Highly Transparent and Multifunctional Polymer Nanohybrid Film with

Article pubs.acs.org/IECR

Highly Transparent and Multifunctional Polymer Nanohybrid Film with Superhigh ZnO Content Synthesized by a Bulk Polymerization Method Hai-Tao Liu,† Xiao-Fei Zeng,*,† Hong Zhao,*,‡ and Jian-Feng Chen†,‡ †

State Key Laboratory of Organic−Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P.R. China ABSTRACT: Highly transparent and multifunctional ZnO/poly(n-butyl methacrylate) (ZnO/PBMA) nanohybrid films with superhigh contents of ZnO nanoparticles (NPs) were synthesized by a bulk polymerization method. The structure; morphology; and optical, mechanical, and thermal properties of the obtained nanohybrid films were studied. ZnO NPs were dispersed uniformly and retained their original size (4−6 nm) in the PBMA matrix without any aggregation. The ZnO/PBMA nanohybrid films allowed more than 90% of visible light at 550 nm to pass and maintained almost the same high transparency as pure PBMA film even when the ZnO NP content was as high as 60 wt %. The nanohybrid films embedded with ZnO NPs exhibited significantly enhanced multifunctional performance, including excellent UV-shielding properties, good thermal stability, high surface hardness, and desired flexibility. In particular, the coefficient of thermal expansion (CTE) of the nanohybrid films clearly decreased to 193 from 325 ppm/K for pure PBMA below the glass transition temperature (Tg) and strikingly decreased from 5378 to 1373 ppm/K after the glass transition. The ZnO/polymer nanohybrids synthesized in this work are promising for the fabrication of optical devices with high transparency, such as flexible displays, optical filters, and flexible solar cells.

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INTRODUCTION Transparent inorganic−organic nanohybrid materials have been extensively investigated over the past decade because of their unique properties resulting from the combination of both inorganic nanoparticles (NPs) and organic polymer.1−6 To obtain transparent functional materials with markedly improved performance, such as low coefficient of thermal expansion (CTE),7,8 scratch resistance,9,10 selective optical transmittance, 11 and high refractive index,12,13 some NPs are incorporated into the polymer matrix homogeneously with high nanophase content. These transparent nanohybrid materials have potential to be used for flexible organic lightemitting diode (FOLED) displays, superhard coatings, intelligent optical window materials, ultrathin lenses, and so on.14−17 For example, a low-CTE feature is very important for the substrate materials used to produce the FOLED displays, because a functional material with a low CTE deposited onto a substrate can be destroyed or damaged owing to the mismatch between the CTE of the functional material and that of the substrate. Pure polymer-based flexible substrates often have high CTE values. However, these values could be reduced to meet the substrate material requirements of FOLED displays if sufficient NPs could be added into the polymer matrixes.18−20 Therefore, the preparation of transparent polymer-based nanohybrid materials with high NP contents is in great demand. It is well-known that the more NPs are added into the polymer matrix, the more difficult it is to achieve the uniform dispersion of the NPs in the matrix. The aggregation of NPs would reduce the transparency and has a worse effect on the properties of the obtained composites,21,22 resulting in a huge obstacle for their applications. So it is very crucial to find an effective way to overcome the NP dispersion problem for highly transparent nanohybrids. Employing appropriate syn© 2012 American Chemical Society

thesis methods with surface-functionalized and surface-stabilized nanoparticles can improve the dispersibility of NPs in polymer matrixes.21 For example, Yang et al.12 presented a bulk polymerization approach for the preparation of transparent nanohybrids with high nanophase content using mercaptoethanol-capped ZnS NPs. Förster et al.23 discovered that nanoparticles coated with a brushlike polymer layer showed no signs of aggregation in polymer at high NP content. Zinc oxide (ZnO) has recently attracted increased attention as a promising material for multifunctional inorganic NPs in view of its outstanding properties, including a low CTE, good thermal stability, strong photoluminescence, high mechanical rigidity, and intensive UV light absorption.24,25 Therefore, ZnO/polymer nanohybrids with high ZnO NP contents might exhibit excellent multifunctional performance for a wide range of existing and new applications in industry. To combine these unique properties of ZnO with those of a polymer through the fabrication of transparent nanohybrids, a variety of synthesis methods and techniques have reported, such as sol−gel process,26 melt compounding,27 solution mixing,28−30 and in situ bulk polymerization.31−34 However, the contents of ZnO NPs in the reported transparent ZnO/polymer nanohybrids have not yet exceed 30 wt %, and the problem of ZnO NP aggregation has occurred for nanohybrids with over 30 wt % NPs. For example, Lü et al.31 synthesized transparent ZnO/ polymer nanohybrid films by an ultraviolet-radiation-initiated polymerization method. When the ZnO content was less than 7 wt %, the nanohybrid films showed high transmittance of over Received: Revised: Accepted: Published: 6753

