7098
J. Phys. Chem. B 2007, 111, 7098-7104
Effect of Substituents in Aromatic Amines on the Activation Energy of Epoxy-Amine Reaction Yanxi Zhang and Sergey Vyazovkin* Department of Chemistry, UniVersity of Alabama at Birmingham, 901 South 14th Street, Birmingham, Alabama 35294 ReceiVed: February 5, 2007; In Final Form: April 18, 2007
Differential scanning calorimetry has been used to study the kinetics of epoxy-amine curing reaction between diglycidyl ether of 4,4′-bisphenol and aromatic amines with different electron-withdrawing/-donating substituents. The substituents include -NO2, -CN, -OCH3, -OH, and -CH3 groups. An advanced isoconversional method has been employed to determine the effective activation energy of the respective processes. An attempt has been made to link the experimental values with the results of quantum chemistry calculations. It has been found that regardless of the electron-withdrawing/-donating properties the presence of a substituent of a large negative charge in the ortho position causes an increase in the activation energy to ≈100 kJ mol-1 from the normally observed values of 50-60 kJ mol-1.
Introduction The mechanism and kinetics of epoxy-amine reaction (or curing) have been widely studied.1-4 During the curing an amine undergoes two major reactions illustrated in Scheme 1. Epoxy monomer initially reacts with a primary amine and produces a secondary alcohol and secondary amine. The secondary amine reacts with an epoxy monomer to give a tertiary amine and another secondary hydroxyl group. Other reactions, such as etherification, only occur at high temperature or in the presence of a catalyst.5-7 While frequently studied, epoxy-amine curing continues to attract interest because many effects remain unexplained. For example, the influence of the amine structure is sometimes unexpected and cannot be explained exclusively by electron-donating or electron-withdrawing characteristics of substituents.8 Noncatalytic mechanism of epoxy-amine reaction is believed to involve the formation of a trimolecular transition state. Shechter et al.,6 Smith,9 and Lee et al.10 suggested a trimolecular transition state with a hydroxyl group formed during the reaction (Scheme 2a). Rozenberg4 proposed a mechanism based on the formation of a more reactive amine dimer (Scheme 2b). The amine can act both as a nucleophilic and an electrophilic reagent. In the dimer the two amines form a hydrogen bond that allows for enhancing the electrophilic activity of one amine with a simultaneous increase in the nucleophilic activity of the other amine when the transition state is formed.4 Both mechanisms reflect the same fact that the amine acts as a nucleophilic reagent when attacking the epoxy ring in an epoxy-amine reaction. An effect of aromatic diamine structure on the rate of epoxyamine reaction has been thoroughly studied by Girard-Reydet et al.11 The focus of that work has been on the ratio of the reactivity of secondary to primary amine hydrogen. The ratio has been found to change with the amine structure, whereas the primary amine hydrogen reaction has been reported to have a markedly larger rate constant than that of the secondary one. Note that the reported difference appears to be due to the * To whom correspondence should be addressed. E-mail: vyazovkin@ uab.edu.
