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Ex situ synthesis of high-refractive-index polyimide hybrid films containing TiO2 chelated by 4-aminobenzoic acid
European Polymer Journal 50 (2014) 54–60
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Macromolecular Nanotechnology
Ex situ synthesis of high-refractive-index polyimide hybrid films containing TiO2 chelated by 4-aminobenzoic acid Bo-Tau Liu a,⇑, Pei-Shan Li a, Wen-Chang Chen b,c, Yang-Yen Yu d a
Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan, ROC Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC c Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC d Department of Materials Engineering, Mingchi University of Technology, Taipei 24301, Taiwan, ROC
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b
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 22 October 2013 Accepted 27 October 2013 Available online 9 November 2013 Keywords: High refractive index TiO2 nanoparticles Polyimide 4-Aminobenzoic acid
a b s t r a c t In this study we used the ex situ synthesis method to fabricate high-refractive-index polyimide-TiO2 hybrid films at low temperature. We incorporated pre-made TiO2 nanoparticles (NPs) chelated with 4-aminobenzoic acid into a soluble polyimide synthesized from 2,2bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 3,30 ,4,40 -benzophenonetetracarboxylic dianhydride by one-step polymerization. The refractive index of the hybrid film with theoretic 90 wt% (experimental 79.04 wt%) TiO2 can reach as high as 1.941—close to that of the PI-TiO2 hybrid film fabricated at high temperature. Compared with epoxy-TiO2 hybrids, the polyimide-TiO2 hybrid films maintain their integrity without cracking in the thick-film condition and reveal extraordinarily high refractive indices. Infrared spectra and thermogravimetric analysis suggested that the enhanced properties in refractive indices and structure might result from the organic–inorganic bonding between 4-aminobenzoic-acidchelated TiO2 and polyimide featuring hydroxyl groups. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Transparent high-refractive-index materials play an important role in the optical applications such as antireflection coatings, optical waveguides, holographic recording systems, and optoelectronic devices [1–5]. In most cases, it is expected that the refractive indices of materials are as high as possible because such materials can provide superior performance characteristics. For example, when the refractive index of the encapsulant is increased from 1.5 to 2.0, the extraction efficiency of a light-emitting diode becomes double [6,7]; the brightness of brightness enhancement films for LCD backlight unit increases with the increase of the refractive index of the prism materials [8]. Due to the shortcomings of organic and inorganic materials in nature, polymer-inorganic hybrids have attracted attention as new high-refractive-index materials ⇑ Corresponding author. Tel.: +886 5 534 2601; fax: +886 5 531 2071. E-mail address: [email protected] (B.-T. Liu). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.10.028
because they can feature the advantages of both organic and inorganic materials and result in hybrids having refractive indices that are adjustable over a wide range. Recently, many reports have appeared describing the syntheses, characteristics, and applications of high-refractiveindex polymer-inorganic nanocomposites, including epoxy-TiO2 [9,10], poly(methyl methacrylate)-TiO2 [11– 13], polyimide (PI)-TiO2 [14–17], and others [7,18–22]. The main methods that have been used to fabricate polymer-inorganic hybrid materials are in situ synthesis and ex situ synthesis [23]. In situ synthesis represents one-step fabrication of the nanocomposites with in situ generating inorganic nanoparticles (NPs), whereas ex situ synthesis is to incorporate pre-made inorganic NPs into monomers and, subsequently, polymerize the monomers to form nanocomposites. The in situ synthesis method can result in very fine and well dispersed inorganic NPs within the polymer matrix. Nevertheless, the nanocomposites fabricated by the in situ synthesis method usually need to be cured at >300 °C to complete the sol–gel reaction for
B.-T. Liu et al. / European Polymer Journal 50 (2014) 54–60
2. Experimental 2.1. Synthesis of soluble BT-6FOH-PI BTDA (CHRISKEV) was treated by thermal anhydridization at 140 °C for 24 h or purified by recrystallization from acetic anhydride, qualified from DSC analysis (Fig. S1). No observable difference between thermal anhydridization and recrystallization purification in the following synthesis was found. 6FOH diamine (CHRISKEV) was degased at
110 °C for 24 h. m-Cresol was distilled under vacuum over calcium hydride and stored over 3 Å molecular sieves. The soluble PI was synthesized by one-step high temperature polycondensation as reported elsewhere [28,29]. Briefly, the mixture of BTDA and 6FOH amine in a molar ratio of 1:1 was added into m-cresol (resulting in 19% solid content) under nitrogen purge. Isoquinoline as a catalyst was added in the solution and then reacted at boiling temperature under reflux for 6 h. The resulting sticky solution was precipitated with methanol and then dried at 110 °C, resulting in the soluble BT-6FOH-PI. 2.2. Preparation of PI-TiO2 hybrid films TiO2 NPs chelated by 4ABA (Alfa Aesar) were synthesized as described in our previous report [26]. The as-prepared titania sol was washed with DI water through centrifugation (57,430g, 70 min), decanting the supernatant, and redispersing the precipitate. Because the titania sol could be well dispersed in high polar solvents but not in non-polar solvents (Table S1), the final wet precipitate was redispersed in DMAc (a polar aprotic solvent) through ultrasonication for 1 h. Various amounts of soluble BT6FOH-PI were added into the titania sol solution, as shown in Table 1. The mixture was filtered through a 0.22-lm filter to remove the large aggregates. Silicon wafer and glass substrates were cleaned using an O2 plasma (PDC-23G, Harrick Plasma; 18 W, 20 min). The cleaned substrates were dipped in the various mixtures for 10 min and then raised from the dipping bath at a rate of 50 mm/min. The coated substrates were placed in an oven at 60 °C for 10 min and then in a vacuum oven at 180 °C to remove DMAc thoroughly. For thick-film preparation, the process is similar to the above description except that the mixture of BT-6FOH-PI and titania sol was poured onto a Petri dish (the film thickness is about 100–200 lm). 