CARBON PERGAMON Carbon4l(2003)1399-1409 UV stabilization route for melt-processible PAN-based carbon fibers M. C. Paiva.P Kotasthane. DD. Edie. A.A. Ogale Department of Chemical Engineering, and Center for Advanced Engineering Fibers and Films, Clemson Universit, Clem SC296340910,USA Received 18 January 2003; accepted 29 January 2003 Abstract Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based polymers. An acrylonitrile/ methyl acrylate(AN/MA)copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized o 2003 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon fibers; B. Stabilization; C. Differential scanning calorimetry (DSC); Infrared spectroscopy; D. Mechanical properties 1. Introduction thermally-induced reactions occur, other approaches must be developed to crosslink the precursor fibers. In contrast to wet-spinning, the melt spinning techniq Various grades of melt-spinnable pan precursors converts pure precursor directly into fiber form at high currently being developed and evaluated for carbon fiber process speeds and without added expense of solvent production in a joint Clemson/Virginia Tech project ecovery and recycling [1]. However, the bulk of funded by the US Department of Energy. The research fibers are produced from polyacrylonitrile(PAN) team at Virginia Tech is synthesizing melt-spinnable PAN rs that are converted into fiber form by wet copolymers [10], and the team at Clemson is converting spinning methods [2]. The reason behind the use of wet- these into melt-spun PAN and carbon fibers. The present methods is that commercial PAn copolymer aper reports the stabilization procedure developed for rs thermally decompose below their melting tem- these melt-spun PAn fibers as well as the conversion of making melt spinning impossible these stabilized fibers into carbon fibers Recently, BP Amoco Chemicals produced a melt-spinn able PAn copolymer [3, 4] containing a high amount of methyl acrylate comonomer located irregularly along the lymer chain, which most likely decreases the crys- 2. Background: reactions of polyacrylonitrile precursor fibers allinity of the copolymer. A stabilizing agent was also added to inhibit thermal degradation. Although this melt- 2. 1. Heat stabilization of pan spinnable copolymer might appear to be attractive as a carbon fiber precursor, its thermal stability makes standard The stabilization of polyacrylonitrile fibers for carbon oxidative stabilization techniques [1, 5-9 impractica fiber production involves thermal treatment, usually in air, Since this type of PAN copolymer melts before any at temperatures ranging from 180 to 300C. This part of the process is intended to increase the stiffness of the Pan Corresponding author. Fax: +1-864-656-0784 molecules and hold them together in such a way as to E-mail address: ogale clemson. edu(AA. Ogale). avoid extensive relaxation and chain scission during the On leave of absence from the Department of Polymer Engineer- final carbonization step ing, University of Minho, 4800-058 Guimaraes, Portugal The increase in molecular stiffness is mainly achieved 0008-6223/03/s-see front matter 2003 Elsevier Science Ltd. All rights reserved doi:10.1016/0008-6223(03)00041
Carbon 41 (2003) 1399–1409 U V stabilization route for melt-processible PAN-based carbon fibers 1 M.C. Paiva , P. Kotasthane, D.D. Edie, A.A. Ogale* Department of Chemical Engineering, and Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, SC 29634-0910, USA Received 18 January 2003; accepted 29 January 2003 Abstract Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile/methyl acrylate (AN/MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers. 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Stabilization; C. Differential scanning calorimetry (DSC); Infrared spectroscopy; D. Mechanical properties 1. Introduction thermally-induced reactions occur, other approaches must be developed to crosslink the precursor fibers. In contrast to wet-spinning, the melt spinning technique Various grades of melt-spinnable PAN precursors are converts pure precursor directly into fiber form at high currently being developed and evaluated for carbon fiber process speeds and without added expense of solvent production in a joint Clemson/Virginia Tech project recovery and recycling [1]. However, the bulk of carbon funded by the US Department of Energy. The research fibers are produced from polyacrylonitrile (PAN) precur- team at Virginia Tech is synthesizing melt-spinnable PAN sors that are converted into fiber form by wet and dry copolymers [10], and the team at Clemson is converting spinning methods [2]. The reason behind the use of wet- these into melt-spun PAN and carbon fibers. The present spinning methods is that commercial PAN copolymer paper reports the stabilization procedure developed for precursors thermally decompose below their melting tem- these melt-spun PAN fibers as well as the conversion of perature, making melt spinning impossible. these stabilized fibers into carbon fibers. Recently, BP Amoco Chemicals produced a melt-spinnable PAN copolymer [3,4] containing a high amount of methyl acrylate comonomer located irregularly along the 2. Background: reactions of polyacrylonitrile polymer chain, which most likely decreases the crys- precursor fibers tallinity of the copolymer. A stabilizing agent was also added to inhibit thermal degradation. Although this melt- 2 .1. Heat stabilization of PAN spinnable copolymer might appear to be attractive as a carbon fiber precursor, its thermal stability makes standard The stabilization of polyacrylonitrile fibers for carbon oxidative stabilization techniques [1,5–9] impractical. fiber production involves thermal treatment, usually in air, Since this type of PAN copolymer melts before any at temperatures ranging from 180 to 300 8C. This part of the process is intended to increase the stiffness of the PAN molecules and hold them together in such a way as to *Corresponding author. Fax: 11-864-656-0784. E avoid extensive relaxation and chain scission during the -mail address: ogale@clemson.edu (A.A. Ogale). 