Transparent Urethane Siloxane Hybrid Material for Flexible Cover

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Applications of Polymer, Composite, and Coating Materials

Transparent Urethane Siloxane Hybrid Material for Flexible Cover Window with Ceramic-like Strength, yet Polymer-like Modulus Yun Hyeok Kim, Gwang-Mun Choi, Dahye Shin, Yong Ho Kim, Dongchan Jang, and Byeong-Soo Bae ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18141 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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ACS Applied Materials & Interfaces

Transparent

Urethane

Siloxane

Hybrid

Material for Flexible Cover Window with Ceramic-like

Strength,

yet

Polymer-like

Modulus Yun Hyeok Kim, Gwang-Mun Choi, Dahye Shin, Yong Ho Kim, Dongchan Jang, and ByeongSoo Bae* Y. H. Kim, G.-M, Choi, Y. H. Kim, Prof. B.-S. Bae Wearable Platform Materials Technology Center Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea * E-mail : [email protected] D. Shin, Prof. D. Jang Department of Nuclear and Quantum Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu Daejeon 34141, Republic of Korea

KEYWORDS Flexible cover window; ceramic-like strength; polymer-like modulus; urethane-methacrylate siloxane hybrid; molecular hybridization

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ABSTRACT Any transition toward an era of flexible electronics will have to overcome mechanical limitations of materials. Specifically, the attainment of both strength and flexibility, which are generally mutually exclusive, is required including glass-like wear resistance, plastic-like compliance and a high level of strain. Here, we fabricate a urethane-methacrylate siloxane hybrid material (UMSH). It is found that the UMSH, with molecule-level hybridization of urethane linkage and methacrylate-siloxane co-networks, demonstrates ceramic-like high strength (574 MPa), yet polymer-like low modulus (8.42 GPa), and even high strain (6.3%) at fracture with excellent optical transparency. This combination of high strength, flexibility and optical transparency indicates that this is a suitable material for glass substitution and can be used as a transparent flexible cover window for foldable display.


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1. INTRODUCTION Flexible display is a promising next-generation display because of its lightweight, thin, and foldable properties.1–3 Some electronic companies have demonstrated eye-catching prototypes of the foldable smart phones with a “dynamic” foldable display, convincing that flexible display is in the sight.4,5 However, in attempt to commercialize a high-performance flexible display including rollable and foldable property, high strain resistance is a critical technical issue, especially, in a substitute for glass, such as substrates and cover windows.6 As a cover window of a flexible display, glass can provide sufficient transparency and surface hardness, but is vulnerable to bending and folding due to its brittleness against bending stress. In the era of flexible display, therefore, it is essential to develop an innovative material that exhibits glasslike strength, polymer-like modulus, and high strain without sacrificing the excellent optical transparency of the silicate glass.7 Organic-inorganic (O-I) hybrid material, via chemical hybridization of polymer groups into 3-dimensional siloxane networks without phase separation, is a powerful candidate to overcome the weakness of the glass’ brittleness.8,9 The homogeneous O-I co-networks facilitate synergetic effect of both hard segment (inorganic siloxane network) and soft segment (organic polymer) enabling the O-I hybrid material to exhibit good thermo-mechanical, electrical, and optical properties. Furthermore, we can easily control those properties by selecting polymer part, changing volume fraction of O/I phase, and modifying molecular structure of siloxane network. Especially, O-I hybrid materials with regular siloxane structure, such as polyhedral oligomeric silsequioxanes (POSS) and ladder-structured polysiloxanes, shows superior mechanical and thermal properties even compared to other O-I hybrid materials.10–12 Our group also proposed ladder-like structured siloxane hybrid materials that have unique properties: 1) glass-like high strength, yet polymer-like low modulus, 2) cost-effective synthesis and capacity 3 ACS Paragon Plus Environment