February April 17, April 27, April 27,

16, 2012 2012 2012 2012

dx.doi.org/10.1021/ie300425v | Ind. Eng. Chem. Res. 2012, 51, 6753−6759

Industrial & Engineering Chemistry Research

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As a comparison, we employed a conventional physical mixing method in place of the above bulk polymerization method to fabricate nanohybrid films with 5 wt % ZnO NP content. The ZnO NP dispersion in butyl acetate was directly added to the polymer solution of PBMA in butyl acetate. After being stirred for 3 h at room temperature, the mixture was poured into a glass mold and allowed to dry at 65 °C for 24 h, then at 75 °C for 2 h, and finally at 95 and 115 °C for 1 h each (the same temperature-programmed process as used for the above polymerization). The thicknesses of all fabricated pure PBMA films and ZnO/PBMA nanohybrid films prepared by physical mixing and chemical polymerization were about 100 ± 5 μm. Characterization. Fourier transform infrared (FTIR) spectroscopic measurements were recorded on a Fourier transform spectrometer (Nexus670, Nicolet, Madison, WI) in 4 cm−1 increments. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-3010 microscope (Tokyo, Japan). Ultraviolet−visible (UV−vis) absorption and transmittance spectra of nanohybrid films were recorded on a UV3150 spectrometer (Shimadzu, Kyoto, Japan) in the range from 200 to 800 nm in 0.5-nm increments. Thermogravimetric analysis (TGA) of the nanohybrid films was performed on a thermogravimetric/differential thermal analyzer (STA-449C, NETZSCH, Selb, Germany) under an air atmosphere from 30 to 550 °C at a heating rate of 10 °C/min. The coefficients of thermal expansion of the nanohybrid films were measured on a Q400 thermomechanical analyzer (TMA) (TA Instruments, New Castle, DE) from −35 to 175 °C at a heating rate of 5 °C/ min in a nitrogen atmosphere. Pencil scratch hardness measurements were carried out according to the standard ASTM method D3363-05, using a pencil hardness tester.

95% at 550 nm, but the transmittance declined to 82% when the ZnO content reached 15 wt %. Wegner et al.33 prepared ZnO/PMMA nanocomposite films by in situ bulk polymerization. When the ZnO content in the polymer matrix reached 30 wt %, the transmittance of the film at 550 nm decreased to 87.2% from the 92% value for pure PMMA. Therefore, it is still a great challenge to disperse a large number of ZnO NPs (i.e., more than 30 wt %) into a polymer matrix homogeneously to fabricate highly transparent nanohybrids maintaining almost the same transmittance as the pure polymer in the visible light region. In addition, the effects of ZnO NPs on the mechanical and thermal properties of transparent nanohybrids have rarely been reported. In this work, we synthesized ZnO/poly(n-butyl methacrylate) (ZnO/PBMA) nanohybrid films with superhigh ZnO NP contents by a bulk polymerization route. The obtained ZnO/ PBMA nanohybrid films had novel features as follows: (1) The ZnO NP content was as high as 60 wt %. (2) The nanohybrid films maintained almost the same high transparency of visible light as the pure polymer films. (3) The CTE of the nanohybrid films was reduced dramatically compared to that of the pure polymer, PBMA, from 325 to 193 ppm/K below Tg and from 5378 to 1373 ppm/K above Tg. The synthesized nanohybrid films in this work exhibited outstanding multifunctional performance, including superb UV-shielding properties; good thermal resistance; high surface hardness; and desired flexibility, so that they could be applicable to functional optical devices.

