Preparation of High-Performance Transparent Glass-Fiber Reinforced Composites Based on Refractive Index-Tunable Epoxy-Functionalized Siloxane Hybrid Matrix

Composites Science and Technology 201 (2021) 108527

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Composites Science and Technology

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Preparation of high-performance transparent glass-fiber reinforced composites based on refractive index-tunable epoxy-functionalized siloxane hybrid matrix

Junho Jang, Ph.D. a,1, Hyeon-Gyun Im, Ph.D. b,1, DaeSeop Lim a, Byeong-Soo Bae, Ph.D. a,* a Wearable Materials Technology Center (WMC), Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea b Electrical Materials Research Division, Korea Electrotechnology Research Institute (KERI), Changwon-si, 51543, Republic of Korea

ARTICLE INFO ABSTRACT

Keywords: Fiber reinforced polymer composites (FRPs) are widely utilized in various industrial fields due to their Transparent glass-fiber reinforced composite remarkable mechanical properties and low cost fabrication. However, typical FRPs cannot concurrently fulfill Epoxy-functionalized siloxane hybrid optical transparency and high mechanical strength, which are essential for transparent electronics and auto Refractive index tuning motive applications. Here, we report a transparent FRP (GFRH) that exhibits high opto-mechanical properties by Optical transparency incorporating reinforced glass-fibers into refractive index-tunable epoxy-functionalized siloxane hybrid mate Mechanical properties rials. To achieve transparent composites, we precisely control the refractive index of the siloxane matrix by adding an epoxy cross-linkable hardener. We compare the opto-mechanical properties of the GFRP according to various conditions in terms of curing method (thermal- and UV-curing), fiber content (from 0 to 25 wt% of matrix), fiberlength (from 0.08 to 3 mm), and fibertype (filamentmat, glass-fabric, chopped strands glass-fibers, and milled glass). Finally, to obtain high-performance GFRP, we further optimize the composite structure with short fiber (5 wt% of chopped strands glass-fibers) and long fiber (10 layers of glass-fabric). The optimized composite exhibits high optical transparency (80% @ 550 nm) and mechanical properties (251 MPa flexural strength, 9 GPa flexural modulus and 3 H pencil hardness). Our transparent composite has significant potential for the application of FRPs to transparent electronics and automotive devices.

1. Introduction applications due to their high-temperature durability, chemical intert ness, high mechanical strength, and low thermal expansion [9–12]. Fiber reinforced polymer composites (FRPs) have been widely Despite the various advantages of glass-fiber FRPs (GFRPs), it is not a developed for automotive and electronics applications because of their simple task to prepare transparent GFRPs; a highly compatible interface promising characteristics such as structural reliability, light weight, between the fiber and matrix should be guaranteed to minimize mechanical strength, and cost-effectiveness [1–3]. FRPs consist of a light-scattering at the interface, which is critical to the optical trans polymeric matrix (e.g., polyester, thermostable, vinylester, and epoxy parency of the composites. Transparent GFRPs can be obtained by resin) reinforced with fibers (e.g., carbon, aramid fibers, and matching the refractive index (RI) of the glass-fiber and the polymer glass-fibers) [2–6]. By reinforcing mechanically weak polymeric mate matrix, which allows reduction of refraction and scattering at the rials with fibers, thermo-mechanical properties are greatly enhanced fiber-matrix interface [1,13,14]. If the refractive index (RI) of the two [3]. Typical FRPs are optically translucent or opaque; the materials is not matched, the obtained GFRPs have high optical haze light-scattering at the interface between the polymer matrix and fiber that restricts transparent applications [6,15]. Preparing transparent results in an optical haze that hinders wide use in transparent applica GFRPs using conventional polymer matrices and glass-fiberfillers is not tions such as flexible substrates and screen protection films [7,8]. easy due to the fact that RI-control of a typical thermosetting polymer Glass-fibers are effective reinforcing materials for various such as an epoxide is very limited for viable use [16]. Therefore,

