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Facile preparation of high refractive index polymer films composited with a tungstophosphoric acid
Materials Letters 190 (2017) 236–239
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile preparation of high refractive index polymer films composited with a tungstophosphoric acid Shuichi Matsumoto a, Thiraporn Ishii a, Mutsumi Wada a, Yutaka Kuwahara a, Tomonari Ogata b, Shoji Nagaoka c,d, Makoto Takafuji a,d, Hirotaka Ihara a,d,⇑ a
Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Innovative Collaboration Organization, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Materials Development Department, Kumamoto Industrial Research Institute, 3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan d Kumamoto Institute for Photo-electro Organics (PHOENICS), 3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan b c
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 17 December 2016 Accepted 31 December 2016 Available online 31 December 2016 Keywords: Polyvinyl alcohol Tungstophosphoric acid Titanium oxide Refractive index Polymeric composites Optical materials and properties
a b s t r a c t In this study, transparent, colorless polymer composites of polyvinyl alcohol (PVA) and tungstophosphoric acid (PWA) with high refractive indices were developed. The composite films were prepared on glass, using a facile spin-coating method, from an aqueous mixture of PVA and PWA without surfactants. The optical properties of the obtained films were investigated using an ultraviolet-visible absorption spectrometer and a prism coupler. Resultant PVA/PWA (90 wt%) composite films exhibited high transparency (90% T) in the visible region and had a high refractive index (n = 1.72). These PVA/PWA films were also compared to corresponding composite films of PVA and titanium oxide (TiO2). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, optical materials with a high refractive index (n) have been developed as key materials for optical applications such as lenses, optical waveguides, etc. [1,2]. Many inorganic optical materials show excellent mechanical properties and high n values, but their major drawbacks are their large weight, fragility and limited processability. On the other hand, most conventional polymers are lightweight, and possess high flexibility and processability compared to inorganic materials. However, the n values of conventional polymers are lower (n = 1.4–1.6) [3] than those of inorganic materials, which are typically higher than 2.0 [4]. To overcome this problem, organic-inorganic composite materials have been widely studied to increase n values by incorporating various nanoparticles (NPs), such as those made from titanium oxide (TiO2) [5,6], alumina oxide [7], gold [8], and nano-diamond [9,10]. However, polymer-inorganic NP composites usually suffer from NP agglomeration due to their high surface energies. To prevent this agglomeration, special treatments such as the addition of surfactants and functionalization of NP surfaces are usually used. ⇑ Corresponding author at: Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. E-mail address: [email protected] (H. Ihara). http://dx.doi.org/10.1016/j.matlet.2016.12.136 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Unfortunately, molecules added in such treatments induce a reduction in n value of the composite film, as well as an increase in costs. Therefore, the development of composite films without the need for surfactants is required. In this study, we therefore focused on using the unique combination of solubility and high refractive index of tungstophosphoric acid (PWA), which is commercially available at low cost. Composite polymer films of polyvinyl alcohol (PVA) incorporating PWA, which has a high n value of 2.2 and a NP diameter of ca. 1 nm, were prepared without the use of surfactants, as shown in Fig. 1. The refractive indices and other properties of the prepared composite PVA/PWA polymers were assessed, and we here discuss their optical properties compared to those of PVA composite films doped with TiO2 NPs (PVA/TiO2).
2. Experimental PVA (degree of polymerization 400–600, degree of hydrolysis 96%, Wako Chemical, Tokyo, Japan), polyvinyl pyrrolidone (PVP, Mw 55000, Aldrich, Tokyo, Japan) and PWA (H3PW12O40nH2O, Wako Chemical, Tokyo, Japan) were of analytical grade and used without further purification. Three kinds of TiO2 NPs (TiO2NPs), TiO2-a, TiO2-b and TiO2-c, were obtained from Aldrich (Tokyo,
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Fig. 1. Schematic illustration of the PVA/PWA composite film.