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EXPERIMENTAL SECTION Materials. n-Butyl methacrylate (BMA) and 2,2′-azobisisobutyronitrile (AIBN) (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). BMA was purified by vacuum distillation. AIBN was recrystallized from ethanol to be used as an initiator. A dispersion of ZnO NPs (product code ZNK133D) in butyl acetate was obtained from Nanomaterials Technology Pte. Ltd. (Singapore). The complete preparation process of ZnO NPs is reported in the corresponding patent.35 Ethanol (EtOH) and tetrahydrofuran (THF) (analytical grade) were purchased from Beijing Beihua Fine Chemicals Co., Ltd. (Beijing, China) and used as received. Synthesis of ZnO/PBMA Nanohybrid Films. The synthesis of ZnO/PBMA nanohybrid films by bulk polymerization consisted of the following processes. First, 5.0 g of ZnO NPs was obtained from ZnO NPs dispersion in butyl acetate after the solvent had been removed by reduced-pressure distillation. Then, the ZnO NPs were blended with a certain amount of monomer BMA, and the mixture was stirred at room temperature for 3 h to obtain a dispersion of ZnO NPs in BMA that was kept overnight. The initiator, AIBN (0.5 wt % based on the weight of the dispersion), was subsequently added to the dispersion, and the mixture was stirred at room temperature for another 2 h. Finally, the obtained transparent dispersion was transferred into a glass mold [i.e., dispersion filled the space between two glass sheets separated by a poly(ethylene terephthalate) film seal], and the bulk polymerization reaction was conducted at 65 °C for 24 h, then at 75 °C for 2 h, and finally at 95 and 115 °C for 1 h each. After the thermal polymerization, highly transparent ZnO/PBMA nanohybrid films were obtained. The ZnO NP content in the obtained nanohybrid films varied from 5 to 60 wt %, that is, from 1.0 to 22.3 vol %. Pure PBMA films were prepared by the same process as described above but without using ZnO NPs.

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RESULTS AND DISCUSSION FTIR spectra of ZnO NPs, BMA, and ZnO/PBMA nanohybrid films are shown in Figure 1. The absorption peaks at 2850−

Figure 1. FTIR spectra of ZnO NPs, BMA, and ZnO/PBMA nanohybrid film.

2960, ∼1715, and ∼1640 cm−1 are due to the characteristic vibrations of CH, CO, and CC bonds, respectively. The positions of these absorption peaks were similar to those reported in refs 11, 36, and 37. The strong absorption peaks of CH, CO, and CC bonds indicate that the ZnO NPs were capped by organic molecules and that a large number of CC double bonds were present in the organic molecular structure. The absorption peak at ∼1640 cm−1 disappeared in the nanohybrid films, which revealed that the CC double bonds in the initial raw materials (i.e., monomers and ZnO NPs) had completely polymerized. Meanwhile, the nanohybrid 6754



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films did not dissolve or swell after being immersed in THF at room temperature for one week. This excellent solvent resistance indicates that the synthesized nanohybrid film had a cross-linked structure.38,39 Therefore, it can be deduced that ZnO NPs acted as chemical cross-linking agents during the synthesis of the nanohybrid films using the bulk polymerization method. Nanohybrid films with a three-dimensional network structure were synthesized after the radical-initiated crosslinking polymerization reaction between ZnO NPs and monomers. Schematic illustrations of the synthesis of ZnO/ PBMA nanohybrid films and the presumed structure of the nanohybrid films are shown in Figure 2. The movement and

