Foldable and extremely scratch-resistant hard coating materials from

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Foldable and Extremely Scratch-Resistant Hard Coating Materials from Molecular Necklace-like Cross-Linkers Jiae Seo,† Sung Wook Moon,‡ Heemin Kang,† Byoung-Ho Choi,‡ and Ji-Hun Seo*,† †

Department of Materials Science and Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul, Korea School of Mechanical Engineering, College of Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul, Korea

‡

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S Supporting Information *

ABSTRACT: A flexible hard coating material displaying extreme scratch resistance and foldable flexibility was developed via the design of an organic− inorganic hybrid coating material employing an alkoxysilyl-functionalized polyrotaxane cross-linker (PRX_Si1). PRX_Si1 has a molecular necklace-like structure that can form organic−inorganic cross-linking points and provide large molecular movements. It was postulated that the scratch resistance and flexibility could be simultaneously increased because of the hybrid cross-linking points and dynamic molecular movements. To confirm this hypothesis, the crystalline structure and mechanical properties of the PRX_Si1-based hard coating material were analyzed via transmission electron microscopy, small-angle X-ray diffraction, tensile, pencil hardness, and scratch tests. Finally, the PRX_Si1-based hard coating material could form homogeneously dispersed nanoscale siloxane crystalline domains, and the strain at the break point was 3 times higher than that of a commercial hard coating material, resulting in no defect formation even after 5000 folding test runs. Moreover, the material displayed extremely high pencil hardness (9H) and scratch resistance. KEYWORDS: hybrid, siloxane, polyrotaxane, cross-linker, antiscratch, coating material polyimide derivatives.10,11 However, the flexibility enhancement is clearly theoretically limited for traditional network polymers, the mechanical strength of which depends largely on the cross-linking density; thus, network polymers are also not free from the trade-off problem.12 Therefore, a novel material design concept is required for the development of extremely flexible hard coating materials to overcome the paradoxical trade-off limitations. Polyrotaxane (PRX) is a necklace-like supermolecule composed of a ring-shaped host molecule, for example αcyclodextrin (α-CD), threaded onto a linear guest molecule, for example, poly(ethylene glycol) (PEG), and around 10−80 α-CD molecules can be threaded onto a single PEG chain with a molecular weight of 10000.13−16 Because there are six primary hydroxyl groups and 12 secondary hydroxyl groups in a single α-CD molecule, theoretically, around 180−1440 reactive groups can be introduced into PEG with a molecular weight of 10 000, and this is an ideal molecular structure to maximize the cross-linking density of polymer networks.17−19 Moreover, cross-linked α-CD molecules are expected to display dynamic movement along the PEG axis under physical stretching because PRX is not a covalently, but mechanochemically, synthesized molecule.20 Therefore, PRX-based hard coating materials are anticipated to display high scratch

1. INTRODUCTION Hard coating materials are widely used, not only for information technology (IT) devices but also for automobile and miscellaneous commodities to protect the devices from the physical damages such as scratches, indentation, physical impact, and so forth.1−4 The initial hard coating agent for display devices was developed as a rigid inorganic glassy material based on the condensation reaction of the alkoxysilyl group. However, inorganic hard coating materials display a lack of flexibility and are susceptible to external stresses, resulting in brittleness.5 For this reason, organic−inorganic hybrid coating materials have been actively developed in recent years to more effectively buffer the external impact to the surfaces based on the flexible properties of the organic compounds.6 However, as display materials undergo development from curved to bendable and foldable displays, extreme flexibility of the hard coating materials is required.7 In general, the resistance to scratching and abrasion of the surface is a characteristic that depends on the hardness of the surface and tends to increase with the content of the rigid inorganic component in organic−inorganic hybrid materials.8 On the other hand, flexibility requires a higher content of the organic compound, resulting in a trade-off, whereby the mechanical strength, as well as the hardness, must be sacrificed to obtain greater flexibility. To solve these problems, the use of cross-linked organic materials showing excellent abrasion resistance has been examined.9 These are epoxy-based materials or highly cross-linked acrylic resins combined with © XXXX American Chemical Society

Received: April 1, 2019 Accepted: June 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acsami.9b05738 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Composition of the Coating Materials and Ratio

