Synthesis of Highly Refractive Polyimides Derived from 2,5- Bis(4-aminophenylsulfanyl)-1,4-dithiane and Dianhydrides

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1 Journl of Photopolymer cience nd Technology Volume 21, Numer 1 (28) CPT ynthesis of Highly Refrctive Polyimides Derived from 2,5 Bis(4minophenylsulfnyl)1,4dithine nd Dinhydrides Ysuo uzuki, Ysuhiro Nkmur, hinji Ando, nd Mitsuru Ued* Deprtment of rgnic nd Polymeric Mterils, Grdute chool of cience nd Engineering, Tokyo Institute of Technology, 2121, okym, H12, Meguroku, Tokyo , Jpn ued.m.d@m.titech.c.jp Highly refrctive nd trnsprent polyimides (PIs) sed on 2,5is(4minophenylsulfnyl) 1,4dithine (BADT) hve een developed. BADT ws polymerized with two dinhhydrides such s 1,2,3,4cycloutnetetrcroxylic dinhydride (CBDA) nd 4,4'[pthiois(phenylenesulfnyl)]diphthlic nhydride (3DEA) to fford two PIs. The PI of 3DEA/BADT exhiited the glss trnsition temperture t 156 o C nd the 5 % weight loss tempertures (T 5% ) t 31 ºC. The 1,4dithine moiety endowed the PI films with good opticl trnsprency with high refrctive indices. The opticl trnsmittnce of the PI film from CBDA nd BADT t 4 nm ws higher thn 9% for the thickness of 1. µm. Furthermore, the 1,4 dithine moiety nd flexile thioether linkges in the moleculr chins of the PIs provided them with high refrctive indices of nd smll irefringence of Keywords: Polyimide; High Refrctive Index; 1,4dithine; ulfurcontining Monomers 1. Introduction High refrctive index (highn) polymers hve een incresingly desired in mny hightech opticl fields s functionl components, such s photoresists for 193 nm immersion lithogrphy [1], cotings for longperiodgrting refrctive index sensors [2], sustrtes for dvnced orgnic electroluminescent devices [3], nd functionl cotings for CM imge sensors (CI) [4]. Conventionl highn opticl polymers such s sulfurcontining epoxy [5], poly(methyl methcrylte) (PMMA) [6], nd polyurethne [7] cn meet most of the requirements. However, the reltively poor therml stility of the polymers often limits their wide pplictions. Furthermore, the conventionl highn polymers usully exhiit refrctive indices of round 1.6, which re not high enough for CI pplictions. Polyimide (PI) is one of the most successful cndidtes used in dvnced opticl frictions [8]. As highn polymers, PIs possess mny chrcteristics, including inherent high refrctive index, good comintion of therml, mechnicl, nd dielectric properties [912]. However, the lrge irefringence nd deep colortion of the conventionl PIs hve to e ddressed s opticl mterils. In our previous works, series of sulfurcontining highn PIs hve een developed [1316]. However, the opticl trnsprency of the sulfurcontining PI films in the visile light region is not sufficiently high nd should e improved. The deep colors of PIs re owing to the formtion of intr nd intermoleculr chrgetrnsfer complexes (CTC) etween the electrondonting dimine moiety nd the electronccepting dinhydride moiety. The incorportion of ulky licyclic unit in dimine is effective to reduce the CTC formtion. Moreover, ccording to the LorenzLorenz eqution, high molr refrction per unit moleculr volume should result in higher refrctive index. Thus, to further improve the refrctive indices nd the trnsprency, we focused on the development of new dimine contining n licyclic structure with high sulfur content. In this study, we report opticlly trnsprent sulfurcontining PIs with high refrctive indices. These PIs were prepred from the dinhydrides, 1,2,3,4cycloutnetetrcroxylic dinhydride (CBDA) nd 4,4'[pthiois(phenylenesulfnyl)] diphthlic nhydride (3DEA), nd new sulfurcontining dimine, 2,5is(4minophenyl sulfnyl)1,4dithine (BADT) y twostep polymeriztion procedure vi the solule poly(mic cid) (PAA) precursors. The resulting Received Mrch 26, 28 Accepted My 9,