1 On leave of absence from the Department of Polymer Engineer- final carbonization step. ing, University of Minho, 4800-058 Guimaraes, Portugal. ˜ The increase in molecular stiffness is mainly achieved 0008-6223/03/$ – see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0008-6223(03)00041-1
1400 M C. Paina et al. / Carbon 41(2003)1399-1409 through the cyclization of PAN [1, 5,6, 11. The cyclization teristics. Molecular on significantly affects the is an exothermic reaction during which nitrile groups react, properties of the poly transforming part of the PAN into a ladder-type polymer. maintained as much ple during stabilization if the he precise reaction mechanism for cyclization can differ, final properties of the carbon fibers are to be maximized depending on the experimental conditions and type of opolymer [7]. Numerous reactions can take place during 2.2. Crosslinking reactions of pan heating of PAN, and many are still not well-understood, as described by Bashir [8]. Burland and Parsons [12] showed When pan is irradiated with UV light in vacuum, it that the first step of the stabilization was the cyclization evolves hydrogen, methane, acrylonitrile and hydrogen through reaction of the nitrile groups, dehydrogenation cyanide, leading to chain scission and crosslinking re- being significant only above 300C Grassie and McGuch- actions simultaneously [191, as represented in Fig. 1. The an [6 proposed that dehydrogenation and cyclization crosslinking reactions take place preferentially at the reactions take place simultaneously, the former occurring tertiary carbon atom in the polymer backbone and, thus, both within the non-cyclized polymer chain as well as does not lead to the formation of conjugated imine bonds within the condensed heterocyclic rings. Cyclization re-(C=N-). The photo-oxidation of this polymer, especially actions are extremely exothermic, but this behavior can be at elevated temperatures, is described by ranby and rabek considerably reduced if a co-monomer such as methyl [20 as resulting in the formation of a ladder structure, rylate, vinyl acetate, or itaconic acid, for example, is following a mechanism similar to that observed for thermal ntroduced into the polymer chain. Furthermore, the activa- oxidation 5] tion energy of the cyclization reaction is smaller for the Other radiation sources have been used to achieve the opolymer, relative to the pan homone indicating crosslinking of PAN. Dietrich et al. [21 used electron- cyclization reaction. When PAN fibers are thermally stabi- they observed, using electron spin resonance, the formation ized the amount of co-monomer in the precursor not only of an alkyl radical structure, when there was poor oxyge affects the rate of oxidative stabilization [1, 7], it also diffusion through the fiber, and the formation of a peroxide affects temperature and applied tension requirements [13]. radical structure, for good oxygen diffusion. The authors The kinetic data for the cyclization reaction can be also found that the radicals formed were extremely stable, obtained by differential thermal analysis(differential scan- with a lifetime of several days. Heat treatment of the fibers ing calorimetry, DSC), using the Kissinger method [14] ed to cyclization, and this process was observed to be The method is based on the observation that when the rate faster for the irradiated fibers than for the non-treated of reaction varies with temperature (i.e, when the reaction fibers has an activation energy ) the position of the dsc peak varies with heating rate, if all other variables are kept 2. 3. Mell-spinning of PAN precursors constant At the molecular structure level, recent studies point out As bP discovered. the controlled introduction of a co- the influence of the polymer structure on the final ladder- monomer such as methyl acrylate(MA)into the acrylonit polymer formation. Several authors have discussed the rile(AN) polymer backbone in adequate amounts(higher stereospecificity of the cyclization reaction [15-17.In than 10%)and with an appropriate stabilizing system, fact, the cyclization reaction should be stereospecific, decrease Ts and allows the polymer to melt before ccurring preferentially in isotactic sequences to form a exothermic cyclization reactions occur 3]. Two mai straight rod-like structure. Gupta and Harrison [9, 18 problems arise when trying to produce carbon fibers from observed that intramolecular cyclization reactions occur at this new class of pan copolymer precursors: one lower temperatures(175-230C)in the amorphous phase chemical in nature, and the other is structure-related. The of the polymer, leading to a considerable decrease in introduction of a significant amount of methyl acrylate as a intermolecular interactions due to the decrease in con- centration of the highly polar nitrile groups. This would account for the macroscopic shrinkage observed at this stage. The crystalline regions would act as"bridge""points between the amorphous regions, holding the structure together. The authors report that, at temperatures above 320C, oxidation and intermolecular crosslinking take place, and that oxidative degradation reactions occur above 380°C To summarize, stabilization of pan precursors is a omplex process that depends both on the chemical composition of the copolymer and on its structural charac Fig. 1. Effect of UV irradiation on PAN [19]