SCHEME 1: Reaction of Epoxy Monomer with Primary and Secondary Amine
SCHEME 2: Trimolecular Transition State for EpoxyAmine Reaction
difference in preexponential (entropic) factors, not the activation energies. If the activation energy of the primary amine reaction was lower, then the effective (global) activation energy derived from differential scanning calorimetry (DSC) measurements would have been lower in systems with a large excess of amine than in stoichiometric systems. However, no lowering of the effective activation energy has been observed in the studies using a large excess of aromatic diamine12 as well as of aromatic monoamine.13 The objective of this work is to investigate the effect of the structure of aromatic amines on the effective activation energy of their reactions with a new solid epoxy monomer, diglycidyl ether of 4,4′-bisphenol (DGEBP). Although the activation energy is not the only parameter that determines the reactivity, the present study focuses on this parameter for several reasons. First,
10.1021/jp071001h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007
Activation Energy of Epoxy-Amine Reaction unlike the preexponential factor and reaction model, the effective activation energy can be unambiguously determined by using isoconversional methods that, according to the results of the ICTAC Kinetic Project,14 are among the most reliable techniques of kinetic analysis of thermal data. Second, the effective activation energy may afford a relatively straight theoretical interpretation in terms of energy barriers.15 Third, the effective activation energy characterizes the temperature sensitivity of the reaction rate and, therefore, is a key parameter in practical kinetic predictions. Specifically, the effective activation energies of a series of curing reactions of DGEBP with amines are determined and discussed. These amines have various substituents that include electron-donating groups, -OCH3, -OH, and -CH3, and electronwithdrawing groups, -NO2 and -CN. The activation energies for curing DGEBP with nitrophenyleneamines have been reported earlier13,16,17 and are used here for reference purposes. The effect of substituents in an aromatic amine may be associated with conjugation, inductive, and steric effects as well as with the formation of hydrogen bond. By combining the experimental results with the quantum chemistry calculations, it is expected to elucidate how these factors can affect the experimental values of the activation energy. The experimental activation energies are compared against a reference value derived from the earlier data on reaction of DGEBP with 3and 4-nitro-1,2-phenylenediamine.16,17 Experimental Section (1) Materials. DGEBP is a new solid epoxy monomer that was synthesized according to Mormann and Bro¨cher.18 The details of the procedures were described in a previous paper.16 The molecular weight of DGEBP monomer is 298. Its epoxy equivalent weight is 149 g equiv-1. 2-Aminophenol (2-AP), anthranilonitrile (2-AN), and 2,3,5,6-tetramethyl-p-phenylenediamine (TMPDA) were purchased from Aldrich, and 2,5dimethoxyaniline (2,5-DMA), 3,5-dimethoxyaniline (3,5-DMA), 3-nitro-1,2-phenylenediamine (3-NPDA), 4-nitro-1,2-phenylenediamine (4-NPDA), and 2,4-dinitroaniline (2,4-DNA) were purchased from Acros Organics. All of the amines were used as received. The chemical structures of the epoxy monomer and amines are shown in Scheme 3. (2) Sample Preparation and Experimental and Computational Methods. (2.1) Sample Preparation. The starting components in the form of fine powder were mixed in the solid state in a stoichiometric ratio. The mixtures (about 5-7 mg) were sealed in 40 µL aluminum pans with a pinhole and heated in a Mettler-Toledo DSC 822e. The temperature and heat calibration of DSC were carried out by using an indium standard. The non-isothermal experiments were performed at 2, 4, 8, 10, 12, 14, 16, 18, 20, and 22 °C min-1 under a flow of N2 at 80 mL min-1. Repetitive DSC runs on individual mixtures at a single heating rate has shown good reproducibility. Reproducibility has also been checked by splitting the ten heating rates in two subsets (2, 8, 12, 16, 20 and 4, 10, 14, 18, 22 °C min-1) and using them to evaluate the respective activation energies, which have not demonstrated any significant difference. The mixtures have also been analyzed by heating in a Mettler Toledo TGA/SDTA 851e at 10 °C min-1 under N2 flow at 80 mL min-1. (2.2) Quantum Chemistry Calculations. Quantum chemistry calculations were performed with Spartan ’04 program (PC/x86) (Wavefunction Inc.). The vibrational frequency and optimized geometry were calculated at the Møller-Plesset RMP2 (FC) levels of theory using 6-31G* basis set. The details
J. Phys. Chem. B, Vol. 111, No. 25, 2007 7099 SCHEME 3: Chemical Structures of DGEBP and Amines
of calculation are described elsewhere.19,20 To further improve the experimental correlations, equilibrium geometry was used in the calculations. The equilibrium geometry is the most stable geometry for the molecule that corresponds to the true minimum on the respective potential energy surface. (2.3) Kinetic Analysis. The kinetic analysis of the DSC data was carried out by using an advanced isoconversional method developed by Vyazovkin.21,22 As all isoconversional methods, it combines the single-step kinetic equation
dR -E ) A exp f(R) dt RT
( )
(1)
with the isoconversional principle that states that at a constant extent of conversion the reaction rate is only a function of the temperature.
