1. Introduction

Thin-film optics deals with structures composed of nanoscale layers of different materials that are on the order of the wavelengths of visible light [1]. Layers at this scale can have extraordinary reflective properties because of the light wave interference originating from the differences between the refractive indices of the layers, air, and substrate, which changes how light is reflected and transmitted. In recent years, thin-film optics has been applied to contact lenses, waveguides, nonlinear optics, photochromics, optoelectronic devices, etc [2]. For these applications, it is necessary to use index-matching techniques, such as alternating layers of high and low refractive index materials on a substrate [3]. Currently, inorganic materials, such as Nb₂O₅, SiO₂, ZrO₂, TiO₂, and Al₂O₃, are sputter-coated onto a substrate to fabricate the refractive index-matched layers [4]. However, there are several disadvantages in using inorganic materials and sputter coating. For example, these inorganic materials are not suitable for flexible devices because of their brittleness, and sputter coating damages the substrate because of the high-voltage plasma generated during the sputtering process [5]. To overcome these problems, spin-coatable organic-inorganic hybrid materials have been used to create high and low refractive index materials for refractive index-matched thin-film applications.

The sol-gel fabrication technique has been shown to produce organic-inorganic hybrid materials with novel physical and chemical properties when metal alkoxides, such as tetraethyl orthosilicate (TEOS), are used [6]. The sol-gel process involves the conversion of monomers into a colloidal solution (sol) that acts as a precursor for an integrated network (gel) of either discrete particles or network polymers [7]. In the sol-gel process, an acid like HCl is usually used to accelerate hydrolysis [8]. However, the acid promotes the corrosion of neighboring metals or metal oxides used in display applications [9]. Therefore, acid-free methods of synthesizing organic-inorganic hybrid materials are needed for optical thin-film applications. In the sol-gel synthesis of materials with high refractive indices, Ti-containing alkoxide materials, e.g., Ti(OR)₄, are usually used. It has been reported that the hydrolysis and condensation of Ti(OR)₄ can be achieved via acid-free polymerization because of the high electro-negativity of Ti(OR)₄ compared to other alkoxides, and therefore, it can be easily reacted with H₂O for hydrolysis [6, 10, 11]. In the synthesis of materials with low refractive indices, alkoxide materials that contain Si atoms, e.g., TEOS, are usually used to achieve a low refractive index. However, because of the low electro-negativity of TEOS, it needs a catalyst to accelerate hydrolysis [11]. In order to induce hydrolysis and condensation of TEOS without acids, metal salts are commonly used as catalysts [11, 12].

In this study, a random copolymer of methyl methacrylate (MMA) and 3-(trimethoxysilyl) propyl methacrylate (MSMA) (poly(MMA-co-MSMA)) capped with trialkoxysilane in MSMA units was used as the precursor for the synthesis of inorganic-organic hybrid materials through an acid-free, in situ, sol-gel process. TTIP and TEOS were reacted with the precursor to obtain the high and low refractive index materials (H and L), respectively [6,12]. These materials were used to fabricate an indium tin oxide (ITO)/L/H/poly(ethylene terephthalate) (PET) thin-film substrate for use as a refractive index-matched touch screen to hide patterns of ITO. The optimum thickness of each layer was calculated with Macleod software [2]. This layered substrate opens up a new path for these novel hybrid materials in film-based flexible display applications because of the simple fabrication techniques required, i.e., spin-coating of the sol solutions at a low curing temperature of ~80 °C and no need for plasma sputtering [6].

2. Experimental details

2.1. Materials

MMA (99%, Aldrich), MSMA (98%, Aldrich), titanium (IV) isopropoxide (TTIP, 97%, Aldrich), tetrahydrofuran (THF, 99.9%, Acros), benzoyl peroxide (BPO, 75%, Acros), TEOS (98%, Aldrich), ethanol (99.5%, Aldrich), and sodium chloride (Sigma-Aldrich) were used as received.

