Effects of Gamma Ra di a tion on Various Polyimides

Jour nal of the Chi nese Chem i cal So ci ety, 2000, 47, 583-588 583 Effects of Gamma Ra di a tion on Various Polyimides Dan iel Chun-Hung Liu a ( ), Der-Jang Liaw b ( ), Been-Yang Liaw b ( ), Chih-Hung Tsai b ( ), Jinn-Hsuan Ho c ( ) and Tong-Ing Ho* c ( ) a Centro Tecnológico da Marinha em São Paulo-CTMSP, Av. Prof. Lineu Prestes, 2242 São Paulo, S.P., Brasil b De part ment of Chem i cal En gi neering, Na tional Tai wan Uni ver sity of Sci ence and Technolo gy, Taipei, Tai wan 106, Re pub lic of China c De part ment of Chem is try, Na tional Tai wan Uni ver sity, Tai pei, Tai wan 106, Re pub lic of China Four polyimides (PIa-PId) con tain ing 4,4 -hexafluoroisopropylidene-bisphthalic an hy dride (6FDA) units in the back bone were pre pared via one-step syn the sis. The ef fects of dif fer ent doses of gamma ra di a tion from a co balt-60 source are stud ied. The ir ra di ated and the unirradiated polyimides are char ac ter i zed by us ing ten sile strength, in fra red (IR), vis cos ity, dif fer en tial scan ning cal o rim e ter (DSC), and gel per me ation chro ma tog ra phy (GPC). All polyimides show the in crease of ten sile strength and elon ga tion at low doses (<350 Mrad). The mol - e cule weights de crease as the ir ra di a tion doses in crease. All sam ples showed a dras tic deg ra da tion of the me - chan i cal and ther mal prop er ties at doses above 1400 Mrad. Sam ples ir ra di ated at a dose of 2800 Mrad were com pletely brit tle. The ma jor dif fer ence of the ra di a tion ef fect of the polyimides de pends largely on their struc - tural dif fer ences. PId with an ex tra hexafluoroisopropylidene moi ety is most sen si tive to ir ra di a tion at high doses. IN TRO DUC TION The in ter ac tion be tween poly meric ma te ri als and ra di a - tion can pro voke pro found changes in the mo lec u lar struc ture of poly mers, al ter ing their phys i cal and chem i cal prop er ties. It is well known that this in ter ac tion re sults si mul ta neously in a mol e cule bond cross link ing and scission pro cess. De pending on its struc ture, a cross link ing pro cess or an other one is pre - dom i nant. And this oc cur rence is in ti mately re lated to the mo - lec u lar ar chi tec ture (chem i cal com po si tion, con fig u ra tion and con for ma tion). Nev er the less, in nu mer ous ap pli ca tions poly - meric ma te ri als are spe cif i cally needed in en vi ron ments of ion iz ing ra di a tion, due to the var i ous poly meric in her ent prop - er ties such as elas tic ity, light weight, easy formability, and low per me abil ity. In ad di tion, en gi neer ing poly mers pres ent me chan i cal and ther mal prop er ties as good as met als or ce - ram ics. All these ad van tages of poly meric ma te ri als have con - trib uted to the in crease of in ter est in ap pli ca tions in other prac ti cal fields. For ex am ple, in space ve hi cles, poly mers are used in a ra di a tion en vi ron ment. Nu clear power plants are an - other ex am ple where poly mers have been em ployed on a very large scale in a ra di a tion en vi ron ment. Other ap pli ca tions in - clude in dus trial fa cil i ties deal ing with ra dio ac tive sub stances, or em ploy ing ir ra di a tion for var i ous pur poses, such as ster il - iza tion of food or sew age treat ment. Fur ther more, a num ber of sys tems for stor age and dis posal of ra dio ac tive waste en tail en cap su la tion, or con tain ment us ing poly meric ma te ri als. 1-4 This pa per pres ents the ef fect of gamma ra di a tion on four dif fer ent polyimides, which con tain 6FDA units in the back bone; the polyimides are pre pared from re ac tions between an equimolar of a dianhydride and an ar o matic diamine. 