Study of the Infrared Spectral Features of an EpoxyCuring 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, 200433, 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 CH2/CH

groups, resulting from the ring-opening reaction of epoxy groups. In

addition, 2D MIR3NIR hetero-spectral correlation analysis and second-

derivative 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; Two-

dimensional correlation analysis; 2D correlation analysis.

INTRODUCTION

In situ real-time monitoring for chemical and physicalchanges during processing of reactive polymer-formingmaterials is of crucial importance for scientific as well asindustrial purposes. Because epoxy resins are widely used tomake adhesives and the matrices of composites, the cross-linking processes and reaction mechanisms of different epoxyresins have been studied extensively by means of near-infrared(NIR) spectroscopy,1–24 Raman spectroscopy,2 mid-infraredspectroscopy (MIR),3–6,10 dielectric relaxation spectroscopy(DRS),8,25 rheology,9,26 differential scanning calorimetry(DSC),27–31 fluorescence spectroscopy,32,33 ultraviolet (UV)spectroscopy,34 NIR multispectral imaging,35 etc., amongwhich NIR spectroscopy is the most convenient as well asstraightforward method.

Although the curing mechanisms1–3,6,7,10–16,18–24 and thekinetics of curing reaction1,6,11,13,14,16–22,24,33,36–43 accordingto NIR spectra have been widely reported, the sequence of thechange of the bands, which may reveal the reaction mechanismat the molecular level, has not been accurately described. Inaddition, due to the obscure band assignments resulting fromsimultaneous occurrence of a variety of fundamental andovertone transitions in the epoxy curing process, not all thebands in NIR spectra have been clearly assigned. Therefore, thedevelopment of powerful techniques for unraveling thecomplicated spectra has long been attempted.

Fortunately, generalized two-dimensional (2D) correlationanalysis,44–46 which can handle spectral fluctuations as anarbitrary function of time or any other physical variable, hasproven to be such an attractive tool, emphasizing the spectralfeatures that are not readily observable in conventional one-

dimensional spectra. Particularly, the method is able to probethe specific order of certain events taking place with thedevelopment of a controlling physical variable, which will beof most significance in the dynamic analysis.47–52 As far as thecuring of thermosetting resin is concerned, only Ozaki hasmade an attempt to employ generalized 2D analysis in adiglycidyl ether of bisphenol A (DGEBA)/dicyandiamide(DDA)/Uron system, and he found that the technique couldbe used to investigate the curing of epoxy resins.5

Furthermore, 2D hetero-spectral correlation, which com-bines two separate measurements via some common themesuch as the influence of identical perturbation, is powerful indistinguishing vague band assignments. The first reported caseof 2D hetero-spectral correlation was based on infrared small-angle 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 beresolved and assigned to characteristic absorbances in the mid-infrared (MIR) spectrum. Since then, hetero-spectral correla-tion of IR–NIR,55–58 IR–Raman,59 IR–EPR (electron para-magnetic resonance),60 NIR–Raman,61 NIR–visible,62–64

Raman–circular dichroism,65 Raman–soft X-ray absorption,66

and IR–gas chromatography67 have all been performed, fromwhich spectra obtained by different techniques were analyzedtogether, and the results obtained were much more informativeand convincing. As far as we are aware, however, 2D hetero-spectral correlation has not been employed in the thermosettingsystems.

In the present work, isothermal curing reactions of thediglycidyl ether bisphenol-A/4,4’-diaminodiphenylmethane(DGEBA/DDM) system were studied using MIR and NIRspectroscopy, and the detailed process was described based on2D correlation analysis. Additionally, 2D hetero-spectralcorrelation was also applied, from which the results obtainedwere further confirmed and some obscure band assignmentswere made.

EXPERIMENTAL

Materials. The cure agent 4,40-diaminodiphenylmethane(DDM) (Shanghai Third Reagent Factory) and the epoxyoligomer diglycidyl ether bisphenol-A (DGEBA) (DOWChemical, g ¼ 0.54) were used without further purification.Their structures are shown in Scheme 1.

