Experimental and Theoretical Investigations of PAN Molecular Weight Increase in Precipitation Polymerization as a Function of H 2 O/DMSO Ratio

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1 Carb Letters Vl., N. March 00 pp. -7 Experimetal ad Theretical Ivestigatis f PAN Mlecular Weight Icrease i Precipitati Plymerizati as a ucti f H O/DMSO Rati Jig Zhag, egjig Bu, Ygqiag Dai, Liwei Xue, Zhixia Xu, Seug K Ryu 3 ad Riguag Ji,, State Key Labratry f Chemical Resurce Egieerig, Beijig Uiversity f Chemical Techlgy, Beijig, 0009, Chia Dept. f Chemistry ad Chemical Egieerig, Ayag Nrmal Uiversity, Hea, , Chia 3 Dept. f Chemical Egieerig, Chugam Natial Uiversity, Daeje , Krea ji.riguag@63.cm, skryu@cu.ac.kr (Received ebruary 8, 00; Accepted March 0, 00) Abstract The precipitati plymerizati f acrylitrile (AN) was carried ut i a mixture sluti f dimethyl sulfxide (DMSO) ad water at 50~65 C usig α,α -azbisisbutyritrile (AIBN) as a iitiatr. The icreased mlecular weight plyacrylitrile (PAN) was prepared with icreasig the H O/DMSO rati frm 0/90 t 80/0. The viscsity average mlecular weight f H O/DMSO slvet was 4.4 times larger tha that f H O/DM slvet, ad precipitati plymerizati was accelerlated due t the far decreased chai trasfer effect f DMSO. Based the experimetal results, the icreased PAN mlecular weight was regarded as the summati f tw mechaisms: i) particle-particle aggregati ad ii) particle-radical attachmet. The theretical equati derived frm the mechaisms was well cicided with the experimetal results shwig the liear relatiship betwee the viscsity average mlecular weight ad the H O/DMSO rati. Keywrds : PAN, Precipitati plymerizati, Mlecular weight icrease, H O/DMSO rati. Itrducti As a type f high-perfrmace irgaic fiber, carb fiber has lts f gd superirities such as specific stregth, high mdulus, lightfastess, crrsi resistace, fatigue, creep resistace, electrical cductivity, excellet heat trasfer perfrmace, ad etc. [,]. Whe temperature is as high as 000 C, carb fiber is the ly material that the stregth des t fall i a iert evirmet. It is well kw that the prperties f high perfrmace plyacrylitrile (PAN) based carb fiber are determied strgly by the ature f precursr, ad preparig high mlecular weight plymer is e f the effective ways t btai high stregth fiber [3-6]. I geeral, a secd mmer such as acrylic ester ad a third mmer such as itacic acid were added t mmer i rder t icrease the mlecular weight f PAN i sluti plymerizati. Recetly, precipitati plymerizati is icreasig the iterests i the preparati f high mlecular weight PAN due t the advatage f small chai trasfer reacti tha sluti plymerizati i usig a secd mmer [7-0]. Bahrami [] reprted the effect f H O/dimethyl frmamide (DM) the cversi rate ad itrisic viscsity durig precipitati plymerizati usig acrylitrile ad acrylic acid as mmers ad H O ad DM as mixed slvet system. Wag [] studied the effect f mmer ccetrati, iitiatr ccetrati, rati f H O/dimethyl sulfxide (DMSO) the precipitati plymerizati, ad suggested that precipitati plymerizati was mre effective tha sluti plymerizati i preparati f high perfrmace carb fibers. Zhag [3] als studied the effect f precipitati plymerizati cditis the cversi rate usig H O/DMSO mixture sluti. Wag [4] reprted the relatiship betwee apparet activati eergy ad the rati f H O/DMSO. Hwever, there are few studies the relatiship betwee mlecular weight icrease f PAN ad H O/DMSO rati, especially relatig t the theretical mechaism f PAN mlecular weight icrease. Therefre, the purpse f this study is experimetal ad theretical ivestigatis f PAN mlecular weight icrease i precipitati plymerizati as fuctis f H O/DMSO rati ad itrisic viscsity. T btai the purpse, i) precipitati plymerizati f acrylitrile was carried ut by chagig the slvet rati f H O/DMSO at differet temperature, ii) mlecular weight ad itrisic viscsity f the btaied plymer were measured i DM sluti, ad iii) mdel mechaism f precipitati plymerizati was suggested i relati betwee mlecular weight icrease f PAN ad H O/DMSO rati.

