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Synthesis and characterization of hyper-branched polyimides from 2,4,6-triaminopyrimidine and dianhydrides system
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Materials Chemistry and Physics 108 (2008) 214–219
Synthesis and characterization of hyper-branched polyimides from 2,4,6-triaminopyrimidine and dianhydrides system Soo-Jin Park a,∗ , Kai Li a , Fan-Long Jin b a
b
Department of Chemistry, Inha University, Nam-gu, Incheon 402-751, South Korea Department of Chemical Engineering, Jilin Institute of Chemical Technology, Jilin City 132022, People’s Republic of China Received 18 September 2007; accepted 19 September 2007
Abstract A series of aromatic hyper-branched polyimides were successfully prepared by condensation polymerization of commercially available A2 -type dianhydride monomers 4,4-biphthalic anhydride (BPAD) and 3,3 ,4,4 -diphenylsulfone tetracarboxylic dianhydride (DSDA) with the BB2 -type triamine monomer 2,4,6-triaminopyrimidine (TAP). The hyper-branched polyimides, with two different terminated groups, were obtained from the reactions between different molar ratios of the TAP and the dianhydrides. Both Fourier transform infrared (FT-IR) and 1 H NMR spectroscopy were used to verify the molecular structures of the obtained hyper-branched polyimides. The molecular weights were determined by gel permeation chromatography (GPC). The results suggested that the amine-terminated hyper-branched polyimides displayed lower degrees of branching and molecular weights than the corresponding anhydride-terminated ones. However, the anhydride-terminated hyper-branched polyimides showed a relatively lower glass transition temperature, obtained by differential scanning calorimetry, which could be attributed to the increased free volume and mobility of the macromolecules caused by the absence of chain-end interactions. Thermogravimetric analysis (TGA) results indicated that the hyper-branched polyimides had excellent thermal stabilities, the amine-terminated hyper-branched polyimides showing higher thermal stabilities than those of the anhydride-terminated ones. © 2007 Elsevier B.V. All rights reserved. Keywords: Hyperbranched; Polyimide; Condensation polymerization
1. Introduction In recent years, the synthesis of hyper-branched polymers has received considerable attention in the chemistry, biology, and biomedicine fields, owing to their unique properties compared with their linear analogues, which can be attributed to their highly branched structures [1,2]. Since the first intentional preparation of hyper-branched polymers, many types have been synthesized, including polyimide [3], polyether [4], polymethacrylate [5], polyphenylene [6], poly(ether ketone) [7], polyester [8], and polyurethane [9]. Among these, hyper-branched polyimides are an important class of high-performance polymers offering outstanding properties such as high mechanical strength, high modulus, superior chemical resistance, excellent electrical properties, and good thermoxidative stability. Owing to these advantages, polyimides
∗
Corresponding author. Tel.: +82 42 860 7234; fax: +82 42 861 4151. E-mail address: [email protected] (S.-J. Park).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.017
have received the most attention among polymers, having been applied widely and on large scales. Hyper-branched polyimides have been successfully prepared by self-polymerization of both ABx -type monomers and A2 + B3 monomers. However, both methods are impractical for large-scale preparation of hyper-branched polyimides, given the requirement of special ABx monomers for the former method and, for the latter, strict polymerization conditions such as low monomer concentrations, strictly controlled slow addition rates and molar ratios of monomers, in order to avoid gel formation. Many researchers have investigated the factors involved in the polymerization of A2 + B3 monomer systems [2,10]. Flory’s theory of gelation formation in the polymerization of A2 + B3 monomers is based on the following requirements: (1) the equal reactivity of all A groups as well as B groups, (2) the exclusive reactivity of A groups with B groups, and (3) no intramolecular cyclization or chain termination in the process [11]. However, if any of these requirements is not met, the theory is invalid, and so the relaxation of such strictures on the polymerization process has been sought. Recently, many efforts have been made to avoid
S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219
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gelation during the preparation of hyper-branched polyimides. The most promising approach is to use monomers containing the same functional groups but with different reactivities [12,13]. In this study, we adopted 2,4,6-triaminopyrimidine (TAP) as a BB2 monomer for the preparation of hyper-branched polyimides by the A2 + BB2 approach. Using TAP as a BB2 monomer containing amine groups of different reactivities can effectively prevent gel formation during polymerization [12]. The molecular formations, molecular weights, and thermal properties of a series of hyper-branched polyimides were studied by FT-IR, 1 H NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and TGA.
2.3. Synthesis of amine-terminated DSDA–TAP polyimide
2. Experimental procedure
TAP (0.19 g, 1.5 mmol), BPAD (0.88 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. The yield was 87%. FT-IR (KBr): 1782, 1728 cm−1 (C O, imide), 1384 cm−1 (C–N, imide), 1851 cm−1 (C O, terminal anhydride). 1 H NMR (DMSO-d ): δ = 8.30 (C C–H, aromatic ring), 7.82 (N–H, 4,66 diimide), 7.68 (C C–H, free anhydride), 7.39 (N–H, 2,4- and 2,6-diimide), 6.94 (C C–H, 2,4- and 2,6-diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide).
