Wettability of Al2O3 Surface by Organic Molecules: Insights from

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Wettability of AlO Surface by Organic Molecules: Insights from Molecular Dynamics Simulation Yoshitake Suganuma, Satoru Yamamoto, Tomoyuki Kinjo, Takuya Mitsuoka, and Kazuhiko Umemoto J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07062 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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The Journal of Physical Chemistry

Wettability of Al2O3 Surface by Organic Molecules: Insights from Molecular Dynamics Simulation Yoshitake Suganuma,*,† Satoru Yamamoto,‡ Tomoyuki Kinjo, † Takuya Mitsuoka, † and Kazuhiko Umemoto† †Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan ‡Dassault Systèmes BIOVIA, 2-1-1, Osaki, Shinagawa, Tokyo 141-6020, Japan

ABSTRACT

We use molecular dynamics (MD) simulations to investigate the wettability of Al2O3 (0001) by organic molecules. Diffusion coefficients estimated for organic molecules are clearly correlated with the contact angles observed experimentally. The results of the MD simulations suggest that molecular flexibility influences wettability. In other words, wettability owing to flexible molecules, such as an epoxy tridecamer, improves with increasing temperature because the interaction between the droplet and the surface increases due to changes in molecular conformation. Conversely, for phenylene sulfide tetramer, wettability does not change with temperature because

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of the molecular rigidity. In addition, for epoxy monomers, we analyze the different molecular structures responsible for modifying the droplet–surface interaction. For hydrogens in aromatic rings and in methyl groups, the interaction with the surface clearly decreases with increasing temperature.


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Introduction The control of wettability is a challenging task for industrial processes and applications such as coating, printing, antifouling treatment, and adhesion.1–10 Numerous studies address the question of how chemistry and the topography of solid surfaces affect wettability.11–18 Surface characteristics are important parameters for controlling wettability. In addition, for liquids, several studies report a relation between molecular structure and wettability.19–21 Controlling wettability requires a thorough understanding of the interaction between the liquid and the solid surface, which is also related to molecular conformation.22,23 Recently, molecular dynamics (MD) simulations have been used to study the interactions between droplets and solid surfaces, and the results suggest that MD simulations constitute a powerful method for understanding interactions and conformational dynamics in wetting.24–30 Most such research has focused on the wettability of carbon materials such as graphite and carbon nanotubes by water. However, few studies simulate the process by which organic molecules wet metals and their oxides, which are industrially significant materials. Yet, the conformational dynamics of polymers in the wetting of metals remains poorly understood. To address this issue, we herein report the results of MD simulations for understanding the process of polymer wetting on metals. In addition, the validity of the simulations is confirmed by comparing with experimentally obtained contact angles. We study typical organic molecules such as solvents and oligomers, which are used as model polymers. The solid surface is the Al2O3 (0001) surface, which models the natural oxidation film on aluminum, a typical metal used in industry and an important alternative lightweight metal for iron. Based on the results of the


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simulations, we discuss the interaction between the liquid and the solid surface and conformational dynamics of organic molecules in wetting of the Al2O3 (0001) surface.

Experiment Materials Water was purified with a Milli-Q deionizing system. Diiodomethane, heptadecane, ethylene glycol, and diethylene glycol were purchased from Wako Pure Chemicals Industries Ltd. (Osaka, Japan) and used as received. 1-bromonaphtalene and octylbenzene were obtained from Tokyo Chemical Industry Co. (Tokyo, Japan) and used without further purification. The epoxy oligomers jER 828 and 1009 were obtained from Mitsubishi Chemical Co.(Tokyo, Japan) and used as bisphenol A diglycidyl ether monomer (epoxy monomer) and tridecamer (epoxy tridecamer), respectively, based on molecular weights from catalog value. Phenylene sulfide (PS) tetramer and heptamer were synthesized as model poly-PS compounds.31, 32 A highly polished single crystal (0001) wafer of α-Al2O3 with a rms roughness of about 1 Å, as determined by atomic force microscopy, was obtained from Kyocera Co. (Kyoto, Japan). The wafer was cleaned by following the procedure detailed in previous research.33 The wafer was dipped in 10 mM HNO3 for 30 min and rinsed by Milli-Q water. Next, it was heated to 350 °C in the ambient air for 30 min, washed with Milli-Q water, and finally dried in the ambient air at room temperature.


