Characterization of mechanical degradation in perfluoropolyether film

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Characterization of mechanical degradation in perfluoropolyether film for its application to anti-fingerprint coatings Kyoungmin Min, Jungim Han, Byungha Park, and Eunseog Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13159 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Applied Materials & Interfaces

Characterization of mechanical degradation in perfluoropolyether film for its application to anti-fingerprint coatings

Kyoungmin Min1,*, Jungim Han2, Byungha Park2, and Eunseog Cho1,*

1

Platform Technology Lab, 2Film Materials Lab, Samsung Advanced Institute of Technology, 130 Samsung-ro, Suwon, Gyeonggi-do, 16678, Republic of Korea.

Abstract Enhancing the mechanical durability of anti-fingerprint films is critical for its industrial application on touch-screen devices to withstand friction damage from repeated rubbing in daily usage. Using reactive molecular dynamics simulations, we herein implement adhesion, mechanical, and deposition tests to investigate two durability-determining factors: intrachain and interchain strength, which affect the structural stability of the anti-fingerprint film (perfluoropolyether) on silica. From the intrachain perspective, it is found that the Si-C bond in the polymer chain is the weakest, and therefore prone to dissociation and potentially forming a C-O bond. This behavior is demonstrated consistently, regardless of the crosslinking density between polymer chains. For the interchain interaction, increasing the chain length enhances the mechanical properties of the film. Furthermore, the chain deposition test, mimicking the experimental coating process, demonstrates that placing shorter chains first to the surface of silica and then depositing longer chains is an ideal way to improve the interchain interaction in the film structure. The current study reveals a clear pathway to optimizing the configuration of the polymer chain as well as its film structure to prolong the product life of the coated anti-fingerprint film.

Keywords: Anti-fingerprint coating, PFPE-silane, Molecular dynamics, Adhesion properties, Pulling test

*Corresponding Author: [email protected] (K. Min), [email protected] (E. Cho) 1

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INTRODUCTION Perfluoropolyether (PFPE) film has been widely applied in industrial fields, such as in lubricants for computer hard drives and electric motors and as fingerprint resistance (anti-fingerprint, AF) for touchscreen devices, owing to its good lubricity and oleophobic/hydrophobic properties. In spite of its great versatility and numerous advantages, its structural stability is finite because it can decompose under certain conditions, thereby limiting its usage. 1–8 For example, the thermal decomposition of PFPE can initiate around 300 °C.

1

Catalytic reaction in the presence of reacting agents is another source of

degradation behaviors in PFPE; e.g., increase in humidity oxides

4,9

2–4

and the existence of metal or metal

. The effects of the oxygen, water, and oxide nanoparticles (SiO2, Fe2O3) on the

decomposition behavior of PFPE are investigated and the results suggest that water molecules and nanoparticles exhibit strong effects, whereas oxygen molecules are not influential.

4

In addition, a

study by Pacansky et al. suggested a transition state during the decomposition of perfluorodimethyl ether (CF3OCF3) in the presence of AlF3 using Hartree–Fock calculations. 6 Similarly, a study on the catalytic effect from AlF3 on a longer chain of CF3CF2OCF2CF3 suggested a possible reaction path during decomposition. 7 However, despite the technological importance of this type of material, investigations of the underlying mechanisms of the degradation behaviors from theoretical approaches are still lacking. This is because previous studies mainly focused on the degradation mechanisms under thermal and catalytic effects. However, the mechanical effect is also a critical factor that determines the product life of the film. This is because the use of the PFPE film for providing AF functionality to touchscreen devices such as smartphones and tablets has been rapidly increasing.

10

Hence, this material

should resist repeated shearing/frictional force generated from daily finger-rubbing, which can damage the structure and eventually rip off the whole coated film. An example is shown in Figure 1(a); the optical microscopy image (Nikon Eclipse LV-100N POL) shows that after repeatedly rubbing the PFPE film with a fingertip, the water contact angle decreases significantly from 110° to less than 90°. In addition, a name-pen test (to check whether the film is ink-proof) indicates that several remaining ink spots are now found owing to the damaged area on the film. Previous theoretical approaches have suggested the degradation mechanism based on observation only from the single or a small number of chains, which limits the ability to gain the overall picture when PFPE is applied in real applications. This is because the mechanical degradation not only affects the polymer chain itself, but also the entangled network in the bulk polymer when PFPE is coated on the substrate. In this respect, providing a full atomistic view of the bulk film polymer structure under external mechanical force conditions is important to broaden the understanding of the working 2


