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Study of Metal/Epoxy Interfaces between Epoxy Precursors and Metal Surfaces using a Newly Developed Reactive Force Field for Alumina-Amine Adhesion Fidel Orlando Valega Mackenzie, and Barend J. Thijsse J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5105328 • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Study of Metal/Epoxy Interfaces between Epoxy Precursors and Metal Surfaces Using a Newly Developed Reactive Force Field for Alumina-Amine Adhesion F.O. Valega Mackenzie and B. J. Thijsse∗ Department of Materials Science and Engineering,Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands E-mail: [email protected] Phone: +31 (0)15 2782221. Fax: +31 (0)15 2786730

∗

To whom correspondence should be addressed

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ACS Paragon Plus Environment


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Abstract In this work we study the adhesion between various alumina surfaces and precursors of amine-containing epoxies.To accomplish this we have developed a reactive force field (ReaxFF) parameterization for the aluminum-nitrogen interaction and combined this with the aluminum-water and glycine force fields reported in earlier work. Molecular clusters and reaction paths selected to model interfacial phenomena of amine-containing epoxy precursors on alumina were used in the fitting of the ReaxFF parameters. It is shown how this new force field satisfactorily reproduces equilibrium bond lengths and angles as well as binding energies for the proposed structures. Reaction profiles are also fitted in agreement with ab initio calculations performed making use of the nudge elastic band method. To substantiate the parametrization, room temperature molecular dynamics results of ammonia adsorption on alumina are compared using ab initio and ReaxFF methods. Using this new interaction scheme, adsorption energies of dimethylamine and DETA on differently terminated alumina surfaces are determined for ten different molecule-surface approach conditions each. We find that the mean and rms adsorption energy values vary considerably with the particular molecule/surface combination. It is also shown that the adhesion of the studied epoxy precursors decreases linearly with the hydroxyl coverage on alumina surfaces.

Keywords Molecular dynamics, Adhesives, Corrosion, Coatings

Introduction Understanding the role of molecular interactions is of ubiquitous importance in many sciences. Particularly for the corrosion, coatings and catalysis fields the knowledge of the interaction between organic and metallic compounds is necessary in order to predict, assess and enhance properties like adhesion or failure in the resulting materials. 2


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Many cases of epoxy adhesives together with alumina and similar oxides are encountered from the aerospace 1 to the microelectronics 2,3 industries where they are very attractive due to their physical and chemical properties. Although much experimental research has been done, it is still difficult to predict the adhesion properties of amine-hardened epoxy mixtures to both acidic or basic substrates. 4 It has been suggested that this difficulty can be explained by the multiple factors that contribute to final adhesion in metal-organic interfaces: e.g. hardener nature and mixing ratio, cure time, substrate treatment, interface formation conditions, non-uniform coverage, exposure to aqueous environment and surface contamination. 5–9 Metal-organic interfaces have also been the subject of innumerable theoretical publications. Among the most broadly used methods density functional theory (DFT) and molecular dynamics (MD) are found. DFT in combination with NEB (nudge elastic band method) calculations has been used to determine preferential adsorption of different epoxy components on alumina surfaces. It has been shown for two common epoxy precursors how diglycidyletherbisphenol A (DGEBA) binds exothermically in contrast with an endothermic process when diethylenetriamine (DETA) is adsorbed.

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Comparitive adsorption between carbonyl and

amine groups in glycine can be studied as well using DFT. Furthermore, it allow us to discern the energetics for several conformers of the glycine molecule. Glycine exhibits several ways in which it can be adsorbed at alumina surfaces, mainly via hydrogen bonds with carboxyl and amine groups altogether, but it binds more strongly with only the carbonyl group at low coordinated aluminum sites.

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Theoretical calculations have shown as well that the

mechanism of the bonding between amine groups and alumina is through the nitrogen lone pair. More precisely, it has been concluded that the nature of the Al-N bond is covalent in nature based on electron density build-ups obtained after calculating the adsorption of methylamine on alumina surfaces using DFT. 12 Molecular dynamics simulations can also describe metal-polymer interfaces, in size and time scales which are not accessible to first principle calculations. In this way materials structure evolution is much closer to what happens experimentally and therefore materials

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properties can be derived. For instance, the work of adhesion between epoxy-like amorphous polymers and metal substrates like silica and alumina has been compared. Moreover their variation under dry and wet conditions has been characterized and qualitatively compared with experiments.

