PMMA Nanocomposite Films for Display

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Transparent BaTiO/PMMA Nanocomposite Films for Display Technologies: Facile Surface Modification Approach for BaTiO Nanoparticles 3

Koichi Suematsu, Masashi Arimura, Naoyuki Uchiyama, and Shingo Saita ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00650 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Applied Nano Materials

Transparent BaTiO3/PMMA Nanocomposite Films for Display Technologies: Facile Surface Modification Approach for BaTiO3 Nanoparticles

Koichi Suematsu*1, Masashi Arimura, Naoyuki Uchiyama, Shingo Saita Chemical and Textile Research Institute, Fukuoka Industrial Technology Center, Fukuoka 818-8540, Japan

Keywords: BaTiO3, PMMA, composite film, transparent, dielectric, silane coupling treatment

1

Present address: Department of Advanced Materials Science and Engineering, Faculty of

Engineering Sciences, Kyushu University, Kasuga, Fukuoka, 816-8580, Japan. 1 Environment ACS Paragon Plus

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Fabrication of a transparent film composed of a barium titanate (BaTiO3) and poly(methyl methacrylate) (PMMA) matrix is reported to expand the application field for composite films such as displays and touch panel screens. BaTiO3 nanoparticles are synthesized by sol-gel route with dispersion carried out in 2-methoxyethanol. The synthesized nanoparticles are 10 nm in size and are highly dispersed in the solvent. The surfaces of the obtained nanoparticles are modified by treatment with titanium isopropoxide and by using two silane coupling agents; n-decyltrimethoxysilane and 3-(triethoxysilyl)propyl methacrylate. The surface-modified BaTiO3 nanoparticles were then added to a sample of methyl methacrylate to obtain a transparent BaTiO3 dispersion. Transparent BaTiO3/PMMA nanocomposite films (sheet type, thickness: 150 µm) are obtained by the polymerization of the BaTiO3/MMA dispersion via heat treatment and by using a polymerization initiator. The visual transparency of the BaTiO3/PMMA film is comparable to that of the original PMMA film. Additionally, no difference in the transparency of the BaTiO3/PMMA films is observed when the BaTiO3 weight ratio is varied between 5 and 33%. The dielectric constant of the nanocomposite film improved from 4.6 (PMMA only) to 7.1 by incorporating 10 wt% of BaTiO3. Such improvement in the dielectric properties, whilst maintaining the transparency, flexibility, and workability of PMMA films, allows the expansion of the fields of functional ceramics and polymers.

2 Environment ACS Paragon Plus

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INTRODUCTION Inorganic/organic composites have been widely investigated as alternatives to inorganic materials, such as metal oxides, for electronics devices.

1-6

Integrating both inorganic and organic

material components allows the fabrication of electronic devices with superior hybrid properties. In particular, suspending functional ceramics in a polymer matrix transforms brittle and hard ceramics into highly flexible and workable materials. Barium titanate (BaTiO3) is a popular functional material for dielectric and piezoelectric devices 7,8, which has a high dielectric constant 2,9. Polymer matrices such as poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) have been extensively investigated as parent phases for inorganic/organic composite devices because of their desirable shape-controllability, flexibility, tensibility, and mechanical strength, and ease of processing 2,10-13

. PMMA is further desirable on account of its high transparency

11-13

. However, the

incorporation of BaTiO3 into the PMMA polymer matrix is reported to remove the transparency of PMMA films. If a transparent BaTiO3 and PMMA composite film can be synthesized, the potential application field for composite films can be expanded to include displays and touch panel screens. 13,14

It is common for BaTiO3 particles to aggregate and make larger particles within solvent or polymer matrix. Larger nanoparticles reduce the dielectric and optical properties on the BaTiO3/polymer composite films. Attempts to suppress the aggregation of BaTiO3 particles are the surface modification including the use of various additives, 3,10,15-21 especially, silane coupling agents



