Kai Yi, Qi Fang Li, Li Zhang, Ning Li, You Zhou, Seung Kon Ryu, Ri Guang Jin. Beijing University of Chemical Technology, Beijing CHINA

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1 iffusion Coefficiens of imehyl Sulphoxide (MSO) and H 2 O in PAN We Spinning and Is Influence on Morphology of Nascen Polyacrylonirile (PAN) Fiber Kai Yi, Qi Fang Li, Li Zhang, Ning Li, You Zhou, Seung Kon Ryu, Ri Guang Jin Beijing Universiy of Chemical Technology, Beijing CHINA Correspondence o: Kai Yi yikai926@gmail.com ABSTRACT Wespun nascen PAN fibers were immersed ino a MSO/H 2 O coagulaion bah. iffusion of MSO from nascen fiber and ha of H 2 O ino nascen fiber were sudied a differen emperaures and concenraions of coagulaion bah. The diffusion coefficien of H 2 O is larger han ha of MSO. As he coagulaion bah emperaure increased, he diffusion coefficiens of MSO and H 2 O increased. iffusion acivaion energy of MSO is smaller han ha of H 2 O during he diffusion. Cross secional and surface srucure of nascen PAN fiber were observed in relaion o coagulaion abiliy (he produc of diffusion coefficiens of wo solvens). Coagulaion abiliy was found o have a significan influence on boh he cross secional and surface morphology of nascen fiber. To obain nascen fiber wih circular cross secional and smooh surface morphology, coagulaion abiliy should be conrolled a low value. Keywords: iffusion coefficien, Coagulaion bah, Coagulaion abiliy, Cross Secional and surface srucure, Nascen PAN fiber. INTROUCTION We spinning is he main mehod of manufacuring PAN-based carbon fibers. Coagulaion of nascen PAN fibers exruded from a spinnere ino a coagulaion bah is a very imporan sage in we spinning [1-6]. This is no only a physical process, bu also a phase separaion process. uring coagulaion, mass and hea ransfer, and phase equilibrium beween fibers and coagulaion bah occurs and resuls in he precipiaion of gel srucure PAN fibers. There are wo kinds of concenraion differences beween fibers and coagulaion bah: 1) concenraion of solven (MSO) in he fibers is higher han in he bah, and 2) concenraion of H 2 O in he bah is higher han in he fibers. This leads o counercurren diffusion during he coagulaion of nascen PAN fibers. Therefore, coagulaion rae of nascen PAN fibers is dependen on counercurren diffusion. The moving boundary model [1], consan flow raio model [2], and Crank s equaion [3] are hree main mahemaical models o explain he counercurren diffusion in he coagulaion process. Among hem, Crank s equaion can be used o sudy diffusion coefficien and diffusion rae along he fiber radial direcion under cerain assumpions. iffusion coefficien and diffusion rae are imporan for describing he microsrucure of he fiber in we spinning [7]. A large diffusion coefficien causes quick diffusion as well as inense counercurren diffusion, which gives rise o an insufficien uniform srucure of nascen PAN fibers. However, a low diffusion coefficien indicaes slow and empered counercurren diffusion, which also gives rise o an insufficien coagulaion of nascen PAN fiber. Therefore, a moderae diffusion is very imporan for obaining a desired srucure of PAN fiber. However, here are few sudies on he calculaion of diffusion coefficien in counercurren diffusion during coagulaion of nascen PAN fibers and he relaionship beween diffusion and he srucure of PAN fiber. In his sudy, counercurren diffusion under differen coagulaion bah condiions is repored. By calculaing he diffusion coefficien of MSO and H 2 O, he opimal coagulaion condiions of nascen PAN fiber, and he influence of coagulaion abiliy on morphology of nascen PAN fiber is invesigaed. EXPERIMENTAL Synhesis of High Molecular Weigh PAN Maerials Acrylonirile (AN, Beijing Chemicals Reagens Co.) was disilled a 76 ~ 78 o remove inhibior in reagen. Mehacrylae (MA, analyical reagen) and iaconic acid (IA, analyical reagen) were used as he second and he hird monomer. Azobisisobuyronirile (AIBN, Shandong Xueyin Chemicals Co.), iniiaor, was purified by Journal of Engineered Fibers and Fabrics 107 hp://