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Contact-Angle Measurement. Contact angles were measured by using a Dropmaster DM-500 Hi (Kyowa Interface Science Co., Saitama, Japan), which offers both probe and stage heating. Solvents were placed onto an Al2O3 (0001) surface at room temperature (298 K). For epoxy and PS oligomers, contact-angle measurements were done with the probe and stage at 423 and 498 K in addition to room temperature (298 K). Next, measurements were made to determine whether the oligomers were in the molten state, and, if so, the contact angles were measured at those temperatures. The droplet volume was about 1 μL, which is sufficiently small to neglect gravity.34 The contact angles were measured only after the droplets had reached the steady state on the substrate. The contact angles were determined by using a contour curve-fitting method implemented in FAMAS software (Kyowa Interface Science Co., Saitama, Japan).

Results Table 1 lists the liquids for which the contact angle was measured and the temperatures at which the measurements were made. The contact angles reported in Table 1 are averages of six to ten measurements using two substrates. The contact angle as a function of temperature for epoxy oligomers differs from that of PS oligomers. For epoxy oligomers, the contact angles decrease with increasing temperature. However, for PS oligomers, the contact angle changes only slightly. The different results for epoxy oligomers vis-à-vis PS oligomers were analyzed by using MD simulations in terms of changes in molecular conformation.


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Table 1. Liquids used for contact-angle measurements and temperature at which contact-angle measurements were made.

Liquid water diiodomethane 1-bromonaphthalene heptadecane octylbenzene ethylene glycol diethylene glycol epoxy monomer

epoxy tridecamer PS tetramer PS heptamer

Temperature (K) 298 298 298 298 298 298 298 298 423 493 423 493 423 493 493

Contact angle (°) 12.8 38.2 21.2 10.1 6.4 14.0 12.0 16.7 7.7 2.5 46.8 30.7 23.9 24.6 39.9

Simulations Methodology The following calculations were done by using the software package Materials Studio ver. 8.0 (Dassault Systèmes BIOVIA, San Diego, CA, 2001–2009). The model consisted of an Al2O3 (0001) surface and liquid droplets. To model the Al2O3 (0001) surface, the Al2O3 lattice was first derived from the structural database of Materials Studio. A repeat unit of Al2O3 was cleaved along the (0001) crystallographic orientation so that the surface consists of oxygen atoms based on crystal


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truncation rod diffraction.33 Next, a vacuum slab was created above the surface so that the lattice size along the z axis was 400 Å. The surface geometry was optimized by using the DMol3 module with the PBE functional and numerical DNP basis set. An orthorhombic lattice was prepared by redefining the vector A to (210). Next, a 24×42×1 supercell was constructed to obtain a surface with dimensions of 197.8 × 199.9 × 400 Å3. An amorphous cell was used for constructing solvent and oligomer bulks, which were optimized by MD simulations for 1000 ps in the canonical NPT ensemble. From the bulk constructions, we built droplets with a radius of 20.0 Å, which is sufficient to measure a contact angle in a simulation, as reported in previous research.29 The MD simulations were conducted by the COMPASSII35 force field, which was implemented by using the Forcite module. Finally, the solvent and oligomer droplets were placed onto the Al2O3 (0001) surface to produce the initial construction shown in Figure 1.


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Figure 1. Initial construction for MD simulation of wetting the Al2O3 (0001) surface.

The canonical ensemble NVT was used for MD wetting simulations. The integration step was set to 1.0 fs. The Berendsen loose-coupling thermostat36 was selected for finite-temperature control of the simulated system. The van der Waals interactions and electrostatic interactions were calculated by using the group-based method, and the cutoff distance was 15.5 Å. During the simulations, the Al2O3 surface was fixed because these atoms vibrated only very slightly and so the vibration was neglected. The MD simulations were done in two steps: In the first step, the systems were evolved for 500 ps to assess the wetting process (the first MD simulation). In the second step, the systems were evolved for 1000 ps for an analysis of the steady state (the second


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MD simulation). These MD simulations are also conducted by the COMPASSII force field. This force field seems to be proper for calculating wettability as reported previous research.29

Results and Discussion First, to confirm the validity of the MD simulations, we tried to evaluate contact angles based on the simulation results. However, we could not obtain contact angles because the droplets were fully spread on the surface, so their contact angles were near zero for most solvents and oligomers. We attribute this result to the small scale of MD simulations compared to the experiments. Then, we calculated we calculated the diffusion coefficient D instead of contact angle, which is should be related to wettability, 29 from the first 500 ps in the second MD simulation. The diffusion coefficient is given by following the equation.37

