- Email: [email protected]
Two-dimensional correlation analysis study of the curing process of phenylethynyl end-capped imide model compounds
Vibrational Spectroscopy 60 (2012) 137–141
Contents lists available at SciVerse ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Two-dimensional correlation analysis study of the curing process of phenylethynyl end-capped imide model compounds Boknam Chae a , Sang Hyun Lee b , Seung Bin Kim c , Young Mee Jung d,∗ , Seung Woo Lee b,∗∗ a
Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea c Department of Chemistry, POSTECH, Pohang 790-784, Republic of Korea d Department of Chemistry, Kangwon National University, Chunchon 200-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 September 2011 Received in revised form 24 October 2011 Accepted 28 October 2011 Available online 3 November 2011 Keywords: Phenylethynyl end-capped imide model compounds Thermal cure FTIR spectroscopy Two-dimensional correlation analysis
a b s t r a c t The phenylethynyl end-capped imide model compounds 6F-PDA-PEAP and 6F-ODA-PEAP were prepared for detailed investigation of the thermal cure process. To probe the spectral changes of ethynyl C C moieties as well as imide rings and phenyl rings in PDA and ODA units during the thermal curing, we applied two-dimensional (2D) correlation analysis to the infrared spectra of 6F-PDA-PEAP and 6F-ODAPEAP films. Thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP was influenced by molecular structure, and it induced spectral changes of C C moieties as well as imide rings and phenyl rings in PDA and ODA units. The thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP films induces the intensity changes of bands of imide and phenyl rings in PDA and ODA units before that of C C moieties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ethynyl end-capped polyimide systems have attracted attention because of their ease of processing, good material properties, and potential applications [1,2]. In particular, phenylethynyl endcapped polyimide compounds exhibit processing characteristics (thermal and mechanical properties) that are superior to those of ethynyl end-capped polyimide systems [3]. Thus, much effort has been exerted to better understand the cure process of these materials. Several different methods have been applied to investigate cure kinetics and cure products as a function of molecular structure. Fang et al. [4] intensively studied the cure kinetics of phenylethynyl endcapped imide model compounds using Fourier-transform infrared (FTIR) spectroscopy and thermal analysis. In addition, the cure products of these materials were analyzed using solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy [5]. Takekoshi et al. [6,7] evaluated the cure kinetics and cure product by high performance liquid chromatography (HPLC) and field desorption mass spectroscopy. These results suggested that the molecular structure of phenylethynyl end-capped imide compounds influenced
∗ Corresponding author. Tel.: +82 33 250 8495; fax: +82 33 253 7582. ∗∗ Corresponding author. Tel.: +82 53 810 2516; fax: +82 53 810 4631. E-mail addresses: [email protected] (Y.M. Jung), [email protected] (S.W. Lee). 0924-2031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2011.10.014
the reaction order as well as the cure product. The phenylethynyl end-capped imide compounds with relatively simple molecular structures had a higher reaction order and induced the complicated cure reaction. The phenylethynyl end-capped imide compounds with bulk functional groups reduced the reactivity of the ethynyl moiety and induced the ethynyl to ethynyl addition reaction. Based on these results and other research results, several reaction pathways were proposed to elucidate the thermal cure mechanism of phenylethynyl end-capped imide compounds [5,8]. In addition, it has been proposed that thermal curing of these compounds induced molecular rearrangement to form stable cure products [5]. However, the detailed segmental motion in these compounds has not been fully investigated. Two-dimensional (2D) correlation spectroscopy is a wellestablished analytical technique that has considerable utility and benefits in various spectroscopic studies [9–12]. In particular, 2D correlation spectroscopy is a powerful technique for studying the inter- and intra-molecular interactions between spectral peaks and for determining the sequence of spectral changes. Several researchers have been applied 2D correlation spectroscopy to understand the detailed reaction pathway during thermal reaction such as crosslinking of polyurethane, epoxy curing reaction, imidization of poly(amic acid) [13–15]. However, specific sequence of spectral changes related to cure kinetics and cure mechanism of phenylethynyl end-capped imide compounds has not been fully examined.
138
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
6F PDA PEAP 1:4 6F-PDA-PEAP O N O
O
F 3C
O
CF3
N
O
N
O
N O
O
6F-ODA-PEAP 1:4 O
O N O
O
F 3C
N O
CF3
O N
O N
O
O
O
Fig. 1. Chemical structure of imide model compounds.
In this study, we examined the thermal curing of phenylethynyl end-capped imide model compounds (Fig. 1) prepared from 4,4 -(hexafluoroisopropylidene)diphthalic anhydride (6F) with two different diamines (e.g. 4,4-oxydianilline (ODA) and 1,4phenylenediamine (PDA)) using FTIR spectroscopy. In particular, 2D correlation analysis of FTIR spectra was used to better understand the spectral changes of phenylethynyl and imide moieties in a phenylethynyl end-capped imide model compound during thermal curing. 2. Experimental details 2.1. Materials 4,4 -(Hexafluoroisopropylidene)diphthalic anhydride (6F) was supplied by Chriskev Company and purified by recrystallization from acetic anhydride. ODA, PDA, isoquinoline, and 4-(dimethylamino)pyridine (DMAP) were purchased from Aldrich Company, and 4-phenylethynylphthalic anhydride (PEAP) was purchased from TCI and used without purification. N-methyl-2pyrrolidinone (NMP) was purchased from Aldrich Company and distilled over calcium hydride in a nitrogen atmosphere.
