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Synthesis and characterization of two series of polyimides as nonlinear optical materials
European Polymer Journal 36 (2000) 2417±2421
Synthesis and characterization of two series of polyimides as nonlinear optical materials Zhao Li, Yuxia Zhao, Jiayun Zhou, Yuquan Shen * Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, People's Republic of China Received 22 June 1999; received in revised form 27 August 1999; accepted 7 January 2000
Abstract Using the post-azo coupling reaction in organic solvents, we synthesized two series of nonlinear optical (NLO) polyamic acids with four dierent NLO chromophores. The properties of these polymers were characterized and compared. We found that increasing the electron withdrawing ability or replacing the benzene ring in the p-conjugated system with thiophene will result in the red shift of the maximum absorption band and enhancement of the nonlinear optical response. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: NLO polyimide; Polymers; Organic nonlinear optical materials
1. Introduction Organic second-order nonlinear optical (NLO) polymers have been extensively studied over the past decade. Compared with inorganic crystals (such as LiNbO3 ), organic polymers have several advantages [1,2]: high electro-optic coecient, fast responding time, low dielectric constant, tailorability of molecules, etc. Among all those polymers, such as poly(methacrylate), polyurethane, polyimide, polyester [3±6], polyimide has attracted much attention due to its good thermal stability, high glass transition temperature, use in the semiconductor industry. The common synthetic methods for NLO polyimide include a tedious procedure for the synthesis of the chromophore containing monomers [3,4,6]. In the past few years, several research groups employed the postfunctionalization method for preparing NLO polymers. Chen et al. used the post-Mitsunobu reaction at the last stage, synthesized several NLO side-chain aromatic
*
Corresponding author.
polyimides with dierent polymer backbones and different chromophores [6]. Wang et al. found that the azo coupling reaction between polymer and diazonium salt can be carried out in polar organic solvents with a high yield, and they synthesized several epoxy-based NLO polymers using this method [7]. We found that the postazo coupling reaction can also be employed on polyamic acids in polar organic solvents. The recent trend is using a ®ve-membered heterocyclic ring, especially thiophene as an NLO chromophore [8±10]. Miller et al. reported a series of NLO chromophore containing a single substituted thiazole ring and compared it with a selected imidazole, oxazole and phenyl analogues [8]. Rao et al. increased the secondorder molecular nonlinear optical properties of push± pull stilbene by replacing benzene rings with thiophene rings successfully [9]. In this article, we chose four kinds of nitro-substituted amino thiophene or benzene as NLO chromophores and prepared two kinds of polyamic acids that can undergo the post-azo coupling reaction. We functionalized the two polyamic acids and obtained two series of novel NLO polyimides after imidization. The properties of synthesized polymers were compared in some detail and several interesting conclusions were deduced.