D

lim →

1 6

〈|

0 | 〉

Where Ri(t) is the position of atom i at time t, and n is the number of atoms. This equation is used for three-dimensional systems, although molecules hardly move along the z axis because the organic molecules exist on the solid surface in the present simulations. Therefore, diffusion coefficients are underestimated; however, the degrees of underestimation seem to be almost same in all simulations, so that they are sufficient to confirm the validation of the MD simulations. We compared the diffusion coefficient to the contact angle directly rather than other measures such as the energy of adhesion, which depends on some experimental values and can cause larger


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error bars. Figure 2 shows the experimentally determined contact angle as a function of the logarithm of the simulated diffusion coefficient. The error bars in MD simulations seems to be small because mean square displacements have good linearity with respect to the simulation time. The diffusion coefficients show a clear negative correlation with contact angle. Note that diiodomethane departs significantly from other plots because COMPASSII contains force fields only for I–, I, I2–, and I3 but not for I bonded to carbon. Excluding diiodomethane, the correlation coefficient is –0.79, which is sufficient correlation to estimate the wettability of the Al2O3 (0001) surface by MD simulation.

Figure 2. Experimentally obtained contact angles as a function of the logarithm of the diffusion coefficient obtained from MD simulation. At right are snapshots of droplets of epoxy oligomers and a PS oligomer in contact-angle measurements.


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To study the wettability as a function of temperature, Figure 3 shows the cohesive energy densities of epoxy oligomers and PS oligomers, which is correlated with surface tension.38,39 The cohesive energy density is given by following the equation.40 〈

〉

〈

〉

Where Eintra is the intramolecular energy, and Etotal is the total energy of the system. The brackets represent an average over a NPT ensemble. The cohesive energy densities are calculated from the MD simulations to optimize the construction of oligomers, and the averages over the final 500 ps in the simulations appear in Figure 3.


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Figure 3. Cohesive energy density of (a) epoxy monomers, (b) epoxy tridecamers, and (c) PS tetramers.


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The results indicate that the cohesive energy densities become smaller with increasing temperature for all oligomers, as is well known.41 According to Young’s equation,42 contact angles are determined by the surface tension of a droplet and the interaction between the droplet and the surface, and smaller surface tension leads to a decrease of contact angles. However, for the PS oligomer, the contact angles are almost the same at 423 and 493 K, in spite of the decrease in cohesive energy density. Consequently, we also analyze the interactions of oligomers with the surface to understand the difference between epoxy oligomers and PS oligomers. To evaluate the interaction between droplet and surface, we examine the movement of hydrogen near the surface, which seems to interact with the surface by some interaction including the hydrogen bond or van der Waals. In the first snapshot of the second MD simulation, we define hydrogens less than 3 Å from the top oxygens of the Al2O3 surface as interacting hydrogens (Hint). As an example, Figure 4 shows Hint for one molecule in an epoxy monomer. You can see that most of the nearest hydrogens from the surface are selected as Hint in Figure 4. Then, the 3 Å distance seems to be sufficient to evaluate the interaction with the surface. Next, from the second MD simulation, we obtain the profile of the relative Hint concentration along the z axis over 1000 ps. Figure 5 shows these profiles for epoxy oligomers and PS oligomers.


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Figure 4. Hint (yellow atoms) of a single epoxy monomer molecule.


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Figure 5. Relative concentration of Hint along z axis over 1000 ps from the second MD simulations for (a) epoxy monomer, (b) epoxy tridecamer, and (c) PS tetramer.


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For the epoxy monomer, only a single peak appears at 298 K in Figure 5(a), which indicates that all Hint are bound near the surface over 1000 ps by interaction with the surface. Conversely, at 423 and 493 K, the first peak becomes smaller and additional peaks appear, which suggests that some Hint begin to move against surface interaction. In other words, the interaction strength between epoxy monomer molecules and the surface decreases with increasing temperature. According to Young’s equation,40 a weaker interaction between droplet and surface leads to greater contact angles. Thus, for epoxy monomers, the decrease in contact angles with increasing temperature is attributed not to the change in interaction strength but to the decrease in surface tension (see Figure 3). For epoxy tridecamer, the first peak at 493 K is larger than that at 423 K [see Figure 5(b)], in contrast to the case of the epoxy monomer. This indicates that more hydrogen atoms interact with the surface at 493 K than at 423 K. In addition to the decrease in surface tension shown in Figure 3, this is considered to result in smaller contact angles at 493 K. To investigate the increase in droplet–surface interaction, Figure 6 shows the molecular radius of gyration as a function of time over 500 ps from the first MD simulation. The radius of gyration of epoxy tridecamer at 423 K is less than that at 0 ps, which implies that epoxy tridecamer molecules experience intramolecular interactions at 423 K. Conversely, the radius of gyration at 493 K increases in the first ~100 ps. These results suggest that, at 493 K, thermal energy allows the epoxy tridecamer molecules to interact with the surface against intramolecular interaction. As a result, the interaction between droplet molecule and surface increases and the contact angle decreases with increasing temperature.