In the same manner, the 6F-ODA-PEAP compound was prepared from the reaction of 6F, ODA, and PEAP. Yield: 81.5%. 1 H NMR (ı, DMSO-d6): 8.24–8.18 (d, 2H, ArH), 8.13 (s, 2H, ArH), 8.10–7.96 (m, 6H, ArH), 7.78 (s, 2H, ArH), 7.70–7.64 (s, 4H, ArH), 7.58–7.46 (m, 12H, ArH), and 7.32–7.22 (m, 6H, ArH). 2.3. Film preparation and measurement The 6F-PDA-PEAP films (Fig. 1) were obtained by dropping 1 wt% solutions of these imide model compounds in N-methyl-2pyrrolidone (NMP) on Si wafers for FTIR spectra. These films were dried at 60◦ C for 12 h under vacuum. FTIR spectroscopic measurements were carried out on a Bruker IFS 66/v spectrometer equipped with #2000-A (Aabspec, Fig. 2). IR spectra were recorded at 4 cm−1 resolution with a liquid-nitrogencooled mercury cadmium telluride (MCT) detector under vacuum. The imide model compound films were cured at 330◦ C for 60 min. Thermal cure behaviors were estimated by the following equation [3,4]:
⎛ The extent of cure ˛ = ⎝
AC≡C Aimide C=O AC≡C
⎞
Aimide C=O
2.2. Synthesis of model compounds The thermal curable model compound 6F-PDA-PEAP was synthesized as follows. First, 10 mmol (4.44 g) of FDA and 50 mmol (5.41 g) of PDA were dissolved together with 20 mmol (2.58 g) of isoquinoline and a catalytic amount of DMAP in dry NMP. The solution was gently heated with stirring at 70 ◦ C for 2 h and refluxed with stirring for 12 h. The reaction solution was poured into water with vigorous stirring, giving 6F-amido-N,N -4,4 -dianilline (6FPDA) in the form of a precipitated powder. The precipitated powder was filtered and dried. The crude product was purified by recrystallization from the methanol/H2 O mixture and by drying in vacuo. Second, the thermal curable model compound with ethynyl groups, 6F-PDA-PEAP, was synthesized from the 6F-PDA and PEAP. A mixture of 5 mmol (3.12 g) of 6F-PDA and 11 mmol (2.73 g) of PEAP with 10 mmol (1.49 g) of isoquinoline and a catalytic amount of DMAP in dry NMP was stirred at 70 ◦ C for 2 h; refluxed with stirring for 12 h; and poured into methanol. The precipitated solids were separated by filtration and washed thoroughly with methanol, leading to a precipitated model compound product. The precipitated polymer powder was filtered and dried, producing 6F-PDA-PEAP. Yield: 87.4%. 1 H NMR (ı, DMSO-d6): 8.24–8.22 (d, 2H, ArH), 8.13 (s, 2H, ArH), 8.10–7.98 (m, 6H, ArH), 7.78 (s, 2H, ArH), 7.69–7.58 (m, 12H, ArH), and 7.52–7.44 (m, 6H, ArH).
t
⎠
(1)
t=0
The 2D correlation spectra were obtained using an algorithm based on a numerical method developed by Noda [9–12]. The 2D correlation analyses were carried out after baseline correction of the FTIR spectra. A subroutine, named KG2D and written in Array Basic language (GRAMS/386; Galactic Inc., NH), was employed in the 2D correlation analyses [16].
Source Sample holder
Sample
Heater
Temperature controller
D Detector
Fig. 2. Schematic diagram of the heating cell used for IR study.
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
1.0
Cure time (min) 0 10 20 30 40 50 60
6F-ODA 6F-PDA
0.8
Extent of cure
Absorban nce (arb. unit)
(a)
139
0.6 0.4 04 0.2
2200
2000
1800
1600
1400
1200
-1
0.0
Wavenumber (cm )
10
20
30
40
50
60
time (min)
Cure time (min)
Fig. 4. Extent of cure of the thermal reaction of 6F-ODA-PEAP and 6F-PDA-PEAP films at 330◦ C for 60 min.
0 10 20 30 40 50 60
Absorbancee (arb. unit)
(b)
0
2200
2000
1800
1600
1400
1200
-1
Wavenumber (cm ) Fig. 3. FTIR spectra measured from (a) 6F-PDA-PEAP and (b) 6F-ODA-PEAP films during thermal reaction at 330◦ C for 60 min.
3. Results and discussion Fig. 3 shows the FTIR spectra of 6F-PDA-PEAP films cured at 330◦ C for 60 min. The assignments of the vibrational modes in 6FPDA-PEAP films were carried out according to previously reported results [17–21]. The bands at 1776 and 1724 cm−1 are assigned to the symmetric and asymmetric C O stretching vibrations of the imide ring. The band at 2200 cm−1 is attributed to the C C stretching vibration in the phenylethynyl unit. There are three different substituted phenyl rings in 6F-PDA-PEAP and 6F-ODA-PEAP: mono-substituted phenyl ring in the phenylethynyl unit, tri-substituted phenyl ring in the imide ring, and para-substituted phenyl ring in the PDA and ODA units (Fig. 1). The vibration involving quadrant stretching of the phenyl ring was detected at 1610 and 1591 (sh) cm−1 , and the semicircle stretching vibration of the phenyl ring was observed in the region 1510–1490 cm−1 (Fig. 3). As described in our previous reports [21–23], the bands at 1509 and 1500 cm−1 can be assigned to the para-substituted phenyl ring vibration in the PDA and ODA units, respectively. However, it is difficult to differentiate the mono and tri-substituted phenyl rings in the phenylethynyl unit and imid ring from the bands at 1610 and 1592 cm−1 . The quadrant stretching vibrational mode of the phenyl ring in the 1620–1565 cm−1 region is less frequency sensitive for mono-, di-, and tri-substitution of phenyl rings compare to semicircle stretching vibration of the phenyl ring. In addition, the stretching vibration of C C conjugated with ethynyl C C is observed at 1610 cm−1 , as proposed by Li and Morgan [3]. Thus, analysis of IR spectra itself does not provide direct information about the individual molecular motion of the phenyl ring in the phenylethynyl unit and the tri-substituted phenyl ring in the imide ring in this study. As shown in Fig. 3, the intensity of the band at 2200 cm−1 corresponds to the ethynyl C C stretching vibration decreases with increasing cure time. The intensity drop in the C C stretching band
