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Enhancement of adhesion strength of perfluoroalkylpolyethers (PFPE) on rough glassy silica for anti-smudge coatings Sung Hoon Lee, Yong Nam Ahn, Venkatesh Botu, Ross J. Stewart, and SangYoon Oh ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00499 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on September 2, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Polymer Materials

Enhancement of adhesion strength of perfluoroalkylpolyethers (PFPE) on rough glassy silica for anti-smudge coatings Sung Hoon Lee1,*,Yong Nam Ahn1,Venkatesh Botu2, Ross J. Stewart2, SangYoon Oh1

1Corning

Technology Center Korea, Corning Precision Materials Co., Ltd., 212 Tangjeong-ro, Asan, Chungcheongnam-do, 31454, Republic of Korea

2Science

and Technology Division, Corning Research & Development Corporation, 1 Science Center Drive, Corning, New York 14831, United States

* Sung Hoon Lee E-mail: [email protected]; Tel +82-41-520-5691 ORCID: 0000-0002-4320-4834

© 2019 Corning Incorporated. All Rights Reserved. ACS Paragon Plus Environment

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The adhesion behavior of perfluoropolyether (PFPE) on rough silica surfaces is investigated by steered molecular dynamics simulations. To reproduce bond breakage during sliding of PFPE, a dynamic bond breaking method is developed and applied to the PFPE – silica interface. Calculated results reveal that nano-scale roughness is a critical parameter that affects the adhesion strength due to the PFPE film thickness of 1 nm, and adhesion strength also depends on the molecular density of PFPE on the silica surface. The effect of roughness amplitude, spacing, and molecular weight of PFPE are individually analyzed to find a key parameter for adhesion enhancement. Adhesion strength on a flat surface is highest and decreases with increasing roughness within the considered conditions. When Ra = 17.5 Å, adhesion strength is 3 times lower than the flat surface, and vacant pores at the interface are observed which implies a reduced molecular density of PFPE from 0.31 to 0.27 molecules/nm2. Increasing the roughness spacing removes vacant pores at the interface and hence, adhesion enhances up to 50 % of the original interface. Decreased molecular weight is another way to increase surface density of PFPE, and the highest adhesion is observed with the lowest molecular weight of PFPE. Comparing the change of adhesion strength, both roughness amplitude and molecular weight are determined as key parameters for enhancing adhesion strength.

Keywords: Anti-smudge, PFPE, Roughness, Adhesion, Molecular dynamics, Sliding test


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1. INTRODUCTION

Due to both the hydrophobicity and oleophobicity of perfluoropolyether (PFPE), PFPE is largely used as functional coatings for water and oil repellant surface treatments.1-9 It has been considered as anti-fouling or anti-finger print coating materials for self-cleaning of touch screen applications or used as a lubricant for hard disk drives.8, 10-12 Thus, PFPE undergoes shear stress in its applications, 10-12 and suffers from low mechanical stability which results in the decrease of PFPE durability. Therefore, organic-inorganic hybrid coatings were suggested to enhance the mechanical stability of PFPE.9,

13-15

Oldani et al., prepared multilayer coatings of PFPE with

ceramic oxides such as TiO2 or ZrO2 on metal surfaces, and reported an enhanced resistance against liquid erosion.15 PFPE on silica surfaces were also synthesized by Fabbri et al., and friction coefficient values were remarkably reduced with hybrid coatings.9, 13, 14

Bonding and adhesion properties between organic and inorganic surfaces are affected by surface conditions such as surface chemistry, hydroxylation density, and roughness.16-19 Nanoscale roughness, especially, can increase adhesion energy by about 90% between silica and polyimide when silica roughness changes from 1.0 to 8.4 Å.17 Thus, due to the 1 nm thickness of the PFPE layer,20 nano-scale roughness would be critical on the adhesion between PFPE and inorganic surfaces. Although many previous works reported the dependence of roughness on the



PFPE performances such as wettability or contact angle,14, 21, 22 detailed reports that focus on the adhesion property with roughness is rare and hence, a fundamental study for the adhesion is required to enhance the durability of PFPE on the surface.