dried and dispersed again in ethanol (35 mL). Thereafter, DMT-MM (1 g) and of Z-Tyr-OH (1 g) were added, and the mixture was stirred for 24 h. After dialysis in Mw = 12k dialysis tube, the product was recovered by freeze-drying to obtain PRX powder. The process of introducing triethoxysilyl propyl group into PRX was then performed as follows. The recovered PRX (100 mg) was dissolved in dimethyl sulfoxide (DMSO) (2 mL), and 2-(triethoxysilyl)propyl isocyanate (0.5 mmol) was added and allowed to react. Subsequently, the mixture was dialyzed in methyl isobutyl ketone (MIBK) for 2 days. The silane-induced PRX powder was collected by evaporating the solvent. 2.2.3. Synthesis of Quaternary Random Copolymer (4P). 3(trimethoxysilyl) propyl methacrylate (3.75 mmol), acrylic styrene (7.5 mmol), n-butyl methacrylate (1.5 mmol), and 2-hydroxypropyl acrylate (2.25 mmol) were added to MIBK (15 mL). Polymerization was performed for 24 h using AIBN as an initiator at 60 °C. After the polymerization, the 4P solution was dialyzed with MIBK for 3 days to remove the remaining monomer and confirmed by NMR (Figure S2). 2.2.4. Preparation of the Coating. The coating mixture consisted of quaternary random copolymer (4P), hexa(methoxymethyl)melamine (M), Paraloid B-72 for dispersant (N), and the crosslinker components was mixed homogeneously according to the ratio shown in Table 1. To achieve homogenous mixing of the PRX series of additives, the cross-linkers were dissolved in the minimum amount of DMSO and mixed in the coating mixture. A bar coater (KP-3000, KIPAE Corp., Suwon, Republic of Korea) was used to uniformly spread out the coating solution using the no. 4 bar (coating thickness = 10 μm), and the product was cured at 80 °C for 24 h under humid conditions in an oven and postcured for 4 h at 120 °C. Through this series of processes, the cured clear coating could be obtained. 2.2.5. Transmission Electron Microscopy. Each prepared coating solution was prepared by diluting 8 times. Then, drop on a carbon mesh grid and completely cure it by curing process at 80 °C for 24 h and 120 °C for 4 h. It was then observed using a Transmission Electron Microscopy (TEM) (Tecnai 20, FEI, Japan). 2.2.6. Fabrication of the Free-Standing Film. The water-soluble PVP (Mw = 10k) was coated onto a glass substrate and cured at 100 °C for 24 h. The coating solution was then coated onto the hardened PVP film and cured at 80 °C for 24 h and postcured at 120 °C for 4 h. Finally, the cured substrate was immersed in 37 °C water for 6 h, and the free-standing film was obtained by sacrificing the PVP film. 2.2.7. UV−vis Spectroscopy. The transmittance of the each single film was measured using a UV-vis spectroscopy (Varian Cary 50 and Agilent Cary 5000). 2.2.8. X-ray Photoelectron Spectroscopy. Surface elemental analysis of the single-film was conducted using X-ray photoelectron spectroscopy (XPS) (X-TOOL, ULVAC-PHI, Japan) with Al Kα sources. The X-ray detector was placed at an angle of 45° to the surface of the samples. 2.2.9. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using a TGA-Q500 system obtained from TA Instruments at a heating rate of 10 °C/min under an N2 atmosphere.

resistance due to formation of the cross-linked network, and extremely flexible nature derived from the movable crosslinking points. To prove this hypothesis, a PRX-based organic−inorganic hybrid material is introduced and evaluated in this study. A part of the primary hydroxyl group in α-CD is substituted by siloxane precursors (alkoxysilyl groups) to form a highly scratch-resistant inorganic glassy cross-linking point on the PRX framework. In addition, melamine agents capable of forming organic cross-linking points with the residual primary and secondary hydroxyl groups in α-CD are further introduced to maximize the cross-linking points on the dynamic PRX framework.21 Based on this experimental design, the probability of developing a flexible, but extremely scratchresistant, hard coating material is examined.

2. MATERIALS AND METHODS 2.1. Materials. PEG (10k), dehydrated chloroform, indium tin oxide (ITO) film, and poly(ethylene terephthalate) (PET) film were purchased from Sigma-Aldrich (St. Louis, MO, USA). N,N′Carbonyldiimidazole (CDI), ethylenediamine, α-CD, 4-(4,6-dimethoxy-1,3,5-tiazin-2-yl)-4-methylmorpholinium chloride (DMT-MM), N-carbobenzoxy-L-tyrosine (Z-Tyr-OH), hexa(methoxymethyl)melamine (M), 1,2-bis(triethoxysilyl)ethane (Si), 2-(triethoxysilyl)propyl isocyanate, 3-(trimethoxysilyl) propyl methacrylate, acrylic styrene, n-butyl methacrylate, and 2-hydroxypropyl acrylate were purchased from TCI Co., Ltd. (Nihonbashi-honcho, Chuo-ku, Japan). Poly(vinylpyrrolidone) (PVP) (58k) was purchased from Alfa Aesar (Massachusetts, State, USA). All of the organic solvents were purchased from Samchun Chemical (Gangnam-gu, Seoul, Korea). Paraloid B-72 (N) was received from Connell Brothers Co. (Mapogu, Seoul, Korea). The SHPC-100KI denoted “Commercial” solution for the flexible display coating was received from Special Materials Source (Hwaseong, Gyeonggi-Do, Korea). Clear Coat, a commercially available hard coating material used in the automotive industry, was kindly provided by KCC Central Research Institute (Yongin, Gyeonggi-Do, Korea). 2.2. Experimental Procedure. 2.2.1. Synthesis of PEG-bis(amine) Mn = 10 000. Polyethylene glycol (0.03 M; Mn = 10 000) (PEG) was stirred in dehydrated chloroform. When it was completely dissolved, N,N′-carbonyldiimidazole (CDI; 4 mol equiv relative to PEG) was added and the solution was stirred vigorously for 24 h. Ethylenediamine was then added at 12 times the number of moles of PEG. After another 24 h, the solution was poured into diethylether and washed with fresh ether three times. Thereafter, the PEGbis(amine) powder was collected and confirmed by NMR (Figure S1). 2.2.2. Synthesis of PRX and Alkoxysilyl-Functionalized PRX (PRX_Si1 and PRX_Si4). PRX was synthesized by a method similar to that reported in a previous study.10 First, α-CD (7 g) and PEGbis(amine) (1 g) were dissolved in distilled water (35 mL) and allowed to react for 1 day. If a white paste developed, it was freezeB