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mechanism in degradation behaviors, so as to guide the enhancement of the structural integrity of this material. The conventional structural view of the AF film on a touch-screen device is shown in Figure 1(b). A low-refractivity coating material is deposited on the glass, and then a primer material (hydroxylated SiO2) is placed to deposit the AF film (PFPE-silane). The PFPE monomers and the silane agent are hybridized to form the Si-O-Si bond via the reaction of hydrolysis-condensation-hydrogen bonding formation to the surface of SiO2. 11 In this case, three important characteristics need to be addressed to design high-durability AF film, as shown in Figure 1(c). First, enhancing the intrachain strength of the single polymer chain can aid to preserve the chemical properties and the structural stability so as to maintain its bond-connectivity. Second, increased interchain cohesion in entangled polymer chains conserves the overall structural stability in the film structure by preventing slippage between them. Lastly, the deposition of the PFPE-silane chain monomers should be performed such that the condensed morphology (high density) of the film is achieved, leading to an increase of the interchain strength. In this regard, we herein investigate the underlying degradation mechanisms in the PFPE-silane monomer itself (intrachain), as well as the film structure on silica (interchain). Combining reactive molecular dynamics (MD) simulations and density functional theory (DFT) calculations, we reveal the intrinsic bond strength in the polymer chain and the possible direction to enhance the strength of the chain itself. In addition, the size effect on the mechanical properties of the PFPE-silane bulk film is investigated by implementing the pulling and the uniaxial tensile test. Finally, the polymer deposition is conducted by placing each polymer chain on the surface of SiO2 and it demonstrates that the structural morphology of the film structure is highly dependent on the length of the deposited chain.

METHODS For the description of PFPE-silane on an amorphous SiO2 structure, two different types of interatomic potentials are implemented. First, for the investigation of the intrinsic properties of the PFPE-silane chain itself, reactive force-field (ReaxFF)

12–14

is used because it can describe the bond-breaking

behavior, which is suitable for measuring the bond-strength of the PFPE-silane chain by straining. We adopt previously developed parameters 11 for the interaction between C, H, O, and N with the addition of the C-F interaction.

15

In addition, we applied DFT calculations for measuring the intrinsic bond

strength of the PFPE-silane chain to validate the results with ReaxFF. Meanwhile, to obtain the mechanical properties of the bulk PFPE-silane polymer film and also to perform the deposition 3



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process of the polymer monomer on SiO2, INTERFACE force-field (IFF) 16 is employed. We used IFF for this because ReaxFF has not yet been fully validated for the interaction of PFPE-silane + SiO2, and thus unwanted reactions could be involved, leading to an unphysical outcome. Because IFF was developed based on the general description between atomic species, the abovementioned problem could be avoided. More importantly, we note that because measuring the nonbonded interaction between chains is critical for quantifying the decohesion strength in the bulk polymer film, using IFF is still suitable for capturing detailed physical insights in this type of structure under mechanical deformation. In addition, it has been proven that IFF can predict the adsorption behavior representing the surface chemistry as well as surfactant-mediated crystal growth coverage on different facets, which validates the implementation of this potential for describing the chain deposition test and the adhesion behavior. 17 The MD simulations are performed within the LAMMPS package.

18

The Nosé–Hoover thermostat

and barostat are applied for the temperature and pressure control with damping parameters of 100 fs and 1,000 fs, respectively. The velocity-Verlet time stepping scheme with a time step of 0.5 fs is used. For the single polymer chain, it is relaxed under the NVT ensemble for 0.5 ns at 300 K with open periodic boundary conditions (PBC) in all directions with a vacuum region of 20 Å to prevent any unwanted interaction. For the bulk polymer film, the structure is constructed with the Amorphous cell generator in Materials Studio (ver. 8.0, Accelrys Inc.) and the structural relaxation proceeds for 0.5 ns at 500 K under NVT followed by 1.0 ns at 300 K and 1 atm under NPT. To implement the deposition process of each chain on SiO2, the structure of the PFPE-silane is initially bonded to an arbitrary location on the surface of amorphous SiO2 and the structural relaxation for 1 ns at 300 K is followed. We adopt amorphous silica, whose surface type is Q3 with OH density of 4.7 /nm2, from previous references, as extensive studies have already been performed for this type of structure. 16,19 Then, the deposition process is performed by placing each chain above SiO2 with the frequency of 1 chain every 5,000 time steps. The initial downward velocity of 1.5 Å/fs is applied to the deposited chains to displace them on the SiO2 surface. The NVT ensemble at 300 K is used during the deposition and then the structure is further relaxed for 1.0 ns at 300 K. After the structural relaxation for the chain-only structure and the hybrid structure (PFPE-silane on SiO2), the pulling test is performed using steered molecular dynamics (SMD). This method has been widely applied for investigating inorganic–organic interfaces for measuring adhesion behaviors.

