Flexible Protective Film: Ultrahard, Yet Flexible Hybrid Nanocomposite Reinforced by 3D Inorganic Nanoshell Structures
Gwangmin Bae, Gwang-Mun Choi, Changui Ahn, Sang-Min Kim, Wonsik Kim, Youngjun Choi, Dawon Park, Dongchan Jang, Jung-Wuk Hong,* Seung Min Han,* Byeong-Soo Bae,* and Seokwoo Jeon*
Emerging flexible optoelectronics requires a new type of protective material that is not only hard but also flexible. Organic–inorganic (O–I) hybrid materials have been used as a flexible cover window to increase wear resistance and polymer-like flexibility. However, the hardness of O–I hybrid materials is much lower than that of metals and ceramics due to the low intrinsic hardness of the organic matrix and limited volume fraction of inorganic reinforcement. Herein, a new type of hybrid nanocomposite combining an O–I hybrid material with continuous and ordered 3D inorganic nanoshell as an additional reinforcement is proposed. The 3D alumina nanoshell uniformly embedded in the epoxy-siloxane molecular hybrid (ESMH) enables a rule of mixture without a loss in flexibility. Two types of reinforcements comprising siloxane molecules and 3D alumina shell ensure a metal-like hardness (1.3 GPa), which is significantly higher than that of the typical polymers and polymer nanocomposites. The 3D hybrid nanocomposite films show superb impact resistance due to the 3D alumina nanoshell that effectively suppresses crack propagation. Inch-scale 3D hybrid nanocomposite films also endure 20 000 bending cycles without failure and maintain high transparency (>82.0% at 550 nm) in the visible regions.
DOI: 10.1002/adfm.202010254
G. Bae, Prof. S. JeonDepartment of Materials Science and EngineeringKAIST Institute for the Nanocentury (KINC)Korea Advanced Institute of Science and Technology (KAIST)Daejeon 34141, Republic of KoreaE-mail: [email protected]. G.-M. ChoiICT Creative Research LaboratoryElectronics and Telecommunications Research Institute (ETRI)Daejeon 34141, Republic of KoreaDr. C. AhnEngineering Ceramic CenterKorea Institute of Ceramic Engineering and Technology (KICET)Icheon, Gyeonggi 17303, Republic of Korea
Dr. S.-M. KimDepartment of Nano-MechanicsKorea Institute of Machinery and Materials (KIMM)156 Gajeongbuk-Ro Yuseong-Gu, Daejeon 34103, Republic of KoreaW. Kim, Prof. S. M. Han, Prof. B.-S. BaeDepartment of Materials Science and EngineeringKorea Advanced Institute of Science and Technology (KAIST)Daejeon 34141, Republic of KoreaE-mail: [email protected]; [email protected]. Choi, D. Park, Prof. J.-W. HongDepartment of Civil and Environmental EngineeringKorea Advanced Institute of Science and Technology (KAIST)Daejeon 34141, Republic of KoreaE-mail: [email protected]. D. JangDepartment of Nuclear and Quantum EngineeringKorea Advanced Institute of Science and Technology (KAIST)Daejeon 34141, Republic of Korea
foldable displays.[1,2] Unlike traditional flat-panel displays, foldable displays require hard yet flexible protective films with high hardness, excellent wear resistance, and relatively low stiffness to efficiently protect the devices from repeated bending. How-ever, as these mechanical properties are well known to be mutually exclusive, it is difficult to achieve both properties simul-taneously.[3] Currently, ultrathin tough-ened glass has been utilized to screen protector materials for foldable electronic devices due to its good transparency, hard-ness, and flexibility. However, inorganic materials, like glass, cannot be the best option for the production of flexible and foldable devices because of their intrinsi-cally low toughness, resulting in unex-pected cracking and shattering failure under long-term bending stresses.[4] On the other hand, several organic mate-rials with excellent flexibility, low density (<2 g cm–3), and high transparency have
been proposed for the protective film of flexible devices.[5–7] Despite these advantages, organic materials have significant problems of very low hardness (H < 0.5 GPa), impact, and wear resistance. An effective strategy to resolve the issues of
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202010254.
