and VT) were carbonized in an Astro The fibers were placed inside a temperature-controlled furnace, at 1500 8C, under a constant flow of He. oven equipped with a window that allowed exposure to The thermal stability and reactivity of the precursors, UV radiation (100 W Hg arc lamp, Oriel). The lamp was as-spun and UV irradiated fibers were studied by DSC, mounted in a Series Q housing equipped with a rear using a Pyris 1 DSC (Perkin-Elmer). Isothermal experi￾reflector and a condenser, to concentrate the radiation on a ments were performed in which the polymer was heated to circle of approximately 60 mm of diameter. The distance a given temperature and was held at that temperature for T able 1 Conditions for UV and heat stabilization of the M and VT fibers studied at constant load and constant length Sample UV irradiation (h) (T5130 8C) Heat oxidation Stabilization performed at constant load M – 2 h (230 8C) a M 2.5 45 min (230 8C) b M 2.5 1 h (230 8C) c M 2.5 2 h (230 8C) d M 2.5 – e Stabilization performed at constant length* M 2.5 – 1 M 2.5 2 h (230 8C) 2 VT 1 – 1 VT 2 – 2 VT 2.5 – 3 VT 2.5 2 h (180 8C), 2 h (200 8C), 4 2 h (210 8C), 1 h (220 8C) * UV irradiation performed at T5150 8C