5 The pres ence of 6FDA in ar o matic ether link ages poly mer main chains gen er ally have lower glass tran si tion and crys tal line melt ing tem per a tures, as well as sig nif i cant im - prove ment in sol u bil ity in or di nary or ganic sol vents, due to the re duc tion of the chain stiff ness. How ever, these ef fects do not mod ify sig nif i cantly the ther mal sta bil ity. 6-8 EX PER I MEN TAL Ma te rials N,N-Dimethylacetamide (DMAc), N,N-dimethyl form - amide (DMF) and pyridine were de hy drated by CaH 2 over - night and then dis tilled un der re duced pres sure. Ace tic an hy - dride was pur chased from Merck and was used as re ceived. 4,4 -hexafluoroisopropylidene bisphthalic an hy dride (6FDA) was recrystallized from ace tic an hy dride. 2,2-bis[4-(4- aminophenoxy)phenyl]pro pane (DAc) and 2,2-Bis[4-(4- aminophenoxy)-phenyl]hexafluoropropane (DAd) were sup - plied from Wakayama Seika Co. Ltd and used with out fur ther pu ri fi ca tion. 1,3-bis(4-aminophenoxy)ben zene (DAa) was pur chased from CHRISKEV and used as re ceived. 2,2 - dimethyl-4,4 -bis(4-amino-phenoxy)biphenyl (DAb) was syn the sized and pu ri fied ac cord ing to the method re ported. 8 The ir ra di a tion of polyimide sam ples was car ried out in air, us ing a co balt 60 source, at a dose rate of 0.7 Mrad/h to give re ceived doses of 80, 350, 700, 1400 and 2800 Mrad.

584 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Liu et al. Mea sure ments The four polyimides were char ac ter ized by ten sile strength, in fra red (IR), in her ent vis cos ity, Dif fer en tial Scanning Cal o rim e ter (DSC) to get the glassy tran si tion tem - per a ture (Tg) and Gel Per me ation Chromatograph (GPC) to ob tain the av er age mo lec u lar weight (Mn). IR spec tra were re - corded within the range of 4000-400 cm -1 for the poly mers on a JASCO IR-700 spec trom e ter. The in her ent vis cos ity of all poly mers was mea sured us ing a Ubbelohde viscometer at 30 o C. El e men tal Anal y sis was made on a Perkin-Elmer 2400 in - stru ment, with a scan ning rate of 3 deg. min -1. Dif fer en tial scan ning cal o rim e try (DSC) anal y sis was per formed with a dif fer en tial scan ning cal o rim e ter (Du Pont 2100) at a heat ing rate of 20 o C min -1. Mo lec u lar weight of the polyimides was mea sured by gel per me ation chro ma tog ra phy (GPC), us ing 0.1 mg of poly mer in 1 ml of Tetrahydrofuran (THF) so lu tion as an eluent, af ter cal i bra tion with the stan dard poly sty renes. Ten sile prop er ties were de ter mined by stress-strain curves, ob tained from a Orientec Tensilon de vice with a load cell of 10 kgf. A gauge of 3 cm and a strain rate of 2 cm. min -1 were used for this study. The mea sure ments were car ried out at room tem per a ture with film spec i mens of 0.5 cm wide, 6 cm long and thick ness of ap prox i mately 0.06 mm. Polyimide films were ex posed to a 60 Co source with dose rate of 0.7 Mrad/h and at room tem per a ture (28 C) ob tain ing dif fer ent doses (80, 350, 700, 1400, and 2800 Mrad). sev eral hours. RE SULTS AND DIS CUS SION In or der to mea sure the ten sile strength, the polyimides were pre pared in thin films by dis solv ing in N,N- dimethyl - acetamide (DMAc) sol vent and then evap o rated on a hor i zon - tal sur face in a oven at 70 o C. Figs. 2, 3 and 4 in di cate the ef - fects of dif fer ent doses of gamma ra di a tion on the change of the prop er ties of ten sile strength, elon ga tion and elas tic modu - lus of polyimides, re spec tively. Each value is an av er age value of five mea sure ments. This al lows us to have a gen eral view of the me chan i cal prop er ties and the ten dency as a func