2.2. Synthesis of the precursor, and high and low refractive index materials

The synthesis of the precursor (P), high (H) and low (L) refractive index materials are summarized in Fig. 1.For the synthesis of P, MMA (1.001 g, 10.01 mmole) and MSMA (0.828 g, 0.003 mole) were polymerized at 60 °C with BPO (0.121 g, 0.001 mole, a reaction initiator) in a three-neck flask containing THF (21 mL) under nitrogen flow for 2 h. The mole ratios of the solution were fixed at [MSMA]/([MMA][MSMA]) = 0.25 and [BPO]/([MMA][MSMA]) = 0.038. THF was chosen as the solvent because of the good solubility of TTIP in THF, even though the by-products of the sol-gel process are alcohols. For the synthesis of H, TTIP (0.203 g (0.001 mole) to 16.461 g (0.058 mole)), deionized water (0.01 to 1.04 mL), and THF (4.2 to 338.7 mL) were added to the prepared P solution in the three-neck flask with mechanical stirring to avoid local inhomogeneities, and reacted for 2 h at 60 °C. The amount of water was controlled by the 1:1 mole ratio of TTIP to water. Meanwhile, the amount of THF was controlled by the concentration of TTIP in THF being fixed at 1.71 MmL⁻¹ to avoid fast gelation, which would lead to the formation of an inhomogeneous solution [6]. For the synthesis of L, ethanol (17 mL) was used as a solvent because the by-products of the sol-gel process are alcohols and the solubility of TEOS in ethanol is good. The process for synthesizing the P for L was the same as that of H except the solvent used was ethanol (instead of THF), and a higher reaction temperature of 70 °C was used (instead of 60 °C) because of ethanol’s higher boiling point. Deionized water (0.18 mL), TEOS (2.778 g, 0.013 mole), and NaCl (0.003 g (0.5 M) to 0.015 g (2.5 M)) were added to the P solution in ethanol with mechanical stirring to avoid local inhomogeneities, and reacted for 2 h at 70 °C. The refractive index of L was controlled by the mole fraction of NaCl instead of the amount of TEOS, as discussed in the Introduction. The H and L solutions were spin-coated onto bare glass substrates at 1000 to 5000 rpm for 40 sec. To remove NaCl, low refractive index thin film was washed by water several times. The thicknesses of the thin-films were controlled by the rpm of the spin coater. The coated films were then cured at 80 °C for 2 h. For comparison to evaluate suitability of acid-free method for touch screen panel, reported methods were adopted to make acid-based high and low refractive index thin films. Briefly for the acidic synthesis of the hybrid material with a high refractive index, the coating solution was prepared with TTIP (97%, Aldrich), toluene (99.8%, Aldrich), HCl (36.5~38%, Aldrich), and a UV-curable resin, MINS-311RM (MINUTA Tech, Korea). A mixture of 1-(2) TTIP (29.6 mL), toluene (29.6 mL), and HCl (0.03 g) in a three-neck flask was stirred at 100 rpm for 1 day at room temperature with a magnetic bar. Then, MINS-311RM (0.3 g) was added to the mixture (2.7 g) and stirred for another day at room temperature. All films were spin-coated onto glass substrates at 1000 to 5000 rpm. Curing was performed at 120 °C for 2 h. For the acidic synthesis of the hybrid material with a low refractive index, the coating solution was prepared with TEOS (98%, Aldrich), water, ethanol (99.5% Aldrich), and HCl (36.5~38%, Aldrich). A mixture of water (0.54 mL), ethanol (52.55 mL), and HCl (0.09 mL) was stirred for 40 min with a magnetic bar at room temperature. Then, the TEOS (6.70 mL) was added dropwise to the prepared mixture. The reaction mixture was stirred for a further 2.5 h at room temperature. All films were spin-coated at 1000 to 5000 rpm. Curing was performed 120 °C for 2 h.

 

Fig. 1 Synthesis of H and L.