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In addition, MD has been used to obtain insight in interface forma-

tion 14 , the importance of the direct surrounding for stabilizing reactions 15 , and tribological phenomena in interfacial juntions 16 , all being crucial in applications where it is necessary to know structural and kinetic properties of the metal/polymer interface. Nowadays atomistic simulations provide a powerful tool that allows us to characterize materials and their properties with an insight that is hardly attainable with current experimental techniques. Traditionally the use of force fields has been the choice over more accurate methods due to the time and length scales involved in interfacial phenomena. Nevertheless, the use of more computationally demanding, i.e. ab initio, calculations is still important to obtain fundamental bonding information and in the development of reliable parameters for force fields. In this paper we present a parameterization of the aluminum-nitrogen interaction for the reactive force field known as ReaxFF 17 based on ab initio data. Such parametrization is then embedded in the combination of two previously defined force fields

18,19

in order

to study epoxy/alumina interfaces. This combination is explained in more detail in the following section. Obtained parameters after the fitting of the ab initio data, together with a comparison of several resulting energy profiles with ab initio calculations, are presented. Finally application of the force field to adsorption of amine-containing epoxy precursors is analyzed.

ReaxFF optimization The study of combined organic and inorganic matrices often involves considering a wide range of elements. For instance, in the case of epoxy polymers we take into account that H,

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C, N and O are present. On the other side of the interface, when looking at oxides we find, besides oxygen, metals such as Cu or Al but also semiconductors like Si. For the specific interfacial study of epoxy polymers on aluminum oxides we need to take into account 5 different elements in total: H, C, N, O, Al. Building a parameterization for ReaxFF that includes this variety can be obtained by merging a pair of existing force fields between a subset of the elements plus a training set describing the other interactions that would emerge when the organic and inorganic parts are brought into contact. Different ReaxFF parameterizations have been developed for organic materials. Examples include the simple hydrocarbons 17,20 and glycine force fields 21 . They have been successfully applied to phenomena such as oxidation, solvent effects and tautomerization. Also for aluminum oxides parameters are available that describe, for instance, contact adhesion 22 and protonation of aluminum surfaces 23 . Using the concept of transferability of ReaxFF potentials it is possible to combine preexisting sets of parameters. To effectively achieve this it is required to take into account the new interaction terms that arise as a consequence of this combination. In the present case the elements involved are Al, O, N, C and H. Glycine parametrization 21 includes the subset O, N, C and H, and the aluminum oxide parametrization 19 deals with Al, O and H within its subset. It is clear from this that to make a parametrization suited for epoxy/alumina the terms where Al-C and Al-N appear need to be parametrized and added to the above mentioned potentials. In the present work we focus on Al-N terms which are of greater importance and contribute the most to the bonding of epoxy/alumina interfaces. A full set of parameters regarding the bonding of carbon to aluminum was not developed. However these elements are still allowed to interact via van der Waals and coulombic interactions. The training set used in our study made use of small molecular clusters containing combinations of the above mentioned elements and also bigger periodic systems so to include surfaces. To effectively describe interfacial phenomena of epoxy/alumina we have constructed

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a set of reactions between amine-containing epoxy primers and the (0001) face of α−alumina. These reactions are based on similar adsorption processes that have been observed previously by experimental techniques. 24 Among these we highlight the deprotonation of ammonia and dimethylamine on the (0001) α−alumina surface. The ReaxFF model is already well established. 25 This is illustrated by its contribution in the modeling and understanding of physico-chemical processes like proton migration in fuel cells 26 , hydroxylation of metal surfaces 27 , surface etching of metal oxides 28 and hydrocarbon combustion 29 . Also the methods that are used to fit data to it and obtain a parameterization have been described elsewhere. 30 Here we merged two previously introduced parameterizations by using the carbon and nitrogen atomic parameters from the glycine force field 21 and combined them with the aluminum-water force field 19 in a systematic manner. From there we construct the remaining interactions based on the training set calculated using ab initio density functional theory (DFT). Our training set can be divided into two parts. The first one consists of small molecular clusters of aluminum-nitrogen alkane analogues. This set contains the dissociation energies of Al-N bonds, as well as angle bending energies for the atom triplets H-Al-N, H-Al-C, O-Al-N, O-Al-C, N-Al-C, N-Al-N. Calculations on this set were performed using density functional theory, with the hybrid functional B3LYP 31,32 and Pople’s basis set 6-311G**