sity in organic solvent. Parts b and c of Figure 3 show TEM images of the ZnO/PBMA nanohybrid films synthesized by the bulk polymerization method with 5 and 60 wt % ZnO NPs, respectively. For all nanohybrids with low or high ZnO contents, the ZnO NPs exhibited a homogeneous dispersion in the polymer matrix at the nanoscale level and retained their original size of 4−6 nm without any aggregation, which indicates that the ZnO NPs were successfully incorporated into the polymer matrix by the bulk polymerization method. A TEM image of a nanohybrid film fabricated by the physical mixing method with 5 wt % ZnO NPs is also presented in Figure 3d. It can be seen that the ZnO NPs exhibited serious aggregation in the polymer matrix, with the largest aggregated diameter reaching more than 200 nm, even though the ZnO NP content was much lower than that of the nanohybrids synthesized by the bulk polymerization method. The dispersion states of ZnO NPs in the polymer matrix were notably different between the nanohybrids prepared by physical mixing or by bulk chemical polymerization. When the nanohybrids were prepared by the mixing method, the compatibility and interaction forces between the ZnO NPs and the polymer matrix were relatively low. As a result, the NPs aggregated severely in the nanohybrids when the solvent evaporated. When the nanohybrids were synthesized by bulk polymerization, the ZnO NPs could covalently attach to the surrounding polymer chains through the cross-linking polymerization reaction, as shown in the schematic in Figure 2. The cross-linking reaction could effectively increase the interaction force between the NPs and the polymer matrix and prevent the aggregation of ZnO NPs during the polymerization process. Therefore, increasing the interaction force between the NPs and the polymer matrix appropriately is an effective way to avoid the aggregation of NPs during the preparation of nanohybrids. Figure 4a shows the UV−vis absorbance spectra of pure PBMA film and ZnO/PBMA nanohybrid films synthesized by the bulk polymerization method containing ZnO NPs from 10 to 60 wt % in 10 wt % increments. The pure PBMA film absorbed UV light only below 240 nm. In contrast, the ZnO/ PBMA nanohybrid films could absorb UV light not only below 240 nm but also in the 240−355-nm range with a very sharp absorption edge around 355 nm, which reveals that the ZnO NPs embedded in the nanohybrids had a wider band gap than the intrinsic band gap of bulk ZnO (3.3 eV) because of quantum confinement effects.40 The very sharp UV absorption edge moved to slightly longer wavelengths as the ZnO NP content in the nanohybrid films was increased from 10 to 60 wt %. The absorbance spectra clearly indicate that the UVshielding properties were improved markedly after the incorporation of ZnO NPs into the PBMA matrix. Meanwhile, none of the ZnO/PBMA nanohybrid films absorbed light in the visible range, which was also the case for the pure PBMA film. The data extracted from the UV−vis absorbance spectra were used to calculate the average size (D) of ZnO NPs according to the expression proposed by Meulenkamp41 as follows

Figure 2. Schematic representation of the synthesis of ZnO/PBMA nanohybrid films by bulk polymerization and the presumed structure of the nanohybrid films.

dissolution of polymer chain segments in the organic solvent were strongly arrested by the chemical cross-linking points of ZnO NPs in the nanohybrid films, which induced their properties of organic solvent resistance. Figure 3a shows a TEM image of ZnO NPs in butyl acetate. ZnO NPs with diameters of 4−6 nm had a spherical and uniform particle morphology and exhibited high monodisper-

1240 294.07 1.09 = 3.301 + + 2 λ1/2 D D

(1)

where λ1/2 (nm) is the wavelength corresponding to the shoulder of the absorption profile at the half-height of the intensity and D (Å) is the particle diameter.41,42 As the ZnO NP content increased from 10 to 60 wt %, λ1/2 increased from

Figure 3. TEM images of (a) ZnO NPs in butyl acetate, ZnO/PBMA nanohybrid films synthesized by bulk polymerization with (b) 5 and (c) 60 wt % ZnO NPs, and (d) a nanohybrid film fabricated by conventional physical mixing with 5 wt % ZnO NPs. 6755



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Figure 5. Photographs of (a) pure PBMA film, ZnO/PBMA nanohybrid films synthesized by bulk polymerization with ZnO NP contents of (b) 5 and (c) 60 wt %, and (d) a nanohybrid film fabricated by a physical mixing method with 5 wt % ZnO NPs. The flexibility of the nanohybrid film containing 60 wt % ZnO NPs is shown in the inset in part c. The thickness of each film was about 100 ± 5 μm.

Figure 4. (a) UV−vis absorbance spectra of pure PBMA film and ZnO/PBMA nanohybrid films synthesized by bulk polymerization containing ZnO NPs from 10 to 60 wt % in 10 wt % increments. (b) UV−vis transmittance spectra of pure PBMA film and ZnO/PBMA nanohybrid films prepared by bulk polymerization and physical mixing method. The thickness of each film was about 100 ± 5 μm.

the dispersion state of ZnO NPs in the polymer matrix (as shown in the TEM images of Figure 3). Figure 6 shows the transmittance values at 550, 400, and 350 nm for a pure PBMA film and ZnO/PBMA nanohybrid films