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for mass production, 3) deformability and fast-polymerization, and 4) excellent optical transparency.13,14 However, they still have brittleness and low strain resistance, i.e., low toughness, that is undesirable for flexible displays. To enhance the strain resistance of the materials, researchers usually make composite materials with elastomers.15–17 Among them, polyurethane (PU) has been emerging as a coating material due to its high mechanical strength, good adhesion, abrasion resistance, toughness and strain resistance.18 However, incorporation of PU into siloxane generally results in micro-phase separation and optical degradation, especially, in case of polydimethylsiloxanes (PDMS) and phenyl-contained PU, due to the great difference between the solubility parameter of highly nonpolar siloxane and polar urethane hard segments.19,20 Here, we demonstrate a urethane-methacrylate siloxane hybrid material (UMSH) that shows a ceramic-like high strength (574MPa), yet a polymer-like low modulus (8.42GPa), and even a high strain at fracture (6.3%). We obtained molecule-level hybridization by copolymerization of ladder-like structured methacrylate oligo-siloxane and urethane dimethacrylate monomer, which has short alkyl groups (butylene) that give it as small an organic portion as possible. The homogeneous incorporation of the urethane linkage, into high performance ladder-like structured siloxane, place UMSH in a new area on the strengthmodulus Ashby map for engineering materials, and overcomes a mutual exclusivity of high strength and strain.

2. RESULTS AND DISCUSSION A schematic fabrication process of urethane-methacrylate siloxane hybrid material (UMSH) is shown in Figure 1 and experimental section in supporting information. We fabricated UMSH 4 ACS Paragon Plus Environment

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via free-radical polymerization of a mixture of ladder-like structured methacrylate oligosiloxane (LMO) resin and butylene urethane dimethacrylate (BUDM) monomer. LMO is a methacrylate-functionalized siloxane resin that was synthesized via hydrolytic sol-gel reaction, and provides high hardness and strength due to its ladder-like siloxane structure. The detailed synthesis procedure and molecular structure were described in the previous report and the supporting information (Figure 1a, Figure S1 and Table S1).14 To achieve high strain of the final product with ceramic-like high strength, a newly synthesized di-functional monomer with urethane bond, BUDM, was co-polymerized with LMO to fabricate UMSH (Figure 1b). LMO and BUDM were chemically bonded and formed methacrylate-siloxane co-networks with urethane bond by UV-initiated free-radical polymerization through their methacrylate groups (Figure 1c). Incorporation of the urethane bond into the siloxane hybrid material enhances the strain of UMSH, enables a high strength / high strain / low modulus material. To compare the effect of urethane-containing monomer, we fabricated 4 kinds of materials: LMSH (fabricated by only LMO), UMSH50, UMSH100 (fabricated by LMO/BUDM, the ratio of LMO and BUDM is 2:1 and 1:1 respectively), and butylene-urethane methacrylate polymer (BUMP, fabricated by only BUDM) Figure 1b illustrates the synthetic procedure of BUDM. To obtain the di-methacrylate monomer with urethane bonding, 2-isocyanatoethyl methacrylate was reacted with 1,4butanediol with the equivalent ratio. It is well-known that isocyanate group (-NCO) is highly reactive with hydroxyl group (-OH) because of its high electrophilicity, but we added triethylamine as a catalyst for the fast completion of the reaction. The synthesis of BUDM was confirmed by FT-IR (Figure 2a). The isocyanate (2260 cm-1) and hydroxyl groups (3260 cm-1) totally disappeared after the urethane reaction, and new secondary amine groups (N-H, 1530 and 3380 cm-1) were formed. The FT-IR analysis shows the complete reaction between the monomers, as evidenced by no remaining reactive sites (-OH and –NCO) that were in the 5 ACS Paragon Plus Environment