* Corresponding author. E-mail addresses: [email protected] (J. Jang), [email protected] (H.-G. Im), [email protected] (D. Lim), [email protected] (B.-S. Bae). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compscitech.2020.108527 Received 18 September 2020; Received in revised form 19 October 2020; Accepted 25 October 2020 Available online 6 November 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved. J. Jang et al. Composites Science and Technology 201 (2021) 108527

Fig. 1. a) Schematic illustration of synthesis process of epoxy-Functionalized siloxane hybrid (EPSH) matrix resin. Inset photograph represents synthesized EPSH 29 matrix resin. b) Si-NMR spectrum of EPSH matrix resin. Insets: chemical structures representing Si atoms according to bonding states. R1: cycloaliphatic epoxide group; R2: phenyl group. The calculated degree of siloxane bond formation is about 90%. c) FT-Raman analysis of EPSH matrix resin. transparent GFRPs using a RI controllable matrix resin should be monohydrate (Ba(OH)2∙H2O, Sigma-Aldrich, U.S.A.), methylhexahy developed. In addition, inherent chromatic aberration, and other defects drophthalic anhydride (MHHPA, Sigma-Aldrich, U.S.A.), 3-ethyl-3 [(3- such as residual stress and micropores in composites have remained ethyloxetane-3-yl)methoxy]methyl oxetane (DOX, Toagosei, Japan), unsolved problems [14]. triarylsulfonium hexafluoroantimonate salt (Sigma-Aldrich, U.S.A.), Sol-gel derived siloxane-based hybrid materials have been widely octadecyl (trichloro)silane (OTS, Sigma-Aldrich, U.S.A.) and tetrabu studied in the past few decades due to several advantages including high tylphosphonium methanesulfonate (Sigma-Aldrich, U.S.A.) were used optical transparency, and excellent thermo-mechanical characteristics without any further purification. E-glass fibers (mats, fabrics, and [17,18]. These properties allow utilization in a broad spectrum of ap strands) were used as fillers, and the RT value and the density are plications, for example, optical waveguides [19], functional coatings 1.54–1.55 and 2.6 g cm 1, respectively, according to the datasheet (E- [20,21], encapsulation of nanocrystals [22], microlenses [23], and di glass fiber, Nitto Boseki, Japan). electrics [24]. In particular, the RI of siloxane hybrid materials can be easily controlled by changing the composition of organic moieties in the 2.2. Synthesis of epoxy-functionalized siloxane hybrid (EPSH) matrix siloxane back-bone or incorporating a cross-linker. Furthermore, their resin other physicochemical properties such as excellent thermal stability, chemical inertness, and mechanical durability can be retained while RI ESPH matrix resin was synthesized by a non-hydrolytic sol-gel re is tuned, and thus transparent or haze GFRPs can be prepared for a va action [10,25]. ECTS and DPSD were blended in a two-neck flask by a riety of uses [1,3,25]. mechanical stirrer for a few minutes. The molar ratio of ECTS to DPSD Based on the above considerations, we fabricated transparent GFRPs was 1 : 1.5. After blending, Ba(OH)2∙H2O was added in an amount of (hereafter GFRH) composed of a sol-gel derived cycloaliphatic epoxy- 0.2 mol% of the total silane precursors as a base-catalyst to promote the functionalized siloxane hybrid (EPSH) resin (as a matrix) and various sol-gel reaction between ECTS and DPSD, and further mechanically ◦ glass-fibers(as a filler).To obtain the maximum optical transparency in mixed at 80 C under N2 purging. A clear viscous resin was obtained the visible region, we manipulated the RI of the EPSH matrix by con after 6 h. Epoxy-hardener (MHHPA and DOX) of varying amounts was trolling the cross-linker content. Both thermal- and photo-curing were then mixed, and an initiator (tetrabutylphosphonium methanesulfonate; used to prepare the GFRHs to study physical properties of the composites thermal-initiator, triarylsulfonium hexafluoroantimonate salt; in terms of the curing mechanism. Optimizing the structure and photo-initiator) was added in an amount of 2 wt% of the total resin composition of glass fillersin the composites, we obtained a transparent mixture. In the optimization process of the transparent GFRH, composite with high optical transparency and desirable mechanical glass-fibers (chopped glass strands and milled glass) were mixed with characteristics. In addition, we investigated the structure-property the as-synthesized EPSH matrix resin. relationship between the fillers and opto-mechanical properties of the composites in terms of length, content, and type of the glass-fibers. 2.3. Fabrication of transparent glass-fiber reinforced hybrid composite (GFRH) 2. Materials and methods Using glass-fabric or mats, the entire fabrication process was similar 2.1. Materials with previously reported methods [3,10,25]. Briefly, glass-fabrics or ◦ mats were placed on the 1st donor glass plate, and heated up to 80 C. 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECTS, Gelset, U.S. The EPSH resin was then dispensed onto the various glass-fibers for A.), diphenylsilanediol (DPSD, Gelest, U.S.A.), barium hydroxide impregnation. Those samples were then covered with the 2nd donor glass