2.1. Materials TAP as a triamine monomer, as well as 4,4-biphthalic anhydride (BPAD) and 3,3 ,4,4 -diphenylsulfonetetracarboxylic dianhydride (DSDA) as dianhydride monomers were purchased from Tokyo Kasei Kogyo Co. N,NDimethylacetamide (DMAc) and m-xylene, used as the reaction media, were also supplied by Tokyo Kasei Kogyo Co. The solvent methyl sulfoxide was kindly provided by Aldrich Chem. The chemical structures of TAP, BPAD, and DSDA are shown in Fig. 1.
2.2. Synthesis of amine-terminated BPAD–TAP polyimide TAP (0.38 g, 3 mmol), BPAD (0.88 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask. The mixture was stirred with a magnetic stirring bar at 30 ◦ C for 15 h under N2 flow. Then, 10 ml of m-xylene was added, and the mixture was heated to 160 ◦ C for 7 h with a Dean-Stark apparatus. After cooling to room temperature, the mixture was poured into methanol, and yellow precipitate was formed. The products were then obtained by filtration followed by drying at 80 ◦ C for 24 h. The yield was 83%. FT-IR (KBr): 1782, 1728 cm−1 (C O, imide), 1384 cm−1 (C–N, imide). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.82 (N–H, 6 4,6-diimide), 7.39 (N–H, 2,4- and 2,6-diimide), 6.94 (C C–H, 2,4- and 2,6diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide), 5.81 (C C–H, 4- and 6-monoimide), 5.44 (C C–H, 2-monoimide).
TAP (0.38 g, 3 mmol), DSDA (1.08 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. The yield was 82%. FT-IR (KBr): 1782, 1727 cm−1 (C O, imide), 1384 cm−1 (C–N, imide). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.81 (N–H, 6 4,6-diimide), 7.39 (N–H, 2,4- and 2,6-diimide), 6.95 (C C–H, 2,4- and 2,6diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide), 5.80 (C C–H, 4- and 6-monoimide), 5.44 (C C–H, 2-monoimide).
2.4. Synthesis of anhydride-terminated BPAD–TAP polyimide
2.5. Synthesis of anhydride-terminated DSDA–TAP polyimide TAP (0.19 g, 1.5 mmol), DSDA (1.08 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. Yield is 88%. FT-IR (KBr): 1782, 1727 cm−1 (C O, imide), 1383 cm−1 (C–N, imide), 1851 cm−1 (C O, terminal anhydride). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.82 (N–H, 4,66 diimide), 7.69 (C C–H, free anhydride), 7.39 (N–H, 2,4- and 2,6-diimide), 6.95 (C C–H, 2,4- and 2,6-diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide).
2.6. Characterization and measurements Fourier transform infrared (FT-IR) spectroscopy on KBr pellets was performed with a Bio-Rad Co. digilab FTS-165. 1 H NMR spectra were determined on a Bruker DRX300 spectrometer operating at 500 MHz using deuterium methyl sulfoxide (DMSO-d6 ) as solvent. Number-average molecular weights and weight-average molecular weights were evaluated by GPC carried out on a Waters 2690 apparatus with a column of Mixed-B X2. DMF containing 0.05 M LiBr was used as the eluant, and the molecular weights were calibrated against standard polystyrenes. The glass transition temperature was studied by differential scanning calorimetry (DSC, Perkin-Elmer, DSC 6) under nitrogen gas. TGAs were performed, using a du Pont TGA-2950 analyzer, to investigate the thermal stabilities of the hyper-branched polyimides from 25 ◦ C to 800 ◦ C at the heating rate of 10 ◦ C/min in a nitrogen atmosphere.
3. Results and discussion 3.1. Synthesis of hyper-branched polyimides
Fig. 1. Chemical structures of the materials used.
In our work, the amine- and anhydride-terminated hyperbranched polyimides were prepared according to 1:1 and 1:2 triamine-to-dianhydride feed ratios, respectively. Fig. 2 shows the chemical structures of both the amine- and anhydrideterminated hyper-branched polyimides, using BPAD–TAP as an example. Polymerization of dianhydride with TAP was performed in a mixture of triamine and dianhydride with DMAc
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Fig. 2. Chemical structures of BPAD–TAP hyper-branched polyimides: (a) amine-terminated and (b) anhydride-terminated.
as a solvent medium. Polymerization involves two steps, as shown in Scheme 1, illustrating the formation of hyper-branched polyamic acid precursors in the first step and imidization in the second step. In the first step, pure TAP reacts with dianhydride to form the soluble polyamic acid precursor. Then, further imidization occurs when the temperature is increased to 160 ◦ C while water is removed by adding m-xylene to the mixture. 3.2. Characterization of hyper-branched polyimides The molecular weights of the obtained hyper-branched polyimides were measured by GPC. The results, shown in Table 1, indicate that the anhydride-terminated polyimides had higher molecular weights than the amine-terminated polyimides.