In this regard, we aim to understand the adhesion behavior with respect to surface roughness during sliding processes. Steered molecular dynamics (SMD) simulations23, 24 are implemented to investigate the fundamentals of adhesion during sliding tests. The Interface force field (FF) is adopted for the molecular interactions in the current work since it well reproduces the interfacial interactions and successfully predicts adhesion energies, especially, for non-bonded interactions between silica and polymers.16, 17, 25, 26 In order to describe the breakage of covalent bonds during the sliding process, a dynamic bond breakage method is developed by utilizing first-principles calculations and complements Interface FF.

During the sliding process, i.e. applying shear stress, a structural analysis is performed to understand the PFPE – silica adhesion mechanism with respect to the roughness. First, the effect of roughness amplitude on the adhesion is investigated by analyzing the force for bond breakage, molecular density, and distribution of siloxane bonds. Then, adhesion dependence on roughness spacing and molecular weight of PFPE are studied. Finally, a key parameter for the adhesion is determined by comparing the effects of the considered conditions on the adhesion strength.


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2. METHODS

2.1. Model generation

2.1.1. Polymer and rough surface

The general structure of PFPE for anti-fouling coatings is a combination of perfluoroether chains with functional end groups. The anti-fouling property is exhibited with the perfluoroether chain, and the functional end group binds the molecule to the glass surface. As a functional end group, alkoxysilanes are typically used, and covalent siloxane bonds which act as an anchor for the PFPE durability are formed between silanes of PFPE and hydroxyl groups on glass surfaces through hydrolysis and condensation reaction as represented in Fig. 1(a).27-29 A single PFPE chain which is used in the current work is represented in Fig. S1. The backbone structure of PFPE contains twenty perfluoropropylenoxy units, and a hydrocarbon moiety connects the backbone to a trialkoxysilane end group. The molecular structure of PFPE in this work is as follows,

F-(CF2CF2CF2O)20- CH2CH2 CH2-Si(OH)3



Rough glassy silica surfaces are generated by cleaving the bulk structure with a pre-defined rough surface. For a systematic analysis of roughness, a deterministic roughness topography is defined by a sine function. Four different amplitudes of roughness; flat, Ra = 4.5, 8.5, and 17.5 Å, are prepared as shown in Fig. S2. The prepared surface is 85 x 85 Å2 in the x-y plane and the thickness of the bulk area under the rough surface is set at 30 Å applying an identical depth of bulk area regardless of roughness. For the case of Ra = 17.5 Å, the period of roughness is adjusted as 85, 128, and 172 Å to study the effect of the roughness spacing. The hydroxylation density of the surfaces are adjusted to the experimental value of 4.6 OH/nm2.30 More detailed procedures of rough surface generation and surface hydroxylation are depicted in our previous publication.17

2.1.2. Interface

To construct a PFPE film on a silica surface deposited by a thermal evaporation process, each individual PFPE molecule in the model is repeatedly inserted near the silica surface with a 10 ps time interval of relaxation between insertion events. This process is continued until the entire surface is covered with PFPE molecules. Since more PFPE molecules are required to cover rougher silica surfaces, 25, 50, 66, and 98 PFPE molecules are deposited on the models: flat, Ra = 4.5, 8.5, and 17.5 Å surfaces, respectively.


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The main adsorption reaction at the interface is siloxane bond formation between a silanol group on the PFPE end group and hydroxyls of the silica surface. To mimic the bond formation, when the distance between a silanol group and hydroxyl is shorter than 9 Å, a siloxane bond is created between the two structures generating a water molecule, which is removed from the surface. After equilibration, the PFPE layer consists of bonded PFPE at the interface and additional non-bonded PFPE on top of them.

2.2. Details of sliding modeling

2.2.1. Initial relaxation and force calculation during sliding

All molecular dynamics (MD) simulations in this study are conducted with the LAMMPS package.31 Initial structures are relaxed for 1 ns at 300 K with the NVT ensemble and for another 1 ns at 300K and 1 atm with the NPT ensemble maintaining 100 Å of vacuum separation in the z-direction. The Nosé–Hoover thermostat32, 33 and Parrinello-Rahman barostat34 are applied with 100 fs and 1000 fs for damping parameters, respectively. The time step is 0.5 fs, and the Particle–Particle Particle–Mesh (PPPM) method is used for the summation of long-range Coulomb interactions with the precision of 10-4.