might result from the loss of C C moieties due to the thermal cure reaction. The extent of cure in the 6F-ODA-PEAP and 6F-PDA-PEAP films was determined by Eq. (1) during the thermal reaction. In this study, the bands at 1776 and 1724 cm−1 assigned to the imide C O stretching vibration were used as an internal standard to calculate the extent of cure in the 6F-ODA-PEAP and 6F-PDA-PEAP films, as reported by Fang et al. [4]. As shown in Fig. 4, the extent of cure in the model compound films increases with increasing cure time. In addition, it shows that the conversion extent of 6F-ODA-PEAP at 330◦ C for 60 min is smaller than that of 6F-PDA-PEAP. The extent of cure of 6F-PDA-PEAP is 65% and that of 6F-ODA-PEAP is 57%. This different reactivity could be explained by the molecular structure of the PDA and ODA units in model compounds, as described by Li and Morgan [3]. As shown in Fig. 1, 6F-PDA-PEAP has a smaller molecular structure than 6F-ODA-PEAP, and it is expected that the bulk nature of the ODA unit relative to the PDA unit could contribute to restricting the cure reaction of phenylethynyl moieties. Thus, the steric effect from the ODA unit could decrease the reactivity of the phenylethynyl moieties. To further examine the thermal cure reaction of the phenylethynyl end-capped imide model compounds, FTIR spectra were analyzed using 2D correlation spectroscopy. Fig. 5 shows the synchronous and asynchronous 2D correlation of FTIR spectra in the region 2250–1180 cm−1 generated from the FTIR spectra of 6F-PDA-PEAP film cured at 330◦ C for 60 min. Corresponding 2D correlation spectra of 6F-ODA-PEAP films cured at 330◦ C for 60 min are shown in Fig. 6. A power spectrum extracted along the diagonal line on the synchronous 2D correlation spectrum also is shown at the top of Figs. 5(a) and 6(a). According to their power spectra, the band at 1724 cm−1 due to the imide C O stretching vibration in 6F-PDA-PEAP and 6F-ODA-PEAP films is strongly influenced by the thermal cure reaction. In addition, the band at 1500 cm−1 due to the para-substituted phenyl ring vibration in the ODA unit also is influenced by the thermal cure reaction. Thus, these results indicate that the thermal cure of phenylethynyl end-capped moieties induces the strong spectral change of the rod-like imide ring for 6F-PDA-PEAP and that of the imide ring as well as the ODA unit for 6F-ODA-PEAP. The larger units in 6F-ODA-PEAP and 6F-PDAPEAP films such as imide rings and the ODA unit are rather strongly influenced by the thermal cure reaction. To better understand the thermal cure process of phenylethynyl end-capped imide model compounds, the sequence of the spectral peaks in 6F-PDA-PEAP and 6F-ODA-PEAP films also was determined using the synchronous and asynchronous 2D correlation FTIR spectra. We examined the sequences of intensity changes of C C moieties as well as imide rings and phenyl rings in PDA units in the
140
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
(a)
(a)
1200
1400
1200 1300
1600
1400
1800
1500 1600
2000 1700
ber (cm-1) Wavenumb
Wavenumbeer (cm-1)
1200
2200
2000
1800
1600
1400
1200
1400
1800
1500 1600
2000 2200
1200
2200
Wavenumber (cm-1) (b) 1200
2000
1800
1600
1400
Wavenumber (cm-1)
1200
1800 1800 1700 1600 1500 1400 1300 1200
(b) 1200
1400
1200 1300
1600
1400
1800
1500 1600
2000 1700 1800 1800 1700 1600 1500 1400 1300 1200
2200 2200 2000
1800
1600
1400
1200
Wavenumber (cm-1)
Waveenumber (cm m-1)
Waveenumber (cm m-1)
1300
1600
1700
1800 1800 1700 1600 1500 1400 1300 1200
2200
1400
1400 1200 1300
1600
1400
1800
1500 1600
2000
1700 1800 1800 1700 1600 1500 1400 1300 1200
2200 2200
2000
1800
1600
1400
1200
Wavenumber (cm-1)
Fig. 5. (a) Synchronous and (b) asynchronous 2D IR correlation spectra of the 6FPDA-PEAP film during thermal reaction at 330◦ C for 60 min. Solid and dashed lines indicate positive and negative cross peaks, respectively.
Fig. 6. (a) Synchronous and (b) asynchronous 2D IR correlation spectra of the 6FODA-PEAP films during thermal reaction at 330◦ C for 60 min. Solid and dashed lines indicate positive and negative cross peaks, respectively.
6F-PDA-PEAP film during thermal curing (Fig. 5). The cross peaks at (2200, 1509) and (1724, 1509) cm−1 for the ethynyl C C stretching and imide C O stretching vibrations with para-substituted phenyl ring vibration in the PDA unit revealed that the intensity change of the para-substituted phenyl ring in PDA occurred before that of the imide ring and the ethynyl C C unit (data not shown). The cross peak at (2200, 1724) cm−1 suggests that the imide ring changes before the unreacted C C unit. Collectively, thermal cure reaction of the 6F-PDA-PEAP film induces spectral changes of the parasubstituted phenyl ring in the PDA unit, imide ring, and unreacted C C bond sequentially. We also investigated the sequence of the bands related to C C moieties as well as imide rings and phenyl rings in ODA unit molecular reorientation in the 6F-ODA-PEAP film during thermal curing (Fig. 6). The cross peaks at (2200, 1724) and (1724, 1500) cm−1 for the ethynyl C C stretching and para-substituted phenyl ring vibrations in the PDA unit with the imide C O stretching vibration indicate that the imide ring changes before the para-substituted phenyl ring in the PDA unit and the ethynyl C C unit. The cross peak at (2200, 1500) cm−1 reveals that the para-substituted phenyl ring in the PDA unit changes before the unreacted C C bond (data not shown). Conclusively, thermal cure reaction of the 6F-ODA-PEAP film induces spectral change of the imide ring, para-substituted phenyl ring in the ODA unit, and unreacted C C bond sequentially. Several researchers have proposed that the thermal cure reaction containing the phenyl ethynyl C C moiety induces phenyl ring migration as well as molecular rearrangement to form cure products [3–5]. Results of two-dimensional correlation analyses of 6F-PDA-PEAP and 6F-ODA-PEAP FTIR spectra during thermal cure reaction are in good agreement with these previous studies. The thermal cure reaction of phenylethynyl end-capped model
compounds induces the segmental motion of ethynyl C C as well as the adjacent imide ring and phenyl ring in the PDA and ODA units. Further, the spectral change of the imide and phenyl rings in the PDA and ODA units occur before the thermal cure of the C C unit. Rather bulky unit changes occur before changes to the ethynyl C C unit when forming cure product.