2 recrysallizaion. imehyl sulphoxide (MSO, Liaoning Panjin Chemicals Co.) and deionized waer (H 2 O) were used as polymerizaion solvens. imehyl formamide (MF) was used o compare he inrinsic viscosiy of he polymer wih ha of MSO soluion. Polyvinyl alcohol (PVA, analyical reagen, Beijing Chemicals Reagens Co.) was used as dispersan. Synhesis of PAN Synhesis of high molecular weigh PAN was carried ou following he previous repor [8]. A mixure of 92 ml MF and 496 ml H 2 O (mass raio: MF/H 2 O=15/85) was added ino a 3 L polymerizaion reacor equipped wih sirrer, reflux condenser, circulaion and emperaure conrol devices. According o he weigh raio of AN: MA: IA = 96:3:1, a mixure of 298 ml AN, 7.89 ml MA, and 2.5 g IA was added o he reacor, and 0.25 g PVA was also added ino he reacor as a dispersan. Afer sirring for 1.5 h a 60, 2.5 g AIBN as an iniiaor was added, and polymerizaion was run for 3 h under he nirogen amosphere. Then, high molecular weigh PAN was obained. The viscosiy-average molecular weigh is 560,000 according o he Ubbelohde viscomeer. We Spinning A 150 g high molecular weigh PAN powder and 1227 ml MSO were added ino a reacor equipped wih waer heaing circulaion bah and spiral agiaor. Afer dissoluion and deaeraion, spinnable soluion conaining 10% high molecular weigh PAN was obained. This PAN soluion was we-spun o nascen PAN fiber hrough a spinnere wih 0.06 mm spinnere diameer and immersed ino a coagulaion bah, which was composed of 77.5% MSO and 22.5% H 2 O. The spin draw raio is se o -10%. Afer fiber immersing ino a coagulaion bah, mass and hea ransfer, and phase equilibrium movemen ake place beween fiber and coagulaion bah, resuling in he precipiaion of PAN fiber. Characerizaion Calculaion of iffusiviy The coagulaion bah was prepared by mixing MSO o H 2 O a weigh raios of 77.5:22.5, 70:30, and 65:35, respecively. The quaniy of MSO in he PAN fiber during he coagulaion was measured via he following procedure. Immerse nascen PAN fiber in boiling waer for 0.5 h, followed by S elemen measuremen in boiled waer by using ICPS-7500 Sequenial Plasma Emission Specromeer. Then he quaniy of PAN polymer was obained afer adequae drying. The quaniy of H 2 O was calculaed by subracing he quaniy of PAN and MSO from he oal mass. iffusiviy of H 2 O ( H2O ) and MSO ( MSO ) were calculaed according o Crank s equaion [3]. H2O 2 r c 16 c (1) Where, c is he H 2 O weigh in he PAN fiber a ime, c is he equilibrium weigh of H 2 O in he compleely coagulaed PAN fiber, r is he radius of he PAN fiber, and is he coagulaion ime. iffusiviy of solven MSO ( ) was also MSO calculaed from MSO weigh ouflow of he nascen PAN fiber wih ime inervals of in he coagulaion bah, c is he MSO weigh ouflow of nascen PAN fiber a ime, c is he equilibrium weigh ouflow of MSO in he compleely coagulaed nascen PAN fiber. Coagulaion Abiliy The rae of coagulaion process was deermined by double diffusion coefficiens; coagulaion abiliy (Ca) in his sysem can be defined as he produc of he wo diffusion coefficiens: Ca (2) MSO H2O Morphology Observaions Scanning elecron microscopy (SEM) was used o observe he cross secional and surface srucure of nascen PAN fiber. The dried nascen