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2. Experimental section 2.1. Characterization The UV±VIS absorption spectra of the polymers as spin-coated ®lms were determined on a U-3000 Hitachi spectrophotometer. 1 H nuclear magnetic resonance (NMR) spectra of samples in dimethyl-d6 sulfoxide were obtained on a Varian Gemini 300 spectrometer operation at 300 Hz. The refractive indices and thickness of the polymer ®lms were measured with an ellipsometer Model L116B and Alpha-step 250 TENCOR, respectively. The viscosity of the polymer was measured in DMF solution with a Ubbelohde type viscometer. 2.2. Materials Dimethyl formamide and dimethyl acetamide were distilled over di-phosphorus pentoxide before use. 5nitro-2-aminothiophene and 3,5-dinitro-2-aminothiophene were synthesized in our laboratory, and the details of synthesis will be reported elsewhere. All other starting materials, reagents and solvents, purchased from Aldrich, were used without further puri®cation unless otherwise mentioned. 2.3. Preparation of PA and PB Polyamic acids PA and PB were synthesized as described in detail in Refs. [11,12]. After polymerization, the polymer solution was precipitated in water. The polymer was collected by ®ltration and washed with plenty of water. It was then dried in vacuum at 50°C overnight to give the precursor polymers PA and PB. 2.4. Preparation of PA-1 An aqueous solution of sodium nitrite ( 0.138 g, 0.3 ml H2 O) was added dropwise to a solution of 4-nitroaniline in 18% hydrochloric acid solution (1.2 ml ) at 0°C. The resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. Then, the solution was stirred below 5°C overnight. PA-1 was obtained by precipitation of the above solution in water and was washed with plenty of water until a neutral stage was achieved. The polymer was further puri®ed by Soxhlet extraction with acetone for 4 h. Yield: 0.876 g (75%) ; kmax 478 nm, 1 H-NMR, d 2.71(s, 4H), 2.86(s, 4H), 7.04(s, 2H), 7.24(s, 1H), 7.75(s, 2H), 7.86(s, 2H), 8.13(s, 1H), 8.25(s, 2H), 8.73(d, 2H). 2.5. Preparation of PA-2 Sodium nitrite (0.138 g, 2.0 mmol) was added slowly to concentrated sulfuric acid (5 ml), and the mixture was
heated to 70°C under stirring. Sodium nitrite dissolved slowly and a solution of nitrosyl sulfuric acid was formed. After the solution was cooled to 5°C, 2,4-dinitro aniline (0.366 g, 2.0 mmol) was added and the mixture was stirred at 5°C for 40 min. Then, the resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. PA2 was prepared via a procedure similar to that described above for the synthesis of PA-1. Yield: 0.860 g (69%), kmax 523 nm, 1 H-NMR, d 2.72(s, 4H), 2.90(s, 4H), 6.53(s, 1H), 6.79(s, 1H), 7.08(s, 2H), 7.72(s, 2H), 7.92(s, 1H), 8.12(s, 1H), 8.44(d, 1H), 8.78(d, 2H). 2.6. Preparation of PA-3 Sodium nitrite (0.138 g, 2.0 mmol) was added slowly to sulfuric acid (5 ml), and the mixture was heated to 70°C under stirring. After the solution was cooled to 5°C, a mixture of propanoic acid (1.0 ml) and acetic acid (5.0 ml) was added. 5-nitro-2-aminothiophene (0.288 g, 2.0 mmol) was added, and the mixture was stirred at 5°C for 40 min. Then, the resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. PA-3 was prepared via a procedure similar to that described above for the synthesis of PA-1. Yield: 0.958 g (82%), kmax 558 nm, 1 HNMR, d 2.71(s, 4H), 2.89(s, 4H), 7.08(d, 1H), 7.13(d, 1H), 7.75(s, 2H), 7.94(s, 1H), 8.04(s, 1H), 8.15(s, 2H), 8.70(d, 2H). 2.7. Preparation of PB-1 Via a procedure similar to that described above for the synthesis of PA-1. Yield: 1.552 g (75%), kmax 471 nm, 1 H-NMR, d 4.05(s, 4H), 4.60(s, 4H), 7.18(d, 4H), 7.62(d, 2H), 7.71(s, 4H), 7.85(s, 2H), 7.94(d, 2H), 8.04(s, 2H), 8.17(s, 2H), 8.17(s, 2H), 8.32(s, 2H), 8.42(s, 2H), 8.59(d, 2H). 2.8. Preparation of PB-2 Via a procedure similar to that described above for the synthesis of PA-2. Yield: 1.487 g (82%), kmax 513 nm, 1 H-NMR, d 4.07(s, 4H), 4.60(s, 4H), 7.20(s, 4H), 7.58(d, 2H), 7.62(s, 1H), 7.69(s, 2H), 7.78(s, 2H), 7.95(s, 2H), 8.04(s, 2H), 8.09(s, 1H), 8.16(s, 1H), 8.41(s, 2H). 2.9. Preparation of PB-3 Via a procedure similar to that described above for the synthesis of PA-3. Yield: 1.528 g (88%), kmax 550 nm, 1 H-NMR, d 4.08(s, 4H), 4.60(s, 4H), 6.94(s, 2H), 7.19(s, 4H), 7.62(s, 2H), 7.72(s, 4H), 7.96(s, 2H), 8.02(s, 2H), 8.10(s, 1H), 8.15(s, 1H), 8.41(s, 2H).