1400 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 through the cyclization of PAN [1,5,6,11]. The cyclization teristics. Molecular orientation significantly affects the is an exothermic reaction during which nitrile groups react, properties of the polymer fibers, and orientation must be transforming part of the PAN into a ladder-type polymer. maintained as much as possible during stabilization if the The precise reaction mechanism for cyclization can differ, final properties of the carbon fibers are to be maximized. depending on the experimental conditions and type of copolymer [7]. Numerous reactions can take place during 2 .2. Crosslinking reactions of PAN heating of PAN, and many are still not well-understood, as described by Bashir [8]. Burland and Parsons [12] showed When PAN is irradiated with UV light in vacuum, it that the first step of the stabilization was the cyclization evolves hydrogen, methane, acrylonitrile and hydrogen through reaction of the nitrile groups, dehydrogenation cyanide, leading to chain scission and crosslinking rebeing significant only above 300 8C. Grassie and McGuch- actions simultaneously [19], as represented in Fig. 1. The an [6] proposed that dehydrogenation and cyclization crosslinking reactions take place preferentially at the reactions take place simultaneously, the former occurring tertiary carbon atom in the polymer backbone and, thus, both within the non-cyclized polymer chain as well as does not lead to the formation of conjugated imine bonds within the condensed heterocyclic rings. Cyclization re- (–C=N–) . The photo-oxidation of this polymer, especially x actions are extremely exothermic, but this behavior can be at elevated temperatures, is described by Ranby and Rabek considerably reduced if a co-monomer such as methyl [20] as resulting in the formation of a ladder structure, acrylate, vinyl acetate, or itaconic acid, for example, is following a mechanism similar to that observed for thermal introduced into the polymer chain. Furthermore, the activa- oxidation [5]. tion energy of the cyclization reaction is smaller for the Other radiation sources have been used to achieve the copolymer, relative to the PAN homopolymer, indicating crosslinking of PAN. Dietrich et al. [21] used electronthat the co-monomer acts as an alternative initiator for the beam irradiation on PAN fibers. For fiber irradiation in air cyclization reaction. When PAN fibers are thermally stabi- they observed, using electron spin resonance, the formation lized the amount of co-monomer in the precursor not only of an alkyl radical structure, when there was poor oxygen affects the rate of oxidative stabilization [1,7], it also diffusion through the fiber, and the formation of a peroxide affects temperature and applied tension requirements [13]. radical structure, for good oxygen diffusion. The authors The kinetic data for the cyclization reaction can be also found that the radicals formed were extremely stable, obtained by differential thermal analysis (differential scan- with a lifetime of several days. Heat treatment of the fibers ning calorimetry, DSC), using the Kissinger method [14]. led to cyclization, and this process was observed to be The method is based on the observation that when the rate faster for the irradiated fibers than for the non-treated of reaction varies with temperature (i.e., when the reaction fibers. has an activation energy), the position of the DSC peak varies with heating rate, if all other variables are kept 2 .3. Melt-spinning of PAN precursors constant. At the molecular structure level, recent studies point out As BP discovered, the controlled introduction of a cothe influence of the polymer structure on the final ladder- monomer such as methyl acrylate (MA) into the acrylonitpolymer formation. Several authors have discussed the rile (AN) polymer backbone in adequate amounts (higher stereospecificity of the cyclization reaction [15–17]. In than 10%) and with an appropriate stabilizing system, fact, the cyclization reaction should be stereospecific, decrease T and allows the polymer to melt before g occurring preferentially in isotactic sequences to form a exothermic cyclization reactions occur [3]. Two main straight rod-like structure. Gupta and Harrison [9,18] problems arise when trying to produce carbon fibers from observed that intramolecular cyclization reactions occur at this new class of PAN copolymer precursors: one is lower temperatures (175–230 8C) in the amorphous phase chemical in nature, and the other is structure-related. The of the polymer, leading to a considerable decrease in introduction of a significant amount of methyl acrylate as a intermolecular interactions due to the decrease in concentration of the highly polar nitrile groups. This would account for the macroscopic shrinkage observed at this stage. The crystalline regions would act as ‘‘bridge’’ points between the amorphous regions, holding the structure together. The authors report that, at temperatures above 320 8C, oxidation and intermolecular crosslinking take place, and that oxidative degradation reactions occur above 380 8C. To summarize, stabilization of PAN precursors is a complex process that depends both on the chemical composition of the copolymer and on its structural charac- Fig. 1. Effect of UV irradiation on PAN [19]