fiber was embedded in he mixure of a cerain proporion epoxy resin and curing agen, cured for 24 h a 60 and hen immersed in liquid nirogen for 10 s and fracured carefully. Afer spuered wih gold, cross secional morphology was observed. The surface morphology of nascen fiber was observed via Scanning Elecron Microscopy (SEM). RESULTS AN ISCUSSION Calculaion of iffusion Coefficiens Figure 1 shows he residual conens of MSO and H 2 O in PAN fiber a differen coagulaion imes under he coagulaion bah condiions of 70% concenraion, 40 emperaure, and -10% minus draw raio. Residual MSO conen decreases wih he ime, and becomes consan o 353 mg/g afer 20s, 60.8 % of MSO is exraced from he nascen fiber. Meanwhile, he amoun of H 2 O linearly increases up o 150 mg/g and becomes consan afer 24s. If =8s, under his coagulaion bah condiions, diffusion Journal of Engineered Fibers and Fabrics 108 hp://

3 coefficien of MSO and H 2 O are = 2.31 MSO m 2. s -1 and = m 2. s -1, H 2 O respecively. The diffusion coefficien of H 2 O is higher han ha of MSO. As we know, PAN does no dissolve in waer, bu PAN does dissolve in MSO. So he diffusion coefficien of MSO should be larger which doesn mach wih he resuls. I appears ha he diffusiviy of H 2 O ino he PAN fiber is based on is affiniy for MSO. Waer can hydrogen bond wih MSO, which is a proon accepor; furher, several H 2 O molecules can coordinae wih a single MSO molecule. In his way, he diffusion coefficien of H 2 O is higher han ha of MSO [9]. Since he coagulaion of nascen fiber is mainly achieved hrough molecular diffusion, as he coagulaion bah emperaure increases, mobiliy of molecular chain increases which is conducive o diffusion of MSO and H 2 O [10,11] and increase of diffusion coefficien of MSO and H 2 O. A he same emperaure, diffusion coefficien of MSO and H 2 O increases as MSO concenraion decreases in coagulaion bah. This is because he concenraion difference beween fiber and coagulaion bah for boh of MSO and H 2 O are larger a lower MSO concenraion. The diffusion is more likely o occur under larger concenraion difference. (a) (a) (b) FIGURE 1. Residual conens of (a) MSO and (b) H 2 O in nascen PAN fiber as a funcion of coagulaion ime. iffusion Coefficiens of MSO and H 2 O in Coagulaion Bah Figure 2 shows diffusion coefficien of MSO ( MSO ) and H 2 O ( H 2O ) as a funcion of emperaure from 40 o 70, MSO concenraion in he coagulaion bah varied from 65% o 77.5%. (b) FIGURE 2. iffusiviies of (a) MSO and (b) H 2 O as a funcion of emperaure in he range of 65 o 77.5% MSO in coagulaion bah. According o Fick s second law, 2 c c 2 x x (3) Journal of Engineered Fibers and Fabrics 109 hp://

4 From Eq. (3), E A exp R T E RT A ln (4) (5) Where, E is diffusion acivaion energy ( k cal / mol ), R is Bolzmann consan, T is absolue emperaure, and A is a consan. iffusion acivaion energies ( E ) of MSO and H 2 O are calculaed via Eq. (5), and lised in Table I. As he concenraion of MSO in he coagulaion bah increases, boh acivaion energies increase. This indicaes ha he diffusion happens difficully for boh of MSO and H 2 O and is also relaed o he decrease of concenraion difference. However, diffusion acivaion energy of MSO is lower han ha of H 2 O, indicaing he diffusion coefficien of MSO should be larger han H 2 O. As menioned above, PAN does no dissolve in waer which makes i have higher diffusion acivaion energy. TABLE I. iffusion acivaion energies of MSO and H 2 O diffusion ino nascen PAN fiber from coagulaion bah. Coagulaion bah concenraion (%) Acivaion energy of MSO ( k cal / mol ) Acivaion energy of H 2 O ( k cal / mol ) Noe: The concenraion