4. Conclusions Thermal cure of 6F-PDA-PEAP and 6F-ODA-PEAP films was investigated in detail by FTIR spectroscopy and 2D correlation spectroscopy. The FTIR spectra of the 6F-PDA-PEAP and 6F-ODAPEAP films revealed that the extent of cure is influenced by the molecular structure of 6F-PDA-PEAP and 6F-ODA-PEAP films. The conversion extent of 6F-ODA-PEAP at 330◦ C for 60 min is smaller than that of 6F-PDA-PEAP because of steric factors of the ODA unit. The analysis of the 2D correlation FTIR spectra indicated that the thermal cure of phenylethynyl end-capped model compounds 6FPDA-PEAP and 6F-ODA-PEAP induced the spectral change of C C and the imide and phenyl rings in the ODA and PDA units. In particular, these results suggest that the imide ring is strongly influenced by thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP films. The phenyl ring in the ODA unit of the 6F-ODA-PEAP films is rather strongly influenced by the thermal cure reaction. Further, 2D correlation analyses of FTIR spectra provided the sequence of intensity changes of phenylethynyl C C moieties and adjacent imide and phenyl rings in the ODA and PDA units during thermal curing. The spectral change of imide and phenyl rings in ODA and PDA groups occurred before the thermal cure of ethynyl C C moieties.
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
Acknowledgements This work was supported by the Industrial Strategic Technology Development Program (No. 10033355) funded by the Ministry of Knowledge Economy (MKE, Korea) and the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of Knowledge Economy. References [1] H. Liou, P.S. Ho, B. Tung, J. Appl. Polym. Sci. 70 (1998) 261–272. [2] H. Liu, C.D. Simone, D.A. Scola, J. Polym. Sci. A: Polym. Chem. 41 (2003) 2630–2649. [3] Y. Li, R.J. Morgan, J. Appl. Polym. Sci. 101 (2006) 4446–4453. [4] X. Fang, D.F. Rogers, D.A. Scola, M.P. Stevens, J. Polym. Sci. A: Polym. Chem. 36 (1998) 461–470. [5] X. Fang, X.-Q. Xie, C.D. Simone, M.P. Stevens, D.A. Scola, Macromolecules 33 (2000) 1671–1681. [6] .J.A. Johnston, F.M. Li, F.W. Harris, T. Takekoshi, Polymer 35 (1994) 4865–4873. [7] T. Takekoshi, J.M. Terry, Polymer 35 (1994) 4874–4880. [8] C.C. Roberts, T.M. Apple, G.E. Wnek, J. Polym. Sci. A: Polym. Chem. 38 (2000) 3486–3497.
141
[9] I. Noda, Appl. Spectrosc. 47 (1993) 1329–1336. [10] I. Noda, Appl. Spectrosc. 54 (2000) 994–999. [11] I. Noda, Y. Ozaki, Two-Dimensional Correlation Spectroscopy: Applications in Vibrational Spectroscopy, John Wiley & Sons Inc., New York, 2004. [12] Y.M. Jung, I. Noda, Appl. Spectrosc. Rev. 41 (2006) 515–547. [13] H. Yamasaki, S. Morita, J. Appl. Sci. 119 (2011) 871–881. [14] S. Radice, S. Turri, M. Scicchitano, Appl. Spectrosc. 58 (2004) 535–542. [15] K.H. Yu, Y.H. Yoo, J.M. Rhee, M.-H. Lee, S.-C. Yu, Bull. Korean Chem. Soc. 24 (2003) 357–362. [16] The program is available from the homepage of Professor Yukihiro Osaki of Kwansei Gakuin University, Sanda, japan, http://science.kwansei.ac.jp/∼ozaki. [17] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman Spectroscopy, Academic, San Diego, 1990. [18] D. Lin-Vien, N.B. Colthup, V.G. Fately, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic, San Diego, 1991. [19] B. Chae, S.W. Lee, B. Lee, W. Choi, S.B. Kim, Y.M. Jung, J. Jung, K.H. Lee, M. Ree, J. Phys. Chem. B 107 (2003) 11911–11916. [20] Y.H. Kim, B. Chae, T.J. Lee, S.B. Kim, Y.M. Jung, S.W. Lee, J. Mol. Struct. 974 (2010) 30–34. [21] S.W. Lee, S.J. Lee, S.G. Hahm, T.J. Lee, B. Lee, B. Chae, S.B. Kim, J.C. Jung, W.C. Zin, B.H. Sohn, M. Ree, Macromolecules 38 (2005) 4331–4338. [22] S.W. Lee, B. Chae, B. Lee, W. Choi, S.B. Kim, S.I. Kim, S. Park, J.C. Jung, K.H. Lee, M. Ree, Chem. Mater. 15 (2003) 3105–3112. [23] B. Chae, S.W. Lee, B. Lee, W. Choi, S.B. Kim, Y.M. Jung, J.C. Jung, K.H. Lee, M. Ree, Langmuir 19 (2003) 9459–9465.