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Table 1 standard with an AGS-X (Shimadzu, Japan) at a rate of 1.0 mm/min.. Refractive indices of materials used in this study (at 589 nm, sodium D SEM images were collected from a Nova 230 (FEI, Germany). For the RI line). measurement, the matrix resin was cast into a 2 mm thick bulk and UV Materials RI (at 589 nm of Wavelength) cured. RI values of the bulk samples were measured by an Abbe

EPSH 1.57 refractometer (Bellingham and Stanley, U.K.). MHHPA 1.47 DOX 1.45 3. Results and discussion E-Glass-Fiber 1.54–1.55 3.1. Synthesis and curing of EPSH matrix resin plate and a molding process was conducted using a vacuum bag press. First, we synthesized the EPSH matrix resin via a non-hydrolytic sol- All glass plates were surface-treated by OTS to facilitate easy separation gel condensation reaction between the methoxy group of ECTS and the of the GFRHs after curing. The pressed sample was UV- (3 min Under ◦ hydroxyl group of DPSD (Fig. 1a) [22,25]. After the sol-gel reaction, we UV-light (λ = 365 nm) irradiation) or thermal-cured (at 180 C for 12 h). can obtain a viscous and transparent matrix resin (inset in Fig. 1a). The GFRHs were obtained after separation from the glass plates. To prepare RI value of the synthesized EPSH resin was about 1.57, confirmedusing short glass-fibers-incorporated composites (chopped strands glass-fiber the Abbe refractometer. To investigate the degree of siloxane bond and milled glass), the resin with glass-fibers dispersed in the matrix formation from the silane precursors, we analyzed the EPSH matrix resin was used instead of bare EPSH resin. To make optimized using 29Si nuclear magnetic resonance (29Si-NMR) (Fig. 1b). The inset in high-performance transparent GFRHs, chopped strands of E-glass-fibers Fig. 1b is the chemical formula of EPSH matrix resin, which represents were used as filler. cycloaliphatic epoxy (R1) and phenyl (R2) groups in the resin. The result showed the absence of unreacted species (i.e., D0 and T0) and mostly the 2.4. Characterization presence of reacted species (D1,2 and T2,3). The calculated degree of siloxane bond formation in 29Si-NMR is about 90%, indicating a highly Optical transmittances of the GFRHs was measured with a UV–visible condensed siloxane networks (Table S1) [25]. From the high intensity of spectrophotometer (SolidSpec-3700, Shimadzu, Japan). Thermal prop the fully reacted species (D2 and T3) peaks, branched siloxane networks ◦ erties were measured by TGA (Q50, TA Instruments, Inc.) with a 5 C can be expected. The siloxane network structure of the EPSH matrix 1 min ramp under a N2 atmosphere. The FT IR spectra were recorded resin can be further confirmed by RAMAN spectroscopy; the results 29 with an FT IR 680 plus (JASCO, U.S.A.). The Si NMR spectrum was showed the formation of cyclic siloxane rings or cage structures (i.e., obtained from a DMX600 FT 600 MHz (Bruker Biospin, Australia). The branched siloxane network), which corresponded to the 29Si-NMR stress strain curves were measured on the basis of the ASTM-D882-12