of MSO in coagulaion bah is he coagulaion bah concenraion. The Influence of Coagulaion Abiliy on Srucure of Nascen PAN Fiber Figure 3 shows coagulaion abiliies of he coagulaion bah a differen condiions. They are calculaed from diffusion coefficien of MSO and H 2 O from Figure 2. The higher Ca, he greaer he abiliy of he coagulaion bah, and he resuling coagulaion process is more inense; he lower Ca, he weaker abiliy of coagulaion abiliy, and he coagulaion process is more moderae. According o Figure 3, a he same MSO concenraion of he coagulaion bah, he coagulaion abiliy increases wih he increase of coagulaion emperaure. As he coagulaion emperaure increases, he double diffusion coefficiens increase, which can be seen from Figure 2, and he coagulaion process can be compleed in a shorer ime. A he same coagulaion emperaure, he coagulaion abiliy decreases wih he increasing concenraion of coagulaion bah, indicaing ha he coagulaion bah wih lower concenraion has higher coagulaion abiliy. According o he coagulaion abiliy values, Figure 3 is divided ino hree regions. In order o direcly sudy he influence of coagulaion abiliy on he morphology of nascen fibers, hree nascen fibers obained from each cerain region were seleced as shown in Figure 3, heir cross secional and surface morphology were observed via SEM, as shown in Figure 4, and he bah condiions are also lised in Table II. FIGURE 3. Coagulaion abiliy of coagulaion bah as a funcion of emperaure in he range of 65% o 77.5% MSO concenraion. Journal of Engineered Fibers and Fabrics 110 hp://

5 Sample No. TABLE II. Bah condiions of hree seleced samples. Coagulaion bah concenraion (%) Coagulaion bah emperaure ( ) The Influence of Coagulaion Abiliy on Cross Secional Morphology of Nascen Fiber Coagulaion abiliy (m 4 /s 2 ) A B C (a) 77.5%,40 E F (b) 70%,60 G H I (c) 65%,70 FIGURE 4. SEM of cross secional srucure of nascen fiber: (a) 77.5%, 40 C, (b) 70%, 60 C, (c) 65%, 70 C. According o SEM of cross secional srucure of nascen fiber, as shown in Figure 4, he coagulaion abiliy has a significan influence on he cross secional srucure of nascen fiber. Under he bah condiion (a), he cross secional srucure of nascen fiber is compac and uniform, as shown in picure C; i can be expeced ha under his condiion, he coagulaion abiliy of coagulaion bah is weak, hus he coagulaion process is mild, he phase separaion ime is relaively long and he fiber srucure is compac and uniform. Under he bah condiion (b), iny holes formed in he cener of nascen fiber, as shown in picure F; his indicaes ha, as coagulaion abiliy increases, he coagulaion process is carried ou faser, he phase separaion ime is relaively shor and he fiber srucure is no as compac and uniform. Journal of Engineered Fibers and Fabrics 111 hp://

6 Under he bah condiion (c), he cross secional srucure of nascen fiber is non-uniform, as shown in picure I; under his condiion, he coagulaion abiliy is he highes, so ha he coagulaion process is very inense, he phase separaion ime is he shores and he cross secional srucure of nascen fiber is he mos non-uniform. Therefore, he use of high concenraion of coagulaion bah, can no only reduce he amoun of replacemen of MSO and H 2 O, bu also inhibi he diffusion. Consequenly, he cross secional srucure of nascen fiber is more likely o be compac and uniform. The Influence of Coagulaion Abiliy on Surface Morphology of Nascen Fiber during coagulaion process, as shown in picure. Under he bah condiion (c), he coagulaion abiliy value is he highes and he coagulaion process is he mos inense which resuls in more rough and non-uniform surface srucure