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Solvent-Free and Highly Transparent SiO2 Nanoparticle−Polymer Composite with an Enhanced Moisture Barrier Property Chan Il Jo,† Jieun Ko,† Zhenxing Yin,† Young-Jae Kim,*,† and Youn Sang Kim*,†,‡ †

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, 08826, Republic of Korea ‡ Advanced Institutes of Convergence Technology, 145 Gwang gyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16229, Republic of Korea S Supporting Information *

ABSTRACT: Solvent-free SiO2−poly(trimethylolpropane triacrylate) (SiO2−PTPT) nanocomposites are successfully fabricated for an encapsulation layer by surface modification of a SiO2 nanoparticle. In order to lead surface-initiated polymerization and introduce structural similarity between a SiO2 surface and polymer, the surface of the SiO2 nanoparticles are modified with [3-(methacryloyloxy)propyl]trimethoxysilane. The modified SiO2 nanoparticle dispersed in trimethylolpropane triacrylate (TPT) is converted to a SiO2−PTPT nanocomposite under UV irradiation. Because SiO2 incorporated in a polymer generates a tortuous path and absorbs a water molecule, the SiO2−PTPT nanocomposite shows a considerably lower water vapor transmission rate than PTPT. All of the nanocomposite films are highly transparent (over 98%) in the visible region before and after humidity aging tests. Furthermore, a light-emitting diode encapsulated by a 20 wt % SiO2−PTPT nanocomposite exhibits a considerably longer lifetime in the accelerated aging condition.

1. INTRODUCTION With the rapid development of both electronic devices and the electronics market, various efforts have been made to improve the stability and life spans of electronic devices.1,2 However, degradation of the electrical performance over the usage period can be observed for such devices. One of the primary causes of this degradation is permeation of the device by oxygen and water under ambient conditions.3−5 According to the literature, oxygen- and water-molecule penetration of a device causes serious problems, such as active layer degradation and electrode oxidation.6,7 In addition, the hydrogen gas generated by water reduction at the electrode interface causes electrode delamination at the interface,8,9 resulting in device failure. Therefore, encapsulation materials with high oxygen and water resistance are considered to be important components in any electronic device design and with regard to device development. In the traditional encapsulation approach, a device is covered with a glass or metal lid using an ultraviolet (UV)-curable epoxy sealant;1 this can effectively protect the device from oxygen and water permeation. However, the application of this approach to flexible devices is restricted because the encapsulating structures (i.e., the glass or metal lids) are rigid. Therefore, the possible use of thin-film-type encapsulation layers composed of organic or inorganic materials in flexible devices has been studied.10−15 Organic materials are flexible compared to glass or metal; however, their permeability to oxygen or moisture renders them ineffective for use as barrier layers. In contrast, various inorganic materials such as SiO2,16,17 Al2O3,18 and Si3N419 have been widely applied as encapsulating materials because of their © 2016 American Chemical Society

excellent barrier properties. However, the fabrication of inorganic encapsulation layers using a vacuum deposition process is expensive and complex. In addition, problems with cracking or pinhole defects on the layer surface are encountered,20,21 along with limited flexibility. To this end, an inorganic/organic multilayer has been proposed as a means of overcoming the individual limitations of both organic and inorganic materials.22−25 These multilayers exhibit acceptable barrier properties for use as an encapsulating material, with the organic interlayer preventing pinholes and crack propagation in the inorganic layer.26,27 Further, an organic/inorganic multilayer has a tortuous pathway, which can delay the permeation of diffusing water and oxygen molecules.28 Despite these benefits, however, these materials have disadvantages with regard to widespread commercial application, such as high cost, complexity, and time-consuming fabrication methods. To overcome the aforementioned limitations, a polymer nanocomposite has been introduced for use in encapsulating materials.29,30 Inorganic nanomaterials in the polymer matrix provide tortuous pathways that hinder permeating molecules and thus decrease the permeation rate31 without affecting the advantages of polymers such as mechanical flexibility and ease of processing. Moreover, the barrier properties of polymer nanocomposites can be enhanced by embedding inorganic Received: Revised: Accepted: Published: 9433

