Theoretical Study of the Interaction between Selected Adhesives and

Adhesion of polyurethanes on native metal surfaces – stability and the role of urea-like species. Christophe Nies , Frank Fug , Christian Otto , Wul...
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J. Phys. Chem. B 2005, 109, 5060-5066

Theoretical Study of the Interaction between Selected Adhesives and Oxide Surfaces Thomas Kru1 ger,*,† Marc Amkreutz,† Peter Schiffels,‡ Bernhard Schneider,‡ Otto-Diedrich Hennemann,‡ and Thomas Frauenheim† Theoretical Physics, Faculty of Science, UniVersity of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany, and Fraunhofer-Institut fu¨r Fertigungstechnik und Angewandte Materialforschung, Wiener Str. 12, 28359 Bremen, Germany ReceiVed: NoVember 9, 2004; In Final Form: January 18, 2005

We investigate the competition of the various organic components of two representative adhesive systems for reactive defect sites at model surfaces of both SiO2 and Al2O3. The reaction energies of resin monomers, curing agents, and in some cases also of additional adhesion promoters with the defects are calculated. We applied a density-functional based tight-binding method including a self-consistent correction of the Mulliken charges, which has already proven to be a useful tool for computational materials science, delivering reliable structural and energetic information.

1. Introduction The generation of stable adhesive joints is an essential task of engineering, because there is an increasing trend to replace rivets and weld seams by organic resins. Especially the automobile and the aircraft industries take stock in this evolving technology. Up to now, however, the basic mechanisms of adhesion on an atomistic scale are still more matters of speculation and chemical intuition than of real knowledge. So it is our goal to unveil some of the essential reactions taking place when technical adhesive agents, consisting of various chemical compounds, come into contact with “real” surfaces. It is accepted in general that if two metal or inorganic semiconductor substrates are glued together, then the adhesion zone consists of the resin (polymer) bulk and two interaction layers, so-called interphases, mediating the contact between the resin and the substrates, which in most practical applications are covered by oxide layers. In the interphases, which emerge when the liquid adhesive is applied to the surface, a complicated interplay between the constituents of the adhesive mixture and the oxide surfaces takes place where displacement reactions, mutual interference of reaction rates, and other phenomena also come into play. The overall performance of the adhesion is basically determined by said interaction layers. The adhesive mixtures we are interested in are the well-known and widely used epoxide (EP) and polyurethane system (PUR), respectively. The former consists exemplarily of the diglycidyl ether of bisphenol A (DGEBA), a resin hardener which, e. g., may be maleic acid anhydride (MA) and an additional adhesion promoter (GOTMS). The latter contains as main ingredients a diisocyanate (MDI) and a bifunctional alcohol (TPG) (Figure 1). As substrates, we chose surfaces of SiO2 because of their relevance in microelectronic applications and surfaces of Al2O3. This is due to the fact that the technically available aluminum surfaces that are used in the automobile and the aircraft * Author to whom correspondence should be addressed. Phone: 49-525160-2333. Fax: 49-5251-60-3435. E-mail: [email protected]. † University of Paderborn. ‡ Fraunhofer-Institut fu ¨ r Fertigungstechnik und Angewandte Materialforschung.

Figure 1. Main components of the EP and PUR systems, respectively.

industries are completely oxidized so that in fact the adhesive mixture will react with an alumina surface and not with the pure metal. Most SiO2 surfaces are experimentally quite well-characterized. This, however, does not apply to either nanoporous or amorphous silica. Therefore, in recent years, several theoretical studies have been published. By use of either molecular dynamics (MD) or Monte Carlo simulations, structural information about nanoporous1-3 and amorphous surfaces4-7 has been obtained. “Real” surfaces, however, are always in contact with atmospheric humidity. While the interaction of water with technical silica glasses leads to a strong adsorption of a H2O double layer accompanied by a significant distortion of the hydrogen bond network,8,9 the contact of water with amorphous surfaces affects an actual hydrolysis, because numerous subcoordinated silicon and oxygen atoms are present.10,11 It has