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=150C
M.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 approximatequences 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 coning 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 followcomonomer 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 experireflector 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
M C. Paina et al. / Carbon 41(2003)1399-1409 27 T=270c sE8品3 25 24 35 Time(min) Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer(AN/MA: 88/12)copolymer at various temperature 60 min. A temperature interval ranging from 220 to 270C was studied, and the scans were performed at heating rates of5,10and20℃c/min Chemical changes as measured by nitrile conve ersion were analyzed using Fourier transform infrared spectros- copy(FT-IR)techniques. The variation in nitrile con- 3 centration across large diameter fibers was studied by IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner 10C/man fibers were analyzed using an Endurance Foundation cmIn Diamond AtR and the nicolet Magna 550 The tensile properties of both fiber types were measured 2502 290310 at four different stages: as-spun fibers, after UV irradiation, Temperature (C after heat stabilization, and after carbonization. The single Fig. 3. DSC thermograms for VT polymer at different heating filament tensile tests were performed on approximately 20 rates fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell 4. Results and discussion 4.1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the 多 er samples to a set temperature, ranging from 220 to 270C, for I h. It was reasoned that if the precursor was thermally stable for approximately I h, it could be melt pun in a batch or a continuous extruder. As displayed in Fig 2, the polymer is very stable up to 230.C At 240C a slow reaction is initiated and a small amount of heat is evolved toward the end of the experiment. At 250C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins Te m perature (c) earlier and the reaction rate increases. These isothermal Fig. 4. Evaluation of the order of reaction from the shape of the tests indicated that the vt precursor can be maintained in a DSC curve
1402 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer (AN/MA: 88/12) copolymer at various temperatures. 60 min. A temperature interval ranging from 220 to 270 8C was studied, and the scans were performed at heating rates of 5, 10 and 20 8C/min. Chemical changes as measured by nitrile conversion were analyzed using Fourier transform infrared spectroscopy (FT-IR) techniques. The variation in nitrile concentration across large diameter fibers was studied by FT-IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner fibers were analyzed using an Endurance Foundation Diamond ATR and the Nicolet Magna 550. The tensile properties of both fiber types were measured at four different stages: as-spun fibers, after UV irradiation, after heat stabilization, and after carbonization. The single Fig. 3. DSC thermograms for VT polymer at different heating filament tensile tests were performed on approximately 20 rates. fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell. 4. Results and discussion 4 .1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the polymer samples to a set temperature, ranging from 220 to 270 8C, for 1 h. It was reasoned that if the precursor was thermally stable for approximately 1 h, it could be meltspun in a batch or a continuous extruder. As displayed in Fig. 2, the polymer is very stable up to 230 8C. At 240 8C a slow reaction is initiated, and a small amount of heat is evolved toward the end of the experiment. At 250 8C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins earlier and the reaction rate increases. These isothermal Fig. 4. Evaluation of the order of reaction from the shape of the tests indicated that the VT precursor can be maintained in a DSC curve
M C. Paina et al. / Carbon 41(2003)1399-1409 1403 Activation energies and rate constants determined by the Kissinger method [8] Precursor Activation energy erage reaction verage frequency (KJ/mol) factor(s) VT 112 7×10 0)FIbers L OOE-O1 8.00E02 E02 UV 2400E02 irradiated 2.00E02 一Mm 0.00E+00 360031002600210016001100600 mber(cm-1 b)VT Fibers 4.00E02 3.00E02 10OE02 -As spu 0.00E+00 38003400300026002200180014001000600 Fig. 5. ATR-FT-IR spectra of (a) M fibers and (b)VT fibers
M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 1403 T able 2 Activation energies and rate constants determined by the Kissinger method [8] Precursor f Tm Activation energy Average reaction Average frequency 21 (K/min) (K) (KJ/mol) order factor (s ) 12 M 5 553.6 157 1.0 3310 10 564.1 20 575.7 7 VT 5 571.5 112 1.0 7310 10 586 20 604 Fig. 5. ATR-FT-IR spectra of (a) M fibers and (b) VT fibers