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as implemented in version 2.8 of the ORCA 34 electronic structure program. The convergence tolerance for the energy was 1 meV. The reason for these choices is twofold. On one hand preliminary studies have shown good agreement among theory and experiments on similar compounds. 35,36 On the other we want to maintain consistency with previous uses of these settings when fitting ReaxFF for the same elements. 20,21 Secondly, we also computed and fitted the reaction path for the deprotonation of aminecontaining epoxy primers on top of the (0001) face of α-alumina. The minimum energy path for such reactions was calculated using the nudge elastic band method (NEB). 37 The climbing image modification to the NEB was used, assuring that the higher energy image will be biased

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towards the saddle point. Optimizations were also carried out with DFT, but this time with the plane-augmented-wave method as they appear in the Vienna ab initio simulation package (VASP). 38–40 The pseudo-potential PBE for gradient corrected calculations reproduced the lattice constant and cohesion energy of α-alumina with better precision that PW91 and even LDA, therefore it was used. The cut-off energy for the plane wave basis set was 450 eV. Forces were converged to values smaller than 0.01 eV/˚ A. The number of k-points used in the sampling was 7 × 7 × 1, with a Gamma centered grid due to the hexagonal symmetry of the α-Al2 O3 crystals, targeting also an energy convergence of 1 meV.

Parameterization Fitting has been done in line with previously discussed methodology. 18,23,41 Tables 1,2 and 3 show the main resulting parameters of our force field. The complete set of potential parameters is available as supporting information. Table 1: Bond energy parameter D(σ) in kcal/mol. Remaining values are bond order parameters. Bond D(σ) pbe1 pbe2 pbo1 pbo2 N-Al 236.0719 -1.5976 4.4084 -0.1268 4.0887

Table 2: Valence angle parameters ka and kb in kcal/mol. The equilibrium angle is 180 − Θ. Remaining values are valence order parameters. Angle Θ ka kb pv1 pv2 N-Al-O 59.4494 35.0309 1.6962 3.2000 1.2653 H-N-Al 76.7396 84.5946 9.4256 -0.9766 2.5448 C-N-Al 43.2932 18.4004 17.9657 0.1001 1.0020 N-O-Al 60.4550 13.5763 2.7670 1.4558 1.0091 O-N-Al 29.7829 1.3475 12.1127 2.3429 1.0003 H-Al-N 66.4729 24.0624 2.1128 -0.0983 1.0105

Table 3: Dihedral bond parameters. V1 , V2 and V3 in kcal/mol. ptor1 and pcot1 are bond order and torsion conjugation parameter, respectively. Dihedral V1 V2 V3 ptor1 pcot1 C-N-Al-O -0.0020 89.5242 1.5000 -4.1018 -3.0872 7



For the optimization of the bond dissociation energy of Al-N, small molecular clusters of aluminum nitrides were studied. Both the singlet and triplet energy states of each molecule were calculated and only the smallest value for corresponding separations were included during the fitting. The equilibrium distance agrees remarkably well for both methods, and is found to be 1.78 ˚ A. The predicted dissociation energy is in good agreement with the DFT result as shown in figure 1, with the ReaxFF energy being just 0.2 eV higher. However our parametrization deviates slightly from DFT in the transition state where the bond is being broken. Since this is a transient state during the fitting a small weight was given which might explain such a mismatch. ReaxFF DFT singlet DFT triplet

7 6 Energy (eV)

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5 4 3 2 1 0 1

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4 5 Distance (Å)

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Figure 1: Dissociation energy profile of the aluminum-nitrogen bond in Al(NH2 )3 Adsorption of ammonia on α-alumina was also studied to investigate the attraction via the nitrogen lone pair to aluminum. Figure 2 shows the adsorption energy of ammonia molecules when oriented with the lone pair towards the surface on an atop position to the aluminum. The equilibrium distance predicted by DFT is 2.01 ˚ A while the value according to ReaxFF is 2.11 ˚ A. Binding energies are respectively 1.47 eV and 1.53 eV, differing only by 3%. These values are far from negligible despite being three times smaller than the binding energy for the single Al-N bond in Al(NH2 )3 . Another difference between DFT and ReaxFF descriptions lies in the shorter separation distances. In this range, ReaxFF shows a somewhat softer interaction: for a 1.7 ˚ A separation the interaction energy is half of that 8


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resulting from DFT. 1.8

ReaxFF DFT

1.6 1.4 Energy (eV)

