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Feb 16, 2011 - ... Wang , Qi An , Xiaofei Zeng , Zhigang Shen , Yajun Zhang , and Feng Shi ... Sha Peng , Ming Zhou , Feiyan Liu , Chang Zhang , Xueqi...
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Effective Solution Mixing Method to Fabricate Highly Transparent and Optical Functional Organic-Inorganic Nanocomposite Film Xiao-Fei Zeng, Xiang-Rong Kong, Jun-Lin Ge, Hai-Tao Liu, Cui Gao, Zhi-Gang Shen, and Jian-Feng Chen* Key Lab for Nanomaterials, MOE; Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China ABSTRACT: The organic-inorganic hybrid optical materials with a high transmittance of visible-light have shown great potential applications. However, it is still in challenge to maintain the transparency of optical materials when the inorganic nanoparticles (NPs) are introduced into the polymer matrix because of Rayleigh scattering caused by the severely aggregation of NPs, which is a huge obstacle for its applications. This article reports a transparent “solution” mixing method to fabricate a highly transparent nanocomposite film of zinc oxide (ZnO)/poly (methyl methacrylate)-co-poly (styrene) (PMMA-PS) with the novel UV-shielding properties. The transparent “solution” containing nanoparticles was prepared by phase-transfer of ZnO NPs suspension in hexane to NPs dispersion in toluene with surface modifier. The latter dispersion had the complete transparency as a solution (so-called “solution”). It was found that the incorporation of ZnO NPs not only provided UV-shielding ability to the PMMA-PS nanocomposite film with the same transparency of the pure PMMA-PS film, but also improved the thermal stability of the film. When 2% of the ZnO NPs were added into the PMMA-PS polymer, the nanocomposite film could block UV radiation at 350 nm up to 97% and allow 98% transmittance of visible light at 400 nm. The SEM and TEM studies further confirmed that the ZnO NPs were well distributed in the PMMA-PS polymer matrix with the maximum aggregated nanoparticle size less than 20 nm in diameter. High thermal stability was achieved with a 37 °C increase in the initial decomposition temperature of such nanocomposite compared to the pure PMMA-PS.

1. INTRODUCTION Organic-inorganic nanocomposite polymers have attracted wide attention over past decade because of its unique property with the combination of both inorganic nanoparticles (NPs) and organic polymer,1,2 which is not available in traditional polymer. In particular, the fabrication of optically transparent nanocomposites via the incorporation of inorganic NPs into a polymer matrix is of extraordinary industrial interest owing to its promising application potential in the fields of light-emitting diodes, optoelectronic packages, transistors, solar cells, and coatings.3-8 Wide bandgap semiconductor such as ZnO is widely used as a gas sensor, transistor, photocatalyst and UV absorber.9-16 Unlike the conventional microsized ZnO particle, homogeneously dispersed ZnO NPs could offer high transparency in the nanocomposite polymer because of its small size. In addition, the large surface area of the ZnO NPs enables intensive interaction between the ZnO NPs and the polymer, and therefore may improve the thermal stability and mechanical property of the nanocomposite polymer. PMMA and PS are well-known for their good optical and chemical properties. Some researches have reported the incorporation of ZnO, SiO2, or TiO2 NPs into PMMA or PS matrices respectively to fabricate the transparent nanocomposites films,10-14,17-23 and they have observed the improvement in the UV, thermal properties and scratch resistance of these nanocomposite films. To make the NPs well dispersed in polymer matrix at nanoscale to avoid Rayleigh scattering and maintain the transparency of the polymer matrix, the in situ polymerization and sol-gel methods have been reported to fabricate the transparent nanocomposites. However, these methods suffer from the drawback to maintain the same r 2011 American Chemical Society