Contents lists available at SciVerse ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Two-dimensional correlation analysis study of the curing process of phenylethynyl end-capped imide model compounds Boknam Chae a , Sang Hyun Lee b , Seung Bin Kim c , Young Mee Jung d,∗ , Seung Woo Lee b,∗∗ a
Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea c Department of Chemistry, POSTECH, Pohang 790-784, Republic of Korea d Department of Chemistry, Kangwon National University, Chunchon 200-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 3 September 2011 Received in revised form 24 October 2011 Accepted 28 October 2011 Available online 3 November 2011 Keywords: Phenylethynyl end-capped imide model compounds Thermal cure FTIR spectroscopy Two-dimensional correlation analysis
a b s t r a c t The phenylethynyl end-capped imide model compounds 6F-PDA-PEAP and 6F-ODA-PEAP were prepared for detailed investigation of the thermal cure process. To probe the spectral changes of ethynyl C C moieties as well as imide rings and phenyl rings in PDA and ODA units during the thermal curing, we applied two-dimensional (2D) correlation analysis to the infrared spectra of 6F-PDA-PEAP and 6F-ODAPEAP films. Thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP was influenced by molecular structure, and it induced spectral changes of C C moieties as well as imide rings and phenyl rings in PDA and ODA units. The thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP films induces the intensity changes of bands of imide and phenyl rings in PDA and ODA units before that of C C moieties. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ethynyl end-capped polyimide systems have attracted attention because of their ease of processing, good material properties, and potential applications [1,2]. In particular, phenylethynyl endcapped polyimide compounds exhibit processing characteristics (thermal and mechanical properties) that are superior to those of ethynyl end-capped polyimide systems [3]. Thus, much effort has been exerted to better understand the cure process of these materials. Several different methods have been applied to investigate cure kinetics and cure products as a function of molecular structure. Fang et al. [4] intensively studied the cure kinetics of phenylethynyl endcapped imide model compounds using Fourier-transform infrared (FTIR) spectroscopy and thermal analysis. In addition, the cure products of these materials were analyzed using solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy [5]. Takekoshi et al. [6,7] evaluated the cure kinetics and cure product by high performance liquid chromatography (HPLC) and field desorption mass spectroscopy. These results suggested that the molecular structure of phenylethynyl end-capped imide compounds influenced
∗ Corresponding author. Tel.: +82 33 250 8495; fax: +82 33 253 7582. ∗∗ Corresponding author. Tel.: +82 53 810 2516; fax: +82 53 810 4631. E-mail addresses: [email protected] (Y.M. Jung), [email protected] (S.W. Lee). 0924-2031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2011.10.014
the reaction order as well as the cure product. The phenylethynyl end-capped imide compounds with relatively simple molecular structures had a higher reaction order and induced the complicated cure reaction. The phenylethynyl end-capped imide compounds with bulk functional groups reduced the reactivity of the ethynyl moiety and induced the ethynyl to ethynyl addition reaction. Based on these results and other research results, several reaction pathways were proposed to elucidate the thermal cure mechanism of phenylethynyl end-capped imide compounds [5,8]. In addition, it has been proposed that thermal curing of these compounds induced molecular rearrangement to form stable cure products [5]. However, the detailed segmental motion in these compounds has not been fully investigated. Two-dimensional (2D) correlation spectroscopy is a wellestablished analytical technique that has considerable utility and benefits in various spectroscopic studies [9–12]. In particular, 2D correlation spectroscopy is a powerful technique for studying the inter- and intra-molecular interactions between spectral peaks and for determining the sequence of spectral changes. Several researchers have been applied 2D correlation spectroscopy to understand the detailed reaction pathway during thermal reaction such as crosslinking of polyurethane, epoxy curing reaction, imidization of poly(amic acid) [13–15]. However, specific sequence of spectral changes related to cure kinetics and cure mechanism of phenylethynyl end-capped imide compounds has not been fully examined.
138
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
6F PDA PEAP 1:4 6F-PDA-PEAP O N O
O
F 3C
O
CF3
N
O
N
O
N O
O
6F-ODA-PEAP 1:4 O
O N O
O
F 3C
N O
CF3
O N
O N
O
O
O
Fig. 1. Chemical structure of imide model compounds.
In this study, we examined the thermal curing of phenylethynyl end-capped imide model compounds (Fig. 1) prepared from 4,4 -(hexafluoroisopropylidene)diphthalic anhydride (6F) with two different diamines (e.g. 4,4-oxydianilline (ODA) and 1,4phenylenediamine (PDA)) using FTIR spectroscopy. In particular, 2D correlation analysis of FTIR spectra was used to better understand the spectral changes of phenylethynyl and imide moieties in a phenylethynyl end-capped imide model compound during thermal curing. 2. Experimental details 2.1. Materials 4,4 -(Hexafluoroisopropylidene)diphthalic anhydride (6F) was supplied by Chriskev Company and purified by recrystallization from acetic anhydride. ODA, PDA, isoquinoline, and 4-(dimethylamino)pyridine (DMAP) were purchased from Aldrich Company, and 4-phenylethynylphthalic anhydride (PEAP) was purchased from TCI and used without purification. N-methyl-2pyrrolidinone (NMP) was purchased from Aldrich Company and distilled over calcium hydride in a nitrogen atmosphere.