d ln(dR/dt)R dT-1
)-
ER R
(2)
A and E are respectively the preexponential factor and the activation energy, f(R) is the reaction model, R is the gas constant, T is the temperature, t is the time, and R is the extent of conversion that can be determined from DSC runs as a fractional heat release. The subscript R denotes values related to a given extent of conversion. Determination of the ER value by using an isoconversional method requires running a series of experiments under different heating programs. The advanced isoconversional method21,22 is based on numerical integration of eq 1 as applied to a series of n different temperature programs, Ti(t). The method allows the effective activation energy to be determined at any particular value of R by finding ER, which minimizes the function n
Φ(ER) )
n
J[ER,Ti(tR)]
∑ ∑ i)1 j*i J[E ,T (t )] R
j R
(3)
7100 J. Phys. Chem. B, Vol. 111, No. 25, 2007
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Figure 1. Variation of effective activation energy with the extent of conversion for stoichiometric DGEBP/3-NPDA and DGEBP/4-NPDA systems. Reference represents the DGEBP/3-NPDA system with a quadruple excess of amine.
Figure 2. DSC curves for curing DGEBP with 2-AP, TMPDA, 2,5-DMA, and 3,5-DMA at 10 °C min-1, respectively.
where
J[ER,Ti(tR)] ≡
∫t t
R
R-∆R
exp
[ ] -ER
RTi(t)
dt
(4)
In eq 4, R varies from ∆R to 1 - ∆R with a step ∆R, typically chosen to be 0.02. The integral, J, is evaluated numerically by using the trapezoid rule. The minimization procedure is repeated for each value of R to find the dependence of the activation energy on the extent of conversion. The mean relative error of the ER values determined in this study was within 15% in accord with the results23 of the earlier analysis of the method. Because the same value of R is accomplished at somewhat different temperatures under different heating programs, the ER value estimated by eqs 2 and/or 3 is associated with some relatively narrow temperature region. Under this circumstance, the single-step rate eq 1 is assumed to hold only for a given extent of conversion and the respective temperature region. It means that isoconversional methods apply a set of independent single-step rate equations to describe the whole range of the extents of conversion. As a result, the isoconversional methods allow for detecting variations in ER with temperature and conversion. If changes in the cure mechanism are associated with changes in the activation energy, they can be detected by these methods.24-26 As shown in a recent review paper,15 analysis of such variations can provide important insights into the mechanism and kinetics of curing, degradation, crystallization, and relaxation of polymers. (2.4) Reference Value of ER. Ideally, the reference value of ER should be derived from a reaction between DGEBP and nonsubstituted aniline. However, performing this reaction is associated with a number of technical issues resulting from the fact that aniline is a liquid and DGEBP is a solid. Therefore a reference value is here derived from a reaction of DGEBP with a solid nitrophenyleneamine.13,16,17 As reported earlier,13 the ER value of the curing reaction between DGEBP and 3-NPDA increases significantly with R (Figure 1). The increase was demonstrated to be due to a competition between the two amino groups in 3-NPDA with the one closer to -NO2 being the reason for the higher ER value. The use of a quadruple excess of the amine resulted in obtaining an almost constant value of ER around 50-55 kJ mol-1 17 that was practically identical (Figure 1) with the value found when curing DGEBP with 4-NPDA, in which none of the amino groups is hindered by -NO2. Therefore it is believed that the ER values for the system with the excess of 3-NPDA can serve as reference values for curing DGEBP with substituted aromatic amines. It should be
Figure 3. TGA curves of curing DGEBP with 2-AP, TMPDA, 2,5-DMA, and 3,5-DMA at 10 °C min-1, respectively.
