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TiO2 optical hybrid films with tunable refractive index prepared via a simple and efficient way
Progress in Organic Coatings 120 (2018) 252–259
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Transparent epoxy/TiO2 optical hybrid films with tunable refractive index prepared via a simple and efficient way Shaomin Dan, Huimin Gu, Jiaojun Tan, Baoliang Zhang, Qiuyu Zhang
T
⁎
Key Laboratory of Applied Physics and Chemistry in Space, Ministry of Education, Department of Applied Chemistry, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid material Epoxy resin TiO2 Optical film Refractive index
A simple and efficient method without any coupling/chelating agents was developed to prepare the transparent epoxy/TiO2 optical hybrid films with tunable refractive index. Instead of adding any coupling agents, the study found out that the lateral hydroxyl groups of epoxy resin E51 could serve as an intermediary to form organic–inorganic bonding between the organic matrix and the hydrolyzed titanium butoxide (TBOT). A series of homogeneous hybrid films were obtained without phase separation via adjusting the mole ratio of TBOT/hydroxyl groups, mass ratio of acid/water and the mixed solvent system. FTIR, XRD and TEM results proved the successful formation of nanoscale crystal-titania domains with the size of 3–5 nm and homogeneously dispersed in organic matrix. Furthermore, SEM and AFM results indicated the relatively good surface smoothness of the resultant optical hybrid films. More importantly, the refractive index of epoxy resin E51/TiO2 optical hybrid films were tunable in the range of 1.512–1.731 at 633 nm, which linearly increased with the content of TiO2 from 0 to 20 wt.%. These hybrid films with continuously tunable refractive index were expected to have great potential application in advanced optical devices.
1. Introduction High-refractive-index (high-n) materials with optical transparency are attracting increasing attention both in scientific and engineering aspects because of their wide application in advanced optoelectronic devices, such as light-emitting diode (LED) encapsulants [1], display devices [2,3], anti-reflective films [4], optical adhesives, holographic recording systems [5], complementary metal oxide semiconductor (CMOS) image sensors [6,7], optical waveguides [6] and so on. Especially, materials with refractive index exceeding 1.7 or even 2.0 are highly desired for LED encapsulants. As the extraction efficiency of LED almost doubles when the refractive index is increased from 1.5 to 2.0, although the typical n values of conventional encapsulants are in the range of 1.4–1.5, being limited by their chemical structures [8]. On the other hand, polymers showing tunable n values (n = 1.3–2.2) and with a thickness of few hundred nanometers are also frequently needed in anti-refractive (AR) coatings for liquid crystal display (LCD). And it is called polymeric refractive index-control technology [9]. By designing a multilayer film stack with a gradually varying refractive index for the LCD surface, the reflection loss on the surface can be reduced dramatically and the negative effects such as reflection of the light sources
and a double image can be effectively avoided [10–12]. However, the n values of most traditional polymer can hardly be continuously adjustable. Incorporating inorganic components with high n values into polymer substrates can overcome those defects and result in hybrid materials with n values that are pretty high and tunable over a wide range. What’s more, this kind of hybrid materials have been defined as an entirely new class of advanced materials because of their improved optoelectronic, thermal and mechanical properties than the corresponding individual organic or inorganic components [9,13,14]. During the past decade, many reports have emerged discussing the design, syntheses and applications of organic-inorganic hybrid materials processing high refractive index [15,16]. One of the most concerned materials are epoxy resins, which are widely applied in optical devices, such as optical adhesives, LED encapsulants and anti-reflective films as a result of their excellent mechanical properties, thermal stability and optical transparency [17,18]. While, the pure epoxy resins dissatisfy the urgent need of applications in the advanced optical devices because of their relative narrow range of available n values (1.4–1.6). Therefore, inorganic domains with high-n values such as titania are employed to improve the refractive index of epoxy resin. Mont et al. reported surfactant-coated-TiO2 nanoparticle loaded epoxy with
Abbreviations: LED, light-emitting diode; n, refractive index; CMOS, complementary metal oxide semiconductor; AR, antirefractive; LCD, liquid crystal display; TiO2, titania; TBOT, titanium butoxide; D-400, poly (propylene glycol) bis (2-aminopropyl ether); HCl, hydrochloric acid; TX, theoretical content (X wt.%) of titania ⁎ Corresponding author. E-mail address: [email protected] (Q. Zhang). https://doi.org/10.1016/j.porgcoat.2018.02.017 Received 1 March 2017; Received in revised form 19 January 2018; Accepted 20 February 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
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morphologies of the resulted films. Thermal behavior, optical transmittance and refractive index distribution of the resultant hybrid films were also revealed in this work.