2.3. Measurements The chemical structures of the soluble BT-6FOH-PI and PI-TiO2 hybrid films were examined using a Fourier transform infrared (FTIR) spectrophotometer (spectrum one, PerkinElmer). The morphologies of the TiO2 NPs chelated by 4ABA and the PI-TiO2 hybrid films were examined using a high-resolution transmission electron microscope (JEM-2010, JEOL) and a field-emission scanning electron microscope (JSM-6700F, JEOL). For TEM observation, BT6FOH-PI/titania sol solution was dropped onto a copper grid, and the coated grid was placed in an oven at 60 °C for 10 min and then in a vacuum oven at 180 °C to remove solvent thoroughly. The solid content was determined using a thermogravimetric analyzer (TGA 2050, TA Instruments) operated at a heating rate of 10 °C/min under a N2 or air flow. The transmittance of the hybrid films was measured at four points on each sample using a UV–Vis spectrophotometer (Lambda 850, PerkinElmer). The samples were qualified by controlling the transmittance at the four points to within an error of 5%. The refractive indices of the hybrids were determined over the wavelength range from 300 to 800 nm using an ellipsometer (GES-5E, SOPRA). The
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inorganic NPs [15,23]. Most organic polymers cannot withstand so high temperature that only a few materials can be used as the substrate or the polymer matrix. On the contrary, ex situ synthesis can be performed at low film-forming temperatures because the inorganic NPs have been fabricated beforehand [24]. However, the large amounts of chelating agents must be used to cover the surface of NPs in order to remain the stabilization and avoid the aggregation. Chelating agents may reduce not only the maximum attainable refractive indices but also the mechanical properties of the polymer-inorganic nanocomposites [25]. In the previous study, we found that 4-aminobenzoic acid (4ABA) featuring an amino group and a carboxylic acid could be a better chelating ligand to synthesize the anatase TiO2 NPs than acetic acid for the high-refractive-index epoxy-TiO2 nanocomposites prepared by the ex situ synthesis [26]. This is because of the facts that 4ABA features a higher refractive index and less demand due to the higher coordinating stability to titanium alkoxides. Sue et al. reported the carboxylic acid as an end-cap on the polyimide backbones can make it possible to prepare the homogeneous PI-TiO2 films with very high TiO2 content (99 wt%) and refractive index (1.99) by in situ synthesis [15]. Liou et al. further found that the PI-TiO2 nanocomposites featuring both high TiO2 content and thick thickness can be prepared by the in situ synthesis method if the polyimide chains possess lateral hydroxyl groups, providing the organic–inorganic bonding with titanium alkoxide [27]. Either ex situ or in situ syntheses, the chelating stability of inorganic NPs plays a key role in the polymer-inorganic hybrid system. In this study, combining the advantages of the ex situ synthesis and the stabilization of polymer matrix to inorganic particles, we synthesized a soluble polyimide with hydroxyl group derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FOH diamine) and 3,30 ,4,40 benzophenonetetracarboxylic dianhydride (BTDA) by virtue of the one-step polymerization and then incorporated pre-made TiO2 NPs chelating with 4ABA into the polyimide matrix to fabricate high-refractive-index hybrid films. Characteristics of the hybrid films and the mechanism of enhancement on refractive index via the interaction between TiO2 NPs and PI matrix were investigated. The performances of such hybrid films prepared at low temperature (ex situ synthesis) were found to reach as well as those prepared at high temperature (in situ synthesis), featuring the very high refractive index and the capacity to make thick films.
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Table 1 Compositions and properties of the PI-TiO2 hybrid films.
a b c d
Code
Theoretical titania content (wt%)
Experimental titania contenta (wt%)
Tdb (°C)
Tfc (°C)
Refractive indexd
BT-6FOH-PI PI30 PI50 PI70 PI80 PI90
– 30 50 70 80 90
– 26.63 47.41 65.53 73.13 79.04
499 453 443 439 410 398
688 561 552 530 508 499
1.596 1.661 1.778 1.858 1.896 1.941
Determined through TGA analysis at 750 °C, shown in Fig. S3. Temperature at which 10% weight loss occurs. Temperature at which weight loss stops increasing. Measured at 633 nm.
samples for ellipsometric analysis were prepared by coating specimens on silicon wafers.
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3. Results and discussion Fig. 1 shows the SEM images of PI30, PI50, PI70, and PI90. Under low-magnitude observation (10k), regardless of low or high TiO2 loading, the films exhibit uniform surfaces without any evident titania aggregates or macro-structural separation. However, observing from the high-magnitude images (100k), the film surfaces are nano-porous when the TiO2 loading is high. We suppose that the nano-pores are formed when the polymer–matrix content reduces to insufficiently fill the voids among the TiO2 NPs. The nano-pores may result in that the refractive indices of the nanocomposites with high nanoparticle content are lower than the theoretical values. This may explain why the refractive indices of the polymer-inor-
ganic hybrid films are always lower than theoretical values [30]. The TEM image of the hybrid film shown in Fig. 2 reveals the TiO2 was well dispersed in the PI matrix. Observing from the high-resolution TEM image, the TiO2 NPs chelated by 4ABA were of pure anatase phase and their crystallite size was about 5 nm, being consistent with the calculation from the peak of (1 0 1) reflection using Sherrer’s equation (Fig. S2). In our previous experiment [26], the titania colloids coordinated by both amino and carboxyl groups of 4ABA can be also well dispersed in the epoxy matrix synthesized from diglycidyl ether of bisphenol A and methyl hexahydro phthalic anhydride. However, when the film became thick, the epoxy-TiO2 hybrid film was broken down to small debris, whereas BT-6FOH-PITiO2 maintained the integrity of the films (Fig. 3). We infer that the cracking may arise from no affinity of polymer chain to TiO2 NPs [31]. On the contrary, the soluble BT6FOH-PI, possessing the lateral carbonyl and hydroxyl
Fig. 1. SEM images of (a) PI30, (b) PI50, (c) PI70, and (d) PI90. Insert: 100k images.