April 17, 2016 August 8, 2016 August 15, 2016 August 15, 2016 DOI: 10.1021/acs.iecr.6b01470 Ind. Eng. Chem. Res. 2016, 55, 9433−9439

Article

Industrial & Engineering Chemistry Research Scheme 1. Schematic Illustration of SiO2−PTPT Nanocomposite Fabricationa

a

MPS−SiO2 in a TPT mixture was subjected to UV curing to yield a SiO2−PTPT nanocomposite.

transmission and optical transmittance are characterized. Finally, a 20 wt % SiO2−PTPT nanocomposite is applied as an encapsulation layer on a blue light-emitting diode (BLED); effective encapsulation with no optical transmittance degradation is observed.

nanomaterials, which could interact with the incoming permeant. In this regard, several polymer nanocomposites with nanomaterials such as MgO,32 Al2O3,33 and SiO234 have been demonstrated because hydroxyl groups on the surface of nanomaterials can absorb the moisture by hydrogen bonding. However, a decrease in the transmittance of polymer nanocomposites caused by nanoparticle aggregation is still limiting especially upon application in optical devices. Aggregation of nanoparticles also tends to generate voids or defects in the polymer matrix, which can lead to unintended higher permeability values.35 Therefore, to obtain the desired properties, transmittance and water vapor transmission rate (WVTR), of a polymer nanocomposite for use in encapsulation, preparing homogeneously disperse nanoparticle−polymer composites is essential. For the fabrication of a polymer-based encapsulation layer, a solvent-free method is suitable for minimizing damage to the device such as degradation of the active layer and unwanted modification during the encapsulation process.36,37 In addition, defects or pinholes can be generated during the solution process, which lead to poor barrier properties of the polymer layer.38 A UV-curing method that converts a liquid monomer to a solid polymer under UV irradiation has been considered as one of the effective ways of protecting a device because of its solvent-free and fast processing, low energy consumption, and ambient temperature operations.39,40 In this work, we synthesized a solvent-free UV-curable polymer nanocomposite [SiO2−poly(trimethylolpropane triacrylate (PTPT)] encapsulation layer, which consists of SiO2 as a filler and PTPT as a matrix (Scheme 1). Trimethylolpropane triacrylate (TPT) has three acrylate groups in a molecule, which generate densely cross-linked polymer networks under UV irradiation in the presence of a photoinitiator. This dense polymer structure is expected to aid in the prevention of water permeation.41,42 Silica nanoparticles in a polymer matrix are expected to provide a tortuous pathway and act as a desiccant by absorbing water molecules via hydrogen bonding.43,44 To enhance the compatibility of SiO2 with the polymer, the silica nanoparticle surfaces are modified using [3-(methacryloyloxy)propyl]trimethoxysilane (MPS). The ligands are copolymerized with TPT through surface-initiated polymerization in a solventfree UV-curing process,45 and the modified SiO2 nanoparticles are homogeneously incorporated in the PTPT matrix at up to 20 wt % content. To confirm the applicability of the nanocomposite as an encapsulation layer, the water vapor