2 Experimetal ad Theretical Ivestigatis f PAN Mlecular Weight Icrease i Precipitati Plymerizati Experimetal.. Materials Acrylitrile (AN, Beijig Chemical Reagets C.) was freed frm ihibitrs by distillati at 76~78 C befre plymerizati. α,α -azbisisbutyritrile (AIBN) was purified by recrystallizati. Dimethyl sulfxide (DMSO) ad deiized water (H O) were used as plymerizati slvets. Dimethyl frmamide (DM) sluti was used t cmpare the itrisic viscsity f the plymer with that f DMSO sluti... Precipitati plymerizati SEM images f PAN btaied frm HO/DMSO rati f (a) 0/90 ad (b) 80/0 i precipitati plymerizati at 55C fr h. ig.. Precipitati plymerizati f AN was carried ut i a.0 L three-ecked glass reactr by addig 0.3 L differet H O/DMSO rati frm 0/90 t 80/0 (wt/wt) as slvet t 0. L AN fr h at 50, 55, 60 ad 65 C uder pure itrge atmsphere. After plymerizati, eugh distilled water was added it the reactr t precipitate the prduced plymer. The precipitated plymer was washed with distilled water ad ethal successively, filtered with Bucher fuel, ad dried uder vacuum at 40 C till a cstat weight was btaied..3. Itrisic viscsity measuremet Itrisic viscsity [η] f the btaied plymers was measured i DM sluti usig a Ubbelhde viscmeter i a water bath at cstat temperature f 35 ± 0. C fllwig the Kashyap s methd [5]. The viscsity average mlecular weight Mη was calculated frm the fllwig equati [6]: [η].78 0 Mη () Results ad Discussi ig. shws the SEM images f PAN btaied frm H O/ DMSO rati f 0/90 ad 80/0 i precipitati plymerizati at 55 C fr h. The mrphlgy f PAN btaied frm small rati f H O (0/90) shws lsely aggregated large particles. While that f btaied frm 80/0 shws big spheres cmpsed f tightly aggregated small particles. These images suggested that there might be immese umber f micr-drplet mmers i 80/0 rati slvet which was divided by H O at the begiig f precipitati plymerizati. Ad these immese micr-drplet mmers prceeded the precipitati plymerizati, the geerated plety f precipitati particles, fially resulted i the icrease f itrisic viscsity ad viscsity average mlecular weight. The size ad shapes f aggregati were depeded the H O/DMSO rati, ad higher mlecular weight PAN precursr ca be btaied frm icreasig the H O/DMSO rati. ig. shws the viscsity average mlecular weights f PAN as a fucti f H O/DMSO rati btaied at 50, 55, 60 ad 65 C. The viscsity mlecular weight liearly icreased as the icrease f H O/DMSO rati frm 0/90 t 80/0. This was because the umber f micr-drplet mmer icreased as the icrease f H O/DMSO rati, ad the icreased micr-drplet were aggregated util they became big precipitati particles, resulted i the mlecular weight icrease, which made the sluti mre turbid ad viscus. Wag [] ad Zhag [3] have reprted the similar results the effect f H O/DMSO rati. O the ther had, the viscsity average mlecular weight decreased as the icrease f reacti temperature at the fixed rati f H O/DMSO. This was believed high 3.. Experimetal study Relatiship betwee HO/DMSO rati ad viscsity average mlecular weights f PAN. ig..