The molecular structures of the amine- and anhydrideterminated BPAD–TAP hyper-branched polyimides were analyzed by FT-IR, and the results are shown in Fig. 3. The bands around 1782 cm−1 (C O asymmetrical stretching) and 1728 cm−1 (C O symmetrical stretching) are the characteristic absorption bands of polyimide. However, the characteristic band of polyamic acid around 1670 cm−1 was not found [14]. Also, the band around 1851 cm−1 attributed to the stretching of C O of the terminal anhydride group was observed in the spectrum of the anhydride-terminated polyimide, but not in that of the amine-terminated polyimide, indicating that there was no free anhydride group remaining in the amine-terminated polyimide. Fig. 4 represents all of the possible TAP molecule structures in the obtained polymers. Monoimide, diimide, and triimide stand
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Scheme 1. Two-steps synthesis of hyper-branched polyimides. Table 1 Characterization of the prepared hyper-branched polyimides Polyimide
Terminal group
Mn a
Mw a
PDIa
DBb
Anhydride:TAP in productb
BPAD–TAP
Amine-terminated Anhydride-terminated
8,851 11,024
10,540 13,229
1.19 1.20
0.32 0.75
1.2:1.0 1.9:1.0
DSDA–TAP
Amine-terminated Anhydride-terminated
8,123 10,873
9,341 12,613
1.15 1.16
0.34 0.71
1.1:1.0 1.8:1.0
a b
Determined by GPC in DMF with 0.05 M LiBr. Determined by 1 H NMR spectra.
for terminal (T), linear (L), and dendritic (D) units, respectively. Fig. 5 shows the two types of anhydride, the imide–imide type and the imide–anhydride type, existing in the polymers. Fig. 6 shows the 1 H NMR spectra of both the amine- and anhydrideterminated BPAD–TAP polyimides. The 1 H shifts of H5 in the terminal and linear units are summarized in Table 2. Those in the dentritic TAP are overlapped with those of BPAD around
7.90 ppm. The peak at 8.29 ppm should be assigned to H5 in fully imidized BPAD in the 1 H NMR spectra [15]. The peaks appearing at 7.55 and 7.68 ppm in Fig. 6(b) represent the H atoms (H5 units in Fig. 5(b)) in the form of free anhydride groups existing in the anhydride-terminated polyimide, while no such characteristic peaks appeared in Fig. 6(a), indicating that there was no free anhydride group remaining in the amine-terminated polyimide. Meanwhile, the spectra, according to the data listed in Table 2, suggest that both the amine- and anhydride-terminated polyimides contain free amine groups in the forms of both linear and terminal TAPs. This result deviates from the theoretical design, which suggests that there should be no free amine groups in anhydride-terminated polyimide. The existence of free amine groups in anhydrideterminated polyimide might be attributed to steric hindrance and the deactivation of the amine group in TAP, preventing the total imidization of all of the amine groups [16]. The structural perfection of hyper-branched polymers is generally characterized by the degree of branching (DB), which is defined by Frey and co-workers [17] as DB =
Fig. 3. FT-IR spectrums of BPAD–TAP hyper-branched polyimides: (a) amineterminated and (b) anhydride-terminated.
D+T D+T +L
(1)
where D and L represent the numbers of dendritic and linear units in the polymer, respectively. Experimentally, the DB is
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S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219
Fig. 4. Possible structures of TAP in hyper-branched polyimides. Table 2 Structural characteristics of TAP units, determined by 1 H NMR Structure unit
Chemical shift (ppm) H5
NH2
2-Monoimide 4-, 6-Monoimide
5.44 5.81
6.53 6.49
4,6-Diimide 2,4-, 2,6-Diimide
6.73 6.94
7.82 7.39
T
L
usually determined from 1 H NMR or 13 C NMR spectroscopy by comparing the integration of the peaks for the respective units in the hyper-branched polymers [18]. The DB results are listed in Table 1. It can be seen that the DB of the amine-terminated BPAD–TAP polyimide is 0.32, whereas that of the anhydride-terminated BPAD–TAP polyimide is 0.75. The same trend was observed in the case of
Fig. 5. Possible structures of dianhydride in hyper-branched polyimides.
the DSDA–TAP polyimides, the DB of which is also shown in Table 1. The results indicate that anhydride-terminated polyimides have better-branched structures than amine-terminated polyimides, which might be a consequence of the lesser reactivity of the TAP unit. The compositions of the products, the molar ratios of BPAD:TAP, can also be calculated on the basis of 1 H NMR spectra, with Eq. (2) [12]: BPAD : TAP =
I7.5−8.3 − ID − 2I6.73 6 : (ID + IL + ID )
(2)
where ID , IL , and IT are the same as in Eq. (2), and I7.5–8.3 and I6.73 represent the peak intensities at 7.5–8.3 and 6.73 ppm, respectively.
Fig. 6. 1 H NMR spectra of BPAD–TAP hyper-branched polyimides: (a) amineterminated and (b) anhydride-terminated.