noted that the reference values for DGEBP are similar to the activation energies found for curing diglycidyl ether of bisphenol A (DGEBA) or phenyl glycidyl ether (PGE) with aniline,27-30 as well as to the activation energies of other epoxy-amine cure reactions.31-36 Results and Discussion (1) Effect of Electron-Donating Substituents: 2-AP, TMPDA, 2,5-DMA, and 3,5-DMA. Figures 2 and 3 show the DSC and TGA data of DGEBP/2-AP, DGEBP/TMPDA, DGEBP/ 2,5-DMA, and DGEBP/3,5-DMA systems. The DGEBP/2-AP system demonstrates one sharp exothermic peak between 100 and 150 °C. The thermogravimetric analysis (TGA) curve displays a mass loss of ≈0.5% in this temperature region. The DGEBP/TMPDA system demonstrates one endothermic peak immediately followed by an exothermic peak from 120 to 200 °C in the DSC curve. In this temperature region the TGA curve shows less than 0.1% mass loss. In the DGEBP/2,5-DMA system, two endothermic peaks at 75 and 120 °C are followed by one exothermic peak from 125 to 250 °C in the DSC curve. A mass loss in this temperature region is less than 2%. For DGEBP/3,5-DMA system, two endothermic peaks at 55 and 115 °C are followed by one exothermic peak from 125 to 225 °C. The TGA curve shows 0.75% mass loss in this temperature region. According to earlier DSC and TGA data,13,16,17 neat DGEBP does not demonstrate any obvious signs of degradation or homopolymerization in the above temperature ranges. Therefore, the exothermic peaks observed for the mixtures (Figure 2) represent the process of curing. Figure 4 presents a variation of the effective activation energy with the extent of conversion. For the DGEBP/2-AP system, the cure process is characterized by a quick decrease in the activation energy from ≈105 to the value of ≈85 kJ mol-1 within R ) 0-0.3, which is followed by an increase to
Activation Energy of Epoxy-Amine Reaction
J. Phys. Chem. B, Vol. 111, No. 25, 2007 7101
Figure 4. Variation of effective activation energy with the extent of conversion for reaction of DGEBP with 2-AP, TMPDA, 2,5-DMA, and 3,5-DMA.
SCHEME 4: Graphic Representation of π-Molecular Orbitals for Benzene and Orbitals for Lone-Pair Electrons of Oxygen and Nitrogen in Hydroxy and Amino Groups (Positive Lobes Are Shaded)
110 kJ mol-1 at a higher value of R. All the values of ER for this system are obviously much larger compared to the reference system. For other systems, the deviations of the ER values from the reference (50-55 kJ mol-1) are not nearly as dramatic. The DGEBP/TMPDA system demonstrates the ER values that increase slightly (from 60 to 70 kJ mol-1). That is, the ER values are ≈10 kJ mol-1 greater than the respective reference values. For the DGEBP/2,5-DMA system, a slightly decreasing dependence of ER (from 50 to 40 kJ mol-1) is observed. All the values of ER for this system are about 10 kJ mol-1 smaller than the respective reference values. The DGEBP/3,5-DMA system shows practically invariable values of ER centered around 55 kJ mol-1. In the DGEBP/2-AP system, -OH is expected to exhibit an electron-donating conjugation effect that should increase the charge on -N in -NH2 and make it a stronger nucleophile. As a result, one may expect the effective activation energy of the epoxy-amine reaction to decrease. However, the data show significantly larger values of the activation energy compared to the reference values. Scheme 4 shows the π-molecular orbital for benzene and the orbitals for the lone-pair electrons of -N in -NH2 and -O in the -OH group. From Scheme 4, the direction of the orbital of the lone-pair electrons on -N of -NH2 group is the same as that of π- electrons of benzene ring. That gives rise to a strong delocalization of the lone-pair electrons on -N into the π-electron cloud of the ring. This results in the strong conjugation effect between -NH2 and the benzene ring. But an overlap of the lone-pair orbital of the -OH group with the orbital of π-electrons of the benzene ring is weak because of the different direction of the orbitals. For this reason, the electron-