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Fig. 2. TEM image of PI90.
groups (see the reaction scheme shown in Scheme S1), may interact with the titania colloids to resist the cracking (explained later in the text). Fig. 4 presents the FTIR spectra of BT-6FOH-PI and PI50 films; both exhibit typical imide characteristics: an absorption peak at 743 cm1 for imide ring deformation, a peak at 1380 cm1 for CAN stretching vibration, and a pair of peaks at 1718 and 1782 cm1 ascribed to the symmetric and asymmetric stretching vibrations of COO unit, respectively [14]. An absorption peak neat 3400 cm1 for OAH stretching vibration and a peak near 1633 cm1 for HAOAH bending vibration [32], and a peak at 1251 cm1 for CAF stretching vibration are also shown in the spectra, arising from the molecular structure of 6FOH diamine [27]. The FTIR spectrum of BT-6FOH-PI also shows a free OAH stretching band at 3789 cm1 [33], indicating some OAH groups of BT-6FOH-PI do not participate in the inter-/intra-hydrogen bonding. The broad and strong adsorption
band near 600–700 cm1 in the spectrum of the PI50 films corresponds to Ti–O stretching vibration. The refractive indices of the PI-TiO2 hybrid films were determined using best-fit curves for the two spectroscopic ellipsometric parameters W and D. The dispersion law type and term are the standard dielectric function and the modified cauchy, respectively. In the experiments, all the regression quantitative indices R2 were greater than 0.997 and the film thickness is in the range of 60– 100 nm. Fig. 5a reveals the variations in refractive indices of the PI-TiO2 hybrid films as a function of the incident wavelength; the variation of the refractive index at 633 nm with respect to various TiO2 NP content is shown in Fig. 5b and Table 1. We observe a typical peak at 300– 350 nm for anatase TiO2 in each of these curves. The refractive index of the hybrid film increases monotonically upon increasing the TiO2 NP content. As mentioned before, an ex situ synthesis usually lead to large residues of chelating agents on the surface of TiO2 NPs, resulting in the decrease of the refractive indices of the hybrid films. However, we have exhibited that 4ABA was better than carboxylic acid
Fig. 3. Photograph images of (a) the epoxy-TiO2 hybrid films with 30-wt% TiO2 NP and (b) PI30.
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Fig. 4. FTIR spectra of BT-6FOH-PI and PI50 with and without thermal treatment at 425 °C for 1 h in N2.
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Fig. 6. TGA curves of BT-6FOH-PI and PI50 films in nitrogen.
Fig. 5. Refractive indices of the PI-TiO2 hybrid films incorporating various TiO2 NP contents (a) as a function of incident wavelength and (b) plotted with respect to the TiO2 NP content at 633 nm.
as a chelating agent due to the fact that it featured a higher refractive index and higher coordinating stability [26]. The refractive index of the BT-6FOH-PI-TiO2 with theoretic 90 wt% (experimental 79.04 wt%) TiO2 prepared by the ex situ synthesis can reach as high as 1.941––close to that of the PI-TiO2 hybrid film fabricated by in situ synthesis at high temperature ([email protected] wt%) [15]. Fig. 5b reveals that the refractive indices of the PI-TiO2 hybrid films are higher than the epoxy-TiO2 ones; the difference increases upon increasing the TiO2 NP content. Although pure PI features a higher refractive index than epoxy resin, the difference in the refractive index between the two polymer-TiO2 nanocomposites should decrease in theory with the increase of the TiO2 NP content. The contradiction implies that there is certain synergetic effect for the PI-TiO2 nanocomposites to increase their refractive index compared with the epoxy-TiO2 hybrids. To more clearly comprehend the interaction between BT-6FOH-PI and TiO2 NPs, we recorded TGA curves of BT-6FOH-PI and PI60 under a nitrogen atmosphere, shown in Fig. 6. Compare with the results testing in air (Fig. S3), the BT-6FOH-PI sample tested in N2 presents a remarkable difference in the shape of the TGA curve. A neat 13% weight loss of BT-6FOH-PI in N2 at 516 °C indicates that two CO2 were released from one PI repeat unit due to the reaction of the imide cycle with
the hydroxyl group to be rearranged to a benzoxazole (Scheme S2) [29]. The FTIR spectrum of the BT-6FOH-PI sample thermally treated at 425 °C for 1 h in N2 (Fig. 4) also confirms the existence of benzoxazoles: a typical band of benzoxazoles at 1556 cm1 [34] and an adsorption peak of C@N at 1667 cm1; the disappearance of the free OAH may result from the formation of benzoxazoles. However, the initial weight loss is not found in the TGA analysis of the PI-TiO2 nanocomposites tested in N2 (Fig. 6) and the benzoxazole characteristic bands are also not found in the corresponding FTIR spectrum (Fig. 4). The suppression of benzoxazole formation may arise from the organic-inorganic bonding during the in situ PI-TiO2 synthesis [27]. As a result, we infer such bonding also exist between BT-6FOHPI and 4ABA-chelated TiO2 in the ex situ preparation, resulting in the resistance to cracking for the thick films. The refractive index nm of a polymer can be evaluated using the Lorentz–Lorenz equation [14]:
n2m 1 4p a ¼ n2m þ 2 3 V mol
ð1Þ
where a and Vmol are the mean polarizability and the molecular volume, respectively. Moreover, the effective refractive index of a well-mixed polymer–inorganic nanocomposites ne can be expressed as
ne ¼ /p np þ /f nf þ ð1 /p /f Þnm
ð2Þ
where np, nf, and nm are the refractive indices of NPs, free space, and polymer matrix, respectively. /p and /f are the volume fraction of NPs and free space, respectively. Based on Eqs. (1) and (2), when the polymer and NPs become compacter, either the decrease of Vmol or /f , the refractive index of the nanocomposites increases. As a result, we infer that the chemical bonding between BT6FOH-PI and TiO2 NPs will result in the increase of refractive index. According to Fresnel’s theory for a single-layer model [35], the reflectance increases with the increase of the refractive index of the coating layer. Fig. 7a shows the reflection spectra of the PI-TiO2 hybrid films. In contrast to blank glass, the hybrid films exhibit higher reflectance and their values increase with the increase of the TiO2 NP
B.-T. Liu et al. / European Polymer Journal 50 (2014) 54–60
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Fig. 8. Photograph image of PI90 coated on glass.
