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Degree of imidization for polyimide films investigated by evolved gas analysis-mass spectrometry
Thermochimica Acta 551 (2013) 184–190
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Degree of imidization for polyimide films investigated by evolved gas analysis-mass spectrometry Byoung-Hyoun Kim a,∗ , Huijung Park a , Heeyong Park a , Dong Cheul Moon b a b
Analytical Science, LG Chem. Research Park, LG Chem. Ltd., 104-1 Moonji-Dong, Yusong-Gu, Daejeon 305-380, South Korea College of Pharmacy, Chungbuk National University, 12 Gaeshin-Dong, Heungduk-Gu, Cheongju 361-763, South Korea
a r t i c l e
i n f o
Article history: Received 30 July 2012 Received in revised form 27 October 2012 Accepted 31 October 2012 Available online 16 November 2012 Keywords: Degree of imidization Polyimide Evolved gas analysis-mass spectrometry
a b s t r a c t The evolved gas analysis-mass spectrometry (EGA-MS) method is described as a new approach for determining the degree of imidization (DOI) in polyimide (PI) films. Partially imidized PI films allowed water to release through the re-imidization process at a sufficiently high imidization temperature. Evolved water from the re-imidization process was quantitatively detected by EGA-MS. From the obtained water content, the number of moles of residual amic acid groups in repeating units of PI was found. Consequently, the DOI of the PI films could be found from the mole ratio of PI and the sum of the PI and the residual polyamic acid (PAA). A water content of 0.018% and a DOI of 99.85% can be measured from 40 mg of PI films using this method. In this study, it was found that rigid PIs showed fast imidization reactions at relatively lower temperatures, while flexible PIs were activated and showed fast imidization reactions at relatively higher temperatures. In addition, the end point of the imidization process in multi-layer PI films was determined by this method. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that polyimides (PIs) not only have excellent thermal and mechanical properties, but also a relatively low dielectric constant and superior chemical resistance [1,2]. Hence, PIs have been widely used in many applications such as in the aerospace, microelectronics, and optoelectronics industries [3,4]. Among them, applications in the field of microelectronics have been steadily increasing owing to increasing industrial demand for electrical insulators, substrates for flexible printed circuits, passivation layers, and alignment layers for liquid crystal displays [5,6]. Many methods have been studied and developed for the thermal behavior of PI film itself [5,7–9], the kinetics of imidization [10,11], the computer-added molecular simulation of imidization [12], the surface modification of PI [2,13], fabrication and structural analysis [2,14,15], and determination of the degree of imidization (DOI) [7,8,14,16–18]. In most cases, Fourier transform infrared spectroscopy (FT-IR) has been widely used to determine the DOI of PI films. There are many advantages to determine the DOI by FT-IR, such as simple and fast analysis, in situ analysis, and the characterization of PI besides the DOI. In the FT-IR method, the DOI has been generally acquired by calculating the ratio between the infrared absorption intensity of the characteristic peaks corresponding to
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (B.-H. Kim). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.029
the imide ring and the internal standard peak corresponding to the C–C stretching of benzene which does not change during imidization. The calculation was undertaken on the assumption that PI films are fully 100% imidized when the calculated ratio is constant at high imidization temperature. Thereafter, the DOI of PIs at different imidization temperatures represents the relative values. Therefore, the determination of the DOI using FT-IR is an indirect method [8,11]. In addition, the DOI is not the absolute value. In some cases, the DOI is increased or decreased above the fully imidized temperature of the PI films due to the polymer chain orientation of the PI through the imidization process [17]. In these respects, it is necessary to establish an adequate analytical method which provides the absolute values for the determination of the DOI in PI films. Partially imidized PI films have residual polyamic acids (PAAs) which consist of amic acid groups. These residual amic acid groups are able to re-imidize completely at sufficiently high temperature. Re-imidization is defined by the conversion process of residual PAAs of PI films in a pyrolyzer furnace for the DOI analysis. In the furnace, water is released from the PI films through the reimidization process. If we can determine the water content in this re-imidization process, the content of the residual PAA in the repeating unit of the partially imidized PI will be measured. In this paper, a new approach is suggested to obtain the absolute values of the DOI of PI films by investigating the evolved water content through the re-imidization process at sufficiently high temperatures. The evolved water content was analyzed by evolved gas
B.-H. Kim et al. / Thermochimica Acta 551 (2013) 184–190
185
Fig. 1. Chemical structures of PAAs and PIs in this study. (A) PI-I consists of BPDA-ODA-PDA; (B) PI-II consists of BPDA-PMDA-BTDA-PDA-ODA; (C) PI-III consists of BPDA-PDA; (D) PI-IV consists of BPDA-PMDA-BTDA-PDA. All the alphanumeric characters above the chemical structures indicate the protons of PAAs as shown in Fig. 2.
analysis-mass spectrometry (EGA-MS). We describe the results of the DOI and the imidization rate in various PI films consisting of different monomers.
2. Experimental 2.1. Chemicals and reagents The monomers for the synthesis of PAA as a precursor of PI preparation which were used were of technical grade, and consisted of 3,3 ,4,4 -biphenyltetracarboxylic dianhydride (BPDA), 3,3 ,4,4 benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), 4,4 -oxydianiline (ODA), and p-phenylene diamine (PDA). Sodium tungstate dihydrate (Na2 WO4 ·2H2 O) was used as a calibration standard for the analysis of the water content and purchased from Aldrich (Milwaukee, WI, USA).
2.2. Preparation of PAAs PAAs were prepared by adding the equimolar amount of dianhydrides into the N,N-dimethylacetamide (DMAc) solution of diamines according to a procedure described elsewhere [5,6,10]. The PAAs consist of BPDA-ODA-PDA for PI-I, BPDA-PMDA-BTDAPDA-ODA for PI-II, BPDA-PDA for PI-III, and BPDA-PMDA-BTDAPDA for PI-IV, respectively. Fig. 1 shows the chemical structures of the PAAs and PIs used in this study.