donating conjugation effect of -OH is rather weak and the electron-withdrawing inductive effect outweighs the electrondonating conjugation effect. This is confirmed by quantum chemistry calculations (Table 1), according to which the electrostatic charges on -N, -H1, and H2 in the -NH2 group are -0.797, 0.373, and 0.347, respectively. The electrostatic charge on -N is more positive than that in aniline (-0.854 in Table 1), so that it is expected to obtain the values of activation energy larger than the reference values. The possibility of the formation of an intramolecular hydrogen bond can be judged from quantum chemistry calculations (Scheme 5) as an increase in the H-N-H bond angle compared to that in aniline as well as the difference in electrostatic charge and the distance between the -NH2 group and a substituent. Generally hydrogen bond length is 1.8-3.0 Å,37-40and its energy is estimated to be 4.3 40 or 6.8 kcal mol-1 (2.80 Å) or 14.4 kcal mol-1 (2.50 Å) for O-H‚‚‚O and 12.0 kcal mol-1 (2.80 Å) for O-H‚‚‚N.39 In 2-AP, the H-N-H bond angle is 111° that is 1° larger than in aniline. In addition, considering the difference in electrostatic charge on O, H1 and the distance between -O and -H1, 2.284 Å, it is possible to form a hydrogen bond between -NH2 and -OH. However, the formation of the hydrogen bond would make the amine hydrogen more acidic, and the nitrogen more basic, therefore, decreasing the energy barrier to the reaction. Since the experimental activation energy shows the opposite trend, the effect of formation of hydrogen bond does not appear to be significant. A factor that may contribute to an increase in the activation energy is the hindrance effect of the -OH group in the ortho position. An -OH group in the ortho position may exert an electrostatic hindrance effect by providing an alternative nucleophilic center. Its electrostatic interaction with the epoxy carbon would create an extra energy barrier to the nucleophilic substitution by the amine nitrogen that should contribute to an increase in the activation energy. The effect of electrostatic hindrance may be estimated by determining the charge on the alternative nucleophilic center in the ortho position. As seen in Scheme 5, the electrostatic charge on -O in -OH group is -0.641, which is comparable to the charge on -N in -NH2. It is, therefore, reasonable to expect a significant increase in the values of ER compared to those for the reference reaction. It should be noted that, in principle, the -OH group can react with epoxy monomer. This possibility has been checked by analyzing DSC data on heating DGEBP with phenol. As seen from Figure 5, a slow exothermic process takes place in the temperature region of 140-230 °C. By applying the advanced isoconversional method to DSC data at different heating rates, it has been determined that ER remains practically constant around 65 kJ mol-1. Because the process occurs at temperatures markedly higher than in the case of 2-AP and demonstrates significantly lower ER values, it has been concluded that a possible contribution of the -OH reaction in the case of 2-AP is practically negligible. In TMPDA, two -NH2 groups are strong electron donors for each other and the -CH3 group also has electron-donating
TABLE 1: Electrostatic Charge, Bond Angle, and Activation Energy in Different Amine Systems 2-AP TMPDA 2,5-DMA 3,5-DMA 2-AN 2,4-DNA aniline
N in -NH2
H in -NH2
negative charge on substituent group
bond angle H-N-H, deg
ER, kJ mol-1
-0.797 -1.012, -1.188 -1.104 -1.275 -1.189 -1.145 -0.854
0.347, 0.373 0.442, 0.457, 0.431, 0.431 0.506, 0.419 0.435, 0.420 0.473, 0.479 0.436, 0.485 0.352, 0.352
-0.641 -0.482, -0.576, -0.464, -0.569 -0.272, -0.338 -0.269, -0.529 -0.541 -0.585, -0.512 n/a
111.25 108.38 110.91 109.59 111.81 116.77 110.05
85-110 60-70 40-50 50-60 90-120 90-115 50-60
7102 J. Phys. Chem. B, Vol. 111, No. 25, 2007