Japan), Arosil (Tokyo, Japan) and Architect Company (Kumamoto, Japan), respectively. The diameters of PWA, TiO2-a, TiO2-b and TiO2-c NPs were 1, 21, 21, and 32 nm, respectively. PVA and PVP solutions were prepared by dissolving 5 g of PVA and PVP in 100 mL of distilled water at 90 °C. Then, predetermined amounts of PWA, TiO2-a, TiO2-b, or TiO2-c were added to each solution, and mixtures were sonicated in an ultrasonic bath. Thin films were prepared on glass by spin-coating 300 lL solution at 1000 rpm. In a typical preparation, films were allowed to dry slowly at 25 °C for 24 h. Transmittance spectra of the obtained films were measured using a V-560 spectrophotometer (JASCO, Tokyo, Japan). Their refractive index values and thickness were evaluated using a prism coupler SPA-4000 (Sairon Technology, Inc., Gwangju, Korea) equipped with a He-Ne laser (wavelength 632.8 nm) and gadolinium gallium garnet (GGG) prism (n = 1.965). Dynamic light scattering (DLS) and thermogravimetric (TG) analyses were also performed with a Zetasizer-ZS (Malvern, Tokyo Japan) and TG/ DTA-6200 (Hitachi-Hitech, Tokyo, Japan), respectively.
3. Results and discussion The solubility of 10 wt% PWA or TiO2 inorganic NPs in aqueous solution was first investigated. Transparent solutions were obtained for aqueous solutions of PWA and TiO2-c, while dispersions of TiO2-a and TiO2-b were turbid, even when decreasing the concentrations to 1 wt%. From DLS analysis, the dispersed size of the TiO2-a and TiO2-b NPs was evaluated to be larger than 5 lm in the cloudy solutions, suggesting aggregation in solution due to
their primary particle size of 21 nm. Therefore, only PWA and TiO2-c NPs were employed to prepare polymer solutions for subsequent film fabrication. Fig. 2a and b show aqueous solutions of PVA polymer with various concentrations of PWA and TiO2-c, respectively. Aqueous solutions with up to 90 wt% PWA were transparent and colorless (Fig. 2a). On the other hand, aqueous solutions of TiO2-c became pale yellow in color and turbid upon increasing the TiO2-c concentration to over 30 and 60 wt%, respectively. The pale yellow color was likely caused by the absorption of TiO2-c NPs around 400 nm [6]. In all cases, PVP polymer mixed with PWA or TiO2 NPs resulted in turbid solutions. Thus, only PVA/PWA and PVA/TiO2 composite thin films were prepared using the aqueous PVA solutions of PWA and TiO2-c, respectively, on a 1 mm-thick glass substrate. The composite films were obtained with a few micrometer of film thickness (Table S1). The transparency of the obtained composite films with up to 90 wt% PWA was 90% throughout the visible light spectrum (transmittance spectra are shown in Fig. S1a). An obtained transparent PVA/PWA (90 wt%) film is shown on the right image of Fig. 2c. Fig. 2c further shows the differential transmittance at 600 nm between a glass substrate and the obtained PVA composite films with the substrate. The transmittance of PVA/ PWA films decreased by up to 2% and 5% as the amount of PWA reached 80 wt% and 90 wt% , respectively. When using up to 50 wt% TiO2-c NPs, a transparent film was also obtained, as shown in the left image of Fig. 2c. However, the transmittance spectra of PVA/TiO2, as shown in Fig. S1b, show that the transmittance decreased to around 70% at 600 nm with 60 wt% TiO2 because of the turbidity. This turbidity in solid films of PVA/TiO2 (60 wt% ) corresponded to the observed turbidity in aqueous solution
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Fig. 2. Photographs of (a) PVA/PWA solutions and (b) PVA/TiO2 solutions at various amount of PWA or TiO2 (c) Concentration dependence of the transmittance losses of PVA/ PWA (solid circles) and PVA/TiO2 (open circles) polymer composite films. Photographs of the PVA composite films with 90 wt% of PWA (right image) and with 50 wt% of TiO2 (left image) on glass substrates.