S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219 Table 3 Thermal properties of hyper-branched polyimides Polyimide
Terminal group
Tg (◦ C)
T5% (◦ C)
T10% (◦ C)
BPAD–TAP
Amine-terminated Anhydride-terminated
272 261
464 460
494 487
DSDA–TAP
Amine-terminated Anhydride-terminated
270 267
457 451
486 480
The results, listed in Table 1, suggest that the compositions of the obtained polyimides are almost identical to those of the feeds. 3.3. Thermal properties The glass transition temperatures (Tg ) of the hyper-branched polyimides determined by DSC are shown in Table 3. It can be seen that the anhydride-terminated polyimides had lower Tg than the amine-terminated ones, which can be attributed to the increased free volume and mobility of the macromolecules caused by the absence of chain-end interactions [19–21]. Another possible explanation is that the amine-terminated polyimides had a stronger macromolecular interaction resulting from hydrogen bonding between the carbonyl groups and the terminal amine groups [10]. TGAs were used to study the thermal stabilities of the hyperbranched polyimides. Fig. 7 shows the TGA curves of both the amine- and anhydride-terminated polyimides. The thermal stability factors determined from the TGA thermograms are shown in Table 3 [22,23]. The results show the excellent thermal stabilities of the polyimides, with 5% weight loss at temperatures ranging from 451 ◦ C to 464 ◦ C and 10% weight loss at temperatures ranging from 480 ◦ C to 494 ◦ C. The amine-terminated polyimides showed slightly higher thermal stabilities compared with the corresponding anhydride-terminated ones, which can be attributed to the different terminal functional groups and the anhydride residues. Furthermore, the fact that the BPAD–TAP polyimides showed a slightly higher thermal stability than the DSDA–TAP polyimides possibly can be explained by the differ-
Fig. 7. TGA curves of BPAD–TAP hyper-branched polyimides in nitrogen.
219
ent chemical structure of the anhydrides, meaning that the free volume of hyper-branched polyimides is increased by the bulky –SO2 groups in DSDA–TAP, which can account for a lower thermal property. 4. Conclusions A new series of aromatic hyper-branched polyimides were successfully synthesized from A2 + BB2 monomer systems using a series of commercially available A2 -type dianhydride monomers and a BB2 -type triamine monomer. No gelation occurred during polymerization in high-monomer concentrations, which can be attributed to the different reactivities of the amine groups in TAP. This made it easier to prepare hyperbranched polyimides on a large scale, because it allowed the monomers to be mixed together at high concentrations in the solvent. The DB values of the obtained hyper-branched polyimides differed according to different end groups, which indicated that the anhydride-terminated polyimides had a higher DB than the amine-terminated ones. The increased DB was responsible for the increased free volume and macromolecular mobility, which led to the relatively lower Tg . TGA results suggested that the obtained polyimides had good thermal stabilities, with 5% losses at temperatures over 450 ◦ C in a nitrogen atmosphere, the amine-terminated polyimides showing higher values than the corresponding anhydride-terminated ones. These results probably can be attributed to the different terminal functional groups. References [1] G.C. Behera, S. Ramakrishnan, Macromolecules 37 (2004) 9814. [2] J. Hao, M. Jikei, M. Kakimoto, Macromol. Symp. 199 (2003) 233. [3] M. Jike, S.H. Chon, M. Kakimoto, S. Kawauchi, T. Imase, J. Watanebe, Macromolecules 32 (1999) 2061. [4] H. Magnusson, E. Malmstrom, A. Hult, Macromolecules 34 (2001) 5786. [5] C.Y. Hong, C.Y. Pan, Y. Huang, Z.D. Xu, Polymer 42 (2001) 6733. [6] K. Xu, H. Peng, Q. Sun, Y. Dong, F. Salhi, J. Luo, J. Chen, Y. Huang, D. Zhang, Z. Xu, B.Z. Tang, Macromolecules 35 (2002) 5821. [7] S.Y. Cho, Y. Chang, J.S. Kim, S.C. Lee, C. Kim, Macromol. Chem. Phys. 202 (2001) 263. [8] M. Trolls˚as, J. Hedrick, D. Mecerreyes, R. J´erˆome, P.H. Dubois, J. Polym. Sci. A: Polym. Chem. 36 (1998) 3187. [9] H.W. Lu, S.H. Liu, X.L. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Mater. Chem. Phys. 81 (2003) 104. [10] J. Fang, H. Kita, K. Okamoto, Macromolecules 33 (2000) 4639. [11] P.J. Flory, J. Am. Chem. Soc. 74 (1952) 2718. [12] Y. Liu, T.S. Chung, J. Polym. Sci. A: Polym. Chem. 40 (2002) 4563. [13] D. Yan, C. Gao, Macromolecules 33 (2000) 7693. [14] R.W. Snyder, B. Thomson, B. Bartges, D. Czerniawski, P.C. Painter, Macromolecules 22 (1989) 4166. [15] J. Xu, C. He, T.S. Chung, J. Polym. Sci. A: Polym. Chem. 39 (2001) 2998. [16] D.G. Hawthorne, J.H. Hodgkin, High Perform. Polym. 11 (1999) 315. [17] D. Holter, A. Burgath, H. Frey, Acta Polym. 48 (1997) 30. [18] C.J. Hawker, R. Lee, J.M.J. Frechet, J. Am. Chem. Soc. 113 (1991) 4583. [19] L.J. Markoski, J.S. Moore, I. Sendijarevic, A. Lee, A.J. Mchugh, Macromolecules 34 (2001) 2695. [20] H. Stuta, J. Polym. Sci. B: Polym. Phys. 33 (1995) 333. [21] K.L. Wooley, C.J. Hawker, J.M. Pochan, Macromolecules 26 (1993) 1514. [22] S.J. Park, F.L. Jin, J.R. Lee, Macromol. Chem. Phys. 205 (2004) 2048. [23] S.J. Park, H.C. Kim, H.L. Lee, D.H. Suh, Macromolecules 34 (2001) 573.