Fourier Transform Infrared Spectroscopy Measure-ments. Fourier transform infrared spectroscopy (FT-IR) wasperformed using a Thermo Nicolet Nexus 440 Spectrometerwith spectral range coverage from 11 000 to 400 cm�1. MIRdata were obtained via an Ever-Glo source along with an XT-KBr beam splitter and a DTGS-KBr detector, while for NIR awhite light source and a MCT detector were employed.

The DDM was dissolved in DGEBA at 80 8C for 10

Received 17 January 2008; accepted 29 July 2008.* Author to whom correspondence should be sent. E-mail: peiyiwu@fudan.edu.cn.

Volume 62, Number 10, 2008 APPLIED SPECTROSCOPY 11290003-7028/08/6210-1129$2.00/0

� 2008 Society for Applied Spectroscopy

minutes, and the blend was used for FT-IR experimentsimmediately. The sample was sandwiched between two NaClwindows, the thickness of which was 27 lm. The mid-infraredspectroscopic details were recorded in the 4000–650 cm�1

range at 4 cm�1 resolution with 64 scans. For the near-infraredstudy, the thickness of the sample cell was 2 mm. NIRspectroscopic details were recorded in the region of 11 000–4000 cm�1 at 4 cm�1 resolution with 128 scans. The timeinterval between each spectra collected was 70 s for both IRand NIR measurements.

Two-Dimensional Correlation Analysis. Seven spectra atequal time intervals in a certain wavenumber range wereselected for application of 2D correlation analysis using 2DShige software. Time-averaged one-dimensional (1D) refer-ence spectra are shown at the side and top of the 2D correlationmaps (see later figures) for comparison. In the 2D correlationmaps, unshaded regions indicate positive correlation intensi-ties, while shaded regions indicate negative correlationintensities.

RESULTS AND DISCUSSION

Figure 1 shows the typical MIR and NIR spectra of theDGEBA/DDM system. In the NIR spectrum (Fig. 1A), peaks at6850, 6655, 6551, and 5050 cm�1 can be assigned to theovertones/combinations of NH2 groups, and peaks at 7000,4890, and 4786 cm�1 can be assigned to the overtones/combinations of OH related groups. The overlapped peaksfrom 6100 to 5600 cm�1 are assigned to the CH/CH2/CH3

overtones of epoxy groups, benzene groups, and backbone. It isworth noticing that the intensity change of the band at 5890cm�1 during the curing process has seldom been mentioned inpublications,7 and therefore the band assignment has not beenmade.

In the corresponding MIR spectrum (Fig. 1B), peaks at 3460and 3370 cm�1 can be assigned to the vibration of NH2 groups,and peaks at 3550 and 3409 cm�1 can be assigned to thevibration of OH groups. The overlapped peaks from 3100 to2800 cm�1 are assigned to the CH/CH2/CH3 vibrations ofepoxy groups, benzene groups, and backbone.68

More detailed band assignments1–24,68 are listed in Table I.To perform further analysis, the curing extent was calculatedby the integral of two typical epoxy bands, centered at 916cm�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 4530cm�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 916and 4530 cm�1 all decrease rapidly upon curing, whichsuggests that the consumption of epoxy groups in the initialstage of curing is very fast. In addition, it is noted that thedecrease of the band at 916 cm�1 is linear with time, while thatat 4530 cm�1 is not linear with time at first, due to varying

SCHEME 1. Chemical structure of DGEBA and DDM.

FIG. 1. (A) NIR and (B) MIR spectra of DGEBA/DDM isothermally cured at125 8C. The arrows presented in the figure indicate the increase or decrease ofthe intensities of bands during the isothermal curing.

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sample thicknesses requiring different amounts of time to reachthe temperature equilibrium.

In order to apply 2D hetero-spectral correlation, the diversityinduced by different sample thicknesses should be avoided.Therefore, spectra before thermo-equilibrium were dropped,and the adjusted curves of the normalized absorbance afterthermo-equilibrium at 125 8C are shown in Fig. 2B. The initialseven spectra of Fig. 2B were selected for application of 2Dcorrelation analysis.

