Study of the Infrared Spectral Features of an Epoxy Curing Mechanism

Transcription

1 Study of the Infrared Spectral Features of an Epoxy Curing Mechanism LIANG LI, QILI WU, SHANJUN LI, and PEIYI WU* The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai, , China In this work, the isothermal curing process of diglycidyl ether of bisphenol A (DGEBA) cured with 4,4 0 -diaminodiphenylmethane (DDM) was monitored in situ by mid-infrared (MIR) and near-infrared (NIR) spectroscopy. With the help of generalized two-dimensional (2D) correlation analysis, the results obtained showed that, during curing, the change of amine and epoxy groups was simultaneous, taking place prior to the change of hydroxyl groups, followed by the change of CH 2 /CH groups, resulting from the ring-opening reaction of epoxy groups. In addition, 2D MIR3NIR hetero-spectral correlation analysis and secondderivative analysis were also employed, by means of which direct evidence of the curing mechanism could be obtained and obscure NIR band assignments in the overlapped CH combination region could be made. Index Headings: Epoxy; Curing; Hetero-spectral correlation; Mid-infrared spectroscopy; Near-infrared spectroscopy; NIR spectroscopy; Twodimensional correlation analysis; 2D correlation analysis. Received 17 January 2008; accepted 29 July * Author to whom correspondence should be sent. peiyiwu@ fudan.edu.cn. INTRODUCTION In situ real-time monitoring for chemical and physical changes during processing of reactive polymer-forming materials is of crucial importance for scientific as well as industrial purposes. Because epoxy resins are widely used to make adhesives and the matrices of composites, the crosslinking processes and reaction mechanisms of different epoxy resins have been studied extensively by means of near-infrared (NIR) spectroscopy, 1 24 Raman spectroscopy, 2 mid-infrared spectroscopy (MIR), 3 6,10 dielectric relaxation spectroscopy (DRS), 8,25 rheology, 9,26 differential scanning calorimetry (DSC), fluorescence spectroscopy, 32,33 ultraviolet (UV) spectroscopy, 34 NIR multispectral imaging, 35 etc., among which NIR spectroscopy is the most convenient as well as straightforward method. Although the curing mechanisms 1 3,6,7,10 16,18 24 and the kinetics of curing reaction 1,6,11,13,14,16 22,24,33,36 43 according to NIR spectra have been widely reported, the sequence of the change of the bands, which may reveal the reaction mechanism at the molecular level, has not been accurately described. In addition, due to the obscure band assignments resulting from simultaneous occurrence of a variety of fundamental and overtone transitions in the epoxy curing process, not all the bands in NIR spectra have been clearly assigned. Therefore, the development of powerful techniques for unraveling the complicated spectra has long been attempted. Fortunately, generalized two-dimensional (2D) correlation analysis, which can handle spectral fluctuations as an arbitrary function of time or any other physical variable, has proven to be such an attractive tool, emphasizing the spectral features that are not readily observable in conventional onedimensional spectra. Particularly, the method is able to probe the specific order of certain events taking place with the development of a controlling physical variable, which will be of most significance in the dynamic analysis As far as the curing of thermosetting resin is concerned, only Ozaki has made an attempt to employ generalized 2D analysis in a diglycidyl ether of bisphenol A (DGEBA)/dicyandiamide (DDA)/Uron system, and he found that the technique could be used to investigate the curing of epoxy resins. 