10,15

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and surfactant agents 3,16,17 have been well used as the additives. Dispersion of BaTiO3 particles,

in both solvent and a polymer matrix, improves the dielectric properties of the final material. 10,16,18 High-temperature sintering is required for efficient crystallization of BaTiO3 nanoparticles during synthesis, which is the mechanical driving force for aggregation and creation of large nanoparticle sizes. Even with ideal dispersion of BaTiO3 into the polymer matrix, the resultant composite film is white and opaque. Recently, researchers have tried to produce small BaTiO3 nanoparticles using liquid phase preparation processes, such as hydrothermal method solvothermal method

24,25

, sol-gel method

26-28

, and precipitation method

22,23

,

29

. A few of the stated

methods produce small BaTiO3 nanoparticles, which can be highly dispersed in a host solvent. According to a previously reported work, highly transparent 10 nm BaTiO3 nanoparticles dispersion can be obtained with liquid phase preparation processes.

24,25,29

It implies that if BaTiO3

nanoparticles were highly dispersed in the polymer matrix, the fabricated BaTiO3/polymer composite films could become transparent. Recently, using the sol-gel method, we prepared BaTiO3 nanoparticles that could be highly dispersed in solvent without the need for surface modification. We reported that the particle size could be controlled between 15 and 60 nm.

27,28

In this work, a BaTiO3 nanoparticle in 10 nm size

dispersion was obtained by optimizing the experimental conditions for crystallization. The surfaces of the nanoparticles were then modified using silane-coupling agents, which were then dispersed into methylmethacrylate (MMA) using a sol-gel approach to form a transparent BaTiO3/PMMA


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nanocomposite film. Notably, transparent BaTiO3/PMMA nanocomposite sheet type films were obtained by surface modification of BaTiO3 nanoparticles. Here, n-decyltrimethoxysilane (DTMS; C13H30O3Si) and 3-(triethoxysilyl)propyl methacrylate (MPTES; C13H26O5Si) were used as silane coupling agents for the surface modification of the BaTiO3 nanoparticles; DTMS is commonly used to improve the dispersity of inorganic particles in organic solvents because of its hydrophobic 30

character MMA

, while MPTES facilitates an improved connection between the inorganic particles and

31

. Additionally, the surfaces of the BaTiO3 nanoparticles were treated using titanium

isopropoxide (TIIP; Ti(OCH(CH3)2)4), before the surface modification with the silane coupling agents in order to improve their effectiveness. The dielectric constant of the composite film was improved whilst maintaining the transparency of the film. Development of such BaTiO3/PMMA nanocomposite films is promising for the functional use of dielectric films. To our knowledge this is the first report on a transparent nanocomposite film of BaTiO3/PMMA fabricated through nanoparticle surface modification.

EXPERIMENTAL Preparation of BaTiO3 nanoparticles dispersion The BaTiO3 nanoparticle dispersion was prepared using a sol-gel method based on previous literature.

28,32

A flowchart detailing the synthesis route of the BaTiO3 nanoparticles dispersion is

shown in Figure 1. Barium ethoxide (Ba(OCH2CH3)2; Kojundo Chemical Laboratory Co., Ltd.) and



TIIP (Kojundo Chemical Laboratory Co., Ltd.) were dissolved into a solution of 40% methanol (MeOH, CH3OH, Wako Pure Chemical Industries, Ltd.) and 60% 2-methoxyethanol (EGMME, CH3O(CH2)2OH; Wako Pure Chemical Industries, Ltd.) under a dry N2 atmosphere. The amount of Ba(OCH2CH3)2 and TIIP were set at 1 mol∙L-1, respectively. The solution was cooled to -30 °C under N2. The mixed solution of H2O and MeOH in a 1:1 volumetric ratio was then added to the solution. The volume of the H2O/MeOH solution was tuned so that the molar ratio of H2O/Ti is 10. The solution was heated to 60 °C at a rate of 2 °C∙min-1 to obtain a monolithic gel. The gel was continuously heated at 60 °C for 5 days in a N2 atmosphere to crystallize the BaTiO3 crystals. The gel was then dispersed in EGMME using ultrasonication at 50 kHz and 10 °C for 2 h. Impurities in the dispersion, such as H2O and MeOH, were removed using a rotary evaporation at 35 °C for 10 min under vacuum conditions. The concentration of BaTiO3 in the nanoparticle dispersion was tuned to 10 wt%. Surface modification of BaTiO3 nanoparticles and preparation of BaTiO3/PMMA nanocomposite films The BaTiO3/PMMA nanocomposite films were prepared by modifying the surfaces of the BaTiO3 nanoparticles. A schematic of the surface modification is shown in Figure 2. To prepare the composite films, an appropriate amount of TIIP was added into 2 g of the BaTiO3 dispersion, which formed a TIIP layer on the surface of the BaTiO3 nanoparticles. The molar ratio of TIIP and BaTiO3 was set at 30 mol%, which was then stirred at 70 °C for 2 h. Subsequently, 0.04 mL HCl solution