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2.10. Preparation of PB-4 Via a procedure similar to that described above for the synthesis of PA-3. Yield: 1.187 g (65%), kmax 630 nm, 1 H-NMR, d 3.87(s, 4H), 4.51(s, 4H), 6.94(s, 2H), 7.16(s, 2H), 7.68(s, 4H), 7.76(s, 4H), 7.98(s, 2H), 8.03(s, 2H), 8.17(s, 1H), 8.41(s, 2H). 2.11. Polymer ®lm preparation and poling The polymers were dissolved in dry distilled dimethyl acetamide. The homogeneous solutions were ®ltered through a 0.2 lm Te¯on ®lter. Glass slides were rinsed with a cleaning solution, water, deionized water, respectively, and then dipped in iso-propyl alcohol before use. The polymer ®lms were dried under vacuum at 50°C for 8 h. The thickness of the ®lm varied from 0.2 to 1.8 lm, which depends upon the solution concentration (15 wt.%) and the spin speed (1200±2000 rpm). Poling was performed by the corona poling technique in a typical setup. A tungsten needle was used as electrode. The poling condition is as follows: temperature 200°C, high voltage 10 kV at needle point, poling time 30 min, gap distance 1.5 cm, poling current < 2:5 lA. The polymer ®lms were cooled down to room temperature in the presence of an electrical ®eld. 2.12. Nonlinear optical property measurement The second-order NLO coecient (d33 ) of the poled ®lms was measured by second harmonic generation (SHG). The source was a Q-switched Nd±YAG laser operating at a wavelength of 1064 nm. The SHG signal at 532 nm, selected with an interference ®lter, was detected by a photomultiplier tube and measured with a boxcar integrator. A Y-cut quartz crystal (d11 0:4 pm=V) was used as the reference. By comparing the SHG intensity from the poled polymer sample with that from the quartz crystal the NLO coecient d33 of the poled polymers was determined. 3. Results and discussion 3.1. Polymer synthesis and characterization The structure of precursor polyamic acids PA, PB is shown in Scheme 1 (structure of NLO polymers from port-azo coupling reaction) and they were synthesized as described in Refs. [11,12]. The intrinsic viscosity measurements indicated values of 0.072 and 0.063 dl=g for polyamic acids PA and PB, respectively. The precursor polymers PA and PB all have good solubility in polar organic solvents such as DMF, NMP, etc. The two polyamic acids were functionalized to introduce dierent chromophores at the ®nal stage of the synthetic route,
Scheme 1.
and the successful preparation of the stable diazo salt solution was critical in this step. The diazo salt solution 4-nitro aniline can be prepared easily in the HCl/H2 O medium. But the other three amino compounds cannot dissolve in this medium. Thus, nitrosyl sulfuric acid solution was used, and the diazo salt solution of these compounds was obtained, which can stay stable below 5°C. The polymer nomenclature includes two parts: the ®rst part refers to the main chain polyamic acid, and the second part refers to the dierent NLO chromophores. It should be pointed out that we also prepared polymer PA-4. But after puri®cation, it cannot redissolve back into organic solvents. This can be explained by the chromophore±chromophore interaction. Compound PA-4 has dinitro thiophene as the side chain, and the interaction between NLO chromophores may be greater than other three chromophores. Compound PB-4 can dissolve in organic solvents, because the spacer between its chromophores is larger than PA-4. The structures and purity of the polymers were investigated and con®rmed by various spectroscopic methods. The dierent NLO chromophores of the polymers were characterized by 1 H-NMR spectroscopy as shown in Section 2. The degree of functionalization or the chromophore loading density was determined by
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Table 1 Nonlinearities of synthesized NLO polymers Polymers
d33 a (pm/v)
Tb (lm)
kmax c (nm)
/d (%)
d33 //e
PA-1 PA-2 PA-3 PB-1 PB-2 PB-3 PB-4
50.4 52.2 51.0 17.5 22.1 32.5 19.5
0.13 0.27 0.13 0.33 0.30 0.36 1.39
478 523 558 471 513 550 630
100 58.6 84.4 100 92.1 100 50.2
50.4 86.0 60.4 17.5 24.0 32.5 38.8
a
d33 value of the ®lms measured directly by SHG method. Thickness of the ®lms. c kmax measure in ®lm form. d Chromophore loading density. e d33 value divided by chromophore loading density. b
comparing the areas of methyl 1 H proton and aromatic proton in 1 H-NMR spectroscopy, which is listed in Table 1. All the polymers have a high-chromophore loading density, and the dierence in chromophore loading density was caused mainly by the dierent stability of the relative azo salt and the eciency of the azocoupling reaction. The UV±VIS spectra of the thin ®lms of the series of polymer are given in Fig. 1. The color of the ®lms changes from red, purple to green, and all the polymers exhibit strong absorption in the visible range. The kmax values of all the polymers are listed in Table 1. Increasing the electron withdrawing eect or replacement of the benzene ring with thiophene both result in a red shift of the kmax and an intensity increase. As thiophene has a lower delocalization energy than that of benzene, it can oer better eective conjugation than that of benzene in a donor±acceptor system. For the series of PA and PB polymers, all PA polymers have relative red shift kmax values. The reason may be the dierent chemical environment of the two series of polymers. 3.2. Thermal properties The thermal properties of side chain NLO polymers can be considered as two parts: thermal properties of the main chain and the side chain. The former accounts for the thermal properties of the polymer backbone and the relaxation eect of the NLO chromophores. The latter refers to the thermal properties of the NLO chromophore itself. We found that the two series of polymers can be imidized at 180°C, which was con®rmed by FTIR spectra. After imidization, the new absorption peak near 1780 cmÿ1 appeared, which was the typical absorption for the imide structure. TGA trace studies of the polymers indicate that after imidization, the polyimides were thermally stable up to 300°C under a nitrogen atmosphere. The thermal properties of the side chain NLO chromophore were indicated with UV±VIS spectroscopy.