Fig. 7. (a) Reflectance and (b) transmittance spectra of the PI-TiO2 hybrid films.
content, implying the increase of refractive index, confirming with the measuring results from the ellipsometry. Fig. 7b shows the transmittance spectra of the PI-TiO2 hybrid films. The cutoff in the UV region was attributed to the adsorption of titania. There is trade-off between reflectance and transmittance. The higher reflectance (or the higher refractive index), the lower transmittance. As a result, the higher TiO2-loading film shows little lower transmittance than the lower TiO2-loading one due to higher refractive index and consequently higher reflection. As is known to all, particle aggregation is easy to occur at the polymer-nanoparticle system, resulting in the light-scattering drawback. In our experiments, the haze values of the PI-TiO2 hybrid films were lower than 0.1, which indicated lighter scattering could be ignored. Fig. 8 shows our sample reveals high quality in appearance, also implying that no manifest light scattering was found. Although PI can be been hydrolyzed under alkaline solution due to imide structure, we found that the PI-TiO2 hybrid films are stable for several months or years under atmospheric environment. Moreover, the feature of the alkaline hydrolysis may be a benefit for recycle. It should be mentioned that the study is the first time to report the preparation of PI-TiO2 nanocomposites at the low film-forming temperature, featuring the very high refractive index and the capacity to form thick films, which are very close to the characteristics of those fabricated at high temperature.
We have synthesized a soluble polyimide with hydroxyl group derived from 6FOH diamine and BTDA by virtue of the one-step polymerization. The high-refractive-index PI-TiO2 hybrid films were prepared incorporating premade TiO2 NPs chelating with 4ABA into the polyimide matrix at low temperature. The refractive index of the PITiO2 hybrid films can reach as high as 1.941—close to that of the PI-TiO2 hybrid film fabricated by in situ synthesis at high temperature and exhibit extraordinarily higher than that of the epoxy-TiO2 hybrid films. The PI-TiO2 hybrid films also reveal a better resistance to cracking in the thick-film condition than the epoxy-TiO2 films. We suspect that the enhanced performances in refractive indices and structure may result from the organic–inorganic chemical bonding between 4ABA-chelated TiO2 and BT-6FOH-PI. Acknowledgments This work was financially supported by the National Science Council of the Republic of China and the Ministry of Economic Affairs of Taiwan.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2013.10.028. References [1] Cui ZC, Lu CL, Yang B, Shen JC, Su XP, Yang H. The research on syntheses and properties of novel epoxy/polymercaptan curing optical resins with high refractive indices. Polymer 2001;42:10095–100. [2] Liu B-T, Yeh W-D, Wang W-H. Preparation of low refractive index fluorinated materials for antireflection coatings. J Appl Polym Sci 2010;118:1615–9. [3] Okutsu R, Ando S, Ueda M. Sulfur-containing poly(meth)acrylates with high refractive indices and high Abbe’s numbers. Chem Mater 2008;20:4017–23. [4] Wu S, Zhou G, Gu M. Synthesis of high refractive index composites for photonic applications. Opt Mater 2007;29:1793–7.