2.3. Preparation of PI films PAA solution was cast on a glass plate in an air convection oven with a spin-coater and then pre-baking was performed at 140 ◦ C for 30 min. After that, the PAA films were imidized under nitrogen atmosphere for 1 h at 200 ◦ C, 250 ◦ C, 300 ◦ C, 350 ◦ C, and 390 ◦ C, respectively. The resulting PI films were isolated carefully from the glass plate and stored in a desiccator until analysis of PI samples was performed.
2.4.
1H
Nuclear magnetic resonance (NMR) spectroscopic analysis
The 1 H NMR spectra were obtained by a Varian VNMRS 500 MHz NMR spectrometer (Agilent Technologies Korea, Seoul, South Korea). Dimethyl sulfoxide-d6 as the deuterated solvent was used in NMR spectroscopic analysis. Chemical shifts were given in parts per million (ppm) from tetramethylsilane (TMS) at 0 ppm and each spectrum range was from −1 to 16 ppm. 2.5. Attenuated total reflection (ATR) Fourier-transform infrared (FT-IR) spectroscopic analysis ATR FT-IR spectra were recorded on a Varian 660-IR (Agilent Technologies Korea, Seoul, South Korea) attached GladiATR (equipped with diamond crystal plate, PIKE technologies, WI, USA) and measurements were performed in the wavenumber range of 4000–400 cm−1 at a resolution of 4 cm−1 . The absorption band at 1340 cm−1 corresponds to the C N stretching of the imide ring. In addition, the band at 1510 cm−1 is characteristic of the benzene ring and used as the internal standard. The DOIs were obtained from the relative ratio of the two absorption bands. In the FT-IR analysis, it is necessary that PI films under the condition of 390 ◦ C are assumed to be fully 100% imidized PI films. 2.6. EGA-MS analysis In order to analyze the water content from the re-imidization process, a pyrolyzer-gas chromatography/mass spectrometer (PyGC/MS) was employed in this study. The Py-GC/MS consists of a PY-2020iD double-shot pyrolyzer (Frontier Laboratories, Fukushima, Japan) and an Agilent 7890A GC system with a 5975C inert XL mass selective detector (Agilent Technologies Korea, Seoul, South Korea). The capillary column for the pyrolyzer linking to the mass selective detector was an UADTM-2.5M deactivated metal column (0.15 mm I.D. × 2.5 mL with a coated film thickness <0.01 m, Frontier Laboratories, Fukushima, Japan). As the EGA-MS experiment started to run, the furnace temperature of the pyrolyzer was increased, and evolved gases from the
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Fig. 2. 1 H NMR spectra of PAAs as the precursor of PIs. (A) PI-I; (B) PI-II; (C) PI-III; (D) PI-IV; (a) the protons of carboxylic acid groups; (b) the protons of amide groups. The numeric indicates aromatic protons of PAAs as shown in Fig. 1.
sample were directly introduced into the mass selective detector. Thus, EGA-MS provides the total ions as well as the individual ions abundance versus the increasing temperature of the furnace in the pyrolyzer. The curves of the EGA-MS were generated by detecting the evolved gases. Here, water is a target evolved gas through the re-imidization process in the furnace of the pyrolyzer. The EGA-MS conditions were as follows; carrier gas: He; column flow rate: 1 mL/min; split ratio of injector: 1/10; temperature profile of the pyrolyzer furnace: 50 ◦ C (5 min), 10 ◦ C/min to 450 ◦ C (5 min), total run time: 50 min; injector, oven, and interface temperature of the GC/MS: 300 ◦ C; and the interface temperature of pyrolyzer: 320 ◦ C. In addition, selected ion monitoring (SIM) was used at m/z 18.
The amounts of 2, 20, and 40 mg of sodium tungstate dihydrate were used for standard calibration of water. In addition, 40 mg of PI films was used for determining the water content by EGA-MS. The DMAc content, as the residual solvent in PI films, was analyzed by purge and trap gas chromatography/mass spectrometry. The pure weight of the PI films was then corrected by the content of residual solvent. The content of residual solvent ranged from 0.3% to 3% (w/w, %). The results of the residual solvent in the PI films are not presented in this paper. The absorbed water content of the inner and/or surface of the PI films ranged from 0.2 mg to 0.3 mg. These values are negligible because small amounts of absorbed water do not affect the pure weight of the PI films.
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Table 1 Results of the calibration for the determination of the water content in PI films. Sodium tungstate dihydrate
Repetition
Calibration range (mg) 1 2 3
2–40 a
LOD (g)a
Calibration curves 2
Slope
Intercept
r
2 × 108 2 × 108 2 × 108
−535103 −327099 −536297
0.9998 0.9999 0.9999
7
LOD (limit of detection): S/N = 3.
3. Results and discussion It is found that the DOI can be obtained from the mole ratio of the PI and the sum of the PI and the residual PAA, as shown in Eq. (1).