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(0.5 mol∙L-1) and 0.04 mL CH3COOH were stirred into the solution to control the pH at room temperature. The silane coupling agents, DTMS (Shin-Etsu Chemical Co., Ltd.) and MPTES (Shin-Etsu Chemical Co., Ltd.), were added into the dispersion at a concentration of 1.28 mmol∙g-1 to BaTiO3 nanoparticles, at room temperature. The final dispersion was collected using centrifugation after stirring the mixed solution of BaTiO3 nanoparticles and silane coupling agents for 2 h at 70 °C. The appropriate concentration of methyl methacrylate (monomer, MMA; Wako Pure Chemical Industries, Ltd.) was mixed into the dry nanoparticle precipitate. This was then dispersed in the MMA using ultrasonication. The BaTiO3 nanoparticles in MMA were set at 5, 10, 20, and 33wt%. A polymerization initiator (0.02 g), dilauroyl peroxide (C24H46O4, Peroyl L; NOF Corporation), was added into the BaTiO3/MMA dispersion. This was then poured into a polytetrafluoroethylene (PTFE)-sealed template with a thickness of 0.2 mm. The BaTiO3/MMA dispersion was left in the template at 70 °C for 12 h. The temperature was then increased to 100 °C for 2 h under a N2 atmosphere to polymerize the MMA, resulting in the fabrication of the BaTiO3/PMMA nanocomposite films. Schematic images of the BaTiO3/MMA dispersion and BaTiO3/PMMA film are described in Figure 2. A handmade PMMA film was also synthesized using the PTFE-sealed template and the same heat treatment procedure by mixing 2 mL of MMA with 0.02 g of Peroyl L. Evaluation of nanoparticles and films The crystal structure of the obtained nanoparticles was analyzed using X-ray diffraction



using CuKα radiation (XRD; Empyrean, PANalytical), and the crystallite size was estimated by using the Scherrer equation. The colloidal particle size distribution of BaTiO3 dispersion was analyzed by using dynamic light scattering (DLS) analysis with a DLS spectrometer (DLS; Zetasizer Nano-ZS, Malvern). The surfaces of the modified nanoparticles were imaged using transmission electron microscopy (TEM; JEM-ARM200F, JEOL Ltd.), and the elemental analysis was carried out using EDS spectroscopy. Surface modification of the BaTiO3 nanoparticles was confirmed by Fourier-transform infrared spectroscopy (FT-IR; NICOLET 6700, Thermo Scientific), additionally, the obtained films were also confirmed using FT-IR measurement. UV-visible spectra were measured using UV-vis spectroscopy (V-550; JASCO Corporation) at wavelengths between 200 and 800 nm under an ambient atmosphere. The dielectric constant and dielectric loss of the films were measured with an impedance analyzer (4192A; Agilent Technologies) in the frequency range between 10 to 104 kHz at room temperature. A gold electrode (φ = 1 mm) was fabricated for the analysis using an ion-sputtering device (JEC-550; JEOL Ltd.).