Curing Process Monitored by Fourier Transform Near-Infrared Spectroscopy from Two-Dimensional CorrelationAnalysis. Generalized 2D correlation spectra on the evolutionof NIR spectra of the DGEBA/DDM cured at 125 8C arepresented in Fig. 3. According to the publications of Noda,44

the intensity of synchronous cross peaks (U(m1, m2)) becomespositive (unshaded area) if the spectral intensities at the twospectral variables corresponding to the coordinates of the crosspeak are either increasing or decreasing together as functions ofthe external variable during the observation interval. On theother hand, a negative sign (shaded area) of cross peaksindicates that one of the spectral intensities is increasing whilethe other is decreasing. In the asynchronous spectrum, a crosspeak between two dynamic spectral features can develop onlyif their intensities vary out of phase with each other, and theintensity of the asynchronous correlation peak W(m1, m2) is ableto give information about the sequential order of intensitychanges between band m1 and band m2. If W(m1, m2) is positive,band m1 varies prior to band m2, and if W(m1, m2) is negative,band m1 varies after band m2. However, the above rules arereversed if the sign of a synchronous peak at the samecoordinate (U(m1, m2)) is negative.

Figure 3 shows synchronous and asynchronous correlationspectra of the system in the region of 7250–5400 cm�1 duringisothermal curing.

When the region 7250–6400 cm�1 is considered, in thesynchronous 2D spectrum (Fig. 3A), two autopeaks centered at7000 and 6655 cm�1 are found, which indicates the significantchange of OH and NH2 bands. Also, there are four distinctcross peaks observed on the upper left side of the correlationspectrum, among which U(7000, 6850 cm�1) and U(6655,6551 cm�1) are positive, while U(7000, 6655 cm�1) andU(7000, 6551 cm�1) are negative ((m1, m2 cm�1) refers to thecross peak on the upper left side of the correlation spectrum,and m1 is the x-axis wavenumber while m2 is the y-axiswavenumber). These peaks, combined with Fig. 1A, indicate

FIG. 2. Normalized absorbance of two typical bands of epoxy groups centeredat 916 cm�1 (—&—) and 4530 cm�1 (—u—)1 during the isothermal curing.

TABLE I. Tentative assignments of spectral bands.1–24,68

Position cm�1 Assignmentsa (tentative) Position cm�1 Assignments (tentative)

3550 OH str. 7000 OH str. overtone3409 OH str. 6850 NH2 asym. str. overtone3460 NH2 asym. str. 6655 NH2 sym.þasym. str.3370 NH2 sym. str. 6551 NH2 sym. str. overtone3054 Epoxy CH2 asym. and phenyl CH str. 6069 Epoxy terminal CH2 str. overtone3034 Phenyl CH str. 5990 Phenyl CH str. overtone2997 Epoxy CH2 sym. 5890 Epoxy CH/CH2 comb.2969 CH3 asym. str. 5764, 5654 CH/CH2 overtones2930 CH2 asym. str. 5050 NH2 comb.2871 CH3 sym. str. And CH str. 4890–4786 OH comb.2840 CH2 sym. str. 4623 Phenyl CH str.þ phenyl conjugated str.916 Epoxy deformation 4530 Epoxy CH2 str. þ epoxy CH2 deformation

a (Str.) stretch; (comb.) combination; (sym.) symmetric; and (asym.) asymmetric.

APPLIED SPECTROSCOPY 1131

that the bands at 7000 and 6850 cm�1 increase while the bandsat 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, whichare assigned to OH and NH2 vibrations, respectively.

On the basis of Noda’s rule, the intensities of bands changein the following sequence: 6655, 6551! 7000! 6850 cm�1.( ‘‘!’’ means ‘‘prior to’’) The sequence 6655, 6551 ! 7000cm�1 means that NH2 vibrations change prior to the OHvibrations, which suggests that the increase of hydroxyl groupsis at the expense of the decrease of amine groups due to thecuring reaction. However, the sequence 7000 ! 6850 cm�1

means that OH vibrations change prior to the NH2 vibrations,which is just the opposite of the results obtained above. Thisphenomenon can be well explained by the fact that the bandaround 6850 cm�1 is totally overlapped by the band around7000 cm�1, and its decrease of intensity is nearly covered bythe increase of intensity of OH vibrations (as shown in Fig.1A), which also leads to the positive U(7000, 6850 cm�1) peakin the synchronous 2D spectrum (Fig. 2B). Therefore, in theasynchronous spectrum, the positive W(7000, 6850 cm�1) peakdoes not suggest that the earlier change took place with OHvibrations, but indicates a slower intensity increase at around6850 cm�1 due to the intensity decrease of the amine group atthe same position.