DOI (%) =
the number of moles of PI the number of moles of PAA + the number of moles of PI
× 100
(1)
The number of moles of residual amic acid groups per repeating unit corresponds to the number of moles of water which is evolved through the re-imidization process in EGA-MS, as shown in Fig. 1. For example as shown in Fig. 1C, a mole (MPAA ) of PI-III generates two moles (MH2 O ) of water in the imidization process. In this case, two molecules (NH2 O ) of water can theoretically be generated in the imidization process. Therefore, the number of moles of residual PAA per repeating unit can be represented, as shown in Eq. (2). MPAA =
MH2 O NH2 O
=
WH2 O
(2)
18 × NH2 O
where MPAA is the number of moles of PAA per repeating unit, MH2 O is the number of moles of H2 O, WH2 O is the weight of H2 O which is evolved through the re-imidization process in EGA-MS, and NH2 O is the number of molecules of H2 O which can theoretically be generated in the imidization process. Consequently, the DOI can be expressed by Eq. (3). The DOI is obtained from the water content (WH2 O ) by EGA-MS analysis. The equation for the DOI is
DOI (%) =
WPI /MWPI (WH2 O /(18 × NH2 O ) + (WPI /MWPI ))
× 100
(3)
WPI = Wsample − WPAA , WPAA = MWPAA × WH2 O /(18 × where NH2 O ), WPI and WPAA are the weight of PI and PAA per repeating unit, Wsample is the weight of the sample for EGA-MS analysis, WH2 O is the weight of H2 O which is evolved through the re-imidization process in EGA-MS, and MWPI and MWPAA are the molecular weights of PI and PAA per repeating unit, respectively. In order to verify the proposed polymer structures shown in Fig. 1, PAAs, as the precursor of PIs, were employed in a 1 H NMR experiment and the results are given in Fig. 2. The 1 H NMR spectra shows signals for the protons of carboxylic acid groups at 13.3 ppm and the protons of amide groups at 10.5 ppm. Signals of aromatic protons for the polymer backbone are from 7.0 ppm to 8.5 ppm. From the results, the mole ratios between the protons of carboxylic acid groups and the protons of amide groups were approximately equal. Therefore, the total ratios of dianhydride and diamine were 1:1 and the proposed polymer structures can be used to perform DOI calculations in the EGA-MS analysis. Fig. 3 and Table 1 represent the calibration data for determining the water content in the PI films using sodium tungstate dihydrate. All correlation coefficients (r2 ) are better than or equal to 0.9998. The limit of detection (LOD) is 7 g of water at a signal-to-noise ratio better than or equal to 3. A water content of 0.018% and a DOI of 99.85% can be measured with 40 mg of PI films. These values are acceptable for the DOI analysis of PI films. In selected ion monitoring (m/z 18) of sodium tungstate dihydrate, water is detected from 50 ◦ C (5 min) to 120 ◦ C (12 min) as shown in Fig. 3. Hence, peak integration was performed from 5 min to 12 min.
Fig. 4 shows the results of selected ion monitoring (m/z 18) for PI films prepared at various imidization temperatures. In PI films, water was detected by two separated regions. Adsorbed water of the inner and/or surface of PI films might be detected before the temperature reached 150 ◦ C (15 min). Evolved water through the re-imidization process was detected from 150 ◦ C to 450 ◦ C. Therefore, this region was integrated as the peak area for DOI analysis. The results of the DOI for various PI films are given in Table 2. In addition, the overlaid graphs of the DOI are illustrated in Fig. 5. The DOI is increased with an increase of the imidization temperature in all the experiments. Imidization was completed at 300 ◦ C for PII and PI-II, 350 ◦ C for PI-IV, and 390 ◦ C for PI-III, respectively. The absolute values of the DOI were acquired by EGA-MS analysis without relative calculations of the DOI. In order to compare with the traditional method, the typical DOIs of PI-III and PI-IV obtained from FT-IR analysis are represented in Table 3. These values are quite different from the results of the EGA-MS analysis because the FT-IR data are relative values. Therefore, the absolute values of the DOI are considered to be useful. As shown in Fig. 5, interesting results were found in this study. The imidization increased in the order of PI-I < PI-II < PI-III < PI-IV at 200 ◦ C, which represents a relatively lower imidization temperature. On the other hand, the imidization increased in the order of PI-IV < PI-III < PI-II < PI-I at 250 ◦ C and 300 ◦ C, which represent a relatively higher imidization temperature. The results of the DOIs show opposite imidization rates between the lower and higher imidization temperatures. Meanwhile, the flexibility of the polymer chains in the PIs increases in the order of PI-IV < PI-III < PI-II < PI-I. Compared to other monomers, ODA plays the role of a relatively flexible monomer in the polymer chain due to its ether linked bond [8]. In general, a flexible polymer shows a fast imidization reaction because the increased flexibility of the polymer chain will be mobilized more easily. However, fast imidization occurred in rigid PIs at an imidization temperature of 200 ◦ C as shown in Fig. 5. Flexible PIs provided fast imidization
Fig. 3. Selected ion monitoring (m/z 18) of calibration standards in EGAMS. (a) 1.79 mg, (b) 14.6 mg, and (c) 31.68 mg of sodium tungstate dihydrate, y = (2 × 108 )x − 771556, r2 = 0.9998.
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Fig. 4. Selected ion monitoring (m/z 18) of EGA-MS for PI films prepared at various imidization temperatures. (A) PI-I; (B) PI-II; (C) PI-III; (D) PI-IV; (E) multi-layer PI film which consists of PI-I, PI-IV and PI-II.
Table 2 Degree of imidization of PI films prepared at various imidization temperatures by EGA-MS analysis. Temperatures of imidization (◦ C)
Degree of imidization (%)a ± S.D.b PI-I
200 250 300 350 390
74.4 97.8 >99.9 >99.9 >99.9
PI-II ± ± ± ± ±
For detailed analysis conditions, refer to the experimental part. a Three replicates analysis. b S.D.: standard deviation.