20–23

The velocity of 50

2

m/s is applied with the spring constant of 50 kcal/mol/Å . Regarding the DFT calculations, the Vienna ab-initio simulation package (VASP) 24,25 is used. For the exchange-correlation functional, the generalized gradient approximation (GGA) with Perdue, Burke, and Ernzerhof (PBE) is applied. 26 The plane-wave cutoff is set to 500 eV and the gamma-only k-point 4


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grid is implemented. The structural optimization is performed with the force criterion of 0.03 eV/Å. To measure the bond-strength change during deformation, a simulation cell containing a single polymer chain is strained in the longitudinal direction of the chain backbone atoms and energy minimization is performed until the energy release (bond-breaking) occurs.

RESULTS AND DISCUSSIONS Intrachain strength: Single chain When the PFPE-silane is bonded on SiO2, numerous possible atomic configurations could exist, owing to the many degrees of freedom of the single chain. To enumerate these possibilities, the structural change during relaxation and the strength of the polymer is obtained. First, the atomic structure of the single chain of PFPE-silane is shown in Figure 2(a). As a model structure, we construct the PFPE-silane monomer with the repeating unit (m, n) of 10 bonded to the surface of SiO2, as shown in Figure 2(b). Then, the structural relaxation is conducted, followed by the pulling test. From the macroscopic point of view, the chain seems to maintain its initial morphology during relaxation and no major modification is found. When pulled, as shown in Figure 2(c), the chain is first stretched in the direction of pulling and finally, it reaches the breakage at the bottom region in the chain. It is noted that the chain dissociation occurs at the C-O bond. More importantly, the microscopic observation shows that the local structure of the chain is partially modified from its original bond configuration; the MD snapshots are depicted in Figure 2(d). 1) It is interesting that during the structural relaxation, OH in PFPE-silane forms the hydrogen bond with O on the SiO2. 2) Simultaneously, the Si-C bond in the chain (which is originally in the silane agent) breaks and the end (C) of the broken residue forms a new bond with O in the silane agent (C-O bond). 3) During pulling, OH in the silane is dissociated first. Then, 4) the C-O bond, which originally connects the PFPE and silane agent, finally breaks. This finding suggests that the pristine structure of PFPE-silane is not strong enough (particularly the silane agent) to maintain its structural integrity, even during the initial coating process on SiO2, and hence, an enhancement in the intrachain strength is required for its better durability. Because PFPE-silane on SiO2 initially exhibits Si-C bond breaking during the relaxation process, it is important to elucidate the mechanism of such behavior and to suggest a possible path toward improving the strength of the bond-configuration in the PFPE-silane. Thus, the pulling test is performed using ReaxFF on the monomer itself (m, n = 10), and its force vs. pulling distance response is shown in Figure 3(a). (The pulling force information is also shown in Figure S1, Supporting Information (SI).) As expected, the Si-C bond breaks first at the pulling distance of approximately 5 Å, 5



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proving that it is the weakest. We perform the same test on chains with different repeating units (m, n = 1, 3, 5, and 8) and the results are consistent. In addition, the bond-strength of each constituting bond in PFPE-silane is also measured to quantitatively confirm the weakness of the Si-C bond. Each bond species is shown in Figure 2(a); the pulling test is performed only on the bond of interest while the others are treated as rigid bodies, such that the pulling force is only applied to that bond. The obtained pulling force to break each bond is shown in Table S1, SI, confirming that the Si-C bond is the weakest, and the rest of the order is as follows: C-O, C(F)-O, C-C, …, and Si-O. To further validate these results, DFT calculations are also conducted by applying strain to the chain and the energy variation with respect to the strain is shown in Figure 3(b). Not surprisingly, the Si-C bond breaks first at the strain of 0.2. These two consistent results from different methodologies strongly support that the Si-C bond is the weakest in PFPE-silane, and hence, the chain-dissociation could start to occur here, leading to structural changes in the PFPE-silane chain.