In the same manner, the 6F-ODA-PEAP compound was prepared from the reaction of 6F, ODA, and PEAP. Yield: 81.5%. 1 H NMR (ı, DMSO-d6): 8.24–8.18 (d, 2H, ArH), 8.13 (s, 2H, ArH), 8.10–7.96 (m, 6H, ArH), 7.78 (s, 2H, ArH), 7.70–7.64 (s, 4H, ArH), 7.58–7.46 (m, 12H, ArH), and 7.32–7.22 (m, 6H, ArH). 2.3. Film preparation and measurement The 6F-PDA-PEAP films (Fig. 1) were obtained by dropping 1 wt% solutions of these imide model compounds in N-methyl-2pyrrolidone (NMP) on Si wafers for FTIR spectra. These films were dried at 60◦ C for 12 h under vacuum. FTIR spectroscopic measurements were carried out on a Bruker IFS 66/v spectrometer equipped with #2000-A (Aabspec, Fig. 2). IR spectra were recorded at 4 cm−1 resolution with a liquid-nitrogencooled mercury cadmium telluride (MCT) detector under vacuum. The imide model compound films were cured at 330◦ C for 60 min. Thermal cure behaviors were estimated by the following equation [3,4]:
⎛ The extent of cure ˛ = ⎝
AC≡C Aimide C=O AC≡C
⎞
Aimide C=O
2.2. Synthesis of model compounds The thermal curable model compound 6F-PDA-PEAP was synthesized as follows. First, 10 mmol (4.44 g) of FDA and 50 mmol (5.41 g) of PDA were dissolved together with 20 mmol (2.58 g) of isoquinoline and a catalytic amount of DMAP in dry NMP. The solution was gently heated with stirring at 70 ◦ C for 2 h and refluxed with stirring for 12 h. The reaction solution was poured into water with vigorous stirring, giving 6F-amido-N,N -4,4 -dianilline (6FPDA) in the form of a precipitated powder. The precipitated powder was filtered and dried. The crude product was purified by recrystallization from the methanol/H2 O mixture and by drying in vacuo. Second, the thermal curable model compound with ethynyl groups, 6F-PDA-PEAP, was synthesized from the 6F-PDA and PEAP. A mixture of 5 mmol (3.12 g) of 6F-PDA and 11 mmol (2.73 g) of PEAP with 10 mmol (1.49 g) of isoquinoline and a catalytic amount of DMAP in dry NMP was stirred at 70 ◦ C for 2 h; refluxed with stirring for 12 h; and poured into methanol. The precipitated solids were separated by filtration and washed thoroughly with methanol, leading to a precipitated model compound product. The precipitated polymer powder was filtered and dried, producing 6F-PDA-PEAP. Yield: 87.4%. 1 H NMR (ı, DMSO-d6): 8.24–8.22 (d, 2H, ArH), 8.13 (s, 2H, ArH), 8.10–7.98 (m, 6H, ArH), 7.78 (s, 2H, ArH), 7.69–7.58 (m, 12H, ArH), and 7.52–7.44 (m, 6H, ArH).
t
⎠
(1)
t=0
The 2D correlation spectra were obtained using an algorithm based on a numerical method developed by Noda [9–12]. The 2D correlation analyses were carried out after baseline correction of the FTIR spectra. A subroutine, named KG2D and written in Array Basic language (GRAMS/386; Galactic Inc., NH), was employed in the 2D correlation analyses [16].
Source Sample holder
Sample
Heater
Temperature controller
D Detector
Fig. 2. Schematic diagram of the heating cell used for IR study.
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
1.0
Cure time (min) 0 10 20 30 40 50 60
6F-ODA 6F-PDA
0.8
Extent of cure
Absorban nce (arb. unit)
(a)
139
0.6 0.4 04 0.2
2200
2000
1800
1600
1400
1200
-1
0.0
Wavenumber (cm )
10
20
30
40
50
60
time (min)
Cure time (min)
Fig. 4. Extent of cure of the thermal reaction of 6F-ODA-PEAP and 6F-PDA-PEAP films at 330◦ C for 60 min.
0 10 20 30 40 50 60
Absorbancee (arb. unit)
(b)
0
2200
2000
1800
1600
1400
1200
-1
Wavenumber (cm ) Fig. 3. FTIR spectra measured from (a) 6F-PDA-PEAP and (b) 6F-ODA-PEAP films during thermal reaction at 330◦ C for 60 min.
3. Results and discussion Fig. 3 shows the FTIR spectra of 6F-PDA-PEAP films cured at 330◦ C for 60 min. The assignments of the vibrational modes in 6FPDA-PEAP films were carried out according to previously reported results [17–21]. The bands at 1776 and 1724 cm−1 are assigned to the symmetric and asymmetric C O stretching vibrations of the imide ring. The band at 2200 cm−1 is attributed to the C C stretching vibration in the phenylethynyl unit. There are three different substituted phenyl rings in 6F-PDA-PEAP and 6F-ODA-PEAP: mono-substituted phenyl ring in the phenylethynyl unit, tri-substituted phenyl ring in the imide ring, and para-substituted phenyl ring in the PDA and ODA units (Fig. 1). The vibration involving quadrant stretching of the phenyl ring was detected at 1610 and 1591 (sh) cm−1 , and the semicircle stretching vibration of the phenyl ring was observed in the region 1510–1490 cm−1 (Fig. 3). As described in our previous reports [21–23], the bands at 1509 and 1500 cm−1 can be assigned to the para-substituted phenyl ring vibration in the PDA and ODA units, respectively. However, it is difficult to differentiate the mono and tri-substituted phenyl rings in the phenylethynyl unit and imid ring from the bands at 1610 and 1592 cm−1 . The quadrant stretching vibrational mode of the phenyl ring in the 1620–1565 cm−1 region is less frequency sensitive for mono-, di-, and tri-substitution of phenyl rings compare to semicircle stretching vibration of the phenyl ring. In addition, the stretching vibration of C C conjugated with ethynyl C C is observed at 1610 cm−1 , as proposed by Li and Morgan [3]. Thus, analysis of IR spectra itself does not provide direct information about the individual molecular motion of the phenyl ring in the phenylethynyl unit and the tri-substituted phenyl ring in the imide ring in this study. As shown in Fig. 3, the intensity of the band at 2200 cm−1 corresponds to the ethynyl C C stretching vibration decreases with increasing cure time. The intensity drop in the C C stretching band