tion of ra - di a tion doses. One can see the dif fer ent be hav iors among ir ra - di ated and unirradiated polyimides, in di cat ing the in flu ence of gamma ra di a tion. How ever, at the dose of 2800 Mrad, the sam ples be came com pletely brit tle, im ped ing its ten sile strength eval u a tion, which meant a dras tic deg ra da tion of the ma te ri als. Fig. 2 shows the be hav ior of the ten sile strength of the four syn the sized polyimides as a func tion of the ir ra di a tion doses. The rea son why PIa shows (Fig. 2) de creas ing of ten sile strength at 80 Mrad ir ra di a tion might be due to the struc ture ef fect for PIa which con tains the meta disubstituted ben zene Poly mer iza tion 1.261 g (5 mmol) of diamine (DAa) and 10 ml of sol - vent DMAc were added to a 100 ml three-necked flask equipped with a mag netic stir rer, a re flux con denser, ther - mom e ter and ar gon in let. Un der ar gon at mo sphere, 2.220 g (5 mmol) of dianhyidride (6FDA) was added into the re ac tion mix ture, with vig or ous stir ring. Dur ing this time in ter val, an in crease of the vis cos ity of the so lu tion was ob served. In or der to fa cil i tate the stir ring of the so lu tion, the sol vent DMAc was added as nec es sary. The re ac tion re mained at room tem per a - ture for 3 hours. Dur ing this pe riod, polyamic acid was formed as shown in Fig. 1. In or der to cyclodehydrate the amic acid, 2 ml of pyridine and 4 ml of ace tic an hy dride were added to the re ac tion mix ture. The re ac tion mix ture was main tained at room tem per a ture for a pe riod of one hour, then heated up to 120 o C for an other two hours. The re ac tion mix ture was trick - led into ex cess meth a nol with vig or ous stir ring. The white pre cip i tate was col lected and washed thor oughly with hot wa - ter and meth a nol. Sub se quently, it was dried in an oven at 100 o C. In or der to form poly mer films, the poly mer was dis solved in DMAc (0.5 g of poly mer/10 ml of sol vent) form ing a yel - low so lu tion, then the so lu tion was poured in a 20-cm di am e - ter glass cul ture dish, and dried in a vac uum oven at 200 o C for Fig. 1. Scheme of re ac tions of four dif fer ent in ves ti - gated polyimides.

Gamma Ra di a tion on Polyimides J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 585 ring. At 80 Mrad ir ra di a tion, the struc tural ef fect may pre vent the for ma tion of semi-crystalline struc ture. How ever, PIa shows the nor mal ef fect at a higher ir ra di a tion dos age. It is noted that up to doses of 350 Mrad there is a slight in crease of the ten sile strength, then af ter 700 Mrad it tends to de crease sig nif i cantly. As men tioned above, the in ter ac tion be tween gamma ra di a tion and poly mer re sults si mul ta neously in a bond scission and a cross link ing pro cess. De pending on the struc ture of the poly mer, one or an other pro cess is pre dom i - nant. When the cross-link pro cess is dom i nant, it re flects an im prove ment of the prop er ties of the poly mer. This fact very com monly oc curs in a poly meric ma te rial. For in stance, up to ap prox i mately 0.2 Mrad, the cross-linking pro cess oc curs pre - dom i nantly in low den sity poly eth yl ene and for doses over 0.2 Mrad the deg ra da tion pro cess is pre dom i nant. The deg ra da tion of the me chan i cal prop er ties of the poly mer, at higher ir ra di a tion doses, can also be ex plained by the ir ra di a tion to be car ried out at a tem per a ture be low Tg of the amor phous poly mers; nor mally the vol a tile prod ucts of low mo lec u lar weight are likely to be trapped in the net of the poly mers, and the in ter nal stresses will cause post-irradiation de te ri o ra