0.04 0.06 0.00 0.00 0.00
77.4 97.5 >99.9 >99.9 >99.9
PI-III ± ± ± ± ±
0.04 0.08 0.00 0.00 0.00
80.4 96.7 99.0 99.7 >99.9
PI-IV ± ± ± ± ±
0.05 0.03 0.08 0.03 0.00
85.9 96.0 98.1 >99.9 >99.9
± ± ± ± ±
0.02 0.18 0.15 0.00 0.00
B.-H. Kim et al. / Thermochimica Acta 551 (2013) 184–190
189
A 105
B
101
Increasing flexibility of PIs
Degree of imidization (%)
100
99
95 97
90 95
85 93
PI-I
80
PI-I
PI-II 75
PI-II 91
PI-III
Increasing flexibility of PIs 70 150
200
250
PI-III
PI-IV 300
350
PI-IV
400
89 230
250
Temperature (ºC)
270
290
310
Temperature (ºC)
Fig. 5. Degree of imidization of PI films prepared at various imidization temperatures. (A) full scaled data and (B) expanded data for imidization temperatures from 230 ◦ C to 320 ◦ C. Table 3 Degree of imidization of PI films prepared at various imidization temperatures by ATR FT-IR analysis. Temperatures of imidization (◦ C)
PI-III
PI-IV a
200 250 300 350 390
b
Ratio of band area
Degree of imidization (%)
Ratio of band areaa
Degree of imidization (%)b
2.01 2.05 2.18 2.20 2.21
91.0 92.8 98.6 99.5 100.0
2.93 3.13 3.14 3.15 3.16
92.7 99.1 99.4 99.7 100.0
For detailed analysis conditions, refer to the experimental part. a The ratio of the C N stretching band at 1340 cm−1 and the benzene ring band at 1510 cm−1 . b The relative ratio versus the ratio of the band area at 390 ◦ C.
above 250 ◦ C of imidization temperature. In addition, an intersection point of the DOIs was ca. 240 ◦ C. The bond length and dihedral angle between the amine of the amide group and the carbon of the carboxylic acid group will exist variously near or far together in flexible PIs. On the other hand, the bond length and dihedral angle in rigid PIs will be relatively stiff. Hence, we suggest that a fast reaction might occur in rigid PIs at a relatively lower imidization temperature, while the imidization reaction of flexible PIs is activated at a relatively higher imidization temperature. The EGA-MS method was applied to the multi-layer PI film for determining the DOI. However, the EGA-MS analysis does not provide the result of the DOI in the case of multi-layer PI film because the source of the evolved water cannot be known through re-imidization. It can, however, be found to complete the imidization reaction of the multi-layer PI film using EGA-MS analysis. Multi-layer PI film consists of PI-I, PI-IV and PI-II. In this experiment, PI-IV is centered in the multi-layer PI film. The results of the DOI of
Table 4 Results of the water content of the multi-layer PI film at 300 and 320 ◦ C. Temperatures of imidization (◦ C) 300 320
Repetition
Weight of sample (mg)
Water content (%)a
1 2 1 2
43.25 42.04 37.98 35.26
0.05 0.05
The multi-layer PI film consists of PI-I, PI-IV and PI-II. a Water detected by re-imidization for residual amic acid groups of PI films. b LOD (limit of detection, S/N = 3): 7 g of water, water content can be estimated to be 0.018%.
PI-I and PI-II show the completion of imidization at 300 ◦ C, and the completion of imidization of PI-IV at 350 ◦ C, as shown in Table 2. Therefore, the temperature of imidization completion of a multilayer PI film might be from 300 ◦ C to 350 ◦ C. Table 4 and Fig. 4E show the results of the water content of the multi-layer PI film at 300 and 320 ◦ C. From the results, the imidization completion of the multi-layer PI film was at 320 ◦ C. 4. Conclusions We have developed a simple EGA-MS method for the determination of the DOI in various PI films. Absolute values of the DOI were obtained without relativization by EGA-MS analysis. In this study, it was found that fast reaction occurred in rigid PIs at relatively lower temperatures, while flexible PIs were activated and showed fast imidization reaction at relatively higher temperatures. In a multi-layer PI film, the results of the DOI cannot be obtained, but information about imidization completion is provided by EGAMS analysis. This is the first reported result for the DOI analysis of PI films using EGA-MS. In addition, this method may help to obtain absolute values of the DOI for various PI films. References [1] K. Xu, J. Economy, Hyperbranched thermosetting poly(imide-ester): synthesis and properties, Macromolecules 37 (2004) 4146–4155. [2] K. Akamatsu, S. Ikeda, H. Nawafune, Site-selective direct silver metallization on surface-modified polyimide layers, Langmuir 19 (2003) 10366–10371. [3] F. Yang, Y. Li, Q. Bu, S. Zhang, T. Ma, J. Zhao, Characterizations and thermal stability of soluble polyimide derived from novel unsymmetrical diamine monomers, Polym. Degrad. Stab. 95 (2010) 1950–1958.
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[4] S. Diaham, M.L. Locatelli, T. Lebey, D. Malec, Thermal imidization optimization of polyimide thin films using Fourier transform infrared spectroscopy and electrical measurements, Thin Solid Films 519 (2011) 1851–1856. [5] M. Hasegawa, T. Matano, Y. Shindo, T. Sugimura, Spontaneous molecular orientation of polyimides induced by thermal imidization, Macromolecules 29 (1996) 7897–7909. [6] T.J. Shin, B. Lee, H.S. Youn, K.B. Lee, M. Ree, Time-resolved synchrotron X-ray diffraction and infrared spectroscopic studies of imidization and structural evolution in a microscaled film of PMDA-3,4 -ODA poly(amic acid), Langmuir 17 (2001) 7842–7850. [7] M. Li, Q.H. Lu, J. Yin, Y. Qian, Z.G. Wang, Effects of post-thermal treatment on preparation of surface microstructures induced by polarized laser on polyimide film, Mater. Chem. Phys. 77 (2002) 895–898. [8] M.B. Saeed, M.S. Zhan, Effects of monomer structure and imidization degree on mechanical properties and viscoelastic behavior of thermoplastic polyimide films, Eur. Polym. J. 42 (2006) 1844–1854. [9] T.J. Shin, M. Ree, Thermal imidization and structural evolution of thin film of poly(4,4 -oxydiphenylene p-pyromellitamic diethyl ester), J. Phys. Chem. B 111 (2007) 13894–13900. [10] Y. Seo, S.M. Lee, D.Y. Kim, K.U. Kim, Kinetic study of the imidization of a poly(ester amic acid) by FT-Raman spectroscopy, Macromolecules 30 (1997) 3747–3753.