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SCHEME 5: Electrostatic Charge of 2-AP, TMPDA, 2,5-DMA, 3,5-DMA, and Aniline
conjugation effect, which is shared by two -NH2 groups. These factors make the electrostatic charge on -N in -NH2 groups more negative than that in aniline. Quantum chemistry calculations support this expectation. The respective electrostatic charges on N are -1.012 and -1.188 (Table 1). Generally, the increased nucleophilic strength should lead to lowering of the value of activation energy. However, Figure 4 shows that the activation energy of DGEBP/TMPDA is somewhat greater than that for the reference, and the difference increases with the extent of conversion. Quite likely the larger value of ER for DGEBP/ TMPDA reflects the fact that two -CH3 groups in the ortho position of the -NH2 group may present some electrostatic hindrance for the reaction with the bulky DGEBP molecule. Quantum chemistry calculations (Scheme 5) show that the electrostatic charge of four -Cs in -CH3 group is -0.576, -0.569, -0.482, and -0.464. Although these values are about two times smaller than the charges on -N, the respective electrostatic interaction may add some extra energy barrier to the epoxy-amine reaction. However, the charge on -C in the ortho position is not as large as in the case of the -O in the -OH in 2-AP, and the carbon does not have a lone pair to produce alternative nucleophilic center. As a result an increase in ER is quite modest compared to the reference. In 2,5-DMA, the electron-donating conjugation effect from the -OCH3 group will increase the electrostatic charge of -N in -NH2 group, which should increase the nucleophilic strength of the amine and decrease the activation energy. On the other hand, the -OCH3 group in the ortho position may exert electrostatic hindrance. It is also possible that the -O in -OCH3 group can form a hydrogen bond with -H in the -NH2 group, which would make the -N in -NH2 more basic and would therefore contribute to lowering of ER. Quantum chemistry calculations show that the electrostatic charge of -N, -H1, and H2 in -NH2 group and O1 in the ortho -OCH3 group is -1.104, 0.419, 0.506, and -0.272, respectively (Table 1). The electrostatic charge of -N in -NH2 group is more negative than that in aniline. It suggests that 2,5-DMA is a strong nucleophile. The bond angle H-N-H is 111°, which is increased by 1° compared with 110° in aniline. As mentioned above, because of the bond angle, the difference in electrostatic charge and the distance
between -O1 and -H1 (2.221 Å), it is possible to form a hydrogen bond between H1 in -NH2 and O1 in -OCH3 group.37-40 As to electrostatic hindrance, because the charge on -O1 is ≈4 times smaller than that on -N, its effect is very weak. As a result, the increase in nucleophilic strength, possibly enhanced by a hydrogen bond, outweighs the hindrance effect so that the ER values are smaller than the reference values. In 3,5-DMA, -OCH3 is an electron-donating group, and two -OCH3 groups on the benzene ring exert electron-donating conjugation effect, which makes the -N in the -NH2 group more basic so that the activation energy should be expected to decrease compared to the reference. However, the kinetic analysis in Figure 4 shows that there is no difference in activation energy between DGEBP/3,5-DMA and reference systems. Quantum chemistry calculations (Table 1) demonstrate that the electrostatic charge of -N in the -NH2 group is -1.275, which is more negative than that in aniline. It is obviously a strong nucleophile, for which it could be certainly expected to obtain activation energies that are lower than for the reference reaction. The electrostatic charge of -H1, H2 in the -NH2 group and -O1, O2 in the -OCH3 group (Scheme 5) are 0.420, 0.435 and -0.269, -0.529, respectively. The bond angle of H-N-H is 110°. Compared with aniline, the difference in the bond angle is insignificant. And the distance between -H in the -NH2 group and -O in the -OCH3 is 4.955 Å and 4.786 