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ACS Applied Materials & Interfaces

Scheme 1. Reaction Scheme for Formation of Organic−Inorganic Hybrid Cross-Linking Points by Silane-Induced PRX and Coating Material

Figure 1. TEM images of the different coating composition, the combination of the samples as followed, (a) PRX_Si1, (b) N_PRX_Si1, (c) 4PN_PRX_Si1, (d) MN_PRX_Si1, and (e) 4PMN_PRX_Si1. (f) Schematic explanation of nanocluster formation (N = dispersant, 4P = random copolymer, M = melamine). The samples were heated from room temperature to 800 °C to compare the organic and inorganic content. 2.2.10. Dynamic Mechanical Analysis. The single-film was prepared by already mentioned. The fully cured specimens were then subjected to dynamic mechanical analysis (DMA) treatment (Seiko Exstar 6000, SEICO INST., Japan), under bending mode, at a frequency of 1 Hz and an elevating temperature of 5 °C/min. The sample size was determined according to ASTM D4065. Threefold measurements were recorded for each sample, and statistical processing was performed using the t-test. 2.2.11. Microtensile Test. The free-standing film or coated ITO film was prepared in the same manner using the bar coater. The sample size is according to ASTM D 822 for thin plastic films. The

hardened specimen was subjected to a tensile test at a speed of 0.1 mm/min. Triplicate measurements were performed for each sample, and statistical processing was performed using the t-test. 2.2.12. Progressive Scratch Test. The coated ITO film was prepared in the same manner using the bar coater. The sample size was 50 × 150 mm, and the scratch test was conducted using a Scratch Tester (Kato Tech KK-01, Kato Tech. Corp., LTD., Minami-ku, Kyoto, Japan). The scratch velocity was maintained at 18.05 mm/s, and the tip load was increased from 2 to 3 N (0−10 mm distance). Triplicate measurements were performed for each sample, and statistical processing was performed using the t-test. 2.2.13. Pencil Hardness Test. Pencil hardness tests were conducted using the KS M ISO 1518-1 method. The pencil set used for the test C


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ACS Applied Materials & Interfaces was made by Mitsubishi. The pencil hardness results were visually confirmed. 2.3. Experimental Analysis. 2.3.1. Small-Angle X-ray Scattering Analysis. The homogeneous coating solution was coated on the ITO substrate with a bar coater and cured at 80 °C for 24 h and postcured at 120 °C for 4 h. The sample used for small-angle X-ray diffraction (SAXS) (Smart Lab, Rigaku, Texas, USA) analysis had dimensions of 10 × 10 mm. The X-ray source was operated at 9 kW (45 kV/200 mA) using a Cu target (wavelength: 1.5412 Å) and the data were acquired from 0° to 1°. 2.3.2. 3D-Confocal Analysis. In order to analyze the surface damaged by the scratch test, the three-dimensional surface morphology was observed by confocal laser scanning microscopy (VK 8710, Keyence, Osaka, Japan) after the scratch test. The confocal images were fixed by using a 3D image analysis program (VK analyzer, Keyence, Osaka, Japan) by selecting upper and lower thresholds. Measurements were performed for three different points on each sample.