(Fig. 2b) of PVA mixed with 60 wt% TiO2-c NPs. From these results, we can conclude that higher amounts of PWA could be embedded as they had higher dispersibility in the PVA polymer matrix than TiO2 NPs, and the prepared PVA/PWA polymer composite films as a result were colorless and transparent. The concentration effects of PWA and TiO2-c on the refractive index n of PVA composite films are shown in Fig. 3a and b, respectively. The n values of the composite films increased with an increase in the amount of PWA or TiO2-c. As mentioned above, higher amounts of PWA could be included in PVA than TiO2-c. The n value (1.70) of PVA/PWA (90 wt%) was close to the n value of 1.68 of PVA/TiO2-c (50 wt%). These n values of PVA composite films around 1.70 were remarkably improved from the n value of 1.53 of the original PVA films. In Fig. 3a and b, the observed n values were compared with n values (dotted lines) calculated by the Lorentz-Lorenz equation [11]. The observed n values for PVA/ TiO2-c films were similar to the theoretical values. On the other hand, in the case of PVA/PWA films, the observed n values were lower than the theoretical values at high concentrations of PWA. This difference suggests a decrease in refractive index caused by the presence of water derived from the PWA hydrate compound used as original material. Therefore, a thermal treatment method was applied to remove water. From the results of thermal investigations at several temperatures, the optimal temperature was determined to be 50 °C. At this temperature, the n values of the composite films increased while the films retained their transparency and colorlessness. After thermal treatment at 50 °C for 12 h, the n values of PVA/PWA composite films with 60, 80, and 90 wt% PWA increased from 1.56, 1.62, and 1.70 to 1.60, 1.70,
and 1.72, respectively (solid lines in Fig. 3c). Their n values after thermal treatment (open circles) could be increased close to the theoretically-predicted values (dotted line), as shown in Fig. 3a. The TG analysis results, shown in Fig. 3d, indicate that the weight loss around 100 °C in the PVA/PWA films with 80 wt% PWA after thermal treatment was smaller than that for the film before thermal treatment. This indicates water loss from the composite films, which corresponds with water removal being the likely cause for the increase in n value after heat treatment. These results suggest that composite films with a high refractive index close to the predicted value could be obtained after thermal treatment. The results indicated that the proposed method could be extended to many kinds of hybrid polymers with optical properties suitable for applications such as lenses and optical waveguides.
4. Conclusions We have demonstrated the fabrication of highly refractive polymer composites made of PVA and two materials with high refractive indices, PWA and TiO2NP. Composite films of PVA and PWA could be obtained by a simple procedure without surfactants, spin-coating a mixture of PVA and PWA on glass. The PVA/PWA polymer composites could achieve a refractive index of 1.70, higher than the original PVA index of 1.53, while maintaining useful optical properties such as transparency in the visible light region and colorlessness. For composites of PVA and TiO2-c, on the other hand, aqueous solutions containing TiO2-c changed to a pale yellow color and became turbid at high concentrations over
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1.8
(c)
PWA
Refractive index, n
Refractive index, n
(a)
1.7
1.6
1.5 0
1.8
(d)
TiO2
1.7
Turbid 1.6
80 wt.%
60 wt.% 0
20 Time (h)
40
100
Weight (%)
Refractive index, n
(b)
20 40 60 80 100 Concentration (wt.%)
1.75 1.73 1.71 1.69 1.67 1.65 1.63 1.61 1.59 1.57 1.55
95 90 After treatment
85 Before treatment
1.5
80 0
20
40
60
80
100
0
Concentration (wt.%)
100 200 300 Temperature (° C )
400
Fig. 3. (a) Dependence of the refractive indexes of PVA/PWA polymer composite films on PWA concentration. (b) Dependence of the refractive indexes of PVA/TiO2 polymer composite films on TiO2 NP concentration. Observed refractive indexes before (solid circles) and after thermal treatment (open circles) and calculated refractive indexes (dotted line). (c) Time dependence of the refractive indexes of PVA polymer composite films with 60 (triangle) and 80 wt% (circle) of PWA under thermal treatment at 25 °C (open symbol) and 50 °C (solid symbol). (d) TG curves of PVA/PWA (80 wt%) polymer composite films before (dotted line) and after the thermal treatment (solid line).