Among the numerous methods of adhesion measurements,35-43 sliding is adopted to measure the adhesion since shear stress is the dominant force for PFPE applications. Force variations © 2019 Corning Incorporated. All Rights Reserved. ACS Paragon Plus Environment


during sliding processes are measured and compared with respect to roughness in this work. To model the sliding process, steered molecular dynamics (SMD) simulations have been conducted in this work because it is verified that SMD can successfully model the adhesion behavior between glassy silica and polymer. 16, 17, 19, 39, 44, 45

During the sliding process, the spring constant, K, is usually selected to ensure the atoms are in an equilibrium state, and 100 kcal/mol/Å2 was typically used in our previous SMD modeling. 16, 17, 19, 44

However, the bond breakage process is a non-equilibrium state which requires a larger

stiffness spring, K, than that in equilibrium. Therefore, we tested five different spring constants of 100, 500, 5000, 10000, and 50000 kcal/mol/Å2. During bond breakage simulations, we monitored potential energy (U) and potential of mean force (P) with different K values. Then, the K value is selected when U and P exhibit the same value with bond dissociation energy from density functional theory. In this way, 50,000 kcal/mol/Å2 was selected in this work to reproduce bond dissociation energy of PFPE and the silica interface. In addition, a constant sliding velocity of 5 m/s is applied after testing several velocities in the range of 1 – 100 m/s to obtain converged behaviour of the PMF.

2.2.2. Dynamic bond modification during sliding


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The silanol groups of PFPE act as an adhesion promoter, wherein, they covalently bind to the silica surface. Schematics of hydrolysis and condensation reactions are exhibited in Fig. 1 (a), and covalent siloxane bonds are created between glass surface and PFPE. In order to accurately predict failure modes that might occur during the sliding process – cohesive failure within the bulk polymer, or adhesive failure at the interface between polymer and glass, it is necessary to account for the dynamic breaking of bonds within the material. To mimic this behavior in our simulations, we rely on a combination of ab initio based Density Functional Theory calculations (DFT) and classical molecular dynamics. DFT calculations provide access to the true bond dissociation energies (BDE) for any given bond, given its neighboring environment, e.g., a C – C BDE is weakened by the presence of electronegative atoms. To account for these variations, a fragment of PFPE and the glass surface is generated, as illustrated in Fig. 1 (b). Here the atom labeled as “sc4” represents the top most layer of the glass surface along with the bridging O. To assist with the DFT calculations, these O atoms are terminated with H’s. Figure 1 (b) labels all the unique bonds that can be broken during a SMD simulation, and thus each of which requires an independent DFT calculation to obtain all possible BDEs. For each bond, a sweep of the potential energy surface (PES) was carried out by varying the distance of the bond in consideration while keeping all other degrees of freedom constant. The depth of the PES is then used as an estimate for the bond dissociation energy. As an example, Fig. S3 represents the



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potential energy surface (PES) between sio and c2 which corresponds to bond3 in Fig. 1 (b). In this figure, we can deduce BDE and Req as 116 kcal/mole and 1.887 Å, respectively. The inflection point, RbrDFT, can be regarded as a pseudo-transition state, beyond which the atom is more probable to break rather than form. All of the calculated results are listed in Table 1.

In the INTERFACE force field, the bonding energy contribution is represented using the quartic harmonic approximation as follows,

E = K2 × (R ― 𝑅𝑒𝑞)2 + K3 × (R ― 𝑅𝑒𝑞)3 + K4 × (R ― 𝑅𝑒𝑞)4

where, K2, K3 and K4 are bond constants, and Req is the equilibrium distance. Using this equation, we find a value of R when E equals the BDE computed by DFT, then the distance becomes the Rbr at which the bond is explicitly broken in the simulation. All the obtained bond breaking distances (RbrMD) are listed in the last column in Table 1, which is used as a look up table to dynamically break the bonds during the molecular dynamics simulation.

Here, we implemented and used a new bond breaking fix for the LAMMPS codebase. The DFT calculations in this work were done using B3LYP hybrid XC functional,46, 47 with

the

combination of the double numerical plus polarization with addition of diffuse functions (DNP+) basis set available in Materials Studio 2018 package.48


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3. RESULTS and DISCUSSION

3.1. Force variation during sliding and convergence test of sliding velocity

During the sliding process, shear stress is applied to the PFPE layer. Fig. 2 (a) illustrates the snapshots at the time of initial, intermediate, and bond breaking. With a constant velocity, force is applied to the x axis on the polymer to detach it from the silica surface, and the generated force during the sliding process is presented in Fig. 2 (b). At the beginning of sliding, bent PFPEs are stretched out and a relatively small amount of force is required at this stage. After the PFPE is completely stretched, covalent bonds between atoms start to expand along the sliding direction. Compared to the chain re-orientation, bond expansion requires very high force and thus, an abrupt increase of force starts to be observed when the sliding distance is around 120 Å. As the sliding distance increases, the weakest bond of PFPE is broken, and the corresponding force is defined as the maximum force (Fmax) for the bond breakage of PFPE on the silica surface.