2. EXPERIMENTAL SECTION 2.1. Materials. [3-(Methacryloyloxy)propyl]trimethoxysilane (MPS; 98%), 2-hydroxy-2-methylpropiophenone (97%), and trimethylolpropane triacrylate (TPT) were purchased from Aldrich. Ethanol (99.9%), tetrahydrofuran (THF; 99%), and ammonium hydroxide solution (25−28%) were purchased from Daejung. Tetraethyl orthosilicate (TEOS; 96%) was purchased from TCI. All organic solvents were used without purification. 2.2. SiO2 Synthesis and Modification. A total of 5 mL of an aqueous ammonium hydroxide solution, 5 mL of deionized water, and 4 mL of TEOS were sequentially mixed with 236 mL of ethanol. The mixture was stirred for 12 h at room temperature. The resultant SiO2 solution was centrifuged for 20 min at 20000 rpm. Thereafter, the precipitate was redispersed in ethanol. This procedure was repeated three times. Then, the SiO2 dispersed in ethanol solution was centrifuged for 10 min at 4000 rpm and the supernatant was collected for the next reaction. To modify the SiO2 nanoparticles, 6 mL of MPS was added to the SiO2 solution (600 mg of SiO2 in 30 mL of ethanol) and stirred for 12 h at 70 °C. The mixture was centrifuged for 20 min at 20000 rpm, and the precipitate was then redispersed in THF. This procedure was repeated three times. Then, the solution was centrifuged for 10 min at 4000 rpm, and the supernatant containing MPS-modified SiO2 nanoparticles (MPS−SiO2) was collected. 2.3. Preparation of SiO2−PTPT Nanocomposites. SiO2−TPT mixtures were prepared by mixing MPS−SiO2 dispersed in a THF solution (16 mg/mL) with TPT. The SiO2−TPT mixtures contained 5 and 20 wt % of MPS−SiO2, respectively. After vigorous mixing of the solutions for 1 h, THF was gently removed using an evaporator. Any residual THF was then eliminated under high-vacuum conditions. 2hydroxy-2-methylpropiophenone, a photoinitiator, was added to each mixture to adjust the composition to a 5 wt % SiO2−TPT mixture. Nanocomposite films were then fabricated on a glass substrate via the bar-coating method, followed by UV irradiation for 1 h using a UV curing system (Minuta Tech., MT-GJ28, 365 nm). The nanocomposite film thickness ranged from 8 to 10 μm. For WVTR measurement, polymer films were prepared on 5 × 5 cm2 and 190-μmthick poly(ether sulfone) (PES) film specimens. Each PES film specimen was treated with UV ozone for 30 min. Then, 0.2 mL of the SiO2−TPT mixture was dropped on the PES film and uniformly pressed using a poly(dimethylsiloxane) (PDMS) mold. PTPT and SiO2−PTPT nanocomposite films on PES were obtained by UV curing the samples for 1 h and then peeling off the PDMS mold. The polymer film thickness on PES was ∼30 μm. 9434

DOI: 10.1021/acs.iecr.6b01470 Ind. Eng. Chem. Res. 2016, 55, 9433−9439

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Industrial & Engineering Chemistry Research

Figure 1. (a) TGA curves and (b) FT-IR spectra of bare SiO2 and MPS−SiO2. where Ei (1141 GPa) and νi (0.07) are the elastic modulus and Poisson’s ratio of the diamond indenter. Es and νs are the elastic modulus and Poisson’s ratio of the sample.

2.4. Characterization. Transmission electron microscopy (TEM) images of the SiO2 nanoparticles and nanocomposite films were obtained using a Hitachi-7600 microscope operated at 100 kV. The hydrodynamic size of MPS−SiO2 in THF was measured three times and averaged at room temperature using dynamic light scattering (DLS; Zetasizer, Malvern Instruments). IR spectra were recorded using a Fourier transform infrared (FT-IR) spectrometer (Nicolet iS10) in order to study the SiO2 nanoparticle surfaces. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were performed by preheating the samples at 100 °C for 1 h and then heating them from 100 to 800 °C at a 10 °C/min heating rate under an N2 atmosphere using an SDT Q600 (TA Instruments Inc.). The nanocomposite film transmittance was measured using an ultraviolet−visible (UV−vis) spectrometer (PerkinElmer, US/Lambda 35), and thickness measurements were conducted using a surface profiler (KLA-Tencor, Alpha-Step IQ). The WVTR tests were performed using a Permatran-W 3/33 from Mocon Inc., in accordance with the ASTM F1249 standard. The temperature and relative humidity (RH) of the test were 38 ± 2 °C and 100%, respectively. The electroluminescence (EL) intensity of the BLED at 1 W power was measured using a fluorescence spectrometer (FluoroMate FS-2, Scinco). For the water absorption test of MPS−SiO2, MPS−SiO2 dispersed in a THF solution was centrifuged, and the precipitation was dried in vacuo at 100 °C for 12 h to remove residual THF and absorbed water. The water absorption test was performed by placing the sample at 38 ± 2 °C and 100% RH. After 7 h, the weight change of the sample was then measured. The water absorption percent of MPS−SiO2 was calculated as follows:

water absorption percent (%) =

3. RESULTS AND DISCUSSION 3.1. Surface Modification of SiO2 Nanoparticles. SiO2 nanoparticles having diameters of 32 ± 3 nm were prepared, and these particles were found to be well-dispersed in an ethanol solution (Figure S1). This small size of the SiO2 nanoparticles is expected to effectively prevent the diffusion of water vapor by generating a compact nanocomposite film.46 In order to initiate polymerization of TPT from the SiO2 surface, surface modification of SiO2 was performed using MPS, so as to generate CC groups on the SiO2 surface. Note that the structural similarity between MPS and TPT can enhance the compatibility between SiO2 and PTPT.47 As shown in Figure S1b, the MPS-modified SiO2 nanoparticles (MPS−SiO2) were well-dispersed in THF, which was used as a dispersion solvent to achieve homogeneous mixing with TPT and to facilitate removal of the solvent for the solvent-free encapsulation process. As expected, no significant changes in the nanoparticle sizes or shapes were observed in the TEM images obtained after surface modification (Figure S1b). Furthermore, the hydrodynamic sizes of as-prepared MPS− SiO2 in THF and MPS−SiO2 in THF after being stored for 6 months measured by DLS were 80.8 ± 0.5 and 81.5 ± 0.7 nm, respectively (Figure S1c,d). This result indicates that MPS− SiO2 in THF has long-term stability with no aggregation over 6 months. To verify the occurrence of surface modification, the SiO2 nanoparticle surfaces were characterized using TGA and FT-IR (Figure 1). The MPS content on the SiO2 surface was determined via TGA (Figure 1a). In the results shown in Figure 1a, the TGA curves indicate bare SiO2 and MPS−SiO2 weight losses of 10.3% and 12.8%, respectively, for temperatures ranging from 100 to 800 °C. Therefore, the additional 2.4% weight loss of MPS−SiO2 compared to that of bare SiO2 indicates thermal degradation of the organic substance on the SiO2 surface. From the TGA results, the mole percent of MPS on SiO2 could be calculated as ∼0.61 mol % (Table S1). The FT-IR spectra of bare SiO2 and MPS−SiO2 are shown in Figure 1b. Here, an absorption peak can be observed at 1706 cm−1 in the MPS−SiO2 spectrum, in contrast with that of bare SiO2. This characteristic absorption band corresponds to CO stretching vibration, which is from MPS present on the SiO2 surface. Therefore, it can be concluded that the SiO2 nanoparticle surfaces were well modified by MPS.

[(W2 − W1)] × 100 W1

where W1 is the weight of the dried sample and W2 is the weight of the sample after humidity exposure. Nanoindentation tests were performed on PTPT and SiO2−PTPT nanocomposite films by Nano Indentation System (Nano Indenter XP: MTS). A holding time at maximum load was 1 s. Nine indentation tests were carried out on each sample, and average values were determined. In a nanoindentation test, the reduced elastic modulus, Er, is calculated as follows: Er =

π dP 2β A dh

where dP/dh is the contact stiffness, S, which is obtained from the slope of the initial unloading. A is the projected area. The indenter geometry parameter β is 1.034 for a Berkovich indenter. The elastic modulus of the sample, Es, is obtained from the reduced elastic modulus

1 − νs 2 1 − νi 2 1 = + Er Es Ei 9435

DOI: 10.1021/acs.iecr.6b01470 Ind. Eng. Chem. Res. 2016, 55, 9433−9439

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Industrial & Engineering Chemistry Research 3.2. Fabrication of SiO2−PTPT Nanocomposites. Because electronic devices can be damaged by organic solvents during solution-based encapsulation processes, a solvent-free encapsulation material is required for effective device protection.36,37 Therefore, we prepared solvent-free SiO2− TPT mixtures by mixing TPT and MPS−SiO2 dispersed in a THF solution for 1 h and then removing the THF solvent. Next, 5 wt % photoinitiator was added to the SiO2−TPT mixture. When appropriate amounts of MPS−SiO2 were mixed in the THF solution with TPT, 5 and 20 wt % SiO2−TPT mixtures were prepared. As shown in the inset of Figure 2, all of the SiO2−TPT mixtures without photoinitiator were transparent, and no precipitation was observed in the solutions even after 1 month in a refrigerator (Figure S2).