3 4 Jig Zhag et al. / Carb Letters Vl., N. (00) Theretical study Based the experimetal results, summati f tw mechaisms, particle-particle aggregati ad particleradical attachmet ca be suggested i the frmati f the icreased mlecular weight PAN. Mechaism I was regarded as a particle-particle aggregati. I the precipitati plymerizati f acrylitrile, the btaied plymer was t disslved i the H O/DMSO mixture sluti. Oce it was geerated, it became ascet particle ad suspeded i the sluti. As the cre, the ascet particle aggregated with ther particles util it became a larger particle i the precipitati plymerizati. This prcess ca be regarded as -dimesial aggregati as shw i ig. 4. The umber f micr-drplet mmer icreased as the icrease f H O/DMSO rati due t the divisi f drplet by H O, which made the sluti mre turbid ad viscus. rm this csiderati, the fllwig equati ca be prpsed. () [P] [ ] Where, [P] is the ccetrati f the ascet precipitati particle, ad fllwig hypthesis ca be suggested. (P) is the umber f aggregated mmers i multi-dimesial directi i precipitati plymerizati. Whe the ascet particle aggregated with ther e particle, (P) will be prprtial t [P] shwig the fllwig equati. (3) k [ ] ( ) Where, (P) is the umber f aggregated mmers i multi-dimesial directi i precipitati plymerizati, : aggregati rate cstat, i : dimesi, j : serial umber f particles. Whe the ascet precipitati particle aggregated with ther tw particles, (P) will be prprtial t the square f Viscsity average mlecular weights f PAN frm HO/ DMSO ad HO/DM slvet i precipitati plymerizati at 60C. ig. 3. H O reacti temperature prmted the suspesi f micrdrplet rather tha particles aggregati, resulted i the decrease f viscsity. ig. 3 shws the viscsity average mlecular weights f the PAN as a fucti f H O/DMSO ad H O/DM rati btaied frm precipitati plymerizati at 60 C. The viscsity average mlecular weight f H O/DMSO slvet was 4.4 times larger tha that f H O/DM slvet. Bahrami [] reprted very similar data t thse f ur H O/DM shw i ig. 3. The higher value f H O/DMSO system was due t the decreased chai trasfer effect f the DMSO. The chai trasfer cstat f water is almst zer ad that f DMSO is much less tha DM [0]. Therefre, H O/ DMSO slvet system is far better tha H O/DM system fr the precipitati plymerizati t btai icreased mlecular weight f PAN. ig. 4. DMSO P P kij Aggregati f ascet PAN particles i precipitati plymerizati.

4 Experimetal ad Theretical Ivestigatis f PAN Mlecular Weight Icrease i Precipitati Plymerizati... 5 [P], ad the aggregati rate cstat f the secd particle will be half f the first particle i the ctributi f aggregati. Therefre, the (P) ca be expressed as the fllwig equati. k ( P) k [ (4) P ] k k [ P]! Whe the ascet precipitati particle aggregated with ther three particles, (P) will be prprtial t the cube f [P], ad the aggregati rate cstat f the third particle will be e-third f the first particle i ctributi f aggregati. k ( P) k 3 k 3 3 [ (5) 3 P ] 3 k 3 k 3 k [ P ] 3 Whe the ascet precipitati particle aggregated with ther particles, (P) will be prprtial t [P], ad the aggregati rate cstat f the th particle will be e- th f the first particle i ctributi f aggregati. ( P) k k ( ) k k ( ) [ P ] k k ( ) k k [ P ] (6) If there was particle t aggregate, (P) will be prprtial t [P] 0, ad expressed as the fllwig equati. ( P) 0 k 00 [ p] 0 k 00 (7) Therefre, all the aggregated ascet particles will be the summati f (P) + (P) (P), ad the relatiship betwee (P) ad [P] ca be btaied as fllws. ( P) ( P) i k k 00 k [ P] k [ P]! i k 3 k 3 k [ P ] 3 k k ( ) k k [ P ] (8) If each ascet precipitati particle has same ability fr precipitati ad k ij is the fucti f temperature, it ca be regarded t k ij k ji k 00 k k, ad the equati (8) ca be simplified as fllws. ( P) ( P) i k + k[ P] [ P] k [ P]! i The equati is exactly part f the Taylr series expasi, s the equati (9) ca be chaged as the fllwig frm: ( P) e K [ P ] (9) (0) Therefre, the ttal aggregated umber f ascet precipitati particles is ( P) e K P [ ] Q () Where Q is the quatity f precipitates ad K is the k ig. 5. Attachmet f particles t PAN free radicals i precipitati plymerizati. verall aggregati rate cstat. Mechaism II was regarded as particle-radical attachmet. Whe the weight rati f H O/DMSO was small, the umber f ascet precipitati particle will be limited. The