Download Full Size | PDF

2.3. Index-matching

The ITO/L/H/PET thin-film substrate was tested for refractive index-matched layers, which is a requirement for touch screens. The optimum thicknesses for index-matched layers were calculated using Macleod software (version 9.2.337, DigiClassic, U.S.A.), in which the measured refractive indices and extinction coefficients were used for calculating the reflectance, R = {(1−n)² + k²}/{(1 + n)² + k²}, where n and k are the refractive index and extinction coefficient, respectively. The values of n and k were measured with a spectroscopic ellipsometer (Elli-SE-U, Ellipso. Tech, Korea) over a wavelength range of 233 to 1033 nm [2]. To prepare the layered samples according to the simulated data, a calibration curve of the layer thickness as a function of the rpm of the spin coater was created, and then the ITO/L/H/PET thin-film substrate was prepared with the optimum layer thicknesses. The reflectance and transmittance of the substrate were evaluated with a spectrophotometer (CM-3600d, Konica Minolta Co., Japan).

2.4. Characterization

Proton nuclear magnetic resonance spectroscopy (¹H NMR, AVANCE digital 400, Bruker, German) was used to analyze the structure of the synthesized P. The P was precipitated in methanol for ¹H NMR analysis. The ¹H NMR sample of P was prepared by dissolving the precipitate in deuterated chloroform (CDCl₃, 16.7 mg/mL), with tetramethylsilane (TMS) used as the standard. Fourier transform infrared spectroscopy (FT-IR, Spectrum GX & AutoImage, PerkinElmer, U.S.A.) was used to analyze the structure of the synthesized H and L with an attenuated total reflectance (ATR) attachment. For ATR measurements, the FT-IR samples were prepared by placing a drop of the solution on a plate. All of the spectra obtained were the accumulation of 16 scans over a wavenumber range of 650 to 4000 cm⁻¹. The microstructure of the hybrid materials was examined with field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan). The thermogravametric analysis (TGA) was carried out in nitrogen using a thermal analyzer system (TA instruments, Q600, United State) with a temperature scan rate of 10 °C/min. The transmittance and reflectance of the films were determined with a UV spectrophotometer (CM-3600d, Konica Minolta Co., Japan) over a wavelength range of 360 to 740 nm. The samples for FE-SEM, spectroscopic ellipsometer, and UV-spectrophotometer measurements were prepared by spin coating the solution onto a glass substrate. A bending test was conducted to evaluate the flexibility of the hybrid thin-film substrate with a bending machine (ZBT-200, Zeetech, Korea). The sample size was 100 × 100 mm², and the test conditions were 10,000 cycles at a rate of 100 mm/sec for a bending range of 40 to 80 mm. The lifetime of the sample was measured with a lifetime measurement system (Polaronix M 6000s, Mcscience, Korea) in a chamber maintained at 85 °C and 85% relative humidity.

3. Results and discussion

3.1. Synthesis and morphology of P, H, and L

Figure 2 shows the ¹H NMR spectrum and labeled chemical structure of P. The peaks for the protons of the CH₂ groups of the main chain (a and d), α-CH₃groups (b and e, triplet), and –OCH₃ groups (c and i) are at 1.6-1.9, 0.7-1.2, and 3.5-3.6 ppm, respectively [13,14]. The peaks for the protons of the –CH₂ groups in the spacer of the MSMA (f, g, and h) are at 3.8, 1.55, and 0.6 ppm, respectively [15,16]. The three singlet peaks for b and e are because of three different tacticities: syndiotactic, atatic, and isotactic [13,14]. All of the peaks were assigned to the chemical structure of P, indicating that P was prepared correctly. NMR spectra of H and L could not be obtained because of the insolubility of these cross-linked films.

 

Fig. 2 NMR spectrum of P.