RESULTS AND DISCUSSION BaTiO3 nanoparticle dispersion The initial BaTiO3 nanoparticles dispersion is pictured in Figure 3. The dispersion shows high transparency with a visible white haze and had good thermodynamic stability; no sedimentation of the BaTiO3 nanoparticles was observed after several months. The distribution of particles in the


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dispersion is shown in Figure 3b. The BaTiO3 nanoparticles were dispersed uniformly throughout the surrounding solution, and no aggregation of the nanoparticles was observed. The average colloidal particle diameter was approximately 9.3 nm, which is comparable to the results reported in recent literature 24,25. The XRD pattern of the dried BaTiO3 nanoparticles powder of the dispersion is shown in Figure 3c. The obtained peaks match the cubic perovskite-type BaTiO3 crystal structure (ICSD ID: 28851), with no impurity peaks. Essentially, BaTiO3 primarily has the tetragonal phase perovskite structure at room temperature. However, Han et al. proposed that the fraction of tetragonal BaTiO3 decreases with particle size, and that the cubic phase structure dominates for nanoparticle sizes less than 12 nm in diameter at room temperature.

33

The estimated average crystallite size using XRD

patterns at (011) peak was 9.6 nm, and thereby the obtained XRD patterns show the cubic crystal structure. The TEM image indicates that the particles size of BaTiO3 is approximately 10 nm as shown in Figure 3d. The average measured sizes of the BaTiO3 nanoparticles using DLS, XRD, and TEM were also in the same size range. Hence in summary, using the sol-gel method, we successfully obtained crystallized BaTiO3 nanoparticles that were 10 nm in size and highly dispersed in the EGMME solution.

Surface modification of BaTiO3 nanoparticles and evaluation of nanocomposite films Images of the BaTiO3 nanoparticles in the MMA solvent (10 wt% BaTiO3) with and without surface modification are shown in Figure 4. Figure 4a shows unmodified BaTiO3 nanoparticles in



MMA solution. The dispersion is opaque and milky white due to the aggregation of BaTiO3 nanoparticles. The surface of the BaTiO3 nanoparticles were modified by two different approaches. In the first approach, silane coupling agents (DTMS and MPTES) were used without TIIP. In the second approach, the modification was achieved using silane coupling agents after treatment with TIIP. The BaTiO3/MMA dispersions produced by the first and second modification approaches are shown in Figures 4b and c, respectively. The dispersion using surface-modified BaTiO3 nanoparticles without TIIP is white and opaque, but the addition of TIIP is shown to improve the transparency of the solution. Hence, surface modification using silane coupling agents with TIIP treatment is an effective approach to disperse the BaTiO3 nanoparticles in MMA. A HAADF-STEM image of the surface-modified BaTiO3 nanoparticles using silane coupling agents with TIIP treatment is shown in Figure 5a. The particle size of the BaTiO3 nanoparticles were observed not to be significantly influenced by the surface modification procedure. A cloudy layer is formed on the surface of the BaTiO3 nanoparticles with this approach, which we attribute to the surface of these nanoparticles being modified. EDS spectra were measured at two different locations in the samples: (i) at the point between the particle and the surface layer (ii) just at the surface layer (Figure 5b). The peaks in both spectra are attributed to the carbon and copper of the micro-grid used to capture the nanoparticles. The EDS spectra (i) and (ii) show peaks for carbon (C), oxygen (O), silicon (Si), titanium (Ti), and barium (Ba). No other impurity elements were observed. The relative intensity of Si to Ti and Ba in the spectrum in region (ii) are stronger than those in the spectrum taken in region (i) of the sample.


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This indicates that the ratio of the silane-coupling agent to the BaTiO3 nanoparticles in region (ii) is larger than that in region (i). The EDS spectra also show that the surface layer of the depicted samples is mainly formed of Si and Ti from the silane coupling agent and TIIP. Thus, we have shown that surfaces of the BaTiO3 nanoparticles can be successfully modified by the combination of TIIP and silane-coupling agents. This result indicates that an amorphous TIIP layer, formed by TIIP, promotes the interaction with silane coupling agents. Generally, silane coupling agents work best under weakly acidic conditions. However, the Ba2+ sites in the BaTiO3 nanoparticles are strongly basic and, thus, cannot facilitate good connections between the nanoparticles and silane coupling agents. This implies that the TIIP layer must cover the BaTiO3 nanoparticle, including the Ba2+ site, thus modifying the surfaces of the BaTiO3 nanoparticles such that improved connections between the nanoparticles and silane coupling agents are obtained. However, further understanding and investigation of how TIIP supports the silane coupling is necessary in future work. The FT-IR spectra of the dried BaTiO3/EGMME and surface-modified BaTiO3/MMA powders are shown in Fig. 6. Each powder was obtained by drying at 120 °C. First, the peaks of the hydroxyl groups, which appear between 3000 and 3600 cm-1 in the spectrum of the unmodified BaTiO3 nanoparticles, decreased in intensities upon surface modification due to the change in the solvent from EGMME to MMA. The characteristic IR peaks of the BaTiO3 nanoparticles include the carbonate adsorbate at ~1440 cm-1 34, O-H deformation of the alcohols around 1350 cm-1, and C-O stretching vibration of the alcohols around 1100 cm-1