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Figure 6. Radius of gyration as a function of time for epoxy tridecamer.

Conversely, for PS tetramer, the profiles of Hint are nearly same at 423 and 493 K [see Figure 5(c)], which implies that little change occurs in the droplet–surface interaction. To assess the difference vis-à-vis the epoxy tridecamer, Figure 7 shows the radius of gyration as a function of time for PS tetramer and heptamer. The profiles of PS tetramer at 423 and 493 K are also nearly the same and remain almost constant over 500 ps. Even for the PS heptamer, which is longer than the PS tetramer, the radius of gyration remains constant in contrast to the epoxy tridecamer. These results suggest that, for the epoxy tridecamer, molecular flexibility allows it to change conformation so as to increase the interaction with the surface and decrease the contact angle with increasing temperature. However, because of its molecular rigidity, PS tetramer cannot change its conformation and interaction with the surface. Thus, increasing temperature has little effect on the contact angle for PS tetramer.


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Figure 7. Radius of gyration as a function of time for PS oligomers.

Indeed, based on the results for the epoxy tridecamer and PS tetramer, the flexibility of molecules seems to effect the droplet–surface interaction and lead to the different changes in contact angle with temperature for the epoxy tridecamer and PS tetramer. Subsequently, the effect of molecular flexibility on the interaction is also examined for epoxy monomer. Figure 8 shows the radius of gyration as a function of time for the epoxy monomer at 298, 423, and 493 K. All profiles are almost the same and remain constant because the epoxy monomer molecule is so short that it is at maximum extension, as shown in Figure 4. Accordingly, for the epoxy monomer, molecular flexibility does not affect the droplet–surface interaction strength.


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Figure 8. Radius of gyration as a function of time for epoxy monomer.

We now analyze the decrease in interaction strength between epoxy monomer molecules and the surface by evaluating the changes in molecular conformation. For this purpose, all hydrogens (Hall) and Hint in Figure 4 are classified as three types: hydrogens bonded to a carbon next to an oxygen (HnearO), hydrogens in aromatic rings (Hring), and hydrogen in methyl groups (Hmethyl). Next, as for the profiles shown in Figure 5, Figure 9 shows profiles of relative concentration along the z axis over 1000 ps from the second MD simulation for the three types of hydrogens. The profiles of HnearO in Hall and HnearO in Hint are almost the same [Figures 9(a) and 9(b)], which indicates that most of the HnearO exists near the surface and interacts with it. Moreover, temperature has little effect on the profiles of HnearO, which suggests that most of the HnearO are bound close to the surface regardless of temperature; that is, HnearO seems to interact strongly with the surface.


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Figure 9. Relative concentration profiles of Hall and Hint, classified by their bonding position, along z axis over 1000 ps from the second MD simulation.


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Two peaks appear for Hring in Hall in Figure 9(c), which suggests that aromatic rings in the epoxy monomer molecule are not parallel to the surface but tilted, as shown in Figure 4. In addition, the profile of Hring in Hint has only one peak at 298 K [Figure 9(d)], which indicates that Hring is bound to the surface due to interactions with the surface. In other words, the aromatic rings hardly move at 298 K. However, the profiles at 423 and 498 K change significantly compared with that at 298 K: the small-distance peaks shrink and, simultaneously, the new peaks grow [Figure 9(d)]. This suggests that the hydrogens in aromatic rings begin to move over the temperature transition, which seems to happen because they cannot resist thermal motion and so interact more weakly with the surface. As a result, the aromatic rings begin to tilt to the other side at 423 and 493 K. Finally, two split peaks appear in the profiles of Hmethyl in Hall [Figure 9(e)] that seem to correspond to two methyl groups in the molecule and their rotational movement. The profiles of Hmethyl in Hint show only a single split peak [Figure 9(f)], in contrast with the profiles in Figure 9(e). This means that only one methyl group in the molecule is adsorbed onto the surface. Furthermore, the first peak shrinks and the second peak increases in Figure 9(f), which indicates that the methyl groups rotate more freely at 423 and 493 K than at 298 K. On the whole, for the epoxy monomer, the interaction of Hring and Hmethyl with the surface decreases significantly with increasing temperature, so that aromatic rings begin to tilt to the other side or the methyl groups begin to rotate freely at the higher temperatures. Thus, the wettability of Al2O3 (0001) by organic molecules can be simulated by MD simulation. In addition, the change in conformation of organic molecules influences the liquid-surface interaction and contact angle.