Fig. 1. UV±VIS absorption spectra of (a) PA-n and (b) PB-n spin-coated ®lms.
Each thin ®lm of the polymers was heated to 160°C, 180°C, 200°C, 220°C, 240°C and kept at that temperature for 15 min, respectively. Each UV±VIS spectroscopy was recorded, and the changes in absorbance are shown in Fig. 2. All these polymers retained 85% or 70% of its original absorbance intensity at 180°C or 200°C. We also found that the thermal stability of those four kinds of NLO chromophore is 1 > 2 > 3 > 4. As thiophene has a less aromatic stabilization energy than the benzene ring, replacement of one nitro group with two will also aect aromatic stabilization. It has been pointed out that if the Tg of the polymer is roughly 150± 200°C above the ultimate operating temperature, decay of the orientation would be negligible over the device lifetime [13]. So, our NLO polymers could all satisfy the demand of the thermal stability for the device fabrication. 3.3. Nonlinear optical properties All the NLO polymers were spin cast into thin ®lms, and the thermal imidization and poling process were
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conjugation than benzene in donor±acceptor compounds. Its substitution in donor±acceptor compounds should be expected to result in enhanced charge-transfer properties and nonlinear responses than in benzenoid rings. The two series of NLO polymers exhibited a good nonlinear optic response. The stability of the nonlinear property at elevated temperatures was very important for the NLO polymers. We also tested one kind of our polymer: PB-1. After keeping the poled sample of PB-1 in an oven for 1000 h at 1000°C, the d33 value retained 50% of its original value. It is reasonable to assume that other kinds of polymers have a good nonlinear stability at elevated temperatures. In summary, two series of NLO polyamic acids with dierent chromophores were prepared and characterized. The optical and thermal properties of these polymers were compared. The d33 value of these polymers was measured using the SHG method. Increasing the electron-withdrawing eect or replacing the benzene ring with thiophene will result in a red shift of kmax and an enhancement of the nonlinear property. Considering the relatively simple synthetic method, the good thermal stability and processibility, the high nonlinear response, our NLO polymers are promising candidates as electrooptic materials for device fabrication.
References Fig. 2. Relationship between temperature and the relative intensity of UV±VIS spectra for the synthesized NLO polymer: (a) PA-n and (b) PB-n.
accomplished at the same time. As during the imidization process, the polymer will lose water and become tighter. This is of bene®t to the orientation of the dipole. The second-order NLO coecient (d33 ) was measured by the SHG. The d33 value is related directly to the chromophore loading density. In order to compare it eectively, we divided the d33 value with chromophore loading density and the result was listed in Table 1. The weight percent of chromophore of the series of PA (30%) was higher than that of PB (20%), so PA polymers all have a relatively greater d33 value than the corresponding PB polymers. For dierent NLO chromophores, the d33 value increased with an increase in the electronwithdrawing ability or replacement of benzene ring with thiophene. Because the second-order molecular nonlinear hyperpolarizability (bl) increases with increasing donor and acceptor strengths, and thiophene has a lower delocalization energy than benzene, it can oer better
[1] Burland DM, Miller RD, Walsh CA. Chem Rev 1994;94:31. [2] Dalton LR, Harper AW, Ghosn R. Chem Mater 1995;7:1063. [3] Robello DR, Dao PT, Schildkraut JS, Scozzafava M, Urankar EJ, Willand CS. Chem Mater 1995;7:284. [4] Chen M, Dalton LR, Yu LP, Shi YQ, Sterier WH. Macromolecules 1992;25:4032. [5] Yu D, Gharavi A, Yu L. J Am Chem Soc 1995;117:11680. [6] Chen T, Jen AK-Y, Cai Y. Macromolecules 1996;29:535. [7] Wang X, Kumar J, Tripathy SK. Macromolecules 1997;30:219. [8] Miller RD, Lee VY, Moylan CR. Chem Mater 1994;6:1023. [9] Rao VP, Jen AK-Y, Wong KJ, Drost KJ. Tetrahedron Lett 1993;34:1747. [10] Jen AK-Y, Rao VP, Wong KJ, Drost KJ. J Chem Soc Chem Commun 1993:91. [11] Shen Y, Ma H, Duan X, Qiu L, Zhai J, Li Z. SPIE 1996;2897(Suppl):10±20. [12] Jen AK-Y, Drost KJ, Rao YP, Cai YM. Polym Preprints 1994;35(2):130. [13] Marder SR, Kipplen B, Jen AK-Y, Peyghambarian N. Nature 1997;388:849.