Fig. 2. a) Schematic illustration of curing process of EPSH matrix resin via i) thermal- and ii) UV-curing methods. Comparison of FT-IR spectra of EPSH matrix before and after curing via b) thermal- and c) UV-curing methods.

3 J. Jang et al. Composites Science and Technology 201 (2021) 108527 results [26,27]. Therefore, the EPSH is successfully synthesized, having a highly condensed siloxane networks with a branched structure. The RI value of the EPSH matrix at 589 nm wavelength was 1.57, as depicted in Table 1. Two types of EPSH with cross-linkers were fabricated via thermal- or UV-induced curing methods [20]. Cross-linkable hardeners with a low RI (anhydride or dioxetane monomers) were added to the EPSH resin, respectively, to match the RI of the matrix resin such that it is similar to that of glass-fiber (RI: 1.54–1.55, Table 1). For thermal curing, the an hydride hardener (i.e., MHHPA, RI: 1.47, Table 1) was mixed, and then ◦ cured at elevated temperature (175 C) (method (i) in Fig. 2a). As confirmedby FT-IR spectra, the epoxide characteristic peak at 882 cm 1 was completely removed after thermal curing, retaining the siloxane bond characteristic peak at 1000–1150 cm 1 (i.e., ladder-like siloxane structure) (Fig. 2b). In the TGA analysis, the 5 wt% decomposition ◦ temperature is above 350 C, which indicates high thermal stability as shown in Fig. S1a. For UV-curing, a dioxetane cross-linker (i.e., DOX, RI: 1.45, Table 1) was added, and then cured under UV irradiation (method (ii) in Fig. 2a). That also exhibited the elimination of epoxide peak as shown in the FT-IR spectra (Fig. 2c), and high thermal stability (TGA profile in Fig. S1b).