1. Introduction
A flexible protective film is an essential element for next-generation optoelectronic device applications, such as flexible and
pure materials (glass or polymer) is to realize hard and flexible nanocomposites by incorporating hard reinforcements such as ceramics,[8–10] graphene,[11,12] and carbon nanotubes (CNTs)[13–15] into soft polymeric matrices. The mechanical properties (e.g., strength, wear resistance, and toughness) of the nanocomposite materials are controlled by changing the size, volume fraction, and morphology of the reinforcement. Nevertheless, it is still difficult to achieve optimized mechanical properties in state-of-the-art nanocomposites for flexible protective materials due to the degradation of their mechanical performance by aggre-gation and discontinuity of the reinforcement.[16,17] Another approach involves layered nanocomposites that contain a con-tinuous interface between the matrix and reinforcement.[18,19] However, the mechanical performance of layered nanocompos-ites indicates a high anisotropy that strongly depends on the direction of the load.[20]
To overcome the limitations of conventional nanocompos-ites, two relatively new approaches by the advance of mate-rial synthesis and nanofabrication have been proposed. First, organic–inorganic (O–I) hybrid materials possess excellent mechanical, optical, and thermal properties for flexible cover windows due to the molecular-level distribution of the inorganic reinforcements that do not aggregate.[21–23] Choi et al. reported that improved wear resistance and polymer-like flexibility can be simultaneously achieved by using the molecular-level hybridiza-tion of siloxane and epoxy molecules.[24] However, the hardness of O–I hybrid materials (H < 1 GPa) is still much lower than that of ceramics and metals due to the low intrinsic hardness of the organic component and limited volume fraction of the inorganic phase. Therefore, an additional strategy is needed to maximize the mechanical properties of nanocomposites while maintaining flexibility. Unlike conventional reinforcements, well-ordered and continuous 3D nanostructures provide good dispersion and percolation properties for the reinforcements, enabling efficient load transfer between the reinforcement and matrix.[25] In our previous work, the strength of nanocompos-ites with 3D ordered and continuous oxide nanoshell ensured a rule of mixture, which theoretically maximized the mechanical properties of the nanocomposite at very high volume fraction (19 vol%) of the reinforcements without aggregation and a loss in flexibility.[26] Therefore, hybridization of the 3D nano-structure with an O–I hybrid material containing molecular-level reinforcement is an efficient approach to maximize the mechanical properties of composite materials.
Herein, we propose a new concept for hybrid nanocompos-ites that are ultrahard yet flexible, which is applicable as flexible protective films. First, we use epoxy-siloxane molecular hybrid (ESMH) as the matrix, which shows superior performance as a flexible protective material due to the molecular-level hybridi-zation of hard siloxane nanobricks chemically linked by soft epoxy chains.[24] Furthermore, 3D nanoshell-structured alu-mina with a continuous and an ordered foam is incorporated as an additional reinforcement using proximity-field nanopat-terning (PnP),[27–31] and atomic layer deposition (ALD).[32–35] The additional 3D alumina nanoshell is uniformly distributed throughout the ESMH up to 17.1 vol% and ensures the rule of mixture without a loss in the flexibility. By combining an O–I hybrid material with continuous and ordered 3D inorganic nanoshell, the 3D hybrid nanocomposites have metal-like
hardness (1.3 GPa for 17.1 vol% alumina) and low stiffness (18.8 GPa for 17.1 vol% alumina), which were previously con-sidered to be incompatible. Furthermore, the inch-scale nano-composite film shows a higher damage tolerance than the pure ESMH film due to the blocking of propagating cracks by the 3D alumina nanoshell. Last, the 3D alumina/ESMH nanocom-posite film maintains high transmittance (>82.0% at 550 nm) in the visible region after 20 000 bending cycles without failure, showing excellent transparency and flexibility. These unprec-edented properties demonstrate that the 3D alumina/ESMH nanocomposite can be a new option for flexible protective films.