Materials Chemistry and Physics 108 (2008) 214–219
Synthesis and characterization of hyper-branched polyimides from 2,4,6-triaminopyrimidine and dianhydrides system Soo-Jin Park a,∗ , Kai Li a , Fan-Long Jin b a
b
Department of Chemistry, Inha University, Nam-gu, Incheon 402-751, South Korea Department of Chemical Engineering, Jilin Institute of Chemical Technology, Jilin City 132022, People’s Republic of China Received 18 September 2007; accepted 19 September 2007
Abstract A series of aromatic hyper-branched polyimides were successfully prepared by condensation polymerization of commercially available A2 -type dianhydride monomers 4,4-biphthalic anhydride (BPAD) and 3,3 ,4,4 -diphenylsulfone tetracarboxylic dianhydride (DSDA) with the BB2 -type triamine monomer 2,4,6-triaminopyrimidine (TAP). The hyper-branched polyimides, with two different terminated groups, were obtained from the reactions between different molar ratios of the TAP and the dianhydrides. Both Fourier transform infrared (FT-IR) and 1 H NMR spectroscopy were used to verify the molecular structures of the obtained hyper-branched polyimides. The molecular weights were determined by gel permeation chromatography (GPC). The results suggested that the amine-terminated hyper-branched polyimides displayed lower degrees of branching and molecular weights than the corresponding anhydride-terminated ones. However, the anhydride-terminated hyper-branched polyimides showed a relatively lower glass transition temperature, obtained by differential scanning calorimetry, which could be attributed to the increased free volume and mobility of the macromolecules caused by the absence of chain-end interactions. Thermogravimetric analysis (TGA) results indicated that the hyper-branched polyimides had excellent thermal stabilities, the amine-terminated hyper-branched polyimides showing higher thermal stabilities than those of the anhydride-terminated ones. © 2007 Elsevier B.V. All rights reserved. Keywords: Hyperbranched; Polyimide; Condensation polymerization
1. Introduction In recent years, the synthesis of hyper-branched polymers has received considerable attention in the chemistry, biology, and biomedicine fields, owing to their unique properties compared with their linear analogues, which can be attributed to their highly branched structures [1,2]. Since the first intentional preparation of hyper-branched polymers, many types have been synthesized, including polyimide [3], polyether [4], polymethacrylate [5], polyphenylene [6], poly(ether ketone) [7], polyester [8], and polyurethane [9]. Among these, hyper-branched polyimides are an important class of high-performance polymers offering outstanding properties such as high mechanical strength, high modulus, superior chemical resistance, excellent electrical properties, and good thermoxidative stability. Owing to these advantages, polyimides
∗
Corresponding author. Tel.: +82 42 860 7234; fax: +82 42 861 4151. E-mail address: [email protected] (S.-J. Park).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.017
have received the most attention among polymers, having been applied widely and on large scales. Hyper-branched polyimides have been successfully prepared by self-polymerization of both ABx -type monomers and A2 + B3 monomers. However, both methods are impractical for large-scale preparation of hyper-branched polyimides, given the requirement of special ABx monomers for the former method and, for the latter, strict polymerization conditions such as low monomer concentrations, strictly controlled slow addition rates and molar ratios of monomers, in order to avoid gel formation. Many researchers have investigated the factors involved in the polymerization of A2 + B3 monomer systems [2,10]. Flory’s theory of gelation formation in the polymerization of A2 + B3 monomers is based on the following requirements: (1) the equal reactivity of all A groups as well as B groups, (2) the exclusive reactivity of A groups with B groups, and (3) no intramolecular cyclization or chain termination in the process [11]. However, if any of these requirements is not met, the theory is invalid, and so the relaxation of such strictures on the polymerization process has been sought. Recently, many efforts have been made to avoid
S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219
215
gelation during the preparation of hyper-branched polyimides. The most promising approach is to use monomers containing the same functional groups but with different reactivities [12,13]. In this study, we adopted 2,4,6-triaminopyrimidine (TAP) as a BB2 monomer for the preparation of hyper-branched polyimides by the A2 + BB2 approach. Using TAP as a BB2 monomer containing amine groups of different reactivities can effectively prevent gel formation during polymerization [12]. The molecular formations, molecular weights, and thermal properties of a series of hyper-branched polyimides were studied by FT-IR, 1 H NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and TGA.
2.3. Synthesis of amine-terminated DSDA–TAP polyimide
2. Experimental procedure
TAP (0.19 g, 1.5 mmol), BPAD (0.88 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. The yield was 87%. FT-IR (KBr): 1782, 1728 cm−1 (C O, imide), 1384 cm−1 (C–N, imide), 1851 cm−1 (C O, terminal anhydride). 1 H NMR (DMSO-d ): δ = 8.30 (C C–H, aromatic ring), 7.82 (N–H, 4,66 diimide), 7.68 (C C–H, free anhydride), 7.39 (N–H, 2,4- and 2,6-diimide), 6.94 (C C–H, 2,4- and 2,6-diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide).
2.1. Materials TAP as a triamine monomer, as well as 4,4-biphthalic anhydride (BPAD) and 3,3 ,4,4 -diphenylsulfonetetracarboxylic dianhydride (DSDA) as dianhydride monomers were purchased from Tokyo Kasei Kogyo Co. N,NDimethylacetamide (DMAc) and m-xylene, used as the reaction media, were also supplied by Tokyo Kasei Kogyo Co. The solvent methyl sulfoxide was kindly provided by Aldrich Chem. The chemical structures of TAP, BPAD, and DSDA are shown in Fig. 1.