Table 1. Glass Transition Temperature (Tg), Stiffness, Modulus, and WVTR of PTPT and SiO2−PTPT Nanocomposites

bare PES PTPT 5 wt % SiO2− PTPT 20 wt % SiO2− PTPT

Tg (°C)

stiffness (N/mm)

modulus (GPa)

WVTR (g/ m2·day)

164.7 166.9

52.7 ± 0.3 57.7 ± 0.2

5.03 ± 0.01 5.53 ± 0.01

364.1 213.3 196.1

167.9

60.8 ± 0.2

5.75 ± 0.02

174.1

It is well-known that nanoparticles incorporated in the polymer matrix can disturb the movement of the polymer chain, resulting in increased Tg.48,49 Because the SiO2 surface was copolymerized with TPT, the thermal mobility of PTPT was restricted by SiO2. Furthermore, the enhanced rigidity of the polymer by silica nanoparticles is expected to help in the prevention of diffusing moisture.50 TGA curves for PTPT and SiO2−PTPT nanocomposites with various SiO2 content values are shown in Figure 3b. All of the nanocomposites exhibited good thermal stability up to 250 °C and then gradually decomposed in the 300−500 °C range. Additionally, the remaining content at 800 °C was proportional to the amount of SiO2 in the PTPT matrix. In order to investigate the mechanical properties of PTPT and SiO2−PTPT nanocomposites, nanoindentation tests were performed. As shown in Figure S4, the similarities of the load versus indenter depth curves indicate that the nanocomposites are based on PTPT matrices. The values of the stiffness (S) extracted from the initial unloading slope of the curves are listed in Table 1. The stiffness increases with the silica content due to the addition of a stiff silica nanoparticle to the PTPT matrix. In addition, the elastic modulus (Es) calculated using the Oliver−Pharr method51 also enhanced with the silica content (Table 1). This result indicates that the mechanical properties of SiO2−PTPT nanocomposites were improved through the reinforcement effect of the silica nanoparticle against mechanical deformation. To evaluate the moisture barrier properties of SiO2−PTPT nanocomposite films, WVTR was measured using a permeation testing instrument at 38 ± 2 °C and 100% RH. PTPT and SiO2−PTPT nanocomposite films were fabricated on a 190μm-thick PES film, and the thickness of the nanocomposite film on PES was ∼30 μm. The WVTR value of bare PES was 361.4 g/m2·day. The WVTRs of the various films as a function of time are shown in Figure S5, indicating that WVTR decreased

Figure 2. TEM images of SiO2−PTPT nanocomposites with (a) 5 and (b) 20 wt % SiO2. The TEM images were obtained by spin-coating the SiO2−TPT mixture onto a TEM grid with subsequent UV curing. Inset: Photographs of SiO2−TPT mixtures.

SiO2−PTPT nanocomposites were fabricated via surfaceinitiated polymerization from MPS−SiO2 under UV irradiation. A schematic illustration of SiO2−PTPT nanocomposite fabrication is shown in Scheme 1. Figure 2 shows TEM images of SiO2−PTPT nanocomposites with 5 and 20 wt % SiO2, respectively. The SiO2 nanoparticles were well-dispersed in the PTPT, and no significant aggregation was found. Although the distance between the particles decreased with increasing SiO2 concentration, the SiO2−PTPT nanocomposite exhibited good dispersibility up to 20 wt % SiO2. TEM images of SiO2−PTPT nanocomposites under low magnification are shown in Figure S3. 3.3. Characterization of SiO2−PTPT Nanocomposites. Figure 3a shows DSC curves of PTPT and SiO2−PTPT nanocomposites. The glass transition temperature (Tg) values obtained from the curves are listed in Table 1. With increased SiO2 content in PTPT, the Tg value of SiO2−PTPT nanocomposites increased slightly compared to that of PTPT.