2. Results and Discussions
A schematic illustration for fabricating the 3D alumina/ESMH nanocomposite is shown in Figure 1a. Alumina is uti-lized as a 3D nanostructured reinforcement due to its high mechanical properties (1.8 GPa compressive strength[36]) and negligible refractive index difference with the ESMH matrix ( / 0.9ESMH Al O2 3 >n n ). The 3D alumina nanostructure is fabri-cated using PnP and ALD techniques based on our previous work (Figure S1, Supporting Information).[35] Next, the ESMH consisting of epoxy and siloxane molecular networks that are chemically bonded is infiltrated (Figure 1b) into the 3D alumina nanostructures using spin casting (see Experimental section). Figure 1c shows the cross-sectional SEM image of the 3D alu-mina nanostructure before ESMH infiltration. The 3D alumina comprises a unit cell in the 3D nanostructure that has a body-centered tetragonal (BCT) symmetry, and the size of the unit cell is 600 nm x 600 nm x 1.9 µm. The 3D continuous and ordered nanostructure significantly improves the strengthening effect of the composite material by maximizing load transfer between matrix and reinforcement in the 3 axial directions. In addition, the interconnected and open pore structure of the 3D alumina allows the matrix materials to densely infiltrate into and out of the 3D nanoshell (Figure 1d). This 3D nanocomposite platform is facile for selecting target materials for suitable applications and combining two heterogeneous materials into a nanocom-posite.[37,38] The cross-sectional SEM image of the 3D alumina/ESMH nanocomposite using a focused ion beam (FIB) indi-cates that the 3D alumina nanostructures in the nanocom-posite are well maintained and do not experience structural collapse (Figure 1e). While the 3D alumina film is optically opaque due to light scattering between air and the matter in the complex nanostructures (Figure 1f, top and Figure S7, Supporting Information), the 3D alumina/ESMH nanocom-posite becomes transparent due to refractive index matching between the alumina ( 1.6)Al O ,2 3 ≈n r and ESMH (nESMH ≈ 1.5).
To maximize the mechanical properties of a nanocomposite, it is important not only to realize the uniform distribution of the reinforcements in three dimensions but also to enhance the intrinsic strength of the reinforcement materials. The addi-tional heat treatment of the 3D alumina nanoshell can be a very simple and efficient approach to achieve both reinforcing strategies simultaneously through the anisotropic shrinkage as well as crystallization of the 3D alumina nanoshell during the heat treatment. As shown in Figure 2a, after heat treatment at 700 °C, the 3D alumina maintains well-ordered and continuous
nanostructures without significant collapse. It is noteworthy that shrinkage of 12% occurs in the out-of-plane direction (Z) in the 3D alumina, while the shrinkage for the in-plane direc-tion (X or Y) is negligible due to the strong adhesion between the SiO2 substrate and alumina.[39] This anisotropic shrinkage reduces the aspect ratio (Z/X) of the BCT unit cell from 3.2 to 2.8 (Figure 2b). The transition of the 3D nanostructure to a more isotropic symmetry improves the mechanical properties of the nanocomposite due to the increased load transfer capa-bility from the matrix to the reinforcement. Figure 2c shows X-ray diffraction (XRD) spectra of the 3D alumina before and after heat treatment. Before heat treatment, the 3D alumina has an amorphous phase with low strength and modulus because the alumina coating is prepared using the ALD process at a low temperature (90 °C) to prevent the structural collapse of the 3D polymeric template. The 700 °C heat treatment changes the amorphous phase of the 3D alumina to a mechanically stronger γ phase. The crystallization of the alumina is also accompanied by the densification of the nanoshell. In TEM images of an alu-mina nanoshell (inset of Figure 2c and Figure S2, Supporting Information), the shell thickness decreases to 16% after the heat treatment.
To analyze the mechanical behavior of the 3D alumina/ESMH nanocomposite, a nanoindentation experiment is performed. Five sets of samples with different alumina thicknesses
from 0 nm (pure ESMH) to 60 nm and heating conditions (no heat treatment and 700 °C heat treatment) are prepared on the SiO2 substrate (Table S1, Supporting Information). In the work of Na et al., the hollow BCT alumina nanoarchitecture (55 nm shell thickness) without polymeric matrix is rapidly col-lapsed at around 15% strain. In contrast, the nanoarchitectures with a thinner shell thickness withstand more strain up to 50% without brittle failure due to the size effect of oxide materials.[35] However, the 3D nanocomposites with 60 nm alumina thick-ness aren’t catastrophically collapsed due to infiltrated flexible polymer matrix, relieving external stress.[26] Considering the previous works, the optimal shell thickness of 3D alumina for the best combination of hardness and flexibility can be pre-dicted to around 60 nm (17.1 vol%). The overflowed ESMH layer is selectively removed using a plasma-assisted ashing system for accurate measurement of the mechanical properties of the nanocomposites (Figure S3, Supporting Information). Samples are indented using a Berkovich tip and the load–displacement curves of each sample are shown in Figure 3a. Interestingly, the pop-in phenomenon is consistently observed in the pure ESMHs (Figure S4, Supporting Information), whereas smooth and continuous paths are observed in the 3D nanocomposites during nanoindentation. The pop-in during nanoindenta-tion is attributed to crack formation in the pure ESMH. As shown in Figure 3b, two fracture modes, namely, radial and
Figure 1. Fabrication of 3D alumina/ESMH nanocomposites. a) A schematic illustration of the fabrication of the 3D alumina/ESMH nanocomposite. b) ESMH molecular structure. c,d) Cross-sectional SEM image of the 3D alumina nanoshell and 3D nanocomposite (inset: top view). e) Cross-sectional SEM image of the 3D nanocomposite by using FIB. f) Digital image of the 3D alumina nanoshell (top) and 3D nanocomposite film (bottom) on the SiO2 substrate.