prpagati radical (M ) ca maily mve i a large rage i DMSO slvet, resulted i the reacti easily termiate ad viscsity average mlecular weight (M η ) small. Whe the rati f H O/DMSO icreased, the umber f ascet precipitati particles als rapidly icreased. I this prcess, each prpagati radical will be sited everywhere f ascet precipitati particles. These prpagati radicals ly react i DMSO, ad the prpagati radicals may t easily mve with the decrease f DMSO. Therefre, the termiati pprtuity f adjacet tw prpagati radicals will be reduced. I this case, prpagati radical will have much mre pprtuity t react ad frm lager mlecule. ially, mlecular weight will be icreased cstatly as the icrease f H O/DMSO rati as shw i ig. 5. Based the physical ccept abve, it ca be put frward a mdel similar t the previus. Whe the ascet particle attached t e ther particle, (M ) will be prprtial t [M ] ad expressed as the fllwig equati. ( M. ) k [ M. ] () Where, (M ): umber f attachmet mmer i multidimesial directi i precipitati plymerizati,

5 6 Jig Zhag et al. / Carb Letters Vl., N. (00) -7 k : attachmet rate cstat, i : dimesi, j : serials umber f particles. Whe the ascet particle reacted with tw ther particles, (M ) will be prprtial t the square f [M ], ad the attachmet rate cstat f the secd particle will be half f the first particle i the ctributi f attachmet. ( M. ) k k [ (3) M. ] k k [ M. ]! Whe the ascet particle attached t three ther particles, (M ) will be prprtial t the cube f [M ], ad the attachmet rate cstat f the third particle will be ethird f the first particle i ctributi f attachmet. ( M. ) k 3 k 3 k 33 [ 3 M. ] 3 k 3 k 3 k [ M. ] 3 (4) Whe the ascet mmer particle attached t ther particles, (M ) will be prprtial t [M ], ad the attachmet rate cstat f the th particle will be e- th f the first particle i ctributi f attachmet. ( M. ) k k ( ) k k ( ) [ M. ] k k ( ) k k [ M. ] (5) If there is mmer particle fr attachmet, (M ) will be prprtial t [M ] 0. ( M. ) 0 k 00 [ M. ] 0 k 00 (6) Therefre, all the attached ascet mmer particles will be the summati f (M ) + (M ) + + (M ) ad the relatiship betwee (M ) ad [M ] ca be btaied as fllws. ( M. ) ( M. ) i k 00 k [ M. k ] k [ M. ] +! i k 3 k 3 k [ M. ] 3 k k ( ) k k [ M. ] (7) If each first br particle has same ability fr precipitati ad k ij is the fucti f temperature, it ca be regarded as k ij k ji k 00 k k, ad the equati (7) ca be simplified as fllw as : ( M. ) ( M. ) i k + k[ M. ] [ M. ]! i k [ M. ] (8) The equati (8) was exactly part f the Taylr series expasi, s it ca be chaged as the fllwig frm: k ( M. ) e k M. [ ] (9) The ttal attached mmer f ascet precipitati particles are ( M. ) e K M. [ Q ] (0) Where Q is the quatity f precipitates ad K is the verall attachmet rate cstat. Therefre, frm () ad (0): e K [ P ] e K M. [ ] Takig lgarithm bth sides f the equati: [ P] [ M. ] Mη () () The viscsity average mlecular weight ca be calculated by the summati f () ad (), ad the fial equati ca be suggested as fllws. [ Mη] K 3 + K 4 [ H O DMSO] K Where K 3 is iversely prprtial t K ad K (3) rm (3), it ca be fud that there is a liear relatiship betwee the viscsity average mlecular weight ad the H O/DMSO rati. Therefre, the theretical equati derived frm the mechaism was well cicided with the experimetal results shwig the liear relatiship betwee the viscsity average mlecular weight ad the H O/DMSO rati. 4. Cclusis I the precipitati plymerizati PAN mlecular weight icrease was depeded the icrease f H O/DMSO rati due t the icreased micr-drplets, which made the sluti mre turbid ad viscus, resulted i the mlecular weight icrease. Usig H O/DMSO slvet system was far better tha usig H O/DM system i precipitati plymerizati ad btaied icreased mlecular weight PAN precursr because f decreased chai trasfer effect f DMSO. Based the experimetal results, the icreased mlecular weight f PAN was regarded as the summati f tw mechaisms: particle-particle aggregati ad particleradical attachmet. The theretical equati derived frm the mechaisms ca be used t estimate the viscsity average mlecular weight f the ewly plymerized plyacrylitrile (PAN) precursr. Refereces [] Wu, S. Z. Plym. Adv. Techl. 005, 6, 64. [] Bajaj, P. Idia J. f Appl. Plym. Sci. 00, 79, 640. [3] H, C. T.; Chug, Carb 990, 8, 83. K

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