3.2. Refractive index matching between glass-fiber and matrix

We used typical E-glass-fibers (ca. 1.54–1.55 of RI) due to their reliable mechanical properties and cost-effectiveness, and thus the RI of the matrix siloxane resin should be tuned to be similar with E-glass-fi bers to prepare transparent composites [1,3]. Because EPSH matrix resin has a relatively higher RI value compared to E-glass-fiber (a large number of π-electrons which stems from phenyl groups of siloxane oligomers may induce a high RI value over 1.57), low RI hardeners are required [28]. The higher the RI difference is, the larger the light-scattering at the interface between fiber and matrix will be; this results in a huge degradation of the optical property of the GFRPs [1,29]. According to the light transmission model of a single fibercomposite involved with scattering under a negligible refraction condition, the light can traverse the fiber-matrixinterface, and reduce the intensity due Fig. 3. Comparison of transmittance of GFRH according to contents of a) to the phase lag (ρ), which is defined as [30]. MHHPA and b) DOX. ⃒ ⃒ 2π ⃒ ⃒ ρ = D nf nm (1) strands of E-glass-fiber in the matrix. Fig. 3a depicts the change of λ transmittance of thermal-cured GFRHs (T-GFRHs) by increasing the where λ, D, nf, and nm are the wavelength of light, the diameter of the MHHPA content (from 0.4 mol% to 1.2 mol% of matrix resin). The T- glass-fiber, the RI of the glass-fiber, and the RI of the matrix, respec GFRH with low MHHPA content (0.4 mol%, green line in Fig. 3a) tively. Following the equation (1), the phase lag increases with an exhibited low transmittance (36.6% @ 550 nm), indicating a mis increasing RI difference between the fiberand matrix. To determine the matched RI value between the glass-fiber and the matrix. The trans relationship between the phase lag and optical transparency, Iba and mittance of the T-GFRH exhibited the highest value in the case of Kagawa modified equation (1) as follows: incorporating 0.55 mol% of MHHPA (80% @ 550 nm). The trans mittance diminished with increasing MHHPA contents over 0.55 mol% ( )( )1 2 V [ ( )1] 2t f due to the enlarged RI difference between the glass-fiber and matrix as 2 D π Vf expected from equation (1). We also confirmed the change of trans T = 1 2Qext(ρ) (2) π mittance of UV-cured GFRH (U-GFRHs) with varying DOX content (from 0.07 mol% to 0.12 mol% of matrix resin). These samples also exhibited π/2 ∫ the highest transmittance when the DOX content was 0.08 mol% in the 2 Qext (ρ) = 2ρ sin(ρ cos γ)sin γ dγ (3) matrix (60% @ 550 nm), while the lowest value was shown when the 0 DOX content was 0.12 mol% (54.8% @ 550 nm) due to the same reason as in the case of T-GFRH. Therefore, we successfully optimized the RI where Qext is the extinction (or attenuation) efficiency (i.e., scattering values of the matrix resin with the maximized optical transparency of efficiency factor), γ is the angle of incident light traversing the glass- GFRHs when MHHPA (via thermal-curing) and DOX (via UV-curing) fiber, Vf is the volume fraction of glass-fiber, and t is the total thick were 0.55 mol% and 0.08 mol%, respectively. ness of the GFRP [31]. By equation (2), increased phase lag induces a reduction of the optical transparency of the GFRPs. This indicates that optimizing the RI value of the matrix is very important to achieve high 3.3. Dependence of opto-mechanical properties on curing methods optical transparency of the composites. Thus, we optimized the RI value of the EPSH matrix with varying To investigate the effect of the curing methods on the opto- hardener content and traced the optical transmittance of the samples. mechanical properties of the GFRHs, optical and mechanical analyses For testing, we fabricated 1 mm thick GFRHs with 15 wt% of chopped were utilized according to the curing methods (thermal- and UV-curing).

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Fig. 4. a) Comparison of transmittance of T-GFRH (red) and U-GFRH (black). b) Photographs of T-GFRH (upper) and U-GFRH (down). c) Stress-strain curves of UV- cured bare EPSH (black), U-GFRH (blue), thermal-cured bare EPSH (brown), and T-GFRH (red). (For interpretation of the references to colour in this figurelegend, the reader is referred to the Web version of this article.)

