Role of Edge Oxygen Atoms on the Adhesive Interaction between

Nov 8, 2013 - Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface ...
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Role of Edge Oxygen Atoms on the Adhesive Interaction between Carbon Fiber and Epoxy Resin Takayuki Semoto, Yuta Tsuji, Hiromasa Tanaka, and Kazunari Yoshizawa* Institute for Materials Chemistry and Engineering and International Research Center for Molecular System, Kyushu University, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: A mechanism of the adhesion between carbon fiber and epoxy resin is studied by using density functional theory (DFT) calculations. Surface structures of carbon fiber were modeled by the armchair-edge structure of graphite functionalized with OH and COOH groups. DFT calculations were performed to construct two realistic models of adhesion interface consisting of the functionalized carbon surface and a fragment of epoxy resin. Adhesive properties of the model interfaces were evaluated based on the binding energy (Eb) between the carbon surface and the resin as well as the maximum adhesive force (Fmax) acting at the interface. Calculated values of Eb are 13.8 kcal/mol for the OH-functionalized surface and 19.1 kcal/mol for the COOHfunctionalized surface. The binding energy per hydrogen bond is calculated to be 6.9 kcal/mol (OH model; two H-bonds) and 6.3 kcal/mol (COOH model; three H-bonds), both of which are virtually similar and reasonable for the bond energy of a typical OH···O hydrogen bond. Analysis of adhesive force−displacement curves derived from energy−displacement plots revealed that Fmax is 0.52 nN for the OH model and 0.70 nN for the COOH model. Calculated adhesive properties are in good agreement with those previously reported for the interface between an aluminum oxide surface and an epoxy resin [J. Phys. Chem. C 2011, 115, 11701], strongly suggesting that hydrogen bonds between the oxygen-containing functional groups play a crucial role in the adhesive interaction in the carbon fiber/epoxy resin system.

1. INTRODUCTION Carbon-fiber-reinforced polymer composites have been widely used in many fields, such as the aerospace industry, transportation, civil construction, and sports equipment. In the composites, carbon fibers are impregnated in a polymer matrix composed of epoxy resin.1−5 Mechanical properties of carbon-fiber-reinforced polymer composites are mainly controlled by interfacial adhesion between the carbon fibers and the polymer matrix used. In general, a strong interfacial adhesion gives rigid composite materials. A number of experimental efforts2−15 have been devoted to strengthen adhesive interactions between carbon fiber and epoxy resin. These experimental works shed light on surface chemical structures of carbon fiber and demonstrated the importance of surface modification to achieve a strong interfacial adhesion. Surface modifications such as by plasma treatment,6−8 ozone treatment,3 electrochemical oxidation,9,10 and chemical oxidation11,12 partially transform surface carbon atoms into oxygencontaining functional groups such as hydroxyl (OH), carboxyl (COOH), ether (C−O−C), and carbonyl (CO) groups. After the modification processes, hydrophobic carbon fibers acquire surface hydrophilicity for enhanced adhesive properties to epoxy resin. For the understanding of adhesion phenomena, mechanical interlocking theory, 16,17 diffusion theory, 18 electronic theory,19−21 and adsorption theory22 have been proposed so © 2013 American Chemical Society

far. While application of the former three theories is limited to some particular systems, adsorption theory predicated on chemical bonding and intermolecular interaction is applicable for various adhesion systems. We have recently investigated the mechanism of adhesion between aluminum oxide surface and epoxy resin using density-functional-theory (DFT) calculations.23−26 In these studies, we constructed realistic surface models consisting of an aluminum oxide surface functionalized with OH groups and elucidated molecular interactions at the adhesion interface. Good agreement between calculated and measured adhesive forces suggests that hydrogen bonds between the oxygen-containing functional groups at the interface play a dominant role in the adhesion. The mechanism of adhesion based on hydrogen bonding should be applicable for understanding the adhesion between carbon fiber and epoxy resin since its adhesive property is improved by functionalization of the carbon-fiber surface with oxygen-containing functional groups. In the present study we theoretically investigate the adhesive interactions between carbon fiber and epoxy resin by constructing realistic models of the adhesive interface. A plausible mechanism of the adhesion will be Received: August 5, 2013 Revised: November 6, 2013 Published: November 8, 2013 24830

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Figure 1. Proposed chemical structures of carbon-fiber surface after surface modification taken from (a) ref 11 and (b) ref 15.