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4. Conclusions
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[5] Zhang GY, Zhang JB, Yang B. Design and fabrication of high refractive index polymer-inorganic optical hybrid materials and their practical application. Acta Polym Sinica 2013:589–99. [6] Mont FW, Kim JK, Schubert MF, Schubert EF, Siegel RW. Highrefractive-index TiO2-nanoparticle-loaded encapsulants for lightemitting diodes. J Appl Phys 2008;103:083120. [7] Yang XF, Shao Q, Yang LL, Zhu XB, Hua XL, Zheng QL, et al. Preparation and performance of high refractive index silicone resintype materials for the packaging of light-emitting diodes. J Appl Polym Sci 2013;127:1717–24. [8] Wang MW, Tseng CC. Analysis and fabrication of a prism film with roll–to–roll fabrication process. Opt Express 2009;17: 4718–25. [9] Guan C, Lü C-L, Liu Y-F, Yang B. Preparation and characterization of high refractive index thin films of TiO2/epoxy resin nanocomposites. J Appl Polym Sci 2006;102:1631–6. [10] Nguyen DH, Kim HN, Lee DS. Transparent nanocomposites of high refractive index based on epoxy resin and TiO2 nanoparticle. J Nanosci Nanotechnol 2012;12:4207–10. [11] Lee LH, Chen WC. High refractive-index thin films prepared from trialkoxysilane-capped poly(methyl methacrylate)-titania materials. Chem Mater 2001;13:1137–42. [12] Yuwono AH, Liu BH, Xue JM, Wang J, Elim HI, Ji W, et al. Controlling the crystallinity and nonlinear optical properties of transparent TiO2-PMMA nanohybrids. J Mater Chem 2004;14:2978–87. [13] Chen WC, Lee SJ, Lee LH, Lin JL. Synthesis and characterization of trialkoxysilane-capped poly(methyl methacrylate)-titania hybrid optical thin films. J Mater Chem 1999;9:2999–3003. [14] Liu JG, Nakamura Y, Ogura T, Shibasaki Y, Ando S, Ueda M. Optically transparent sulfur-containing polyimide-TiO2 nanocomposite films with high refractive index and negative pattern formation from poly(amic acid)-TiO(2) nanocomposite film. Chem Mater 2008;20:273–81. [15] Su HW, Chen WC. High refractive index polyimide-nanocrystallinetitania hybrid optical materials. J Mater Chem 2008;18:1139–45. [16] Liou GS, Lin PH, Yen HJ, Yu YY, Chen WC. Flexible nanocrystallinetitania/polyimide hybrids with high refractive index and excellent thermal dimensional stability. J Polym Sci, Part A: Polym Chem 2010;48:1433–40. [17] Huang XH, Huang W, Fu LC, Yan DY. Synthesis and characterization of thioether-containing polyimides with high refractive indices. J Polym Res 2012:19. [18] Nakayama N, Hayashi T. Preparation and characterization of TiO2 and polymer nanocomposite films with high refractive index. J Appl Polym Sci 2007;105:3662–72. [19] Nakayama N, Hayashi T. Preparation and characterization of TiO2– ZrO2 and thiol-acrylate resin nanocomposites with high refractive index via UV-induced crosslinking polymerization. Composites Part A 2007;38:1996–2004. [20] Liu Y, Lü C, Li M, Zhang L, Yang B. High refractive index organic– inorganic hybrid coatings with TiO2 nanocrystals. Colloids Surf, A 2008;328:67–72.
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Ex situ synthesis of high-refractive-index polyimide hybrid films containing TiO2 chelated by 4-aminobenzoic acid Bo-Tau Liu a,⇑, Pei-Shan Li a, Wen-Chang Chen b,c, Yang-Yen Yu d a
Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan, ROC Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC c Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC d Department of Materials Engineering, Mingchi University of Technology, Taipei 24301, Taiwan, ROC
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b
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 22 October 2013 Accepted 27 October 2013 Available online 9 November 2013 Keywords: High refractive index TiO2 nanoparticles Polyimide 4-Aminobenzoic acid
a b s t r a c t In this study we used the ex situ synthesis method to fabricate high-refractive-index polyimide-TiO2 hybrid films at low temperature. We incorporated pre-made TiO2 nanoparticles (NPs) chelated with 4-aminobenzoic acid into a soluble polyimide synthesized from 2,2bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 3,30 ,4,40 -benzophenonetetracarboxylic dianhydride by one-step polymerization. The refractive index of the hybrid film with theoretic 90 wt% (experimental 79.04 wt%) TiO2 can reach as high as 1.941—close to that of the PI-TiO2 hybrid film fabricated at high temperature. Compared with epoxy-TiO2 hybrids, the polyimide-TiO2 hybrid films maintain their integrity without cracking in the thick-film condition and reveal extraordinarily high refractive indices. Infrared spectra and thermogravimetric analysis suggested that the enhanced properties in refractive indices and structure might result from the organic–inorganic bonding between 4-aminobenzoic-acidchelated TiO2 and polyimide featuring hydroxyl groups. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Transparent high-refractive-index materials play an important role in the optical applications such as antireflection coatings, optical waveguides, holographic recording systems, and optoelectronic devices [1–5]. In most cases, it is expected that the refractive indices of materials are as high as possible because such materials can provide superior performance characteristics. For example, when the refractive index of the encapsulant is increased from 1.5 to 2.0, the extraction efficiency of a light-emitting diode becomes double [6,7]; the brightness of brightness enhancement films for LCD backlight unit increases with the increase of the refractive index of the prism materials [8]. Due to the shortcomings of organic and inorganic materials in nature, polymer-inorganic hybrids have attracted attention as new high-refractive-index materials ⇑ Corresponding author. Tel.: +886 5 534 2601; fax: +886 5 531 2071. E-mail address: [email protected] (B.-T. Liu). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.10.028
because they can feature the advantages of both organic and inorganic materials and result in hybrids having refractive indices that are adjustable over a wide range. Recently, many reports have appeared describing the syntheses, characteristics, and applications of high-refractiveindex polymer-inorganic nanocomposites, including epoxy-TiO2 [9,10], poly(methyl methacrylate)-TiO2 [11– 13], polyimide (PI)-TiO2 [14–17], and others [7,18–22]. The main methods that have been used to fabricate polymer-inorganic hybrid materials are in situ synthesis and ex situ synthesis [23]. In situ synthesis represents one-step fabrication of the nanocomposites with in situ generating inorganic nanoparticles (NPs), whereas ex situ synthesis is to incorporate pre-made inorganic NPs into monomers and, subsequently, polymerize the monomers to form nanocomposites. The in situ synthesis method can result in very fine and well dispersed inorganic NPs within the polymer matrix. Nevertheless, the nanocomposites fabricated by the in situ synthesis method usually need to be cured at >300 °C to complete the sol–gel reaction for