tion of the poly mer through crack ing. 9 Ac cord ing to Chapiro, 10 in radiolysis of or ganic com - pounds, gas evo lu tion, such as H 2, CO, CO 2 and CH 4, and dou - ble-bond for ma tion oc cur with com pa ra ble yields in most poly mers. The range of doses in which a given poly mer still keeps its valu able prop er ties de pends on sev eral fac tors, mainly the chem i cal struc ture of the poly mer. The elon ga tion prop erty is clearly shown to be af fected by gamma ra di a tion for the four stud ied polyimides. Note that above 700 Mrad, there is a dras tic de crease of this prop erty. Equally im por tant, this be hav ior in di cates the in crease of the brit tle ness of the ma te rial. These ef fects may be ex plained by the oc cur rence of deg ra da tion, mainly by an ox i da tive pro - cess. The in crease of the elas tic modu lus at lower doses is due to the change of the struc ture from an amor phous re gion to that of a semi-crystalline as a re sult of the tem per a ture ris ing of the ma te rial dur ing the ir ra di a tion. Fig. 5 pres ents the ef fect of the ra di a tion dose in the glassy tran si tion tem per a ture. One ob serves that the four poly - mers Tg de creased with an in crease of doses, in di cat ing the re duc tion of the mo lec u lar weight due to a scission mech a - nism, i.e., the deg ra da tion of poly mer. It is ob served that, up to the doses of 700 Mrad, the be hav ior of this prop erty var ies slightly and at doses of 1400 Mrad the prop er ties de cay more sig nif i cantly. Sim i larly, it is ob served that in the be hav ior of Fig. 2. The ef fect of gamma ra di a tion on the ten sile strength of polyimides:, PIa;, PIb;, PIc;, PId. Fig. 4. Modu lus of Elas tic ity of the polyimides as a func tion of doses:, PIa;, PIb;, PIc;, PId. Fig. 3. The elon ga tion of polyimides as a func tion of doses:, PIa;, PIb;, PIc;, PId. Fig. 5. Ef fect of ra di a tion on glass tran si tion tem per a - ture (Tg):, PIa;, PIb;, PIc;, PId.

586 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Liu et al. Table 1. Inherent Viscosity (dl/g) a Doses(Mrad) Polymer 0 80 350 700 1400 2800 PIa 1.20 0.90 0.80 0.81 0.51 * PIb 1.20 1.03 0.79 0.79 0.55 * PIc 0.92 0.74 0.85 0.75 0.57 * PId 0.83 0.77 0.71 0.62 0.43 * * not evaluated a In DMAc at 30 o C, c = 0.5 g/dl the vis cos ity prop erty, Ta ble 1, there is a dras tic re duc tion of vis cos ity val ues only at doses of 1400 Mrad; the re duc tion of the vis cos i ties is strongly re lated to the de creas ing of mo lec u - lar weight. This be hav ior is con sis tent with the re ported re - sults, where polyimides have been men tioned to be de graded at doses ap prox i mately of 1000 Mrad. 11 In fra red re sults The spec tra of IR shows that all the stud ied poly mers pre sented strong char ac ter is tic peaks of a typ i cal polyimide, such as near 1770 and 1720 cm -1, which cor re spond to asym - met ric and sym met ric stretch ing of the car bonyl group of imide, re spec tively. The band near 1370 cm -1 is at trib uted to the stretch ing of C-N (C-N-C) and, near band 1250 cm -1, it cor re sponds to C-O-C. Fur ther more, the tra di tional bands of poly(amic acid), an in ter me di ate prod uct of syn the sis of polyimides, did not ap pear in the spec tra, which can be ver i - fied in Fig. 6 for polyimide PIa. This fig ure also pres ents the spec tra of PIa ir ra di ated with doses of 1400 Mrad, and no ob - vi ous changes were ob served, mean ing that no changes oc - curred in re la tion to the func tional group of polyimides. This be hav ior is also ob served for the other polyimides stud ied (PIb, PIc, PId). How ever, we did not mea sure the IR spec tra of the polyimides at 2800 Mrad ra di a tion