35

. Most significant is the peak at 1450 cm-1,



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which corresponds to the formation of –CO32- on the BaTiO3 surface. In contrast, in the case of the surface-modified BaTiO3 nanoparticles, various peaks attributed to the organic content and Si-O bonds are observed. The peaks of the typical C-H symmetric and asymmetric stretching vibrations (-CH3 and -CH2) appeared between 2800 and 3000 cm-1. These were associated with the silane coupling agents, DTMS

30

and MPTES

31

. Additionally, the peaks at 1670-1730 cm-1, 1600-1440

cm-1, 1400-1300 cm-1, and 1200-1000 cm-1 were attributed to the C=O axial deformation

31

,

carbonate adsorbate 34,36, CH2 and CH3 bending or scissoring vibration 31,37, and Si-O-Si cross-linked Si-O stretching vibration

30,31,37

, respectively. Additionally, a small peak attributed to the Si-O-Ti

stretching vibration is observed at 935 cm-1 38. These results in combination with the HAADF-STEM image and EDS analysis indicate that the surfaces of the BaTiO3 nanoparticles were successfully covered by TIIP and the silane coupling agents. Images of the PMMA, BaTiO3 (5 wt%)/PMMA and BaTiO3 (10 wt%)/PMMA films are shown in Figures 7a-c, respectively. The thickness of these films was approximately 150 µm. These images show that the films visually appear transparent and the logo “FITC” can be seen through the films regardless of the composition of BaTiO3. Additionally, the high flexibility of the depicted films was also consistent, as demonstrated in the inset picture in Figure 7c. The TEM image of the composite film, BaTiO3(10 wt%)/PMMA, is shown in Figure 7d. Here, black spots in the image show BaTiO3 nanoparticles. This image shows the BaTiO3 nanoparticles to be successfully dispersed in the films and gives no evidence of nanoparticle aggregation. This TEM image is consistent with


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the schematic image of the BaTiO3/PMMA film presented in Figure 2. This indicates that the silane coupling agents on the BaTiO3 surface promote the dispersion of the BaTiO3 nanoparticles in the PMMA film; DTMS improves the dispersity of the nanoparticles in the organic solvent, and MPTES facilitates the interaction between the BaTiO3 surface and the MMA monomer

30,31

. Thus, we have

successfully obtained transparent and flexible BaTIO3/PMMA composite films by dispersing the BaTiO3 nanoparticles throughout the PMMA film. The FT-IR spectra of the PMMA and BaTiO3 (10 wt.%)/PMMA films are shown in Fig. 8. The characteristic peaks obtained for the PMMA film, such as the carbonyl adsorption at 1726 cm-1 and the C-O stretching vibration at 1483, 1435, and 1387 cm-1, are consistent with those typically reported in literature

39

. These peaks are also observed in the BaTiO3 (10 wt.%)/PMMA film,

indicating that the MMA solvent was successfully polymerized regardless of whether the BaTiO3 nanoparticles were composited or not. The XRD patterns of the BaTiO3/PMMA nanocomposite films are shown in Figure 9a. The XRD patterns of the handmade PMMA film is also shown in Figure 9a. PMMA is amorphous,

40

thus, its XRD pattern shows a broad with peak between 13° and 29°. This

pattern corresponds with those previously reported in literature.