in Figure 1a. Without additives, the PRX_Si1 molecules undergo extensive agglomeration, generating tube-shaped siloxane crystals, as revealed by more detailed observations. Because alkoxysilyl groups are introduced in every α-CD molecule threaded on PRX_Si1, the siloxane crystal was generated by self-condensation of the alkoxysilyl groups positioned on neighboring α-CDs. As a result, (100) siloxane planes with an interplanar spacing of 0.417 nm are formed parallel to the central PEG axis. To avoid agglomeration of the PRX_Si1 molecules, PRX_Si1 was mixed with a nonreactive dispersant polymer (Paraloid B-72, denoted as N).26 As shown in Figure 1b, no significant agglomeration was observed in the N_PRX_Si1 sample, and a cylindrical crystal structure similar to that of the PRX_Si1 sample was also observed for the N_PRX_Si1 sample, which indicates that PRX_Si1 molecules could be uniformly dispersed and the same self-condensation reaction could be induced, even in the dispersant polymers. The 100 nm length rodlike structure could be a unique nanostructure of self-condensed PRX_Si1 molecules, considering that the length of PEG (Mn = 10 000) is about 100 nm. Although the characteristic cylindrical crystalline structure formed from self-condensed PRX_Si1 was well defined, this could not be the networked structure resulting from the crosslinking process that was anticipated to confer the flexible and scratch-resistance properties. Therefore, the alkoxysilyl and hydroxyl groups in the PRX_Si1 molecules must be consumed to form inorganic and organic cross-linking points with other molecules such as 4P or melamine.27 To confirm the possibility of forming inorganic cross-linking points by reaction of the alkoxysilyl groups on the α-CDs with the external alkoxysilyl groups belonging to the polymeric 4P molecules, PRX_Si1 was mixed with the polymeric 4P in the dispersant and TEM was conducted. As shown in Figure 1c, rodlike crystalline structures with the characteristic 0.339 nm interplanar spacing of the (011) crystalline plane of siloxane were observed throughout the materials. Interestingly, the direction of the crystalline plane was perpendicular to the PEG axis in PRX_Si1, whereas parallel (100) planes were observed when the film was made by using only PRX_Si1 without 4P (Figure 1a,b). Note that the (100) and (011) planes are perpendicular to each other.28,29 The reason why specific (100) and (011) planes are mainly observed from the TEM images was not yet clearly understood. Probably, mechanism studies for crystallization process induced by the PRX molecules must be conducted. In any event, it is thought that the alkoxysilyl groups in the α-CDs of PRX_Si1 undergo condensation with those in the 4P polymer chains, thereby forming the siloxane crystalline structure in the direction perpendicular to the PEG axis of PRX_Si1, as shown in the reaction scheme in Figure 1c. To confirm the possibility of forming organic cross-linking points by reaction of the residual hydroxyl group on the α-CDs in the PRX_Si1 with external molecules, PRX_Si1 was reacted with the melamine derivative (M) in the dispersant polymer. Melamine derivatives are known to react with hydroxyl groups to form amino ether bonds.30 Therefore, many kinds of synthetic resins containing hydroxyl groups employ melamine derivatives to increase the cross-linking density of the cured resin. In this study, the melamine derivative (M) was added to confirm the formation of an amino ether bond with the residual hydroxyl groups on the α-CDs of the PRX derivatives. Figure 1d shows the TEM images of the MN_PRX_Si1 sample after the curing process. Unlike the PRX_Si1 samples without

3. RESULTS AND DISCUSSION To develop high-performance flexible hard coating materials, a PRX-based organic−inorganic hybrid network material is introduced in this study. To this end, alkoxysilyl-functionalized linear polymer (4P) and alkoxysilyl-functionalized necklacelike structure PRX derivatives were synthesized, each of which was mixed with a melamine additive (M) and dispersant (N) to fabricate the coating materials (Table 1). The composition and ratio shown in Table 1 are the optimized composition and ratio to secure the solubility and transparency for preparing transparent hard coating materials. The number of α-CDs contained in one (PEG) molecule was determined by 1H NMR and gel permeation chromatography analysis22 (Figure S1, Table S1), and the number of alkoxysilyl groups introduced into the α-CD was calculated through analysis of the 1H NMR peaks (Figure S1, Table S1). PRX cross-linkers containing different numbers of alkoxysilyl groups were thus synthesized (PRX_Si1 and PRX_Si4). The anticipated reaction mechanism and overall procedure for preparing the PRX-based hard coating materials is illustrated in Scheme 1. A quaternary random copolymer (4P) containing alkoxysilyl and hydroxyl functional groups was designed to form the inorganic and organic cross-linking points with the PRX derivatives. The alkoxysilyl groups in 4P could form an inorganic (−Si−O−Si−) cross-linking point with the alkoxysilyl groups introduced in the PRX derivatives by condensation,23 and a hydroxyl group could form an organic crosslinking point (−CH2−O−CH2−N−) with the hydroxyl groups in the PRX derivatives by mediation with hexamethoxymethylmelamine (M).24 Through these reactions, inorganic and organic hybrid networks were anticipated to be formed on the flexible PRX molecules, giving rise to flexible, but extremely scratch-resistant properties. To obtain flexible and scratch-resistant properties, uniform dispersion of the organic and inorganic nanophases in the coating material is very important because uniformity of the multiphase is responsible for the enhancement of the contradictory physical properties, such as flexibility and surface hardness, induced by different phases.25 In this study, the optimal conditions for formation of the uniform organic− inorganic hybrid structure were examined by mixing PRX_Si1 with various additives. Figure 1 shows the TEM images of the PRX_Si1-containing coating materials prepared with different combinations of various additives (Table 1). First, a film was made using only PRX_Si1, the TEM image of which is shown D


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Figure 2. (a) S−S curve of the free-standing film of the coating materials according to the different composition. The bar graph of (b) tensile strength and (c) tensile strain at breaking point extracted from S−S curve. (d) Scratch resistance data for coating materials on ITO film, and (e) bar graph of a maximum coefficient of static friction measured from scratch tester. The confocal image after scratching of the samples: (f) N_PRX_Si1, (g) 4PN_PRX_Si1, (h) MN_PRX_Si1, and (i) 4PMN_PRX_Si1.