2.2. Synthesis of amine-terminated BPAD–TAP polyimide TAP (0.38 g, 3 mmol), BPAD (0.88 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask. The mixture was stirred with a magnetic stirring bar at 30 ◦ C for 15 h under N2 flow. Then, 10 ml of m-xylene was added, and the mixture was heated to 160 ◦ C for 7 h with a Dean-Stark apparatus. After cooling to room temperature, the mixture was poured into methanol, and yellow precipitate was formed. The products were then obtained by filtration followed by drying at 80 ◦ C for 24 h. The yield was 83%. FT-IR (KBr): 1782, 1728 cm−1 (C O, imide), 1384 cm−1 (C–N, imide). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.82 (N–H, 6 4,6-diimide), 7.39 (N–H, 2,4- and 2,6-diimide), 6.94 (C C–H, 2,4- and 2,6diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide), 5.81 (C C–H, 4- and 6-monoimide), 5.44 (C C–H, 2-monoimide).
TAP (0.38 g, 3 mmol), DSDA (1.08 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. The yield was 82%. FT-IR (KBr): 1782, 1727 cm−1 (C O, imide), 1384 cm−1 (C–N, imide). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.81 (N–H, 6 4,6-diimide), 7.39 (N–H, 2,4- and 2,6-diimide), 6.95 (C C–H, 2,4- and 2,6diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide), 5.80 (C C–H, 4- and 6-monoimide), 5.44 (C C–H, 2-monoimide).
2.4. Synthesis of anhydride-terminated BPAD–TAP polyimide
2.5. Synthesis of anhydride-terminated DSDA–TAP polyimide TAP (0.19 g, 1.5 mmol), DSDA (1.08 g, 3 mmol), and DMAc (15 ml) were added to a four-neck flask, and the procedures that followed were the same as in the preparation of the amine-terminated BPAD–TAP polyimides. Yield is 88%. FT-IR (KBr): 1782, 1727 cm−1 (C O, imide), 1383 cm−1 (C–N, imide), 1851 cm−1 (C O, terminal anhydride). 1 H NMR (DMSO-d ): δ = 8.29 (C C–H, aromatic ring), 7.82 (N–H, 4,66 diimide), 7.69 (C C–H, free anhydride), 7.39 (N–H, 2,4- and 2,6-diimide), 6.95 (C C–H, 2,4- and 2,6-diimide), 6.73 (C C–H, 4,6-diimide), 6.53 (N–H, 2-monoimide), 6.49 (N–H, 4- and 6-monoimide).
2.6. Characterization and measurements Fourier transform infrared (FT-IR) spectroscopy on KBr pellets was performed with a Bio-Rad Co. digilab FTS-165. 1 H NMR spectra were determined on a Bruker DRX300 spectrometer operating at 500 MHz using deuterium methyl sulfoxide (DMSO-d6 ) as solvent. Number-average molecular weights and weight-average molecular weights were evaluated by GPC carried out on a Waters 2690 apparatus with a column of Mixed-B X2. DMF containing 0.05 M LiBr was used as the eluant, and the molecular weights were calibrated against standard polystyrenes. The glass transition temperature was studied by differential scanning calorimetry (DSC, Perkin-Elmer, DSC 6) under nitrogen gas. TGAs were performed, using a du Pont TGA-2950 analyzer, to investigate the thermal stabilities of the hyper-branched polyimides from 25 ◦ C to 800 ◦ C at the heating rate of 10 ◦ C/min in a nitrogen atmosphere.
3. Results and discussion 3.1. Synthesis of hyper-branched polyimides
Fig. 1. Chemical structures of the materials used.
In our work, the amine- and anhydride-terminated hyperbranched polyimides were prepared according to 1:1 and 1:2 triamine-to-dianhydride feed ratios, respectively. Fig. 2 shows the chemical structures of both the amine- and anhydrideterminated hyper-branched polyimides, using BPAD–TAP as an example. Polymerization of dianhydride with TAP was performed in a mixture of triamine and dianhydride with DMAc
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Fig. 2. Chemical structures of BPAD–TAP hyper-branched polyimides: (a) amine-terminated and (b) anhydride-terminated.
as a solvent medium. Polymerization involves two steps, as shown in Scheme 1, illustrating the formation of hyper-branched polyamic acid precursors in the first step and imidization in the second step. In the first step, pure TAP reacts with dianhydride to form the soluble polyamic acid precursor. Then, further imidization occurs when the temperature is increased to 160 ◦ C while water is removed by adding m-xylene to the mixture. 3.2. Characterization of hyper-branched polyimides The molecular weights of the obtained hyper-branched polyimides were measured by GPC. The results, shown in Table 1, indicate that the anhydride-terminated polyimides had higher molecular weights than the amine-terminated polyimides.