the improved n value of 1.67 at the wavelength of 500 nm, which was markedly higher than that of the traditional epoxy resin (n = 1.53) [19]. Besides, Chau et al. successfully prepared the epoxy/TiO2 hybrid materials, of which the TiO2 nanoparticles were modified by acetic acid and the n values ranged from 2.18 to 2.38 [20]. In these works, surface modification of the nanoparticles was implemented to prevent aggregation and improve their compatibility with the organic phase. However, the pretreatment made the preparation tedious and changed the properties of the nanoparticles, such as a decrease in density and refractive index or an increase in the hydrophobic nature of the surface [21,22]. It is worth noting that the size of the inorganic domains with high-n values plays a vital role in the performance of the hybrid materials, which is required to be less than 40 nm (one-tenth of the wavelength of visible light) to avert scattering loss and maintain the optical transparency [23]. To control the size of titania domains, chelating agents or coupling agents were necessarily introduced to the system in many cases to produce strong interfacial force between organic and inorganic [24]. Unfortunately, the comprehensive performance such as mechanical, optical and thermal properties of the resultant hybrid materials might deteriorate due to the coupling or chelating agents [25,26]. In this study, a simple and efficient synthesis route without adding any coupling/chelating agents was developed to prepare the epoxy/ TiO2 hybrid films with good optical transparence and tunable refractive index. Inspired by Liou et. al [25], our group applied the lateral hydroxyl groups in epoxy resin E51 chains as an intermediary to provide organic–inorganic bonding with titanium butoxide instead of surface modification of titania nanoparticles or adding coupling/chelating agents to avoid phase separation of the epoxy/TiO2 optical hybrid films. As a result, highly homogeneous solution with different TiO2 concentration were successfully prepared and then thermal cured to transparent hybrid films as shown in Scheme 1. Then the structure of the resultant epoxy-TiO2 hybrid materials were investigated by FTIR and XRD. Moreover, SEM, AFM and TEM were employed to reveal the
2. Materials and methods 2.1. Materials Titanium butoxide (TBOT, 97%, Aladdin) was used as the precursor. Epoxy resin E51 resin (Wuxi Resin Factory of Blue Star New Chemical Materials) was used as the organic matrix. Poly (propylene glycol) bis (2-aminopropyl ether) (D-400, Aladdin) was used as curing agent. Benzyl alcohol purchased from Aladdin (99%) was used as accelerator. Aqueous hydrochloric acid (HCl, 37 wt.%) was purchased from J.T. Baker. n-Butanol (99.5%) purchased from Aldrich was used as the solvent. Ultrapure water was used throughout the work (Aquapro Co. Ltd.). 2.2. Preparation of Epoxy/TiO2 hybrid films The synthesis route to prepare the epoxy/TiO2 optical hybrid films is shown in Scheme 1 and the formation mechanism of crystal-titania domains as well as the hybrid materials are shown in Schemes 2 and 3, respectively. The hybrid films are prepared according to Chen’s work [10] with modification. According to Scheme 1, D-400 and a small amount of benzyl alcohol are chosen as the curing system of E51 (mass ratio:E51/D400/benzyl alcohol:100/64/4), which are added to the system before Ti-precursor. Amine hardener D-400 is a medium and low temperature curing agent. It takes 24 h to cure epoxy at room temperature and can also quickly solidify the system when heating at 40–60 °C. Meanwhile, the hydrolysis rate of TBOT is so fast that hydrolytic stabilizers such as acetic acid, hydrochloric acid and acetone are necessary to control the hydrolysis rate. Therefore, the addition of D-400 in advance does not make the epoxy matrix cured before the
Scheme 1. Reaction scheme for the preparation of epoxy/TiO2 optical hybrid films. 253
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Scheme 2. Formation procedure of TiO2 by sol-gel process.
corresponding properties of the obtained epoxy/TiO2 hybrid films are exhibited in Table 1. Particularly, TX indicates the theoretical content (X wt.%) of titania hydrolyzed by TBOT within the hybrid films. Take T20 as an instance to illustrate the procedure of the preparation of
inorganic precursors hydrolyzed sufficiently at room temperature. What’s more, adding D-400 before Ti-precursor avoids disturbing the hydrolysis stability created by hydrolytic stabilizers HCl because of the amino group in the structure of D400. The compositions and the
Scheme 3. Illustration of preparation process of epoxy/TiO2 hybrid films: (i) sol-gel reaction under catalyzed conditions, (ii) Spin-coated and precuring, (iii) thermosetting and obtaining hybrid film T20. 254
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the spectra of T10, T15 and T20, despite there are some overlap with the stretching vibrations of CeO at 1250 cm−1. All the information indicate that the hydrolyzed TiO2 particles are grafted to epoxy resin via covalent bonds. As for T5, the peak of TieOeC is almost invisible and the possible reason is the low content of TBOT or kinetic trapping of the small domain at low concentration. Fig. 2 shows the XRD spectra of the pure epoxy (T0), epoxy/TiO2 hybrid films (T5–T20) and TiO2 powder prepared by sol-gel method. A broad hump centered at 20° is observed in the XRD pattern of T0, which comes from the amorphous characteristic peak of epoxy resin E51. As the content of TiO2 rises, the intensity of a TiO2 broad peak (101) increases gradually in the range of 21–25°, while the amorphous peak of epoxy decreases accordingly. This result indicates that TiO2 domains are synthesized successfully through sol-gel process and are incorporated into epoxy resin by the solidification of the epoxy resin. What is more, the width of the TiO2 peaks in the hybrid films results from X-ray scattering generated from the petty size of the TiO2 nanocrystalline domains [10]. The morphologies of epoxy/TiO2 hybrid films are characterized by FE-SEM and the results are shown in Fig. 3. It is clear that the hybrid materials have good film-forming property and smoothing surface without any apparent TiO2 aggregations or obvious micro phase separation. Furthermore, the 3D, deflection and friction AFM images of T5 and T20 are depicted in Fig. 4. And the root mean square height (Sq) of all the hybrid films obtained from AFM are listed in Table 1, in which the Sq values of T0, T5, T10, T15 and T20 are 2.12, 1.81, 4.09, 4.79 and 4.97 nm, respectively. According to the results, the surface evenness of the hybrid materials tend to drop with the increase of TiO2 content in the system. The possible reason is that surface roughness of the film is affected by the content of solvent and stabilizer, which increases with the mass percentage of TBOT. As a result, there is high content of solvent and stabilizer in the network of the film with high Ti content. The solvent and stabilizer gradually evaporate during the process of epoxy curing, and some holes would form at the film surface, which could lead to a slight drop in the surface evenness of the films. Overall, the resulted Sq values are low, indicating that the films possess relatively smooth surface. In order to explore the detail morphology and microstructure further, TEM was applied to investigate the hybrid materials. Fig. 5 shows two typical bright-field images of T5 and T20, respectively. Moreover, the enlarged image of an individual TiO2 domain and the selected-area diffraction (SAED) pattern are also shown in Fig. 5(c) and (d), which demonstrate that the nanoparticles are successfully synthesized and oriented with anatae structure. What is more, the average size of the prepared nano-TiO2 is around 3–5 nm and the spatial distribution of T20 is denser than T5. More importantly, the nanocrystals embed uniformly in the polymer matrix which demonstrates that the hydroxyl groups of E51 could serve as active sites for TiO2 grafting onto and keep the morphology stability of the hybrid materials.