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monomers. The synthesis of BUDM was complementary confirmed by LC-MS (Figure 2b). Two types of monomers with different molecular weight (245 and 400 g/mol) can be obtained as reaction products, derived from the half- or complete urethane reaction. The LC-MS spectra shows a sharp peak at 423 g/mol, which means the complete urethane reaction between the monomers. It should be noted that the molecular weight from LC-MS analysis generally larger than expected molecular weight due to the sodium ion adduct (Na+, 23 g/mol). Because the LCMS analysis is in the agreement with the FT-IR analysis, we can conclude that the synthetic reaction of BUDM is a high yield reaction and the most of the reaction product exists as the dimethacrylate functionalized monomer form (400 g/mol). Therefore, we used BUDM without further purification. However, BUDM was in solid-phase at room temperature, we performed DSC analysis to find its melting temperature (Figure 2c). DSC spectrum of as-synthesized BUDM shows a sharp endothermic peak at 52oC, which also means the most of BUDM is composed of one element. The DSC spectrum of BUDM provides a proper processing temperature of BUDM, so we performed all process at 60 oC. Figure 3a shows images of LMSH, UMSH (UMSH50, UMSH100) and BUMP bulks, fabricated by casting the synthesized resins, into disc-shaped mold with 1mm thickness, and UV-initiated free-radical polymerization. Figure S2 shows the FT-IR spectra of the fabricated hybrid materials with the indications of the polymerizable carbon double bonds (C=C, 1636 cm-1) and non-polymerizable carbonyl groups (C=O, 1715-1700 cm-1), before and after the polymerization process. The C=C bond was broken by free radicals during polymerization process, and peaks related to the C=C significantly decreased after polymerization. The degree of methacrylate conversion was calculated from FT-IR spectra based on the Beer-Lambert law.21 While LMSH showed a lower methacrylate conversion (71.8%) due to steric hindrance by super-functionality of LMO, the urethane insertion drastically increased the methacrylate conversion, up to 93.3% in case of UMSH100. This boost of methacrylate conversion is related 6 ACS Paragon Plus Environment

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to hydrogen bonding and Boltzmann-averaged dipole moment.22 The hydrogen bonding in urethane groups, which have high polarity, increases the charge of the propagating radical in methacrylate groups, and it overcomes the steric hindrance of the LMO resin. Even though all of the bulk samples were fabricated in same condition, LMSH was divided into several pieces due to the brittleness and stress-accumulation during radical polymerization (Figure 3a). In the case of UMSH, however, the inserted urethane bonding, having crack resistance and elongation properties, reduced the accumulated stress and made it possible to obtain completely undamaged bulks. All of the hybrid materials show excellent optical transparency over the visible range (>90%); these values are comparable to that of conventional silicate glass (Figure 3b). The outstanding transparency is derived from homogenous O-I hybridization without micro-phase separation. Though the BUMP also exhibited good optical properties, it had a critical problem during fabrication process: it exhibited high haze due to the wrinkle formation at the surface during polymerization and the micro-phase separation was observed in SEM image (Figure S3). Compared to neat LMSH, UMSH shows the enhanced transmittance at 550 nm, and even in the UV region. The high methacrylate conversion of UMSH decreases the remaining free radicals and unreacted methacrylate groups, which act as sources for UV light absorption and as optical degradation sites.23 Incorporation of polyurethane into siloxane matrix generally leads to the aggregation, resulting in poor morphology and immiscibility due to the solubility parameter gap derived from the difference between soft and hard segments in urethane monomers compared to the polysiloxane.24–26 To convincingly evaluate the degree of micro-phase separation, i.e., homogenous hybridization, we performed SEM and SAXS analysis. Figure 3c and 3d show SEM images of the surfaces of LMSH and UMSH100. The images of both LMSH and UMSH100 show clear and smooth surfaces; no visible micro-pores or micro-phase separation were present in the SEM images. The SAXS analysis, which is a powerful quantitative test to 7 ACS Paragon Plus Environment