Figure 3. DSC (a) and TGA (b) curves of PTPT and SiO2−PTPT nanocomposites. 9436

DOI: 10.1021/acs.iecr.6b01470 Ind. Eng. Chem. Res. 2016, 55, 9433−9439

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films were highly transparent, having over 98% transmittance in the visible region (Figure 4a). This result also confirms that the SiO2 nanoparticles were homogeneously dispersed in the PTPT matrix and that light scattering was minimized in the visible region. In addition, the small differences in the refractive indexes of SiO2 and TPT (1.46 and 1.47, respectively) contributed to the high transparency of the nanocomposite films. Figure 4b shows the transmittance of SiO2−PTPT nanocomposite films following exposure at 38 ± 2 °C and 100% RH. No noticeable difference in the transmittance was observed among the samples, with all maintaining transmittance levels of more than 98%. Therefore, the SiO2−PTPT nanocomposite films exhibited good potential with regard to use as encapsulation layers for optical devices. 3.4. SiO2−PTPT Nanocomposites for LED Encapsulation. The above results show that a 20 wt % SiO2−PTPT nanocomposite film exhibits outstanding moisture barrier characteristics while maintaining its high transparency. In order to verify the feasibility of this material as an encapsulation layer, we encapsulated a BLED using the 20 wt % SiO2−PTPT nanocomposite (see the inset in Figure 5a). A stability test was

with increasing SiO2 nanoparticle content in the nanocomposites (Table 1). The WVTR values of PTPT and SiO2−PTPT with 5 and 20 wt % SiO2 were 196.1 and 174.1 g/ m2·day, respectively. We assume that SiO2 nanoparticles in the polymer play two major roles, as shown schematically in Figure S6: tortuous path generation and water molecule absorption. Because the diffusing water molecules travel around the nanoparticles, the SiO2 nanoparticles in the PTPT matrix create a tortuous path for water molecules and extend the permeation time.52 In addition, the nanocomposite permeability can be affected by the nanofiller concentration.31 Moreover, SiO2 effectively absorbs water molecules through the formation of hydrogen bonds between the hydroxyl groups on the SiO2 surface and the water.43,44 Although the SiO2 nanoparticle surfaces were modified with MPS in this study, the O−H stretching (νO−H) apparent in the 3400−3200 cm−1 region in the MPS−SiO2 FT-IR spectrum indicates that hydroxyl groups remained in SiO2 (Figure 1b). Furthermore, a 40% weight change of MPS−SiO2 after humidity exposure indicates that the residual silanol groups on the SiO2 surface confirmed by FT-IR form hydrogen bonds with water molecules. As a result, the moisture barrier property was enhanced through the incorporation of SiO2 nanoparticles in the polymer matrix. The encapsulation layer transmittance is an important characteristic with regard to the application of this layer in optical devices. The optical transmittance performance of the SiO2−PTPT nanocomposite films on the glass substrate was characterized using a UV−vis spectrometer (Figure 4). The transmittance of bare glass was used as a reference. All of the

Figure 5. EL intensity of a BLED with no encapsulation layer, a PTPT-encapsulated BLED, and a BLED encapsulated with a 20 wt % SiO2−PTPT nanocomposite at various exposure times. The temperature and relative humidity of the test were 60 ± 5 °C and 90% RH. The BLEDs were operated at 1 W, and the EL intensities were normalized at 453 nm. Inset: (a) BLED encapsulated with a 20 wt % SiO2−PTPT nanocomposite and (b) a flexible 20 wt % SiO2−PTPT nanocomposite film.

conducted at 60 °C and 90% RH, so as to verify the resistance of the encapsulation layer to high temperature and humidity. Figure 5 shows the normalized EL intensity of the BLED with no encapsulation layer, the PTPT-encapsulated BLED, and the BLED encapsulated with a 20 wt % SiO2−PTPT nanocomposite at 453 nm for various time values. The EL of the BLED with no encapsulation layer gradually decreased and was observed to be quenched after 91 h, while the PTPTencapsulated BLED exhibited a decrease to 11% status of its initial EL intensity. On the other hand, the BLED encapsulated with a 20 wt % SiO2−PTPT nanocomposite maintained over 75% status of its initial EL intensity, despite exposure to harsh conditions. Also, no delamination of the encapsulation film caused by volume expansion was found in the BLED encapsulated with a 20 wt % SiO2−PTPT nanocomposite. The EL spectra of the each BLED are shown in Figure S7. This result indicates that SiO2 nanoparticles in a PTPT matrix effectively protect BLEDs from moisture both by generating tortuous paths and by absorbing water molecules. Even though