other reactive species, 5 nm dot-shaped multicrystalline structures with interplanar distances of 0.369 and 0.317 nm were observed for the MN_PRX_Si1 sample, which do not correspond to the characteristic crystalline structures of siloxanes (Figure 1c). The melamine derivative could form up to six cross-linking points by forming an amino ether bond with the same melamine molecules (M) or other molecules containing hydroxyl groups. As a result, characteristic ordered structures with interplanar distances of 0.369 and 0.317 nm were observed due to self-networking or reaction with the hydroxyl group (Figure S2a). Because α-CD molecules contain a large amount of hydroxyl groups, melamine molecules (M) are thought to react with the α-CDs by disrupting the selfcondensation reactions of the alkoxysilyl groups on the neighboring α-CDs; thus, the cylindrical siloxane crystal structure disappeared. It could thus be concluded that the organic cross-linking points could be formed on PRX_Si1 via mediation by the melamine additive (M), and the alkoxysilyl groups on the α-CD could not induce self-condensation with neighboring α-CDs.31 Finally, PRX_Si1 was mixed with both melamine molecule (M) and 4P polymers in the dispersant to form an organic−inorganic hybrid cross-linked structure (Figure 1e). Thus, dot-shaped siloxane clusters with identical 0.339 nm interplanar spacings were observed. Although the 0.339 nm interplanar spacing is the characteristic spacing of the (011) siloxane planes, as shown in Figure 1d, the rodlike crystalline structure was not observed in this case. Instead, dotshaped clusters with characteristic 0.317 and 0.369 nm interplanar spacings were also observed, which was the same crystalline structure shown in Figure 1d. Because PRX_Si1 contains both alkoxysilyl and hydroxyl groups, condensation reactions with the external alkoxysilyl groups belonging to the 4P polymers are expected, as well as the formation of amino

ether bonds with the hydroxyl groups belonging to the 4P polymers mediated by the melamine additive, as depicted in Figure 1e. Tensile and scratch tests (Figure 2) were conducted to compare the physical properties of the free-standing coating films prepared with different combinations of the additives. The free-standing film could be successfully fabricated by adding the dispersant “N”, which is generally required for the preparation of various free-standing films.32 As shown in Figure 2a−c, the “N_PRX_Si1” film displayed the lowest mechanical strength, but the highest strain, among the samples. Because the cylindrical nanocrystalline structure of PRX_Si1 could not be connected to form a network structure, the high flexibility of the N_PRX_Si1 film is probably due to the flexibility of the dispersant polymer “N”. The “4PN_PRX_Si1” showed decreased strain, but an increase in the tensile strength by 38.6%. This increased strength and decreased strain are presumably due to the formation of a networked structure between PRX_Si1 and 4P via the inorganic cross-linking points, as shown in Figure 1c. On the other hand, the strain of “MN_PRX_Si1” increased than “4PN_PRX_Si1” one, and the tensile strength did not change compared to that of “N_PRX_Si1”. This increased strain is presumably due to the formation of a networked structure between PRX_Si1 and M via the organic cross-linking points, as shown in Figure 1d. In case of the 4PMN_PRX_Si1, the tensile strength further increased to ∼16 MPa, which is the maximum value among the samples; however, the strain did not decrease significantly compared to that of the “4MN_PRX_Si1” sample. This increased strength is attributed to the formation of organic and inorganic hybrid cross-linking points between PRX_Si1, M, and 4P. E


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ACS Applied Materials & Interfaces To confirm the scratch resistance of the films, the samples were coated onto an ITO substrate and a scratch test was conducted. When the tip of the scratch device was moved by applying various loads from 2 to 3 N at a constant speed on the coating surface, “4PMN_PRX_Si1”, which formed organic− inorganic cross-linking points, maintained the highest friction coefficient among the samples (Figure 2d). The friction coefficient at the point generating the initial scratch is defined as the maximum static friction coefficient,33 and the maximum value was also achieved with “4PMN_PRX_Si1” (Figure 2e). The surface morphologies after the scratch test were observed through confocal microscopy (Figure 2f), demonstrating that the “4PMN_PRX_Si1”-coated surface displays excellent scratch resistance. This suggests that the formation of the organic−inorganic hybrid cross-linking points is effective for enhancing the scratch resistance.34 To confirm whether the extreme scratch resistance was induced by the characteristic molecular structure of PRX_Si1, different cross-linker molecules were designed and used instead of PRX_Si1. The polymeric multifunctional 4P and the monomeric di-functional silane (Si) were respectively used to confirm the structural effect of supramolecular PRX_Si1. In addition, PRX_Si4 was used to confirm whether the number of alkoxysilyl groups in the PRX backbone is important for improving the scratch resistance and flexibility of the hard coating materials (Figure 3).