Table 1 Compositions of the recipes and properties of epoxy/TiO2 hybrid films. Sample(TX)
T0 T5 T10 T15 T20 a b c d
Reactant compositions
Properties of Hybrid films
Resin matrix (g)
TBOT (g)
TiO2 contenta (wt.%)
TiO2 contentb (wt.%)
Sqc/nm
Refractive Indexd
5.00 4.63 2.19 1.38 0.98
0 1.06 1.06 1.06 1.06
0 5 10 15 20
0 7.3 14.5 17.6 20.2
2.12 1.80 4.09 4.79 4.94
1.512 1.554 1.618 1.693 1.731
Theoretical content of TiO2 in the hybrid materials. Experimental content of TiO2 determined through TGA at 800 °C. The root mean square height determined by AFM. Measured at 633 nm.
epoxy/TiO2 hybrid films. Firstly, 0.98 g epoxy resin consisting primarily of E51 and D-400 (mass ratio:100/64) with a small amount of benzyl alcohol as a promoter and 3.5 ml acetone were added to 50 ml beaker and then stirred at room temperature for about 30 mins. Then 1.06 g TBOT being dissolved homogenously in 2.5 ml butanol were added dropwise to the mixed solution in the first step and stirred at room temperature for another 30 mins. After that, a well-mixed solvent with 0.14 g of HCl, 2.8 ml of butanol and 0.16 g of H2O were dropwise into the above mixture with a syringe and stirred to form the homogenous precursor solution of T20. Finally, the solution was spin coated on the cleaned silicon wafer with the speed of 1500 rpm for 18 s and then thermal cured at 40 °C for 40 mins and at 60 °C for 90 mins to obtain the final films. 2.3. Characterization Tensor 27 (FTIR) spectrometer (Bruker) was used to record Fourier transform infrared spectra of all hybrid films. The crystalline phase were identified by X-ray diffraction diffractometer (Shimadzu XRD–7000 s) with Cu Ka radiation (k = 1.542 Å) from 10° to 80°. The surface morphologies were imaged through a field emission scanning electron microscope (FE-SEM, ZEISS EVO 18) with an accelerating voltage of 15 kV and an atomic force microscope (AFM, Agilent 5100) with the contact mode. Transmission electron microscope (TEM, JEOL JEM-2010) with the accelerating voltage of 200 kV was used to characterize the detailed microstructure of the hybrid materials. Thermal properties and titania contents were obtained from thermogravimetric analysis (TGA, Q50, TA Instruments) with temperature ranging from 35 ∼ 800 °C and the heating rate of 10 °C min−1 under nitrogen atmosphere. Transmittance of epoxy/TiO2 hybrid films (thickness: 30 ± 3 μm) were measured by UV–vis spectrophotometer (Labtech 2.0). The refractive index (n) and the thickness (h) of the hybrid films were obtained from the ellipsometer (PZ2000) at the wavelength of 633 nm. Finally, tensile properties of the epoxy/TiO2 hybrid materials were conducted according to ASTM D-638.
3.2. Optical properties of epoxy/TiO2 hybrid films Fig. 6 shows the transmittance of the hybrid films, T0-T20. The transmission rate of the obtained hybrid films decreases at the wavelength of 450 nm compared with pure epoxy, which is attributed by the low band gap of TiO2 (3.2 eV) [27]. The transparence of the hybrid films reduces slightly as the TiO2 content increases from 5% to 20%, indicating that the phase size of the nanocrystalline-titania are well controlled and dispersed. The transmittance of all hybrid materials in the range of visible light is over 70%, which is acceptable in engineering applications. The cutoff wavelength of the hybrid films appear in the UV–vis spectra (above 300 nm) while absorbance band of the pure epoxy is below 300 nm. Furthermore, the corresponding band edge of the hybrid films also red shift as the TiO2 content increasing. Similar phenomena could also be observed when the size of TiO2 is less than 10 nm [28]. The refractive index of epoxy/TiO2 hybrid films at the wavelength
3. Results and discussion 3.1. Structure and morphology of epoxy/TiO2 hybrid films The interaction between epoxy resin and different contents of nanocrystalline-titania domains is investigated with FTIR,as shown in Fig. 1. Peaks located at around 927 cm−1 in Fig. 1 are attributed to the residual epoxy groups. Compared with T0, the strong absorption band at 610–630 cm−1 of all the hybrid films correspond to TieOeTi linkages, which prove that TiO2 are successfully obtained and incorporated into the organic matrix as the epoxy resin being cured. Moreover, peaks of TieOeC at 1233 cm−1 could be observed clearly in 255
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Fig. 1. FTIR spectra of the epoxy/TiO2 hybrid films.
of 633 nm with different TiO2 content is shown in Fig. 7. The n values increase almost linearly with the TiO2 contents from 0 to 20 wt.%, revealing that the TieOH groups of the hydrolyzed TBOT successfully transform into the TieOeTi structures and generate the epoxy/TiO2 hybrid films with optical transparence and tunable refractive index from another aspect.
3.3. Thermal properties of epoxy/TiO2 hybrid films TGA curves of epoxy matrix (T0) and the hybrid materials (T5–T20) are shown in Fig. 8. The incorporation of TiO2 causes a slight decrease (45 °C) in Td of hybrid materials compared with the epoxy matrix. The possible reason is that the metallic compounds may oxidize to degrade hybrid films according to Boggess and Taylor [29,30]. Overall, the thermal stability of the hybrid films is still good for the advanced optical application although it is inferior to pure epoxy. Furthermore, the actual residual weight of T0 and T100 are 5.9 and 95.3% at 800 °C, separately, which contain 0 and 100 wt.% TiO2, theoretically. According to the residual weight of T0 and T100, the real TiO2 contents of T5–T20 are estimated as 7.3–20.2 wt.% (the detail contents are shown in Table 1) under assumption of the linear relationship. The actual contents are in conformity with the theoretical ones, which demonstrate that the TBOT has hydrolyzed to TiO2 successfully and been incorporated into E51 in accordance with the original design. DSC curves of pure epoxy resin (T0) and the hybrid materials
Fig. 2. XRD spectra of pure epoxy, epoxy/TiO2 hybrid films and pure TiO2 (anatae).