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2. Experimental 2.1. Synthesis of soluble BT-6FOH-PI BTDA (CHRISKEV) was treated by thermal anhydridization at 140 °C for 24 h or purified by recrystallization from acetic anhydride, qualified from DSC analysis (Fig. S1). No observable difference between thermal anhydridization and recrystallization purification in the following synthesis was found. 6FOH diamine (CHRISKEV) was degased at
110 °C for 24 h. m-Cresol was distilled under vacuum over calcium hydride and stored over 3 Å molecular sieves. The soluble PI was synthesized by one-step high temperature polycondensation as reported elsewhere [28,29]. Briefly, the mixture of BTDA and 6FOH amine in a molar ratio of 1:1 was added into m-cresol (resulting in 19% solid content) under nitrogen purge. Isoquinoline as a catalyst was added in the solution and then reacted at boiling temperature under reflux for 6 h. The resulting sticky solution was precipitated with methanol and then dried at 110 °C, resulting in the soluble BT-6FOH-PI. 2.2. Preparation of PI-TiO2 hybrid films TiO2 NPs chelated by 4ABA (Alfa Aesar) were synthesized as described in our previous report [26]. The as-prepared titania sol was washed with DI water through centrifugation (57,430g, 70 min), decanting the supernatant, and redispersing the precipitate. Because the titania sol could be well dispersed in high polar solvents but not in non-polar solvents (Table S1), the final wet precipitate was redispersed in DMAc (a polar aprotic solvent) through ultrasonication for 1 h. Various amounts of soluble BT6FOH-PI were added into the titania sol solution, as shown in Table 1. The mixture was filtered through a 0.22-lm filter to remove the large aggregates. Silicon wafer and glass substrates were cleaned using an O2 plasma (PDC-23G, Harrick Plasma; 18 W, 20 min). The cleaned substrates were dipped in the various mixtures for 10 min and then raised from the dipping bath at a rate of 50 mm/min. The coated substrates were placed in an oven at 60 °C for 10 min and then in a vacuum oven at 180 °C to remove DMAc thoroughly. For thick-film preparation, the process is similar to the above description except that the mixture of BT-6FOH-PI and titania sol was poured onto a Petri dish (the film thickness is about 100–200 lm). 2.3. Measurements The chemical structures of the soluble BT-6FOH-PI and PI-TiO2 hybrid films were examined using a Fourier transform infrared (FTIR) spectrophotometer (spectrum one, PerkinElmer). The morphologies of the TiO2 NPs chelated by 4ABA and the PI-TiO2 hybrid films were examined using a high-resolution transmission electron microscope (JEM-2010, JEOL) and a field-emission scanning electron microscope (JSM-6700F, JEOL). For TEM observation, BT6FOH-PI/titania sol solution was dropped onto a copper grid, and the coated grid was placed in an oven at 60 °C for 10 min and then in a vacuum oven at 180 °C to remove solvent thoroughly. The solid content was determined using a thermogravimetric analyzer (TGA 2050, TA Instruments) operated at a heating rate of 10 °C/min under a N2 or air flow. The transmittance of the hybrid films was measured at four points on each sample using a UV–Vis spectrophotometer (Lambda 850, PerkinElmer). The samples were qualified by controlling the transmittance at the four points to within an error of 5%. The refractive indices of the hybrids were determined over the wavelength range from 300 to 800 nm using an ellipsometer (GES-5E, SOPRA). The
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inorganic NPs [15,23]. Most organic polymers cannot withstand so high temperature that only a few materials can be used as the substrate or the polymer matrix. On the contrary, ex situ synthesis can be performed at low film-forming temperatures because the inorganic NPs have been fabricated beforehand [24]. However, the large amounts of chelating agents must be used to cover the surface of NPs in order to remain the stabilization and avoid the aggregation. Chelating agents may reduce not only the maximum attainable refractive indices but also the mechanical properties of the polymer-inorganic nanocomposites [25]. In the previous study, we found that 4-aminobenzoic acid (4ABA) featuring an amino group and a carboxylic acid could be a better chelating ligand to synthesize the anatase TiO2 NPs than acetic acid for the high-refractive-index epoxy-TiO2 nanocomposites prepared by the ex situ synthesis [26]. This is because of the facts that 4ABA features a higher refractive index and less demand due to the higher coordinating stability to titanium alkoxides. Sue et al. reported the carboxylic acid as an end-cap on the polyimide backbones can make it possible to prepare the homogeneous PI-TiO2 films with very high TiO2 content (99 wt%) and refractive index (1.99) by in situ synthesis [15]. Liou et al. further found that the PI-TiO2 nanocomposites featuring both high TiO2 content and thick thickness can be prepared by the in situ synthesis method if the polyimide chains possess lateral hydroxyl groups, providing the organic–inorganic bonding with titanium alkoxide [27]. Either ex situ or in situ syntheses, the chelating stability of inorganic NPs plays a key role in the polymer-inorganic hybrid system. In this study, combining the advantages of the ex situ synthesis and the stabilization of polymer matrix to inorganic particles, we synthesized a soluble polyimide with hydroxyl group derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FOH diamine) and 3,30 ,4,40 benzophenonetetracarboxylic dianhydride (BTDA) by virtue of the one-step polymerization and then incorporated pre-made TiO2 NPs chelating with 4ABA into the polyimide matrix to fabricate high-refractive-index hybrid films. Characteristics of the hybrid films and the mechanism of enhancement on refractive index via the interaction between TiO2 NPs and PI matrix were investigated. The performances of such hybrid films prepared at low temperature (ex situ synthesis) were found to reach as well as those prepared at high temperature (in situ synthesis), featuring the very high refractive index and the capacity to make thick films.