Å. In addition, considering the difference in electrostatic charge between -H in -NH2 and -O in the -OCH3 group, there is no formation of a hydrogen bond. The electrostatic charge of -O2 in the -OCH3 group (-0.529) is quite significant. Although -OCH3 can certainly exert some electrostatic hindrance to -NH2, the group is in the meta position so that the electrostatic hindrance is likely to be much smaller than in the case when a substituent is in the ortho position. However, it still may be sufficient to compensate for the increased nucleophilic strength of the amine so that the experimental activation energies do not differ much from the reference values. (2) Effect of Electron-Withdrawing Substituents: 2-AN and 2,4-DNA. DSC and TGA data of the DGEBP/2-AN system are shown in Figure 6. The DSC curve displays two endothermic peaks followed by two overlapped exothermic peaks at 210-
Activation Energy of Epoxy-Amine Reaction
J. Phys. Chem. B, Vol. 111, No. 25, 2007 7103 SCHEME 6: Bond Angle and Electrostatic Charge for 2-AN and 2,4-DNA
Figure 5. DSC curve for a reaction of DGEBP with phenol at the heating rate of 10 °C min-1. (Inset shows the ER values for this reaction.)
Figure 6. DSC and TGA curves of the DGEBP/2-AN system at 10 °C min-1.
Figure 7. Variation of effective activation energy with the extent of conversion for DGEBP/2-AN, DGEBP/2,4-DNA, and DGEBP/2-AP systems.
320 °C. Mass loss reaches ≈15% by 320 °C, suggesting that at higher temperatures there may be a significant contribution of decomposition. However, there is no significant mass loss at the earlier stages of the exothermic process, indicating that it is primarily related to curing. The kinetic analysis of the DSC data is shown in Figure 7. The DGEBP/2,4-DNA system was studied in a previous paper.13 The results for the DGEBP/2AN system are compared with the DGEBP/2,4-DNA system in Figure 7. The DGEBP/2-AN system shows some moderate increase in the effective activation energy with the extent of conversion. Note that the early stages (R < 0.4) of the process, which are associated predominantly with curing, demonstrate ER ≈90110 kJ mol-1. These values are significantly larger than the reference values 50-55 kJ mol-1. It is noteworthy that the behavior of ER for this system is similar to that for DGEBP/ 2,4-DNA system.13 Because both amines contain electronwithdrawing groups (-CN or -NO2) in the ortho position, they
are expected to be weak nucleophiles, the respective activation energies of the reaction should then be larger than for the reference. Due to the existence of electron-donating (-NH2) and electron-withdrawing groups in each amine, the charge of the whole molecule will separate, which results in a negative charge on -N in the -NH2 group and a positive charge on -N in the -NO2 group.41 Quantum chemistry calculations yield the electrostatic charges on -N in 2-AN and -N1 in 2,4-DNA to be -1.189 and -1.145, respectively (Table 1). It suggests that against the expectations both amines are stronger nucleophiles than aniline so that their nucleophilic strength cannot be used to explain the increased values of the experimental activation energies (Figure 7). However, the hindrance effect and the effect of the formation of hydrogen bond on the activation energy cannot be ignored. Scheme 6 shows that, for 2-AN, the H-N-H bond angle is ≈112° and the electrostatic charge of -H1, -H2, and -N2 is 0.473, 0.479, and -0.541, respectively. The C-C2N2 bond angle is 177°; the N in the -CN group bends toward H. The H2-N2 distance is 2.842 Å. For 2,4-DNA, the H-N-H bond angle is ≈117° and the electrostatic charge of -H1, -H2, and -O1, -O2 is 0.485, 0.436 and -0.585, -0.512, respectively. The H1-O1 distance is 1.959 Å. Compared with aniline (Scheme 5), the H-N-H bond angle is increased by 2 and 7° in 2-AN and 2,4-DNA, respectively. In addition, considering the difference in electrostatic charge and the distance between the -NH2 group and the substituent,37-40 a hydrogen