(Figure 1a). Because PRX_Si4 contains four reactive alkoxysilyl groups on a single α-CD, self-condensation reactions with the neighboring α-CDs are thought to be accelerated. Thus, dispersion of PRX_Si4 by the dispersant and reaction with other additives are thought to be ineffective, resulting in the aforementioned morphological similarity (Figures 4d vs 1a). SAXS analysis was used to provide an overview of the microstructures of the hard coating materials prepared using different cross-linkers. SAXS is an analytical method that provides structural data through X-ray irradiation of the bulk film. It can provide more quantitative data about the microstructures and is a method widely used to analyze the microstructures of polymer nanocomposites.36,37 Figure 4a′− d′ shows the average size of the nanocrystalline domains dispersed in the samples, where the domain sizes are similar to those observed from the TEM images shown in Figure 4a−d. In Figure 5, more detailed information about the crystalline structures of the prepared films was obtained from the structure factors, S(q), derived from the SAXS data. Figure 5a,b shows the experimental scattering intensity, I(q), and S(q) data for the samples and the schematic microstructures of the corresponding samples. The peak in the SAXS profile of “4PMN_PRX_Si1” in Figure 5a appeared to the right of the xaxis, indicative of small nanostructured crystalline domains. SASfit analysis of the raw data generated the S(q) data presented in Figure 5b,38,39 confirming that the hard coating materials prepared using PRX_Si1 as a cross-linker (4PMN_PRX_Si1) displayed the finest (∼8.7 nm) and uniformly dispersed crystalline structures with an average distance of 10.7 nm for the crystalline particles in the bulk film. In Figure 5a,b, 4PMN showed a macroscopic structure similar to 4PMN_PRX_Si1, and it was confirmed that the nanostructure was distributed within a short-range. Figure 5c depicted the distribution of nanostructures of bulk films through SASfit analysis. The size and distribution of the nanoclusters observed from the TEM images were quantitatively analyzed using ImageJ, and the results were compared with data obtained from the SAXS. Although the size of nanoclusters was very well consistent in different measurement methods, the distance of nanoclusters was significantly different each other. This difference is presumably due to the difference in sample preparation between TEM and SAXS analysis. In Figure S5, the optical transparency of the four different hard coating materials was analyzed via UV−vis spectroscopy, and the results were compared with those of commercially available flexible hard coating materials, denoted as “Commercial.” Figure S5a shows the images of the free-standing films and the visible light transmittance measured by UV−vis spectroscopy. The transparency to visible light exceeded 85% for all the films (Figure S5b), making the prepared hard coating materials prospectively applicable in optical devices. The XPS for surface element of each free standing film was also analyzed (Figure S6). The binding energy of N 1s induced by the melamine additive was clearly observed at 398 eV. In O 1s and Si 2p, 531 eV of the Si−O−Si bond and 101 eV of the C− Si−O bond, which are induced by the siloxane nanoclusters, were clearly confirmed.40,41 To confirm the contents of the inorganic compound in the film, organic and inorganic fractions were analyzed by TGA study. As shown in Figure S7, lower than 11% of inorganic compounds are included in the prepared sample. Because alkoxysilyl-functionalized groups, which are main chemical group synthesizing inorganic siloxane

Figure 3. Variables for silane cross-linker.

Figure 4 shows the TEM images of the films prepared using different cross-linkers, and the compositions are presented in Table 1. When the polymeric 4P containing functional groups similar to those of the PRX cross-linker was cross-linked without additives by self-condensation reactions, uniformly dispersed organic and inorganic cross-linking points were observed (Figure 4a). Because polymeric 4P contains functional groups, that is, hydroxyl and alkoxysilyl groups, similar to the PRX cross-linker, its microstructure is very similar to that of PRX_Si1. In the case of the film prepared using the monomeric linear cross-linker Si, large agglomerations of polymers were observed with sizes between 350 and 450 nm (Figure 4b). These results could be regarded as a phenomenon that the Si cross-linker was aggregated in a short-range with the polymer 4P.35 Figure 4c also shows the microstructure of the hard coating materials cured with PRX_Si1. As already shown in Figure 1e, siloxane- and melamine-induced crystalline domains were uniformly dispersed over the entire space. In Figure 4d, when PRX_Si4 was used as the cross-linker, large (>100 nm) agglomerates and tubelike crystalline structures were observed in the film. Interestingly, this morphology is very similar to that of the sample prepared using only PRX_Si1 F


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Figure 4. TEM images according to the cross-linking parameters, (a) 4PMN, (b) 4PMN_Si, (c) 4PMN_PRX_Si1, and (d) 4PMN_ PRX_Si4, and the size distribution analysis through the SAXS analysis (a′) 4PMN, (b′) 4PMN_Si, (c′) 4PMN_PRX_Si1, and (d′) 4PMN_ PRX_Si4.

line, as shown in Figure 6d. This temporary folding line immediately disappeared, with recovery of the original shape of the film, and the original film shape was maintained even after more than 5000 repeated folding tests. When the free-standing film was prepared using only the polymeric cross-linker 4P without PRX derivatives (4PMN), the tensile strength was not sufficient, and the value was much lower than that of the commercial film. However, the strain was partially improved compared to that of the commercial film. Because the chemical groups of polymeric 4P are similar to those of PRX_Si1, the unique mechanical properties of the PRX_Si1 film are thought to be derived from the structural singularity of the molecular necklace-like PRX additives. When Si or PRX_Si4 was used as a cross-linker, the tensile strength and flexibility of the films was inadequate. These results indicate that use of the PRX derivatives with the optimized composition of alkoxysilyl groups is important to obtain foldable mechanical flexibility and enhanced mechanical strength. To confirm the crosslinking density of the coating materials, DMA analysis was conducted (Figure S8). The cross-linking density can be calculated from eq 1 using the storage modulus after the glass transition temperature.