transparency as the polymer without nanoparticles, when nanocomposite featuring full blocking properties of UV-light. For example, Walter Caseri9 et al. prepared a ZnO layer in EVA with the thickness of 1-2 μm by exposure of an EVA sheet to the diethyl zinc liquid and in situ hydrolysis. The ZnO/EVA films were transparent and had a strong absorption of UV-light, but there was no reported experimental data of the transmittance of UV light and visible light of the nanocomposite films. Kyung Kim et al.10 synthesized transparent ZnO/PMMA hybrid materials via free-radical polymerization, and the UV-shielding rate of the nanocomposite was up to 98% at 350 nm, while the transmittance of visible light decreased to 47% from 85% of the pure PMMA at 400 nm. Gerhard Wegner et al.11 prepared ZnO/ PMMA nanocomposite films by in situ bulk polymerization. When the ZnO content in matrix was 30 wt %, the film with 2 μm thickness could shield about 65% UV-light at 350 nm. However, the transmission of the film decreased to 87.2% from 92% of pure PMMA at 550 nm. Therefore, the method or process to fabricate such hybrid nanocomposite maintained the same high transparency of visible light as the polymer without nanoparticles is still of great challenge. On the other hand, PMMA-PS material is widely used in architecture, automotive, air and railway transport fields because of its better thermal and mechanical properties than that of PMMA or PS.24,25 The incorporation of ZnO NPs into PMMA-PS Received: August 2, 2010 Revised: January 14, 2011 Accepted: February 2, 2011 Published: February 16, 2011 3253

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Figure 1. Photos of (a) the ZnO NPs suspension in hexane and (b) the ZnO “solution” in toluene.

to make highly transparent hybrid polymer composite has not yet been reported so far. In this paper, we reported a novel transparent “solution” mixing method to fabricate a highly transparent ZnO/PMMA-PS nanocomposite film with well-dispersed NPs in the transparent PMMA-PS polymer matrix. The excellent compatibility between NPs and organic matrix was achieved by selecting a suitable liquid medium and a surfactant on NPs to form “solution” of nanoparticles, which effectively prevented the phase separation in organic medium evaporation. The optical and structural properties of the ZnO/PMMA-PS nanocomposite films fabricated via this method were characterized. The nanocomposite film could block UV radiation at 350 nm up to 97%, while the visible light transmittance of the film with 2 wt % ZnO was the same as that of pure PMMA-PS film at 400 nm.

2. EXPERIMENTAL SECTION 2.1. Materials. PS-PMMA (PS wt% = 70%, PMMA wt% = 30%) purchased from Shanghai Pen Chemical Factory was used as the polymer system. ZnO NPs dispersed in hexane was purchased from Nanomaterials Technology Pte. Ltd., Singapore. Toluene and ethanol were obtained from Beijing Beihua Fine Chemicals Co., Ltd., and used without further purification. The surface modifier, monoethanolamine (MEA, AR), was purchased from Shantou Xilong Chemical Co., Ltd. 2.2. Solution Mixing Method to Fabricate Hybrid Film of ZnO/PMMA-PS. The solution mixing method included two steps. As shown in Scheme 1, the first step was to obtain the “solution” containing ZnO NPs via phase transfer and surface modification, and the second step was to mix the “solution” and the polymer solution with the same solvent, followed by the controlled the evaporation process to fabricate the transparent hybrid films. Step 1: Preparation of “solution” containing ZnO NPs. The photo of raw materials, 41 wt % ZnO suspension in hexane, is shown in Figure 1a with milk-like observation. The ZnO suspension was added into a test tube, and then centrifuged at 2000 rpm for 5 min. After the centrifugation, the upper layer of hexane was removed and ethanol was added for the next centrifugation. After the centrifugation process was performed for several times, the ZnO NPs were redispersed in toluene using sonicator for several hours to obtain a dispersion of ZnO NPs in toluene. MEA was added to the preheated ZnO NPs dispersion of temperature 80 °C under stirring. The mass ratio of MEA/ ZnO was 5/1. This modification process was carried out by refluxing toluene at 80 °C for two hours with continuous stirring. After modification, MEA was wiped off by separatory funnel and the dispersion of modified ZnO in toluene was obtained. Such