2 J. Photopolym. ci. Technol. Vol.21, PIs exhiited high refrctive indices in the rnge of with high trnsprency (>4 nm) nd low irefringence in the rnge of Experimentl 2.1. Mterils CBDA kindly supplied y JR Co. Ltd., ws dried in vcuo t 12 o C for 12 h prior to use. 3DEA ws prepred inhouse ccording to our previous work [15]. All the other regents nd solvents were used without further purifiction ynthesis of 2,5dicetoxy1,4dithine (DADT) To solution of 2,5dihydroxy1,4dithine (1.4 g, 68.4 mmol) in dry pyridine (22 ml) cooled t o C, cetic nhydride (15.4 g, mmol) ws dded dropwise over 1 h using dropping funnel under N 2 tmosphere. During the solution stirred for 1 h, the white solid egn to precipitte. The mixture ws poured into wter. The precipitte ws filtered nd dried under reduced pressure. Recrystlliztion from methnol gve white crystl (9.9 g, yield 61%). M.p.161 o C. IR (KBr, cm 1 ): ν = 2966, 2935 (lkyl CH), 1739 (ester C=), 1431 (lkyl CH), 1369 (cetyl CH), 725 (lkyl CH). 1 H NMR (3 MHz, CDCl 3, ppm): δ = (m, 2H), (dd, 2H), (dd, 2H), (s, 6H). E LEM. A NAL. Clcd for C 8 H : C, 4.66; H, Found: C, 4.75; H ynthesis of 2,5is(4nitrophenylsulfnyl) 1,4dithine (BNDT) To solution of DADT (5.29 g, 2. mmol) nd 4nitrothiophenol (8% pure, 8.14 g, 42. mmol) in dry methylene chloride (5 ml), oron trifluoride ether complex (1 ml) ws dded dropwise under N 2 tmosphere. The rection mixture ws stirred for 3 h t room temperture, nd the precipitte ws collected y filtrtion. Recrystlliztion from toluene gve pleyellow crystl (9.8 g, yield 94%). IR (KBr, cm 1 ): ν = 298 (lkyl CH), 1573 nd 1334 (nitro N 2 ). 1 H NMR (3 MHz, CDCl 3, ppm): δ = (dt, 4H), (m, 4H), (m, 2H), (m, 4H). E LEM. A NAL. Clcd for C 16 H 14 N : C, 45.5; H, 3.31; N Found: C, 44.44; H, 3.42; N, ynthesis of 2,5is(4minophenylsulfnyl) 1,4dithine (BADT) The mixture of BNDT (8.94 g, 21. mmol), ethnol (75 ml), nd ctlytic mount of 1 w % plldium on ctivted cron (1 g) ws refluxed nd then hydrzine monohydrte (45 ml) diluted with ethnol (15mL) ws dded dropwise over period of 1.5 h. After the ddition ws completed, the rection mixture ws refluxed for 24 h. The hot mixture ws filtered to remove the ctlyst, nd the filtrte ws cooled to room temperture to give crystl BADT. The product dsored in the ctlyst ws wshed with DMF three times, nd the DMF ws poured into lrge mount of wter to precipitte the crude BADT. The totl yield ws 6.22 g (8.8%). Recrystlliztion from ethnol gve white crystl. IR (KBr, cm 1 ): ν = 3448, 3355 (mine, NH), 2923, 2854 (lkyl CH). 1 H NMR (3 MHz, CDCl 3, ppm): δ = (m, 4H), (m, 4H), 4.14 (dd, 1H), 4.5 (dd, 1H), (s, 4H), (m, 4H). E LEM. A NAL. Clcd for C 16 H 18 N 2 4 : C, 52.42; H, 4.95; N Found: C, 52.63; H, 4.75; N ynthesis of poly(mic cid)s (PAAs) nd PIs A typicl polymeriztion procedure for the synthesis of PAAs is s follows. BADT (.552 g, 1.5 mmol), predried t 8ºC for 4 h, nd dehydrted NMP (5 ml) were chrged into 3 ml flsk equipped with mgnetic stirrer nd nitrogen inlet. Then, the flsk ws cooled with wter th under nitrogen gs flow. When cler solution ws otined, 3DEA (.814 g, 1.5 mmol) ws dded. The solution ws stirred t room temperture for 24 h to fford viscous PAA solution, which is filtered through.45 µm Teflon syringe filter. For nlysis, the PAA solutions were spincoted onto silicon wfer, nd the thickness ws controlled y spinning rte. For FTIR nd UVvis mesurements, the thicknesses were controlled to e out 2.5 µm. The PI films were prepred y therml imidiztion of PAA films t elevted tempertures for.5 h ech t 8 o C nd 12 o C, nd for 2 h t 2 o C, respectively, under nitrogen, followed y immersing the silicon wfer into wrm wter. 132