41

The XRD patterns of the

BaTiO3/PMMA nanocomposite films show peaks of PMMA and BaTiO3, respectively. The BaTiO3 peaks show a similar pattern to those of the cubic perovskite BaTiO3 crystal structures (ICSD No.: 28851) as shown in Figure 3c. The crystal sizes, which were estimated using the Sherrer equation, were approximately 10 nm, regardless of the films composition. This observation indicates that the



crystal structure of the BaTiO3 nanoparticles is not influenced by the surface modification process or by the polymerization of MMA. The peaks that would be attributed to the TIIP and silane-coupling agents were not observed in the measured XRD patterns. The absence of these peaks indicates that the TIIP and silane-coupling agents are present in films in amorphous phases. The concentration of BaTiO3 nanoparticles in the PMMA film could be increased with no significant change observed in the visual appearance of the sample, as shown in Figures 7 a-c. The UV-vis spectra and total transmittance of the handmade PMMA and BaTiO3/PMMA nanocomposite films are shown in Figure 7b. Here, the reference spectrum was carried out in air. It is common for pure PMMA films to show high transmittance of approximately 93% in visible light region (around 400–800 nm).

12

However, the transmittance of handmade PMMA sample was

measured to be 75 % at 800 nm, and this further decreases with decreasing the wavelength. This reduction in transmittance was caused by our adopted procedure carried out to polymerize the MMA solution. Polymerization temperature, pressure, atmosphere, and surface roughness of the PTFE template are not optimal to obtain the highly transparent PMMA film. The BaTiO3/PMMA nanocomposite films show the similar behavior between 800 and 400 nm. The transmittance of the nanocomposite films is almost 70% at 800 nm. The measured transmittance decreases with concentration of BaTiO3 nanoparticles. The transmittance of the BaTiO3 (5 wt.%)/PMMA film at 400 nm was measured to be 47.5 %, which decreased to 40.5% for the BaTiO3 (33 wt.%)/PMMA film. The observed difference in sample transmittance is caused by the ratio of the BaTiO3 nanoparticles.


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The adsorption of the PMMA film occurs at 243 nm, and this is in agreement with values that have been previously reported.

12

In contrast, the adsorption onsets of the BaTiO3/PMMA nanocomposite

films is increased with increasing concentration of the BaTiO3 nanoparticles from at 321 nm for BaTiO3 (5 wt.%)/PMMA film to 344 nm for BaTiO3 (33 wt.%)/PMMA film. Such increase observed for the adsorption onset is evidence for the presence of BaTiO3 nanoparticles in the sample, and the range of the adsorption onsets using BaTiO3/PMMA nanocomposite films are less than the visible light region. Additionally, no strong adsorption is observed using the nanocomposite films in the visible light range. If we optimize the procedure taken to obtain the PMMA film, we can produce a nanocomposite film with increased transparency. Nevertheless, we have successfully obtained transparent BaTiO3/PMMA nanocomposite films that have the optical properties of PMMA films. Finally, the dielectric properties, including the dielectric constant and dielectric loss, of BaTiO3/PMMA nanocomposite films were measured. Figure 10a and b show the frequency dependence of the dielectric constant and dielectric loss of PMMA and BaTiO3/PMMA nanocomposite films, respectively. The dielectric constant of the handmade PMMA film is approximately 4.5 at 10 kHz. Due to the high dielectric constant of BaTiO3, increasing the concentration of BaTiO3 nanoparticles in the PMMA films enhances the dielectric constant of the composite films. The dielectric constant of the handmade PMMA film at 104 kHz is approximately 91% lower than that of the BaTiO3 (10 wt.%)/PMMA nanocomposite film, which can be further reduced to 89% at 10 kHz. The change in the dielectric constant with frequency is not significantly



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influenced by the concentration of BaTiO3 nanoparticles. Hence, the dielectric constants of the fabricated nanocomposite films are remarkably consistent with frequency variation between 10 and 104 kHz. In general, the influence of the frequency change on the dielectric constant depends on the type of polymer and dipole polarization of the polymer matrix.