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Table 1 Compositions and properties of the PI-TiO2 hybrid films.
a b c d
Code
Theoretical titania content (wt%)
Experimental titania contenta (wt%)
Tdb (°C)
Tfc (°C)
Refractive indexd
BT-6FOH-PI PI30 PI50 PI70 PI80 PI90
– 30 50 70 80 90
– 26.63 47.41 65.53 73.13 79.04
499 453 443 439 410 398
688 561 552 530 508 499
1.596 1.661 1.778 1.858 1.896 1.941
Determined through TGA analysis at 750 °C, shown in Fig. S3. Temperature at which 10% weight loss occurs. Temperature at which weight loss stops increasing. Measured at 633 nm.
samples for ellipsometric analysis were prepared by coating specimens on silicon wafers.
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3. Results and discussion Fig. 1 shows the SEM images of PI30, PI50, PI70, and PI90. Under low-magnitude observation (10k), regardless of low or high TiO2 loading, the films exhibit uniform surfaces without any evident titania aggregates or macro-structural separation. However, observing from the high-magnitude images (100k), the film surfaces are nano-porous when the TiO2 loading is high. We suppose that the nano-pores are formed when the polymer–matrix content reduces to insufficiently fill the voids among the TiO2 NPs. The nano-pores may result in that the refractive indices of the nanocomposites with high nanoparticle content are lower than the theoretical values. This may explain why the refractive indices of the polymer-inor-
ganic hybrid films are always lower than theoretical values [30]. The TEM image of the hybrid film shown in Fig. 2 reveals the TiO2 was well dispersed in the PI matrix. Observing from the high-resolution TEM image, the TiO2 NPs chelated by 4ABA were of pure anatase phase and their crystallite size was about 5 nm, being consistent with the calculation from the peak of (1 0 1) reflection using Sherrer’s equation (Fig. S2). In our previous experiment [26], the titania colloids coordinated by both amino and carboxyl groups of 4ABA can be also well dispersed in the epoxy matrix synthesized from diglycidyl ether of bisphenol A and methyl hexahydro phthalic anhydride. However, when the film became thick, the epoxy-TiO2 hybrid film was broken down to small debris, whereas BT-6FOH-PITiO2 maintained the integrity of the films (Fig. 3). We infer that the cracking may arise from no affinity of polymer chain to TiO2 NPs [31]. On the contrary, the soluble BT6FOH-PI, possessing the lateral carbonyl and hydroxyl
Fig. 1. SEM images of (a) PI30, (b) PI50, (c) PI70, and (d) PI90. Insert: 100k images.
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Fig. 2. TEM image of PI90.
groups (see the reaction scheme shown in Scheme S1), may interact with the titania colloids to resist the cracking (explained later in the text). Fig. 4 presents the FTIR spectra of BT-6FOH-PI and PI50 films; both exhibit typical imide characteristics: an absorption peak at 743 cm1 for imide ring deformation, a peak at 1380 cm1 for CAN stretching vibration, and a pair of peaks at 1718 and 1782 cm1 ascribed to the symmetric and asymmetric stretching vibrations of COO unit, respectively [14]. An absorption peak neat 3400 cm1 for OAH stretching vibration and a peak near 1633 cm1 for HAOAH bending vibration [32], and a peak at 1251 cm1 for CAF stretching vibration are also shown in the spectra, arising from the molecular structure of 6FOH diamine [27]. The FTIR spectrum of BT-6FOH-PI also shows a free OAH stretching band at 3789 cm1 [33], indicating some OAH groups of BT-6FOH-PI do not participate in the inter-/intra-hydrogen bonding. The broad and strong adsorption
band near 600–700 cm1 in the spectrum of the PI50 films corresponds to Ti–O stretching vibration. The refractive indices of the PI-TiO2 hybrid films were determined using best-fit curves for the two spectroscopic ellipsometric parameters W and D. The dispersion law type and term are the standard dielectric function and the modified cauchy, respectively. In the experiments, all the regression quantitative indices R2 were greater than 0.997 and the film thickness is in the range of 60– 100 nm. Fig. 5a reveals the variations in refractive indices of the PI-TiO2 hybrid films as a function of the incident wavelength; the variation of the refractive index at 633 nm with respect to various TiO2 NP content is shown in Fig. 5b and Table 1. We observe a typical peak at 300– 350 nm for anatase TiO2 in each of these curves. The refractive index of the hybrid film increases monotonically upon increasing the TiO2 NP content. As mentioned before, an ex situ synthesis usually lead to large residues of chelating agents on the surface of TiO2 NPs, resulting in the decrease of the refractive indices of the hybrid films. However, we have exhibited that 4ABA was better than carboxylic acid
Fig. 3. Photograph images of (a) the epoxy-TiO2 hybrid films with 30-wt% TiO2 NP and (b) PI30.
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Fig. 4. FTIR spectra of BT-6FOH-PI and PI50 with and without thermal treatment at 425 °C for 1 h in N2.
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Fig. 6. TGA curves of BT-6FOH-PI and PI50 films in nitrogen.
