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 re￾being 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 electron￾that 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 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 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 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 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]