circumferential crack, are observed in the pure ESMH. When a load is applied to the pure ESMH, the tensile stress is applied at the weak film–substrate interface, and radial cracks are gen-erated by exceeding the threshold stress. Increasing the load causes the bending stress to be applied to the film surface, and circumferential cracks are formed.[40] On the other hand, when a load is applied to the 3D nanocomposite, the additional 3D alumina reinforcement dissipates a significant amount of energy in the three axial directions, alleviating the stress con-centration region, which effectively inhibits crack formation.[41] Furthermore, after the nanoindentation test, a large elastic recovery is observed in the 3D nanocomposite film due to the high resilience of the ESMH matrix and 3D nanostructure (Figure 3b).[24] The ability of the 3D nanocomposite to absorb more energy upon impact is further evaluated with ball drop test as discussed below. An average hardness and Young’s mod-ulus of the pure ESMH and the 3D alumina/ESMH nanocom-posites obtained from the load–displacement curves are shown in Figure 3c and Table S1 (Supporting Information). To prevent a substrate effect during the nanoindentation measurement, the hardness and Young’s modulus are taken at a penetration depth that is 10% of the film thickness.[42] The hardness and Young’s modulus of the 3D nanocomposite (700 °C-60 nm) are
1.3 and 18.8 GPa, respectively, which are ≈200% enhancement in comparison to that of the bulk pure ESMH. The strength and modulus of the 3D nanocomposite containing heat-treated alumina (700 °C-60 nm) are higher than those of the 3D nano-composite containing alumina prior to heat treatment with the same shell thicknesses. It should be noted that the hardness and modulus of the 3D nanocomposite containing heat-treated alumina with a thinner shell thickness (700 °C-50 nm) are also higher than those of the 3D nanocomposite containing non-heat-treated alumina (None-60 nm) despite the thinner alumina shell. These results demonstrate that the heat treatment of the 3D alumina significantly increases the strength and modulus of the nanocomposite. To demonstrate the reliability of the hardness and modulus obtained from nanoindentation, finite element analysis (FEA) simulation is conducted (see Experi-mental section). Figure 3d presents both the experimental and simulated modulus of the pure ESMH and 3D alumina/ESMH nanocomposites as a function of the volume fraction of alumina reinforcement. The modulus obtained from the FEA simula-tion shows good agreement with the experimental data and is within 4%. It is noteworthy that the experimental and simulated modulus values of pure ESMH and 3D nanocomposites have the same slope that increases without a plateau up to 17.1 vol%
Figure 2. Heat treatment of 3D alumina nanoshell. a) Cross-sectional SEM images (inset: top view). b) Shrinkage and aspect ratio graph. c) XRD patterns of 3D alumina nanoshell before and after heat treatment process (inset: TEM images of alumina nanoshell).
of alumina. The linear dependence indicates that the modulus of the 3D nanocomposites is consistent with the rule of mix-ture: an optimal performance is obtained due to the uniform dispersion of 3D continuous alumina nanostructures that effectively transfer the external load in the 3 axial directions.[26] Figure 3e and Figures S5 and S6 (Supporting Information) present the von Mises stress and effective strain distributions at 10% uniaxial strain obtained from the FEA simulations. Inter-estingly, both components (ESMH and alumina) carry nearly an identical strain at 10% strain distribution (Figure S6, Sup-porting Information). Thus, the resultant stress distributions of 3D nanocomposites show that the entire alumina reinforcement endures significantly larger stress than the soft ESMH matrix (Figure 3e and Figure S5f, Supporting Information). This near iso-strain condition of the mechanical performance indicates that the 3D alumina/ESMH nanocomposites follow the rule of mixture for the volume fraction of the reinforcement.