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1.2 1 0.8 0.6 0.4 0.2 0 1.5

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Figure 2: Binding energy profile for NH3 molecule approaching an aluminum site on the (0001) face of α−alumina. Energies were also fitted for the angle bending of this molecules. The general procedure started with bending the angles between three consecutive atoms from their equilibrium configurations. After each bending step the angle in question was constrained and the rest of the molecule was fully relaxed. Figures 3 and 4 show examples of the angle bending energies for two triplets of atoms found at the alumina/epoxy interface. The overall behavior is that equilibrium angles are very well reproduced, with the exception of the bending of N-Al-O which DFT predicts to be 117◦ whereas ReaxFF results in the value of 135◦ . The general trend for deviations near the equilibrium angle is small for shallow distortions and an increasing mismatch is observed when the angles are more than 35◦ larger than their equilibrium configurations. We did not attached much weight to this, given the fact that for such a distortion to happen most likely the system will be undergoing a reaction. A dihedral angle is also considered, being justified by the possible bonding of an amine group from a polymeric backbone chain to an aluminum site at the interface. Such bonding will be accompanied by the C-N-Al-O torsional barrier. Its computed DFT and ReaxFF energies are compared in figure 5 as a function of the dihedral angle. It is observed that our ReaxFF parametrization traces the DFT calculation with very good accuracy. Among the reactions considered in this work we present results for the deprotonation 9



1.2

ReaxFF DFT

Energy (eV)

1 0.8 0.6 0.4 0.2 0 70

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100 110 120 130 140 150 160 Angle (°)

Figure 3: Angle bending energy profile for N-Al-O in AlH(NH2 )(OH)

ReaxFF DFT

1.8 1.6 Energy (eV)

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Figure 4: Energy profile when the C-N-Al angle is varied in Al(H2 )N(CH3 )2

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0.2 0.15 0.1 0.05 0 −150

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Figure 5: Torsion energy profile for the dihedral angle C-N-Al-O in Al(OH)2 N(CH3 )2

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of dimethylamine on an aluminum terminated corundum surface. As the initial state a dimethylamine molecule adsorbed on top of an aluminum site at the (0001) face of α-Al2 O3 is chosen. For this reaction to happen, deprotonation of the secondary amine will take place, hydroxylating an adjacent oxygen site, ending in a bond formed between the aluminum site and the now tertiary amine as final state. This can be better visualized with the aid of figure 6.

Figure 6: Schematic of the deprotonation reaction of dimethylamine on aluminum site of α-Al2 O3 . The initial and final states of the reaction are respectively shown at the top and bottom of the figure. Coloring of atoms: H white, C green, N blue, O red and Al turquoise. The energy profile of this reaction is shown in figure 7 for both DFT and ReaxFF. The activation energy and the resulting energy difference between reactants and products fall within less than 10% from values computed using DFT. In particular ReaxFF predicts for an aluminum-terminated surface that the top aluminum plane protrudes 0.8 ˚ A while DFT does not, therefore making the next oxygen plane less accessible in ReaxFF, thus requiring 11



more energy for protonation. 1.2

ReaxFF DFT

1 Relative Energy (eV)

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0.8 0.6 0.4 0.2 0 -0.2 0

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0.4 0.6 Reaction Coordinate (a.u.)

0.8

1

Figure 7: Energy profile for the deprotonation reaction calculated through NEB for dimethylamine on α-alumina. The normalized reaction coordinate is chosen as the cumulative of the difference in the position of each atom between successive images.

Results Molecular dynamics has been used to compare the adsorption of ammonia on aluminum terminated α−alumina. Results for ab initio and reactive molecular dynamics are compared using VASP and ReaxFF, respectively, on similar structures with the same initial conditions, in order to determine different angle distribution profiles of the NH3 molecules when being adsorbed at the surface. Six distinct systems were prepared with single NH3 molecules on top of the same aluminum terminated (0001) face of α-Al2 O3 . The simulation unit cell was 9.7 ˚ A × 9.7 ˚ A × 20.0 ˚ A resulting in 104 atoms of both Al and O, containing 8 oxide layers. Ammonia A from the alumina molecules were randomly placed at an interaction distance between 5-10 ˚ surface. In the case of the simulations with VASP a 1 ps relaxation was taken, after minimization, after which 5 out of the 6 ammonia molecules sticked to the surface. Angle and energy measurements were taken over the remaining 4 ps simulation. The time step used was 0.5 fs and the thermostat applied used the Nosé algorithm. 42