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identify micro-phase separated structures and separation grade, is given in Figure 3e. The scattering peaks, observed at qmax = 1.7 nm-1 in the spectra of both LMSH and UMSH100, suggest that both LMSH and UMSH100 are phase-separated materials. However, the two SAXS profiles show same peak position and slopes, which suggests that the addition of urethane monomer does not affect the degree of phase separation of the UMSH100, compared to LMSH.27 To quantitatively obtain the degree of micro-phase separation, “n”, derived from the fitting curve (q-n) of the SAXS profiles in the range of q > qmax (gray area in Figure 3e), was calculated following Porod’s law. The “n” values in both SAXS spectra were 1.8, lower than 4 (the standard value), which indicates very low degree of micro-phase separation.28 The SAXS analysis is in the agreement with the excellent optical transparency and smooth surface image from SEM; we can conclude that UMSH is a nanoscale-hybridized and homogeneous material. To evaluate the elastic recovery of UMSH, we performed nano-indentation test with a Berkovich tip, followed by Oliver-Pharr method (Figure 4).29 Figure 4a shows representative load (P)-displacement (h) curves with one loading-unloading cycle. The P-h curves illustrate that the deformed LMSH almost recovered during the creep process with 5 mN force after unloading; this indicates that LMSH is a quite resilient material. Elastic recovery (We) was calculated by integrating the P-h curves (Figure 4b); the values were 0.86 (neat LMSH), 0.73 (UMSH50), 0.68 (UMSH100) and 0.48 (BUMP). UMSH did not recover like LMSH, but We value of UMSH is higher than 0.6, which is an appropriate value for the design of crackresistant and flexible cover window, as found in Musil’s study.30,31 Figure 4c is optical microscopy (OM) images of loaded positions after nano-indentation tests. LMSH, UMSH50 and UMSH100 showed no permanent deformation on contacted area that ensures the high elastic recovery of LMSH and UMSH. In case of BUMP, however, residual contacted area was observed due to the poor elastic recovery. The image of BUMP also shows spherical aggregates 8 ACS Paragon Plus Environment

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(dark spots) from the micro-phase separation of urethane polymer. To confirm the mechanical potential of UMSH, we performed in situ micro-beam bending test to measure the flexural properties (Figure 5). Figure 5a provides a perspective image of the fabricated sample for the micro-beam bending tests. The micro-beam bending tests were performed by applying load to the center of the doubly clamped micro-beams with a diamond tip. The micro-beam bending tests were recorded in situ by SEM and the deformation was visibly observed (in situ SEM video, see Video S1 for the LMSH and Video S2 for the UMSH100). We tested eight micro-beam specimens of each material, respectively, to ensure the experimental results. Figure 5a shows the representative flexural stress (σ) – strain (ε) curves of four different materials; in situ SEM images of LMSH and UMSH100 are presented in Figure 5b-g. The all values of the obtained flexural properties, such as flexural strength (σf), strain (ε) and flexural modulus (Ef) of the test specimens, are listed in Table S2-S5. All σ-ε curves of LMSH and UMSH showed an elastic deformation against force until fracture; this phenomenon is similar to the deformation of typical ceramics. It should be noted that the slight convex-like curvatures just prior to the fracture come from the additional force by geometric constraint effect at the round-shaped beam end.13 To quantitatively obtain the values of σf, ε and Ef from the σ-ε curve, we eliminated the excess beam stretching stress from the stress at fracture. The obtained value of Ef / σf of LMSH was 10.55 ± 0.70 GPa / 431 ± 51 MPa, and the values of UMSH50 and UMSH100 were respectively 10.06 ± 1.04 GPa / 480 ± 59 MPa and 8.42 ± 0.96 GPa / 574 ± 67 MPa. UMSH100 showed a 34% increase of the flexural strength, compared to LMSH. Especially, the strain at the fracture increased by 54%, from 0.0413 ± 0.0040 (LMSH) to 0.0634 ± 0.0052 (UMSH100), because of the high crack resistance and the high elongation properties of the urethane bonding. In case of BUMP, the fracture strain was outstanding as 0.0691 ± 0.0118, but both Ef and σf (6.88 ± 0.93