5 Furthermore, 2D hetero-spectral correlation, which combines two separate measurements via some common theme such as the influence of identical perturbation, is powerful in distinguishing vague band assignments. The first reported case of 2D hetero-spectral correlation was based on infrared smallangle X-ray scattering (IR-SAXS) spectra, 53 while the first IR- NIR hetero-spectral correlation was reported by Barton et al., 54 which can be used to allow bands in the NIR spectrum to be resolved and assigned to characteristic absorbances in the midinfrared (MIR) spectrum. Since then, hetero-spectral correlation of IR NIR, IR Raman, 59 IR EPR (electron paramagnetic resonance), 60 NIR Raman, 61 NIR visible, Raman circular dichroism, 65 Raman soft X-ray absorption, 66 and IR gas chromatography 67 have all been performed, from which spectra obtained by different techniques were analyzed together, and the results obtained were much more informative and convincing. As far as we are aware, however, 2D heterospectral correlation has not been employed in the thermosetting systems. In the present work, isothermal curing reactions of the diglycidyl ether bisphenol-a/4,4 -diaminodiphenylmethane (DGEBA/DDM) system were studied using MIR and NIR spectroscopy, and the detailed process was described based on 2D correlation analysis. Additionally, 2D hetero-spectral correlation was also applied, from which the results obtained were further confirmed and some obscure band assignments were made. EXPERIMENTAL Materials. The cure agent 4,4 0 -diaminodiphenylmethane (DDM) (Shanghai Third Reagent Factory) and the epoxy oligomer diglycidyl ether bisphenol-a (DGEBA) (DOW Chemical, g ¼ 0.54) were used without further purification. Their structures are shown in Scheme 1. Fourier Transform Infrared Spectroscopy Measurements. Fourier transform infrared spectroscopy (FT-IR) was performed using a Thermo Nicolet Nexus 440 Spectrometer with spectral range coverage from to 400 cm 1. MIR data were obtained via an Ever-Glo source along with an XT- KBr beam splitter and a DTGS-KBr detector, while for NIR a white light source and a MCT detector were employed. The DDM was dissolved in DGEBA at 80 8C for 10 Volume 62, Number 10, 2008 APPLIED SPECTROSCOPY /08/ $2.00/0 Ó 2008 Society for Applied Spectroscopy

2 SCHEME 1. Chemical structure of DGEBA and DDM. minutes, and the blend was used for FT-IR experiments immediately. The sample was sandwiched between two NaCl windows, the thickness of which was 27 lm. The mid-infrared spectroscopic details were recorded in the cm 1 FIG. 1. (A) NIR and (B) MIR spectra of DGEBA/DDM isothermally cured at 125 8C. The arrows presented in the figure indicate the increase or decrease of the intensities of bands during the isothermal curing. range at 4 cm 1 resolution with 64 scans. For the near-infrared study, the thickness of the sample cell was 2 mm. NIR spectroscopic details were recorded in the region of cm 1 at 4 cm 1 resolution with 128 scans. The time interval between each spectra collected was 70 s for both IR and NIR measurements. Two-Dimensional Correlation Analysis. Seven spectra at equal time intervals in a certain wavenumber range were selected for application of 2D correlation analysis using 2D Shige software. Time-averaged one-dimensional (1D) reference spectra are shown at the side and top of the 2D correlation maps (see later figures) for comparison. In the 2D correlation maps, unshaded regions indicate positive correlation intensities, while shaded regions indicate negative correlation intensities. RESULTS AND DISCUSSION Figure 1 shows the typical MIR and NIR spectra of the DGEBA/DDM system. In the NIR spectrum (Fig. 1A), peaks at 6850, 6655, 6551, and 5050 cm 1 can be assigned to the overtones/combinations of NH 2 groups, and peaks at 7000, 4890, and 4786 cm 1 can be assigned to the overtones/ combinations of OH related groups. The overlapped peaks from 6100 to 5600 cm 1 are assigned to the CH/CH 2 /CH 3 overtones of