351.7 to 360.5 nm, as observed in Figure 4a. Application of the eq 1 to the observed λ1/2 values indicates that the average size of the ZnO NPs in nanohybrid films was less than 5.0 nm, confirming that the ZnO NPs were well-dispersed homogeneously in the polymer matrix despite the nanophase content. The particle size estimated from the UV−vis absorbance analysis is in good agreement with that observed in the TEM images of the nanohybrid films (Figure 3b,c). The UV−vis transmittance spectra of ZnO/PBMA nanohybrid films prepared by the bulk polymerization and conventional physical mixing methods and of pure PBMA film are shown in Figure 4b. Both nanohybrid films synthesized by the bulk polymerization method with ZnO NP contents of 5 and 60 wt % allowed more than 90% of visible light at 550 nm to pass, and photographs of the nanohybrid films are shown in parts b and c, respectively, of Figure 5. The two films maintained almost the same transparency as the pure PBMA film (Figure 5a). Moreover, the nanohybrid film containing 60 wt % ZnO NPs could be bent without any fracture, as shown in the inset of Figure 5c, and exhibited the desired flexibility. In contrast, the transmittance of the nanohybrid film fabricated by physical mixing with 5 wt % ZnO NPs decreased to 10% from the 92% value observed for the pure PBMA film at 550 nm, and the photograph of this translucent nanohybrid film is shown in Figure 5d. It has been reported that NP aggregates, especially those larger than 40 nm in diameter, in a polymer matrix steeply increase the intensity of scattered light as described by Rayleigh’s law, which would induce a dramatic decrease of the transparency of the nanohybrids.21 The transparency in the visible light range was distinctly different for the nanohybrids prepared by physical mixing and those prepared by bulk polymerization, which demonstrates the notable difference in

Figure 6. Transmittance values at 550, 400, and 350 nm of pure PBMA film and ZnO/PBMA nanohybrid films synthesized by bulk polymerization with different ZnO NP contents. The thickness of each film was about 100 ± 5 μm.

synthesized by the bulk polymerization method with ZnO NP contents of 10−60 wt %. In the visible light range from 400 to 800 nm, the transparency values of all of the nanohybrid films were almost the same as that of the pure PBMA film, and all of the transparency values were higher than 90% at 400 and 550 nm regardless of the ZnO NP content. Meanwhile, the nanohybrid film containing 10 wt % ZnO NPs could block up to 99.5% of the UV radiation at 350 nm. These results indicate that the introduction of ZnO NPs significantly increased the UV-shielding properties of pure PBMA while maintaining the high optical transparency of the pure polymer matrix in the visible light range, which is consistent with the UV−vis absorbance properties shown in Figure 4a. Therefore, the synthesized ZnO/PBMA nanohybrids could be used for 6756



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UV-light-switch optical devices such as UV-shielding films, sunglasses, optical filters with specific wavelengths, or windshields and windows of airplanes and space vehicles. The TMA curves of a pure PBMA fillm and a ZnO/PBMA nanohybrid film with 30 wt % ZnO NPs are shown in Figure 7.

Figure 9. TGA curves of pure PBMA, ZnO/PBMA nanohybrids with different ZnO NP contents, and ZnO NPs.

the TGA curves increased with increasing ZnO content, and the amount measured was in good agreement with the theoretical values of pure ZnO in the nanohybrids, thus indicating that all of the ZnO NPs were successfully incorporated into the PBMA matrix. As shown in the TGA curves, the nanohybrid film containing 5 wt % ZnO NP content fabricated by the physical mixing method exhibited almost the same thermal decomposition profile as the pure PBMA matrix. In contrast, the embedding of 5 wt % ZnO NPs into the PBMA matrix through bulk polymerization obviously enhanced the thermal stability of the pure polymer matrix. The decomposition rate was much lower than those of pure PBMA and the nanohybrid fabricated by the physical mixing method. It has been shown that a strong interaction force between NPs and the polymer matrix hinders the movement of the polymer strand and makes the polymer more difficult to decompose.45,46 The higher thermal stability of the ZnO/PBMA nanohybrids also confirms the homogeneous dispersion state of ZnO NPs in the polymer matrix. To quantify the improvement of the thermal stability, two characteristic parameters of the TGA curves, namely, the initial decomposition temperature and the temperature at 30 wt % weight loss, were extracted and are reported in Table 1. These results suggest that, as the ZnO content in the nanohybrids synthesized by the bulk polymerization method increased from 10 to 60 wt %, the initial decomposition temperature increased, and the thermal decomposition rate decreased. The improvement of the thermal resistance might be due to two factors. First, the ZnO NPs had better thermal stability than PBMA. PBMA was

Figure 7. TMA curves of pure PBMA and ZnO/PBMA nanohybrid with 30 wt % ZnO NPs.