Download Full Size | PDF

The molar ratio of the MMA and MSMA units of P was calculated from the NMR data. The integrations of the f, g, and h peaks of the MSMA unit are all 0.04 because they represent the same number of protons. The theoretical integrations of the d, e, and i peaks of the MSMA unit were expected to be 0.04, 0.06, 0.18, respectively, based on the number of protons. However, these peaks are overlapped by the peaks from MMA. The d peak from MSMA is overlapped by the a peak from MMA at 1.6-1.9 ppm, with a measured integration of 0.19 for the overlapping a and d peaks. Using the expected integration of the d peak (0.04), the integration of the a peak is 0.15. Similarly, the e peak from MSMA is overlapped by the b peak from MMA. The measured integration of the overlapping b and e peaks is 0.28, which can be divided into 0.06 from the e peak and 0.22 from the b peak. Finally, the measured integration of the overlapping c and i peaks is 0.36, which can be divided into 0.18 from the c peak and 0.18 from the i peak. Therefore, the calculated ratio of a:b:c of the MMA unit is 0.15:0.22:0.18 (2:2.9:2.4), which is close to the theoretical ratio of the proton numbers (2:3:3). The integrations of the observed peaks are listed in Table 1.The molar ratio of the MSMA unit in P was calculated with the equation ((I_d-i)/20)/((I_a-c)/8 + (I_d-i)/20)), where I_d-i and I_a-c are the sums of the integrations of peaks d to i and a to c, respectively. The molar ratio of MSMA in P was determined to be 22%. This is close to the input molar ratio of MSMA (25%).

Table 1. NMR integration of P; the expected integration and number of protons are based on [MMA]:[MSMA] = 0.78:0.22. The expected integration was normalized with the integration (0.04) of the f (or g or h as they have the same integration) peak

View Table | View all tables in this article

Figure 3 shows the normalized FT-IR spectra of P, H, and L with a standard peak of C = O stretching at 1730 cm⁻¹ which intensity does not change after the addition of TTIP or TEOS to P. The FT-IR spectrum of P shows C-H stretching at 2974 and 2860 cm⁻¹ [17], and C = O stretching, C-H bending, -CH₃ rocking and C-O-C skeletal vibrations at 1726, 1452, 1380 and 1365 cm⁻¹, respectively (inset Fig. 3) [18,19]; the CH₃ rocking and C-O-C skeletal vibrations at ~1370 was overlapped. The peaks at 1068 and 996 cm⁻¹ correspond to the Si-O stretching and C-O-Si skeletal vibrations, respectively [19]. The presence of peaks for Si-containing groups indicates that the trialkoxysilane was successfully incorporated into P. The FT-IR spectrum of H has peaks at 3471, 2970 and 2863, 1722, 1459, and 1365 cm⁻¹, which correspond to –OH stretching, C-H stretching, C = O stretching, C-H bending, and C-O-C skeletal vibrations, respectively [17–19]. In addition, the peaks at 1133, 908, 837, 790, and 686 cm⁻¹ are attributed to the symmetric Si-O-Si ((Si-O-Si)_s) stretching, Si-O-Ti stretching, Si-C stretching vibration, asymmetric Si-O-Si ((Si-O-Si)_as) stretching, and Ti-O-Ti stretching vibrations, respectively [19,20]. The increased intensity of the C-H stretching peak and the appearance of Si-O-Si, Si-O-Ti, and Ti-O-Ti stretching vibrations confirms the successful reaction of TTIP with P, leading to the cross-linked structure with incorporated Ti atoms. The FT-IR spectrum of L shows peaks at 3341, 2974 and 2890, 1723, 1381, and 1274 cm⁻¹, which correspond to the –OH stretching, C-H stretching, C = O stretching, C-H bending, and C-O-C skeletal vibrations [17,18], respectively. In addition, the peaks at 1100 and 880 cm⁻¹ are attributed to the (Si-O-Si)_s and (Si-O-Si)_as stretching vibrations, respectively [19]. The increased intensity of the C-H stretching peak and the appearance of the Si-O-Si stretching vibration confirms the successful reaction of TEOS with P, leading to the cross-linked structure with incorporated Si atoms.