Figure 5. (a) SAXS data according to the samples. (b) SAXS analysis of distribution of nanoparticles in bulk film. Schematic images of the (c) 4PMN, (d) 4PMN_Si, (e) 4PMN_PRXSi1, and (f) 4PMN_PRX_Si4.

compound, in polymers are around 10%, this result is well consistent with the in-feed compositions. Figure 6a−c shows the stress−strain (S−S) curves and resulting tensile strength and maximum strain at break points of the free-standing films. The S−S curve of the “Commercial” flexible hard coating material was typical of brittle materials with a high elastic modulus, where the material had a limited strain value of ∼30%. Thus, the flexibility was sufficient for withstanding the bending stress, but not enough to withstand the folding stress. The free-standing film fractured immediately when folding stress was applied, as shown in Figure 6d. In the case of PRX_Si1, the S−S curve exhibited typical plastic polymer behavior with a greatly enhanced maximum strain (∼110%). When the PRX_Si1 free-standing film was completely folded, the film did not fracture like the commercial film, but maintained the film shape with a temporary folding

ρ=

Er′ 3RT

(1)

where ρ is the cross-linking density (mol/cm3), R is 8.314 (cm3 MPa/mol K), T is Tg + 50 in the rubbery state (K), and Er′ is the storage modulus in the rubbery state (MPa). As shown in Figure S8, the 4PMN_PRX_Si1 shows the highest cross-linking density, and the 4PMN_Si shows the lowest cross-linking density. Based on the TEM result, these results are seemed to be induced by the uniformly dispersed inorganic clusters (cross-linking) formed in the coating materials. When large agglomerations are observed, well-networked structures are hardly formed and it seems to cause the relatively low cross-linking densities. G


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Figure 6. (a) Results of free-standing film tensile test. The bar graph of (b) tensile strength and (c) tensile strain at breaking point extracted from S−S curve and (d) optical images showing flexibility of free-standing film.

Figure 7. (a) Tensile test of the various films coated on ITO and (b) optical images showing flexibility of the coating frame.

The scratch resistance of the coated film was estimated with a scratch tester using a tip-loading system.43 The hard coating materials were coated onto the ITO film, and a scratch test was conducted with a constant tip-velocity with variation of the load from 2 to 3 N (Figure 8). The friction coefficients of the bare ITO, 4PMN, and Commercial films were the lowest. The f r i c t i o n c o e ffic i e n t s o f t h e 4 P M N _ S i - a n d t h e 4PMN_PRX_Si4-coated films were moderate, and the 4PMN_PRX_Si1-coated film had the highest friction coefficient under the applied load (Figure 8a). Because a decrease in the friction coefficient is induced by the permanent deformation (scratch formation) under the applied load, this highest friction coefficient is interpreted as resistance against permanent deformation.44 The scratched area was monitored using laser confocal microscopy. As shown in Figure 8b, negligible surface indentation was observed only for the 4PMN_PRX_Si1-coated surface, indicating excellent scratch resistance. The pencil hardness of the hard coating materials on different substrates, that is PET and ITO, was also confirmed (Figure 8c).45 The commercially available flexible hard coating materials exhibited substrate-dependent pencil hardness, where the maximum hardness of 6H was achieved

To confirm the coating stability of the hard coating materials, the films were coated onto ITO substrates and exposed to tensile stress to observe the crack formation during elongation (Figure 7a). The transparently coated ITO films were converted to white opaque films upon elongation. This is due to the diffraction of visible light by the microcracks formed inside the ITO.42 Interestingly, the macroscale surface cracks and deformations observed for the other samples were absent f o r t h e 4 P M N _ P R X _ S i 1 - c o a te d I T O fil m. T h e 4PMN_PRX_Si1-coated ITO film showed a 65.8% increase in the tensile strain compared to that of the bare ITO film. This indicates that the outstanding coating stability and flexibility of the 4PMN_PRX_Si1 hard coating, and even the flexibility of the ITO film, could be improved by coating with PRX_Si1. Figure 7b shows the folding stability data for the “Commercial” and PRX_Si1-coated ITO films. When bent and folded completely, the commercial coating film completely peeled off from the substrate, whereas the “4PMN_PRX_Si1” coating remained stable (Figure 7b). This indicates the outstanding coating stability of 4MN_PRX_Si1, even in the completely folded state. H


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Figure 8. (a) Scratch resistance data and the confocal image of the (b) ITO, (c) Commercial, (d) 4PMN, (e) 4PMN_Si, (f) 4PMN_PRX_Si1, and (g) 4PMN_PRX_Si4 after scratching on ITO. (h) Bar graph of the maximum coefficient of static friction and the pencil hardness value of the each sample.