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GPa and 368 ± 47 MPa) were lower than those of LMSH. Therefore, it can be concluded that the mechanical properties of the UMSH are ignored a Lever rule, and UMSH shows synergetic properties of methacrylate-siloxane network and urethane linkage. In addition, we calculated the modulus of resilience (Ur) as an indicator of the toughness, by integrating the strength with strain from zero to yield strain (εy). The calculated Ur of LMSH / UMSH100 was 9.05 ± 1.62 / 18.11 ± 2.19 MJm-3, that is, UMSH100 is a tougher material than LMSH. These results suggest that the homogeneous chemical insertion of urethane bonding into siloxane networks can greatly enhance the toughness and elongation of O-I siloxane hybrid materials. The values related to mechanical properties of LMSH, UMSH and BUMP are arranged in Table 1. To compare the mechanical superiority with common materials, the obtained E and σf values of LMSH, UMSH and BUMP from micro-beam bending tests were indicated in Ashby’s E-σf plot (Figure 5h). All fabricated materials show new mechanical properties that located in the new area of Ashby’s plot; the fabricated O-I hybrid materials demonstrate ceramic-like high σf and polymer-like low E. Interesting mechanical properties can be attained by the hybridization of organic methacrylate groups and the inorganic siloxane network at the molecule-level. Although LMSH exhibited the interesting mechanical properties, UMSH shows higher σf and lower E values, even better than those of the epoxy-siloxane molecular hybrid (ESMH) (E = 10.8 ± 0.5 GPa, and σf = 544.09 ± 104.08 MPa) reported in the previous paper.13 The strain (σf / E) of UMSH100 is 0.068, which is a novel value and higher than the values of engineering polymers and the ESMH (~0.05); UMSH100 is comparable to dragline silk, well known as a mechanically outstanding material in nature.32 Moreover, Ur of UMSH100 is higher by 38% than that of ESMH (13.06 ± 3.74 MJm-3). To elucidate the mechanical novelty of UMSH100, we further compared the mechanical properties with various


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synthesized O-I nanocomposites, for which the literature values were plotted in the Ashby plot.13,32–39 Most of the synthesized O-I hybrid materials are located in or near the general region of engineering polymers and composites. Especially, (hydroxylethyl)methacrylate/silica, polyurethane/silica composite, and rodlike silicate/cyanate ester nanocomposite which have similar concepts to this work, show lower values of both σf and E due to separation between O-I phases and lack of the homogeneity of hybridization.33–35 The key distinction of this work is that the molecule-level hybridization, between the methacrylate-siloxane co-networks and urethane cross-linker monomer, leading to the interesting combination of ceramic-like high values of σf and high flexibility (polymer-like low E and high strain at fracture), which is known to be mutually exclusive. To complementally evaluate toughness, denoting the resistance to crack propagation or energy consumption against fracture, we measured fracture toughness by employing microcantilever tests with in situ SEM (Figure 6). Although UMSH100 shows sufficient flexural properties for the hard coating, the toughness is considered more essential characteristic for the practical hard coating.40 The high toughness has benefits of retarded crack formation and propagation, and high wear resistance. We fabricated five micro-cantilevers for each specimen with the dimensions of 10 (length) x 2 (width, B) x 3 (thickness, W) with 600 nm-thick prenotch (a). The crack length to width (a/W) ratio was 0.25 and the pre-notch was introduced about 2 μm apart from the beam support.41 After preparation of micro-cantilevers, force was applied to the cantilevers at a distance (L) of 6 μm from the pre-notch. Figure 6a indicates in situ recorded SEM image during the tests, and the representative load-displacement curves are showed in Figure 6b. Even though the fracture displacement was increased by increasing amount of BUDM, UMSH100 shows the highest load value among fabricated materials. The fracture toughness (KIC) and toughness (GC) were calculated by the following equation.42



KIC =

𝑃max𝐿 3

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𝑎

𝑓(𝑊)

𝐵𝑊2

( ) = 1.46 + 24.36(

f

𝑎

𝑊

GC ≅

𝑎

𝑊)