Intrachain strength: Crosslinked chain Because the C-O formation after the bond-break of Si-C is only demonstrated for the single chain on SiO2, it is important to obtain the statistical overview from the cases when a number of chains are placed, which could further confirm the possibility of such behavior. In this respect, a total of 16 chains (single, not-crosslinked) are bonded to the surface of SiO2 as an initial structural setup, shown in Figure 4(a). Then, the structural relaxation is followed to investigate whether the same behavior (Si-C break then C-O formation) occurs. Interestingly, three different atomic configurations are finally obtained. Among 16 single chains, 11 chains reserve their initial structure but three chains experience Si-C break then C-O formation and one chain exhibits the Si-C break without further reaction. This observation strongly supports that the Si-C break is prone to occur in the PFPE-silane structure. Furthermore, the crosslinking effect, which typically occurs in the PFPE-silane chain,

27

is also

investigated. Two chains are connected via a siloxane bond and a total of three crosslinked chains are attached to the surface of SiO2 (Figure 4(b)). During structural relaxation, one crosslinked chain remains in its initial configuration, while the other two experience the Si-C break followed by C-O bond formation. Similarly, 3-crosslinked chains are bonded to SiO2 and the chain configuration is modified in the same way as observed in previous cases (Figure 4(c)). To summarize, current results indicate that some pristine and crosslinked PFPE-silane chains could experience the dissociation of the Si-C bond during initial coating process and they either form a C-O bond or remain as residue without further reaction. This means that the PFPE-silane structure could be easily degraded under ambient conditions, and thus bond-enhancement in Si-C is necessary to 6


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maintain the structural integrity. A possible design guideline could be to replace the weak Si-C bond with a stronger bond such as Si-O. To validate this hypothesis, we conducted the same simulation as that shown in Figure 4, but replaced Si-C with Si-O. Surprisingly, this Si-O bond is not broken during relaxation, regardless of the crosslinking density, such that the overall structural stability of PFPEsilane is now significantly improved (Figure S2, SI). Although further experimental validation is necessary, we believe that this approach can be realized by designing/finding a new type of silane agent molecule, which is the future direction of our research.

Interchain strength The interaction cohesive strength between PFPE-silane chains is another important aspect of determining the structural integrity of the film structure, because their weak interaction results in the disentanglement of chains, which significantly degrades its durability. In order to elucidate this, it is required to understand the film deposition process: how the film structure is formed from the accumulation of the single chains. In this regard, we mimic the spray coating method by depositing the PFPE-silane chain by chain on SiO2 in a timely manner (Figure S3, SI). As explained in the method section, we implemented IFF for atomic interaction. Because IFF cannot describe the bond formation, we started with the SiO2 structure whose surface is already bonded with PFPE-silane chains. Once the structure is constructed, three principal adhesion characteristics could exist, which govern the tribology of the hybrid (SiO2 + PFPE-silane film) structure in Figure S3, SI: A. Interchain interaction in overcoated chains themselves. B. Adhesion between overcoated and SiO2-bonded chains. C. Adhesion between bonded chains and the SiO2 surface. It is clear that the strength for cases A and B is mainly affected by the nonbonded interaction between chains and is thereby weaker than that of case C, whose interaction is ruled by the bonded interaction (chemically bonded). We note that construction of the overcoated region is unavoidable to provide a sufficient number of reactive PFPEsilane chains that can be bonded onto the SiO2 surface. In this regard, the overcoated region will be damaged first during the actual usage of the touch-screen device and thus its role as a protecting layer is also critical. Therefore, investigating cases A and B could bring important perspectives on how to enhance the overall durability of the entire AF film. First, to elucidate the properties from case A, the bulk film structure of the PFPE-silane is constructed as shown in Table I. The size effect is also considered by varying the number of repeating units (m, n) in the chain from 1 to 10 while maintaining the total molecular weight of the bulk structure so that they are close each other. First, it is important to note that the structure with longer chains is more tightly packed based on its larger density (1.88 to 2.39 g/cm3 when (m, n) is increased from 1 to 10). This could be an indication of stronger chain interactions in such a structure. 7



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To verify this hypothesis, two different mechanical tests are performed: the uniaxial tensile test and the decohesion test. We note that the rate of the tensile test, 1010/s, and the pulling velocity of 50 m/s are much faster than the range of regular experiments. However, it is common and practical to use deformation rates of several orders of magnitude higher in MD simulations.

28,29

For example, the

pulling and sliding method implemented by Lee et al. used the rate of 20 m/s within the MD framework and captured the adhesion properties for the system of PVDF on graphite. 29 Another case is for investigating mechanical properties under tensile deformation. Many previous studies using computational approaches applied strain rates of 108–1012/s, which are much faster than those in the experiment (around 100/s).