10.1021/jp0448651 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/24/2005

Interaction between Adhesives and Oxide Surfaces been shown12 that H2O can cleave both the oxygen bridge between a 4-fold and a 3-fold coordinated silicon atom and the Si2O2 four-membered ring, a unit which is found frequently on amorphous SiO2 surfaces, thereby yielding two adjacent OH groups. We have concentrated our efforts on models of amorphous, hydroxylated SiO2, because this system offers the broadest spectrum of possible reaction sites. Meanwhile, an enormous number of alumina modifications is known, but only R-Al2O3 is thermodynamically stable. Its simplest nonpolar {0001} surface has been characterized experimentally in detail by Ahn and Rabalais.13 The surface is terminated by an Al layer with 3-fold coordinated Al atoms while O atoms comprise the second layer. The interlayer spacing amounts to only ∼0.3 Å, and the bond lengths essentially are conserved. Hydroxyl groups have been observed on the surface even at 1100 °C. Theoretical calculations are in satisfactory agreement with the experimental findings.14-16 Surface properties of the metastable alumina modifications γ-Al2O3 and κ-Al2O3 have been examined theoretically as well.17,18 Due to its enormous technological relevance, many investigations have been performed regarding adsorption on technical alumina surfaces, demonstrating the pronounced reactivity for H2O and polar organic molecules.19-22 It has been shown that, e. g., carboxylic acids react with surface OH groups forming carboxylates. The bonding of a monofunctional carboxylic acid is not stable in the presence of water, but bifunctional carboxylic acids lead to a stable bidentate conformation if a second OH group is in reach.21 Previous theoretical approaches treat the reaction of small molecules mostly with cluster models of crystalline alumina surfaces. The adsorption of a water molecule on a model of the R-Al2O3 {0001} surface has been investigated by Shapovalov and Truong.23 They obtained two energy minima corresponding to molecular adsorption and dissociative hydroxylation with reaction energies of -23.4 and -31.6 kcal/mol, respectively, in agreement with experimental observations. Also, results obtained for the reactions of methanol, H2O, H2S, and CO with γ-Al2O3 match the experimental findings.24,25 Because of the eminent progress of surface analytic methods in the past years, precise investigations of the interaction between adhesives and surfaces became possible. Dieckhoff, e. g., characterized the adhesion of organic molecules containing triazine and cyanate groups on the natively oxidized Si {100} surface,26 while Schneider investigated the reaction between maleic acid anhydride and natively oxidized aluminum substrates.27 Both works emphasize the necessity to complement the experimental results with theoretical calculations to obtain a deeper understanding of the processes in the interaction layers. 2. Methods 2.1. DFTB. For the treatment of both the inorganic and the organic components, we used the DFTB method, which is based upon a density-functional tight-binding approximation. The basic idea is as follows. Starting from the Kohn-Sham total energy

where µ ) nv - nV is the magnetization density; we represent the total density n as the sum of a reference density n0 and fluctuations δn around n0. Then the exchange-correlation energy

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5061 is expanded around n0 at µ ) 0 up to the second order in δn, and we finally obtain occ

Etotal ) 1 2

∑ i,σ

〈ψiσ|H ˆ 0|ψiσ〉 -

∫∫

[

1

+

n0n′0

1

db r db′ r + Enn + ∫∫ |br - b′| 2 r

|]

δ2Exc[n0,0]

|b r - b′ r |

δnδn′

n0

Exc[n0,0] -

δnδn′ db r db′ r +

∫ Vxc[n0,0] n0 dbr

(2)

where all one-electron terms as well as those Coulomb and exchange-correlation (xc) terms, which depend on n0 only, are merged in H ˆ 0. The Coulomb-like second term and the last term correct for the double counting and are combined with Exc[n0,0] and Enn to a repulsive potential Erep. Now, the following approximations are made: • Erep is determined as a sum of interatomic two-body potentials that are obtained by a fit on small molecules. • The molecular orbitals are expanded in a minimal set of atomic valence orbitals {φ} that are obtained by a superposition of up to five Slater-type functions per orbital. ˆ 0pq ) • The one-center terms H ˆ 0pq are approximated by H p δpq, where p is the energy corresponding to the atomic orbital φp in a pseudoatom calculation. • The two-center terms (φp on atom j, φq on atom k) are calculated according to

〈|

H ˆ 0pq ) φp -

|〉

∇2 + Veff[n0j + n0k ] φq 2

where Veff is the effective Kohn-Sham potential depending on the superposed densities of the neutral pseudoatoms j and k. • All integrals are tabulated by use of the Slater-Koster representation. • The energy correction in the second order is determined self-consistently