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made dispersion of ZnO NPs was highly transparent solutionlike, which we called a “solution”. The photo of such transparent “solution” of 5 wt % ZnO in toluene is shown in Figure 1b. We found that the “solution” could maintain stable and transparent for more than 6 months. Step 2: Preparation of ZnO/PMMA-PS hybrid film. Ten grams of granular PMMA-PS and 90 g of toluene were weighted and added to a round-bottom flask (RBF), and then stirred for 2 h until PMMA-PS dissolved completely to form a polymer solution. A certain amount of the ZnO dispersion from above step was added slowly to the PMMA-PS solution and the mixture was sonicated for 10 min until a homogeneous and transparent solution was obtained. The above ZnO/PMMA-PS solutions were loaded into a self-developed mold and were then baked in an oven at 60 °C for 5 h and 120 °C for 30 min to obtain the PMMA-PS films. The films were kept in room temperature for 72 h to ensure the organic solvent fully evaporated. The thickness of the obtained nanocomposite films was about 200 ( 10 μm. The content of ZnO was 2 wt % based on the polymer matrix. As a comparison, we used a traditional mixing method, slurry dispersion mixing in place of the above solution mixing method to prepare the nanocomposite films. The ZnO NPs suspension in hexane was directly added into the toluene medium with the polymer because hexane and toluene are miscible. Other steps of the slurry dispersion method to fabricate the hybrid film were the same as that of the solution mixing method. 2.3. Characterization of NPs and Nanocomposite Films. The thickness of the films was characterized with a micrometer screw-gauge at 5 different locations of each film. The particle size distribution (PSD) of ZnO NPs in toluene was investigated with a laser particle size analyzer (Zetasizer 3000HS, Ma1ver, Britain). The morphologies of ZnO NPs and their dispersion in the PMMA-PS matrix were recorded by a transmission electron microscope (TEM) (JEM-3010, JEOL, Japan) at an accelerating voltage of 100 kV. The X-ray powder diffractions of the ZnO NPs were measured with a X-ray diffractor (Cu KR = 1.5406) (XRD6000, Shimadzu, Japan). The scanning range was from 5-80° with 5°/min increment. The thermal decomposition profile of the ZnO/PMMA-PS and PMMA-PS films were recorded on a thermo-gravimetric/differential thermal analyzer (STA-449C, Netzsch, Germany) under argon atmosphere from 30-500 °C with heating rate of 10 °C/min. The UV-vis spectra of the ZnO/PMMA-PS and PMMA-PS films were measured with a spectrometer (UV-2501, Shimadzu, Japan) in the range from 200 nm-800 nm with 1 nm increment. The surface structural characteristics of the ZnO/PMMA-PS nanocomposites and PMMA-PS films were studied with a scanning electron microscopy (SEM) (JSM-6306, JEOL, Japan).

3. RESULTS 3.1. Characterization of the ZnO NPs. Figure 2 and inset of Figure 2 show the TEM and the high-resolution TEM images of the ZnO NPs. The diameter of the ZnO NPs was about 4-16 nm and the particles were distributed uniformly in toluene. It can be found from the lattice-resolved plane of the NPs that the NPs had the d-spacing of nearly 2.6 Å corresponding to the (002) plane and 2.48 Å corresponding to the (101) plane of the wurtzite phase of ZnO. Figure 3 exhibits the narrow size distribution of the ZnO NPs. The D90 diameter of the ZnO NPs was about 30 nm. The XRD spectrum of the ZnO NPs is shown in Figure 4. The diffraction pattern well matched the standard diffraction 3254

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Scheme 1. Fabrication of the Transparent Hybrid Film Using the Solution Mixing Method by a Two-Step Procedure

Figure 2. TEM images of the ZnO NPs.

Figure 4. XRD pattern of the ZnO NPs.

Figure 3. Particle size distribution of the ZnO NPs.

pattern of the wurtzite phase ZnO.26 Figure 5 shows the TGA curve of the ZnO NPs. It could be noted that the surface modifier

of ZnO NPs was completely decomposed at about 420 °C and the content of it was about 9 wt %. 3.2. Optical Properties and Morphology of ZnO/PMMA-PS Nanocomposite Films. The optical properties of the pure PMMA-PS film, the film prepared by the traditional slurry dispersion method, and the film by the solution mixing method are shown in Figure 6a-c, respectively. The thickness of the obtained films was about 200 μm. A clear UV cut off around 350 nm could be observed for the film by the solution mixing method. Compared with the optical properties of the film by slurry 3255

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Figure 5. TGA curve of the ZnO NPs.