Synthesis and characterization of two series of polyimides as nonlinear optical materials Zhao Li, Yuxia Zhao, Jiayun Zhou, Yuquan Shen * Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, People's Republic of China Received 22 June 1999; received in revised form 27 August 1999; accepted 7 January 2000
Abstract Using the post-azo coupling reaction in organic solvents, we synthesized two series of nonlinear optical (NLO) polyamic acids with four dierent NLO chromophores. The properties of these polymers were characterized and compared. We found that increasing the electron withdrawing ability or replacing the benzene ring in the p-conjugated system with thiophene will result in the red shift of the maximum absorption band and enhancement of the nonlinear optical response. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: NLO polyimide; Polymers; Organic nonlinear optical materials
1. Introduction Organic second-order nonlinear optical (NLO) polymers have been extensively studied over the past decade. Compared with inorganic crystals (such as LiNbO3 ), organic polymers have several advantages [1,2]: high electro-optic coecient, fast responding time, low dielectric constant, tailorability of molecules, etc. Among all those polymers, such as poly(methacrylate), polyurethane, polyimide, polyester [3±6], polyimide has attracted much attention due to its good thermal stability, high glass transition temperature, use in the semiconductor industry. The common synthetic methods for NLO polyimide include a tedious procedure for the synthesis of the chromophore containing monomers [3,4,6]. In the past few years, several research groups employed the postfunctionalization method for preparing NLO polymers. Chen et al. used the post-Mitsunobu reaction at the last stage, synthesized several NLO side-chain aromatic
*
Corresponding author.
polyimides with dierent polymer backbones and different chromophores [6]. Wang et al. found that the azo coupling reaction between polymer and diazonium salt can be carried out in polar organic solvents with a high yield, and they synthesized several epoxy-based NLO polymers using this method [7]. We found that the postazo coupling reaction can also be employed on polyamic acids in polar organic solvents. The recent trend is using a ®ve-membered heterocyclic ring, especially thiophene as an NLO chromophore [8±10]. Miller et al. reported a series of NLO chromophore containing a single substituted thiazole ring and compared it with a selected imidazole, oxazole and phenyl analogues [8]. Rao et al. increased the secondorder molecular nonlinear optical properties of push± pull stilbene by replacing benzene rings with thiophene rings successfully [9]. In this article, we chose four kinds of nitro-substituted amino thiophene or benzene as NLO chromophores and prepared two kinds of polyamic acids that can undergo the post-azo coupling reaction. We functionalized the two polyamic acids and obtained two series of novel NLO polyimides after imidization. The properties of synthesized polymers were compared in some detail and several interesting conclusions were deduced.