3 J. Photopolym. ci. Technol. Vol.21, 2.5. Mesurements. The FTIR spectr were otined on Hori FT72 spectrometer. The 1 H nd 13 C NMR spectr were recorded with Bruker DPX3 spectrometer using CDCl 3 or DMd 6 s solvent nd trimethylsilne s the reference. The UVvis trnsmittnce spectr were recorded on JAC V56 UV/Vis spectrometer in the rnge 25 8 nm. Therml nlysis ws performed on eiko EXTAR 6 TG/DTA 63 therml nlyzer t heting rte of 1 o C/min for thermogrvimetry (TG) nd eiko EXTAR 6 DC 62 t heting rte of 1 o C/min for differentil scnning clorimetry (DC) under nitrogen. The outofplne (n TM ) nd inplne (n TE ) refrctive indices of PI films were mesured with prism coupler (Metricon, model PC21) equipped with HeNe lser light source (wvelength: nm). The polriztion of the incident light guided from linerly polrized lser ws controlled y inserting hlfwveplte in the light pth. The prism ttchle to the surfce of PI films ws mde of single crystl of glliumgdoliniumgrnet (GGG). In plne (n TE )/outofplne (n TM ) irefringence ( n) ws clculted s difference etween n TE nd n TM. The verge refrctive index (n v ) ws clculted ccording to eqution (1): 2 2 n = (2n + n ) / 3 (1) AV TE 3. Results nd Discussion 3.1. Monomer ynthesis As monomer hving high sulfur content (35. wt %) with n licyclic ring structure, BADT ws selected, nd it ws prepred vi three steps from 2,5dihydroxy1,4dithine s shown in cheme 1. 2,5Dihydroxy1,4dithine ws converted to DADT y tretment with cetic nhydride, which ws rected with 4 nitrothiophenol in the presence of oron trifluoride ether complex to produce BNDT. Finlly, the BNDT ws hydrogented to BADT with hydrzine in the presence of Pd ctlyst. BADT ws otined s the mixture of trns nd cis stereoisomers. The structure of BADT ws chrcterized on the sis of elementl nlysis s well s FTIR nd 1 H NMR spectroscopies. The IR spectrum of BADT showed chrcteristic sorptions t 3448 nd 3355 cm 1 which re TM ssignle to the mino group. Figure 1 shows the 1 H NMR spectrum of BADT, nd the inset indictes the ssignment of ech resonnce. All peks greed with the ssignment to the expected structure of BADT. The chrcteristic singlet signl due to the mino group resonted t 3.8 ppm nd douletdoulet signls t 4.14 nd 4.5 ppm ssignle to the methine protons next to the thioether group of the trns nd cis stereoisomers, respectively, re clerly oserved. Using these signls, the rtio of trns nd cis isomers in the mixture ws estimted to e 1.8 : 1. The structure of BADT ws lso confirmed y the elementl nlysis. H 2 N H BF 3. Et 2, CH 2 Cl 2,r.t.,3h Ac 2 pyridine, r.t., 1 h H 2 N H 2 NNH H 2 2 N Pd/C, EtH, reflux, 24 h Ac DADT BNDT BADT Ac N 2 NH 2 cheme 1. ynthesis of dimine (BADT). CHCl 3 c (cis, trns) H 2 N (cis, trns) d (cis, trns) e d c PPM Figure 1. 1 H NMR spectrum of BADT (cis nd trns mixture). e (c is, trns) H 2 NH 2 TM 3.2. Polyimide ynthesis Two PIs were prepred from two dinhydrides of CBDA nd 3DEA, nd BADT vi the PAA precursors, followed y therml imidiztion t 2 o C for 2 h (cheme 2). Tle 1 summrizes the results of polymeriztion. Reltively high moleculr weight PAAs with inherent viscosities in the rnge of dl/g were prepred esily. Flexile PI films were otined y heting the 133