Figure 6. UV-vis spectra of (a) pure PMMA-PS film and nanocomposite films fabricated by (b) the traditional slurry dispersion method, and (c) the solution mixing method.

dispersion method, the UV light transmittance of the film by the solution mixing method was lower, while the visible light transmittance was higher. For example, the transmittance of the film with 2 wt % ZnO by the solution mixing method at 350 nm in the UV region was 3%, whereas the transmittance of the film by the slurry dispersion method was about 50%. In the visible region, the transmittance of the film by the solution mixing method was 98% at 400 nm, which was slightly higher than that of the pure PMMA-PS film. However, the transmittance of the film by the slurry dispersion method decreased to 89% from 97% of the pure PMMA-PS film. It could be concluded that the film fabricated by the slurry dispersion method had lower transparency and UV-shielding capability than the film prepared by the “solution” mixing method. In conclusion, the film with 2 wt % ZnO NPs prepared by the “solution” mixing method could absorb 97% of the UV-light at 350 nm, and the visible light transmittance is 98% at 400 nm. The transparent ZnO/PMMA hybrid composite fabricated by Kyung Kim et al.9 also could shield 98% of the UV-light at 350 nm. However, the transmittance of visible light decreased from 85% to 47% at 400 nm. Although the thickness of the hybrid

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composite reported in the literature was 1 cm, the ZnO content in the PMMA matrix was from 0.017 to 0.11 wt %, which was much lower than 2 wt % in this paper. Therefore, the total quality of ZnO NPs in the matrix was similar between the film reported in the literature and the film in this paper, and the data of the optical properties was comparable. The SEM images of the surface of the 2 wt % incorporated ZnO/PMMA-PS nanocomposite films are shown in Figure 7. It can be seen that the ZnO NPs prepared by the traditional slurry dispersion method aggregated severely in the polymer matrix (Figure 7a), whereas the ZnO NPs prepared by the solution mixing method were homogeneously distributed at nanoscale without phase separation (Figure 7b). From the TEM image, Figure 7c, it also can be seen that the ZnO NPs were well dispersed in the polymer matrix with the largest aggregated diameter less than 20 nm. The dispersion state of ZnO NPs in the PMMS-PS matrix was notably different between those NPs prepared by the traditional slurry dispersion method and the solution mixing method. When the film was fabricated by the traditional slurry dispersion method, the compatibility between ZnO in hexane and polymer in toluene was relatively low, and the ZnO NPs could not maintain stable in toluene with polymer. Therefore, the NPs aggregated severely in the film. Moreover, there was no cross-link between the particles and the polymer, and as a result, when the organic medium evaporated, the NPs would move with the organic medium and cause two phases separated. When the film was fabricated by the solution mixing method, the compatibility between the NPs and the polymer matrix was improved and the NPs were well dispersed in the matrix, because the addition of MEA as a coupling agent between ZnO and the nonpolar PMMS-PS polymer molecules caused the NPs to lose their conformational entropy and to inhibit particle agglomeration in the polymer matrix by steric repulsion. To obtain transparent nanocomposite materials, the diameter of NPs should be below 40 nm and without particle aggregation in the matrix to maintain the transparency of the raw materials.27 The TEM studies further confirmed that our proposed “solution” mixing method could solve the key problems of NPs aggregation and phase separation in the polymer matrix to fabricate nanocomposites, and the good dispersion of NPs with the largest aggregated diameter below 20 nm in the matrix would achieve the supertransparency of hybrid nanocomposite, as shown in Figure 8b. Such hybrid film had the same transparency as the film without NPs (Figure 8a). The traditional mixing method would lead to opaque nanocomposite due to light scattering caused by larger NPs aggregates in the matrix, and the photo of such nanocomposite film is shown in Figure 8c. 3.3. Thermal Gravimetric Analysis of ZnO/PMMA-PS Nanocomposite. Figure 9a-c presents TGA curves of the PMMA-PS polymer, ZnO/PMMA-PS nanocomposite films with 2 wt % ZnO using solution mixing method and slurry dispersion method respectively. The pure PMMA-PS film started to decompose at about 133 °C (here the decomposition temperature was defined as 2% weight loss of samples). The film fabricated by slurry dispersion method began to decompose at 135 °C. In contrast, the ZnO/PMMA-PS nanocomposite film fabricated by solution mixing method exhibited a decomposition temperature of 170 °C. This finding revealed that the ZnO/PMMA-PS nanocomposite film fabricated by “solution” mixing method had higher initial decomposition temperature than the pure PMMA-PS film and the film fabricated by slurry dispersion 3256

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Figure 7. SEM images of the surface of the nanocomposite films fabricated by (a) the traditional slurry dispersion method, (b) the solution mixing method, and (c) TEM image of the middle site species of the cross-section of the nanocomposite film fabricated by the solution mixing method.