𝑎

𝑎

– 47.21(𝑊)2 + 75.18(𝑊)3

𝐾𝐼𝐶2 𝐸

The average KIC and GC values are arranged in Figure 6c and Table 1. The fracture toughness of UMSH100 is the highest value of 0.360 ± 0.011 MPam1/2, increasing by 20 % compared to LMSH (0.299 ± 0.004). In case of toughness, the GC increases by 82% from 8.47 J/m2 to 15.39 J/m2 by incorporating the urethane groups. Micro-cantilever tests also substantiate that the UMSH100 is tougher than LMSH and the most appropriate for the flexible hard coating material. To demonstrate the mechanical durability of UMSH100 as the substitute of a cover window for flexible optoelectronic devices, LMSH and UMSH100 were coated on commercial 75 μm thick polyethylene terephthalate (PET) film (Figure 7). Since the methacrylate groups exhibited high shrinkage and stress accumulation due to the high functionality of LMO during polymerization, LMSH coatings showed peeling with cracks and inward warpage problems even at 10 μm thickness (Figure 7a-c). However, UMSH100 coatings (Figure 7d-f) showed decreased the inward warpage problems due to the stress relaxation by inserted urethane groups. Figure S4 shows pencil hardness of UMSH100; pencil hardness increased based on thickness, showing 3H for the 10 μm thickness and 8H for 60 μm thickness, which values are slightly lower than silicate glass but comparable to that of the silicate glass (9H). Furthermore, UMSH100 on the PET film tolerated a 104-times repeated folding test with 1.5 mm radius without any cracking; this was confirmed by SEM micrograph of the surface (Figure 7g-h). Especially, the 10 μm thick UMSH100 coating was so flexible that it could be rolled up inward 12 ACS Paragon Plus Environment

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and even outward onto a 3.2 mm mandrel bar without any cracking, as shown in Figure 7i-j. Figure 7k provides results of a steel-wool test of UMSH100 (Figure S5), UMSH100 was absolutely free from the scratches after the 103-times steel-wool test. This results suggest that UMSH100 has outstanding wear-resistance compared to scratch-vulnerable bare PET film. Water repellency is also important for flexible cover windows due to the dirt retention, selfcleanability and longer life expectancy of the cover itself. To evaluate the water repellency, we measured the water contact angles of LMSH, UMSH100 and conventional silicate glass for the comparison (Figure S6). The water contact angles of LMSH and UMSH100 were 81.82o and 86.26 o, respectively, but that of the silicate glass was 16.17o. We can conclude that UMSH100 has resistance to dust or fingerprint, which is an important factor for flexible cover window materials, without any surface modification.

3. CONCLUSION In summary, we have demonstrated a combination of ceramic-like high strength (8.42 GPa), polymer-like low modulus (574 MPa), and even high strain at fracture (6.3%), obtained by hybridization of methacrylate, siloxane, and urethane groups at molecule-level without intraand inter-phase separation. Our mass-producible material, with high strength, outstanding optical transparency and low modulus, suggests that urethane incorporated methacrylatesiloxane hybrid material can be used in flexible cover window for commercialization of foldable optoelectronic devices.



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ASSOCIATED CONTENT Supporting Information Molecular structures of LMO, FT-IR spectra before and after polymerization for calculating a methacrylate conversion, 1 mm-thick bulk and SEM image of BUMP, pencil hardness results of UMSH versus coating thickness, steel-wool tests, and water contact angle analysis.

AUTHOR INFORMATION Corresponding Author *E-mail : [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Wearable Platform Materials Technology Center (WMC) supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2016R1A5A1009926). This work was also supported by a grant from the Korea Evaluation Institute of Industrial Technology (Project number : 10051337). We gratefully thank to Korea Basic Science Institute (KBSI) for the

29Si

NMR spectra

measurement.


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Figures

Figure 1. Design concept of UMSH. (a) Synthesis and representative molecular structure of LMO resin via hydrolytic sol-gel reaction. (b) Synthesis of BUDM via triethylamine-catalyzed urethane reaction. (c) Fabrication of UMSH by free-radical polymerization between methacrylate groups of LMO and BUDM.



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Figure 2. Synthesis of BUDM. (a) FT-IR spectra of mixture of 2-isocyanatoethyl methacrylate and 1,4-butanediol before and after catalyst insertion. (b) LC-MS peaks of BUDM for yield of urethane reaction. (c) DSC spectra of BUDM for identifying melting point and uniformity. Inner image indicates BUDM at room temperature and 60 oC.


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Figure 3. Hybridization without micro-phase separation enables excellent optical properties. (a) Optical image and (b) total transmittance of 1 mm-thick LMSH, UMSH50, UMSH100 and BUMP bulks. SEM micrographs of surfaces of (c) LMSH and (d) UMSH100. (e) SAXS profiles of LMSH and UMSH100. The degree of micro-phase separation is calculated by Porod’s law in range of the gray region (q > qmax).