An Ashby’s mechanical properties chart plotting the hard-ness (H) against the effective modulus (E*) for engineering materials is shown in Figure 4 to compare the mechanical properties of the 3D nanocomposite with the known data from existing materials and nanocomposites; E* = E/(1 − ν2), where E is the Young’s modulus and ν is Poisson’s ratio. In the chart, the 3D alumina/ESMH nanocomposite shows the highest mechan-ical properties among the polymer and polymer-based com-posites[43–49] positioned close to the metal region. In general, to realize the flexibility, materials with a low effective modulus at a given hardness should be developed.[50] Therefore, it is well known that the hardness-to-effective modulus ratio (H/E*) is one of the key parameters of flexible and hard nanocomposites.[3]
The 3D alumina/ESMH nanocomposite has a much higher H/E* ratio (0.064) than metal alloys that have the same hardness (Ti and Ni alloys range from ≈0.008 to 0.014). In addition, the dashed line indicates the value of H3/E*2, which represents con-tact loads required to induce plastic deformation, i.e., the wear resistance.[51] The 3D nanocomposite shows an excellent H3/E*2 ratio (5.4) that is comparable to that for ceramics (alumina and zirconia), which indicates that the 3D nanocomposite is suitable for wear-resistant film. The excellent wear resistance of the 3D alumina/ESMH nanocomposite film is revealed by a steel wool wear test and compared to that for the 3D alumina/epoxy nano-composite (Figure S7, Supporting Information).
To further investigate the ability of the 3D alumina/ESMH nanocomposite to absorb energy upon impact, which can be encountered during the actual application in flexible protective film, a ball drop test is performed (Figure 5a). Inch-scale pure ESMH and 3D nanocomposite films with a thickness of 10 µm are fabricated on a SiO2 substrate. Figure 5b shows digital images of the samples immediately after the collision with a 16.6 g steel ball that fell from the same height (0.16 m). While the bare SiO2 wafer and pure ESMH-coated SiO2 are easily fractured, the 3D nanocomposite film effectively protects the SiO2 substrate without fracture due to its enhanced resilience (Movie S1, Supporting Information). To further analyze the strengthening mechanism of the 3D nanocomposite film, inten-tionally fractured surfaces are investigated (Figure 5c). While a smooth fracture surface is observed in the pure ESMH film, a rough surface where cracks change the direction at the interface is observed in the 3D nanocomposite film. This complicated pattern found in the 3D nanocomposite demonstrates that the
Figure 3. Mechanical properties of 3D alumina/ESMH nanocomposites. a) Load–displacement curves. b) Top and cross-sectional SEM images of pure ESMH (top) and 3D alumina/ESMH nanocomposite (700 °C-50 nm, bottom) by using FIB after nanoindentation test. c) Hardness and Young’s modulus of pure ESMH and 3D nanocomposites. d) Comparison of experimental and simulated modulus of the pure ESMH and the 3D nanocom-posites. e) Distributions of von Mises stress of pure ESMH and 3D nanocomposites at 10% strain.
continuous 3D alumina nanoshell acts as an effective obstacle to crack propagation. Generally, cracks formed within the matrix will be hindered by the presence of the interface that requires the crack to change its propagation pathway along the interface direction, which requires additional stress to be applied.[52] In comparison to 2D interfaces, the 3D structuring of the alumina nanoshell will be effective in hindering crack propagation in all directions, thereby resulting in higher impact energy absorption via suppression of crack propagation.