Fig. 3. FE-SEM images of the epoxy/TiO2 hybrid films. (a) T5, (b) T20. (Coated on silicon wafers). 256
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Fig. 4. AFM images of the epoxy/TiO2 hybrid films coated on silicon wafers:.
Fig. 5. TEM images of the epoxy/TiO2 hybrid films (a) T5, (b) T20, (c) an individual TiO2 domain, (d) SAED pattern of TiO2 domains.
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Fig. 6. Optical transmission spectra of epoxy/TiO2 hybrid films (thickness: 30 ± 3 μm).
Fig. 9. DSC curves of the prepared films T0, T5, T10, T15 and T20.
Fig. 10. Tensile strength of the epoxy/TiO2 resin hybrid films T0, T5, T10, T15 and T20 as a function of strain.
Fig. 7. Effect of TiO2 content on the refractive index of epoxy/TiO2 hybrid films.
Table 2 Mechanical properties of epoxy/TiO2 hybrid films. Sample
Tensile Strength/ MPa
Elongation at break/%
Tensile yield stress/ MPa
T0 T5 T10 T15 T25
57.59 24.20 25.99 44.35 47.16
4.47 60.76 17.08 7.15 5.44
31.49 22.44 25.75 44.15 39.26
at 50 °C. These suggest that there are substantial amount of solvent/ unreacted functional groups trapped in the films, which will come out/ react during heating. These events will suppress the heat absorbance associated with the Tg during DSC measurement and make the Tg disappear. 3.4. Mechanical property of epoxy/TiO2 hybrid films Fig. 8. TGA curves of the prepared films T0, T5, T10, T15 and T20.
In order to explore mechanical property of the hybrid films, samples with different contents of nanoparticles are prepared and tested according to the standard of ASTM D-638. The results are shown in Fig. 10. and the specific strength values are listed in Table 2. It is observed that the elongation at break of T5 is 14 times higher than the pure epoxy system. Meanwhile, the tensile strength reduces from 57.59 MPa to 24.2 MPa when the loading of TiO2 increased from 0 to
(T5–T20) are shown in Fig. 9. It can be seen that the Tg is about 42 °C, which is in good agreement with the literature records[31]. However, there is not any obvious thermal transition in the range of −25–200 °C for T5–T20 reveled in the DSC curves. While according to Fig. 8, the TGA curves of T5-T20 start to lose weight right after the heating begin 258
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5%. What’s more, the tensile strength of T10, T15 and T20 tend to rise gradually, although the values are still lower than the pure epoxy. The elongation at break decreases in the meantime. It is known that strength of the interface in nanocomposites plays a significant role in the hybrid materials’ ability to transfer stresses and elastic deformation from the matrix resin to the nanoparticles [32]. Nanoparticles can dramatically improve the toughness of organic matrix when the content is within 5%. Because TiO2 can disperse well in the system and form strong interface strength, which could hinder and passivate the cracks produced by the external force and improve the toughness of the materials. However, further increase of TiO2 loading decreases the mechanical properties may possibly indicates poor interface as the result of high solvent. Overall, the mechanical property of the hybrid film are acceptable for the application of optical devices.
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4. Conclusions High refractive index epoxy/TiO2 optical hybrid films are successfully synthesized using the lateral hydroxyl groups of epoxy resin E51 as an intermediary to form organic–inorganic bonding with the hydrolyzed titanium butoxide (TBOT), which is a simple and efficient way without any coupling/chelating agents. The hybrid films are in good surface smoothness without any phase separation according to AFM images, which indicates that the hybrid materials are homogeneous in nano scale. The covalent bonds between the organic- inorganic phases play a significant role in controlling the size of the titania nanocrystals, which guarantees the hybrid films a good optical property. The refractive index of the hybrid films could increase lineally with TiO2 weight fraction and could reach as high as 1.731 with the TiO2 content of 20 wt.%. In summary, the resultant optical hybrid films with advantages of good thermal property, optical transparency and tunable refractive index possess a vast commercial application prospect in advanced optical devices. Acknowledgment This work was supported by the fund of National Natural Science Foundation of China (No. 51433008 and 51673156). References [1] F.W. Mont, J.K. Kim, M.F. Schubert, E.F. Schubert, R.W. Siegel, High-refractiveindex TiO2-nanoparticle-loaded encapsulants for light-emitting diodes, J. Appl. Phys. 103 (2008) (2365-2302). [2] K. Kitamura, K. Okada, N. Fujita, Y. Nagasaka, M. Ueda, Y. Sekimoto, Y. Kurata, Fabrication method of double-microlens array using self-alignment technology, Jpn. J. Appl. Phys. 43 (2004) 5840–5844. [3] T. Nakamura, H. Fujii, N. Juni, N. Tsutsumi, Enhanced coupling of light from organic electroluminescent device using diffusive particle dispersed high refractive index resin substrate, Opt. Rev. 13 (2006) 104–110. [4] K.C. Krogman, T. Druffel, M.K. Sunkara, Anti-reflective optical coatings incorporating nanoparticles, Nanotechnology 16 (2005) 338–343. [5] N. Suzuki, Y. Tomita, T. Kojima, Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films, Appl. Phys. Lett. 81 (2002) 4121–4123. [6] M. Suwa, H. Niwa, M. Tomikawa, High refractive index positive tone photo-sensitive coating, J. Photopolym. Sci. Technol. 19 (2006) 275–276. [7] J.G. Liu, M. Ueda, High refractive index polymers: fundamental research and
259
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Transparent epoxy/TiO2 optical hybrid films with tunable refractive index prepared via a simple and efficient way Shaomin Dan, Huimin Gu, Jiaojun Tan, Baoliang Zhang, Qiuyu Zhang
T
⁎
Key Laboratory of Applied Physics and Chemistry in Space, Ministry of Education, Department of Applied Chemistry, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid material Epoxy resin TiO2 Optical film Refractive index
A simple and efficient method without any coupling/chelating agents was developed to prepare the transparent epoxy/TiO2 optical hybrid films with tunable refractive index. Instead of adding any coupling agents, the study found out that the lateral hydroxyl groups of epoxy resin E51 could serve as an intermediary to form organic–inorganic bonding between the organic matrix and the hydrolyzed titanium butoxide (TBOT). A series of homogeneous hybrid films were obtained without phase separation via adjusting the mole ratio of TBOT/hydroxyl groups, mass ratio of acid/water and the mixed solvent system. FTIR, XRD and TEM results proved the successful formation of nanoscale crystal-titania domains with the size of 3–5 nm and homogeneously dispersed in organic matrix. Furthermore, SEM and AFM results indicated the relatively good surface smoothness of the resultant optical hybrid films. More importantly, the refractive index of epoxy resin E51/TiO2 optical hybrid films were tunable in the range of 1.512–1.731 at 633 nm, which linearly increased with the content of TiO2 from 0 to 20 wt.%. These hybrid films with continuously tunable refractive index were expected to have great potential application in advanced optical devices.