Figure 2. Structures of the (a) zigzag-edge and (b) armchair-edge surfaces of graphene.

local density approximation (LDA) with the Ceperley−Alder exchange-correlation potential as parametrized by Perdew and Zunger30 and the Perdew−Burke−Ernzerhof form of generalized gradient approximation31 (GGA). The bulk structure of carbon fiber was modeled by stacked graphene sheets. The unit cell of graphite is optimized by using the LDA and GGA methods. The plane-wave basis set energy cutoff was set to be 380 eV, and a k-point set of 7 × 7 × 2 was used. Electron−ion interactions were treated with ultrasoft pseudopotentials. Graphite has a hexagonal crystal structure, and thus the lattice parameters are given by a and c. The a axis is in the plane of the graphene sheets. The c axis is normal to the plane with the ABAB stacking sequence, where the interlayer distance between the graphene sheets is c/2. Calculated values of a are 2.44 Å by LDA and 2.46 Å by GGA, both of which are consistent with the experimental value (2.46 Å).32 Optimized values of c are 6.71 Å by LDA and 10.23 Å by GGA. While the LDA method well reproduces the experimental value (6.70 Å),32 the GGA method overestimates the interlayer distance of the graphene sheets. This tendency was reported in a previous DFT study.33 Thus, in the following calculations, we adopted a graphite unit cell optimized with the LDA method. The unit cell of graphite was cleaved at the (110) face (armchair-edge surface), and 15 Å thickness of vacuum layer was added to build a slab model. The carbon atoms at the edge surface were capped by hydrogen atoms. A 1 × 3 × 1 supercell of which the volume is 6.74 × 12.68 × 24.30 Å3 was used. As described before, on the edge surface of carbon fiber, there should be oxygen-containing functional groups such as OH, COOH, C−O−C, and CO groups. While both OH and COOH groups can work as a hydrogen donor and a hydrogen acceptor, the C−O−C and CO groups can work only as a hydrogen acceptor. Thus, the OH and COOH groups should contribute to the formation of hydrogen bonds with epoxy resin to a larger extent than the C−O−C and CO groups. In this study, we will focus on the OH- and COOH-functionalized surfaces. To model an OH-functionalized surface, four hydrogen atoms on the armchair-edge surface were replaced

proposed on the basis of the evaluation of the binding energy and adhesive force at the interface.

2. METHOD 2.1. Modeling of Carbon-Fiber Surfaces. The surface environment of carbon fiber has been experimentally characterized by X-ray photoelectron spectroscopy (XPS) and FT-IR measurements,8,11,12,15 which provide useful information on the concentration of functional groups distributed on the surface. Figure 1 shows possible surface chemical structures proposed in the literature.11,15 It is difficult to construct a computational model of carbon-fiber surfaces because their atomic geometries cannot be described in a uniform way. We have built some realistic models of a carbon-fiber surface consistent with the experimental findings. Carbon fibers usually contain more than 90 wt % carbon atoms; its chemical structure can be treated as that of bulk graphite consisting of stacked graphene sheets. In the present study, the bulk structure of carbon fiber was modeled by that of graphite. As possible candidates of the interaction site with epoxy resin, a basal face, a zigzag edge, and an armchair edge of graphite are considered at first. The basal face [(001) face] is the plane of a graphene sheet composed of the π-conjugated system of carbon atoms. This face is hydrophobic and inert, exhibiting weak adhesive properties to epoxy resin. The structures of the zigzag edge [(100) face] and armchair edge [(110) face] surfaces are shown in Figure 2. The relative stability of these edge surfaces is understood from their electronic structures. The armchair-edge structure has a large band gap compared with the zigzag-edge one, and thereby the armchair-edge structure is energetically more stable. We thus adopted the armchair-edge surface of graphite as an interaction site with epoxy resin. The edge surface model employed in the present study is applicable for a polyacrylonitrile-based carbon fiber that exhibits either edge or basal face of graphene sheets for outcropping the external surface of the fiber.27 All calculations were performed with the CASTEP28 software package of Materials Studio 5.5.29 The functionals used here are 24831