E(2) )

1 2

γjk∆qj∆qk ∑ j, k

where γii is the Hubbard parameter of atom i. ∆qi describes the change of the Mulliken charge at i; i.e., we have replaced the density fluctuations by charge differences. The inclusion of this correction into the DFTB scheme is denoted as self-consistentcharge DFTB (SCC-DFTB). The details of this method, which has proven extremely useful and reliable in a broad variety of applications from defects in solids to ion transport in proteins, can be found in the literature.28,29 We employ our own code, which is available on request. 2.2. Generation of Model Structures. The model of the R-SiO2 surface has been prepared in the following way. We start from a stoichiometrically composed R-Cristobalite-type crystal consisting of 51 SiO2 units with periodic boundary conditions in all three dimensions. In a MD simulation, this bulk model is heated to 2000-6000 K while expanding the volume by a factor of 1.5-2.0 until any crystal information is lost. By subsequent compression (final density 2.6 g/cm3) and cooling to ambient temperature, an amorphous cell is obtained where, after cleaving it into halves, the cell axis perpendicular to the cutting area is stretched to about 100 Å. Final relaxation yields the model in the form of a slab with two surface segments

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Kru¨ger et al.

Figure 2. Model of the partially hydroxylated R-SiO2 surface, side view (gold, Si; red, O; white, H).

of about 14 × 14 Å2 each (only one of them is used for the reactions in question). The details of this procedure can be found in ref 30. The partial hydroxylation of the surface was achieved by adding three H2O molecules to the surface and performing a complete geometry optimization. Two of the molecules were chemisorbed while only the third one dissociated, forming two OH groups (site 1 and site 2) on the surface. Although the chemisorbed molecules may play a certain role in the series of possible reactions, we have removed them in order to keep the situation clear. The consideration of chemisorbed H2O will be the topic of a subsequent paper. After removal of the molecules and a further geometry relaxation, the final model shown in Figure 2 was obtained. It has an hydroxylation degree of about 1 per 100 Å2, which is well below the value of 4.6 derived from the assumptions of the Zhuravlev model,31 giving us the possibility to study the reactions without being disturbed by adjacent groups. The energy gain of the hydroxylation amounts to 101.1 kcal/mol. The main difference between the two sites is that in site 1 the hydrogen atom forms an additional bridge to another oxygen whereas in site 2 this is not the case. The Al2O3 surface we used was obtained in a similar manner without, however, the amorphization steps mentioned above. (With the exception of R-Al2O3, all other alumina modifications show a considerable degree of disorder in the vacancy distribution, which makes it difficult to distinguish them from the amorphous oxide if a model of only a few hundred atoms is used. We therefore decided to concentrate our efforts on the crystalline R-Al2O3 at first.) It represents the oxygen terminated cut of R-Al2O3 along [0001] with subsequent complete hydrogenation of the surface oxygen atoms (Figure 3); i.e., in contrast to the SiO2 case, we now employ an OH group saturated surface, because the higher polarity of Al2O3 compared to that of SiO2 will lead to a pronounced attachment of H2O. We have chosen only one of the central sites for the reactions with the organic compounds, because they are quite similar both sterically and with respect to the Mulliken charge distributions. Again details can be found elsewhere.30 The results of the reactions have been obtained by modeling the possible products and, consecutively, complete geometry optimization using the conjugate gradient approach implemented in our DFTB code. 3. Results 3.1. Reactions with the R-SiO2 Surface. 3.1.1. Components of the EP System. The resin monomer of the EP system, DGEBA, will react with a Sisurface-OH unit predominantly by additive opening of one of the oxirane rings, forming Sisurface-

Figure 3. Model of the completely hydrogenated R-Al2O3 [0001] surface, side view (blue, Al; red, O; white, H).

TABLE 1: Geometry of the DGEBA Surface Bridgea parameter bonds angles dihedrals a

Sisurface-O O-CH(-CH2-OH) CH(-CH2-OH) -R Sisurface-O-CH(-CH2-OH) O-CH(-CH2-OH) -R Sisurface-O-CH(-CH2-OH) -R

site 1

site 2

1.617 1.443 1.521 127.2 105.6 167.6

1.626 1.431 1.526 122.0 112.0 150.7

Distances are given in Å, angles in deg.