In addition, the 3D alumina/ESMH nanocomposites show excellent optical and flexible properties that are applicable to transparent protective films for flexible devices. To evaluate the potential usage of the 3D alumina/ESMH nanocomposite, 10 µm thick 3D nanocomposite films were fabricated on a com-mercial 125 µm thick polyethylene terephthalate (PET) film (see Experimental section), and a dynamic bending test with a 5 mm bending radius was conducted (Figure 5d). Figure 5e indicates the transmittance spectra of the pure ESMH and the 3D alu-mina/ESMH nanocomposites with different alumina shell thicknesses before the bending test. The 3D nanocomposites sustain high transmittance (85.1% at 550 nm), which is slightly lower than that for the pure ESMH (88.7% at 550 nm) due to the even infiltration of the ESMH and refractive index matching between the ESMH (nESMH ≈ 1.5) and alumina ( 1.6)Al O ,2 3 ≈n r
. The transmittance of the nanocomposite strongly depends on the light scattering at the matrix–reinforcement interfaces. Since the effective interface areas of the 3D nanocomposites with 40, 50, and 60 nm alumina nanoshell are similar, the transmittance difference of 3D nanocomposites with different alumina shell
thicknesses is negligible.[53] In addition, the continuous 3D alu-mina nanoshell evenly infiltrated in the ESMH matrix effectively suppresses the haze of 3D nanocomposites (≈12.8%) even at an extremely high-volume fraction of reinforcement (Table S2, Supporting Information). After convex bending cycles, the trans-mittance at 550 nm is measured to quantify the flexibility of the 3D nanocomposites (Figure 5f). The excellent flexibility of a pure ESMH film with a thickness of 50 µm was already shown in ref. [24]. What is intriguing is that the transmittance of the 3D nanocomposites does not degrade dramatically after 20 000 bending cycles without failure. The transmittance decreases after repeated bending are dependent upon the alumina shell thickness. The transmittance for the 700 °C-60 nm condition after 20 000 bending cycles is degraded by 3% (from 85.1% to 82.0%), while that for the 700 °C-40 nm condition is degraded by only 1% (from 85.6% to 84.3%). This degradation is due to the formation of internal voids created by fracture of the 3D alu-mina nanoshell during the repeated bending motion (Figure S8, Supporting Information). In brittle materials, such as inorganic materials, a fracture behavior is governed by the size of the flaw, which can be commonly explained using Weibull statistics.[54] Since the probability of finding the weakest flaw inversely scales with the thickness of the inorganic nanoshell (t), the fracture strength (σf) of the hollow 3D alumina nanoshell is approxi-mately proportional to (1/t)1/2 considering Weibull statistics and confinement effects in nanomaterials.[55] Thus, as the thick-ness of the 3D alumina shell increases, the 3D alumina/ESMH nanocomposite is more likely to create nanovoids induced by fracture during the bending motion. Nevertheless, the 3D
Figure 4. Ashby’s material properties chart that plots the hardness versus effective modulus, Reproduced with permission.[57] Copyright 2007, Elsevier.
alumina nanoshells are not catastrophically collapsed after 20 000 bending cycles due to the ESMH matrix infiltrated in and out of the nanoshell, relieving the bending stress (Figure S8, Supporting Information). As shown in Figure 5g, the transmit-tance of the 3D alumina/ESMH nanocomposites after 20 000 bending cycles sustains a high transmittance (82.0% at 550 nm), indicating excellent transparency and flexibility.
3. Conclusion
In summary, we developed an unconventional 3D hybrid nano-composite that is hard yet flexible by combining an ESMH
as the matrix with continuous 3D alumina nanoshell as addi-tional reinforcements. The 3D alumina nanoshell was evenly embedded in the ESMH matrix up to 17.1 vol% and sustained the optimized strengthening condition without a loss in flex-ibility. The 3D alumina/ESMH nanocomposite showed an increased hardness (1.3 GPa with 17.1 vol% alumina) and Young’s modulus (18.8 GPa with 17.1 vol% alumina), a two-fold enhancement over that for the pure ESMH. The mechanical properties obtained by FEA simulation that had a high cor-respondence with the experimental results demonstrated that the harder alumina phase carried as much stress as possible, which is known as the rule of mixture. As shown on the Ashby plot in Figure 4, we demonstrated that the 3D alumina/ESMH
Figure 5. Potential application of 3D alumina/ESMH nanocomposite film. a) A schematic illustration of the ball drop test. b) Digital images of pure ESMH and 3D nanocomposite (700 °C-40 nm) on SiO2 substrate right after collision at the same height. c) Fracture surface of pure ESMH (left) and 3D nanocomposite film (right) after intentional failure. d) Dynamic bending test of 3D nanocomposites on a PET film (bending diameter: 10 mm). e) Transmittance of pure ESMH and 3D nanocomposites on PET film (inset: digital images of 3D nanocomposite film with different shell thicknesses, scale bar: 1 cm). f) Transmittance of 3D nanocomposite films after each bending cycle (inset: bent image of 3D nanocomposite on PET film). g) Digital images of 3D nanocomposite film (700 °C-50 nm) before and after 20 000 bending cycles (scale bar: 1 cm).
nanocomposite had a metal-like hardness, high H/E* value, and high wear resistance, which are key components of the flexible protective film. Moreover, the inch-scale 3D alumina/ESMH nanocomposite films had a higher impact energy absorption than the pure ESMH film due to the 3D continuous alumina nanoshell acting as an obstacle to crack propagation. The 3D nanocomposite films also showed excellent transparency (>85.1% at 550 nm) and flexibility up to 20 000 bending cycles without a loss of transparency. This is attributed to the size-induced strengthening of 3D alumina nanoshells preventing failure. These unique and excellent properties, including high mechanical properties, optical transparency, and flexibility, make the 3D hybrid nanocomposite useful for protective films for flexible optoelectronic device.
