Layered Double Hydroxide Film with

Feb 20, 2014 - Bioinspired Nacre-like Heparin/Layered Double Hydroxide Film with Superior Mechanical, Fire-Shielding, and UV-Blocking Properties...
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Bioinspired Nacre-like Heparin/Layered Double Hydroxide Film with Superior Mechanical, Fire-Shielding, and UV-Blocking Properties Yingqi Shu,† Penggang Yin,*,‡ Jianfeng Wang,‡ Benliang Liang,‡ Hao Wang,*,† and Lin Guo*,‡ †

The College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, People’s Republic of China School of Chemistry and Environment, Beihang University, Beijing, 100191, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The combination of two or more seemingly distinct properties into a unique composite is an exciting direction for the fabrication of novel multifunctional materials. A vacuum-filtration method was used to fabricate strong and multifunctional heparin/layered double hydroxide (HEP/LDH) films mimicking nacre. The experimental results confirm that the prepared films show a layered nano/microscale-hierarchical structure, in which the LDHs are aligned, with a very high loading amount of LDHs closely comparable to that in the natural nacre, up to 87.5 wt %. Both the modulus (Er ≈ 23.4 GPa) and hardness (H ≈ 0.27 GPa) of the HEP/LDH films are remarkably high. Furthermore, the hybrid films show a combination of outstanding properties of UV-blocking and fire-resistance properties. Therefore, this work provides a way of fabricating multifunctional organic− inorganic hybrid films, which have potential applications in the areas of optical applications, transportation, and construction.

1. INTRODUCTION As conventional structural materials reach their performance limits, one of the major scientific challenges for the 21st century is the development of new composite materials with good mechanical performance and multifunctionality to support advances in diverse strategic fields.1 The combination of two or more seemingly distinct properties into a unique composite is an exciting direction for the fabrication of novel multifunctional materials. Billions of years of evolution have produced extremely efficient natural materials, which provide some inspiration for scientists and engineers to design artificial materials. In nature, nacre (mother-of-pearl), made of inorganic and organic constituents, possesses a unique combination of properties that include light weight, strength, stiffness, and toughness. Because of the great advantages provided by nacre compared with conventional artificial materials, it has been one of the most extensively attractive biological models to inspire the design and fabrication of artificial materials with high mechanical performance. It is clear now that hierarchical organization and a precisely designed organic−inorganic interface play an important role in the amazing mechanical properties of this natural nanocomposite. If suitable pathways can be developed to transfer nacre’s multiscale structures and properties into future materials, synthetic high-performance materials will undoubtedly benefit a wide area of fields, such as advanced constructions, transportation, biomedical implant technology, and even the defense sector, which is increasingly relevant to future societies. Hence, the structure−function harmony of nacre has inspired a large class of biomimetic advanced materials1−5 and organic−inorganic composites.6−12 In previous reports, natural clay mineral platelets were always used to make composites with biologically inspired structures, simultaneously, with the development of assembly techniques of montmorillonite nanosheets with polymers. These obtained hybrid films behaved with interesting fire retardant8,13 and gasbarrier properties.14 © 2014 American Chemical Society

Among these numerous platelet-like inorganic materials, layered double hydroxides (LDHs) are receiving increasingly attention in the field of organic−inorganic artificial composites because they have a number of advantages over other inorganic layered materials: (i) LDHs, exhibiting swelling behavior and the interlayer spacing can be adjusted based on the size, shape, and the number of the guest anions; (ii) the layer charge density and elemental composition of LDHs can be tuned during the synthesis process, which facilitates fine control over the properties of the host layer. On the other hand, LDHs are among the most studied advanced functional materials and have been widely applied in the field of catalysts and catalyst support,15−17 anion exchangers, molecular containers,18−20 and electrical and optical functional materials.21−23 Until recent years, with well-defined LDH nanosheets now readily available, there has been a rapid growth in publications related to the application of LDH platelets as the inorganic bricks in the fabrication of biologically inspired organic−inorganic hybrid films.24−31 For example, Yao32 assembled LDH platelets on the air/water interface and developed spin-coating into the fabrication of lamellar LDH/chitosan hybrid films; Han33 took an alternative approach in the design of an LDH platelet− poly(vinyl alcohol) matrix nacre-mimetic artificial nanocomposite; Zhu34 obtained layered LDH−PEO thin films by vacuum filtration. Unfortunately, the mechanical properties of the bioinspired LDH−polymer films are still significantly lower than those of nacre. This is attributed to low volume fraction of inorganic constituents, random orientation, weak bonding at the inorganic−organic interface, and weak interfacial load transfer in LDH−polymer composites. Moreover, the exploitation of the extraordinary properties of artificial LDH− Received: Revised: Accepted: Published: 3820

December 5, 2013 February 13, 2014 February 20, 2014 February 20, 2014 dx.doi.org/10.1021/ie404115g | Ind. Eng. Chem. Res. 2014, 53, 3820−3826

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Scheme 1. Fabrication of the Artificial Nacre-like HEP/LDH Filma