1. Introduction High-refractive-index (high-n) materials with optical transparency are attracting increasing attention both in scientific and engineering aspects because of their wide application in advanced optoelectronic devices, such as light-emitting diode (LED) encapsulants [1], display devices [2,3], anti-reflective films [4], optical adhesives, holographic recording systems [5], complementary metal oxide semiconductor (CMOS) image sensors [6,7], optical waveguides [6] and so on. Especially, materials with refractive index exceeding 1.7 or even 2.0 are highly desired for LED encapsulants. As the extraction efficiency of LED almost doubles when the refractive index is increased from 1.5 to 2.0, although the typical n values of conventional encapsulants are in the range of 1.4–1.5, being limited by their chemical structures [8]. On the other hand, polymers showing tunable n values (n = 1.3–2.2) and with a thickness of few hundred nanometers are also frequently needed in anti-refractive (AR) coatings for liquid crystal display (LCD). And it is called polymeric refractive index-control technology [9]. By designing a multilayer film stack with a gradually varying refractive index for the LCD surface, the reflection loss on the surface can be reduced dramatically and the negative effects such as reflection of the light sources
and a double image can be effectively avoided [10–12]. However, the n values of most traditional polymer can hardly be continuously adjustable. Incorporating inorganic components with high n values into polymer substrates can overcome those defects and result in hybrid materials with n values that are pretty high and tunable over a wide range. What’s more, this kind of hybrid materials have been defined as an entirely new class of advanced materials because of their improved optoelectronic, thermal and mechanical properties than the corresponding individual organic or inorganic components [9,13,14]. During the past decade, many reports have emerged discussing the design, syntheses and applications of organic-inorganic hybrid materials processing high refractive index [15,16]. One of the most concerned materials are epoxy resins, which are widely applied in optical devices, such as optical adhesives, LED encapsulants and anti-reflective films as a result of their excellent mechanical properties, thermal stability and optical transparency [17,18]. While, the pure epoxy resins dissatisfy the urgent need of applications in the advanced optical devices because of their relative narrow range of available n values (1.4–1.6). Therefore, inorganic domains with high-n values such as titania are employed to improve the refractive index of epoxy resin. Mont et al. reported surfactant-coated-TiO2 nanoparticle loaded epoxy with
Abbreviations: LED, light-emitting diode; n, refractive index; CMOS, complementary metal oxide semiconductor; AR, antirefractive; LCD, liquid crystal display; TiO2, titania; TBOT, titanium butoxide; D-400, poly (propylene glycol) bis (2-aminopropyl ether); HCl, hydrochloric acid; TX, theoretical content (X wt.%) of titania ⁎ Corresponding author. E-mail address: [email protected] (Q. Zhang). https://doi.org/10.1016/j.porgcoat.2018.02.017 Received 1 March 2017; Received in revised form 19 January 2018; Accepted 20 February 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
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morphologies of the resulted films. Thermal behavior, optical transmittance and refractive index distribution of the resultant hybrid films were also revealed in this work.