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Conclusion The process by which organic molecules wet an Al2O3 (0001) surface is analyzed by using MD simulations. The diffusion coefficients calculated by these simulations provide satisfactory estimates of the wettability of the Al2O3 (0001) surface. Moreover, the effect of changes in molecular conformation is evaluated for several oligomers. The results indicate that the molecular flexibility affects the molecule–surface interaction. In addition, for the epoxy monomer, the interaction with the surface of hydrogens in aromatic rings and in methyl groups decreases dramatically with increasing temperature. Thus, this method can aid in understanding the consequences of molecular structure for the wettability of Al2O3 (0001) surfaces and thereby help to control the wettability of this surface.


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AUTHOR INFORMATION Corresponding Author [email protected]

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(17) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Dynamic Contact Angle Measurement of Temperature-Responsive Surface Properties for Poly(N-isopropylacrylamide) Grafted Surfaces. Macromolecules 1994, 27, 6163–6166. (18) Reiter, G.; Khanna, R. Kinetics of Autophobic Dewetting of Polymer Films. Langmuir 2000, 16, 6351–6357. (19) Reiter, G.; Auroy, P.; Auvray, L. Instabilities of Thin Polymer Films on Layers of Chemically Identical Grafted Molecules. Macromolecules 1996, 29, 2150–2157. (20) van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Additive and Nonadditive Surface Tension Components and the Interpretation of Contact Angles. Langmuir 1988, 4, 884–891. (21) Kwok, D. Y.; Neumann, A. W. Contact Angle Measurement and Contact Angle Interpretation. Adv. Colloid Interface Sci. 1999, 81, 167–249. (22) Tao, Y. T. Structural Comparison of Self-Assembled Monolayers of n-Alkanoic Acids on the Surfaces of Silver, Copper, and Aluminum. J. Am. Chem. Soc. 1993, 115, 4350–4358. (23) Steiner, T.; Saenger, W. Geometry of C-H-O Hydrogen Bonds in Carbohydrate Crystal Structures. Analysis of Neutron Diffraction Data. J. Am. Chem. Soc. 1992, 114, 10146–10154. (24) Cao, Z.; Stevens, M. J.; Dobrynin, A. V. Elastocapillarity: Adhesion and Wetting in Soft Polymeric Systems. Macromolecules 2014, 46, 6515–6521. (25) Nieto, D. R.; Santese, F.; Toth, R.; Posocco, P.; Pricl, S.; Fermeglia, M. Simple, Fast, and Accurate In silico Estimations of Contact Angle, Surface Tension, and Work of Adhesion of Water and Oil Nanodroplets on Amorphous Polypropylene Surfaces. Appl. Mater. Interfaces 2012, 4, 2855−2859.


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TOC Graphic


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Figure 1. Initial construction for MD simulation of wetting the Al2O3 (0001) surface. 68x103mm (150 x 150 DPI)



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Figure 2. Experimentally obtained contact angles as a function of the logarithm of the diffusion coefficient obtained from MD simulation. At right are snapshots of droplets of epoxy oligomers and a PS oligomer in contact-angle measurements. 177x80mm (150 x 150 DPI)


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Figure 3. Cohesive energy density of (a) epoxy monomers, (b) epoxy tridecamers, and (c) PS tetramers. 79x147mm (150 x 150 DPI)



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Figure 4. Hint (yellow atoms) of a single epoxy monomer molecule. 80x47mm (150 x 150 DPI)


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Figure 5. Relative concentration of Hint along z axis over 1000 ps from the second MD simulations for (a) epoxy monomer, (b) epoxy tridecamer, and (c) PS tetramer. 76x170mm (150 x 150 DPI)



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Figure 6. Radius of gyration as a function of time for epoxy tridecamer. 81x55mm (150 x 150 DPI)


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Figure 7. Radius of gyration as a function of time for PS oligomers. 81x57mm (150 x 150 DPI)



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Figure 8. Radius of gyration as a function of time for epoxy monomer. 83x57mm (150 x 150 DPI)


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Figure 9. Relative concentration profiles of Hall and Hint, classified by their bonding position, along z axis over 1000 ps from the second MD simulation. 175x171mm (150 x 150 DPI)


Wettability of Al2O3 Surface by Organic Molecules: Insights from

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