due to its com plete deg ra da tion. GPC results Gel per me ation chro ma tog ra phy (GPC) is one of the quick est meth ods to de ter mine the dis tri bu tion of the poly mer mo lec u lar weight. The ef fects of ra di a tion doses on the dis tri - bu tion of mol e cule weight for the four polyimides PIa-PId are re corded in Fig. 7, re spec tively. The num ber-average mo lec u - lar weight (Mn) of the four polyimides stud ied as a func tion of the ir ra di a tion doses is also shown in Ta ble 2. This shows the de pend ence of the chem i cal struc ture and the gamma ra di a - tion doses on the dis tri bu tion of the mo lec u lar weight upon ir - ra di a tion of dif fer ent doses. In gen eral, the mo lec u lar weights have re duced re mark ably with the in creas ing of the ra di a tion doses for all four polyimides stud ied. It is ob vi ous that the gamma ra di a tion has caused the polyimide to cleave. The scission pro cess re sulted in the deg ra da tion of the poly mer and this will also lower the Mn value. This ef fect re flects di - rectly in the poly mer prop er ties, which can be the orig i nal rea - son for the be hav iors of me chan i cal and ther mal prop er ties as dis cussed above. The dis cussed poly mer mol e cules flow through the po - rous bead of the col umn, so they can dif fuse into the in ter nal Fig. 6. IR Spec tra of polyimide PIa unirradiated (a) and ir ra di ated with 1400 Mrad (b). Fig. 7. The ef fect of ra di a tion doses on the mo lec u lar weight of polyimide PIa: a, 0 Mrad; b, 350 Mrad; c, 700 Mrad; d, 1400 Mrad; e, 2800 Mrad.

Gamma Ra di a tion on Polyimides J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 587 Table 2. Mn of Polyimides with Different Gamma Ray Doses (Mrad) Doses Polymer 0 80 350 700 1400 2800 PIa 52000 42000 18000 15000 13000 7000 PIb 58000 33000 26000 15000 7000 3000 PIc 43000 37000 32000 24000 10000 4000 PId 69000 47000 25000 11000 9000 5000 pore struc ture of the gel de pend ing on their size and pore-size dis tri bu tion of the gel. Larger mol e cules have more dif fi culty pen e trat ing into the pores so they flow di rectly through the col umn and get out within a short time in ter val. Smaller poly - mer mol e cules can pen e trate into the in te rior of the gel; there - fore they stay in side the gel lon ger. It is shown in Fig. 7 that with the doses of 1400 Mrad and 2800 Mrad, be sides the ma - jor ity of the deg ra da tion pro cess, there are also in di ca tions of larger mo lec u lar weight poly mer for ma tion due to the crosslinking pro cess for the polyimide PIa. This cross-linking pro - cess is also found for polyimide Pib, how ever to a lesser ex - tent. For polyimides PIc and PId this cross-linking high mo - lec u lar weight poly mer is sel dom found. Thus the pres ence of a dimethyl or di-(hexafluoro)methyl-methylene group in polyimides PIc and PId en hanced the cleav age of the mainchain of the poly mer re sult ing in a lesser cross-linking prop - erty. At lower ra di a tion doses (<700 Mrad), all polyimides (PIa-PId) show the ob vi ous de crease in mol e cule weight (Mn from 51874 at 0 Mrad to 14780 a 700 Mrad for PIa as an ex am - ple; the ef fect of ra di a tion doses on the Mn change is re corded on Ta ble 2). Thus the in crease of the ten sile strength (Fig. 2) and the elon ga tion prop erty (Fig. 3) are the re sult of the cross-linking which has oc curred at the de graded main-chain poly mer which do not show the in crease in the Mn of the polyimides (Ta ble 2). How ever from the GPC chart (Fig. 7) we ob serve the for ma tion of small new peaks at a re ten tion time at 20 min utes. These large mol e cule weight polyimides are formed by the cross-linking pro cess. Fig. 