4 J. Photopolym. ci. Technol. Vol.21, corresponding PAAs cst on fused silic sustrte in nitrogen, followed y immersion in wrm wter. The resulting PI films were dried in vcuo t 1 o C for 1 h prior to mesurements. H 2 N H R + BADT R NH 2 NMP (2 wt%), r.t., 24 h H N H Poly(mic cid) R: 8 o C,.5 h; 12 o C,.5 h; 2 o C, 2 h N R N Polyimide (CBDA/BADT, 3DEA/BADT) cheme 2. ynthesis of polyimides. CBDA/BADT 3DEA/BADT refcbda/3da ref3dea/3da [η] inh (dl/g) Film color lmost colorless pleyellow CBDA 3DEA Tle 1. Polymeriztion nd therml properties of polyimides. N H n n T 5% ( o C) Mesured with PAA t concentrtion of.5 g/dl in NMP t 3 o C. Mesured y DC. T g ( o C) The successful therml conversion from PAAs to PIs cn e confirmed y the FTIR spectr (Figure 2), in which the chrcteristic sorptions due to the imide moiety t 1774 cm 1 (υ s,c= ), 172 cm 1 (υ s,c= ), nd 1365 cm 1 (υ CN ) re clerly identified. No peks due to the N H stretching t 326 cm 1 nd C= stretching of the mide groups t 156 cm 1 re oserved. Furthermore, elementl nlysis lso supported the formtion of PIs. Trnsmittnce 4 35 CBDA/BADT 3DEA/BADT Wvenumer (cm 1 ) 1 5 Figure 2. IR spectr of CBDA/BADT nd 3DEA/BADT Therml Properties The therml stility of PIs ws evluted y TG nd DC under nitrogen tmosphere. The results re summrized in Tle 1, in which the therml properties of the reference PIs from 4,4 thiois[(pphenylenesulfnyl)niline] (3DA) tht is representtive sulfurcontining dimine giving highly refrctive PI [1316] re presented. Figure 3 shows the trces of TG for PIs. All the PIs show therml stility with the 5% weight loss tempertures (T 5% ) in the rnge of 3 31 ºC. Weight residue (%) CBDA/BADT 3DEA/BADT Temperture ( o C) Figure 3. TG curves of polyimides (CBDA/BADT, 3DEA/BADT). The glss trnsition tempertures (T g s) were evluted y DC mesurements (Figure 4). The PI from 3DEA nd BADT shows T g t 151 ºC. n the other hnd, no T g ws oserved for CBDA/BADT elow 2 o C. 134

5 J. Photopolym. ci. Technol. Vol.21, Exo CBDA/BADT 3DEA/BADT Temperture ( o C) Figure 4. DC curves of Polyimides. The temperture dependences of the dynmic storge modulus (E ), loss modulus (E ), nd tn δ of 3DEA/BADT re shown in Figure 5, which indictes tht the modulus remins constnt elow the T g. Wheres, the modulus drops drmticlly ove the T g,. The E curve exhiits T g t 156 ºC. These results revel tht the het deformtion tempertures of the PIs were decresed y the incorportion of the licyclic structure. Modulus trge modulus Loss modulus Temperture ( o C) tnd Figure 5. DMA curves of polyimide films (3DEA/BADT). tnδ 3.4. pticl Properties Figure 6 shows the UV/vis opticl trnsmission spectr of CBDA/BADT nd 3DEA/BADT (pleyellow, 2 3 µm thick) with those of the reference PIs (CBDA/3DA nd 3DEA/3DA). The sornce of the films ws normlized to 2.4 µmthick. The CBDA/BADT exhiits higher trnsprency in the UV region thn tht of the reference CBDA/3DA, nd the trnsmittnces in the visile region re higher thn 9% even t 4 nm. Note tht the experimentl trnsmission spectr of the PIs gree well with the clculted spectr (Figure 7) sed on the TDDFT [17], in which the higher trnsprency of CBDA/BADT t the shorter wvelengths is well reproduced, which is presumly due to the sulfurcontining licyclic structure. n the other hnd, 3DEA/BADT shows similr spectrum to 3DEA/3DA, in which the licyclic structure does not improve the trnsprency of the resulting PIs ecuse the sornce of the romtic PIs minly depends on the dinhydrides structure [18]. Trnsmittnce (%) CBDA/BADT 3DEA/BADT CBDA/3DA 3DEA/3DA Wvelength (nm) Figure 6. UVvis spectr of polyimides. CBDA/BADT, 3DEA/BADT, CBDA/3DA, 3DEA/3DA (normlizing to 2.4 µm thickness). Clculted Asornce (r.unit) 2 1 CBDA/BADT 3DEA/BADT CBDA/3DA 3DEA/3DA Wvelength (nm) Figure 7. Clculted opticl sorption spectr of polyimides using the TDDFT method. Tle 2 summrizes the inplne (n TE ), outplne (n TM ), verge refrctive indices (n v ), nd the irefringences ( n) for the two PIs with the reference PIs exhiiting high n v s. The CBDA/BADT nd 3DEA/BADT show the significntly high n v vlues of nd , 135