for the film on the PET substrate. In contrast, the 4PMN_PRX_Si1-coated surface showed the maximum pencil hardness (9H) on the PET and ITO surfaces. These results are thought to be due to the unique molecular structure of 4PMN_PRX_Si1. When the polymeric cross-linker 4P containing chemical groups similar to those of 4PMN_PRX_Si1 was exclusively used to prepare the hard coating materials, the scratch resistance was inferior. However, when PRX_Si1 was added, 5−10 nm sized organic and inorganic cross-linking points were uniformly formed along the flexible PRX backbone. This microstructure could effectively buffer the external stress to enhance the scratch resistance, as depicted in Scheme 2.46 In contrast, the linear cross-linker including polymeric 4P or monomeric Si showed lower scratch resistance because the cross-linking points could be broken when external stress was applied due to the lack of flexibility (Scheme 2). Finally, to confirm the potential of the PRX_Si1 cross-linker to enhance the scratch resistance in different applications, the PRX_Si1 was added to the commercially available topcoat paint (clear coat) used on the automotive substrate (Figure 9b,c). When a load is applied to a substrate in increased manner, scratches are generated step by step. In Figure 9a,

Scheme 2. Schematic Diagram of Application of Scratch to a Substrate

during scratch tests, critical points are observed based on the change of scratch mechanisms. The 1st critical load is defined at the beginning of localized scratch damages and refers to the load value at the time of generating a whitening damage. The 2nd critical load is defined at the transition of damage mechanisms and refers to the load value at the beginning point of the cross-shaped whitening damage. The 3rd critical load refers to the load at the point where the coating is peeled off from the substrate.47,48 Based on these analytical methods, the I


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Figure 9. (a) Type of load value by scratch shape by tip. The scratch resistance data for (b) synthesized coating material with silane cross-linker coated on the automotive substrate and (c) commercial coating material (clear coat) with silane cross-linker coated on automotive substrate.

critical loads were analyzed on the automotive substrate coated with a paint containing different additives (Figure 9b). In Figure 9b, the load value of 4PMN_PRX_Si1, which was defined by Figure 9a, was increased by coating the synthesizedhard coating material on the automotive substrate. This could be seen as a result of preventing the occurrence of initial scratches because of the flexibility and high strength of the structure of PRX_Si1. In Figure 9c, the critical load of the commercial topcoat material (clear coat) was greatly enhanced by simply adding PRX_Si1. Because most of the commercial topcoat materials are formed by the condensation reaction of alkoxysilyl derivatives, adding PRX_Si1 is thought to be effective for enhancing the scratch resistance via a mechanism similar to that depicted in Scheme 2. This indicates that the usefulness of PRX_Si1 is not limited to the designed flexible hard coating materials, but is also applicable to the commercially available antiscratch materials based on the condensation reactions of alkoxysilyl derivatives.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ji-Hun Seo: 0000-0001-6193-4008 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This research was partially supported by the KCC Research Institute and Korea University-Future Research Unit (K1822141). This research was also supported by the XProject, funded by the National Research Foundation, Republic of Korea (NRF-2018R1E1A2A02086660).

4. CONCLUSIONS In this study, a molecular-necklace-like organic−inorganic hybrid cross-linker was proposed for the development of flexible and extremely scratch-resistant hard coating materials. In general, increasing the strength and hardness of hard coating materials is achieved at the expense of the flexibility. As display technologies move from curved to bendable and foldable, foldable but extremely scratch-resistant materials must be developed. By exploiting the structural characteristics of PRX derivatives, the flexibility and the scratch resistance were simultaneously enhanced without any trade-off issue. We believe that the concept suggested in this work contributes to the development of novel types of cross-linkers that can overcome the traditional drawbacks of many polymeric materials in the engineering and industrial fields.

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the siloxane cluster size and distance between siloxane clusters by TEM and SAXS analysis; optical images of free-standing film and UV−vis transmittance data; and XPS data analysis, TGA raw data, and DMA test results of the Commercial, 4PMN, 4PMN_Si, 4PMN_PRXSi1, and 4PMN_PRX_Si4 (PDF)

Notes

The authors declare no competing financial interest.

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ABBREVIATIONS PEG, poly(ethylene glycol) ITO, indium tin oxide CDI, N,N′-carbonyldiimidazole α-CD, α-cyclodextrin Z-Tyr-OH, N-carbobenzoxy-L-tyrosine DI, deionized DMT-MM, 4-(4,6-dimethoxy-1,3,5-tiazin-2-yl)-4-methylmorpholinium chloride M, hexamethoxymethylmelamine Si, 1,2-bis(triethoxysilyl)ethane N, Paraloid B-72 MIBK, methyl isobutyl ketone SAXS, small-angle X-ray scattering PVP, poly(vinylpyrrolidone) PRX, polyrotaxane PRX_Si1, 1-silane-induced polyrotaxane PRX_Si4, 4-silane-induced polyrotaxane 4P, quaternary random copolymer TEM, transmission electron microscopy

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05738. 1 H NMR results of the PEG-bis(amine) and quaternary random copolymer (4P); 1H NMR results of the PRX derivatives; molecular profile of the synthesized PRX derivatives; TEM image of the composition of the PRX without silane and coating materials (4PMN_PRX), melamine and dispersant (MN), and quaternary random polymer 4P and melamine (4PM); correlation between J


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Foldable and extremely scratch-resistant hard coating materials from

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