epoxy groups, benzene groups, and backbone. It is worth noticing that the intensity change of the band at 5890 cm 1 during the curing process has seldom been mentioned in publications, 7 and therefore the band assignment has not been made. In the corresponding MIR spectrum (Fig. 1B), peaks at 3460 and 3370 cm 1 can be assigned to the vibration of NH 2 groups, and peaks at 3550 and 3409 cm 1 can be assigned to the vibration of OH groups. The overlapped peaks from 3100 to 2800 cm 1 are assigned to the CH/CH 2 /CH 3 vibrations of epoxy groups, benzene groups, and backbone. 68 More detailed band assignments 1 24,68 are listed in Table I. To perform further analysis, the curing extent was calculated by the integral of two typical epoxy bands, centered at 916 cm 1 in the MIR region and 4530 cm 1 in the NIR region, which are always used to analyze epoxy curing conversion. The normalized absorbances of the bands at 916 and 4530 cm 1 (normalized by the band area before and after fully cured) are shown in Fig. 2. In Fig. 2A, the normalized absorbances of the bands at 916 and 4530 cm 1 all decrease rapidly upon curing, which suggests that the consumption of epoxy groups in the initial stage of curing is very fast. In addition, it is noted that the decrease of the band at 916 cm 1 is linear with time, while that at 4530 cm 1 is not linear with time at first, due to varying 1130 Volume 62, Number 10, 2008

3 TABLE I. Tentative assignments of spectral bands. 1 24,68 Position cm 1 Assignments a (tentative) Position cm 1 Assignments (tentative) 3550 OH str OH str. overtone 3409 OH str NH 2 asym. str. overtone 3460 NH 2 asym. str NH 2 sym.þasym. str NH 2 sym. str NH 2 sym. str. overtone 3054 Epoxy CH 2 asym. and phenyl CH str Epoxy terminal CH 2 str. overtone 3034 Phenyl CH str Phenyl CH str. overtone 2997 Epoxy CH 2 sym Epoxy CH/CH 2 comb CH 3 asym. str. 5764, 5654 CH/CH 2 overtones 2930 CH 2 asym. str NH 2 comb CH 3 sym. str. And CH str OH comb CH 2 sym. str Phenyl CH str.þ phenyl conjugated str. 916 Epoxy deformation 4530 Epoxy CH 2 str. þ epoxy CH 2 deformation a (Str.) stretch; (comb.) combination; (sym.) symmetric; and (asym.) asymmetric. sample thicknesses requiring different amounts of time to reach the temperature equilibrium. In order to apply 2D hetero-spectral correlation, the diversity induced by different sample thicknesses should be avoided. Therefore, spectra before thermo-equilibrium were dropped, and the adjusted curves of the normalized absorbance after thermo-equilibrium at 125 8C are shown in Fig. 2B. The initial seven spectra of Fig. 2B were selected for application of 2D correlation analysis. Curing Process Monitored by Fourier Transform Near- Infrared Spectroscopy from Two-Dimensional Correlation Analysis. Generalized 2D correlation spectra on the evolution of NIR spectra of the DGEBA/DDM cured at 125 8C are presented in Fig. 3. According to the publications of Noda, 44 the intensity of synchronous cross peaks (U(m 1, m 2 )) becomes positive (unshaded area) if the spectral intensities at the two spectral variables corresponding to the coordinates of the cross peak are either increasing or decreasing together as functions of the external variable during the observation interval. On the other hand, a negative sign (shaded area) of cross peaks indicates that one of the spectral intensities is increasing while the other is decreasing. In the asynchronous spectrum, a cross peak between two dynamic spectral features can develop only if their intensities vary out of phase with each other, and the intensity of the asynchronous correlation peak W(m 1, m 2 ) is able to give information about the sequential order of intensity changes between band m 1 and band m 2.IfW(m 1, m 2 ) is positive, band m 1 varies prior to band m 2, and if W(m 1, m 2 ) is negative, band m 1 