the improved n value of 1.67 at the wavelength of 500 nm, which was markedly higher than that of the traditional epoxy resin (n = 1.53) [19]. Besides, Chau et al. successfully prepared the epoxy/TiO2 hybrid materials, of which the TiO2 nanoparticles were modified by acetic acid and the n values ranged from 2.18 to 2.38 [20]. In these works, surface modification of the nanoparticles was implemented to prevent aggregation and improve their compatibility with the organic phase. However, the pretreatment made the preparation tedious and changed the properties of the nanoparticles, such as a decrease in density and refractive index or an increase in the hydrophobic nature of the surface [21,22]. It is worth noting that the size of the inorganic domains with high-n values plays a vital role in the performance of the hybrid materials, which is required to be less than 40 nm (one-tenth of the wavelength of visible light) to avert scattering loss and maintain the optical transparency [23]. To control the size of titania domains, chelating agents or coupling agents were necessarily introduced to the system in many cases to produce strong interfacial force between organic and inorganic [24]. Unfortunately, the comprehensive performance such as mechanical, optical and thermal properties of the resultant hybrid materials might deteriorate due to the coupling or chelating agents [25,26]. In this study, a simple and efficient synthesis route without adding any coupling/chelating agents was developed to prepare the epoxy/ TiO2 hybrid films with good optical transparence and tunable refractive index. Inspired by Liou et. al [25], our group applied the lateral hydroxyl groups in epoxy resin E51 chains as an intermediary to provide organic–inorganic bonding with titanium butoxide instead of surface modification of titania nanoparticles or adding coupling/chelating agents to avoid phase separation of the epoxy/TiO2 optical hybrid films. As a result, highly homogeneous solution with different TiO2 concentration were successfully prepared and then thermal cured to transparent hybrid films as shown in Scheme 1. Then the structure of the resultant epoxy-TiO2 hybrid materials were investigated by FTIR and XRD. Moreover, SEM, AFM and TEM were employed to reveal the
2. Materials and methods 2.1. Materials Titanium butoxide (TBOT, 97%, Aladdin) was used as the precursor. Epoxy resin E51 resin (Wuxi Resin Factory of Blue Star New Chemical Materials) was used as the organic matrix. Poly (propylene glycol) bis (2-aminopropyl ether) (D-400, Aladdin) was used as curing agent. Benzyl alcohol purchased from Aladdin (99%) was used as accelerator. Aqueous hydrochloric acid (HCl, 37 wt.%) was purchased from J.T. Baker. n-Butanol (99.5%) purchased from Aldrich was used as the solvent. Ultrapure water was used throughout the work (Aquapro Co. Ltd.). 2.2. Preparation of Epoxy/TiO2 hybrid films The synthesis route to prepare the epoxy/TiO2 optical hybrid films is shown in Scheme 1 and the formation mechanism of crystal-titania domains as well as the hybrid materials are shown in Schemes 2 and 3, respectively. The hybrid films are prepared according to Chen’s work [10] with modification. According to Scheme 1, D-400 and a small amount of benzyl alcohol are chosen as the curing system of E51 (mass ratio:E51/D400/benzyl alcohol:100/64/4), which are added to the system before Ti-precursor. Amine hardener D-400 is a medium and low temperature curing agent. It takes 24 h to cure epoxy at room temperature and can also quickly solidify the system when heating at 40–60 °C. Meanwhile, the hydrolysis rate of TBOT is so fast that hydrolytic stabilizers such as acetic acid, hydrochloric acid and acetone are necessary to control the hydrolysis rate. Therefore, the addition of D-400 in advance does not make the epoxy matrix cured before the
Scheme 1. Reaction scheme for the preparation of epoxy/TiO2 optical hybrid films. 253
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Scheme 2. Formation procedure of TiO2 by sol-gel process.
corresponding properties of the obtained epoxy/TiO2 hybrid films are exhibited in Table 1. Particularly, TX indicates the theoretical content (X wt.%) of titania hydrolyzed by TBOT within the hybrid films. Take T20 as an instance to illustrate the procedure of the preparation of
inorganic precursors hydrolyzed sufficiently at room temperature. What’s more, adding D-400 before Ti-precursor avoids disturbing the hydrolysis stability created by hydrolytic stabilizers HCl because of the amino group in the structure of D400. The compositions and the
Scheme 3. Illustration of preparation process of epoxy/TiO2 hybrid films: (i) sol-gel reaction under catalyzed conditions, (ii) Spin-coated and precuring, (iii) thermosetting and obtaining hybrid film T20. 254
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the spectra of T10, T15 and T20, despite there are some overlap with the stretching vibrations of CeO at 1250 cm−1. All the information indicate that the hydrolyzed TiO2 particles are grafted to epoxy resin via covalent bonds. As for T5, the peak of TieOeC is almost invisible and the possible reason is the low content of TBOT or kinetic trapping of the small domain at low concentration. Fig. 2 shows the XRD spectra of the pure epoxy (T0), epoxy/TiO2 hybrid films (T5–T20) and TiO2 powder prepared by sol-gel method. A broad hump centered at 20° is observed in the XRD pattern of T0, which comes from the amorphous characteristic peak of epoxy resin E51. As the content of TiO2 rises, the intensity of a TiO2 broad peak (101) increases gradually in the range of 21–25°, while the amorphous peak of epoxy decreases accordingly. This result indicates that TiO2 domains are synthesized successfully through sol-gel process and are incorporated into epoxy resin by the solidification of the epoxy resin. What is more, the width of the TiO2 peaks in the hybrid films results from X-ray scattering generated from the petty size of the TiO2 nanocrystalline domains [10]. The morphologies of epoxy/TiO2 hybrid films are characterized by FE-SEM and the results are shown in Fig. 3. It is clear that the hybrid materials have good film-forming property and smoothing surface without any apparent TiO2 aggregations or obvious micro phase separation. Furthermore, the 3D, deflection and friction AFM images of T5 and T20 are depicted in Fig. 4. And the root mean square height (Sq) of all the hybrid films obtained from AFM are listed in Table 1, in which the Sq values of T0, T5, T10, T15 and T20 are 2.12, 1.81, 4.09, 4.79 and 4.97 nm, respectively. According to the results, the surface evenness of the hybrid materials tend to drop with the increase of TiO2 content in the system. The possible reason is that surface roughness of the film is affected by the content of solvent and stabilizer, which increases with the mass percentage of TBOT. As a result, there is high content of solvent and stabilizer in the network of the film with high Ti content. The solvent and stabilizer gradually evaporate during the process of epoxy curing, and some holes would form at the film surface, which could lead to a slight drop in the surface evenness of the films. Overall, the resulted Sq values are low, indicating that the films possess relatively smooth surface. In order to explore the detail morphology and microstructure further, TEM was applied to investigate the hybrid materials. Fig. 5 shows two typical bright-field images of T5 and T20, respectively. Moreover, the enlarged image of an individual TiO2 domain and the selected-area diffraction (SAED) pattern are also shown in Fig. 5(c) and (d), which demonstrate that the nanoparticles are successfully synthesized and oriented with anatae structure. What is more, the average size of the prepared nano-TiO2 is around 3–5 nm and the spatial distribution of T20 is denser than T5. More importantly, the nanocrystals embed uniformly in the polymer matrix which demonstrates that the hydroxyl groups of E51 could serve as active sites for TiO2 grafting onto and keep the morphology stability of the hybrid materials.