4. Experimental SectionPhotoresist Coating on the SiO2 Substrate: A SiO2–Si substrate was
cleaned for 2 min with an oxygen-plasma (45 sccm, 40 motors, and 40 W). To maintain adhesion between the 3D polymeric template and substrate, a photoresist film (SU-8, 2; Microchem) was spin-coated at 3000 rpm for 30 s with ≈2 µm thickness on the cleaned SiO2 substrate and then baked at 95 °C for 3 min to evaporate the solvent. For direct chemical bonding between the SiO2 substrate and alumina, an inch-scale chrome mask was placed on the film and irradiated with UV light for 1 min. Then, cross-linking occurred only in the irradiated part, and the adhesion layer under the chrome mask was removed by SU-8 developer (Microchem). After rinsing with ethyl alcohol and hard baking at 210 °C for 5 min, the photoresist film (SU-8, 10; Microchem) with a 10 µm thickness was spin-coated on the substrate at 1700 rpm for 30 s. The photoresist coated on the substrate was soft-baked at 95°C for 60 min.
Fabrication of the 3D Alumina Nanoshell Structure: A 3D polymeric template was fabricated through the PnP technique using a conformal phase mask. A detailed procedure of the PnP technique in the previous paper was already included.[27] The alumina was coated on the surface of the 3D polymeric template by using ALD (Atomic classic, CN1) at 90 °C (Figure S1, Supporting Information). Trimethylaluminum (TMA, UP chemical) and H2O gas were used as the precursor and reactant, respectively. The polymeric template was then ashed by using a microwave plasma system (R3T, MUEGGE) for 2 hours. A total of 825 sccm of oxygen, 50 sccm of nitrogen, and 70 sccm of tetrafluoromethane radicals were used for template ashing.
Heat Treatment of the 3D Alumina: Fabricated 3D alumina samples were heated at 700 °C for 2 h in air and slowly cooled to room temperature. To prevent the collapse of the 3D alumina nanostructure due to thermal shock, the samples were preheated at a relatively low temperature (350 °C) for 2 h before direct heating, and the temperature was slowly raised at a rate of 3 °C min–1.
Fabrication of the 3D Alumina/ESMH Nanocomposite: The 3D alumina/ESMH nanocomposites were fabricated by entirely infiltrating the void in the 3D alumina with the ESMH, which was synthesized by UV-initiated cationic ring-opening polymerization of the cycloaliphatic epoxy-functionalized oligosiloxanes (CEOSs) according to the previously reported method.[24] The CEOSs were mixed with triarylsulfonium hexafluoroantimonate (TSHFA, Sigma-Aldrich) salts as the UV initiator and propylene glycol monomethyl ether acetate (PGMEA, Sigma-Aldrich) as the diluent solvent with a 1:0.01:1 (CEOS:TSHFA:PGMEA) weight ratio. The solution was spin-coated on the 3D alumina nanostructure at 3000 rpm for 30 s. The CEOS solution-coated substrate was subsequently soft baked at 90 °C for 60 min to evaporate the PGMEA solvent, and then, the samples were exposed under UV-A irradiation at 2 J cm–2 followed by humid annealing (85 °C, 85% RH) for 6 h. After curing, 3D alumina/ESMH nanocomposites were obtained. For comparison, a 10 µm thick ESMH film was fabricated on the SiO2 substrate. The CEOS solution was spin-coated on the SiO2 wafer at 1400 rpm for 30 s. The
next fabrication processes were performed identically to those of the 3D alumina/ESMH nanocomposites.
Nanoindentation Test: Before nanoindentation, the overflowed ESMH layer was partially removed using a microwave plasma system (R3T, MUEFFE) (Figure S3, Supporting Information). An MTS nanoindenter XP was utilized to perform nanoindentation tests on the fabricated pure ESMH and 3D nanocomposites. Ten times of nanoindentation tests were performed for each sample. A Berkovich tip was used to perform indentation of each sample up to a load of 500 mN using continuous stiffness mode (CSM) at a nominal strain rates of 0.05 s–1. The Oliver-Pharr method was used to analyze the indentation data to calculate the hardness and modulus.