O-CH(-CH2-OH)-R where R here denotes the remnant of the DGEBA moiety. The geometric parameters of this bridge are quite independent of the reaction site as can be seen from Table 1. Figure 4 shows the molecule docking at site 2 of the R-SiO2 cell. The five-membered ring of maleic acid anhydride will be cleaved by reaction with Sisurface-OH to yield the acid Sisurface-O-C(dO)-CHdCH-COOH. Some geometry information can be found in Table 2. In contrast to the previous case, the reaction products with MA differ significantly from each other. While at site 2 a simple addition of MA has taken place, at site 1 further effects can be observed. The carbonyl carbon is coordinated to another silicon atom of the surface, and the CdO distance indicates more a single than a regular double bond. In fact, the carbonyl oxygen is bound to a further silicon atom, thereby anchoring the MA unit efficiently on the surface. It is generally assumed that the adhesion promoter GOTMS is already completely hydrolyzed to GOTHS before it can attack a surface OH group. This reaction can proceed both as a condensation and as an oxirane ring opening. We therefore have investigated both processes with both possible reaction sites. The condensation with site 1 first led to a partial segmentation of the adhesion promoter, representing a high-lying minimum of the potential energy surface, which, however, will not be populated because of the enormous endothermicity of this reaction (+95.9 kcal/mol). In contrast, a slight distortion of the initial structure allows the expected product to be achieved. At site 2, the structure Sisurface-O-Si(OH)2-(CH2)3-O-CH2oxirane is formed immediately. Some geometric data are given in Table 3. By comparison of sites 1 and 2, the large differences between both the Sisurface-O-Si bond angle and the Sisurface-

Interaction between Adhesives and Oxide Surfaces

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5063 TABLE 4: Additive Ring Opening of GOTHSa parameter bonds angles dihedrals a

Sisurface-O O-CH(-CH2-OH) CH(-CH2-OH)-CH2 Sisurface-O-CH(-CH2-OH) O-CH(-CH2-OH)-CH2 Sisurface-O-CH(-CH2-OH)-CH2

site 1

site 2

1.622 1.438 1.544 139.0 109.4 112.4

1.619 1.441 1.526 124.8 108.8 165.5

Distances are given in Å, angles in deg.

TABLE 5: Reaction energies ∆E (kcal/mole)

Figure 4. DGEBA docking at site 2 of the R-SiO2 cell, side view (gold, Si; red, O; black, C; white, H).

TABLE 2: Reaction Products of Surface + MAa parameter bonds

angles dihedrals a

Sisurface-O O-C(dO) CdO C(dO)-CHd Sisurface-O-C(dO) O-C(dO)-CHd Sisurface-O-C(dO)-CHd

site 1

site 2

1.630 1.473 1.425 1.500 144.9 114.1 132.5

1.672 1.383 1.222 1.480 119.7 114.8 179.7

Distances are given in Å, angles in deg.

TABLE 3: Condensation of GOTHS with Sites 1 and 2a parameter bonds angles dihedrals a

Sisurface-O O-Si Si-CH2 Sisurface-O-Si O-Si-CH2 Sisurface-O-Si-CH2

site 1

site 2

1.601 1.622 1.862 160.9 121.5 120.5

1.608 1.636 1.873 135.6 115.7 162.6

Distances are given in Å, angles in deg.

O-Si-CH2 dihedral angle are eye-catching. At site 1, the distortion relative to site 2 enables the formation of a hydrogen bridge and an additional bond between a GOTHS oxygen and an undercoordinated surface silicon atom. In this way, a remarkable stabilization of the reaction product is obtained. The additive ring opening leads to the chain Sisurface-O-CH(-CH2OH)-CH2-R. Some geometric parameters differ significantly depending on the reaction site (Table 4). In the case of site 1, the dihedral angle is relatively low, indicating that the GOTHS moiety is bound toward the surface, whereas the high dihedral angle in the case of site 2 shows that here the moiety extends into the free space above the surface. Thus, in case 1, four hydrogen bridges can emanate from the Si(OH)3 and the CH2OH unit, respectively, to further stabilize the reaction product, while in case 2 only one hydrogen bridge is formed. Therefore, assuming a mean bond energy of 7-8 kcal/mol per hydrogen