[11] W.G. Dong, G.H. Kim, J.Y. Choi, J.R. Yoon, Y.H. Kim, A study of the curing behavior of polyamic acid coated on a Cu or Zn layer, Colloid Surf. A 324 (2008) 122–125. [12] J.A. Young, J.A. Hinkley, B.L. Farmer, Molecular simulations of the imidization of adsorbed polyamic acid, Macromolecules 33 (2000) 4936–4944. [13] J.Y. Choi, W. Dong, Y.H. Kim, Formation of Cu-based nanoparticle during imidization of PAA into PI in the various environments, Colloid Surf. A 313–314 (2008) 335–338. [14] Y. Suzuki, Y. Maekawa, M. Yoshida, K. Maeyama, N. Yonezawa, Ion-beaminduced dual-tone imaging of polyimide via two-step imidization, Chem. Mater. 14 (2002) 4186–4191. [15] Y.J. Kang, H. Kim, E. Kim, I. Kim, C.S. Ha, Fabrication and structural analysis of aromatic polyimide ultra-thin films with different fluorine contents, Colloid Surf. A 313–314 (2008) 585–589. [16] H.J. Lee, J. Won, H.C. Park, H. Lee, Y.S. Kang, Effects of poly(amic acid) imidization on solution characteristics and membrane morphology, J. Membr. Sci. 178 (2000) 35–41. [17] X. Jin, D. Zhu, Influence of curing temperature on the optical properties of fluorinated polyimide thin films, Eur. Polym. J. 44 (2008) 3571–3577. [18] G.H. Kim, W. Dong, C.S. Yoon, Y.H. Kim, Synthesis of duplex nanoparticles in polyimide by the reaction of polyamic acid with Cu·Zn alloy films, Colloid Surf. A 321 (2008) 292–296.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Degree of imidization for polyimide films investigated by evolved gas analysis-mass spectrometry Byoung-Hyoun Kim a,∗ , Huijung Park a , Heeyong Park a , Dong Cheul Moon b a b
Analytical Science, LG Chem. Research Park, LG Chem. Ltd., 104-1 Moonji-Dong, Yusong-Gu, Daejeon 305-380, South Korea College of Pharmacy, Chungbuk National University, 12 Gaeshin-Dong, Heungduk-Gu, Cheongju 361-763, South Korea
a r t i c l e
i n f o
Article history: Received 30 July 2012 Received in revised form 27 October 2012 Accepted 31 October 2012 Available online 16 November 2012 Keywords: Degree of imidization Polyimide Evolved gas analysis-mass spectrometry
a b s t r a c t The evolved gas analysis-mass spectrometry (EGA-MS) method is described as a new approach for determining the degree of imidization (DOI) in polyimide (PI) films. Partially imidized PI films allowed water to release through the re-imidization process at a sufficiently high imidization temperature. Evolved water from the re-imidization process was quantitatively detected by EGA-MS. From the obtained water content, the number of moles of residual amic acid groups in repeating units of PI was found. Consequently, the DOI of the PI films could be found from the mole ratio of PI and the sum of the PI and the residual polyamic acid (PAA). A water content of 0.018% and a DOI of 99.85% can be measured from 40 mg of PI films using this method. In this study, it was found that rigid PIs showed fast imidization reactions at relatively lower temperatures, while flexible PIs were activated and showed fast imidization reactions at relatively higher temperatures. In addition, the end point of the imidization process in multi-layer PI films was determined by this method. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is well known that polyimides (PIs) not only have excellent thermal and mechanical properties, but also a relatively low dielectric constant and superior chemical resistance [1,2]. Hence, PIs have been widely used in many applications such as in the aerospace, microelectronics, and optoelectronics industries [3,4]. Among them, applications in the field of microelectronics have been steadily increasing owing to increasing industrial demand for electrical insulators, substrates for flexible printed circuits, passivation layers, and alignment layers for liquid crystal displays [5,6]. Many methods have been studied and developed for the thermal behavior of PI film itself [5,7–9], the kinetics of imidization [10,11], the computer-added molecular simulation of imidization [12], the surface modification of PI [2,13], fabrication and structural analysis [2,14,15], and determination of the degree of imidization (DOI) [7,8,14,16–18]. In most cases, Fourier transform infrared spectroscopy (FT-IR) has been widely used to determine the DOI of PI films. There are many advantages to determine the DOI by FT-IR, such as simple and fast analysis, in situ analysis, and the characterization of PI besides the DOI. In the FT-IR method, the DOI has been generally acquired by calculating the ratio between the infrared absorption intensity of the characteristic peaks corresponding to
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (B.-H. Kim). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.029
the imide ring and the internal standard peak corresponding to the C–C stretching of benzene which does not change during imidization. The calculation was undertaken on the assumption that PI films are fully 100% imidized when the calculated ratio is constant at high imidization temperature. Thereafter, the DOI of PIs at different imidization temperatures represents the relative values. Therefore, the determination of the DOI using FT-IR is an indirect method [8,11]. In addition, the DOI is not the absolute value. In some cases, the DOI is increased or decreased above the fully imidized temperature of the PI films due to the polymer chain orientation of the PI through the imidization process [17]. In these respects, it is necessary to establish an adequate analytical method which provides the absolute values for the determination of the DOI in PI films. Partially imidized PI films have residual polyamic acids (PAAs) which consist of amic acid groups. These residual amic acid groups are able to re-imidize completely at sufficiently high temperature. Re-imidization is defined by the conversion process of residual PAAs of PI films in a pyrolyzer furnace for the DOI analysis. In the furnace, water is released from the PI films through the reimidization process. If we can determine the water content in this re-imidization process, the content of the residual PAA in the repeating unit of the partially imidized PI will be measured. In this paper, a new approach is suggested to obtain the absolute values of the DOI of PI films by investigating the evolved water content through the re-imidization process at sufficiently high temperatures. The evolved water content was analyzed by evolved gas
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Fig. 1. Chemical structures of PAAs and PIs in this study. (A) PI-I consists of BPDA-ODA-PDA; (B) PI-II consists of BPDA-PMDA-BTDA-PDA-ODA; (C) PI-III consists of BPDA-PDA; (D) PI-IV consists of BPDA-PMDA-BTDA-PDA. All the alphanumeric characters above the chemical structures indicate the protons of PAAs as shown in Fig. 2.
analysis-mass spectrometry (EGA-MS). We describe the results of the DOI and the imidization rate in various PI films consisting of different monomers.