might result from the loss of C C moieties due to the thermal cure reaction. The extent of cure in the 6F-ODA-PEAP and 6F-PDA-PEAP films was determined by Eq. (1) during the thermal reaction. In this study, the bands at 1776 and 1724 cm−1 assigned to the imide C O stretching vibration were used as an internal standard to calculate the extent of cure in the 6F-ODA-PEAP and 6F-PDA-PEAP films, as reported by Fang et al. [4]. As shown in Fig. 4, the extent of cure in the model compound films increases with increasing cure time. In addition, it shows that the conversion extent of 6F-ODA-PEAP at 330◦ C for 60 min is smaller than that of 6F-PDA-PEAP. The extent of cure of 6F-PDA-PEAP is 65% and that of 6F-ODA-PEAP is 57%. This different reactivity could be explained by the molecular structure of the PDA and ODA units in model compounds, as described by Li and Morgan [3]. As shown in Fig. 1, 6F-PDA-PEAP has a smaller molecular structure than 6F-ODA-PEAP, and it is expected that the bulk nature of the ODA unit relative to the PDA unit could contribute to restricting the cure reaction of phenylethynyl moieties. Thus, the steric effect from the ODA unit could decrease the reactivity of the phenylethynyl moieties. To further examine the thermal cure reaction of the phenylethynyl end-capped imide model compounds, FTIR spectra were analyzed using 2D correlation spectroscopy. Fig. 5 shows the synchronous and asynchronous 2D correlation of FTIR spectra in the region 2250–1180 cm−1 generated from the FTIR spectra of 6F-PDA-PEAP film cured at 330◦ C for 60 min. Corresponding 2D correlation spectra of 6F-ODA-PEAP films cured at 330◦ C for 60 min are shown in Fig. 6. A power spectrum extracted along the diagonal line on the synchronous 2D correlation spectrum also is shown at the top of Figs. 5(a) and 6(a). According to their power spectra, the band at 1724 cm−1 due to the imide C O stretching vibration in 6F-PDA-PEAP and 6F-ODA-PEAP films is strongly influenced by the thermal cure reaction. In addition, the band at 1500 cm−1 due to the para-substituted phenyl ring vibration in the ODA unit also is influenced by the thermal cure reaction. Thus, these results indicate that the thermal cure of phenylethynyl end-capped moieties induces the strong spectral change of the rod-like imide ring for 6F-PDA-PEAP and that of the imide ring as well as the ODA unit for 6F-ODA-PEAP. The larger units in 6F-ODA-PEAP and 6F-PDAPEAP films such as imide rings and the ODA unit are rather strongly influenced by the thermal cure reaction. To better understand the thermal cure process of phenylethynyl end-capped imide model compounds, the sequence of the spectral peaks in 6F-PDA-PEAP and 6F-ODA-PEAP films also was determined using the synchronous and asynchronous 2D correlation FTIR spectra. We examined the sequences of intensity changes of C C moieties as well as imide rings and phenyl rings in PDA units in the
140
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
(a)
(a)
1200
1400
1200 1300
1600
1400
1800
1500 1600
2000 1700
ber (cm-1) Wavenumb
Wavenumbeer (cm-1)
1200
2200
2000
1800
1600
1400
1200
1400
1800
1500 1600
2000 2200
1200
2200
Wavenumber (cm-1) (b) 1200
2000
1800
1600
1400
Wavenumber (cm-1)
1200
1800 1800 1700 1600 1500 1400 1300 1200
(b) 1200
1400
1200 1300
1600
1400
1800
1500 1600
2000 1700 1800 1800 1700 1600 1500 1400 1300 1200
2200 2200 2000
1800
1600
1400
1200
Wavenumber (cm-1)
Waveenumber (cm m-1)
Waveenumber (cm m-1)
1300
1600
1700
1800 1800 1700 1600 1500 1400 1300 1200
2200
1400
1400 1200 1300
1600
1400
1800
1500 1600
2000
1700 1800 1800 1700 1600 1500 1400 1300 1200
2200 2200
2000
1800
1600
1400
1200
Wavenumber (cm-1)
Fig. 5. (a) Synchronous and (b) asynchronous 2D IR correlation spectra of the 6FPDA-PEAP film during thermal reaction at 330◦ C for 60 min. Solid and dashed lines indicate positive and negative cross peaks, respectively.
Fig. 6. (a) Synchronous and (b) asynchronous 2D IR correlation spectra of the 6FODA-PEAP films during thermal reaction at 330◦ C for 60 min. Solid and dashed lines indicate positive and negative cross peaks, respectively.
6F-PDA-PEAP film during thermal curing (Fig. 5). The cross peaks at (2200, 1509) and (1724, 1509) cm−1 for the ethynyl C C stretching and imide C O stretching vibrations with para-substituted phenyl ring vibration in the PDA unit revealed that the intensity change of the para-substituted phenyl ring in PDA occurred before that of the imide ring and the ethynyl C C unit (data not shown). The cross peak at (2200, 1724) cm−1 suggests that the imide ring changes before the unreacted C C unit. Collectively, thermal cure reaction of the 6F-PDA-PEAP film induces spectral changes of the parasubstituted phenyl ring in the PDA unit, imide ring, and unreacted C C bond sequentially. We also investigated the sequence of the bands related to C C moieties as well as imide rings and phenyl rings in ODA unit molecular reorientation in the 6F-ODA-PEAP film during thermal curing (Fig. 6). The cross peaks at (2200, 1724) and (1724, 1500) cm−1 for the ethynyl C C stretching and para-substituted phenyl ring vibrations in the PDA unit with the imide C O stretching vibration indicate that the imide ring changes before the para-substituted phenyl ring in the PDA unit and the ethynyl C C unit. The cross peak at (2200, 1500) cm−1 reveals that the para-substituted phenyl ring in the PDA unit changes before the unreacted C C bond (data not shown). Conclusively, thermal cure reaction of the 6F-ODA-PEAP film induces spectral change of the imide ring, para-substituted phenyl ring in the ODA unit, and unreacted C C bond sequentially. Several researchers have proposed that the thermal cure reaction containing the phenyl ethynyl C C moiety induces phenyl ring migration as well as molecular rearrangement to form cure products [3–5]. Results of two-dimensional correlation analyses of 6F-PDA-PEAP and 6F-ODA-PEAP FTIR spectra during thermal cure reaction are in good agreement with these previous studies. The thermal cure reaction of phenylethynyl end-capped model
compounds induces the segmental motion of ethynyl C C as well as the adjacent imide ring and phenyl ring in the PDA and ODA units. Further, the spectral change of the imide and phenyl rings in the PDA and ODA units occur before the thermal cure of the C C unit. Rather bulky unit changes occur before changes to the ethynyl C C unit when forming cure product.