Figure 4. Transmittance spectra of PTPT and SiO2−PTPT nanocomposite films with various SiO2 content values on glass substrates (a) before and (b) after aging at 38 ± 2 °C and 100% RH. The transmittance curve of bare glass was used as a baseline. Insets: Digital photographs of (a) PTPT and (b) 5 and (c) 20 wt % SiO2−PTPT nanocomposite films on glass substrates. The film thickness ranged from 8 to 10 μm. The films are indicated by red lines. 9437

DOI: 10.1021/acs.iecr.6b01470 Ind. Eng. Chem. Res. 2016, 55, 9433−9439

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Industrial & Engineering Chemistry Research SiO2−PTPT nanocomposites have relatively higher WVTR values compared with other reports, a 20 wt % SiO2−PTPT nanocomposite film exhibits a high barrier property as an encapsulation layer of BLED at accelerated aging conditions. In addition, the 20 wt % SiO2−PTPT nanocomposites exhibited mechanical flexibility, as shown in the inset of Figure 5b. Therefore, we suggest that the SiO2−PTPT nanocomposite has potential as an encapsulation layer for various flexible electronic devices.

Performance of Organic Photovoltaics with Low-Costing Encapsulation. ChemPhysChem 2015, 16, 1134−1154. (3) Aziz, H.; Popovic, Z.; Xie, S.; Hor, A. M.; Hu, N. X.; Tripp, C.; Xu, G. Humidity-induced crystallization of tris (8-hydroxyquinoline) aluminum layers in organic light-emitting devices. Appl. Phys. Lett. 1998, 72, 756−758. (4) Aziz, H.; Popovic, Z.; Tripp, C. P.; Hu, N. X.; Hor, A. M.; Xu, G. Degradation processes at the cathode/organic interface in organic light emitting devices with Mg: Ag cathodes. Appl. Phys. Lett. 1998, 72, 2642−2644. (5) Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochak, L. S.; Mccarty, D. M.; Thompson, M. E. Reliability and Degradation of Organic Light-Emitting Devices. Appl. Phys. Lett. 1994, 65, 2922− 2924. (6) Kolosov, D.; English, D. S.; Bulovic, V.; Barbara, P. F.; Forrest, S. R.; Thompson, M. E. Direct observation of structural changes in organic light emitting devices during degradation. J. Appl. Phys. 2001, 90, 3242−3247. (7) Do, L. M.; Han, E. M.; Niidome, Y.; Fujihira, M.; Kanno, T.; Yoshida, S.; Maeda, A.; Ikushima, A. J. Observation of Degradation Processes of Al Electrodes in Organic Electroluminescence Devices by Electroluminescence Microscopy, Atomic Farce Microscopy, Scanning Electron-Microscopy, and Anger Electron-Spectroscopy. J. Appl. Phys. 1994, 76, 5118−5121. (8) Schaer, M.; Nuesch, F.; Berner, D.; Leo, W.; Zuppiroli, L. Water vapor and oxygen degradation mechanisms in organic light emitting diodes. Adv. Funct. Mater. 2001, 11, 116−121. (9) Czerw, R.; Carroll, D. L.; Woo, H. S.; Kim, Y. B.; Park, J. W. Nanoscale observation of failures in organic light-emitting diodes. J. Appl. Phys. 2004, 96, 641−644. (10) Jeong, J. A.; Kim, H. K. Al2O3/Ag/Al2O3 multilayer thin film passivation prepared by plasma damage-free linear facing target sputtering for organic light emitting diodes. Thin Solid Films 2013, 547, 63−67. (11) Choi, D. W.; Kim, S. J.; Lee, J. H.; Chung, K. B.; Park, J. S. A study of thin film encapsulation on polymer substrate using low temperature hybrid ZnO/Al2O3 layers atomic layer deposition. Curr. Appl. Phys. 2012, 12, S19−S23. (12) Park, J. S.; Chae, H.; Chung, H. K.; Lee, S. I. Thin film encapsulation for flexible AM-OLED: a review. Semicond. Sci. Technol. 2011, 26, 034001. (13) Fahlteich, J.; Fahland, M.; Schonberger, W.; Schiller, N. Permeation barrier properties of thin oxide films on flexible polymer substrates. Thin Solid Films 2009, 517, 3075−3080. (14) Granstrom, J.; Swensen, J. S.; Moon, J. S.; Rowell, G.; Yuen, J.; Heeger, A. J. Encapsulation of organic light-emitting devices using a perfluorinated polymer. Appl. Phys. Lett. 2008, 93, 193304. (15) Liu, Y. F.; Feng, J.; Zhang, Y. F.; Cui, H. F.; Yin, D.; Bi, Y. G.; Song, J. F.; Chen, Q. D.; Sun, H. B. Polymer encapsulation of flexible top-emitting organic light-emitting devices with improved light extraction by integrating a microstructure. Org. Electron. 2014, 15, 2661−2666. (16) Mandlik, P.; Gartside, J.; Han, L.; Cheng, I. C.; Wagner, S.; Silvernail, J. A.; Ma, R. Q.; Hack, M.; Brown, J. J. A single-layer permeation barrier for organic light-emitting displays. Appl. Phys. Lett. 2008, 92, 103309. (17) Lin, M. C.; Tseng, C. H.; Chang, L. S.; Wuu, D. S. Characterization of the silicon oxide thin films deposited on polyethylene terephthalate substrates by radio frequency reactive magnetron sputtering. Thin Solid Films 2007, 515, 4596−4602. (18) Charton, C.; Schiller, N.; Fahland, M.; Hollander, A.; Wedel, A.; Noller, K. Development of high barrier films on flexible polymer substrates. Thin Solid Films 2006, 502, 99−103. (19) Wuu, D. S.; Lo, W. C.; Chiang, C. C.; Lin, H. B.; Chang, L. S.; Horng, R. H.; Huang, C. L.; Gao, Y. J. Water and oxygen permeation of silicon nitride films prepared by plasma-enhanced chemical vapor deposition. Surf. Coat. Technol. 2005, 198, 114−117. (20) Jamieson, E. H. H.; Windle, A. H. Structure and Oxygen-Barrier Properties of Metallized Polymer Film. J. Mater. Sci. 1983, 18, 64−80.