16

PMMA films are non-polar; thus,

the electronic properties of PMMA strongly affect the dielectric constant of the nanocomposite films. The dielectric losses of the handmade PMMA film and nanocomposite films, as a function of the frequency, are shown in the Figure 10b. The dielectric losses of each film are approximately consistent, regardless of the concentration of BaTiO3, and decrease with increasing frequency. The measured dielectric loss of BaTiO3/PMMA nanocomposite films is comparable to the dielectric loss reported in previous literature.

17

It is common for BaTiO3 ceramics to show lower dielectric losses

than the results obtained for these films in the same frequency range. 17,42 Thus, the dielectric loss of these films is caused by the electronic and structural properties of the PMMA film. The dielectric constant and dielectric loss at 10 kHz as a function of the BaTiO3 fraction are shown in Figure 10c. The dielectric constant is shown to increase when the BaTiO3 fractional concentration is increased from 0 to 10 wt.%. This trend is caused by the high dielectric constant of BaTiO3 nanoparticles. However, the dielectric constant is also shown to remain consistent when the BaTiO3 fractional concentration is increased from 10 to 33 wt%. According to the theoretical calculation using the Lichtenecker model, increasing the BaTiO3 fractional concentration leads to enhance dielectric constants. 28,43 In contrast to our reported results in previous paper, we proposed that the aggregation


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and random dispersion of the BaTiO3 particles in thin films reduces the dielectric constant.

44

Thereby, if the BaTiO3 nanoparticles aggregated during MMA polymerization, the dielectric constant of the fabricated nanocomposite films should be decreased. According to the results presented in this paper, the measured UV-vis spectra, transmittance, and dielectric properties of the nanocomposite films that contain over 10 wt.% of BaTiO3 are similar. These results therefore indicate that large aggregated particles are not formed in these films. However, an ordering or uniformity of the BaTiO3 nanoparticles, which were embedded into the polymer matrix, was not clearly observed. Increasing the BaTiO3 concentration in the nanocomposite films increases the surface area of the interfaces between the BaTiO3 nanoparticles and PMMA. Thus, the constant value of the dielectric constant on the BaTiO3 fraction over 10 wt.% is likely to be caused by a random distribution of the nanoparticles throughout the PMMA, suggesting that above this concentration threshold, the dispersion is no longer uniform. However, no experimental evidence to support this theory has been obtained as yet. Nevertheless, the dielectric constant of the transparent PMMA-based films was successfully improved by incorporating BaTiO3 nanoparticles that were less than 10 nm in size. Finally, no significant change in the dielectric loss of the nanocomposite film is observed by changing the concentration of BaTiO3 nanoparticles.

CONCLUSIONS BaTiO3 nanoparticles, which were measured to be 10 nm in size, were synthesized and



dispersed in EGMME solvent using the sol-gel approach. Transparent BaTiO3/PMMA nanocomposite sheet type films were then fabricated via surface modification of BaTiO3 nanoparticles. The average size of the BaTiO3 nanoparticles was measured to be 9.3 nm, and the particle size distribution was observed to be narrow. XRD patterns indicate that the BaTiO3 nanoparticles have a cubic perovskite-type crystal structure with an average crystallite size of 9.6 nm. TEM images further confirmed the measured size of the BaTiO3 nanoparticles. The surfaces of the BaTiO3 nanoparticles were successfully modified by silane coupling agents (DTMS and MPTES) after being treated with TIIP. These surface-modified BaTiO3 nanoparticles were then dispersed in MMA solution. In the FT-IR spectra, the appearance of the Si-O and Si-O-Ti peaks indicated that the surfaces of the BaTiO3 nanoparticles were modified by the silane coupling agents. The BaTiO3/PMMA nanocomposite sheet type films were prepared via polymerization of the BaTiO3/MMA dispersion with a varying weight ratio of BaTiO3 of 5, 10, 20 and 33%. The visual transparency of the fabricated BaTiO3/PMMA films was comparable to that of the handmade PMMA film. Presence of BaTiO3 nanoparticles in PMMA film was confirmed by XRD patterns. UV-vis spectra confirm that the integration of the BaTiO3 nanoparticles into the PMMA polymer matrix leads to a red shift on the onset wavelength of the total transparency. The dielectric constants of the BaTiO3/PMMA nanocomposite films were stable with frequency changes between 10 and 104 kHz at room temperature. The dielectric constant of the nanocomposite films increased when the BaTiO3 nanoparticle concentration is increased from 0 to 10 wt.%, but remained constant for films with 10–