varies after band m 2. However, the above rules are reversed if the sign of a synchronous peak at the same coordinate (U(m 1, m 2 )) is negative. Figure 3 shows synchronous and asynchronous correlation spectra of the system in the region of cm 1 during isothermal curing. When the region cm 1 is considered, in the synchronous 2D spectrum (Fig. 3A), two autopeaks centered at 7000 and 6655 cm 1 are found, which indicates the significant change of OH and NH 2 bands. Also, there are four distinct cross peaks observed on the upper left side of the correlation spectrum, among which U(7000, 6850 cm 1 ) and U(6655, 6551 cm 1 ) are positive, while U(7000, 6655 cm 1 ) and U(7000, 6551 cm 1 ) are negative ((m 1, m 2 cm 1 ) refers to the cross peak on the upper left side of the correlation spectrum, and m 1 is the x-axis wavenumber while m 2 is the y-axis wavenumber). These peaks, combined with Fig. 1A, indicate FIG. 2. Normalized absorbance of two typical bands of epoxy groups centered at 916 cm 1 ( & ) and 4530 cm 1 ( u ) 1 during the isothermal curing. APPLIED SPECTROSCOPY 1131

4 FIG. 4. 2D correlation spectra derived from the NIR spectra of the isothermal DGEBA/DDM curing process at 125 8C. Negative peaks are marked by shading. FIG. 3. 2D correlation spectra derived from the NIR spectra of the isothermal DGEBA/DDM curing process at 125 8C. Negative peaks are marked by shading. that the bands at 7000 and 6850 cm 1 increase while the bands at 6655 and 6551 cm 1 decrease during isothermal curing. In the corresponding asynchronous 2D spectrum (Fig. 3B), only three positive cross peaks, W(7000, 6850 cm 1 ), W(7000, 6655 cm 1 ), and W(7000, 6551 cm 1 ), can be identified, which are assigned to OH and NH 2 vibrations, respectively. On the basis of Noda s rule, the intensities of bands change in the following sequence: 6655, 6551! 7000! 6850 cm 1. (! means prior to ) The sequence 6655, 6551! 7000 cm 1 means that NH 2 vibrations change prior to the OH vibrations, which suggests that the increase of hydroxyl groups is at the expense of the decrease of amine groups due to the curing reaction. However, the sequence 7000! 6850 cm 1 means that OH vibrations change prior to the NH 2 vibrations, which is just the opposite of the results obtained above. This phenomenon can be well explained by the fact that the band around 6850 cm 1 is totally overlapped by the band around 7000 cm 1, and its decrease of intensity is nearly covered by the increase of intensity of OH vibrations (as shown in Fig. 1A), which also leads to the positive U(7000, 6850 cm 1 ) peak in the synchronous 2D spectrum (Fig. 2B). Therefore, in the asynchronous spectrum, the positive W(7000, 6850 cm 1 ) peak does not suggest that the earlier change took place with OH vibrations, but indicates a slower intensity increase at around 6850 cm 1 due to the intensity decrease of the amine group at the same position. When the region cm 1 is considered, in the synchronous 2D spectrum (Fig. 3C), three autopeaks centered at 6069, 5890, and 5654 cm 1, two positive peaks U(6069, 5890 cm 1 ) and U(5990, 5764 cm 1 ), and four negative peaks U(6069, 5990 cm 1 ), U(6069, 5764 cm 1 ), U(5990, 5890 cm 1 ), and U(5890, 5764 cm 1 ) are observed. The autopeak at 5654 cm 1 is very broad, which demonstrates that this autopeak consists of more than one peak. These results prove that the bands at 6069 and 5890 cm 1 (assigned to the CH 2 /CH 1132 Volume 62, Number 10, 2008