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2. Experimental section 2.1. Characterization The UV±VIS absorption spectra of the polymers as spin-coated ®lms were determined on a U-3000 Hitachi spectrophotometer. 1 H nuclear magnetic resonance (NMR) spectra of samples in dimethyl-d6 sulfoxide were obtained on a Varian Gemini 300 spectrometer operation at 300 Hz. The refractive indices and thickness of the polymer ®lms were measured with an ellipsometer Model L116B and Alpha-step 250 TENCOR, respectively. The viscosity of the polymer was measured in DMF solution with a Ubbelohde type viscometer. 2.2. Materials Dimethyl formamide and dimethyl acetamide were distilled over di-phosphorus pentoxide before use. 5nitro-2-aminothiophene and 3,5-dinitro-2-aminothiophene were synthesized in our laboratory, and the details of synthesis will be reported elsewhere. All other starting materials, reagents and solvents, purchased from Aldrich, were used without further puri®cation unless otherwise mentioned. 2.3. Preparation of PA and PB Polyamic acids PA and PB were synthesized as described in detail in Refs. [11,12]. After polymerization, the polymer solution was precipitated in water. The polymer was collected by ®ltration and washed with plenty of water. It was then dried in vacuum at 50°C overnight to give the precursor polymers PA and PB. 2.4. Preparation of PA-1 An aqueous solution of sodium nitrite ( 0.138 g, 0.3 ml H2 O) was added dropwise to a solution of 4-nitroaniline in 18% hydrochloric acid solution (1.2 ml ) at 0°C. The resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. Then, the solution was stirred below 5°C overnight. PA-1 was obtained by precipitation of the above solution in water and was washed with plenty of water until a neutral stage was achieved. The polymer was further puri®ed by Soxhlet extraction with acetone for 4 h. Yield: 0.876 g (75%) ; kmax 478 nm, 1 H-NMR, d 2.71(s, 4H), 2.86(s, 4H), 7.04(s, 2H), 7.24(s, 1H), 7.75(s, 2H), 7.86(s, 2H), 8.13(s, 1H), 8.25(s, 2H), 8.73(d, 2H). 2.5. Preparation of PA-2 Sodium nitrite (0.138 g, 2.0 mmol) was added slowly to concentrated sulfuric acid (5 ml), and the mixture was
heated to 70°C under stirring. Sodium nitrite dissolved slowly and a solution of nitrosyl sulfuric acid was formed. After the solution was cooled to 5°C, 2,4-dinitro aniline (0.366 g, 2.0 mmol) was added and the mixture was stirred at 5°C for 40 min. Then, the resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. PA2 was prepared via a procedure similar to that described above for the synthesis of PA-1. Yield: 0.860 g (69%), kmax 523 nm, 1 H-NMR, d 2.72(s, 4H), 2.90(s, 4H), 6.53(s, 1H), 6.79(s, 1H), 7.08(s, 2H), 7.72(s, 2H), 7.92(s, 1H), 8.12(s, 1H), 8.44(d, 1H), 8.78(d, 2H). 2.6. Preparation of PA-3 Sodium nitrite (0.138 g, 2.0 mmol) was added slowly to sulfuric acid (5 ml), and the mixture was heated to 70°C under stirring. After the solution was cooled to 5°C, a mixture of propanoic acid (1.0 ml) and acetic acid (5.0 ml) was added. 5-nitro-2-aminothiophene (0.288 g, 2.0 mmol) was added, and the mixture was stirred at 5°C for 40 min. Then, the resulting diazonium salt solution was added to a solution of PA (0.858 g, 2.0 mmol) in 50 ml of dimethyl formamide. PA-3 was prepared via a procedure similar to that described above for the synthesis of PA-1. Yield: 0.958 g (82%), kmax 558 nm, 1 HNMR, d 2.71(s, 4H), 2.89(s, 4H), 7.08(d, 1H), 7.13(d, 1H), 7.75(s, 2H), 7.94(s, 1H), 8.04(s, 1H), 8.15(s, 2H), 8.70(d, 2H). 2.7. Preparation of PB-1 Via a procedure similar to that described above for the synthesis of PA-1. Yield: 1.552 g (75%), kmax 471 nm, 1 H-NMR, d 4.05(s, 4H), 4.60(s, 4H), 7.18(d, 4H), 7.62(d, 2H), 7.71(s, 4H), 7.85(s, 2H), 7.94(d, 2H), 8.04(s, 2H), 8.17(s, 2H), 8.17(s, 2H), 8.32(s, 2H), 8.42(s, 2H), 8.59(d, 2H). 2.8. Preparation of PB-2 Via a procedure similar to that described above for the synthesis of PA-2. Yield: 1.487 g (82%), kmax 513 nm, 1 H-NMR, d 4.07(s, 4H), 4.60(s, 4H), 7.20(s, 4H), 7.58(d, 2H), 7.62(s, 1H), 7.69(s, 2H), 7.78(s, 2H), 7.95(s, 2H), 8.04(s, 2H), 8.09(s, 1H), 8.16(s, 1H), 8.41(s, 2H). 2.9. Preparation of PB-3 Via a procedure similar to that described above for the synthesis of PA-3. Yield: 1.528 g (88%), kmax 550 nm, 1 H-NMR, d 4.08(s, 4H), 4.60(s, 4H), 6.94(s, 2H), 7.19(s, 4H), 7.62(s, 2H), 7.72(s, 4H), 7.96(s, 2H), 8.02(s, 2H), 8.10(s, 1H), 8.15(s, 1H), 8.41(s, 2H).