reaction of the R-SiO2 surface with

site 1

site 2

GOTHS (condensation) GOTHS (additive ring opening) MA DGEBA

-105.3 -71.2 -40.2 -35.4

-18.0 -45.8 -6.3 -41.6

bridge, the GOTHS attack at site 1 should be energetically favored by about 21-24 kcal/mol. The energies of all reactions described above can be found in Table 5. Obviously both the condensation and the addition of GOTHS to the surface, combined with the opening of the oxirane ring, are the preferred reactions within the EP system, and it depends on the specific structure of the reactive site which product is more favorable. This is a remarkable fact, because over a long time it was taken for granted that only the condensation reaction takes place, yielding a siloxane bridge. In the case of the condensation reaction, the enormous energy difference between site 1 and site 2 can be explained by the fact that an additional, stretched O-Si bond between a GOTHS oxygen and an undercoordinated silicon atom is formed. Note that the reaction of the radicals SiH3• and OH• to silamethanol is exothermic by about 130 kcal/mol.32 In the case of the addition reaction, the energy difference between the sites matches the amount of energy obtained by three hydrogen bridges (21-24 kcal/mol); i.e., the reaction energy depends strongly on the possibility of further stabilization of the product by formation of O-H‚‚‚O bridge bonds. The addition of DGEBA, which is structurally analogous to the GOTHS addition, can compete against the addition of the adhesion promoter. The hardener maleic acid anhydride is very site sensitive. If the possibility to attract additional coordination partners for the carbonyl group is given, then the reaction energy can attain values comparable to those of the other components in the EP mixture. We conclude that the overall energy of said reactions depends not only on the intrinsic bond breaking and bond forming processes but also on the possibility of a further stabilization of the generated entity by hydrogen (or other dipolar) bridges. 3.1.2. Components of the PUR System. The PUR system consists essentially of an isocyanate (MDI) and an alcohol (TPG). However, the reactivity of the isocyanate moiety with respect to a Sisurface-OH group is orders of magnitude larger than that of the alcohol so that isocyanate and alcohol will not compete for the surface sites. TPG will only lower the overall concentration of MDI so that a separate analysis of the reaction of TPG + surface is not necessary. MDI reacts with a surface OH group under formation of a urethane Sisurface-O-C(dO)-NH-R. The geometry data of this moiety can be found in Table 6, and the reaction product with site 2 is shown in Figure 5. First of all, the facts deserve attention that (i) the carbonyl bonds are stretched and that (ii) the SisurfaceO-C(dO) angles are decreased. This indicates the presence of an additional coordination partner of the carbonyl oxygen stabilizing the whole arrangement. It can be seen in Figure 5

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Kru¨ger et al.

TABLE 6: Urethane Formationa bonds

angles dihedrals a

parameter

site 1

site 2

Sisurface-O O-C(dO) CdO C(dO)-NH NH-R Sisurface-O-C(dO) O-C(dO)-NH C(dO)-NH-R Sisurface-O-C(dO)-NH O-C(dO)-NH-R

1.735 1.433 1.407 1.445 1.417 92.9 116.3 118.5 139.3 90.0

1.766 1.326 1.280 1.365 1.431 95.3 123.7 122.5 175.7 7.8

Distances are given in Å, angles in deg.

Figure 6. DGEBA docking at the R-Al2O3 cell, side view (blue, Al; red, O; black, C; white, H). Figure 5. MDI docking at site 2 of the R-SiO2 cell, side view (gold, Si; red, O; black, C; green, N; white, H).

TABLE 7: Geometry of the DGEBA Surface Bridgea parameter

that said partner is the surface silicon atom originally bearing the reactive OH group; i.e., a skewed four-membered ring has been formed. This central unit is the same for both sites so that no relevant difference of the reaction energies is to be expected. In fact, they amount to -27.8 and -25.6 kcal/mol, respectively. The O-C(dO)-NH-R dihedral angles show how the orientation of the two benzene rings coupled via a CH2 link changes with respect to the surface if we go from site 1 to site 2, but obviously this change has no significant influence on the reaction energy. 3.2. Reaction with the r-Al2O3 Surface. 3.2.1. Components of the EP System. Also, in the case of the completely hydroxylated alumina surface, the resin monomer DGEBA will react by additive opening of an oxirane ring forming Alsurface-OCH(-CH2-OH)-R. Apart from the Alsurface-O bond, which of course is much longer than the corresponding Sisurface-O bond, the geometry parameters are close to those in the silica case (Table 1). However, due to many hydrogen-hydrogen repulsions, the main part of the DGEBA moiety is directed more pronouncedly perpendicular to the surface (Figure 6). The reaction is about 7 kcal/mol more exothermic than that in the silica case (Table 5). The five-membered ring of maleic acid anhydride will be cleaved by reaction with Alsurface-OH to yield the corresponding acid. The most important geometry data can be found in Table 8. The comparison to the silica case deserves special attention. The similarity of the bond lengths of the alumina product and the silica site 2 (Table 2) indicate the presence of essentially