5 SCHEME 2. Sequence of band intensity change analyzed by 2D NIR correlation spectra. Tentative assignments are listed above or below panes. vibration of epoxy) vary in the same direction, and the bands at 5990 (assigned to benzene vibration), 5764, and 5654 cm 1 (assigned to the CH 2 /CH vibration of backbone) vary in the opposite direction. In the corresponding asynchronous correlation spectrum in this region (Fig. 3D), one positive cross peak W(5990, 5890 cm 1 ) and four negative cross peaks W(6069, 5990 cm 1 ), W(6069, 5764 cm 1 ), W(5990, 5674 cm 1 ), and W(5890, 5764 cm 1 ) are observed. On the basis of Noda s rule, the intensities of bands change in the following sequence: 6069, 5890! 5764, 5654! 5990 cm 1. This sequence means that the CH 2 /CH vibrations of the epoxy groups change first, followed by the CH/CH 2 groups of the backbone, and then the benzene groups, respectively. As benzene rings are not directly involved in the reaction, the change of phenyl CH vibrations is somewhat confusing. Indeed, Lachenal et al. 5 also found an unexpected increase in the intensity of combination modes due to the phenyl rings when investigating the curing of epoxy and dicyanamide. They suggested that the unexpected intensity increase might be attributed to the modification of the packing of the phenyl rings during polymerization. 5 In addition, as the intensity change of the phenyl CH vibration is very small, the phenomenon might also be attributed to the slight shift of the baseline due to the change of the color of the sample. Therefore, the change of benzene vibrations is not further considered as a part of the reaction. Correlating with different spectral regions always can provide additional information. Therefore, regions and cm 1 are correlated as shown in Fig. 4. In the synchronous correlation spectrum (Fig. 4A), four positive peaks U(7000, 5654 cm 1 ), U(7000, 5990 cm 1 ), U(6655, 5890 cm 1 ), and U(6655, 6069 cm 1 ), and four negative peaks U(7000, 5890 cm 1 ), U(7000, 6069 cm 1 ), U(6655, 5654 cm 1 ), and U(6655, 5990 cm 1 ) can be observed. These peaks indicate that OH and CH/CH 2 of the backbone change in the same direction, while NH 2 and epoxy vibrations change in the opposite direction. In the corresponding asynchronous correlation spectrum (Fig. 4B), four positive cross peaks W(7000, 5654 cm 1 ), W(7000, 5890 cm 1 ), W(7000, 5990 cm 1 ), and W(7000, 6069 cm 1 ), and four negative cross peaks W(6655, 5654 cm 1 ), W(6655, 5990 cm 1 ), W(6551, 5654 cm 1 ), and W(6551, 5990 cm 1 ) are identified. It is worthwhile to remark here that there are no cross peaks at (6655, 5890 cm 1 ), (6655, 6069 cm 1 ), (6551, 5890 cm 1 ), and (6551, 6069 cm 1 ), which suggests that the NH 2 vibrations and epoxy vibrations vary simultaneously. In this case, the intensities of bands change in the following sequence: 6655, 6551, 6069, 5890!7000! 5654 cm 1, which suggests that the NH 2 and epoxy groups change first, followed by the change of OH groups and CH/CH 2 of the backbone. The sequence of band intensity changes is summarized in Scheme 2. The above results of 2D correlation analysis correspond well with the mechanism of the curing reaction, as shown in Scheme 3. Although the change of band intensity in NIR spectra during the curing process has been widely observed 1 3,6,7,10 16,18 24 and the mechanism as well as the curing kinetics can also be obtained according to the analysis of NIR spectra, 1,6,11,13,14,16 22,24,33,36 43 the sequence of change of band intensities was not easily elucidated by traditional analytic methods. Curing Process Monitored by Fourier Transform Mid- Infrared Spectroscopy from Two-Dimensional Correlation Analysis. Generalized 2D correlation spectroscopy was applied to study the evolution of MIR spectra of the DGEBA/DDM system cured at 125 8C, and the results are presented in Fig. 5. Figures 5A and 5B show, respectively, synchronous and asynchronous correlation spectra in the region cm 1. In the synchronous correlation spectrum (Fig. 5A), three SCHEME 3. Schematic curing reaction. APPLIED SPECTROSCOPY 1133