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In the case of ReaxFF relaxation simulations were run for 15 ps and measurements were taken from the remaining 5 ps run. The time step used in this case was 0.1 fs and a Berendsen thermostat was utilized, having a 10 fs time constant. The small time step was chosen to ensure that no details would be missed (the light atom H is part of the system) and to maintain consistency over many simulation runs. In this occasion only 4 out of the 6 molecules were adsorbed. This slight difference with the VASP simulation can be accounted by the different assignments of velocities in each implementation. In all cases the systems were maintained at a temperature of 300 K. When at equilibrium, random behavior is found for the energy fluctuations using both methods. The only accountable difference lies in the fact that in ReaxFF the fluctuations are are different from the ab initio results. This is ascribed to different settings of the thermostat dynamics. The results are plotted in figure 8. 0.8

0.8

Ab Initio

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Energy(eV)

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Figure 8: Energy variations for both VASP and ReaxFF when ammonia is equilibrated on α-alumina surface. The angle distribution was examined for two triplets of atoms, namely H-N-H and NAl-O, to investigate their difference when ammonia is already adsorbed. The mean angle for H-N-H (see figure 9) is in good agreement between both methods (and also with their equilibrium values as isolated molecules, which are 106.4◦ and 105.5◦ , using VASP and ReaxFF respectively). However their values are distributed somewhat differently. They seem to follow a quite symmetrical distribution when VASP is used. The distribution is 13


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biased towards greater values in the case of ReaxFF. This can be due to a stronger O-H interaction in ReaxFF causing the net broadening of the H-N-H angle. The O-Al-N angle exhibits equilibrium values of 102.5◦ and 115.0◦ for ab initio and ReaxFF respectively, as shown in figure 10. These values differ from the ones shown in figure 3. This is explained by the different coordination of the aluminum atoms when connecting the surface of α-alumina to the molecule than when being on the bare α-alumina surface.

0.1 ReaxFF

Frequency (a.u.)

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Figure 9: NH3 molecules adsorbed on aluminum oxide surface. Comparison of the H-N-H angle distribution in MD runs using ab initio and ReaxFF. 0.06

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Frequency (a.u.)

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Frequency (a.u.)

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0.02

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Figure 10: NH3 molecules adsorbed on aluminum oxide surface. Comparison of the O-Al-N angle distribution in MD runs using ab initio and ReaxFF.

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Molecular adsorption for two amine-containing epoxy precursors was examined for five α-alumina surfaces. These molecules were dimethylamine and diethylenetriamine (DETA). Due to its crystallographic stacking various arrangements are possible for α-Al2 O3 faces in terms of their surface endings: two aluminum terminated (b-Al-O-Al and b-O-Al-Al) and one oxygen terminated (b-Al-Al-O) configuration. Here b symbolizes the bulk. Additionally two modifications were applied to the b-Al-O-Al surface by attaching OH groups to random Al sites producing coverages of 12% and 75% hydroxyl groups. Since full exploration of the energy landscape for each molecule is unfeasible and the relaxation times are of the order of tens of picoseconds we have studied the amine precursor adsorption using only ReaxFF. Equilibrium configurations at 300 K were obtained by using molecular dynamics in the NVT ensemble for each surface and precursor molecule separately during 100 ps, long enough for the energy to relax. After this initial relaxation step, the molecules were moved towards the surfaces at random positions. A small push was given to them by adding a bias velocity towards the surface, to force molecule and surface to come into contact. This translational velocity never exceeded the equivalent of 30 K. Further relaxation of the system was attained after a total time of 200 ps. The adsorption energy is then measured as the difference of the final (adsorbed) and the initial (separated) relaxed energies. The choice for the value of the time step was 0.05 fs, this small value due mainly to the presence of hydrogen in the systems. Adsorption simulations were repeated 10 times for each surface in order to obtain statistical information. The initial position, approach velocity and incident angle were varied randomly in all repetitions. In tables 4 and 5 we show the mean values of the adsorption energies for dimethylamine and diethylenetriamine, together with their standard deviations. Table 4: Adsorption energy (eV) of dimethylamine on five different α-alumina surfaces b-Al-Al-O b-O-Al-Al b-Al-O-Al 12% OH 75% OH −4 6.9±2.4 < 10 0.52±0.10 0.40±0.20 0.06±0.01