30–32

However, they have all proven that their results can

qualitatively/quantitatively reproduce the stress–strain curves that are similar to experiments and can conserve the physics behind how the intrinsic structural/mechanical properties are affected. The atomic view of how each method is conducted to deform the structure is shown in Figure 5(a) and 5(c). Both methods can measure the degree of chain-to-chain interaction either by straining (tensile test) or tearing apart (decohesion test). First, the stress vs. strain response from the tensile test is obtained as shown in Figure 5(b), and it exhibits that as the length of the chain increases, the yield strength also increases from 217 to 319 MPa, as shown in Table I. (The yield point is selected where the strain softening is initiated, around 0.18 strain for all three structures.) The same trend is obtained from the decohesion test, whose maximum pulling force value is the largest from the longest chain (Figure 5(d) and Table I). Overall, the current result clearly validates that employing a longer chain is preferable to enhance the interchain interaction in the overcoated film structure, thus potentially improving the durability of the coated film.

Chain deposition and interfacial adhesion strength Although forming longer chains in the overcoated region is desirable for high durability in the AF film, it is also important to investigate how placing chains of different length could affect the morphology of the film during the deposition process. In this case, two regions are constructed on the top of SiO2; i.e., the bonded chain and the overcoated chain, as illustrated in Figure S3, SI. When the maximum degree of bond formation between PFPE-silane and the hydroxyl groups on SiO2 surface is reached during deposition, additionally deposited chains are accumulated on top of pre-existing PFPE-silane; hence, the interaction between these two regions is an important factor for determining the film durability. To consider this effect more clearly, we study the case when chains with different lengths are employed and the corresponding structural setup is shown in Table S2, SI. The repeating unit varies from 1 to 10 for both regions while maintaining the total molecular weight of the bonded chain + deposited film to be similar, at approximately 40,000 u. It is assumed that, regardless of what 8


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length of chain is deposited, the number of bonded chains to the surface of SiO2 (total of nine chains) is maintained as constant. In other words, when shorter chains are bonded, the molecular weight of the deposited chains is larger and vice versa. Figure 6 shows the atomic snapshots after the deposition process for each configuration. First, it is noticeable that when longer chains, especially with m, n = 10, are initially bonded to the SiO2 surface, the final film structure contains void areas, unlike when the shortest chains (m, n = 1) are bonded. In other words, as the length of the bonded chains decreases, denser chain arrangement can be achieved. This is because the short chains are aligned parallel to the surface of SiO2 during relaxation. Such behavior is possible because larger free space is available for shorter chains and the interaction between them is weak; hence, they can move around more easily. However, for longer chains, their movement is limited because adjacent chains already occupy the free space and thus, they can be easily accumulated on or entangled with each other. It is important to note that the entangling capability of the longer chains is desirable when constructing the overcoated region because such functionality can increase the mechanical properties of the film structure, as discussed before (Figure 5). Once the region with the bonded chain is wellconstructed; i.e., even and dense morphology formation, depositing longer chains could lead to a densely packed overcoated structure. Unlike the case of the bonded chains, deposited chains have more degrees of freedom because they do not need to form chemical bonds with the surface of SiO2. In addition, the end-to-end distance (REE) of the deposited chains is calculated and divided by its original length (REE0) for all cases in Table S2, SI. This shows that when the longer chains are deposited, a smaller value of the ratio (REE/REE0) is obtained, indicating a higher degree of entanglement in those chains. It is also important to note that when longer chains are initially bonded on the silica surface, a larger value of REE/REE0 is shown when the chains with the same length are deposited. Based on this result and the atomic snapshots in Figure 6, it seems that short (m, n = 1) chains should initially be bonded to the surface, which should then be followed by deposition with long (m, n = 10) chains to construct an ideal morphology (increased entanglement) of the film (dotted box). Because the above finding is mostly based on the qualitative observation of the film morphology, it is beneficial to measure the actual interfacial adhesion in a quantitative way. After the deposition, two interfacial regions, which affect the structural integrity of the hybrid structure, are available where the interaction between the bonded and deposited film (interfacial strength) and the strength of the deposited film (interchain strength) is critical, as shown in Figure 7(a). Both of these regions should resist the external forces from daily usage. We note that the interaction at the interface (Figure 7(a), left) is more important because if the interfacial strength of this region is too weak, then the entire 9



