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
1404 M C. Paina et al. / Carbon 41(2003)1399-1409 molten state for a sufficiently long duration( I h)below the order of the reaction was obtained from Eqs. (3)and 240C to allow further melt spinning (4), where a and b were estimated as described in Fig 4 From the dsc thermograms obtained at different heat- ing rates(5-20C/min), Fig. 3, it was observed that the maximum of the reaction peak was dependent on heating In k=In p +In rate. Based on these data, the Kissinger method [14 was used to estimate reaction kinetics parameters. The activa- tion energy was derived from the temperature dependence S=b of the peak maximum on the heating rate, as described by q(1) n=126√S The results for kinetic parameters, summarized in Table 2, suggest that the activation energy is smaller for the Vt (1) polymer than that of M polymer. This would be expected because the Vt polymer has a higher methyl acrylate content, and methyl acrylate is known to act as an initiator The frequency factors were calculated using Eq (2), and for the cyclization reaction [6]. However, the frequency M Fibers,lCN川H(CH2) spun VT Fibers, I(C N)/(CH2) A∈6pun Uv25h uv ht Fig. 6. Nitrile ratio for samples treated at various conditions, as determined by FT-IR-ATR spectroscopy
1404 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 molten state for a sufficiently long duration (| 1 h) below the order of the reaction was obtained from Eqs. (3) and 240 8C to allow further melt spinning. (4), where a and b were estimated as described in Fig. 4. From the DSC thermograms obtained at different heating rates (5–20 8C/min), Fig. 3, it was observed that the E f E maximum of the reaction peak was dependent on heating ln k 5 ln ] 1 ln ]1 ]] (2) 2 R RT T m m rate. Based on these data, the Kissinger method [14] was used to estimate reaction kinetics parameters. The activa- a S 5 ] (3) tion energy was derived from the temperature dependence b of the peak maximum on the heating rate, as described by ] Eq. (1): n 5 1.26ŒS (4) ] f The results for kinetic parameters, summarized in Table d ln S D2 T m E 2, suggest that the activation energy is smaller for the VT ]]]5 2 ] (1) 1 R polymer than that of M polymer. This would be expected dS D ] because the VT polymer has a higher methyl acrylate Tm content, and methyl acrylate is known to act as an initiator The frequency factors were calculated using Eq. (2), and for the cyclization reaction [6]. However, the frequency Fig. 6. Nitrile ratio for samples treated at various conditions, as determined by FT-IR-ATR spectroscopy
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 graph
M.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 stabiInitial 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 dehydroindicated 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 cyclizalinking. 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 endecreased 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
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 in
1406 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 (temperature and intensity of UV radiation). It is emphasized, 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 penetration 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 stabilization, 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 VTand 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 considersignificant 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
M C. Paina et al. / Carbon 41(2003)1399-1409 1407 (a) (b) Fig 9. SEM micrographs of carbonized fibers obtained from(a)M, and(b)VTa tensile strength results primarily from the critical flaws eems reasonable to anticipate better mechanical properties present in these fiber samples prepared by a batch pre or the v fibers if the molecular weight and orientation In contrast, the flaw-insensitive property, tensile mod can be improved, and the polymer and fibers are produced is not very different for the two types of fibers that have by a continuous process undergone stabilization. It is noted that fibers that were only UV-treated did not depicts the importance of the survive the carbonization step. However, fibers that re- properties of the fibers as related to those of the ceived thermal oxidation treatment after uv irradiation final fibers. The could be successfully carbonized, i., thermal oxidation is orientation in M fibers that went through a post-drawing necessary for fiber stabilization, Table 3 summarizes the step(whereas the VT fibers did not) result in better tensile properties of the M fibers that were stabilized using properties for M fibers. Also, as illustrated in micrographs the dual Uv-thermal treatment and subsequently carbon- of Fig. 9, the larger number and size of flaws in the Vt ized (M. to M,). The results indicate that the introduction fibers manifested into low strength and strain-to-failure. It of a UV irradiation step does not decrease the final Tensile properties of the M fibers Uv irradiated, heat oxidized and carbonized at 1500C No. tests Strain-to-failure (MPa 9.2±0.3 188±26 10±0.4 MMMMMvM 8.8±0.3 674±250 6±17 10±0.3 8.5±0.3 143+28 1.1±0.3 8.6±0.2 020±320 06±30 1.1±0.2 7.1±0.3 15.7+0.7 34+73 0.6±0.1 4410-4900 235-295 1.5-2.0 Range of values from the technical data sheet of Mitsubishi rayon, for a standard PAN-based carbon fiber