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2.10. Preparation of PB-4 Via a procedure similar to that described above for the synthesis of PA-3. Yield: 1.187 g (65%), kmax 630 nm, 1 H-NMR, d 3.87(s, 4H), 4.51(s, 4H), 6.94(s, 2H), 7.16(s, 2H), 7.68(s, 4H), 7.76(s, 4H), 7.98(s, 2H), 8.03(s, 2H), 8.17(s, 1H), 8.41(s, 2H). 2.11. Polymer ®lm preparation and poling The polymers were dissolved in dry distilled dimethyl acetamide. The homogeneous solutions were ®ltered through a 0.2 lm Te¯on ®lter. Glass slides were rinsed with a cleaning solution, water, deionized water, respectively, and then dipped in iso-propyl alcohol before use. The polymer ®lms were dried under vacuum at 50°C for 8 h. The thickness of the ®lm varied from 0.2 to 1.8 lm, which depends upon the solution concentration (15 wt.%) and the spin speed (1200±2000 rpm). Poling was performed by the corona poling technique in a typical setup. A tungsten needle was used as electrode. The poling condition is as follows: temperature 200°C, high voltage 10 kV at needle point, poling time 30 min, gap distance 1.5 cm, poling current < 2:5 lA. The polymer ®lms were cooled down to room temperature in the presence of an electrical ®eld. 2.12. Nonlinear optical property measurement The second-order NLO coecient (d33 ) of the poled ®lms was measured by second harmonic generation (SHG). The source was a Q-switched Nd±YAG laser operating at a wavelength of 1064 nm. The SHG signal at 532 nm, selected with an interference ®lter, was detected by a photomultiplier tube and measured with a boxcar integrator. A Y-cut quartz crystal (d11 0:4 pm=V) was used as the reference. By comparing the SHG intensity from the poled polymer sample with that from the quartz crystal the NLO coecient d33 of the poled polymers was determined. 3. Results and discussion 3.1. Polymer synthesis and characterization The structure of precursor polyamic acids PA, PB is shown in Scheme 1 (structure of NLO polymers from port-azo coupling reaction) and they were synthesized as described in Refs. [11,12]. The intrinsic viscosity measurements indicated values of 0.072 and 0.063 dl=g for polyamic acids PA and PB, respectively. The precursor polymers PA and PB all have good solubility in polar organic solvents such as DMF, NMP, etc. The two polyamic acids were functionalized to introduce dierent chromophores at the ®nal stage of the synthetic route,
Scheme 1.
and the successful preparation of the stable diazo salt solution was critical in this step. The diazo salt solution 4-nitro aniline can be prepared easily in the HCl/H2 O medium. But the other three amino compounds cannot dissolve in this medium. Thus, nitrosyl sulfuric acid solution was used, and the diazo salt solution of these compounds was obtained, which can stay stable below 5°C. The polymer nomenclature includes two parts: the ®rst part refers to the main chain polyamic acid, and the second part refers to the dierent NLO chromophores. It should be pointed out that we also prepared polymer PA-4. But after puri®cation, it cannot redissolve back into organic solvents. This can be explained by the chromophore±chromophore interaction. Compound PA-4 has dinitro thiophene as the side chain, and the interaction between NLO chromophores may be greater than other three chromophores. Compound PB-4 can dissolve in organic solvents, because the spacer between its chromophores is larger than PA-4. The structures and purity of the polymers were investigated and con®rmed by various spectroscopic methods. The dierent NLO chromophores of the polymers were characterized by 1 H-NMR spectroscopy as shown in Section 2. The degree of functionalization or the chromophore loading density was determined by
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Table 1 Nonlinearities of synthesized NLO polymers Polymers
d33 a (pm/v)
Tb (lm)
kmax c (nm)
/d (%)
d33 //e
PA-1 PA-2 PA-3 PB-1 PB-2 PB-3 PB-4
50.4 52.2 51.0 17.5 22.1 32.5 19.5
0.13 0.27 0.13 0.33 0.30 0.36 1.39
478 523 558 471 513 550 630
100 58.6 84.4 100 92.1 100 50.2
50.4 86.0 60.4 17.5 24.0 32.5 38.8
a
d33 value of the ®lms measured directly by SHG method. Thickness of the ®lms. c kmax measure in ®lm form. d Chromophore loading density. e d33 value divided by chromophore loading density. b