 

Fig. 3 FT-IR spectra of P, H, and L. The inset shows the enlarge spectrum of P.

Download Full Size | PDF

Figures 4(a)-4(c) show the SEM images of the top-down views of P, H, and L thin-films on glass substrates. The SEM image of P shows a featureless film, and those of H and L exhibit well-dispersed, uniform, inorganic domains (H) and spheres (L) in the polymer matrix. Inorganic domains and spheres with similar shapes have been reported in the literature [6, 21–26]. The sizes of the particles in H and L were determined to be 61.4 ± 29.4 nm and 126.9 ± 22.5 nm, respectively. These well-dispersed, uniform, inorganic domains in the polymer matrix play a positive role in controlling the refractive index of the H and L films on the substrates.

 

Figures 4 FE-SEM images of the top-down views of (a) P, (b) H, and (c) L thin-films.

Download Full Size | PDF

3.2. Thermal properties

Figure 5 shows TGA thermograms of P, H, and L. The decomposition temperatures determined at the maximum degradation rate for P, H, and L are 300, 352, and 398 °C, respectively. Significant improvement on thermal stability was observed by employing inorganic materials in H and L. The residues of P, H, and L are 9.5, 30.4, and 36.1%, respectively. Increase of residue for H and L is due to non-degradable inorganic portion in H and L. The increased inorganic portions for H and L by reaction with TTIP and TEOS are 20 and 26%, respectively.

 

Fig. 5 TGA thermograms of P, H, and L.

Download Full Size | PDF

3.3. Optical properties

Figures 6(a) and 6(b) show the refractive indices and transmittances of the H and L films on glass substrates as functions of the wt% of TTIP and concentration of NaCl in water, respectively. The transmittance of P on the glass substrate is 92.0%, which is close to that of bare glass (91.3%), indicating that a thin film of P on a glass substrate does not deteriorate the transmittance. In the case of H, the transmittance decreases from 92.0 to 88.5% (100.7 to 96.9% when the transmittance is normalized with respect to bare glass) and the refractive index increases from 1.50 to 1.82 as the wt% of TTIP increases from 0 to 90 wt%. The high refractive index of H is because the incoming light is scattered by the titania molecules, resulting in a low transmittance. When the wt% of TTIP in the reaction mixture was higher than 90 wt%, the resulting TiO₂ particles were precipitated through the gelation of the reactants, and therefore, a homogenous solution could not be obtained. In the case of L, the transmittance remains nearly constant at ~92.0% (100.0% with respect to bare glass) and the refractive index decreases from 1.50 to 1.44 as the NaCl concentration increases from 0 to 2.5 M. When the NaCl concentration was higher than 2.5 M, an inhomogeneous solution was immediately obtained because of the fast gelation caused by fast hydrolysis. The increased transmittance and decreased refractive index of L are because of the existence of the high transmittance Si-O-Si groups and low refractive index of silica atoms, respectively.

 

Figures 6 Transmittances and refractive indices of (a) H and (b) L thin-films as functions of the wt% of TTIP and NaCl concentration, respectively.