M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 1407 Fig. 9. SEM micrographs of carbonized fibers obtained from (a) M and (b) VT . 2 4 tensile strength results primarily from the critical flaws seems reasonable to anticipate better mechanical properties present in these fiber samples prepared by a batch process. for the VT fibers if the molecular weight and orientation In contrast, the flaw-insensitive property, tensile modulus, can be improved, and the polymer and fibers are produced is not very different for the two types of fibers that have by a continuous process. undergone stabilization. It is noted that fibers that were only UV-treated did not Fig. 10a also clearly depicts the importance of the survive the carbonization step. However, fibers that reproperties of the as-spun fibers as related to those of the ceived thermal oxidation treatment after UV irradiation final fibers. The higher molecular weight and degree of could be successfully carbonized, i.e., thermal oxidation is orientation in M fibers that went through a post-drawing necessary for fiber stabilization. Table 3 summarizes the step (whereas the VT fibers did not) result in better tensile properties of the M fibers that were stabilized using properties for M fibers. Also, as illustrated in micrographs the dual UV-thermal treatment and subsequently carbonof Fig. 9, the larger number and size of flaws in the VT ized (M to M ). The results indicate that the introduction a d fibers manifested into low strength and strain-to-failure. It of a UV irradiation step does not decrease the final T able 3 Tensile properties of the M fibers UV irradiated, heat oxidized and carbonized at 1500 8C Sample No. tests Diameter Max. strength Modulus Strain-to-failure (mm) (MPa) (GPa) (%) M 10 9.2 a 60.3 10306260 188626 1.060.4 M 9 8.8 b 60.3 6746250 136617 1.060.3 M 16 8.5 c 60.3 6906230 143628 1.160.3 M 18 8.6 d 60.2 10206320 206630 1.160.2 M 22 7.1 2 60.3 12636309 181620 0.760.2 VT 18 15.7 4 60.7 334673 5866 0.660.1 M – 7 4410–4900 235–295 1.5–2.0 *commercial * Range of values from the technical data sheet of Mitsubishi Rayon, for a standard PAN-based carbon fiber
1408 M.C. Paiva et al. Carbon 41(2003)1399-1409 a)As spun fibe c) Heat oxidized fibe 300 %o stra b) uv irradiated fibers d)Carbonized fibers M 100 %o strain Y strain Fig. 10. The effect of UV irradiation, thermal oxidation, and carbonization on the tensile properties of M and VT fibers properties of the carbon fibers obtained. Comparison of the Acknowledgements mechanical properties of M. Ma and M, shows that this conclusion is valid for fibers stabilized at constant load as The authors well as constant length. from the Department of Energy, through grant No 4500011036 work made use of erc shared Facilities supported by the National Sc cience under Award No. EEC-9731680 5. Conclusions This study establishes the feasibility of producir References carbon fibers from melt-spinnable polyacrylonitrile co- olymers. DSC and solubility tests performed on PAN- [l Grassie N, Hay JN, McNeill IC. Thermal coloration and ret-spun fibers and on experimental melt-spun fib insolubilization in polyacrylonitr showed that UV irradiation effectively crosslinked the copolymers to an extent that enabled subsequent thermal 22] Edie DD. The effect of processing on the structure and oxidation. The experimental fibers could also be success- properties of carbon fibers. Carbon 1998, 36(4): 345-62 ly carbonized. Although the carbon fibers exhibited low 3]Davidson JA, Jung H-T, Hudson SD, Percec S Investigation mechanical properties, there is a potential to substantially f molecular orientation in melt-spun high acrylonitrile fibers, Polymer 2000, 41: 3357-64 improve these properties by optimizing polymer structure 4 Jokarsky R, Ball LE, Wu MM, Uebele CE. Melt spun (copolymer composition and molecular weight) and melt rylonitrile olefinically unsaturated fibers and a process to pinning conditions to form smaller diameter precursor make fibers. US Pat 6, 114, 034(2000) fibers with higher degrees of molecular orientation 5] Brandrup J, Peebles Jr. LH. On the chromophore of poly
1408 M.C. Paiva et al. / Carbon 41 (2003) 1399–1409 Fig. 10. The effect of UV irradiation, thermal oxidation, and carbonization on the tensile properties of M and VT fibers. properties of the carbon fibers obtained. Comparison of the Acknowledgements mechanical properties of M M and M shows that this a, d 2 conclusion is valid for fibers stabilized at constant load as The authors gratefully acknowledge the financial support well as constant length. from the Department of Energy, through grant No. 4500011036. This work made use of ERC Shared Facilities supported by the National Science Foundation under Award No. EEC-9731680. 5. Conclusions This study establishes the feasibility of producing References carbon fibers from melt-spinnable polyacrylonitrile copolymers. DSC and solubility tests performed on PAN- [1] G rassie N, Hay JN, McNeill IC. Thermal coloration and based wet-spun fibers and on experimental melt-spun fibers insolubilization in polyacrylonitrile. J Polym Sci 1962;56:189–202. showed that UV irradiation effectively crosslinked the [2] E die DD. The effect of processing on the structure and copolymers to an extent that enabled subsequent thermal properties of carbon fibers. Carbon 1998;36(4):345–62. oxidation. The experimental fibers could also be success- [3] D avidson JA, Jung H-T, Hudson SD, Percec S. Investigation fully carbonized. Although the carbon fibers exhibited low of molecular orientation in melt-spun high acrylonitrile mechanical properties, there is a potential to substantially fibers. Polymer 2000;41:3357–64. improve these properties by optimizing polymer structure [4] J okarsky RJ, Ball LE, Wu MM, Uebele CE. Melt spun (copolymer composition and molecular weight) and melt acrylonitrile olefinically unsaturated fibers and a process to spinning conditions to form smaller diameter precursor make fibers. US Pat 6,114,034 (2000). fibers with higher degrees of molecular orientation. [5] B randrup J, Peebles Jr. LH. On the chromophore of poly-