The glass transition temperatures (Tg) of PBMA and the nanohybrid were 41.4 and 49.5 °C, respectively, as observed in the inset of Figure 7. The large amount of chemical crosslinking at ZnO NPs restricts the segmental motions of the polymer chains, which might lead to an increase in the Tg value of the polymer.43 The coefficients of thermal expansion of pure PBMA and ZnO/PBMA nanohybrid calculated from the TMA curves are summarized in Figure 8. Apparently, the

Figure 8. Summaries of calculated coefficients of thermal expansion of pure PBMA and ZnO/PBMA nanohybrid with 30 wt % ZnO NPs (a) below and (b) above Tg.

Table 1. Thermal Stability Parameters of pure PBMA, ZnO/ PBMA Nanohybrids Synthesized by Bulk Polymerization, and ZnO NPs

incorporation of ZnO NPs within the nanohybrid effectively reduces the CTE of pure PBMA. Below Tg, the CTE was reduced from 325 to 193 ppm/K, whereas above Tg, the efficiency was even more evident, with the CTE reduced from 5378 to 1373 ppm/K. Compared with that of pure PBMA, the CTE of the nanohybrid was decreased about 41% and 74%, respectively. It can be concluded that the ZnO/PBMA nanohybrids exhibited much higher thermal dimensional stability than pure PBMA. The significant reduction of the CTE might be due to the low CTE performance of ZnO (2.0 ppm/K).20 Meanwhile, the strong interaction force between the ZnO NPs and the polymer matrix in the cross-linked nanohybrids limits the movement of the polymer chains and decreases the thermal expansion of the PBMA matrix.18,44 Figure 9 shows the thermogravimetric analysis (TGA) results for pure PBMA, ZnO/PBMA nanohybrids, and ZnO NPs. It can be seen that about 18 wt % organic molecules capped the surface of the ZnO NPs. The nanohybrid residue observed in

pure PBMA ZnO/10 wt % PBMA ZnO/20 wt % PBMA ZnO/30 wt % PBMA ZnO/40 wt % PBMA ZnO/50 wt % PBMA ZnO/60 wt % PBMA ZnO NPs 6757

initial decomposition temperature (°C)

temperature at 30 wt % weight loss (°C)

214.7 217.1

247.7 259.5

218.4

289.5

224.6

299.8

226.8

314.8

231.7

320.5

239.7

332.6

249.6

−



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found to decompose at about 215 °C, whereas the ZnO NPs exhibited a higher initial decomposition temperature of about 250 °C. Second, the nanohybrids synthesized by bulk polymerization had a three-dimensional network structure that could enhance the thermal stability.36 Figure 10 presents the pencil hardness results for a pure PBMA film and ZnO/PBMA nanohybrid films with different

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 64447274 (X.-F.Z.), +86 10 64443134 (H.Z.). Fax: +86 10 64423474 (X.-F.Z.), +86 10 64448778 (H.Z.). Email: [email protected] (X.-F.Z.), [email protected]. edu.cn (H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21006004, 21121064) and the National ‘863’ Program of China (Nos. 2009AA03Z334, 2009AA033301, and 2012AA030605).