60 wt% TiO2-c. Eventually, the n value (1.70) of the transparent PVA/PWA composite polymer films with 90 wt% PWA was equivalent to the n value (1.68) of the PVA/TiO2C composite polymer with 50 wt% TiO2-c. After removal of excess water by an optimized thermal treatment, the PVA/PWA composite polymers exhibited high n values close to theoretical values. We believe that the transparent, colorless and highly refractive polymers fabricated in this study can contribute to the development of key materials for optical applications. Acknowledgement The present study was partially supported by a Grant-in-Aid for Challenging Exploratory Research of JSPS KAKENHI, Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2016.12. 136.
References [1] T. Higashihara, M. Ueda, Macromolecules 48 (2015) 1915–1929. [2] M. Dosnaillove, L.N. Leonat, J. Patek, D. Roth, P. Bauer, M.C. Scharber, N.S. Sariftci, J.D. Pedarning, Thin Solid Films 591 (2015) 97–104. [3] J. Brandup, E.H. Immergut, E.A. Gruike, A. Abe, D.R. Bloch, Polymer Handbook, fourth ed., John Wiley & Sons, New York, 2005. [4] A.S. Korothov, V.V. Atuchin, Opt. Commun. 281 (2008) 2132–2138. [5] P. Tao, A. Viswanath, Y. Li, R.W. Siegel, B.C. Benicewicz, L.S. Schadler, Polymer 54 (2013) 1639–1646. [6] A.H. Yuwono, B.H. Liu, J.M. Xue, J. Wang, H.I. Elim, W. Ji, Y. Li, J.T. White, J. Mater. Chem. 14 (2004) 2978–2987. [7] E. Ritzhaupt-Kleissl, J. Böhm, J. Haußelt, T. Hanemann, Mater. Sci. Eng., C 26 (2006) 1067–1071. [8] J. Kim, H. Yang, P.F. Green, Langmuir 28 (2012) 9735–9741. [9] S. Morimune, M. Kotera, T. Nishino, K. Goto, K. Hata, Macromolecules 44 (2011) 4415–4421. [10] T. Ogata, R. Yagi, N. Nakamura, Y. Kuwahara, S. Kurihara, ACS Appl. Mater. Interfaces 4 (2012) 3769–3772. [11] J.V. Herráez, R. Belda, J. Solution Chem. 35 (2006) 1315–1328.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile preparation of high refractive index polymer films composited with a tungstophosphoric acid Shuichi Matsumoto a, Thiraporn Ishii a, Mutsumi Wada a, Yutaka Kuwahara a, Tomonari Ogata b, Shoji Nagaoka c,d, Makoto Takafuji a,d, Hirotaka Ihara a,d,⇑ a
Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Innovative Collaboration Organization, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Materials Development Department, Kumamoto Industrial Research Institute, 3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan d Kumamoto Institute for Photo-electro Organics (PHOENICS), 3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan b c
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 17 December 2016 Accepted 31 December 2016 Available online 31 December 2016 Keywords: Polyvinyl alcohol Tungstophosphoric acid Titanium oxide Refractive index Polymeric composites Optical materials and properties
a b s t r a c t In this study, transparent, colorless polymer composites of polyvinyl alcohol (PVA) and tungstophosphoric acid (PWA) with high refractive indices were developed. The composite films were prepared on glass, using a facile spin-coating method, from an aqueous mixture of PVA and PWA without surfactants. The optical properties of the obtained films were investigated using an ultraviolet-visible absorption spectrometer and a prism coupler. Resultant PVA/PWA (90 wt%) composite films exhibited high transparency (90% T) in the visible region and had a high refractive index (n = 1.72). These PVA/PWA films were also compared to corresponding composite films of PVA and titanium oxide (TiO2). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Over the past decade, optical materials with a high refractive index (n) have been developed as key materials for optical applications such as lenses, optical waveguides, etc. [1,2]. Many inorganic optical materials show excellent mechanical properties and high n values, but their major drawbacks are their large weight, fragility and limited processability. On the other hand, most conventional polymers are lightweight, and possess high flexibility and processability compared to inorganic materials. However, the n values of conventional polymers are lower (n = 1.4–1.6) [3] than those of inorganic materials, which are typically higher than 2.0 [4]. To overcome this problem, organic-inorganic composite materials have been widely studied to increase n values by incorporating various nanoparticles (NPs), such as those made from titanium oxide (TiO2) [5,6], alumina oxide [7], gold [8], and nano-diamond [9,10]. However, polymer-inorganic NP composites usually suffer from NP agglomeration due to their high surface energies. To prevent this agglomeration, special treatments such as the addition of surfactants and functionalization of NP surfaces are usually used. ⇑ Corresponding author at: Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. E-mail address: [email protected] (H. Ihara). http://dx.doi.org/10.1016/j.matlet.2016.12.136 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
Unfortunately, molecules added in such treatments induce a reduction in n value of the composite film, as well as an increase in costs. Therefore, the development of composite films without the need for surfactants is required. In this study, we therefore focused on using the unique combination of solubility and high refractive index of tungstophosphoric acid (PWA), which is commercially available at low cost. Composite polymer films of polyvinyl alcohol (PVA) incorporating PWA, which has a high n value of 2.2 and a NP diameter of ca. 1 nm, were prepared without the use of surfactants, as shown in Fig. 1. The refractive indices and other properties of the prepared composite PVA/PWA polymers were assessed, and we here discuss their optical properties compared to those of PVA composite films doped with TiO2 NPs (PVA/TiO2).
2. Experimental PVA (degree of polymerization 400–600, degree of hydrolysis 96%, Wako Chemical, Tokyo, Japan), polyvinyl pyrrolidone (PVP, Mw 55000, Aldrich, Tokyo, Japan) and PWA (H3PW12O40nH2O, Wako Chemical, Tokyo, Japan) were of analytical grade and used without further purification. Three kinds of TiO2 NPs (TiO2NPs), TiO2-a, TiO2-b and TiO2-c, were obtained from Aldrich (Tokyo,
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237
Fig. 1. Schematic illustration of the PVA/PWA composite film.
Japan), Arosil (Tokyo, Japan) and Architect Company (Kumamoto, Japan), respectively. The diameters of PWA, TiO2-a, TiO2-b and TiO2-c NPs were 1, 21, 21, and 32 nm, respectively. PVA and PVP solutions were prepared by dissolving 5 g of PVA and PVP in 100 mL of distilled water at 90 °C. Then, predetermined amounts of PWA, TiO2-a, TiO2-b, or TiO2-c were added to each solution, and mixtures were sonicated in an ultrasonic bath. Thin films were prepared on glass by spin-coating 300 lL solution at 1000 rpm. In a typical preparation, films were allowed to dry slowly at 25 °C for 24 h. Transmittance spectra of the obtained films were measured using a V-560 spectrophotometer (JASCO, Tokyo, Japan). Their refractive index values and thickness were evaluated using a prism coupler SPA-4000 (Sairon Technology, Inc., Gwangju, Korea) equipped with a He-Ne laser (wavelength 632.8 nm) and gadolinium gallium garnet (GGG) prism (n = 1.965). Dynamic light scattering (DLS) and thermogravimetric (TG) analyses were also performed with a Zetasizer-ZS (Malvern, Tokyo Japan) and TG/ DTA-6200 (Hitachi-Hitech, Tokyo, Japan), respectively.