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based structures were optimized with the GGA method with 260 eV of the plane-wave energy cutoff and a 1 × 1 × 1 k-point set. Single-point energy calculations were carried out at the optimized geometries with a higher level of theory, 380 eV of the plane-wave energy cutoff, and a 2 × 1 × 1 k-point set. Finally, for a quantitative evaluation of adhesive properties, the lowest-energy structures of the two interface models were reoptimized with 380 eV of the plane-wave energy cutoff and a 2 × 1 × 1 k-point set. These structures were adopted as adhesion interface models in the following calculations. It is known that DFT methods cannot reasonably treat van der Waals force and hydrophobic interaction in general. Although these interactions could play a role at an adhesive interface, we expect that the hydrogen bond is much stronger than them in the present system.38 2.2. Calculations of Adhesive Properties. The binding energy Eb between a carbon fiber and an epoxy resin is obtained as follows

by OH groups, as shown in Figure 3a. A COOH-functionalized surface was also built in a similar manner, as shown in Figure

Figure 3. Periodic models of carbon-fiber surface functionalized with (a) OH groups and (b) COOH groups (white, H; red, O; green, C).

3b. The concentration of OH and COOH groups in these models is calculated to be 4.68 nm−2. The surface models thus constructed were optimized with the GGA method with 380 eV of the plane-wave energy cutoff and a 2 × 1 × 1 k-point set. The OH and COOH groups, the terminal H atoms, and the armchair-edge carbon atoms were relaxed, while the other carbon atoms were fixed in the geometry optimization. After the optimization, the vacuum layer was removed, and an enlarged 30 Å thick vacuum layer was added to ensure enough space for the following calculations. The heights of the cell along the c axis are 40.22 and 41.37 Å for the OH- and COOHfunctionalized surfaces, respectively. 2.2. Modeling of the Adhesion Interface. Figures 4a and 4b show a polymer chain of diglycidylether of bisphenol A (DGEBA) and a model molecule of the polymer chain employed for the present calculation, respectively. This model molecule contains two benzene rings, two ether groups, and one OH group. Methyl groups were omitted here because they do not interact with a hydrophilic surface, which was demonstrated by molecular dynamics (MD) simulations of a polymerized DGEBA on an aluminum oxide surface in our previous study.24 The fragment was randomly located above the graphite surface, and then MD simulations were performed to prepare 10 initial structures of the adhesion interface for geometry optimization. The MD calculations were carried out with the Discover program of Materials Studio 4.234 with the COMPASS force field.35−37 In all the MD simulations, the system was kept at 300 K by using an NVT ensemble with the Nosé−Hoover thermostat. Total dynamics time was set to be 100 ps with a time step of 1 fs. The atoms of the functionalized surfaces were fixed except for the OH and COOH groups, the terminal H atoms, and the armchair-edge C atoms. The MD-

E b = Eepoxy + ECF − E(epoxy + CF)

(1)

where Eepoxy and ECF are the total energies calculated for the fragment of epoxy resin and the model of the carbon-fiber surface, respectively. E(epoxy+CF) is the total energy of the whole system. The binding energy is interpreted as the energy required for breaking all adhesive interactions at the interface, and thus Eb corresponds to the magnitude of the interaction strength. The adhesive force F is calculated as follows F=

dE dΔr

(2)

where Δr is the displacement of the model molecule of epoxy resin from the equilibrium position and E is the total energy of the adhesion system. The value of Δr was varied in the direction perpendicular to the carbon surface from −0.4 to 2.0 Å at an interval of 0.2 Å, and partial optimization was repeated at each step of Δr. In the partial optimization, the OH groups of the epoxy resin, the OH and COOH groups, the terminal H atoms, and the armchair-edge C atoms of the carbon surfaces were relaxed. The total energy obtained for each step of Δr was plotted as a function of Δr. The obtained E−Δr plots were approximated by the Morse potential curve by using the leastsquares method in the range from 0.0 to 2.0 Å, where the Morse potential is written as follows E = De(1 − e−aΔr )2