increased (from 81% to 86% @ 550 nm) due to the reduction of the total Table 2 light-scattering path in the composite [25]. Because the compatibility Summary of flexural strength, flexural modulus, and pencil hardness of UV- between the glass-fiber and matrix can affect the light-scattering, the Cured EPSH, U-GFRH, Thermal-Cured EPSH, and T-GFRH. glass-fiber was surface-treated using amine-functionalized silane (i.e., Flexural Strength Flexural Modulus Pencil Hardness aminopropyltrimethoxysilane, APTS) to enhance the adhesion of the UV-Cured EPSH 23.95 MPa 1.96 GPa 1 H matrix to the glass-fiber.The surface-treatment of the glass-fibercan be U-GFRH 28.74 MPa 2.41 GPa 2 H confirmed by a TGA analysis, where the decomposition temperature of Thermal-Cured EPSH 42.97 MPa 2.12 GPa 1 H the glass-fiber was reduced due to the incorporation of an organic T-GFRH 48.11 MPa 3.10 GPa 2 H compound on the glass-fiber (Fig. S2). From the results, not only the optical transparency of GFRH (Fig. 5b) but also the mechanical prop Fig. 4a presents the transmittance spectra of RI optimized GFRH with erties (strength and modulus) (Fig. 5c) were slightly improved, indi different curing methods. The T-GFRH exhibited a higher transmittance cating increased adhesion of the matrix to the glass-fiber [33]. value (80% @ 550 nm) than that of U-GFRH (60% @ 550 nm); this To investigate the effect of the glass-fiber content in the composite, might due to the reduced residual stress and micro pores at the fiber- we observed the change of the opto-mechanical properties with varying matrix interface allowed by the thermal relaxation process during loading of glass-fiber. As shown in Fig. 6a, the flexural strength of the thermal curing [29]. In addition, according to the RI-density relation GFRH was maintained until 10 wt%. After this point, the flexural ship, the thermal-cured hybrid matrix would have a relatively higher strength was continuously enhanced according to the increase of the density compared to the UV-cured one, which can also reduce the RI glass-fiber content due to the reinforcing effects [34]. The flexural difference between fibersand matrix [32]. As shown in Fig. 4b, T-GFRH modulus and pencil hardness of the GFRHs also exhibited a similar showed clear and uniform transparency, while U-GFRH was relatively tendency with the flexural strength (Fig. S3 and Table S2). We also translucent. confirmed the change of the transmittance of the GFRHs with varying To compare the mechanical properties, we conducted 3-point glass-fiberloading (Fig. 6b). In this case, the transmittance values of the bending and pencil hardness tests of the bare EPSH matrix and the GFRHs were continuously reduced with increasing glass-fiber loading GFRHs (Fig. 4c). After reinforcing glass-fibers into matrix, surface because the total light-scattering site expanded (i.e., increased interfa hardness and flexuralproperties were enhanced (Table 2). In particular, cial area between the glass-fiber and siloxane matrix). This can be the thermal-cured samples exhibited improved flexural properties directly seen in photographs of a low-loaded GFRH (Fig. S4a) and a compared to the UV-cured samples (higher flexural stress and strain); high-loaded GFRH (Fig. S4b). Thus, optimal loading of glass-fiber is this trend can also be explained by thermal relaxation during thermal required for maximizing the opto-mechanical properties of the GFRH. curing. A reduction of micro pores allowed enhanced wetting of the We also investigated the relationship between the fiber length and matrix resin into the glass-fiber, and resulted in prolonged yield strain the opto-mechanical properties of the GFRHs. Several length of glass- values. Therefore, in this study, for preparing composites with better fibers (3, 1, 0.4, and 0.08 mm) are prepared using a vibration-mill ma optical and mechanical performance, the fabrication process was opti chine as shown in cross-sectional SEM images of Fig. S5. Glass-fibers mized by employing thermal-curing. loading of GFRH samples was fixed at 15 wt% of the matrix resin. The transmittance of the GFRH was reduced according to decreasing fiber length (from 80.7% to 29.5% @ 550 nm); this can be due to the shorter 3.4. Factors affecting opto-mechanical properties of GFRH glass-fiber having an irregular shape and the higher total interfacial area, which induces degradation of the optical property (Fig. 7a). Ac According to equation (2), except for the RI difference, the total light cording to the Cox model, the elasticity of fibrous-based materials is transmittance of the FRPs depends on thickness, the fiber content in enhanced with increasing fiberlength [35]. For effective reinforcement, composite, and the fiberlength. We investigated the changes of the opto- it is necessary to determine critical fiber length, expressed by the mechanical properties of the GFRH with these factors. For the testing, all following equation: GFRHs were fabricated using a filler (chopped strands of E-glass-fiber), ( ) σf d and MHHPA (0.55 mol% in matrix) via a thermal-curing process. Fig. 5a lc = (4) 2τ shows a comparison of the transmittance according to the thickness of c the GFRH samples fixed with glass-fiber content (15 wt% of matrix where l is the critical length, σ is the tensile strength of the fiber,d is the resin). With decreasing thickness, the transmittance of the GFRHs c f

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Fig. 6. Change of a) flexuralstrength and b) transmittance of GFRHs according to fiber loading in EPSH matrix.

mm (maximized uniformity), the experimental results corresponded to the Cox model. The flexural modulus of the GFRHs exhibited a similar tendency to the Cox model (Fig. S6). Therefore, to obtain transparent composites with desirable opto-mechanical properties, we may conclude that using glass-fibers with optimal length and uniform distribution is important.