4. Conclusions Thermal cure of 6F-PDA-PEAP and 6F-ODA-PEAP films was investigated in detail by FTIR spectroscopy and 2D correlation spectroscopy. The FTIR spectra of the 6F-PDA-PEAP and 6F-ODAPEAP films revealed that the extent of cure is influenced by the molecular structure of 6F-PDA-PEAP and 6F-ODA-PEAP films. The conversion extent of 6F-ODA-PEAP at 330◦ C for 60 min is smaller than that of 6F-PDA-PEAP because of steric factors of the ODA unit. The analysis of the 2D correlation FTIR spectra indicated that the thermal cure of phenylethynyl end-capped model compounds 6FPDA-PEAP and 6F-ODA-PEAP induced the spectral change of C C and the imide and phenyl rings in the ODA and PDA units. In particular, these results suggest that the imide ring is strongly influenced by thermal curing of 6F-PDA-PEAP and 6F-ODA-PEAP films. The phenyl ring in the ODA unit of the 6F-ODA-PEAP films is rather strongly influenced by the thermal cure reaction. Further, 2D correlation analyses of FTIR spectra provided the sequence of intensity changes of phenylethynyl C C moieties and adjacent imide and phenyl rings in the ODA and PDA units during thermal curing. The spectral change of imide and phenyl rings in ODA and PDA groups occurred before the thermal cure of ethynyl C C moieties.
B. Chae et al. / Vibrational Spectroscopy 60 (2012) 137–141
Acknowledgements This work was supported by the Industrial Strategic Technology Development Program (No. 10033355) funded by the Ministry of Knowledge Economy (MKE, Korea) and the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of Knowledge Economy. References [1] H. Liou, P.S. Ho, B. Tung, J. Appl. Polym. Sci. 70 (1998) 261–272. [2] H. Liu, C.D. Simone, D.A. Scola, J. Polym. Sci. A: Polym. Chem. 41 (2003) 2630–2649. [3] Y. Li, R.J. Morgan, J. Appl. Polym. Sci. 101 (2006) 4446–4453. [4] X. Fang, D.F. Rogers, D.A. Scola, M.P. Stevens, J. Polym. Sci. A: Polym. Chem. 36 (1998) 461–470. [5] X. Fang, X.-Q. Xie, C.D. Simone, M.P. Stevens, D.A. Scola, Macromolecules 33 (2000) 1671–1681. [6] .J.A. Johnston, F.M. Li, F.W. Harris, T. Takekoshi, Polymer 35 (1994) 4865–4873. [7] T. Takekoshi, J.M. Terry, Polymer 35 (1994) 4874–4880. [8] C.C. Roberts, T.M. Apple, G.E. Wnek, J. Polym. Sci. A: Polym. Chem. 38 (2000) 3486–3497.
141
[9] I. Noda, Appl. Spectrosc. 47 (1993) 1329–1336. [10] I. Noda, Appl. Spectrosc. 54 (2000) 994–999. [11] I. Noda, Y. Ozaki, Two-Dimensional Correlation Spectroscopy: Applications in Vibrational Spectroscopy, John Wiley & Sons Inc., New York, 2004. [12] Y.M. Jung, I. Noda, Appl. Spectrosc. Rev. 41 (2006) 515–547. [13] H. Yamasaki, S. Morita, J. Appl. Sci. 119 (2011) 871–881. [14] S. Radice, S. Turri, M. Scicchitano, Appl. Spectrosc. 58 (2004) 535–542. [15] K.H. Yu, Y.H. Yoo, J.M. Rhee, M.-H. Lee, S.-C. Yu, Bull. Korean Chem. Soc. 24 (2003) 357–362. [16] The program is available from the homepage of Professor Yukihiro Osaki of Kwansei Gakuin University, Sanda, japan, http://science.kwansei.ac.jp/∼ozaki. [17] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman Spectroscopy, Academic, San Diego, 1990. [18] D. Lin-Vien, N.B. Colthup, V.G. Fately, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic, San Diego, 1991. [19] B. Chae, S.W. Lee, B. Lee, W. Choi, S.B. Kim, Y.M. Jung, J. Jung, K.H. Lee, M. Ree, J. Phys. Chem. B 107 (2003) 11911–11916. [20] Y.H. Kim, B. Chae, T.J. Lee, S.B. Kim, Y.M. Jung, S.W. Lee, J. Mol. Struct. 974 (2010) 30–34. [21] S.W. Lee, S.J. Lee, S.G. Hahm, T.J. Lee, B. Lee, B. Chae, S.B. Kim, J.C. Jung, W.C. Zin, B.H. Sohn, M. Ree, Macromolecules 38 (2005) 4331–4338. [22] S.W. Lee, B. Chae, B. Lee, W. Choi, S.B. Kim, S.I. Kim, S. Park, J.C. Jung, K.H. Lee, M. Ree, Chem. Mater. 15 (2003) 3105–3112. [23] B. Chae, S.W. Lee, B. Lee, W. Choi, S.B. Kim, Y.M. Jung, J.C. Jung, K.H. Lee, M. Ree, Langmuir 19 (2003) 9459–9465.