When the region 6250–5400 cm�1 is considered, in thesynchronous 2D spectrum (Fig. 3C), three autopeaks centeredat 6069, 5890, and 5654 cm�1, two positive peaks U(6069,5890 cm�1) and U(5990, 5764 cm�1), and four negative peaksU(6069, 5990 cm�1), U(6069, 5764 cm�1), U(5990, 5890cm�1), and U(5890, 5764 cm�1) are observed. The autopeak at5654 cm�1 is very broad, which demonstrates that thisautopeak consists of more than one peak. These results provethat the bands at 6069 and 5890 cm�1 (assigned to the CH2/CH

FIG. 3. 2D correlation spectra derived from the NIR spectra of the isothermalDGEBA/DDM curing process at 125 8C. Negative peaks are marked byshading.

FIG. 4. 2D correlation spectra derived from the NIR spectra of the isothermalDGEBA/DDM curing process at 125 8C. Negative peaks are marked byshading.

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vibration of epoxy) vary in the same direction, and the bands at5990 (assigned to benzene vibration), 5764, and 5654 cm�1

(assigned to the CH2/CH vibration of backbone) vary in theopposite direction.

In the corresponding asynchronous correlation spectrum inthis region (Fig. 3D), one positive cross peak W(5990, 5890cm�1) and four negative cross peaks W(6069, 5990 cm�1),W(6069, 5764 cm�1), W(5990, 5674 cm�1), and W(5890, 5764cm�1) are observed.

On the basis of Noda’s rule, the intensities of bands changein the following sequence: 6069, 5890! 5764, 5654! 5990cm�1. This sequence means that the CH2/CH vibrations of theepoxy groups change first, followed by the CH/CH2 groups ofthe backbone, and then the benzene groups, respectively. Asbenzene rings are not directly involved in the reaction, thechange of phenyl CH vibrations is somewhat confusing.Indeed, Lachenal et al.5 also found an unexpected increase inthe intensity of combination modes due to the phenyl ringswhen investigating the curing of epoxy and dicyanamide. Theysuggested that the unexpected intensity increase might beattributed to the modification of the packing of the phenyl ringsduring polymerization.5 In addition, as the intensity change ofthe phenyl CH vibration is very small, the phenomenon mightalso be attributed to the slight shift of the baseline due to thechange of the color of the sample. Therefore, the change ofbenzene vibrations is not further considered as a part of thereaction.

Correlating with different spectral regions always canprovide additional information. Therefore, regions 7250–6400and 6250–5400 cm�1 are correlated as shown in Fig. 4. In thesynchronous correlation spectrum (Fig. 4A), four positivepeaks U(7000, 5654 cm�1), U(7000, 5990 cm�1), U(6655,5890 cm�1), and U(6655, 6069 cm�1), and four negative peaksU(7000, 5890 cm�1), U(7000, 6069 cm�1), U(6655, 5654cm�1), and U(6655, 5990 cm�1) can be observed. These peaks

indicate that OH and CH/CH2 of the backbone change in thesame direction, while NH2 and epoxy vibrations change in theopposite 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, 6069cm�1), and four negative cross peaks W(6655, 5654 cm�1),W(6655, 5990 cm�1), W(6551, 5654 cm�1), and W(6551, 5990cm�1) are identified. It is worthwhile to remark here that thereare no cross peaks at (6655, 5890 cm�1), (6655, 6069 cm�1),(6551, 5890 cm�1), and (6551, 6069 cm�1), which suggeststhat the NH2 vibrations and epoxy vibrations vary simulta-neously. In this case, the intensities of bands change in thefollowing sequence: 6655, 6551, 6069, 5890 !7000 ! 5654cm�1, which suggests that the NH2 and epoxy groups changefirst, followed by the change of OH groups and CH/CH2 of thebackbone.