comparing the areas of methyl 1 H proton and aromatic proton in 1 H-NMR spectroscopy, which is listed in Table 1. All the polymers have a high-chromophore loading density, and the dierence in chromophore loading density was caused mainly by the dierent stability of the relative azo salt and the eciency of the azocoupling reaction. The UV±VIS spectra of the thin ®lms of the series of polymer are given in Fig. 1. The color of the ®lms changes from red, purple to green, and all the polymers exhibit strong absorption in the visible range. The kmax values of all the polymers are listed in Table 1. Increasing the electron withdrawing eect or replacement of the benzene ring with thiophene both result in a red shift of the kmax and an intensity increase. As thiophene has a lower delocalization energy than that of benzene, it can oer better eective conjugation than that of benzene in a donor±acceptor system. For the series of PA and PB polymers, all PA polymers have relative red shift kmax values. The reason may be the dierent chemical environment of the two series of polymers. 3.2. Thermal properties The thermal properties of side chain NLO polymers can be considered as two parts: thermal properties of the main chain and the side chain. The former accounts for the thermal properties of the polymer backbone and the relaxation eect of the NLO chromophores. The latter refers to the thermal properties of the NLO chromophore itself. We found that the two series of polymers can be imidized at 180°C, which was con®rmed by FTIR spectra. After imidization, the new absorption peak near 1780 cmÿ1 appeared, which was the typical absorption for the imide structure. TGA trace studies of the polymers indicate that after imidization, the polyimides were thermally stable up to 300°C under a nitrogen atmosphere. The thermal properties of the side chain NLO chromophore were indicated with UV±VIS spectroscopy.
Fig. 1. UV±VIS absorption spectra of (a) PA-n and (b) PB-n spin-coated ®lms.
Each thin ®lm of the polymers was heated to 160°C, 180°C, 200°C, 220°C, 240°C and kept at that temperature for 15 min, respectively. Each UV±VIS spectroscopy was recorded, and the changes in absorbance are shown in Fig. 2. All these polymers retained 85% or 70% of its original absorbance intensity at 180°C or 200°C. We also found that the thermal stability of those four kinds of NLO chromophore is 1 > 2 > 3 > 4. As thiophene has a less aromatic stabilization energy than the benzene ring, replacement of one nitro group with two will also aect aromatic stabilization. It has been pointed out that if the Tg of the polymer is roughly 150± 200°C above the ultimate operating temperature, decay of the orientation would be negligible over the device lifetime [13]. So, our NLO polymers could all satisfy the demand of the thermal stability for the device fabrication. 3.3. Nonlinear optical properties All the NLO polymers were spin cast into thin ®lms, and the thermal imidization and poling process were
Z. Li et al. / European Polymer Journal 36 (2000) 2417±2421
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conjugation than benzene in donor±acceptor compounds. Its substitution in donor±acceptor compounds should be expected to result in enhanced charge-transfer properties and nonlinear responses than in benzenoid rings. The two series of NLO polymers exhibited a good nonlinear optic response. The stability of the nonlinear property at elevated temperatures was very important for the NLO polymers. We also tested one kind of our polymer: PB-1. After keeping the poled sample of PB-1 in an oven for 1000 h at 1000°C, the d33 value retained 50% of its original value. It is reasonable to assume that other kinds of polymers have a good nonlinear stability at elevated temperatures. In summary, two series of NLO polyamic acids with dierent chromophores were prepared and characterized. The optical and thermal properties of these polymers were compared. The d33 value of these polymers was measured using the SHG method. Increasing the electron-withdrawing eect or replacing the benzene ring with thiophene will result in a red shift of kmax and an enhancement of the nonlinear property. Considering the relatively simple synthetic method, the good thermal stability and processibility, the high nonlinear response, our NLO polymers are promising candidates as electrooptic materials for device fabrication.
References Fig. 2. Relationship between temperature and the relative intensity of UV±VIS spectra for the synthesized NLO polymer: (a) PA-n and (b) PB-n.
accomplished at the same time. As during the imidization process, the polymer will lose water and become tighter. This is of bene®t to the orientation of the dipole. The second-order NLO coecient (d33 ) was measured by the SHG. The d33 value is related directly to the chromophore loading density. In order to compare it eectively, we divided the d33 value with chromophore loading density and the result was listed in Table 1. The weight percent of chromophore of the series of PA (30%) was higher than that of PB (20%), so PA polymers all have a relatively greater d33 value than the corresponding PB polymers. For dierent NLO chromophores, the d33 value increased with an increase in the electronwithdrawing ability or replacement of benzene ring with thiophene. Because the second-order molecular nonlinear hyperpolarizability (bl) increases with increasing donor and acceptor strengths, and thiophene has a lower delocalization energy than benzene, it can oer better
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