Figure 8. Photos of (a) the pure PMMA-PS film and the nanocomposite films fabricated by (b) the solution mixing method and (c) the traditional slurry dispersion method (background: tree).

method. Moreover, the decomposition rate of such film was slower than the other two films. The improvement in the thermal property of the ZnO/PMMA-PS nanocomposite film could be benefited from the large surface area and surface modification of the ZnO NPs, which enabled the ZnO NPs to bind the individual polymer chain and further cross-link the matrix polymer chains. The further cross-linking in the ZnO/PMMA-PS nanocomposite film hindered the movement of the PMMA-PS polymer strand and prevented the decomposition of the PMMA-PS polymer at a relatively higher temperature.

4. CONCLUSIONS Highly transparent ZnO/PMMA-PS nanocomposite film had been fabricated by a novel and effective solution mixing

method. The film could block the UV radiation up to 97% and allow 98% of visible light to pass through with 2 wt % ZnO in the matrix. The ZnO NPs in the solution, which had been transferred from hexane to toluene by means of phase transfer, and modified by MEA coupling agent, were well distributed in the PMMA-PS polymer matrix with the aggregates diameter less than 20 nm. Our method could effectively prevent the nanophase separation from polymer when the liquid medium was evaporated. Moreover, the addition of the ZnO NPs improved the thermal stability of the PMMA-PS polymer by increasing the decomposition temperature to 170 °C from 133 °C. The method presented here can be easily adopted in industrial production to make UV-shielding transparent glasses, coating films, or containers to protect from UV damage for long lifetime. 3257

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Figure 9. TGA curves of (a) the pure PMMA-PS polymer and the nanocomposites fabricated by (b) the solution mixing method and (c) the traditional slurry dispersion method.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86 10 64446466. Fax: þ86 10 64434784. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (21006004, 20976008), “863” programs of China (2009AA03Z334), and Chinese Universities Scientific Fund (JD-1003). ’ REFERENCES (1) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Predicting the Mesophases of Copolymer-Nanoparticle Composites. Science 2001, 292, 2469. (2) Balazs, A. C.; Emrick, T.; Russell, T. P. Nanoparticle Polymer Composites: Where Two Small Worlds Meet. Science 2006, 314, 1107. (3) Schmid, A.; Tonnar, J.; Armes, S. P. A New Highly Efficient Route to Polymer-Silica Colloidal Nanocomposite Particles. Adv. Mater. 2008, 20, 3331. (4) Simon, H. S.; Markus, K.; Klaus, M. A Simple and Efficient Route to Transparent Nanocomposites. Adv. Mater. 2008, 20, 929. (5) Sung, J.; Jo, P. S.; Shin, H.; Huh, J.; Min, B. G.; Kin, D. H.; Park, C. Low-Electric-Resistance Nanocomposites of Self-Assembled Block Copolymers and SWNTs. Adv. Mater. 2008, 20, 1505. (6) Schmid, A.; Scherl, P.; Armes, S. P. Synthesis and Characterization of Film-Forming Colloidal Nanocomposite Particles Prepared via Surfactant-Free Aqueous Emulsion Copolymerization. Macromolecules 2009, 42, 3721. (7) Elim, H. I.; Elim, H. I.; Cai, B.; Kurata, Y.; Sugihara, O.; Kaino, T.; Adschiri, T.; Chu, A. L.; Kambe, N. Refractive Index Control and Rayleigh Scattering Properties of Transparent TiO2 Nanohybrid Polymer. J. Phys. Chem. B 2009, 113, 10143. (8) Li, Y. Q.; Fu, S. Y.; Yang, Y.; Mai, Y. W. Facile Synthesis of Highly Transparent Polymer Nanocomposites by Introduction of Core-Shell Structured NPs. Chem. Mater. 2008, 20, 2637. (9) Kyprianidou-Leodidou, T.; Margraf, P.; Caseri, W.; Suter, U. W.; Walther, P. Polymer Sheets with a Thin Nanocomposite Layer Acting as a UV Filter. Polym. Adv. Technol. 1997, 8, 505.

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