3. Results and discussion The solubility of 10 wt% PWA or TiO2 inorganic NPs in aqueous solution was first investigated. Transparent solutions were obtained for aqueous solutions of PWA and TiO2-c, while dispersions of TiO2-a and TiO2-b were turbid, even when decreasing the concentrations to 1 wt%. From DLS analysis, the dispersed size of the TiO2-a and TiO2-b NPs was evaluated to be larger than 5 lm in the cloudy solutions, suggesting aggregation in solution due to
their primary particle size of 21 nm. Therefore, only PWA and TiO2-c NPs were employed to prepare polymer solutions for subsequent film fabrication. Fig. 2a and b show aqueous solutions of PVA polymer with various concentrations of PWA and TiO2-c, respectively. Aqueous solutions with up to 90 wt% PWA were transparent and colorless (Fig. 2a). On the other hand, aqueous solutions of TiO2-c became pale yellow in color and turbid upon increasing the TiO2-c concentration to over 30 and 60 wt%, respectively. The pale yellow color was likely caused by the absorption of TiO2-c NPs around 400 nm [6]. In all cases, PVP polymer mixed with PWA or TiO2 NPs resulted in turbid solutions. Thus, only PVA/PWA and PVA/TiO2 composite thin films were prepared using the aqueous PVA solutions of PWA and TiO2-c, respectively, on a 1 mm-thick glass substrate. The composite films were obtained with a few micrometer of film thickness (Table S1). The transparency of the obtained composite films with up to 90 wt% PWA was 90% throughout the visible light spectrum (transmittance spectra are shown in Fig. S1a). An obtained transparent PVA/PWA (90 wt%) film is shown on the right image of Fig. 2c. Fig. 2c further shows the differential transmittance at 600 nm between a glass substrate and the obtained PVA composite films with the substrate. The transmittance of PVA/ PWA films decreased by up to 2% and 5% as the amount of PWA reached 80 wt% and 90 wt% , respectively. When using up to 50 wt% TiO2-c NPs, a transparent film was also obtained, as shown in the left image of Fig. 2c. However, the transmittance spectra of PVA/TiO2, as shown in Fig. S1b, show that the transmittance decreased to around 70% at 600 nm with 60 wt% TiO2 because of the turbidity. This turbidity in solid films of PVA/TiO2 (60 wt% ) corresponded to the observed turbidity in aqueous solution
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Fig. 2. Photographs of (a) PVA/PWA solutions and (b) PVA/TiO2 solutions at various amount of PWA or TiO2 (c) Concentration dependence of the transmittance losses of PVA/ PWA (solid circles) and PVA/TiO2 (open circles) polymer composite films. Photographs of the PVA composite films with 90 wt% of PWA (right image) and with 50 wt% of TiO2 (left image) on glass substrates.