Table 1 Compositions of the recipes and properties of epoxy/TiO2 hybrid films. Sample(TX)
T0 T5 T10 T15 T20 a b c d
Reactant compositions
Properties of Hybrid films
Resin matrix (g)
TBOT (g)
TiO2 contenta (wt.%)
TiO2 contentb (wt.%)
Sqc/nm
Refractive Indexd
5.00 4.63 2.19 1.38 0.98
0 1.06 1.06 1.06 1.06
0 5 10 15 20
0 7.3 14.5 17.6 20.2
2.12 1.80 4.09 4.79 4.94
1.512 1.554 1.618 1.693 1.731
Theoretical content of TiO2 in the hybrid materials. Experimental content of TiO2 determined through TGA at 800 °C. The root mean square height determined by AFM. Measured at 633 nm.
epoxy/TiO2 hybrid films. Firstly, 0.98 g epoxy resin consisting primarily of E51 and D-400 (mass ratio:100/64) with a small amount of benzyl alcohol as a promoter and 3.5 ml acetone were added to 50 ml beaker and then stirred at room temperature for about 30 mins. Then 1.06 g TBOT being dissolved homogenously in 2.5 ml butanol were added dropwise to the mixed solution in the first step and stirred at room temperature for another 30 mins. After that, a well-mixed solvent with 0.14 g of HCl, 2.8 ml of butanol and 0.16 g of H2O were dropwise into the above mixture with a syringe and stirred to form the homogenous precursor solution of T20. Finally, the solution was spin coated on the cleaned silicon wafer with the speed of 1500 rpm for 18 s and then thermal cured at 40 °C for 40 mins and at 60 °C for 90 mins to obtain the final films. 2.3. Characterization Tensor 27 (FTIR) spectrometer (Bruker) was used to record Fourier transform infrared spectra of all hybrid films. The crystalline phase were identified by X-ray diffraction diffractometer (Shimadzu XRD–7000 s) with Cu Ka radiation (k = 1.542 Å) from 10° to 80°. The surface morphologies were imaged through a field emission scanning electron microscope (FE-SEM, ZEISS EVO 18) with an accelerating voltage of 15 kV and an atomic force microscope (AFM, Agilent 5100) with the contact mode. Transmission electron microscope (TEM, JEOL JEM-2010) with the accelerating voltage of 200 kV was used to characterize the detailed microstructure of the hybrid materials. Thermal properties and titania contents were obtained from thermogravimetric analysis (TGA, Q50, TA Instruments) with temperature ranging from 35 ∼ 800 °C and the heating rate of 10 °C min−1 under nitrogen atmosphere. Transmittance of epoxy/TiO2 hybrid films (thickness: 30 ± 3 μm) were measured by UV–vis spectrophotometer (Labtech 2.0). The refractive index (n) and the thickness (h) of the hybrid films were obtained from the ellipsometer (PZ2000) at the wavelength of 633 nm. Finally, tensile properties of the epoxy/TiO2 hybrid materials were conducted according to ASTM D-638.
3.2. Optical properties of epoxy/TiO2 hybrid films Fig. 6 shows the transmittance of the hybrid films, T0-T20. The transmission rate of the obtained hybrid films decreases at the wavelength of 450 nm compared with pure epoxy, which is attributed by the low band gap of TiO2 (3.2 eV) [27]. The transparence of the hybrid films reduces slightly as the TiO2 content increases from 5% to 20%, indicating that the phase size of the nanocrystalline-titania are well controlled and dispersed. The transmittance of all hybrid materials in the range of visible light is over 70%, which is acceptable in engineering applications. The cutoff wavelength of the hybrid films appear in the UV–vis spectra (above 300 nm) while absorbance band of the pure epoxy is below 300 nm. Furthermore, the corresponding band edge of the hybrid films also red shift as the TiO2 content increasing. Similar phenomena could also be observed when the size of TiO2 is less than 10 nm [28]. The refractive index of epoxy/TiO2 hybrid films at the wavelength
3. Results and discussion 3.1. Structure and morphology of epoxy/TiO2 hybrid films The interaction between epoxy resin and different contents of nanocrystalline-titania domains is investigated with FTIR,as shown in Fig. 1. Peaks located at around 927 cm−1 in Fig. 1 are attributed to the residual epoxy groups. Compared with T0, the strong absorption band at 610–630 cm−1 of all the hybrid films correspond to TieOeTi linkages, which prove that TiO2 are successfully obtained and incorporated into the organic matrix as the epoxy resin being cured. Moreover, peaks of TieOeC at 1233 cm−1 could be observed clearly in 255
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Fig. 1. FTIR spectra of the epoxy/TiO2 hybrid films.
of 633 nm with different TiO2 content is shown in Fig. 7. The n values increase almost linearly with the TiO2 contents from 0 to 20 wt.%, revealing that the TieOH groups of the hydrolyzed TBOT successfully transform into the TieOeTi structures and generate the epoxy/TiO2 hybrid films with optical transparence and tunable refractive index from another aspect.
3.3. Thermal properties of epoxy/TiO2 hybrid films TGA curves of epoxy matrix (T0) and the hybrid materials (T5–T20) are shown in Fig. 8. The incorporation of TiO2 causes a slight decrease (45 °C) in Td of hybrid materials compared with the epoxy matrix. The possible reason is that the metallic compounds may oxidize to degrade hybrid films according to Boggess and Taylor [29,30]. Overall, the thermal stability of the hybrid films is still good for the advanced optical application although it is inferior to pure epoxy. Furthermore, the actual residual weight of T0 and T100 are 5.9 and 95.3% at 800 °C, separately, which contain 0 and 100 wt.% TiO2, theoretically. According to the residual weight of T0 and T100, the real TiO2 contents of T5–T20 are estimated as 7.3–20.2 wt.% (the detail contents are shown in Table 1) under assumption of the linear relationship. The actual contents are in conformity with the theoretical ones, which demonstrate that the TBOT has hydrolyzed to TiO2 successfully and been incorporated into E51 in accordance with the original design. DSC curves of pure epoxy resin (T0) and the hybrid materials
Fig. 2. XRD spectra of pure epoxy, epoxy/TiO2 hybrid films and pure TiO2 (anatae).
