M.S.A. Rahaman er al. Polymer Degradation and Stability 92 (2007)1421-1432 1423 Previously two important reactions occur during stabilize- tion process which can change the chemistry of PAn structure 40]. They are dehydrogenation and cyclization reactions as illustrated in Fig 3. Both are important to form ladder polymer structure which was thermally stable and might be able to withstand high temperature during pyrolysis process. In addi tion, stabilization process also could be present in oxidation reaction which gives an insight about diffusion of oxygen through the reacting polymer [41] Fig 4. Ladder PAN structure [26]- 2 Oxidation reaction he rings are formed. The dehydrogenation reactions have at ast two elementary steps, with oxidation in the first step The oxidation reaction during PAN-based precursor stabili- and elimination of water in the second. Studies have shown zation is the least reaction and is the step which most precur- that either the original pan polymer or cyclized ladder poly sors depend. Commercially, stabilization of PAn fiber is done mer can undergo dehydrogenation [43]. As a conclusion from in an"oxidizing medium which is typically air. The reaction Fig 3, the reactions are usually written in the form of Fig. 6 exotherm when PAN is stabilized in air is partly due to reac- Since oxygen is required for the reaction to proceed, dehydro- tion with oxygen. Although stabilization could be done in an genation does not occur in inert atmosphere. This is different inert atmosphere, a polymer back-bone containing oxygen- from the cyclization reaction. The double bond or unsaturated bearing groups that evolves in PAN ladder structure(Fig. 4) bond that formed in the reaction improves the polymers ther provides greater stability to sustain high temperature carbon- mal stability and reduces chain scission during carbonization ization treatment [42 Fitzer and Muller [43 have concluded that the activation energy and the frequency factor were greater in air than in ni- trogen(inert gas). This indicates that oxygen is an initiator for 23. Cyclization reaction the formation of activated center for cyclization because of the ncrease in the activation energy. Consequently, various struc The last reaction that would be discussed is cyclization tures of oxidized pAN that account for the presence of oxygen which is the most important reaction in the stabilization of have been proposed including those containing bridging ether PAN fiber. Cyclization is the reaction of the nitrile groups in links, those containing carbonyl groups, and those in which the precursor polymer with adjacent groups to form a stable, each nitrogen atom donates its lone pair of electron to an ladder polymer and could be described by first order kinetic oxygen(as shown in Fig. 5)5,] equation [43]. Cyclization is the most important reaction in stabilization process. The cyclization of the nitrile groups is 2. 2. Dehydrogenation process an exothermic reaction and that the evolution of gaseous prod ucts accompanies this reaction [46]. The reaction is necessary Dehydrogenation is the formation of double bonds that sta- to hold molecules in fiber together and increases the stiffness bilizes carbon chain and cyclization is the process by which [47-50]. In addition, the idea of cyclization was conceived by Cyclization CEN C=N CEN H2O Dehydrogenation Dehydrogenation Cyclization Fig. 3. Proposed chemistry of PAN stabilization 19, 40Previously two important reactions occur during stabiliza￾tion process which can change the chemistry of PAN structure [40]. They are dehydrogenation and cyclization reactions as illustrated in Fig. 3. Both are important to form ladder polymer structure which was thermally stable and might be able to withstand high temperature during pyrolysis process. In addi￾tion, stabilization process also could be present in oxidation reaction which gives an insight about diffusion of oxygen through the reacting polymer [41]. 2.1. Oxidation reaction The oxidation reaction during PAN-based precursor stabili￾zation is the least reaction and is the step which most precur￾sors depend. Commercially, stabilization of PAN fiber is done in an ‘oxidizing’ medium which is typically air. The reaction exotherm when PAN is stabilized in air is partly due to reac￾tion with oxygen. Although stabilization could be done in an inert atmosphere, a polymer back-bone containing oxygen￾bearing groups that evolves in PAN ladder structure (Fig. 4) provides greater stability to sustain high temperature carbon￾ization treatment [42]. Fitzer and Muller [43] have concluded that the activation energy and the frequency factor were greater in air than in ni￾trogen (inert gas). This indicates that oxygen is an initiator for the formation of activated center for cyclization because of the increase in the activation energy. Consequently, various struc￾tures of oxidized PAN that account for the presence of oxygen have been proposed including those containing bridging ether links, those containing carbonyl groups, and those in which each nitrogen atom donates its lone pair of electron to an oxygen (as shown in Fig. 5) [5,44]. 2.2. Dehydrogenation process Dehydrogenation is the formation of double bonds that sta￾bilizes carbon chain and cyclization is the process by which the rings are formed. The dehydrogenation reactions have at least two elementary steps, with oxidation in the first step and elimination of water in the second. Studies have shown that either the original PAN polymer or cyclized ladder poly￾mer can undergo dehydrogenation [43]. As a conclusion from Fig. 3, the reactions are usually written in the form of Fig. 6. Since oxygen is required for the reaction to proceed, dehydro￾genation does not occur in inert atmosphere. This is different from the cyclization reaction. The double bond or unsaturated bond that formed in the reaction improves the polymer’s ther￾mal stability and reduces chain scission during carbonization [45]. 2.3. Cyclization reaction The last reaction that would be discussed is cyclization which is the most important reaction in the stabilization of PAN fiber. Cyclization is the reaction of the nitrile groups in the precursor polymer with adjacent groups to form a stable, ladder polymer and could be described by first order kinetic equation [43]. Cyclization is the most important reaction in stabilization process. The cyclization of the nitrile groups is an exothermic reaction and that the evolution of gaseous prod￾ucts accompanies this reaction [46]. The reaction is necessary to hold molecules in fiber together and increases the stiffness [47e50]. In addition, the idea of cyclization was conceived by Fig. 3. Proposed chemistry of PAN stabilization [9,40]. Fig. 4. Ladder PAN structure [26]. M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432 1423