The sequence of band intensity changes is summarized inScheme 2. The above results of 2D correlation analysiscorrespond well with the mechanism of the curing reaction, asshown in Scheme 3. Although the change of band intensity inNIR spectra during the curing process has been widelyobserved1–3,6,7,10–16,18–24 and the mechanism as well as thecuring kinetics can also be obtained according to the analysisof NIR spectra,1,6,11,13,14,16–22,24,33,36–43 the sequence ofchange of band intensities was not easily elucidated bytraditional analytic methods.

Curing Process Monitored by Fourier Transform Mid-Infrared Spectroscopy from Two-Dimensional CorrelationAnalysis. Generalized 2D correlation spectroscopy was appliedto study the evolution of MIR spectra of the DGEBA/DDMsystem cured at 125 8C, and the results are presented in Fig. 5.Figures 5A and 5B show, respectively, synchronous andasynchronous correlation spectra in the region 3650–3170cm�1. In the synchronous correlation spectrum (Fig. 5A), three

SCHEME 2. Sequence of band intensity change analyzed by 2D NIR correlation spectra. Tentative assignments are listed above or below panes.

SCHEME 3. Schematic curing reaction.

APPLIED SPECTROSCOPY 1133

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 thesame direction, while the changes of 3370 (assigned to NH2

vibration) cm�1 are in the opposite direction. The band at 3324cm�1 can be assigned to hydrogen-bonded OH vibration, asOH hydrogen bonds are generally conceived in cured epoxyresin 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 twonegative cross peaks W(3460, 3409 cm�1) and W(3370, 3324cm�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 NH2 vibrations change prior to the OHvibrations. However, the sequence 3550, 3409, 3324 ! 3460cm�1 does not mean that OH vibrations change prior to theNH2 vibrations. It indicates a slower intensity increase at 3460cm�1 due to the intensity decrease of the amine group at thesame position, and the similar phenomenon has been discussedin the region 7250–6400 cm�1.

In the synchronous correlation spectrum of the region 3170–2780 cm�1 (Fig. 5C), three autopeaks at 3054, 2997, and 2871cm�1 are observed. The autopeak at 2871 cm�1 is very broadand seems to consist of more than one peak. One positive peakU(3054, 2997 cm�1) and two negative peaks U(3054, 2871cm�1) and U(2997, 2871 cm�1) show that the intensity changesof 3054 and 2997 cm�1 (assigned to epoxy CH2/CH vibration)are in the same direction, while the intensity changes of 2871cm�1 (assigned to CH vibration of the backbone) are in theopposite direction.

In the corresponding asynchronous correlation spectrum inthe same region (Fig. 5D), negative cross peaks W(3054, 2930cm�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 2997cm�1. Therefore, the intensities of the bands change in thefollowing sequence: 3054 ! 2997 ! 2871 ! 2930 cm�1,which demonstrates that the epoxy CH2 groups change first,followed by the CH/CH2 groups of the backbone.

In conclusion, the analysis of Fig. 5 also provides similarinformation 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 ofhydroxyl groups (band at 3550 and 3309 cm�1), while theintensity change of epoxy CH2 groups (band around 3054 and2997 cm�1) is sooner than that of CH2/CH groups of backbone(band around 2930 and 2871 cm�1), resulting from the ring-opening reaction of the epoxy groups. Although the curingmechanisms as well as the curing kinetics have both beenpreviously demonstrated according to the analysis of MIRspectra,3–5,10,68 the sequence of the changes of intensity hasseldom been reported.

Band Assignments from Two-Dimensional Hetero-Spec-tra Correlation Analysis. The successful applications of 2Dhetero-spectral correlation analysis have proven 2D hetero-spectral correlation to be a very attractive analytical tool thatcombines two separate measurements via some commontheme. In general, different techniques always provide differentinformation about the issue, and results from differenttechniques may validate each other, especially when the datacan be analyzed together. Therefore, 2D MIR3NIR hetero-

FIG. 5. 2D correlation spectra derived from the MIR spectra of the isothermalDGEBA/DDM curing process at 125 8C. Negative peaks are marked byshading.

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spectral correlation analysis was employed, the results of whichwere presented in Fig. 6.