(3)

where De and a are constant values specific to a system. According to eq 2, the differentiation of eq 3 by Δr gives an F−Δr curve. The maximum value of the F−Δr curve is denoted

Figure 4. Structures of (a) a polymer chain of DGEBA and (b) a fragment of epoxy resin used in the present study. 24832

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Figure 5. Optimized structures of two models of adhesion interface between an epoxy resin and a carbon-fiber surface functionalized with (a) OH groups and (b) COOH groups (white, H; red, O; green, C). Dashed lines represent hydrogen bonds between the surface and the resin, where an O−H interatomic distance shorter than 2.5 Å is regarded as a hydrogen bond.

by Fmax, which is the critical value required to break adhesive interactions between adhesive and adherend. The value of Fmax can be regarded as a theoretical cohesive force or cleavage force, which can correspond to the experimentally observable rupture force. It should be noted that the calculated adhesive force does not consider imperfect adhesion and internal stress in adhesive and adherend. Therefore, it may overestimate the adhesive force, but it is useful for a qualitative understanding of the adhesion mechanism.

DGEBA-based epoxy resin contains two different oxygencontaining functional groups in a repeating unit: an OH group on a methylene carbon atom and an ether group sandwiched between a benzene ring and a methylene group. Owing to the steric hindrance between the benzene ring and the carbon surface, a ″short″ OH group on the carbon surface should make it hard to access the ether O atom without a large distortion of the geometry around the ether O atom. Actually, some interface structures having a hydrogen bond between an OH group on the carbon surface and an ether O atom of epoxy resin were chosen as initial structures for optimization, but they were found to be thermodynamically less stable. 3.2. Adhesive Force Calculation. The adhesive force acting on the lowest-energy structure has been evaluated for each surface model. Figure 6 shows computed E−Δr plots, where E and Δr are relative to the values at the equilibrium structure. The plots are nicely approximated by the Morse potential curves in the range from 0.0 to 2.0 Å (R2 > 0.999). The derivatives of the approximated potential curves shown in Figure 7 provide force−displacement curves at the atomic scale. The COOH-functionalized surface exhibits a stronger adhesive force than the OH-functionalized one. The maximum values of adhesive force (Fmax) are 0.52 nN for the OH model and 0.70 nN for the COOH model. These values are very close to the adhesive force (0.73 nN) obtained for an adhesive interface between an aluminum oxide surface and an epoxy resin,24 indicating that hydrogen bond controls the adhesion in the present system. As expected from the binding energy, the adhesive force also indicates that the COOH-functionalized surface provides a stronger adhesive interaction than the OHfunctionalized one. We here define ΔrF as the displacement where Fmax is given. The values of ΔrF are 0.63 and 0.57 Å for the OH- and COOH-functionalized surface models, respectively (0.57 Å for the aluminum oxide surface).24 The adhesive properties calculated for the two surface models are summarized in Table 1. All the adhesive properties strongly suggest that the adhesion between carbon fiber and epoxy resin