2. Experimental 2.1. Chemicals and reagents The monomers for the synthesis of PAA as a precursor of PI preparation which were used were of technical grade, and consisted of 3,3 ,4,4 -biphenyltetracarboxylic dianhydride (BPDA), 3,3 ,4,4 benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), 4,4 -oxydianiline (ODA), and p-phenylene diamine (PDA). Sodium tungstate dihydrate (Na2 WO4 ·2H2 O) was used as a calibration standard for the analysis of the water content and purchased from Aldrich (Milwaukee, WI, USA).
2.2. Preparation of PAAs PAAs were prepared by adding the equimolar amount of dianhydrides into the N,N-dimethylacetamide (DMAc) solution of diamines according to a procedure described elsewhere [5,6,10]. The PAAs consist of BPDA-ODA-PDA for PI-I, BPDA-PMDA-BTDAPDA-ODA for PI-II, BPDA-PDA for PI-III, and BPDA-PMDA-BTDAPDA for PI-IV, respectively. Fig. 1 shows the chemical structures of the PAAs and PIs used in this study.
2.3. Preparation of PI films PAA solution was cast on a glass plate in an air convection oven with a spin-coater and then pre-baking was performed at 140 ◦ C for 30 min. After that, the PAA films were imidized under nitrogen atmosphere for 1 h at 200 ◦ C, 250 ◦ C, 300 ◦ C, 350 ◦ C, and 390 ◦ C, respectively. The resulting PI films were isolated carefully from the glass plate and stored in a desiccator until analysis of PI samples was performed.
2.4.
1H
Nuclear magnetic resonance (NMR) spectroscopic analysis
The 1 H NMR spectra were obtained by a Varian VNMRS 500 MHz NMR spectrometer (Agilent Technologies Korea, Seoul, South Korea). Dimethyl sulfoxide-d6 as the deuterated solvent was used in NMR spectroscopic analysis. Chemical shifts were given in parts per million (ppm) from tetramethylsilane (TMS) at 0 ppm and each spectrum range was from −1 to 16 ppm. 2.5. Attenuated total reflection (ATR) Fourier-transform infrared (FT-IR) spectroscopic analysis ATR FT-IR spectra were recorded on a Varian 660-IR (Agilent Technologies Korea, Seoul, South Korea) attached GladiATR (equipped with diamond crystal plate, PIKE technologies, WI, USA) and measurements were performed in the wavenumber range of 4000–400 cm−1 at a resolution of 4 cm−1 . The absorption band at 1340 cm−1 corresponds to the C N stretching of the imide ring. In addition, the band at 1510 cm−1 is characteristic of the benzene ring and used as the internal standard. The DOIs were obtained from the relative ratio of the two absorption bands. In the FT-IR analysis, it is necessary that PI films under the condition of 390 ◦ C are assumed to be fully 100% imidized PI films. 2.6. EGA-MS analysis In order to analyze the water content from the re-imidization process, a pyrolyzer-gas chromatography/mass spectrometer (PyGC/MS) was employed in this study. The Py-GC/MS consists of a PY-2020iD double-shot pyrolyzer (Frontier Laboratories, Fukushima, Japan) and an Agilent 7890A GC system with a 5975C inert XL mass selective detector (Agilent Technologies Korea, Seoul, South Korea). The capillary column for the pyrolyzer linking to the mass selective detector was an UADTM-2.5M deactivated metal column (0.15 mm I.D. × 2.5 mL with a coated film thickness <0.01 m, Frontier Laboratories, Fukushima, Japan). As the EGA-MS experiment started to run, the furnace temperature of the pyrolyzer was increased, and evolved gases from the
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Fig. 2. 1 H NMR spectra of PAAs as the precursor of PIs. (A) PI-I; (B) PI-II; (C) PI-III; (D) PI-IV; (a) the protons of carboxylic acid groups; (b) the protons of amide groups. The numeric indicates aromatic protons of PAAs as shown in Fig. 1.
sample were directly introduced into the mass selective detector. Thus, EGA-MS provides the total ions as well as the individual ions abundance versus the increasing temperature of the furnace in the pyrolyzer. The curves of the EGA-MS were generated by detecting the evolved gases. Here, water is a target evolved gas through the re-imidization process in the furnace of the pyrolyzer. The EGA-MS conditions were as follows; carrier gas: He; column flow rate: 1 mL/min; split ratio of injector: 1/10; temperature profile of the pyrolyzer furnace: 50 ◦ C (5 min), 10 ◦ C/min to 450 ◦ C (5 min), total run time: 50 min; injector, oven, and interface temperature of the GC/MS: 300 ◦ C; and the interface temperature of pyrolyzer: 320 ◦ C. In addition, selected ion monitoring (SIM) was used at m/z 18.
The amounts of 2, 20, and 40 mg of sodium tungstate dihydrate were used for standard calibration of water. In addition, 40 mg of PI films was used for determining the water content by EGA-MS. The DMAc content, as the residual solvent in PI films, was analyzed by purge and trap gas chromatography/mass spectrometry. The pure weight of the PI films was then corrected by the content of residual solvent. The content of residual solvent ranged from 0.3% to 3% (w/w, %). The results of the residual solvent in the PI films are not presented in this paper. The absorbed water content of the inner and/or surface of the PI films ranged from 0.2 mg to 0.3 mg. These values are negligible because small amounts of absorbed water do not affect the pure weight of the PI films.