8 shows curves of the thermogravimetric anal y sis in air and ni tro gen at mo spheres of polyimides ir ra di ated at 2800 Mrad. All four polyimides pre sented ac cen tu ate weight loss at around 600 o C (873K) in both at mo spheres, which in di cates that all stud ied polyimides are ther mal-oxidative re sis tant. We have not re corded the TGA curve of PIc(N 2); how ever it shows less weight loss com pared to PIc(air) at above 900K. The rea son why PI(air) shows greater weight loss is due to the ox i da tive deg ra da tion ef fect of the air on the poly mers and the re sult of gas evo lu tion 10 in the pres ence of ox y gen. CON CLU SION The polyimides PIa-PId show sim i lar ra di a tion ef fects; the ten sile strength, elon ga tion and modu lus of elas tic ity are all in creased at lower doses. All of them are de graded sig nif i - cantly only at doses above 1400 Mrad. They pre sented good ther mal sta bil ity. Even for the sam ple ir ra di ated up to 2800 Mrad, their Tg are still above 220 o C. They also showed good ther mal ox i da tive sta bil ity, con sid er ing that the ir ra di a tion was car ried out in pres ence of air, and this is con sid ered the worst con di tion to the ir ra di a tion for any poly mers. Among the stud ied polyimides, the PId showed to be more sen si tive to the ra di a tion be cause in its in ves ti gated prop er ties, they showed higher dif fer ences of val ues be tween unirradiated and ir ra di ated with the high est dose sam ples. Our study can pro - vide the chem i cal struc tural in for ma tion for the search of ra di - a tion re sis tant poly mers. AC KNOWL EDG MENTS Fig. 8. Curves of thermogravimetric anal y sis of polyimides ir ra di ated at 2800 Mrad. The au thors thank the Na tional Sci ence Coun cil of the Re pub lic of China for all the sup port of this work, the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) of Brazil for fi nan cial sup port and Dra

588 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Liu et al. Luci Diva Brocardo Machado of Instituto de Pesquisas Energéticas Nucleares (IPEN) for TMA anal y sis. The au thors also thank Dr. P. S. Song and Dr. L. S. Hwang of the In sti tute of Nu clear En ergy Re search of ROCAEC for the gamma ra di - a tion. Re ceived Jan u ary 24, 2000. Key Words Gamma ray; Ra di a tion; Polyimides. REF ER ENCES 1. Clough, R. Ra di a tion-resistant Poly mers, En cy clo pe dia of Poly mer Sci ence and En gi neering, 1985; Vol. 13, p. 667. 2. In ter na tional Atomic En ergy Agency, Im mo bi li za tion of Low and In ter me di ate Level Ra dio ac tive Wastes with Poly mers, Vi enna, 1988. (Tech ni cal Re ports Se ries, 289). 3. Fornes, R. E.; Mem ory, J. D.; Naranong, N. J. Appl. Polym. Sci. 1981, 26, 2061. 4. Zhang, L. L.; Jiang, Q. T.; Jiang, L. X.; Cai, X. X. Poly mer In ter na tional 1996, 39, 289. 5. Wil son, M.; Stenzenberger, H. D.; Hergenrother, P. M. Polyimides, Blackies & Son, Glas gow and Lon don, 1990. 6. Schmidt, W.; Guo, D. Makromol. Chem 1988, 189, 2029. 7. Sinta, R.; Minns, R. A.; Gaudiana, R. A.; Rog ers, H. G. Macromolecules 1987, 20, 2374. 8. (a) Liaw, D. J.; Liaw, B. Y.; Jeng, M. Q. Poly mer 1998, 39, 1597. (b) Negi, Y. S.; Suzuki, Y. -I.; Kawamura, I.; Hagiwara, T.; Takahashi, Y.; Iijima, M.; Kakimoto, M. -A.; Imai, Y. J. Polym. Sci, Part A: Polym. Chem. 1992, 30, 2281. 9. O Donnel, J. H. in Ra di a tion Ef fects on Poly mers, ACS Sym po sium Se ries 475, Clough, R.; Shalaby, S. W., Eds.; ACS Wash ing ton, D. C., 1991; p 402. 10. Chapiro, A. Ra di a tion Chem is try of Poly meric Sys tems; John Wiley & Sons, 1962. 11. Scroog, C. E.; Endrey; A. L.; Abramo, S. V.; Berr, C. E.; Ed wards, W. M.; Olivier, K. L. J. Polym. Sci. A3, 1965, 1373.