3. RESULTS AND DISCUSSION 3.1. Binding Energy at the Adhesive Interface. Figure 5 shows the lowest-energy structures of the adhesion interface among 10 structures optimized for the OH- and COOHfunctionalized surface models. For the OH-functionalized surface shown in Figure 5a, the OH group in the fragment of epoxy resin forms two hydrogen bonds with two OH groups on the carbon surface. The binding energy Eb at the adhesion interface is calculated to be 13.8 kcal/mol. For the COOHfunctionalized surface shown in Figure 5b, on the other hand, an ether O atom in the epoxy resin also forms another hydrogen bond with a COOH group on the carbon surface, which leads to a larger value of Eb (19.1 kcal/mol) than the OH surface model. The binding energy divided by the number of hydrogen bonds is 6.9 kcal/mol (OH model; two H-bonds) and 6.4 kcal/mol (COOH model; three H-bonds), both of which are qualitatively similar and reasonable for the bond energy of a typical OH···O hydrogen bond (about 5 kcal/ mol).38 In addition, the bond energy obtained for each model is very close to that reported for an interface model between an aluminum oxide surface and an epoxy resin (5.3 kcal/mol; Eb = 15.7 kcal/mol), in which the adhesive interaction is governed by hydrogen bonds between the oxygen-containing groups at the interface.24 The number of hydrogen bonds observed at the interface, which determines the ″gross″ binding energy, would be dependent on the height of the oxygen-containing functional groups from the carbon surface. As shown in Figure 4, the 24833

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4. CONCLUSIONS We have investigated the mechanism of adhesion between carbon fiber and epoxy resin by DFT calculations. For the construction of a computational model of the adhesive interface, the surface of a carbon fiber was approximated by graphene sheets, and an epoxy resin was represented by a fragment of DGEBA containing one OH group and two ether O atoms. The armchair-edge surface of graphene was chosen as a possible site for the adhesive interaction, and the edge carbon atoms were functionalized with the OH or COOH groups for a realistic description of the carbon-fiber surface. As a result, two models of the carbon-fiber surface were prepared for further computations. DFT calculations with the GGA method were performed to search equilibrium structures of the adhesive interface consisting of the OH-/COOH-functionalized carbon surface and the model molecule of epoxy resin. In the lowestenergy structure of each interface model, the binding energy at the adhesive interface is calculated to be 13.8 kcal/mol for the OH-functionalized surface model and 19.1 kcal/mol for the COOH-functionalized surface model. These binding energies are in good agreement with the value (15.7 kcal/mol) obtained for the adhesion between an aluminum oxide surface and an epoxy resin.24 The adhesive force−displacement curves derived from the energy−displacement plots give the maximum force of adhesion (Fmax) of 0.52 nN for the OH-functionalized surface and 0.70 nN for the COOH-functionalized surface, both of which are also close to the value (0.73 nN) obtained for the aluminum oxide surface. The good agreement in the adhesive properties between the two adhesion systems suggests that the mechanism of adhesion in the present system can be explained in a similar manner as the aluminum oxide system: Hydrogen bonds between the oxygen-containing functional groups at the interface play a dominant role in the adhesive interaction between carbon fiber and epoxy resin.

Figure 6. Energy−displacement plots for the OH- and COOHfunctionalized surface models.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of optimizations and Cartesian coordinate of computational models. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-92-802-2529. E-mail: [email protected]. jp.

Figure 7. Adhesive force−displacement curves for the OH- and COOH-functionalized surface models.

Notes

The authors declare no competing financial interest.

Table 1. Summary of Theoretical Adhesive Properties of the Interface between Functionalized Carbon-Fiber Surfaces and Epoxy Resin model OH COOH aluminum oxide24

Eb/kcal mol 13.8 19.1 15.7

−1

Fmax/nN

ΔrF/Å

0.52 0.70 0.73

0.63 0.57 0.57

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ACKNOWLEDGMENTS

K.Y. thanks Grants-in-Aid for Scientific Research (nos. 22245028 and 24109014) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the Kyushu University Global COE Project, the Nanotechnology Support Project, the MEXT Project of Integrated Research on Chemical Synthesis, the CREST of the Japan Science and Technology Cooperation, and the MEXT Project of Elements Strategy Initiative. T.S. thanks JSPS for a graduate fellowship. Y.T. thanks JSPS Research Fellowship for Young Scientists.

is derived from hydrogen bonds between the functionalized carbon surface and the oxygen-containing group of the resin, which is similar to the aluminum oxide/epoxy resin system. 24834

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