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33 wt.% of BaTiO3. In contrast, no change in dielectric loss was observed for all films, suggesting that this property is independent of the BaTiO3 concentration. We report that the 10 wt% BaTiO3/PMMA nanocomposite film exhibits the highly dielectric constant with the highest measured and visual transparency. In summary, we have reported successful fabrication of BaTiO3/PMMA nanocomposite films and were successful in maintaining the functional properties of PMMA, such as transparency, flexibility. The dielectric constant could be further improved by using approaches, such as elemental doping of the BaTiO3 nanoparticles. We have shown the initial development stages of BaTiO3/PMMA nanocomposite films in this work, thus paving the way for the expansion of the current application field for transparent and dielectric nanocomposite films.

ACKNOWLEDGMENTS This work was supported by the Foundation for Interaction in Science & Technology, Japan and by the Murata Science Foundation. TEM observation was helped by the Advanced Characterization Platform of the Nanotechnology Platform Japan, which is sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. K.S. was partially supported by the Kyushu University Platform of Inter / Transdisciplinary Energy Research.

AUTHOR INFORMATION



Corresponding author *Koichi Suematsu E-mail: [email protected] Fax: +81-92-583-7538 Notes The authors declare no competing financial interest.


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Figures

Figure 1. Flow diagram detailing the procedure for the synthesis of BaTiO3 nanoparticle dispersion.

Figure 2. Schematic images of BaTiO3 nanoparticle surface modification, BaTiO3/MMA dispersion, and BaTiO3/PMMA film.



Figure 3. Properties of BaTiO3 nanoparticle dispersion. (a) Photographic image with red laser light highlighting the dispersion, (b) nanoparticle size distribution, (c) XRD pattern of the dried nanoparticles, and (d) TEM image of the nanoparticles.

Figure 4. Photographs of BaTiO3 nanoparticle: (a) without surface modification, (b) modified using only silane coupling agents, and (c) modified using titanium isopropoxide and silane coupling agents in the MMA solution.


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Figure 5. (a) A HAADF-STEM image of surface-modified BaTiO3 nanoparticles and (b) EDS spectra at two different regions of the nanoparticle.

Figure 6. FT-IR spectra of as-synthesized BaTiO3 nanoparticles and surface-modified BaTiO3 nanoparticles.



Figure 7. Photographs of (a) a handmade PMMA film, (b) BaTiO3 (5 wt.%)/PMMA and (c) BaTiO3 (10 wt.%)/PMMA nanocomposite films. (d) TEM image of the BaTiO3 (10 wt.%)/PMMA nanocomposite film.

Figure 8. FT-IR spectra of a handmade PMMA film and BaTiO3 (10 wt.%)/PMMA nanocomposite films.


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Figure 9. (a) XRD patterns of the nanocomposite films, and (b) UV-vis spectra of the handmade PMMA film and BaTiO3/PMMA nanocomposite films using PMMA (black line), BaTiO3 (5 wt.%)/PMMA (red line), BaTiO3 (10 wt.%)/PMMA (blue line), BaTiO3 (20 wt.%)/PMMA (green line), BaTiO3 (33 wt.%)/PMMA (purple line).



Figure 10. Frequency dependence of (a) the dielectric constant and (b) the dielectric loss using handmade PMMA film (black line), BaTiO3 (5 wt.%)/PMMA (red line), BaTiO3 (10 wt.%)/PMMA (blue line), BaTiO3 (20 wt.%)/PMMA (green line), and BaTiO3 (30 wt.%)/PMMA (purple line) nanocomposite films. (c) BaTiO3 concentration dependence of the dielectric constant and dielectric loss at 10 kHz.


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Graphical abstract

A transparent BaTiO3/PMMA nanocomposite film is successfully prepared using Ti(OCH(CH3)2)4 and a silane coupling treatment procedure to modify the surfaces of 10 nm BaTiO3 nanoparticles.


PMMA Nanocomposite Films for Display

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