Fig. 3. FE-SEM images of the epoxy/TiO2 hybrid films. (a) T5, (b) T20. (Coated on silicon wafers). 256
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Fig. 4. AFM images of the epoxy/TiO2 hybrid films coated on silicon wafers:.
Fig. 5. TEM images of the epoxy/TiO2 hybrid films (a) T5, (b) T20, (c) an individual TiO2 domain, (d) SAED pattern of TiO2 domains.
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Fig. 6. Optical transmission spectra of epoxy/TiO2 hybrid films (thickness: 30 ± 3 μm).
Fig. 9. DSC curves of the prepared films T0, T5, T10, T15 and T20.
Fig. 10. Tensile strength of the epoxy/TiO2 resin hybrid films T0, T5, T10, T15 and T20 as a function of strain.
Fig. 7. Effect of TiO2 content on the refractive index of epoxy/TiO2 hybrid films.
Table 2 Mechanical properties of epoxy/TiO2 hybrid films. Sample
Tensile Strength/ MPa
Elongation at break/%
Tensile yield stress/ MPa
T0 T5 T10 T15 T25
57.59 24.20 25.99 44.35 47.16
4.47 60.76 17.08 7.15 5.44
31.49 22.44 25.75 44.15 39.26
at 50 °C. These suggest that there are substantial amount of solvent/ unreacted functional groups trapped in the films, which will come out/ react during heating. These events will suppress the heat absorbance associated with the Tg during DSC measurement and make the Tg disappear. 3.4. Mechanical property of epoxy/TiO2 hybrid films Fig. 8. TGA curves of the prepared films T0, T5, T10, T15 and T20.
In order to explore mechanical property of the hybrid films, samples with different contents of nanoparticles are prepared and tested according to the standard of ASTM D-638. The results are shown in Fig. 10. and the specific strength values are listed in Table 2. It is observed that the elongation at break of T5 is 14 times higher than the pure epoxy system. Meanwhile, the tensile strength reduces from 57.59 MPa to 24.2 MPa when the loading of TiO2 increased from 0 to
(T5–T20) are shown in Fig. 9. It can be seen that the Tg is about 42 °C, which is in good agreement with the literature records[31]. However, there is not any obvious thermal transition in the range of −25–200 °C for T5–T20 reveled in the DSC curves. While according to Fig. 8, the TGA curves of T5-T20 start to lose weight right after the heating begin 258
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5%. What’s more, the tensile strength of T10, T15 and T20 tend to rise gradually, although the values are still lower than the pure epoxy. The elongation at break decreases in the meantime. It is known that strength of the interface in nanocomposites plays a significant role in the hybrid materials’ ability to transfer stresses and elastic deformation from the matrix resin to the nanoparticles [32]. Nanoparticles can dramatically improve the toughness of organic matrix when the content is within 5%. Because TiO2 can disperse well in the system and form strong interface strength, which could hinder and passivate the cracks produced by the external force and improve the toughness of the materials. However, further increase of TiO2 loading decreases the mechanical properties may possibly indicates poor interface as the result of high solvent. Overall, the mechanical property of the hybrid film are acceptable for the application of optical devices.
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4. Conclusions High refractive index epoxy/TiO2 optical hybrid films are successfully synthesized using the lateral hydroxyl groups of epoxy resin E51 as an intermediary to form organic–inorganic bonding with the hydrolyzed titanium butoxide (TBOT), which is a simple and efficient way without any coupling/chelating agents. The hybrid films are in good surface smoothness without any phase separation according to AFM images, which indicates that the hybrid materials are homogeneous in nano scale. The covalent bonds between the organic- inorganic phases play a significant role in controlling the size of the titania nanocrystals, which guarantees the hybrid films a good optical property. The refractive index of the hybrid films could increase lineally with TiO2 weight fraction and could reach as high as 1.731 with the TiO2 content of 20 wt.%. In summary, the resultant optical hybrid films with advantages of good thermal property, optical transparency and tunable refractive index possess a vast commercial application prospect in advanced optical devices. Acknowledgment This work was supported by the fund of National Natural Science Foundation of China (No. 51433008 and 51673156). References [1] F.W. Mont, J.K. Kim, M.F. Schubert, E.F. Schubert, R.W. Siegel, High-refractiveindex TiO2-nanoparticle-loaded encapsulants for light-emitting diodes, J. Appl. Phys. 103 (2008) (2365-2302). [2] K. Kitamura, K. Okada, N. Fujita, Y. Nagasaka, M. Ueda, Y. Sekimoto, Y. Kurata, Fabrication method of double-microlens array using self-alignment technology, Jpn. J. Appl. Phys. 43 (2004) 5840–5844. [3] T. Nakamura, H. Fujii, N. Juni, N. Tsutsumi, Enhanced coupling of light from organic electroluminescent device using diffusive particle dispersed high refractive index resin substrate, Opt. Rev. 13 (2006) 104–110. [4] K.C. Krogman, T. Druffel, M.K. Sunkara, Anti-reflective optical coatings incorporating nanoparticles, Nanotechnology 16 (2005) 338–343. [5] N. Suzuki, Y. Tomita, T. Kojima, Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films, Appl. Phys. Lett. 81 (2002) 4121–4123. [6] M. Suwa, H. Niwa, M. Tomikawa, High refractive index positive tone photo-sensitive coating, J. Photopolym. Sci. Technol. 19 (2006) 275–276. [7] J.G. Liu, M. Ueda, High refractive index polymers: fundamental research and
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