bonds angles dihedrals a

Alsurface-O O-CH(-CH2-OH) CH(-CH2-OH)-R Alsurface-O-CH(-CH2-OH) O-CH(-CH2-OH)-R Alsurface-O-CH(-CH2-OH)-R

1.986 1.497 1.520 126.2 106.2 152.4

Distances are given in Å, angles in deg.

TABLE 8: Reaction Product of Surface + MAa parameter bonds

angles dihedrals a

Alsurface-O O-C(dO) CdO C(dO)-CHd Alsurface-O-C(dO) O-C(dO)-CHd Alsurface-O-C(dO)-CHd

1.944 1.388 1.242 1.485 131.5 115.4 33.9

Distances are given in Å, angles in deg.

the same bonding situation. The low dihedral angle, however, shows that here a cis arrangement of the Alsurface-O-C(dO)CHd chain is realized. This is easy to understand because it allows for an additional substantial energy gain by the formation of three hydrogen bridges (Figure 6). In analogy to the silica surface, the reaction with GOTHS can proceed in two different ways, of which the resulting geometries are given in Table 9. In contrast to the silica case, however, the energetic order of the products is different (Table 10). The essential geometric difference between the two products consists of the orientation of the main part of the GOTHS moiety. While for the condensation product, a trans arrangement

Interaction between Adhesives and Oxide Surfaces

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5065

TABLE 9: GOTHS at the r-Al2O3 Surfacea parameter b: b: b: a: a: d: a

condensation

Alsurface-O O-Si Si-CH2 Alsurface-O-Si O-Si-CH2 Alsurface-O-Si-CH2

parameter

1.937 1.652 1.884 126.8 112.9 175.7

b: b: b: a: a: d:

addition 1.926 1.512 1.515 121.6 107.0 84.3

b ) bond, a ) angle, d ) dihedral. Distances are given in Å, angles in deg.

TABLE 10: Reaction Energies ∆E (kcal/mol) reaction of the surface with GOTHS (condensation) GOTHS (additive ring opening) MA DGEBA

-80.6 -33.6 -46.6 -48.7

TABLE 11: Urethane Formationa parameter bonds

angles dihedrals a

Alsurface-O O-CH(-CH2-OH) CH(-CH2-OH)-CH2 Alsurface-O-CH(-CH2-OH) O-CH(-CH2-OH)-CH2 Alsurface-O-CH(-CH2-OH)-CH2

Alsurface-O O-C(dO) CdO C(dO)-NH NH-R Alsurface-O-C(dO) O-C(dO)-NH C(dO)-NH-R Alsurface-O-C(dO)-NH O-C(dO)-NH-R

2.003 1.397 1.246 1.392 1.411 132.3 112.5 124.6 170.8 163.3

Distances are given in Å, angles in deg.

is preferred, which compels the organic unit out of the surface, for the addition product a twisted geometry is realized because of the required space for the CH2-OH group. Comparing the energies of the reactions mentioned above we obtain the following. In contrast to the silica case, the formation of an Alsurface-O-Si bridge by condensation is unambiguously the thermodynamically most favored reaction. None of the other possible reactions can compete with it unless supported by kinetic factors. So it can be assumed that in the case of an hydroxylized alumina surface the additional adhesion promoter in fact acts as it has been expected. 3.2.2. Components of the PUR System. The argument given in subsection 3.1.2 holds here as well so that we may concentrate our considerations on the reaction of MDI with the surface. The geometry data of the emerging urethane moiety are given in Table 11. Comparing the Al-O bond lengths with MA and GOTHS, we see an increase of about 0.06-0.08 Å. This is due to the fact that the urethane group is directly connected with the first benzene ring of the MDI moiety, which has a higher space requirement than the that of the linear fragments in the cases of MA and GOTHS. Also, the prolate Al-O-C angle is a means to increase the distance between the surface terminating OH groups and the first benzene ring, and even the urethane chain adopts an approximate trans orientation for this purpose. Moreover, the product is stabilized by two additional hydrogen bridges so that the high reaction energy of -57.9 kcal/mol is reasonable. 4. Summary We have investigated the reactions of the main components of the industrially relevant adhesive mixtures EP and PUR with (i) an amorphous, partially hydroxylated SiO2 surface and (ii) a completely hydroxylated surface of R-Al2O3 by means of the SCC-DFTB method. The following essential results have been obtained:

• The overall energy of said reactions depends not only on the intrinsic bond breaking and bond forming processes. The possibility of a further stabilization of the generated entity by hydrogen or other dipolar bridges is of decisive importance as well. • On partially hydroxylated SiO2, the addition of the adhesion promoter GOTHS under opening of its oxirane ring can thermodynamically compete with the condensation of two silanol groups. This finding disproves previous assumptions that the adhesion promoter always reacts by condensation, forming a siloxane bridge between the surface and the organic moiety. • Depending on the reaction site, the attack of other ingredients such as the resin monomer (DGEBA) and the hardener (MA) may be competitive to the attack of the adhesion promoter. • In the case of the PUR system, the urethane formation is accompanied by the development of an additional skewed fourmembered Si-O-C-O ring, further stabilizing the reaction product. • In contrast to the SiO2 case and with consideration of thermodynamics, GOTHS reacts with fully hydroxylated Al2O3 by condensation only. • Other ingredients will be able to compete with GOTHS only if they can benefit from kinetic factors substantially. Acknowledgment. The authors wish to express their warmest thanks to Dr. C. Ko¨hler and Dr. A. T. Blumenau for providing us with the aluminum Slater-Koster files33 and the raw surface models, respectively. Financial support by the Deutsche Forschungsgemeinschaft and the Fraunhofer-Gesellschaft is gratefully acknowledged. References and Notes (1) Beckers, J. V. L.; de Leeuw, S. W. J. Non-Cryst. Solids 2000, 261, 87. (2) Burlakov, V. M.; et al. Phys. ReV. Lett. 2001, 86, 3052. (3) Beckers, J. V. L.; de Leeuw, S. W. Int. J. Inorg. Mater. 2001, 3, 175. (4) Wilson, M.; Walsh, T. R. J. Chem. Phys. 2000, 113, 9180. (5) Roder, A.; Kob, W.; Binder, K. J. Chem. Phys. 2001, 114, 7602. (6) Ceresoli, D.; et al. Phys. ReV. Lett. 2000, 84, 3887. (7) Masini, P.; Bernasconi, M. J. Phys.: Condens. Matter 2002, 14, 4133. (8) Hartnig, C.; et al. J. Mol. Liq. 2000, 85, 127. (9) Gallo, P.; Ricci, M. A.; Rovere, M. J. Chem. Phys. 2002, 116, 342. (10) Iarlori, S.; et al. J. Phys. Chem. B 2001, 105, 8007. (11) Litton, D. A.; Garofalini, S. H. J. Appl. Phys. 2001, 89, 6013. (12) Walsh, T. R.; Wilson, M.; Sutton, A. P. J. Chem. Phys. 2000, 113, 9191. (13) Ahn, J.; Rabalais, J. W. Surf. Sci. 1997, 388, 121. (14) Godin, T. J.; LaFemina, J. P. Phys. ReV. B 1994, 49, 7691. (15) Tepesch, P. D.; Quong, A. A. Phys. Status Solidi B 2000, 217, 377. (16) Marmier, A.; Lozovoi, A.; Finnis, M. W. J. Eur. Ceram. Soc. 2003, 23, 2729 (17) Vijay, A.; Mills, G.; Metiu, H. J. Chem. Phys. 2002, 117, 4509. (18) Ruberto, C.; Yourdshahyan, Y.; Lundqvist, B. I. Phys. ReV. B 2003, 67, 195412. (19) Wu, X.; Cong, P.; Mori, S. Appl. Surf. Sci. 2002, 201, 115.

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