The molecular structures of the amine- and anhydrideterminated BPAD–TAP hyper-branched polyimides were analyzed by FT-IR, and the results are shown in Fig. 3. The bands around 1782 cm−1 (C O asymmetrical stretching) and 1728 cm−1 (C O symmetrical stretching) are the characteristic absorption bands of polyimide. However, the characteristic band of polyamic acid around 1670 cm−1 was not found [14]. Also, the band around 1851 cm−1 attributed to the stretching of C O of the terminal anhydride group was observed in the spectrum of the anhydride-terminated polyimide, but not in that of the amine-terminated polyimide, indicating that there was no free anhydride group remaining in the amine-terminated polyimide. Fig. 4 represents all of the possible TAP molecule structures in the obtained polymers. Monoimide, diimide, and triimide stand
S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219
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Scheme 1. Two-steps synthesis of hyper-branched polyimides. Table 1 Characterization of the prepared hyper-branched polyimides Polyimide
Terminal group
Mn a
Mw a
PDIa
DBb
Anhydride:TAP in productb
BPAD–TAP
Amine-terminated Anhydride-terminated
8,851 11,024
10,540 13,229
1.19 1.20
0.32 0.75
1.2:1.0 1.9:1.0
DSDA–TAP
Amine-terminated Anhydride-terminated
8,123 10,873
9,341 12,613
1.15 1.16
0.34 0.71
1.1:1.0 1.8:1.0
a b
Determined by GPC in DMF with 0.05 M LiBr. Determined by 1 H NMR spectra.
for terminal (T), linear (L), and dendritic (D) units, respectively. Fig. 5 shows the two types of anhydride, the imide–imide type and the imide–anhydride type, existing in the polymers. Fig. 6 shows the 1 H NMR spectra of both the amine- and anhydrideterminated BPAD–TAP polyimides. The 1 H shifts of H5 in the terminal and linear units are summarized in Table 2. Those in the dentritic TAP are overlapped with those of BPAD around
7.90 ppm. The peak at 8.29 ppm should be assigned to H5 in fully imidized BPAD in the 1 H NMR spectra [15]. The peaks appearing at 7.55 and 7.68 ppm in Fig. 6(b) represent the H atoms (H5 units in Fig. 5(b)) in the form of free anhydride groups existing in the anhydride-terminated polyimide, while no such characteristic peaks appeared in Fig. 6(a), indicating that there was no free anhydride group remaining in the amine-terminated polyimide. Meanwhile, the spectra, according to the data listed in Table 2, suggest that both the amine- and anhydride-terminated polyimides contain free amine groups in the forms of both linear and terminal TAPs. This result deviates from the theoretical design, which suggests that there should be no free amine groups in anhydride-terminated polyimide. The existence of free amine groups in anhydrideterminated polyimide might be attributed to steric hindrance and the deactivation of the amine group in TAP, preventing the total imidization of all of the amine groups [16]. The structural perfection of hyper-branched polymers is generally characterized by the degree of branching (DB), which is defined by Frey and co-workers [17] as DB =
Fig. 3. FT-IR spectrums of BPAD–TAP hyper-branched polyimides: (a) amineterminated and (b) anhydride-terminated.
D+T D+T +L
(1)
where D and L represent the numbers of dendritic and linear units in the polymer, respectively. Experimentally, the DB is
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Fig. 4. Possible structures of TAP in hyper-branched polyimides. Table 2 Structural characteristics of TAP units, determined by 1 H NMR Structure unit
Chemical shift (ppm) H5
NH2
2-Monoimide 4-, 6-Monoimide
5.44 5.81
6.53 6.49
4,6-Diimide 2,4-, 2,6-Diimide
6.73 6.94
7.82 7.39
T
L
usually determined from 1 H NMR or 13 C NMR spectroscopy by comparing the integration of the peaks for the respective units in the hyper-branched polymers [18]. The DB results are listed in Table 1. It can be seen that the DB of the amine-terminated BPAD–TAP polyimide is 0.32, whereas that of the anhydride-terminated BPAD–TAP polyimide is 0.75. The same trend was observed in the case of
Fig. 5. Possible structures of dianhydride in hyper-branched polyimides.
the DSDA–TAP polyimides, the DB of which is also shown in Table 1. The results indicate that anhydride-terminated polyimides have better-branched structures than amine-terminated polyimides, which might be a consequence of the lesser reactivity of the TAP unit. The compositions of the products, the molar ratios of BPAD:TAP, can also be calculated on the basis of 1 H NMR spectra, with Eq. (2) [12]: BPAD : TAP =
I7.5−8.3 − ID − 2I6.73 6 : (ID + IL + ID )
(2)
where ID , IL , and IT are the same as in Eq. (2), and I7.5–8.3 and I6.73 represent the peak intensities at 7.5–8.3 and 6.73 ppm, respectively.
Fig. 6. 1 H NMR spectra of BPAD–TAP hyper-branched polyimides: (a) amineterminated and (b) anhydride-terminated.