3.5. GFRH reinforced with various types of glass-fibers

To compare the change of properties with the types of glass-fibers, we fabricated GFRHs with various glass-fibers (in order of length: fila ment mat, glass-fabric, chopped strands of glass-fiber,and milled glass) (Fig. 8a). Loading of the glass-fibersand the thickness of the GFRH were fixedat 25 wt% of matrix resin and 1 mm, respectively. Fig. 8b presents a comparison of the transmittance of GFRHs with various glass-fibers. Fig. 5. a) Comparison of transmittance of GFRHs according to thickness. Optical transmittance of the GFRHs with the filament mat (long-fiber) Comparison of b) transmittance and b) mechanical properties of GFRHs with was close to that of bare EPSH, while the GFRHs with the milled glass surface-treated fiber (red) and non-treated fiber (black). (For interpretation of (short-fiber) showed the lowest transmittance due to increased light- the references to colour in this figure legend, the reader is referred to the Web scattering at the interface between the fiber and matrix as previously version of this article.) explained. In addition, GFRHs with the filament mat exhibited higher optical transparency compared to that of GFRHs with the glass-fabric diameter of the fiber,and τc is the shear strength of the bond between the due to the low light-scattering of the filament mat, which is randomly matrix and the fiber. In our study, the flexural strength of the GFRHs arranged in two dimensions, compared to the glass-fabric, although both increased slightly until 1.0 mm glass-fiber length (Fig. 7b), which may fibershave long fiberlength [26]. We conducted a 3-point bending test be a critical length in equation (4). The optimal length of the glass-fiber of GFRHs with various glass-fibers (Fig. 8c). The results showed that provides uniform dispersion, and this may cause effective strengthening long glass-fibers have higher flexural properties than the other of the composite [36]. In cases of shorter length of glass-fiber than 1.0 glass-fibers.The shortest glass-fiber( i.e., milled glass, length: 0.08 mm)

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exhibited the lowest flexuralproperties, which corresponded to the Cox model [35]. In particular, the GFRH with glass-fabric shows higher flexural properties compared to the filament mat, although they have similar fiberlength. Therefore, not only the length of the glass-fibers,but also the orientation and structure of the glass-fibers can severely affect the mechanical properties of the composites. The flexural strength, flexuralmodulus, and pencil hardness of each GFRH are summarized in Table S3. A comparison of the characteristics according to various glass-fibers is summarized in Table S4. We selected the glass-fabric as a filler for a high-performance transparent composite because it showed high optical transparency and desirable mechanical strength compared to the above glass-fibers. With an increasing number of glass-fabric sheets in the composites (from 10 to 30 layers), the GFRHs exhibited decreasing transmittance and increasing mechanical strength (Fig.S7a and b), which corre sponded to previous results and the Cox model. To build up a highly transparent and mechanically robust composite, finally, we designed the optimal structure of the GFRH by combining with short glass-fiber(5 wt% of chopped strands of glass-fiber)and long glass-fiber (10 layers of glass-fabric). As an optimized matrix, an EPSH matrix resin with dispersed chopped glass strands was prepared (Fig. 9a); this uniform dispersion allows enhancement of the mechanical properties of the matrix. As the major reinforcement, glass-fabric was chosen due to its high transmittance and mechanical strength. Fig. 9b illustrates the fabrication process of the optimized GFRH. First, (i) 5 layers of glass-fabrics were placed on the 1st donor glass plate. (ii) The EPSH matrix resin mixed with 5 wt% of chopped strands glass-fiberswas then dispensed, and (iii) an additional 5 layers of glass-fabrics were placed onto the resin impregnated glass-fabrics. (iv) The step (iii) sample was subsequently covered by the 2nd donor glass plate, and then a thermal-curing process was performed. (v) By detachment of the two glass-plates, we can obtain the transparent composite (inset in Fig. 9b). The optimal GFRH showed a smooth surface in the cross-sectional SEM image (Fig. 9c) and low waviness (~ 250 nm) (Fig. S8). In the cross- sectional SEM image of the optimal GFRH, 5 layers of glass-fabric were clearly seen on the top and bottom layers of the GFRH. These Fig. 7. a) Transmittance and b) flexural strength of GFRHs with various layers have 100 μm thickness (Fig. 9d). Moreover, the chopped strands fiber length. of glass-fibersexhibited a uniform dispersion (Fig. 9e), and good wetting with the matrix resin (Fig. 9f). Interestingly, the optimal GFRH exhibited high transmittance (80% @ 550 nm) with almost the same value as the