6 FIG. 5. 2D correlation spectra derived from the MIR spectra of the isothermal DGEBA/DDM curing process at 125 8C. Negative peaks are marked by shading. autopeaks centered at 3550, 3370, and 3409 cm 1 are found. The signs of cross peaks indicate that the changes of 3550, 3409 (assigned to OH vibration), and 3324 cm 1 are in the same direction, while the changes of 3370 (assigned to NH 2 vibration) cm 1 are in the opposite direction. The band at 3324 cm 1 can be assigned to hydrogen-bonded OH vibration, as OH hydrogen bonds are generally conceived in cured epoxy resin systems. In the corresponding asynchronous correlation spectrum (Fig. 5B), three positive cross peaks W(3550, 3370 cm 1 ), W(3550, 3460 cm 1 ), and W(3409, 3370 cm 1 ), and two negative cross peaks W(3460, 3409 cm 1 ) and W(3370, 3324 cm 1 ) are found. The sequence of changes is: 3370! 3550, 3409, 3324! 3460 cm 1. The sequence 3370! 3550, 3409, 3324 cm 1 means that NH 2 vibrations change prior to the OH vibrations. However, the sequence 3550, 3409, 3324! 3460 cm 1 does not mean that OH vibrations change prior to the NH 2 vibrations. It indicates a slower intensity increase at 3460 cm 1 due to the intensity decrease of the amine group at the same position, and the similar phenomenon has been discussed in the region cm 1. In the synchronous correlation spectrum of the region cm 1 (Fig. 5C), three autopeaks at 3054, 2997, and 2871 cm 1 are observed. The autopeak at 2871 cm 1 is very broad and seems to consist of more than one peak. One positive peak U(3054, 2997 cm 1 ) and two negative peaks U(3054, 2871 cm 1 ) and U(2997, 2871 cm 1 ) show that the intensity changes of 3054 and 2997 cm 1 (assigned to epoxy CH 2 /CH vibration) are in the same direction, while the intensity changes of 2871 cm 1 (assigned to CH vibration of the backbone) are in the opposite direction. In the corresponding asynchronous correlation spectrum in the same region (Fig. 5D), negative cross peaks W(3054, 2930 cm 1 ), W(3054, 2871 cm 1 ), W(2997, 2930 cm 1 ), W(2997, 2871 cm 1 ), and W(2930, 2871 cm 1 ) are detected. In addition, the band change at 3054 cm 1 is slightly prior to that at 2997 cm 1. Therefore, the intensities of the bands change in the following sequence: 3054! 2997! 2871! 2930 cm 1, which demonstrates that the epoxy CH 2 groups change first, followed by the CH/CH 2 groups of the backbone. In conclusion, the analysis of Fig. 5 also provides similar information to that discussed in the analysis of the NIR region, which suggests that the intensity change of the amine group (band around 3370 cm 1 ) is sooner than the intensity change of hydroxyl groups (band at 3550 and 3309 cm 1 ), while the intensity change of epoxy CH 2 groups (band around 3054 and 2997 cm 1 ) is sooner than that of CH 2 /CH groups of backbone (band around 2930 and 2871 cm 1 ), resulting from the ringopening reaction of the epoxy groups. Although the curing mechanisms as well as the curing kinetics have both been previously demonstrated according to the analysis of MIR spectra, 3 5,10,68 the sequence of the changes of intensity has seldom been reported. Band Assignments from Two-Dimensional Hetero-Spectra Correlation Analysis. The successful applications of 2D hetero-spectral correlation analysis have proven 2D heterospectral correlation to be a very attractive analytical tool that combines two separate measurements via some common theme. In general, different techniques always provide different information about the issue, and results from different techniques may validate each other, especially when the data can be analyzed together. Therefore, 2D MIR3NIR hetero Volume 62, Number 10, 2008