S.-J. Park et al. / Materials Chemistry and Physics 108 (2008) 214–219 Table 3 Thermal properties of hyper-branched polyimides Polyimide
Terminal group
Tg (◦ C)
T5% (◦ C)
T10% (◦ C)
BPAD–TAP
Amine-terminated Anhydride-terminated
272 261
464 460
494 487
DSDA–TAP
Amine-terminated Anhydride-terminated
270 267
457 451
486 480
The results, listed in Table 1, suggest that the compositions of the obtained polyimides are almost identical to those of the feeds. 3.3. Thermal properties The glass transition temperatures (Tg ) of the hyper-branched polyimides determined by DSC are shown in Table 3. It can be seen that the anhydride-terminated polyimides had lower Tg than the amine-terminated ones, which can be attributed to the increased free volume and mobility of the macromolecules caused by the absence of chain-end interactions [19–21]. Another possible explanation is that the amine-terminated polyimides had a stronger macromolecular interaction resulting from hydrogen bonding between the carbonyl groups and the terminal amine groups [10]. TGAs were used to study the thermal stabilities of the hyperbranched polyimides. Fig. 7 shows the TGA curves of both the amine- and anhydride-terminated polyimides. The thermal stability factors determined from the TGA thermograms are shown in Table 3 [22,23]. The results show the excellent thermal stabilities of the polyimides, with 5% weight loss at temperatures ranging from 451 ◦ C to 464 ◦ C and 10% weight loss at temperatures ranging from 480 ◦ C to 494 ◦ C. The amine-terminated polyimides showed slightly higher thermal stabilities compared with the corresponding anhydride-terminated ones, which can be attributed to the different terminal functional groups and the anhydride residues. Furthermore, the fact that the BPAD–TAP polyimides showed a slightly higher thermal stability than the DSDA–TAP polyimides possibly can be explained by the differ-
Fig. 7. TGA curves of BPAD–TAP hyper-branched polyimides in nitrogen.
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ent chemical structure of the anhydrides, meaning that the free volume of hyper-branched polyimides is increased by the bulky –SO2 groups in DSDA–TAP, which can account for a lower thermal property. 4. Conclusions A new series of aromatic hyper-branched polyimides were successfully synthesized from A2 + BB2 monomer systems using a series of commercially available A2 -type dianhydride monomers and a BB2 -type triamine monomer. No gelation occurred during polymerization in high-monomer concentrations, which can be attributed to the different reactivities of the amine groups in TAP. This made it easier to prepare hyperbranched polyimides on a large scale, because it allowed the monomers to be mixed together at high concentrations in the solvent. The DB values of the obtained hyper-branched polyimides differed according to different end groups, which indicated that the anhydride-terminated polyimides had a higher DB than the amine-terminated ones. The increased DB was responsible for the increased free volume and macromolecular mobility, which led to the relatively lower Tg . TGA results suggested that the obtained polyimides had good thermal stabilities, with 5% losses at temperatures over 450 ◦ C in a nitrogen atmosphere, the amine-terminated polyimides showing higher values than the corresponding anhydride-terminated ones. These results probably can be attributed to the different terminal functional groups. References [1] G.C. Behera, S. Ramakrishnan, Macromolecules 37 (2004) 9814. [2] J. Hao, M. Jikei, M. Kakimoto, Macromol. Symp. 199 (2003) 233. [3] M. Jike, S.H. Chon, M. Kakimoto, S. Kawauchi, T. Imase, J. Watanebe, Macromolecules 32 (1999) 2061. [4] H. Magnusson, E. Malmstrom, A. Hult, Macromolecules 34 (2001) 5786. [5] C.Y. Hong, C.Y. Pan, Y. Huang, Z.D. Xu, Polymer 42 (2001) 6733. [6] K. Xu, H. Peng, Q. Sun, Y. Dong, F. Salhi, J. Luo, J. Chen, Y. Huang, D. Zhang, Z. Xu, B.Z. Tang, Macromolecules 35 (2002) 5821. [7] S.Y. Cho, Y. Chang, J.S. Kim, S.C. Lee, C. Kim, Macromol. Chem. Phys. 202 (2001) 263. [8] M. Trolls˚as, J. Hedrick, D. Mecerreyes, R. J´erˆome, P.H. Dubois, J. Polym. Sci. A: Polym. Chem. 36 (1998) 3187. [9] H.W. Lu, S.H. Liu, X.L. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Mater. Chem. Phys. 81 (2003) 104. [10] J. Fang, H. Kita, K. Okamoto, Macromolecules 33 (2000) 4639. [11] P.J. Flory, J. Am. Chem. Soc. 74 (1952) 2718. [12] Y. Liu, T.S. Chung, J. Polym. Sci. A: Polym. Chem. 40 (2002) 4563. [13] D. Yan, C. Gao, Macromolecules 33 (2000) 7693. [14] R.W. Snyder, B. Thomson, B. Bartges, D. Czerniawski, P.C. Painter, Macromolecules 22 (1989) 4166. [15] J. Xu, C. He, T.S. Chung, J. Polym. Sci. A: Polym. Chem. 39 (2001) 2998. [16] D.G. Hawthorne, J.H. Hodgkin, High Perform. Polym. 11 (1999) 315. [17] D. Holter, A. Burgath, H. Frey, Acta Polym. 48 (1997) 30. [18] C.J. Hawker, R. Lee, J.M.J. Frechet, J. Am. Chem. Soc. 113 (1991) 4583. [19] L.J. Markoski, J.S. Moore, I. Sendijarevic, A. Lee, A.J. Mchugh, Macromolecules 34 (2001) 2695. [20] H. Stuta, J. Polym. Sci. B: Polym. Phys. 33 (1995) 333. [21] K.L. Wooley, C.J. Hawker, J.M. Pochan, Macromolecules 26 (1993) 1514. [22] S.J. Park, F.L. Jin, J.R. Lee, Macromol. Chem. Phys. 205 (2004) 2048. [23] S.J. Park, H.C. Kim, H.L. Lee, D.H. Suh, Macromolecules 34 (2001) 573.