The maximum force during SMD depends on the sliding or pulling velocity,16, 17, 19, 44 and it is important to find a converged value to predict accurate adhesion properties. Therefore, various sliding velocities from 1 to 50 m/s are pretested. As shown in the inset of Fig. 2 (b), Fmax is overestimated when the velocity is 50 m/s, and fluctuates as velocity decreases. The converged value is close to 2900 kcal/mole·Å when the velocity is 1 or 5 m/s, and by considering



computational time, the velocity of 5 m/s is selected as a sliding velocity in this work.

3.2. Effect of roughness on the adhesion between PFPE and silica surface

3.2.1. Adhesion behaviour of PFPE on flat surface

Figure 3 exhibits equilibrated interface structures and distributions of silanol groups on flat and various rough surfaces. Since the major interaction between silica and PFPE is a condensation reaction for siloxane bond formation, a sharp peak of silanol concentration is observed at the interface. In addition, as shown in Fig. 3 (a), silanol groups are evenly distributed on the flat surface with 1 nm thickness, which is consistent with the previous experimental result.20 It implies the PFPE layer consists of several stacks of 1 nm thickness, where the bottom layer is bound to the silica surface with covalent bonds and the others are deposited on top of them without bond formation. Thus, we can expect two types of failure modes during the sliding process. Firstly, non-bonded PFPE layers could be removed from the surface with a relatively small amount of force implying a cohesive failure mode. Bonded PFPE then removes from the surface with much higher force indicating adhesive failure at the interface. Even though we applied pulling force on PFPE layers, similar failure modes are expected due to strong covalent bonds which are major bond source in the current system. The entanglement of PFPEs are very low, and thus the molecular density of PFPE on the silica surface is observed to be only 0.30


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molecules/nm2, which is 15 times lower than the hydroxylation density of silica, 4.6 OH/nm2.30 Thus, unlike adhesion between polyimide and silica,16, 17 the impact of hydroxylation density on the adhesion would be very limited for PFPE on silica surfaces. On the other hand, an increase of molecular density of PFPE could be an effective method to enhance adhesion strength of the current system. Variation of surface roughness or molecular structure can increase molecular density, and thus the effects of roughness amplitude, spacing, and molecular weight on the adhesion are investigated and discussed in the following sections.

3.2.2. Effect of roughness amplitude on the adhesion of PFPE

For higher roughness cases (see in Fig. 3 (b) ~ (d)), a similar PFPE thickness of 1 nm is observed regardless of roughness, but the intensity of the peak at the interface decreases due to the increasing aspect ratio of roughness. In addition, we observed vacant pores when roughness is 17.5 Å (see in Fig. 3 (d)). The existence of vacant pores implies that PFPE is not well attached to the silica surface,17 and hence the molecular density of PFPE would be even lower than that for other roughness cases.

As schematically shown in Fig. 4 (a), shear stress is applied to the top layer of PFPE adopting bond breakage criteria listed in Table 1. Since there exist both bonded and non-bonded PFPE on the surface, the effect of roughness on each of them is individually investigated. Calculated force



variations with roughness for non-bonded and bonded PFPE are presented in Fig. 4 (b) and (c), respectively. When PFPE molecules are adsorbed on the silica surface without bond formation, there exist only non-bonded interactions at the interface. During sliding of PFPE, a sharp increase of force is initially observed to initiate PFPE sliding, but forces saturate around 82 kcal/mol·Å with continuous sliding regardless of surface roughness. This result is consistent with our previous work, the region where the sliding forces are applied was independent of the converged forces.44 It is noteworthy that the initial maximum force for sliding of non-bonded interactions increase with roughness due to the expansion of surface area.