4. CONCLUSIONS A solvent-free and UV-curable SiO2−PTPT nanocomposite as an encapsulation layer was successfully fabricated through surface modification of SiO2 nanoparticles. The SiO2 nanoparticle surfaces were modified with MPS to yield surfaceinitiated polymerization with TPT. The modified SiO 2 nanoparticles were well-dispersed in a PTPT matrix up to 20 wt % SiO2 content. The SiO2−PTPT nanocomposites have higher Tg, stiffness, and elastic modulus values compared to PTPT. Also, the WVTR values decreased with increasing SiO2 concentration, probably because of the two effects of silica nanoparticles: generating a tortuous path and absorbing incoming water molecules. All of the SiO2−PTPT nanocomposite films exhibited high transparency of more than 98% before and after a humidity aging test. Moreover, the 20 wt % SiO2−PTPT nanocomposite encapsulation layer exhibited excellent barrier characteristics on a BLED. We believe that the SiO2−PTPT nanocomposite has considerable potential for use as an encapsulation layer in various flexible electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01470. TEM images of SiO2, MPS−SiO2, and SiO2−PTPT nanocomposites, nanoindentation data, the WVTR values of PTPT and SiO2−PTPT nanocomposites, and EL spectra of the BLEDs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Center for Advanced Soft Electronics as Global Frontier Research Program (Grant 2013M3A6A5073177) of the Ministry of Science, ICT and Future Planning of Korea, and the LG Display Academic Industrial Cooperation Program. The authors appreciate the availability of facilities of the Materials Chemistry Laboratory at Seoul National University.

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