(Fig. 2b) of PVA mixed with 60 wt% TiO2-c NPs. From these results, we can conclude that higher amounts of PWA could be embedded as they had higher dispersibility in the PVA polymer matrix than TiO2 NPs, and the prepared PVA/PWA polymer composite films as a result were colorless and transparent. The concentration effects of PWA and TiO2-c on the refractive index n of PVA composite films are shown in Fig. 3a and b, respectively. The n values of the composite films increased with an increase in the amount of PWA or TiO2-c. As mentioned above, higher amounts of PWA could be included in PVA than TiO2-c. The n value (1.70) of PVA/PWA (90 wt%) was close to the n value of 1.68 of PVA/TiO2-c (50 wt%). These n values of PVA composite films around 1.70 were remarkably improved from the n value of 1.53 of the original PVA films. In Fig. 3a and b, the observed n values were compared with n values (dotted lines) calculated by the Lorentz-Lorenz equation [11]. The observed n values for PVA/ TiO2-c films were similar to the theoretical values. On the other hand, in the case of PVA/PWA films, the observed n values were lower than the theoretical values at high concentrations of PWA. This difference suggests a decrease in refractive index caused by the presence of water derived from the PWA hydrate compound used as original material. Therefore, a thermal treatment method was applied to remove water. From the results of thermal investigations at several temperatures, the optimal temperature was determined to be 50 °C. At this temperature, the n values of the composite films increased while the films retained their transparency and colorlessness. After thermal treatment at 50 °C for 12 h, the n values of PVA/PWA composite films with 60, 80, and 90 wt% PWA increased from 1.56, 1.62, and 1.70 to 1.60, 1.70,
and 1.72, respectively (solid lines in Fig. 3c). Their n values after thermal treatment (open circles) could be increased close to the theoretically-predicted values (dotted line), as shown in Fig. 3a. The TG analysis results, shown in Fig. 3d, indicate that the weight loss around 100 °C in the PVA/PWA films with 80 wt% PWA after thermal treatment was smaller than that for the film before thermal treatment. This indicates water loss from the composite films, which corresponds with water removal being the likely cause for the increase in n value after heat treatment. These results suggest that composite films with a high refractive index close to the predicted value could be obtained after thermal treatment. The results indicated that the proposed method could be extended to many kinds of hybrid polymers with optical properties suitable for applications such as lenses and optical waveguides.
4. Conclusions We have demonstrated the fabrication of highly refractive polymer composites made of PVA and two materials with high refractive indices, PWA and TiO2NP. Composite films of PVA and PWA could be obtained by a simple procedure without surfactants, spin-coating a mixture of PVA and PWA on glass. The PVA/PWA polymer composites could achieve a refractive index of 1.70, higher than the original PVA index of 1.53, while maintaining useful optical properties such as transparency in the visible light region and colorlessness. For composites of PVA and TiO2-c, on the other hand, aqueous solutions containing TiO2-c changed to a pale yellow color and became turbid at high concentrations over
239
S. Matsumoto et al. / Materials Letters 190 (2017) 236–239
1.8
(c)
PWA
Refractive index, n
Refractive index, n
(a)
1.7
1.6
1.5 0
1.8
(d)
TiO2
1.7
Turbid 1.6
80 wt.%
60 wt.% 0
20 Time (h)
40
100
Weight (%)
Refractive index, n
(b)
20 40 60 80 100 Concentration (wt.%)
1.75 1.73 1.71 1.69 1.67 1.65 1.63 1.61 1.59 1.57 1.55
95 90 After treatment
85 Before treatment
1.5
80 0
20
40
60
80
100
0
Concentration (wt.%)
100 200 300 Temperature (° C )
400
Fig. 3. (a) Dependence of the refractive indexes of PVA/PWA polymer composite films on PWA concentration. (b) Dependence of the refractive indexes of PVA/TiO2 polymer composite films on TiO2 NP concentration. Observed refractive indexes before (solid circles) and after thermal treatment (open circles) and calculated refractive indexes (dotted line). (c) Time dependence of the refractive indexes of PVA polymer composite films with 60 (triangle) and 80 wt% (circle) of PWA under thermal treatment at 25 °C (open symbol) and 50 °C (solid symbol). (d) TG curves of PVA/PWA (80 wt%) polymer composite films before (dotted line) and after the thermal treatment (solid line).
60 wt% TiO2-c. Eventually, the n value (1.70) of the transparent PVA/PWA composite polymer films with 90 wt% PWA was equivalent to the n value (1.68) of the PVA/TiO2C composite polymer with 50 wt% TiO2-c. After removal of excess water by an optimized thermal treatment, the PVA/PWA composite polymers exhibited high n values close to theoretical values. We believe that the transparent, colorless and highly refractive polymers fabricated in this study can contribute to the development of key materials for optical applications. Acknowledgement The present study was partially supported by a Grant-in-Aid for Challenging Exploratory Research of JSPS KAKENHI, Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2016.12. 136.
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