6 J. Photopolym. ci. Technol. Vol.21, Tle 2. pticl properties of polyimides thickness λ cutoff (nm) n TE n TM n v n c CBDA/BADT DEA/BADT refcbda/3da ref3dea/3da Cutoff wvelength. Refrctive indices t 632.8nm. c Birefringence. respectively. Ech vlue is comprle to tht of the ech reference PI. These results clerly indicte tht the effect of the high sulfur content in BADT is comprle to the phenyl rings hving high molr refrction. The reduction of the numer of phenyl rings in the repeting unit of BADTderived PIs is dvntgeous to improve the opticl trnsmission t the shorter wvelengths in the UV region. The introduction of 1,4dithine moiety in the PI min chins will led us to new series of highly refrctive semiromtic PIs. 4. Conclusions A dimine of BADT with high sulfurcontent nd n licyclic structure ws newly designed nd prepred to develop highly refrctive trnsprent polyimides (PIs). The experimentl results demonstrted tht the synergic effects of sulfur nd cyclic structure is effective to develop PIs with high refrctive indices, low irefringence, nd good opticl trnsprency. The CBDA/BADT nd 3DEA/BADT PIs show the high vlues of n v of nd , respectively, nd excellent trnsprency higher thn 9% even t 4 nm. The highsulfur content in BADT increses the refrctive indices of the resulting PIs. n the other hnd, reduction of numer of romtic units in BADT does not increse the refrctive index. However, in ll cses, the incresing of sulfur contents in the repeting units is positive in improving the refrctive indices of the PIs without deteriorting the other properties. References 1. A. K. Whittker, I. Blkey, H. P. Liu, D.J. T. Hill, G. A. George, W. Conley, nd P. Zimmermn, Proc PIE, 5753, 827 (25). 2. J. Yng, L. Yng, C. Q. Xu, nd Y. F. J. Li, Lightwve Tech., 25, 372 (27). 3. T. Nkmur, H. Fujii, N, Juni, nd N. Tsutsumi, pt. Rev., 13, 14 (26). 4. M. uw, H. Niw, nd M. Tomikw J. Photopolym. ci. Tech., 19, 275 (26). 5. C. L. Lu, Z. C. Cui, Y. X. Wng, B. Yng, nd J. C. hen, J. Appl. Polym. ci., 89, 2426 (23). 6. R. A. Minns nd R. A. Gudin, J.M..Pure Appl. Chem., A29, 19 (1992). 7. C. L. Lu, Z. C. Cui, Y. Wng, Z. Li, C. Gun, B. Yng, nd J. C. hen, J. Mter. Chem., 13, 2189 (23). 8. C. Feger nd H. Frnke, in Polyimides Fundmentl nd Appliction, M. K. Ghosh, nd K. L. Mittl, Mrcel Dekker: New York, 1996, p 759 (1996). 9. C. P. Yng, Y. Y. u, nd M. Y. Hsu, Polym. J., 38, 132 (26). 1. C. P. Yng, Y. P. Chen, E. M. Woo, nd. H. Li, Polym. J., 38, 457 (26). 11. X. Zho, J. G. Liu, J. M. Rui, L. Fn, nd. Y. Yng, J. Appl. Polym. ci., 13, 1442 (27). 12. M. Zhng, Z. Wng, L. X. Go, M. X. nd Ding, J. Polym. ci., Prt A, Polym. Chem., 44, 959 (26). 13. J. G. Liu, Y. Nkmur, Y. hiski,. Ando, nd M. Ued, Polym. J., 39, 543 (27). 14. J. G. Liu, Y. Nkmur, Y. hiski,. Ando, nd M. Ued, J. Polym. ci. Prt A, Polym. Chem., 45, 566 (27). 15. J. G. Liu, Y. Nkmur, C. A. Terrz, Y. hiski,. Ando, nd M. Ued, Mcromol. Chem. Phys., 29, (28). 16. J. G. Liu, Y. Nkmur, Y. hiski,. Ando, nd M. Ued, Mcromolecules, 4, 4614 (27) Ando, T. Fujigy, nd M. Ued, Jpn. J. Appl. Phys., 41, L15 (22) Ando, T. Mtuur, nd. ski, Polym. J., 29, 69 (1997). 136