Unlike non-bonded PFPE, bonded PFPE exhibits different force variations due to the created siloxane bonds at the interface. As shown in Fig. 4 (c), there exists a maximum force (FMax) for bond breaking during the sliding process, and FMax varies depending on roughness. FMax is more than 10 times higher than that for non-bonded PFPE and thus, bonded PFPE is the origin of the PFPE adhesion on silica surfaces, and non-bonded PFPE would be easily removed from the surface. As shown in the figure, FMax exhibits the highest on flat surface and decreases with increasing roughness.

For more detailed analysis, the number of total bonded PFPE, sliding PFPE, and percentages between them are counted and listed in Table 2. For a flat surface, all the PFPE are sliding with


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the applied force, and thus the highest FMax is observed. However, as the roughness increases, not all PFPE slide with the applied force due to the depth of roughness. If PFPE is randomly distributed or well entangled with one another like polyimides, the sliding force is applied to the whole PFPE regardless of roughness. However, due to the low interaction between PFPE molecules, the PFPE is easily divided into several layers depending on roughness. Thus, only 69 or 33 % of the bonded PFPE are involved in sliding when Ra = 8.5 or 17.5 Å, respectively, and FMax decreases in this condition. This result can be confirmed by analysing the concentration of PFPE tails on rough surfaces as shown in Fig. 5. For the flat surface, the first peak is located at 10 ~ 15 Å above the surface and the second peak is observed with a similar thickness, which is consistent with the predicted PFPE thickness. In this case, since tails are only at the surface, every molecule is involved with sliding, resulting in the highest FMax. For the other rough surfaces, not all PFPE exist above the silica surface. For the surface with Ra = 4.5 or 8.5 Å, more than half the tails are located above the silica surface, but for Ra = 17.5 Å, most of the tails are within the valleys of the silica surfaces. Therefore, when we apply a sliding force on the surface, only a small part of the PFPE are sliding and removed from the surface, which results in the decrease of the PFPE performance.

In addition, molecular density of the bonded PFPE is also calculated and listed in Table 2. The density of the total bonded PFPE is similar for flat or Ra = 4.5 or 8.5 Å as 0.31 molecules/nm2, © 2019 Corning Incorporated. All Rights Reserved. ACS Paragon Plus Environment


but when the roughness is the highest, Ra = 17.5 Å, molecular density is lowered to 0.27 molecules/nm2. This is because of the existence of vacant pores at the interface, and it can be confirmed by analysing the distribution of created siloxane bonds at the interface. As shown in Fig. S4, green dots represent siloxane bonds on the silica surface (red lines). Created bonds are evenly distributed on the silica surface when Ra = 8.5 Å, and similar configurations are observed for the lower roughness surfaces. However, when roughness is the highest, Ra = 17.5 Å, there exists free space at the bottom of the silica surface. It is consistent with the vacant volume as shown in Fig. 3 (d), and for this reason, molecular density at this roughness is lower than the other roughness cases. Even though Fmax exhibits the highest with a flat surface among the considered roughness cases in this study, it is possible that the highest value of Fmax would appear in between flat and Ra = 4.5 Å. Due to the expansion of surface area, the number of bonded PFPE increases and hence, Fmax could increase.

3.2.3. Effect of roughness spacing on the adhesion of PFPE

In our previous adhesion study between polyimide and glassy silica, vacant pores at the interface drastically reduces adhesion energy, and the effect disappears with increasing roughness spacing.17 Since vacant pores affect the surface density of PFPE, various roughness spacing is tested for its influence on the PFPE adhesion. Figure 6 (a) exhibits the force variation


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during the sliding process for three different spacing, 85, 128, 172 Å, with same roughness amplitude, Ra = 17.5 Å. As shown in the figure, FMax increases when spacing is greater than 85 Å. Figure 6 (b) shows the interface between the PFPE layer and silica surface; when the spacing is 85 Å, there exist vacant pores at the interface. On the other hand, we did not observe any pores when the spacing is 128 Å, and hence FMax can increase with this spacing. Interestingly, among the three spacing values, FMax exhibits the highest when the spacing is 128 Å as shown in Fig. 6 (a). This can be explained by considering adhesion energy variation with roughness spacing.17 Figure 7 shows a trend of adhesion energy with roughness spacing. When the spacing decreases, adhesion energy usually increases due to the surface area expansion. However, when there exist vacant pores at the interface, adhesion energy drastically decreases. The adhesion trend (black line) incorporates both effects, and thus it exhibits the maximum adhesion point as a function of spacing as shown in Fig. 7. The maximum adhesion could be obtained near the spacing of 128 Å in this system, thus adhesion energy is reduced for spacing of 85 or 171 Å due to the existence of vacant pores or decrease of surface area, respectively. In addition, it is noteworthy to mention that the variation of FMax with roughness spacing is not significant compared to that with roughness amplitude since the amount of FMax variation with spacing is much lower than that with Ra as can be seen in Fig. 4 (c). Therefore, we can conclude that amplitude is a more dominant parameter than spacing for adhesion enhancement. However, roughness spacing needs