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overcoated film could be easily ripped off. To measure the strength of both cases, the pulling test is performed and the corresponding maximum pulling force value is shown in Figure 7(b). First, it is important that the strength at the interface is stronger (solid bar) than the interchain strength (shaded bar), which is desirable for the overcoated region. This could be because in this case, the bonded AF film can actively participate during the pulling process by directly stretching with respect to the deformation. Hence, the pulling force needs to overcome the bonded as well as the nonbonded interaction between chains. However, when pulling only the top portion of the deposited film, the bottom portion can be also stretched during pulling; thus nonbonded interaction in those chains plays a more significant role and the resulting pulling force could be smaller. More importantly, we note that as the length of the bonded chain increases, the maximum pulling force value generally becomes smaller for all cases. As expected from the atomic snapshots in Figure 6, such behavior is directly associated to the larger void area formed from longer bonded chains. The best case which the pulling force is the largest is obtained from the short chain (m, n = 1) on the bottom and the long chain (m, n = 10) on the top. Again, this results from the fact that depositing longer chains leads to more entanglement in the overcoated chains as well as between the deposited and initially bonded chains. Moreover, the short chains bonded to the SiO2 surface leads to the formation of a dense morphology during the deposition. To summarize, the interchain interaction in the AF film can be largely affected by the length variation of the deposited chains and sophisticated control of this could greatly enhance the durability of the film. Before offering our concluding remarks, we address some important perspectives in terms of the limitations and uncertainties of this computational approach. 1) Although we found the structural change and possible bond-breaking behavior at the interface, it is still important to validate this result with experimental analytical tools to confirm the broken species and the morphology of the coated film. 2) The results from the MD simulations are ensemble-averaged with an error bar of typically 5– 10% (Table I). More importantly, we claim that the actual values of the mechanical and adhesion properties should be used in a qualitatively way. This is because the current MD simulations limit the time and size scales. However, it is still particularly useful to capture the trend of properties in the materials of interest and to develop the design principles. 3) Because the environmental gas species are omitted, such as O2, N2, H2O, and so on, the current results assume perfect conditions (i.e., no environmental effects); hence, including those effects in future studies will be informative to experimentalists.

10


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CONCLUSIONS Degradation mechanism that affects the durability of the AF film is investigated extensively in terms of two aspects: intra- and interchain strength. From the intrachain strength, the Si-C bond, which connects the PFPE and the silane agent, is the weakest point and therefore can be easily dissociated and subsequently form new C-O bond. This behavior is demonstrated consistently, regardless of different crosslinking densities in the film structure. Therefore, substituting the Si-C bond with a stronger one should be considered to prolong the product life of this type of film. For the interchain interaction, increasing the chain length can enhance the interchain strength in the film structure. Furthermore, the deposition test, mimicking the experimental coating process, is performed by placing each chain on the SiO2 surface in a timely manner. This result indicates that it is desirable to first use shorter chains to form the siloxane bond to the surface of SiO2, which should then be followed by longer chain deposition for improved decohesion strength in the chains. The current study reveals a new design principle for optimizing the chemical composition in the AF polymer and the morphology formation during the film-coating process, which can construct high-durability AF film on touchscreen devices.

Supporting Information Intrachain strength, Structural change during relaxation, Chain deposition process, Structural setup, and the degree of entanglement after deposition process

TOC

Intrachain Strength

Interchain Cohesion

Dense Deposition

=

Highly durable AF Film

11



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REFERENCES (1)

Rudnick, L. R.; Shubkin, R. L. Synthetic Lubricants And High- Performance Functional Fluids, Revised And Expanded; Crc Press, 1999.

(2)

Tao, Z.; Bhushan, B. Degradation Mechanisms and Environmental Effects on Perfluoropolyether, Self-Assembled Monolayers, and Diamondlike Carbon Films. Langmuir 2005, 21 (6), 2391–2399.

(3)

Zhao, Z.; Bhushan, B. Humidity Effect on Friction/Stiction and Durability of Head-Disk Interface with Polar Perfluoropolyether Lubricant. J. Appl. Phys. 1997, 81 (8), 5387–5389.

(4)

Lotfi, R.; van Duin, A. C. T.; Biswas, M. M. Molecular Dynamics Simulations of Perfluoropolyether Lubricant Degradation in the Presence of Oxygen, Water, and Oxide Nanoparticles Using a ReaxFF Reactive Force Field. J. Phys. Chem. C 2018, 122 (5), 2684– 2695.