7 TABLE II. 2D correlation cross peaks in Fig. 6. Positive cross peaks (cm 1 ) Negative cross peaks (cm 1 ) 7000, 2871 a 7000, , , , , , , , , , , , , , , , , , , , , , , , , , , , , 2871 a m 1, m 2 refers to the cross peak on the upper left side of the correlation spectrum, and m 1 is the x-axis wavenumber, while m 2 is the y-axis wavenumber. FIG. 6. 2D hetero-spectral (MIR3NIR) correlation spectra of the isothermal DGEBA/DDM curing process at 125 8C. Negative peaks are marked by shading. spectral correlation analysis was employed, the results of which were presented in Fig. 6. In Fig. 6A, the regions and cm 1 are correlated, and in Fig. 6B the regions and cm 1 are correlated. The cross peaks in Fig. 6 are summarized in Table II. It can be concluded from Table II that, during the curing process, the intensities of bands at 6655, 6551, 6069, 5890, 2997, and 3054 cm 1 (assigned to NH 2 vibrations and epoxy CH/CH 2 vibrations) are decreasing, and the intensities of bands at 7000, 5764, 5654, 2930, and 2871 cm 1 (assigned to OH vibrations and CH 2 /CH vibrations of backbone) are increasing. These results are consistent with the aforementioned outcomes. Particularly, there are four decreasing bands related to epoxy CH vibrations in both the NIR region (bands at 6069 and 5890 cm 1 ) and the MIR region (bands at 3054 and 2997 cm 1 ), whose changes of intensity can also be found by secondderivative curves (as shown in Fig. 7). As the epoxy ring is broken at the O CH 2 bond when reacting with an amine group (shown in Scheme 3), the change of NH 2 vibrations should be simultaneous with that of epoxy terminal CH 2 vibrations, whose fundamental vibration in the MIR region is at 3054/2997 cm 1. However, the change of the asymmetric stretching vibration (at 3054 cm 1 ) is prior to that of the symmetric stretching vibration (at 2997 cm 1 ). Thus, it can be deduced that the change of NH 2 vibrations ought to be simultaneous with that of overtones/combinations of the asymmetric stretching vibration of epoxy CH 2 groups. In addition, it has been shown that NH 2 vibrations (bands at 6655 and 6551 cm 1 ) and epoxy CH vibrations (bands at 6069 and 5890 cm 1 ) vary simultaneously. Therefore, bands at 6069 and FIG. 7. Second-derivative curves of (A) NIR and (B) MIR spectra of DGEBA/ DDM isothermally cured at 125 8C APPLIED SPECTROSCOPY 1135

8 5890 cm 1 both should be the overtones/combinations of the asymmetric stretching vibration of epoxy CH 2 groups. This assumption is fully supported by the fact that the band at 6069 cm 1 is the first overtone of the epoxy terminal CH 2 band (at 3054 cm 1 ). 3 Now it can be deduced that the band at 5890 cm 1 is the combination band of the epoxy CH 2 asymmetric stretching vibration (3054 cm 1 ) and the overtone of the epoxy CH 2 deformation vibration ( cm 1 ). CONCLUSION Clear pictures of the curing mechanism of the DDM-cured DGEBA system were elucidated here by NIR and MIR spectroscopy with 2D correlation analysis, which could be the most effective as well as convenient way to perform dynamic analyses on such investigations. The results indicated that, during the curing process, the intensity change of the amine and epoxy groups were simultaneous and occurred sooner than the intensity change of the hydroxyl groups, followed by the change of the CH 2 /CH groups of the backbone, resulting from the ring-opening reaction of the epoxy groups. In addition, 2D hetero-spectral correlation was also employed and the aforementioned conclusions were further confirmed. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the National Science of Foundation of China (NSFC) (Nos , , , ), the Leading Scientist Project of Shanghai (No. 07XD14002), the National Basic Research Program of China (2005CB623800), and PHD Program of MOE ( ). 1. G. A. George, P. Coleclarke, N. St. John, and G. Friend, J. Appl. Polym. Sci. 42, 643 (1991). 2. K. E. Chike, M. L. Myrick, R. E. Lyon, and S. M. Angel, Appl. Spectrosc. 47, 1631 (1993). 3. N. Poisson, G. Lachenal, and H. Sautereau, Vib. Spectrosc. 12, 237 (1996). 4. 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