Fig. 8. a) Schemes of filament mat, glass-fabric, chopped strands glass-fibers, and milled glass. Comparison of b) transmittance and c) mechanical properties of GFRHs with various glass-fibers.

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Fig. 9. a) Fabrication of resin with chopped strands of glass-fibers dispersed in matrix. b) Schematic illustration of entire process for GFRH with optimal structure. Inset is a photograph of the optimal GFRH. c) Cross-sectional scanning electron microscopy (SEM) image of the optimal GFRH. The red and blue boxes indicate 5 layers of glass-fabric and chopped strands glass-fibers dispersed matrix, respectively. d-e) Magnified images of d) red and e) blue area in c). f). Magnified image of glass-fiber dispersed matrix. g) Transmittance of bare EPSH (black) and optimal GFRH (red). h) Comparison of mechanical properties of bare EPSH (black), GFRHs with optimal structure (red), 10 layers of glass-fabric (blue), and 25 wt% of chopped strands glass-fibers(brown). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Table 3 Acknowledgement Comparison of pencil hardness of bare EPSH, and GFRHs with only chopped strands glass-fibers (25 wt%), only glass-fabric (10 layers), and optimal This work was supported by the Wearable Platform Materials Tech structure. nology Center (WMC) funded by the National Research Foundation of Types Pencil Hardness Korea (NRF) grant by the Korean Government (MSIT) Bare EPSH 1 H (2016R1A5A1009926) and a grant from the Korea Evaluation Institute GFRH (25 wt% of Chopped Strands Glass-Fibers) 2 H of Industrial Technology (10051337). This work was also supported by GFRH (10 layers of Glass-Fabric) 1 H the Primary Research Program (20A01024) of the Korea Electro GFRH (Optimal Structure) 3 H technology Research Institute. We would like to thank the Korea Basic Science Institute (KBSI) for help with the 29Si NMR spectral GFRHs with only glass-fabric (Fig. 9g). Meanwhile, we found that the measurements. mechanical properties (251 MPa flexuralstrength; fivetimes that of the bare matrix, 9 GPa flexuralmodulus; four times that of the bare matrix, Appendix A. Supplementary data and 3 H pencil hardness) were significantly enhanced by introducing chopped strands of glass-fibers (Fig. 9h and Table 3); these results Supplementary data to this article can be found online at https://doi. indicate that the synergetic effect of the short fiber (strands in the ma org/10.1016/j.compscitech.2020.108527. trix) and the long fiber(fabric) fillersallows optimal structural integrity. Consequently, unprecedentedly improved mechanical properties and References high optical transparency are concurrently obtained. Therefore, our [1] Y.-W. Lim, J. Jin, B.-S. Bae, Optically transparent multiscale composite films for GFRH has excellent opto-mechanical properties, and its potential use flexible and wearable electronics, Adv. Mater. 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