(5)

Booth, B. D.; Vilt, S. G.; Lewis, J. Ben; Rivera, J. L.; Buehler, E. A.; McCabe, C.; Jennings, G. K. Tribological Durability of Silane Monolayers on Silicon. Langmuir 2011, 27 (10), 5909– 5917.

(6)

Pacansky, J.; Waltman, R. J. The Effect of Lewis Acid Catalysis on the Decomposition of CF3OCF3 to COF2 and CF4. J. Fluor. Chem. 1997, 83 (1), 41–45.

(7)

Jiang, B.; Keffer, D. J.; Edwards, B. J. A Quantum Mechanical Study of the Decomposition of CF3OCF3 and CF3CF2OCF2CF3 in the Presence of AlF3. J. Phys. Chem. A 2008, 112 (12), 2604–2609.

(8)

Jiang, B.; Selvan, M. E.; Keffer, D. J.; Edwards, B. J. A Reactive Molecular Dynamics Study of the Thermal Decomposition of Perfluorodimethyl Ether. J. Phys. Chem. B 2009, 113 (42), 13670–13677.

(9)

Liu, J.; Stirniman, M. J.; Gui, J. Catalytic Decomposition of Perfluoropolyether Lubricants. IEEE Trans. Magn. 2003, 39 (2), 749–753.

(10)

Masuko, M.; Ikushima, F.; Aoki, S.; Suzuki, A. Preliminary Study on the Tribology of an Organic-Molecule-Coated Touch Panel Display Surface. Tribol. Int. 2013, 65, 314–325.

(11)

Deetz, J. D.; Faller, R. Parallel Optimization of a Reactive Force Field for Polycondensation of Alkoxysilanes. J. Phys. Chem. B 2014, 118 (37), 10966–10978.

(12)

Fogarty, J. C.; Aktulga, H. M.; Grama, A. Y.; van Duin, A. C. T.; Pandit, S. A. A Reactive Molecular Dynamics Simulation of the Silica-Water Interface. J. Chem. Phys. 2010, 132 (17).

(13)

van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105 (41), 9396–9409.

(14)

Ostadhossein, A.; Kim, S.-Y.; Cubuk, E. D.; Qi, Y.; van Duin, A. C. T. Atomic Insight into the Lithium Storage and Diffusion Mechanism of SiO2/Al2O3 Electrodes of Lithium Ion Batteries: 12


Page 13 of 22 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


ReaxFF Reactive Force Field Modeling. J. Phys. Chem. A 2016, 120 (13), 2114–2127. (15)

Singh, S. K.; Srinivasan, S. G.; Neek-Amal, M.; Costamagna, S.; van Duin, A. C. T.; Peeters, F. M. Thermal Properties of Fluorinated Graphene. Phys. Rev. B 2013, 87 (10), 104114.

(16)

Heinz, H.; Lin, T.-J.; Mishra, R. K.; Emami, F. S. Thermodynamically Consistent Force Fields for the Assembly of Inorganic, Organic, and Biological Nanostructures: The INTERFACE Force Field. Langmuir 2013, 29 (6), 1754–1765.

(17)

Heinz, H. Adsorption of Biomolecules and Polymers on Silicates, Glasses, and Oxides: Mechanisms, Predictions, and Opportunities by Molecular Simulation. Curr. Opin. Chem. Eng. 2016, 11, 34–41.

(18)

Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117 (1), 1–19.

(19)

Emami, F. S.; Puddu, V.; Berry, R. J.; Varshney, V.; Patwardhan, S. V; Perry, C. C.; Heinz, H. Force Field and a Surface Model Database for Silica to Simulate Interfacial Properties in Atomic Resolution. Chem. Mater. 2014, 26 (8), 2647–2658.

(20)

Min, K.; Rammohan, A. R.; Lee, H. S.; Shin, J.; Lee, S. H.; Goyal, S.; Park, H.; Mauro, J. C.; Stewart, R.; Botu, V.; et al. Computational Approaches for Investigating Interfacial Adhesion Phenomena of Polyimide on Silica Glass. Sci. Rep. 2017, 7 (1), 10475.

(21)

Min, K.; Kim, Y.; Goyal, S.; Lee, S. H.; McKenzie, M.; Park, H.; Savoy, E. S.; Rammohan, A. R.; Mauro, J. C.; Kim, H.; et al. Interfacial Adhesion Behavior of Polyimides on Silica Glass: A Molecular Dynamics Study. Polymer. 2016, 98, 1–10.