bond can form in both systems. This will make -H in the -NH2 group more acidic and prone to the formation of the transition state (Scheme 2) in the epoxy-amine reaction so that the activation energy should rather decrease. However, the electrostatic charge of -O1 and -O2 in the -NO2 group and that of -N2 in the -CN group will provide significant electrostatic hindrance for nucleophilic reaction and, therefore, will create an extra energy barrier. Apparently this is the major factor that outweighs all others, giving rise to a significant increase in the experimental activation energy compared to the reference values. To stress the importance of the effect of the electrostatic hindrance on the activation energy, Figure 7 also shows the data for the DGEBP/2-AP system. Unlike 2-AN and 2,4-DNA, 2-AP contains an electron-donating substituent. However, all three systems have an alternative nucleophilic center in the ortho position and show ER values that are significantly larger than the reference values. It suggests that the electrostatic hindrance can well outweigh the other effects and has the more important effect on the experimental activation energy. Conclusions Although the electron-donating/-withdrawing properties of substituents are routinely invoked in explaining the reactivity of aromatic molecules, the study suggests that these properties
7104 J. Phys. Chem. B, Vol. 111, No. 25, 2007 alone are not sufficient to explain changes in the effective activation energy of the epoxy-amine reactions. The reaction is expected to follow the mechanism of nucleophilic substitution. However, there is no straightforward effect of a charge on the amine nitrogen on the experimentally found activation energy. Its value seems also to be affected by the formation of an intramolecular hydrogen bond as well as by increased charge on the amine hydrogens. Unexpectedly, it has been found that the negative charge on a substituent in the ortho position has the strongest effect on the values of the activation energy, causing its significant increase. Similar increases are observed for both electron-donating/-withdrawing substituents. The discovered effect may be of a practical use when designing epoxyamine formulations. In particular, by introducing substituents of a large negative charge close to the amine group, one can significantly increase the activation energy of the epoxy-amine reaction, shifting it to higher temperatures. Changing the activation energy also means changing the temperature sensitivity of the reaction that may be of relevance for the process of reaction injection molding customarily used on epoxy-amine systems. Acknowledgment. The authors thank Justin Lang for his help with quantum chemistry calculations and Mettler-Toledo, Inc. for donation of the TGA instrument used in this work. References and Notes (1) Lee, H.; Neville, K. Handbook of Epoxy Resins; McGraw-Hill: New York, 1967. (2) Tanaka, Y.; Bauer, R. S. In Epoxy Resins: Chemistry and Technology, 2nd ed.; May, C. A., Ed.; Dekker: New York, 1988; p 285. (3) Barton, J. M. In Epoxy Resins and Composites I; Dusˇek, K., Ed.; Advances in Polymer Science, Vol. 72; Springer-Verlag: Berlin, 1985; p 111. (4) Rozenberg, B. A. In Epoxy Resins and Composites II; Dusˇek, K., Ed.; Advances in Polymer Science, Vol. 75; Springer-Verlag: Berlin, 1986; p 113. (5) Xu, L.; Schlup, J. R. J. Appl. Polym. Sci. 1998, 67, 895. (6) Shechter, L.; Wynstra, J.; Kurkjy, R. P. Ind. Eng. Chem. 1956, 48, 94. (7) Woo, E. M.; Huang, Y.; Chang, L. L.; Kao, H.; Wu, R.; Su, C. J. Macromol. Sci., Part B 2004, B43, 365. (8) Liu, H.; Uhlherr, A.; Varley, R. J.; Bannister, M. K. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3143. (9) Smith, I. T. Polymer 1961, 2, 95. (10) Lee, J. Y.; Shim, M. J.; Kim, S. W. J. Appl. Polym. Sci. 2002, 83, 2419. (11) Girard-Reydet, E.; Riccardi, C. C.; Sautereau, H.; Pascault, J. P. Macromolecules 1995, 28, 7599.
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