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Table 1 Results of the calibration for the determination of the water content in PI films. Sodium tungstate dihydrate
Repetition
Calibration range (mg) 1 2 3
2–40 a
LOD (g)a
Calibration curves 2
Slope
Intercept
r
2 × 108 2 × 108 2 × 108
−535103 −327099 −536297
0.9998 0.9999 0.9999
7
LOD (limit of detection): S/N = 3.
3. Results and discussion It is found that the DOI can be obtained from the mole ratio of the PI and the sum of the PI and the residual PAA, as shown in Eq. (1).
DOI (%) =
the number of moles of PI the number of moles of PAA + the number of moles of PI
× 100
(1)
The number of moles of residual amic acid groups per repeating unit corresponds to the number of moles of water which is evolved through the re-imidization process in EGA-MS, as shown in Fig. 1. For example as shown in Fig. 1C, a mole (MPAA ) of PI-III generates two moles (MH2 O ) of water in the imidization process. In this case, two molecules (NH2 O ) of water can theoretically be generated in the imidization process. Therefore, the number of moles of residual PAA per repeating unit can be represented, as shown in Eq. (2). MPAA =
MH2 O NH2 O
=
WH2 O
(2)
18 × NH2 O
where MPAA is the number of moles of PAA per repeating unit, MH2 O is the number of moles of H2 O, WH2 O is the weight of H2 O which is evolved through the re-imidization process in EGA-MS, and NH2 O is the number of molecules of H2 O which can theoretically be generated in the imidization process. Consequently, the DOI can be expressed by Eq. (3). The DOI is obtained from the water content (WH2 O ) by EGA-MS analysis. The equation for the DOI is
DOI (%) =
WPI /MWPI (WH2 O /(18 × NH2 O ) + (WPI /MWPI ))
× 100
(3)
WPI = Wsample − WPAA , WPAA = MWPAA × WH2 O /(18 × where NH2 O ), WPI and WPAA are the weight of PI and PAA per repeating unit, Wsample is the weight of the sample for EGA-MS analysis, WH2 O is the weight of H2 O which is evolved through the re-imidization process in EGA-MS, and MWPI and MWPAA are the molecular weights of PI and PAA per repeating unit, respectively. In order to verify the proposed polymer structures shown in Fig. 1, PAAs, as the precursor of PIs, were employed in a 1 H NMR experiment and the results are given in Fig. 2. The 1 H NMR spectra shows signals for the protons of carboxylic acid groups at 13.3 ppm and the protons of amide groups at 10.5 ppm. Signals of aromatic protons for the polymer backbone are from 7.0 ppm to 8.5 ppm. From the results, the mole ratios between the protons of carboxylic acid groups and the protons of amide groups were approximately equal. Therefore, the total ratios of dianhydride and diamine were 1:1 and the proposed polymer structures can be used to perform DOI calculations in the EGA-MS analysis. Fig. 3 and Table 1 represent the calibration data for determining the water content in the PI films using sodium tungstate dihydrate. All correlation coefficients (r2 ) are better than or equal to 0.9998. The limit of detection (LOD) is 7 g of water at a signal-to-noise ratio better than or equal to 3. A water content of 0.018% and a DOI of 99.85% can be measured with 40 mg of PI films. These values are acceptable for the DOI analysis of PI films. In selected ion monitoring (m/z 18) of sodium tungstate dihydrate, water is detected from 50 ◦ C (5 min) to 120 ◦ C (12 min) as shown in Fig. 3. Hence, peak integration was performed from 5 min to 12 min.
Fig. 4 shows the results of selected ion monitoring (m/z 18) for PI films prepared at various imidization temperatures. In PI films, water was detected by two separated regions. Adsorbed water of the inner and/or surface of PI films might be detected before the temperature reached 150 ◦ C (15 min). Evolved water through the re-imidization process was detected from 150 ◦ C to 450 ◦ C. Therefore, this region was integrated as the peak area for DOI analysis. The results of the DOI for various PI films are given in Table 2. In addition, the overlaid graphs of the DOI are illustrated in Fig. 5. The DOI is increased with an increase of the imidization temperature in all the experiments. Imidization was completed at 300 ◦ C for PII and PI-II, 350 ◦ C for PI-IV, and 390 ◦ C for PI-III, respectively. The absolute values of the DOI were acquired by EGA-MS analysis without relative calculations of the DOI. In order to compare with the traditional method, the typical DOIs of PI-III and PI-IV obtained from FT-IR analysis are represented in Table 3. These values are quite different from the results of the EGA-MS analysis because the FT-IR data are relative values. Therefore, the absolute values of the DOI are considered to be useful. As shown in Fig. 5, interesting results were found in this study. The imidization increased in the order of PI-I < PI-II < PI-III < PI-IV at 200 ◦ C, which represents a relatively lower imidization temperature. On the other hand, the imidization increased in the order of PI-IV < PI-III < PI-II < PI-I at 250 ◦ C and 300 ◦ C, which represent a relatively higher imidization temperature. The results of the DOIs show opposite imidization rates between the lower and higher imidization temperatures. Meanwhile, the flexibility of the polymer chains in the PIs increases in the order of PI-IV < PI-III < PI-II < PI-I. Compared to other monomers, ODA plays the role of a relatively flexible monomer in the polymer chain due to its ether linked bond [8]. In general, a flexible polymer shows a fast imidization reaction because the increased flexibility of the polymer chain will be mobilized more easily. However, fast imidization occurred in rigid PIs at an imidization temperature of 200 ◦ C as shown in Fig. 5. Flexible PIs provided fast imidization
Fig. 3. Selected ion monitoring (m/z 18) of calibration standards in EGAMS. (a) 1.79 mg, (b) 14.6 mg, and (c) 31.68 mg of sodium tungstate dihydrate, y = (2 × 108 )x − 771556, r2 = 0.9998.