formaion during coagulaion process, as shown in picure F. As menioned before, he morphology of nascen fiber obained in a high concenraion of coagulaion bah, is more likely o be compac and uniform. CONCLUSIONS A cerain MSO concenraions of he coagulaion bah, he diffusion coefficien of H 2 O is higher han ha of MSO, due o is affiniy for MSO. And he diffusion coefficiens of MSO and H 2 O increase wih he increasing of coagulaion bah emperaure. A cerain emperaures of he coagulaion bah, he diffusion coefficiens of MSO and H 2 O decreases wih he increase of coagulaion bah concenraion. A (a)77.5%,40 C (b)70%,60 B As he MSO concenraion of he coagulaion bah increases, diffusion acivaion energies of MSO and H 2 O increase. And E is lower han ha of H 2 O MSO due o he solubiliy of MSO on PAN. Moreover, according o SEM images, coagulaion abiliy has a significan influence on boh he cross secional and surface srucure of nascen fiber. As he coagulaion abiliy increases, he coagulaion process becomes faser and he cross secional and surface morphology of nascen fiber become less compac and uniform. Therefore, o obain nascen fiber wih circular cross secional and smooh surface morphology, coagulaion abiliy should be conrolled a lower values. E (c)65%,70 FIGURE 5. SEM of surface srucure of nascen fiber: (a) 77.5%, 40 C, (b) 70%, 60 C, (c) 65%, 70 C. According o he SEM of surface srucure of nascen fiber, as shown in Figure 5, he coagulaion abiliy also has grea influence on he surface srucure of nascen fiber. Under he bah condiion (a), he coagulaion abiliy value is he lowes and he coagulaion process is mild which can guaranee even double diffusion process and smooh surface srucure of nascen fiber, as shown in picure A and picure B. Under he bah condiion (b), he coagulaion abiliy increases, so ha he rae of double diffusion becomes higher which resuls in rough surface srucure of nascen fiber formaion F REFERENCES [1] Paul R.; iffusion during he coagulaion sep of we-spinning, Journal of applied Polymer Science, Vol12, No.3, 1968, [2] Rende A.; A new approach o coagulaion phenomena in we spinning, Journal of Applied Polymer Science, Vol16, No.3, 1972, [3] Chen H.; Qu R. J.; Liang Y.; Wang C. G.; Kineics of iffusion in Polyacrylonirile Fiber Formaion, Journal of Applied Polymer Science, Vol96, No.5, 2005, [4] Ziabicki A.; Fundamenals of Fiber Formaion, New York, Wiley, Journal of Engineered Fibers and Fabrics 112 hp://

7 [5] Wu W.; Paul R.; Time-Lag Technique of iffusion-coefficien Measuremen in Soluion Spinning, Texile Research Journal, Vol48, No.4, 1978, [6] Whie J. L.; Hancock T. A.; Fundamenal analysis of he dynamics, mass ransfer, and coagulaion in we spinning of fibers, Journal of applied Polymer Science, Vol26, No.9, 1981, [7] Wang Y. X.; Wang C. G.; Yu M. J.; Effecs of differen coagulaion condiions on polyacrylonirile fibers we spun in a sysem of dimehylsulphoxide and waer, Journal of Applied Polymer Science, Vol104, No.6, 2006, [8] J. Zhang, F. J. Bu, Y. Q. ai, L. W. Xue, Z. X. Xu, S. K. Ryu and R. G. Jin. Carbon Leers, Vol11, No.1, 2010, 22. [9] A. Beroluzza, S. Bonora, M. A. Baaglia, P. Moni. Journal of Raman Specroscopy, Vol8, No.5, 1979, [10] Knaul Jonahan z, Creber Kaherine A M. [J]. Journal of Applied Polymer Science, 66, 1997, [11] Liu C K, Cuculo J A. [J]. Journal of Polymer science: Par B: Polymer Physics, 29, 1991, AUTHORS ARESSES Kai Yi Qi Fang Li Li Zhang Ning Li You Zhou Seung Kon Ryu Ri Guang Jin Beijing Universiy of Chemical Technology Box 61, No.15, Eas Road of Bei San Huan, Chaoyang isric Beijing CHINA Journal of Engineered Fibers and Fabrics 113 hp://