to be controlled in the range of no vacant pores for a maximum adhesion since the effectiveness of roughness amplitude also depends on the spacing.17

3.3. Effect of molecular weight of PFPE on the adhesion

Increasing the surface density of PFPE can also be achieved by reducing molecular weight. With a short chain length, more PFPE can be chemically bound to the silica surface, and adhesion could be improved for the same roughness condition. Therefore, adhesion variation with respect to molecular weight is investigated. For longer chain length PFPE of which molecular weight is 5000, twice the relaxation time is applied to ensure the equilibrium condition. Figure 8 (a) exhibits maximum force variation during sliding with various molecular weights and roughness. As shown in the figure, when the molecular weight increases, FMax decreases about two or three times at any roughness, which implies molecular weight is one of the key parameters for PFPE adhesion. In addition, roughness amplitude can be regarded as another key parameter. The priority of each parameter can be determined by comparing the change of FMax within the same condition. FMax similarly varies about two or three times depending on either roughness or molecular weight, and hence the influence of both parameters can be considered as equally critical to the PFPE adhesion.


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Figure 8 (b) exhibits molecular density of sliding PFPE with roughness or molecular weights. When molecular density increases, it requires more energy to breaks bonds of PFPE and hence, FMax increases. Therefore, the trend of molecular density (Fig. 8(b)) is similar with that of FMax (Fig. 8(a)), and thus increase of molecular density can be a primary condition to enhance adhesion strength of PFPE. As discussed, reducing roughness amplitude or molecular weight can be efficient methods to enhance adhesion of PFPE.

4. Conclusion

In this work, the adhesion property of PFPE on rough silica surfaces is investigated by steered molecular dynamics simulations. A dynamic bond breakage method is applied to mimic accurate failure modes and forces for sliding processes, or other processes where bonds are likely to break, and predicts the interface behaviour of PFPE on rough silica. From the calculated results, it was revealed that PFPE is deposited with several 1 nm thick layers during the deposition process. At the interface, covalent siloxane bonds are created, and additional PFPE molecules are deposited on top of the bonded layer. The additional PFPE molecules cannot be bound to the silica surface and remain as several stacks of non-bonded PFPE each with 1 nm thickness. During the sliding modelling, it is found that the created siloxane bonds are a major source of adhesion strength,



and the non-bonded PFPE layer can be easily removed from the surface during sliding processes. In addition, due to the low surface density of PFPE, 0.3 molecules/nm2, the modification of the hydroxylation density on silica surfaces may not be a proper approach to increase the adhesion. With the investigation of surface roughness, nano-scale roughness is determined as a major parameter for the adhesion. When the silica surface is flat, the maximum force for sliding is the highest within the considered roughness values. However, as the roughness increases, the maximum force decreases due to the reduced molecular density of PFPE on the silica surface. Moreover, vacant pores are observed when roughness is 17.5 Å, which further reduces molecular density of PFPE. Based on this result, we can conclude that increasing the surface density of PFPE is a key to enhance adhesion strength. The PFPE surface density is adjusted by changing the roughness spacing and the molecular weight of PFPE. When roughness spacing increases, vacant pores disappear from the surface and hence, adhesion is enhanced. However, the magnitude of adhesion enhancement is much smaller than that caused by roughness amplitude. On the other hand, surface density significantly increases with decreasing molecular weight and consequently, the adhesion strength enhancement is as effective as that caused by roughness amplitude. Therefore, we found that both roughness amplitude and molecular weight are key parameters for adhesion strength of PFPE on silica surface whereas the effect of roughness spacing is limited. Therefore, if the surface roughness can be reduced as low as possible, the


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adhesion will be maximized. Since a change of molecular weight can also affect the anti-fouling performance, a comprehensive study is required to determine optimal molecular weight for higher adhesion with superior anti-fouling performance.