Fig. 5. Refractive indices of the PI-TiO2 hybrid films incorporating various TiO2 NP contents (a) as a function of incident wavelength and (b) plotted with respect to the TiO2 NP content at 633 nm.
as a chelating agent due to the fact that it featured a higher refractive index and higher coordinating stability [26]. The refractive index of the BT-6FOH-PI-TiO2 with theoretic 90 wt% (experimental 79.04 wt%) TiO2 prepared by the ex situ synthesis can reach as high as 1.941––close to that of the PI-TiO2 hybrid film fabricated by in situ synthesis at high temperature ([email protected] wt%) [15]. Fig. 5b reveals that the refractive indices of the PI-TiO2 hybrid films are higher than the epoxy-TiO2 ones; the difference increases upon increasing the TiO2 NP content. Although pure PI features a higher refractive index than epoxy resin, the difference in the refractive index between the two polymer-TiO2 nanocomposites should decrease in theory with the increase of the TiO2 NP content. The contradiction implies that there is certain synergetic effect for the PI-TiO2 nanocomposites to increase their refractive index compared with the epoxy-TiO2 hybrids. To more clearly comprehend the interaction between BT-6FOH-PI and TiO2 NPs, we recorded TGA curves of BT-6FOH-PI and PI60 under a nitrogen atmosphere, shown in Fig. 6. Compare with the results testing in air (Fig. S3), the BT-6FOH-PI sample tested in N2 presents a remarkable difference in the shape of the TGA curve. A neat 13% weight loss of BT-6FOH-PI in N2 at 516 °C indicates that two CO2 were released from one PI repeat unit due to the reaction of the imide cycle with
the hydroxyl group to be rearranged to a benzoxazole (Scheme S2) [29]. The FTIR spectrum of the BT-6FOH-PI sample thermally treated at 425 °C for 1 h in N2 (Fig. 4) also confirms the existence of benzoxazoles: a typical band of benzoxazoles at 1556 cm1 [34] and an adsorption peak of C@N at 1667 cm1; the disappearance of the free OAH may result from the formation of benzoxazoles. However, the initial weight loss is not found in the TGA analysis of the PI-TiO2 nanocomposites tested in N2 (Fig. 6) and the benzoxazole characteristic bands are also not found in the corresponding FTIR spectrum (Fig. 4). The suppression of benzoxazole formation may arise from the organic-inorganic bonding during the in situ PI-TiO2 synthesis [27]. As a result, we infer such bonding also exist between BT-6FOHPI and 4ABA-chelated TiO2 in the ex situ preparation, resulting in the resistance to cracking for the thick films. The refractive index nm of a polymer can be evaluated using the Lorentz–Lorenz equation [14]:
n2m 1 4p a ¼ n2m þ 2 3 V mol
ð1Þ
where a and Vmol are the mean polarizability and the molecular volume, respectively. Moreover, the effective refractive index of a well-mixed polymer–inorganic nanocomposites ne can be expressed as
ne ¼ /p np þ /f nf þ ð1 /p /f Þnm
ð2Þ
where np, nf, and nm are the refractive indices of NPs, free space, and polymer matrix, respectively. /p and /f are the volume fraction of NPs and free space, respectively. Based on Eqs. (1) and (2), when the polymer and NPs become compacter, either the decrease of Vmol or /f , the refractive index of the nanocomposites increases. As a result, we infer that the chemical bonding between BT6FOH-PI and TiO2 NPs will result in the increase of refractive index. According to Fresnel’s theory for a single-layer model [35], the reflectance increases with the increase of the refractive index of the coating layer. Fig. 7a shows the reflection spectra of the PI-TiO2 hybrid films. In contrast to blank glass, the hybrid films exhibit higher reflectance and their values increase with the increase of the TiO2 NP
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Fig. 8. Photograph image of PI90 coated on glass.
Fig. 7. (a) Reflectance and (b) transmittance spectra of the PI-TiO2 hybrid films.
content, implying the increase of refractive index, confirming with the measuring results from the ellipsometry. Fig. 7b shows the transmittance spectra of the PI-TiO2 hybrid films. The cutoff in the UV region was attributed to the adsorption of titania. There is trade-off between reflectance and transmittance. The higher reflectance (or the higher refractive index), the lower transmittance. As a result, the higher TiO2-loading film shows little lower transmittance than the lower TiO2-loading one due to higher refractive index and consequently higher reflection. As is known to all, particle aggregation is easy to occur at the polymer-nanoparticle system, resulting in the light-scattering drawback. In our experiments, the haze values of the PI-TiO2 hybrid films were lower than 0.1, which indicated lighter scattering could be ignored. Fig. 8 shows our sample reveals high quality in appearance, also implying that no manifest light scattering was found. Although PI can be been hydrolyzed under alkaline solution due to imide structure, we found that the PI-TiO2 hybrid films are stable for several months or years under atmospheric environment. Moreover, the feature of the alkaline hydrolysis may be a benefit for recycle. It should be mentioned that the study is the first time to report the preparation of PI-TiO2 nanocomposites at the low film-forming temperature, featuring the very high refractive index and the capacity to form thick films, which are very close to the characteristics of those fabricated at high temperature.
We have synthesized a soluble polyimide with hydroxyl group derived from 6FOH diamine and BTDA by virtue of the one-step polymerization. The high-refractive-index PI-TiO2 hybrid films were prepared incorporating premade TiO2 NPs chelating with 4ABA into the polyimide matrix at low temperature. The refractive index of the PITiO2 hybrid films can reach as high as 1.941—close to that of the PI-TiO2 hybrid film fabricated by in situ synthesis at high temperature and exhibit extraordinarily higher than that of the epoxy-TiO2 hybrid films. The PI-TiO2 hybrid films also reveal a better resistance to cracking in the thick-film condition than the epoxy-TiO2 films. We suspect that the enhanced performances in refractive indices and structure may result from the organic–inorganic chemical bonding between 4ABA-chelated TiO2 and BT-6FOH-PI. Acknowledgments This work was financially supported by the National Science Council of the Republic of China and the Ministry of Economic Affairs of Taiwan.
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4. Conclusions
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