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Fig. 4. Selected ion monitoring (m/z 18) of EGA-MS for PI films prepared at various imidization temperatures. (A) PI-I; (B) PI-II; (C) PI-III; (D) PI-IV; (E) multi-layer PI film which consists of PI-I, PI-IV and PI-II.
Table 2 Degree of imidization of PI films prepared at various imidization temperatures by EGA-MS analysis. Temperatures of imidization (◦ C)
Degree of imidization (%)a ± S.D.b PI-I
200 250 300 350 390
74.4 97.8 >99.9 >99.9 >99.9
PI-II ± ± ± ± ±
For detailed analysis conditions, refer to the experimental part. a Three replicates analysis. b S.D.: standard deviation.
0.04 0.06 0.00 0.00 0.00
77.4 97.5 >99.9 >99.9 >99.9
PI-III ± ± ± ± ±
0.04 0.08 0.00 0.00 0.00
80.4 96.7 99.0 99.7 >99.9
PI-IV ± ± ± ± ±
0.05 0.03 0.08 0.03 0.00
85.9 96.0 98.1 >99.9 >99.9
± ± ± ± ±
0.02 0.18 0.15 0.00 0.00
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A 105
B
101
Increasing flexibility of PIs
Degree of imidization (%)
100
99
95 97
90 95
85 93
PI-I
80
PI-I
PI-II 75
PI-II 91
PI-III
Increasing flexibility of PIs 70 150
200
250
PI-III
PI-IV 300
350
PI-IV
400
89 230
250
Temperature (ºC)
270
290
310
Temperature (ºC)
Fig. 5. Degree of imidization of PI films prepared at various imidization temperatures. (A) full scaled data and (B) expanded data for imidization temperatures from 230 ◦ C to 320 ◦ C. Table 3 Degree of imidization of PI films prepared at various imidization temperatures by ATR FT-IR analysis. Temperatures of imidization (◦ C)
PI-III
PI-IV a
200 250 300 350 390
b
Ratio of band area
Degree of imidization (%)
Ratio of band areaa
Degree of imidization (%)b
2.01 2.05 2.18 2.20 2.21
91.0 92.8 98.6 99.5 100.0
2.93 3.13 3.14 3.15 3.16
92.7 99.1 99.4 99.7 100.0
For detailed analysis conditions, refer to the experimental part. a The ratio of the C N stretching band at 1340 cm−1 and the benzene ring band at 1510 cm−1 . b The relative ratio versus the ratio of the band area at 390 ◦ C.
above 250 ◦ C of imidization temperature. In addition, an intersection point of the DOIs was ca. 240 ◦ C. The bond length and dihedral angle between the amine of the amide group and the carbon of the carboxylic acid group will exist variously near or far together in flexible PIs. On the other hand, the bond length and dihedral angle in rigid PIs will be relatively stiff. Hence, we suggest that a fast reaction might occur in rigid PIs at a relatively lower imidization temperature, while the imidization reaction of flexible PIs is activated at a relatively higher imidization temperature. The EGA-MS method was applied to the multi-layer PI film for determining the DOI. However, the EGA-MS analysis does not provide the result of the DOI in the case of multi-layer PI film because the source of the evolved water cannot be known through re-imidization. It can, however, be found to complete the imidization reaction of the multi-layer PI film using EGA-MS analysis. Multi-layer PI film consists of PI-I, PI-IV and PI-II. In this experiment, PI-IV is centered in the multi-layer PI film. The results of the DOI of
Table 4 Results of the water content of the multi-layer PI film at 300 and 320 ◦ C. Temperatures of imidization (◦ C) 300 320
Repetition
Weight of sample (mg)
Water content (%)a
1 2 1 2
43.25 42.04 37.98 35.26
0.05 0.05
The multi-layer PI film consists of PI-I, PI-IV and PI-II. a Water detected by re-imidization for residual amic acid groups of PI films. b LOD (limit of detection, S/N = 3): 7 g of water, water content can be estimated to be 0.018%.
PI-I and PI-II show the completion of imidization at 300 ◦ C, and the completion of imidization of PI-IV at 350 ◦ C, as shown in Table 2. Therefore, the temperature of imidization completion of a multilayer PI film might be from 300 ◦ C to 350 ◦ C. Table 4 and Fig. 4E show the results of the water content of the multi-layer PI film at 300 and 320 ◦ C. From the results, the imidization completion of the multi-layer PI film was at 320 ◦ C. 4. Conclusions We have developed a simple EGA-MS method for the determination of the DOI in various PI films. Absolute values of the DOI were obtained without relativization by EGA-MS analysis. In this study, it was found that fast reaction occurred in rigid PIs at relatively lower temperatures, while flexible PIs were activated and showed fast imidization reaction at relatively higher temperatures. In a multi-layer PI film, the results of the DOI cannot be obtained, but information about imidization completion is provided by EGAMS analysis. This is the first reported result for the DOI analysis of PI films using EGA-MS. In addition, this method may help to obtain absolute values of the DOI for various PI films. References [1] K. Xu, J. Economy, Hyperbranched thermosetting poly(imide-ester): synthesis and properties, Macromolecules 37 (2004) 4146–4155. [2] K. Akamatsu, S. Ikeda, H. Nawafune, Site-selective direct silver metallization on surface-modified polyimide layers, Langmuir 19 (2003) 10366–10371. [3] F. Yang, Y. Li, Q. Bu, S. Zhang, T. Ma, J. Zhao, Characterizations and thermal stability of soluble polyimide derived from novel unsymmetrical diamine monomers, Polym. Degrad. Stab. 95 (2010) 1950–1958.
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