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Figure 4. Nano-indentation characterization of LMSH, UMSH50, UMSH100 and BUMP. (a) Representative load-displacement curves, and (b) elastic recovery (We). (c) Optical microscopy (OM) images of residual impressions after the nano-indentation tests.


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Figure 5. Flexural properties from micro-beam bending tests of LMSH, UMSH50, UMSH100 and BUMP. (a) Perspective image of samples and presentative flexural stress-strain curve of micro-beam bending tests. The micro-beam bending tests are recorded by in situ SEM images: (b, e) initial loading, (c, f) loading force just before and (d, g) after crack formation. (h) Ashby plot of Young’s modulus versus strength. All samples exhibit ceramic-like high strength and polymer-like low modulus in the white area, not covered by common engineering materials. Hybridization of urethane into methacrylate-siloxane co-networks allows UMSH to have lower modulus and higher strength. Literature values for other comparative materials are given: reported

epoxy-siloxane

molecular

hybrid

(ESMH),13

natural

dragline

silk,32

(hydroxylethyl)methacrylate/silica nanocomposite,33 polyurethane/silica nanocomposite,34 rodlike silicate/cyanate ester nanocomposite,35 poly(vinyl alcohol)/montmorillonite clay,36 chitosan/silica nanocomposite,37 iron oxide/ oleic acid nanocomposite,38 and layered silicate/glass/epoxy hybrid nanocomposite.39

F: flexural modulus , T: tensile modulus



Page 26 of 29

Figure 6. Fracture behavior from micro-cantilever tests of LMSH, UMSH50, UMSH100 and BUMP. (a) in situ SEM images of UMSH100 during micro-cantilever tests. Samples are fabricated with dimensions of 10 x 2 (width) x 3 (thickness) μm by FIB, having pre-notch with crack length of 600 nm (a/W = 0.2). The fracture is penetrated from the pre-notched crack. (b) Presentative load-displacement curve of micro-cantilever tests. (c) Fracture toughness and toughness values of four hybrid specimens.


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Figure 7. Application of UMSH as a flexible hard coating on a plastic substrate. (a-c) Photograph of LMSH coated on 75 𝜇m-thick PET film: (a) 10 um, (b) 20 um and (c) 30um. (d-e) Photograph of UMSH coated on PET film: (d) 10 um, (e) 30um and (f) 50 um. (g) Folding test of UMSH coated on PET film. The bending radius was 1.5 mm. (h) SEM image of UMSH film, after 104-times bending cycles, shows a smooth surface without any cracking. Image of UMSH (10 um) coated film rolled up (i) inward and (j) outward in a 3.2 mm diameter mandrel without any cracking (ASTM D552-93a). (k) Steel-wool test of UMSH coated on PET film. Photographs (top) show scratches only on uncoated bare PET film (right). The optical 3D surface profiles (bottom) clearly indicate that UMSH is absolutely free from scratches after 103-times steel-wool test.



Sample

Methacrylate conversion

Transmittance at 550 nm

Max. Strain, 𝜀

Page 28 of 29

Flexural strength, 𝜎

Flexural modulus, Ef

f

Fracture toughness, K1C 1/2

Toughness, GC 2

%

%

%

MPa

GPa

LMSH

71.8

90.6

4.13±0.40

431±51

10.55±0.70

0.299±0.004

8.47

UMSH50

80.8

91.3

4.72±0.43

480±59

10.06±1.04

0.313±0.015

9.74

UMSH100

93.3

91.7

6.34±0.52

574±67

8.42±0.96

0.360±0.011

15.39

BUMP

96.1

88.0

6.91±1.18

368±47

6.88±0.93

0.310±0.009

13.97

MPa m

J/m

Table 1. Values related to optical and mechanical properties of LMSH, UMSH50, UMSH100 and BUMP.


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Abstract Graphic


Transparent Urethane Siloxane Hybrid Material for Flexible Cover

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