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In the case of dimethylamine, we find that it bonds most strongly to the b-Al-Al-O surface, with many different values ranging from 4.5 to 9.0 eV. Such variation indicates a type of non-localized interaction that is consistent with hydrogen bonding. For the bAl-O-Al surface most interaction occurs with nitrogen, since the first oxygen layer is not as exposed as in b-Al-Al-O thus hindering the number of hydrogen bonds. The doubly aluminum-terminated surface presented itself repulsive to the dimethylamine. Besides its vanishing adsorption energy this was evidenced by the average separation distance from the surface 9 ˚ A, more than three times that for the O-terminated surface (≈2.5 ˚ A). Table 5: Adsorption energy (eV) of diethylenetriamine on five different αalumina surfaces b-Al-Al-O b-O-Al-Al b-Al-O-Al 12% OH 75% OH 7.6±4.9 1.3±0.2 0.85±0.03 0.70±0.30 0.13±0.03 Diethylenetriamine adsorption energies for each surface are presented in table 5. Similarly to dimethylamine the strongest interaction occurs with oxygen-terminated surfaces. This time the net energy difference is greater, correlating with the fact that there are more hydrogen atoms present in the molecule. Its standard deviation is also large, accentuating the importance of the orientation when the DETA molecule is adsorbed. Contrary to the dimethylamine molecules there exists a non-negligible adsorption of DETA to the aluminum-terminated surfaces, especially b-O-Al-Al, with a well defined bonding nature (which seems obvious from the small standard deviation). Here the primary amines at the end of the molecule have their nitrogen atoms oriented towards the surface (see figure 11) and are charged more negatively than the secondary amine. Such configuration stabilizes the structure by gaining approximately 1.3 eV. For b-Al-O-Al analogous configurations are encountered but in this case the adsorption energy is smaller due to the greater separation with the second aluminum layer. The small role that secondary amines plays in adhesion of both dimethlyamine and the central amine group in diethylenetriamine can be explained in part by the different charge transferred to its nitrogen. ReaxFF predicts a more basic character for primary than for 16


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Figure 11: DETA molecule adsorbed by two different surface terminations. In the case of b-Al-Al-O surfaces (left) the molecule binds mainly by directing the hydrogen atoms towards it. For aluminum terminated ones (right) adsorption via Al-N interactions is favored. secondary amines. Steric effects are also important. Secondary amines have their motion greatly affected by the presence of more than one massive atom (e.g. carbon), an effect which tends to be less pronounced in primary amines. Finally, hydroxylation of the b-Al-O-Al surface is found to follow a linear reduction effect on the mean molecular bonding energy U . For both molecules we find U (Θ) = U (0)(0.97 − γΘ) with Θ the fractional coverage and γ = 1.22 (see figure 12).

Conclusions We have developed a ReaxFF description of the aluminum-nitrogen bonding suitable for amine-containing epoxy precursors on alumina. The equilibrium configurations and energetics are well reproduced by ReaxFF and consistent with density functional theory calculations. Reaction barriers are reproduced within 10% as well. Equilibration and adsorption of ammonia molecules on aluminum-terminated (0001) αalumina by molecular dynamics have shown satisfactory agreement with ab initio simulations 17



1.1

Dimethylamine Diethylenetriamine

1 0.9 0.8 U/U(0)

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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

OH fraction

Figure 12: Normalized energy (U/U(0)) as a function of the OH coverage at the b-Al-OAl surface for adsorption of dimethylamine and diethylenetriamine molecules. U(0) is the energy at zero coverage. Uncertainties where taken as one standard deviation resulting from the statistical sampling. taking into account that the fitting was done for molecular configurations at 0 K. ReaxFF was also used to bring insight into the initial adsorption of amine-containing epoxy precursors on alumina surfaces. We show how the different coordinations of nitrogen in amine groups affect adhesion. This work opens the possibility of studying curing or deposition of epoxies on alumina surfaces. Since water has been inherited from the aluminum-water force field in ReaxFF, it is also possible to get insight into the role that previously deposited water can have on adhesion. Chemical reactions where bonds are constantly broken and formed lie within the core of ReaxFF. Thus exploring the different mechanisms for the onset of mechanical failure in larger structures becomes attainable. In future work rather complex polymer matrices will be built and their adhesion to alumina surfaces will be characterized. Using the current parametrization it is possible to study different types of interfacial adhesion between epoxy and alumina from purely nonbonded to covalently-bonded surfaces. This will be the subject of forthcoming reports.

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Acknowledgment This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organization for Scientific Research (NWO) and partly funded by the Ministry of Economic Affairs, Agriculture and Innovation. The authors thank Adri C. T. van Duin for the assistance in the fitting, and also for the discussions and suggestions when preparing the training set.

Supporting Information Available The parameter file resulting from the fitting and used for obtaining results within this work can be found as supplementary information. Also, a file with the mathematical description of the potential functions involving parameters that were fitted in this work has been added. This material is available free of charge via the Internet at http://pubs.acs.org/.

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