(22)

Park, S.; Khalili-Araghi, F.; Tajkhorshid, E.; Schulten, K. Free Energy Calculation from Steered Molecular Dynamics Simulations Using Jarzynski’s Equality. J. Chem. Phys. 2003, 119 (6).

(23)

Park, S.; Schulten, K. Calculating Potentials of Mean Force from Steered Molecular Dynamics Simulations. J. Chem. Phys. 2004, 120 (13).

(24)

Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6 (1), 15–50.

(25)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab-Initio Total Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54 (16), 11169–11186.

(26)

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865–3868.

(27)

Haensch, C.; Hoeppener, S.; Schubert, U. S. Chemical Modification of Self-Assembled Silane Based Monolayers by Surface Reactions. Chem. Soc. Rev. 2010, 39 (6), 2323–2334.

(28)

Jose, J.; T, B. V.; Swaminathan, N. Insights into Traction-Separation Phenomena of GrapheneCis-1,4-Polyisoprene Interface Using Molecular Dynamics. Polymer. 2017, 122, 280–295.

(29)

Lee, S.; Park, J.; Yang, J.; Lu, W. Molecular Dynamics Simulations of the Traction-Separation Response at the Interface between PVDF Binder and Graphite in the Electrode of Li-Ion Batteries. J. Electrochem. Soc. 2014, 161 (9), A1218–A1223.

(30)

Hossain, D.; Tschopp, M. A.; Ward, D. K.; Bouvard, J. L.; Wang, P.; Horstemeyer, M. F. Molecular Dynamics Simulations of Deformation Mechanisms of Amorphous Polyethylene. Polymer. 2010, 51 (25), 6071–6083.

(31)

Lu, C.-T.; Weerasinghe, A.; Maroudas, D.; Ramasubramaniam, A. A Comparison of the Elastic Properties of Graphene- and Fullerene-Reinforced Polymer Composites: The Role of Filler Morphology and Size. 2016, 6, 31735.

(32)

Echtermeyer, I. H. S. and A. T. Effects of Temperature and Strain Rate on the Deformation of Amorphous Polyethylene: A Comparison between Molecular Dynamics Simulations and Experimental Results. Model. Simul. Mater. Sci. Eng. 2013, 21 (6), 65016. 13



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Figure 1. (a) Optical microscopy image for the structural degradation of PFPE-silane film after rubbing with fingertip. Cyan indicates the film material and the blue spots in the right figure indicate remaining name-pen ink. (b) Schematic view of a touch-screen device to which anti-fingerprint (AF) film is applied. (c) Design principle for achieving high durability in AF film.

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ACS Applied Materials & Interfaces 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

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Figure 2. (a) Atomic structure of PFPE-silane structure and bond-species. (b) Structural setup for PFPE-silane bonded on SiO2 and (c) its atomistic view during relaxation and subsequent pulling test. (d) Detailed view of how the bond-breaking and formation occurs during relaxation and the pulling test.

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Figure 3. Intrachain strength measured by the pulling test (a) with ReaxFF and (b) with DFT calculations.

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Figure 4. Structural change and possible snapshots during relaxation when a number of single (top), 2crosslinked (middle), and 3-crosslinked (bottom) chains are placed on SiO2. The number in parentheses indicates the number of chains satisfying each condition.

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Structure Setup Repeating unit (m, n)

1

5

10

# of chains

45

18

10

Total molecular weight [u]

21077.1

21539.16

21069.2

Structural/Mechanical/Adhesion Properties Density [g/cm3]

1.8818

2.2285

2.3968

Yield stress [MPa]

217 ± 25

273 ± 23

319 ± 19

Maximum pulling force [MPa]

362 ± 13

642 ± 19

741 ± 23

Table I. Structural setup for constructing bulk film structures and corresponding properties of PFPEsilane with different numbers of repeating units.

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Figure 5. (a) Atomistic view of how the uniaxial tensile test is performed and (b) its corresponding stress vs. strain response under the strain rate of 1010/s. (c) Atomistic view of how the decohesion test is performed and (d) its corresponding pulling force vs. distance response.

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Figure 6. Atomic snapshots after chain deposition process for PFPE-silane chains on SiO2 (bottom, brown) with different numbers of repeat units. The black region is where the bonded chains are located. The number on the top of the structure indicates the degree of entanglement (REE/REE0).

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Figure 7. (a) Schematic view of the interfacial and interchain strength between deposited (overcoated) and bonded chains and (b) their maximum pulling force when the number of repeating units (m, n) varies.

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Characterization of mechanical degradation in perfluoropolyether film

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