AUTHOR INFORMATION Corresponding Author * Sung Hoon Lee E-mail address: [email protected]; Tel +82-41-520-5691 ORCID: 0000-0002-4320-4834 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors would like to thank Jeong-Hong Oh and Aravind R. Rammohan for their technical discussions and Hong Yoon for his support of this project.


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Table 1. Defined force field type, bond dissociation energy (BDE), and bond breaking distance (Rbr)

DFT calculation Bond #

Atom 1 /FF type

Atom 2 /FF type

a

BDE (kcal/mol)

Req

b

MD Rbr

DFT c

Rbr

MD d

(Å)

(Å)

(Å)

1

Si

sc4

O

oc23

158

1.624

2.42

2.425

2

O

oc23

Si

sio

147

1.676

2.47

2.400

3

Si

sio

C

c2

116

1.887

2.78

2.695

4

C

c2

C

c2

125

1.540

2.37

2.210

5

C

c2

C

c2

104

1.522

2.32

2.180

6

C

c2

O

oc

91

1.433

2.18

1.955

7

O

oc

C

c2

104

1.401

1.98

1.980

8

C

c2

C

c

99

1.533

2.33

2.165

9

C

c

O

oc

90

1.424

2.17

1.950

10

O

oc

C

c

91

1.359

2.08

1.955

11

C

c

C

c

86

1.561

2.36

2.140

a

BDE : Bond dissociation energy Req : Equilibrium bond distance c R DFT : Bond breaking distance from DFT br d R MD : Bond breaking distance for MD run br b



Page 28 of 37

Table 2. Measured force for bond breaking, number of bonded PFPE, and molecular density on silica surface

Ra (Å)

a

Fmaxa

Number of bonded PFPE

Molecular density for bonded PFPE 2

(molecules/nm )

(kcal/mol·Å) Total

Sliding layer

Total

Sliding layer

Flat surface

3069

25

25 (100%)

0.31

0.30

4.5

1798

28

25 (89%)

0.30

0.26

8.5

1835

42

29 (69%)

0.31

0.22

17.5

1049

51

17 (33%)

0.27

0.09

Fmax : Maximum force for bond breaking © 2019 Corning Incorporated. All Rights Reserved. ACS Paragon Plus Environment

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


(a)

(b)

Figure 1. (a) Schematics of surface reactions between silica surface and PFPE; Created Si-O-Si bond is colored as red (b) PFPE fragment to reproduce bond break. Force field types are represent as a blue color and different types of bond are numbered as a green color



Figure 2. (a) Snapshot of PFPE – silica interface during sliding process (b) Force for bond breakage of PFPE depending on sliding velocity


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Figure 3. Concentration profile of silanol along with z axis for (a) flat surface (b) Ra = 4.5 Å (c) Ra = 8.5 Å (d) Ra = 17.5 Å



(a)

(b)

(c)

Figure 4. Force variation for bond breaking with respect to different roughness of silica (a) Schematics of sliding (b) Force variation for non-bonded PFPE (c) Force variation for bonded PFPE. Here, the force value at the peak position in each roughness corresponds to the maximum force (Fmax) for bond breaking


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Figure 5. Relative concentration of PFPE tails for various rough surfaces; Green dotted line represents top surface of silica (a) flat surface (b) Ra = 4.5 Å (c) Ra = 8.5 Å (d) Ra = 17.5 Å



Figure 6. (a) Force variation for bond breaking with respect to different roughness spacing (b) Interface structure with different roughness spacing


Page 34 of 37

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Figure 7. Adhesion trend with roughness spacing


Page 36 of 37

7000

Ra= 1 Å

6000

Ra= 8.5 Å Ra= 17.5 Å

5000 4000 3000 2000 1000 2000

3000

4000

5000

Molecular weight of PFPE (a)

0.7 0.6

Molecular density (molecules/nm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Max. Force (kcal/mole·Å)


0.5

Ra= 1 Å Ra= 8.5 Å Ra= 17.5 Å

0.4 0.3 0.2 0.1 2000 2500 3000 3500 4000 4500 5000

Molecular weight of PFPE (b) Figure 8. (a) Max. force variation on different roughness and PFPE molecular weight (b) Molecular density (molecules/nm2) on various roughness and molecular weight


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