In Fig. 6A, the regions 7250–6400 and 3170–2780 cm�1 arecorrelated, and in Fig. 6B the regions 7250–5400 and 3170–2780 cm�1 are correlated. The cross peaks in Fig. 6 aresummarized in Table II.

It can be concluded from Table II that, during the curingprocess, the intensities of bands at 6655, 6551, 6069, 5890,2997, and 3054 cm�1 (assigned to NH2 vibrations and epoxyCH/CH2 vibrations) are decreasing, and the intensities of bandsat 7000, 5764, 5654, 2930, and 2871 cm�1 (assigned to OHvibrations and CH2/CH vibrations of backbone) are increasing.These results are consistent with the aforementioned outcomes.

Particularly, there are four decreasing bands related to epoxyCH vibrations in both the NIR region (bands at 6069 and 5890cm�1) and the MIR region (bands at 3054 and 2997 cm�1),whose changes of intensity can also be found by second-derivative curves (as shown in Fig. 7).

As the epoxy ring is broken at the O–CH2 bond whenreacting with an amine group (shown in Scheme 3), the changeof NH2 vibrations should be simultaneous with that of epoxyterminal CH2 vibrations, whose fundamental vibration in theMIR region is at 3054/2997 cm�1. However, the change of theasymmetric stretching vibration (at 3054 cm�1) is prior to thatof the symmetric stretching vibration (at 2997 cm�1). Thus, itcan be deduced that the change of NH2 vibrations ought to besimultaneous with that of overtones/combinations of theasymmetric stretching vibration of epoxy CH2 groups. Inaddition, it has been shown that NH2 vibrations (bands at 6655and 6551 cm�1) and epoxy CH vibrations (bands at 6069 and5890 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

TABLE II. 2D correlation cross peaks in Fig. 6.

Positive cross peaks (cm�1) Negative cross peaks (cm�1)

7000, 2871a 7000, 29977000, 2930 7000, 30546655, 2997 6655, 28716655, 3054 6655, 29306551, 2997 6551, 28716069, 3054 6069, 29306069, 2997 6069, 28715890, 3054 5890, 29305890, 2997 5890, 28715990, 2930 5764, 30545990, 2871 5764, 29975890, 3054 5654, 30545890, 2997 5654, 29975764, 28715764, 29305654, 29305654, 2871

a ‘‘m1, m2’’ refers to the cross peak on the upper left side of the correlationspectrum, and m1 is the x-axis wavenumber, while m2 is the y-axis wavenumber.

FIG. 6. 2D hetero-spectral (MIR3NIR) correlation spectra of the isothermalDGEBA/DDM curing process at 125 8C. Negative peaks are marked byshading.

APPLIED SPECTROSCOPY 1135

5890 cm�1 both should be the overtones/combinations of theasymmetric stretching vibration of epoxy CH2 groups. Thisassumption is fully supported by the fact that the band at 6069cm�1 is the first overtone of the epoxy terminal CH2 band (at3054 cm�1).3 Now it can be deduced that the band at 5890cm�1 is the combination band of the epoxy CH2 asymmetricstretching vibration (3054 cm�1) and the overtone of the epoxyCH2 deformation vibration (2 3 1430 cm�1).

CONCLUSION

Clear pictures of the curing mechanism of the DDM-curedDGEBA system were elucidated here by NIR and MIRspectroscopy with 2D correlation analysis, which could be themost effective as well as convenient way to perform dynamicanalyses on such investigations. The results indicated that,during the curing process, the intensity change of the amineand epoxy groups were simultaneous and occurred sooner thanthe intensity change of the hydroxyl groups, followed by thechange of the CH2/CH groups of the backbone, resulting fromthe ring-opening reaction of the epoxy groups. In addition, 2Dhetero-spectral correlation was also employed and the afore-mentioned conclusions were further confirmed.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support by the NationalScience of Foundation of China (NSFC) (Nos. 20774022, 20573022,20425415, 20490220), the ‘‘Leading Scientist’’ Project of Shanghai (No.07XD14002), the National Basic Research Program of China(2005CB623800), and PHD Program of MOE (20050246010).

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1136 Volume 62, Number 10, 2008