Download Full Size | PDF

Table 2 shows a summary of the optical properties of bare glass, P, H, and L. The H and L films tested were produced with 90 wt% TTIP and 2.5 M NaCl to obtain the highest and lowest refractive indices for H and L, respectively. The transmittances of the bare glass, P, H, and L were 91.3 (100.0), 92.0 (100.1), 88.3 (96.9), and 92.3% (100.8%), respectively; the numbers in parenthesis are the transmittances normalized with respect to bare glass. As can be seen, all of the materials exhibit high transmittances on glass substrates, indicating that they are suitable for transparent optical device applications. The reflectances of the bare glass, P, H, and L are 8.4, 7.6, 9.9, and 7.4%, respectively, and the extinction coefficients of P, H, and L are 5.4 × 10⁻⁴, 2.4 × 10⁻², and 9.58 × 10⁻³, respectively. The absorbance is proportional to the extinction coefficient through the equation α = 4πk/λ, where α is the absorbance and k is the extinction coefficient [2]. All of the materials exhibit low extinction coefficients (low absorbance). The resultant refractive indices of the bare glass, P, H, and L were 1.52, 1.50, 1.82, and 1.44, respectively. Thus, these new spin-coatable hybrid materials with high and low refractive indices (1.82 and 1.44, respectively) and high transparency were synthesized by an acid-free sol-gel process. Therefore, these materials are suitable for optical, index-matched, flexible thin-film applications.

Table 2. The optical properties at 550 nm of the bare glass, P, H, and L. The H and L films tested were produced with 90 wt% TTIP and 2.5 M NaCl, respectively

View Table | View all tables in this article

3.4. Application of the H and L thin-films as index-matched touch screens

Table 3 shows the calculated optimum thicknesses of the index-matched layers of the thin-film substrate tested for touch screen applications. For comparative purposes, two thin-film substrates were tested: one composed of ITO/SiO₂/Nb₂O₅/PET and the other composed of ITO/L/H/PET. The ITO/SiO₂/Nb₂O₅/PET layered substrate was prepared with a conventional sputtering method [27]. The sputtering of Nb₂O₅ was performed in 20 sccm (standard cubic centimeter per minute) of Ar and 1.0 sccm of O₂ with a radio frequency power (RF) of 700 W, while the sputtering of SiO₂ was carried out in 20 sccm of Ar with a RF power of 800 W using an inorganic evaporator (Sunic ELpuls 200, SUNIC SYSTEM, Korea). Figure 7(a) shows the ΔR of the ITO/SiO₂/Nb₂O₅/PET and ITO/L/H/PET thin-film substrates as a function of wavelength from 360 to 740 nm. ΔR represents R₁−R₂, where R₁ and R₂ are the reflectances on ITO and L (or SiO₂), respectively, of the ITO/L/H/PET (or ITO/SiO₂/Nb₂O₅/PET) layered substrate [28]. When ΔR becomes lower than 1%, the patterned ITO is hidden and the visibility of the touch screen is improved [29]. The values of ΔR at 550 nm are 0.55, and 0.37% and the percentages of the visible light spectrum with ΔR < 1% are 71.65 and 65.12% for ITO/SiO₂/Nb₂O₅/PET and ITO/L/H/PET, respectively. Even though the wavelength percentage of the hybrid substrate (ITO/L/H/PET) is slightly smaller than that of ITO/SiO₂/Nb₂O₅/PET, it is still satisfactory for touch screen applications. Thus, the small ΔR of the ITO/L/H/PET over a wide range of the visible spectrum indicates that L and H can replace SiO₂ and Nb₂O₅, respectively, for index-matching applications in flexible touch screens. ΔR for ITO/L/H/PET is oscillated in Fig. 7(a). Compared to ITO/SiO₂/Nb₂O₅/PET which was made by a sputtering method, ITO/L/H/PET was fabricated by a spin-coating method. Thus, the layer thickness of ITO/L/H/PET is ~four times thicker than that of ITO/SiO₂/Nb₂O₅/PET as shown in Table 3. When each layer in the multilayer film is thick, optical properties such as reflectance and transmittance are known to be actively oscillated [1,2]. Figs. 7(b) and 7(c) show photographs of the patterned ITO/PET and patterned ITO/L/H/PET thin-films, respectively. The patterned ITO of the ITO/L/H/PET is not visible, demonstrating that the ITO/L/H/PET substrate has a similar performance to that of the ITO/SiO₂/Nb₂O₅/PET substrate in an index-matching technique.