Finite Element Analysis: Uniaxial compression tests of the unit cells were simulated using a nonlinear finite element analysis (FEA) platform LS-DYNA. The 3D alumina/ESMH nanocomposites and pure ESMH with identical geometries were modeled, and the thicknesses of the alumina nanoshell were changed from 0 nm to 40 nm, 50 nm, and 60 nm, respectively. The material properties and unit cell geometry of the pure ESMH and 3D alumina/ESMH nanocomposites used in the FEA simulation were experimentally obtained on samples (Figure S5, Supporting Information). In each 3D nanocomposite, three geometric parts of the ESMH with a body-centered tetragonal (BCT) symmetry, alumina nanoshell, and ESMH infiltration were individually constructed. A contact condition between alumina and ESMH phases was imposed on the surface of each part to achieve the bonding effect under the assumption that the parts were completely bonded. The 3D models were discretized with tetrahedral solid elements to accurately describe the complex geometry and allow large deformation. The plastic-kinematic material model, which employs a bilinear elastoplastic constitutive relationship, was used for the alumina and ESMH models. The pure ESMH and 3D nanocomposite unit cells were compressed in the longitudinal uniaxial direction, and stress–strain curves were obtained. The modulus was calculated from the slope of the stress-strain curve in the elastic deformation region. At a strain of 10%, the distributions of the von Mises stress and the effective strain were presented to enrich the simulation results. All simulations were performed on a multicore system equipped with a Dual Intel Xeon(R) CPU (E5-2687W v4) running at 3.40 GHz for 48 threads and 128 GB RAM.
Fabrication of the 3D Alumina/ESMH Nanocomposite on a Transparent Substrate: A schematic illustration of the fabrication process is shown in Figure S9 (Supporting Information). The free-standing 3D polymeric template/alumina nanostructure was delaminated from the SiO2 substrate using a water-soluble poly(acrylic acid) (PAA) sacrificial layer.[56] The polymeric template was subsequently ashed using a microwave plasma system (R3T, MUEFFE) for 2 h. Before infiltrating the ESMH, a thin ESMH layer was coated on a transparent substrate using spin casting (3000 rpm and 30 s) and an attached 3D alumina nanostructure. After soft baking at 65 °C for 30 min, the ESMH was infiltrated into the 3D alumina nanostructure when it was spin-coated (3000 rpm, 30 s). The next fabrication processes were performed identically to those of the 3D alumina/ESMH nanocomposites.
Other Characterization Methods: The cross-sectional and top surface images of the samples were analyzed using field emission scanning electron microscopy (FESEM, S-4800, Hitachi). The thicknesses of the alumina nanoshell were measured by transmission electron microscopy (TEM, Tecnai F20, FEI). A high-resolution focused ion beam (FIB) (Helios G4, FEI) was utilized to obtain cross-sectional images of the samples. The crystal structure of the alumina was identified using X-ray diffraction (XRD, D/MAX-2500, Rigaku) with CuKα radiation. The refractive index of alumina was measured by using ellipsometry (ALPHA-SE, J. A. Woollam). The ball drop tests were conducted using a custom-made free-fall tester (Figure 5a). A UV–vis spectrophotometer (SolidSpec-3700, Shimadzu) was used to measure the total transmittance of the samples on the PET film with no reference mode. A dynamic bending test was conducted using a folding tester (DMLHP-CS, YUASA) with a radius of curvature of 5 mm for over 20 000 cycles. The steel wool wear test was equipped with a Liberon grade 0000, where the load was 1 kgf, the loading area was 1 cm2, the migration length was 2.5 cm, scratching repeating time was 1000 cycles, and speed was 160 mm s–1.
Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.
AcknowledgementsG.B., G.-M.C., and C.A. contributed equally to this work. This research was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT for the Bioinspired Innovation Technology Development Project (NRF-2018M3C1B7021997), and Korea Ministry of Land, Infrastructure and Transport(MOLIT) as Innovative Talent Education Program for Smart City. This work was also supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF-2020M3D1A1110522 and NRF-2020M3D1A1110524) and Multi-Ministry Collaborative R&D Program (Development of Techniques for Identification and Analysis of Gas Molecules to Protect Against Toxic Substances) through the National Research Foundation of Korea (NRF) funded by KNPA, MSIT, MOTIE, ME, NFA (Grant No. 2017M3D9A1073501).
Conflict of InterestThe authors declare no conflict of interest.
Data Availability StatementResearch data are not shared.
Received: November 30, 2020Revised: December 26, 2020
Published online:
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