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REFERENCES

(1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid NanorodPolymer Solar Cells. Science 2002, 295, 2425. (2) Lu, Y.; Yang, Y.; Sellinger, A.; Lu, M.; Huang, J.; Fan, H.; Haddad, R.; Lopez, G.; Burns, A. R.; Sasaki, D. Y.; Shelnutt, J.; Brinker, C. J. Self-Assembly of Mesoscopically Ordered Chromatic Polydiacetylene/Silica Nanocomposites. Nature 2001, 410, 913. (3) Wang, G. F.; Tao, X. M.; Wang, R. X. Flexible Organic LightEmitting Diodes with a Polymeric Nanocomposite Anode. Nanotechnology 2008, 19, 145201. (4) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics. Nat. Mater. 2005, 4, 138. (5) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and Stiff Layered Polymer Nanocomposites. Science 2007, 318, 80. (6) Wang, L.; Yoon, M. H.; Lu, G.; Yang, Y.; Facchetti, A.; Marks, T. J. High-Performance Transparent Inorganic−Organic Hybrid ThinFilm n-Type Transistors. Nat. Mater. 2006, 5, 893. (7) Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Optically Transparent Nanofiber Paper. Adv. Mater. 2009, 21, 1595. (8) Zhi, C.; Bando, Y.; Terao, T.; Tang, C.; Kuwahara, H.; Golberg, D. Towards Thermoconductive, Electrically Insulating Polymeric Composites with Boron Nitride Nanotubes as Fillers. Adv. Funct. Mater. 2009, 19, 1857. (9) Veprek, S.; Veprek-Heijman, M. G. J.; Karvankova, P.; Prochazka, J. Different Approaches to Superhard Coatings and Nanocomposites. Thin Solid Films 2005, 476, 1. (10) Di Gianni, A.; Trabelsi, S.; Rizza, G.; Sangermano, M.; Althues, H.; Kaskel, S.; Voit, B. Hyperbranched Polymer/TiO2 Hybrid Nanoparticles Synthesized via an in Situ Sol−Gel Process. Macromol. Chem. Phys. 2007, 208, 76. (11) Yuwono, A. H.; Liu, B.; Xue, J.; Wang, J.; Elim, H. I.; Ji, W.; Li, Y.; White, T. J. Controlling the Crystallinity and Nonlinear Optical Properties of Transparent TiO2−PMMA Nanohybrids. J. Mater. Chem. 2004, 14, 2978. (12) Lü, C.; Cheng, Y.; Liu, Y.; Liu, F.; Yang, B. A Facile Route to ZnS-Polymer Nanocomposite Optical Materials with High Nanophase Content via γ-Ray Irradiation Initiated Bulk Polymerization. Adv. Mater. 2006, 18, 1188. (13) Sciancalepore, C.; Cassano, T.; Curri, M. L.; Mecerreyes, D.; Valentini, A.; Agostiano, A.; Tommasi, R.; Striccoli, M. TiO 2 Nanorods/PMMA Copolymer-Based Nanocomposites: Highly Homogeneous Linear and Nonlinear Optical Material. Nanotechnology 2008, 19, 205705. (14) Nogi, M.; Yano, H. Transparent Nanocomposites Based on Cellulose Produced by Bacteria Offer Potential Innovation in the Electronics Device Industry. Adv. Mater. 2008, 20, 1849. (15) Sangermano, M.; Messori, M. Scratch Resistance Enhancement of Polymer Coatings. Macromol. Mater. Eng. 2010, 295, 603.

Figure 10. Pencil hardness of pure PBMA film and ZnO/PBMA nanohybrid films with different ZnO NP contents.

ZnO NP contents. It can be seen that the pencil hardness showed a continuous improvement with increasing ZnO NP content and the hardnesses of the nanohybrids increased obviously from 4B for PBMA to 2B−3H depending on the amount of NPs added. This tremendous improvement in hardness can be attributed to the stiffness of the inorganic particles. In addition, the strong interaction force between the ZnO NPs and the polymer matrix and the increased crosslinking density with increasing ZnO NP content were also beneficial to the improvement of the surface hardness.47 Moreover, the improved mechanical properties also demonstrate that the ZnO NPs were dispersed homogeneously in the polymer matrix.

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CONCLUSIONS In conclusion, highly transparent and multifunctional nanohybrid films with ZnO NP contents ranging from 10 to 60 wt % were successfully synthesized by a bulk polymerization method. The ZnO NPs covalently attached to the surrounding polymer chains, which improved the compatibility and interaction force between the NPs and the polymer matrix. The ZnO NPs were dispersed homogeneously in the polymer matrix without any aggregation, even when the ZnO NP content was as high as 60 wt %. The synthesized ZnO/PBMA nanohybrid films maintained almost the same transparency as the PBMA film and exhibited more than 90% visible light transmittance regardless of the nanophase content. Meanwhile, the synthesized nanohybrid films showed excellent multifunctional features, including superb UV-shielding properties, good thermal resistance, high surface hardness, and desired flexibility. The embedding of ZnO NPs into the nanohybrids effectively increased the thermal dimensional stability of PBMA. Compared with that of pure PBMA, the CTE of the nanohybrids decreased by about 41% below Tg and by 74% above Tg. The synthesis process used in this work is simple, environmentally friendly, and easy to scale up for the potential industrialization of ZnO/polymer nanohybrids, which have great potential for use in the fabrication of multifunctional optical devices with superhigh transparency such as flexible displays, optical filters, and flexible solar cells. 6758



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Highly Transparent and Multifunctional Polymer Nanohybrid Film with

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