Table 3. Refractive indices and thicknesses of each layer of ITO/SiO₂/Nb₂O₅/PET and ITO/L/H/PET substrates tested

View Table | View all tables in this article

 

Figures 7 (a) ΔR of the ITO/SiO₂/Nb₂O5/PET (solid line) and ITO/L/H/PET (dotted line) substrates plotted as a function of wavelength. Photographs of the ITO patterns on (b) ITO/PET and (c) ITO/L/H/PET substrates.

Download Full Size | PDF

3.5. Acid resistance and bending stability of the hybrid substrate

Bending tests were conducted to evaluate the mechanical flexibility of the hybrid thin-film substrates. Figure 8(a) shows the percentage increase of resistance from the initial value of the ITO/SiO₂/Nb₂O₅/PET and ITO/L/H/PET substrates as a function of the number of bending cycles. As can be seen, the ITO/SiO₂/Nb₂O₅/PET exhibits higher resistance increases compared to that of ITO/L/H/PET for all bending cycles tested. The ITO/SiO₂/Nb₂O₅/PET shows a 55% increase in resistance after 10,000 cycles, while the ITO/L/H/PET exhibits an 18% increase, indicating that the ITO/L/H/PET is more suitable for flexible display applications than the typical inorganic ITO/SiO₂/Nb₂O₅/PET. Figure 8(b) shows a photograph of the ITO/L/H/PET at minimum width of 40 mm during a bending test, with the substrate demonstrating good bending performance. Lifetime measurements were also conducted to evaluate the longevity of the acid-free hybrid material in a touch screen display application when subjected to harsh conditions. For comparison, H and L were synthesized with acid by the reported method and spin-coated following the same method as those without acids [30,31]. Fig. 8(c) shows the percentage increase of the resistance as a function of the duration in a chamber maintained at 85 °C and 85% relative humidity. The ITO/L/H/PET prepared with acid shows greater increases in resistance than that of the ITO/L/H/PET prepared without acid. The ITO/L/H/PET prepared without acid exhibits a 4% increase after 30 days, while the ITO/L/H/PET with acid shows a 15% increase after the same period of time. Thus, the ITO/L/H/PET substrate prepared by the acid-free method is suitable for flexible display applications with harsh environmental conditions.

 

Figures 8 (a) Plot of the percentage resistance increase of the ITO/SiO₂/Nb₂O₅/PET and ITO/L/H/PET substrates as a function of the number of bending cycles ^[32–35^]. (b) Photograph of the ITO/L/H/PET substrate during the bending test at minimum width of 40 mm. (c) Plot of the percentage resistance increase of the ITO/L/H/PET substrates prepared with and without acid as a function of the duration in a chamber maintained at 85 °C and 85% relative humidity ^[33–35^].

Download Full Size | PDF

4. Conclusions

The precursor copolymer, poly(MMA-co-MSMA), capped with trialkoxysilane in MSMA units was successfully used to synthesize index-matching materials through an acid-free sol-gel process by reacting the precursor with TTIP and TEOS for high and low refractive index materials, respectively. The refractive indices of H and L were controlled by the amount of TTIP and concentration of NaCl used, respectively. A film of H on a glass substrate exhibited a high optical transparency of 96%, with respect to bare glass at 550 nm, and high refractive index of 1.82 when synthesized with 90 wt% of TTIP. The L film exhibited a high optical transparency of ~100%, with respect to bare glass at 550 nm, and low refractive index of 1.44 when synthesized with 2.5 M of NaCl. The ITO/L/H/PET layered substrate had a low ΔR (<1%) for a wide range of the visible spectrum (65%), as well as good bending flexibility and a long lifetime. Thus, the prepared L and H, which can be spin-coated onto a flexible substrate, can replace the typical inorganic high and low refractive index materials of SiO₂ and Nb₂O₅, respectively, which are used for flexible display applications. By using the optical refractive-index matching technique with these acid-free high and low refractive-index materials, any applications related to flexible display other than touch screen panel (e.g., anti-reflective coatings) can be possible.