The NiAl−NO3 LDH powder is first delaminated by formamide solution. Then, the exfoliated LDH nanosheet and an aqueous solution of HEP are stirred together to guarantee full adsorption of HEP on LDH nanosheets. Finally, the HEP−LDH hybrid building blocks are aligned into the nacrelike structured composite by self-assembly induced by vacuum filtration. a

polymer films is limited. Meanwhile, multifunctional biologically inspired LDH−polymer films have been rarely reported. Hence, it still remains a significant technical challenge to fabricate high performance composites with high LDH loading and multifunctionalities. In this study, heparin (HEP), a green, biocompatible, and environmentally friendly glycosaminoglycan, which has the highest negative charge density of all known biological polyanions, is chosen as “mortar” to interact with positive charged LDH nanosheets. Since each HEP molecule contains ca. 20 sulfonic groups and ca. 10 carboxylic groups,35 it can be associated with the positively charged LDH monolayers to produce an organic/inorganic film by strong electrostatic interactions. Simultaneously, a great number of hydroxy groups on the surface of LDH nanoplatelets as well as in the HEP molecules will facilitate the formation of a hydrogen bond network at the organic−inorganic interface. Bioinspired, nacrelike HEP/LDH films in practical sizes are fabricated by the vacuum-filtration method (Scheme 1). The large size films help us to exploit more multifunctional applications when compared with our group’s previously reported ultrathin films36 by the layer-by-layer (LBL) fabrication technique. Moreover, this meets the requirements of future construction and coating applications. In contrast to the previous reports on polymer/ LDH composites,24,25,32−34 LDH nanosheet in this paper is used as the matrix phase. The experimental results confirm that the inorganic nanosheets are aligned well with a high loading amount of LDHs, up to 87.5 wt %, which is very closely comparable to that of natural nacre. In addition, both the modulus (Er ≈ 23.4 GPa) and hardness (H ≈ 0.27 GPa) of the HEP/LDH films are remarkably high. Furthermore, the hybrid films show a combination of outstanding properties of UVblocking and fire-resistance properties. We hope these advanced HEP/LDH bioinspired films could prove beneficial for transportation, construction, or protection, or as a replacement for conventional petroleum based plastics.

heparin salt was dissolved in a small amount of deionized water. Then it was diluted with formamide to an indicated anion concentration (water/formamide volume ratio of 1/3). The exfoliated, positively charged LDH nanosheets were synthesized by vigorously agitating 0.1 g of Ni−Al−NO3 LDH in 100 mL of formamide in a two-necked flask under a N2 gas flow for 24 h. This solution was allowed to settle for 24 h, and the supernatant fraction was then employed for the negatively charged HEP molecule adsorption. To adsorb one monolayer of HEP onto the LDH platelets, the LDH dispersion was slowly added to a stirred solution of HEP. When 0.1 wt % HEP solution was added to exfoliated LDH colloids, LDH nanosheets were aggregated into flocculent precipitates. This process was easily performed on a 1−2 L scale and the solution was stirred for at least overnight to complete polymer adsorption and a fine dispersion of the stabilized LDH platelets. Then, the colloidal dispersion was vacuum filtered through polyamide membrane filter (220 nm pore size, Shanghai Xinya). In the presence of vacuum pressure, LDH nanosheets coated with HEP molecules were deposited on the membrane filter with packed interlocking structure and preferential plane orientation. Finally, the membrane filter was torn off and the HEP/LDH film was dried under room temperature. The thickness of the films can be controlled by the volume of the dispersion loaded onto the filter. 2.3. UV-Shielding Test. The UV-shielding performance of the HEP/LDH film was evaluated by the photocatalytic degradation of Rhodamine B aqueous solution in the presence of TiO2 nanoparticles using a 400 W high-pressure mercury lamp as the source of irradiation.37 The experiments were conducted as follows: a suspension consisting of 30 mL of Rhodamine B aqueous solution (0.01 g L−1) with 20 mg of TiO2 nanoparticles was stirred in the dark for 1 h to ensure complete dispersion of TiO2 nanoparticles and adsorption/ desorption equilibrium. The suspension was then charged into a single-necked flask covered by a layer of aluminum foil to exclude light. The mouth of the flask was left open, allowing light to enter. The photocatalytic degradation of Rhodamine B was carried out under constant stirring. Then, 3 mL of the suspension was sampled every 10 min and centrifuged to remove TiO2 nanoparticles. After that, the absorbance at 552 nm was determined. Nacre-like HEP/LDH film and weighing paper of the same thickness as the film served as controls. 2.4. Characterization. X-ray diffraction (XRD) data of HEP/LDH film was recorded by a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 40 mA,

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Analytical grade reagents heparin (HEP, Mw = 6−20 kDa), Ni(NO3)2·6H2O, Al(NO3)3· 9H2O, and NaOH were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. Polyamide membrane filters (220 nm pore size) were purchased from Shanghai Xinya. 2.2. Fabrication of Highly Oriented HEP/LDH Film. The synthesis process and exfoliation of Ni−Al LDH were similar to that described in our previous work.36 A 0.1 g sample of 3821

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band in the wavenumber range 3300−3600 cm−1 and the adsorption around 1630 cm−1 were associated with the stretching vibration and bending vibration of O−H in the brucite-like layer and water molecules;38 (ii) the absorption band at 1384 cm−1 provided evidence about the presence of NO3−1.39 From the FTIR spectra of the HEP/LDH film, the NO3− feature peak of the LDH was not observed after the fabrication, because the LDH was exfoliated to positively charged unilamellar LDH nanosheets to interact with HEP. A great number of hydroxy groups on the surface of LDH nanoplatelets as well as in the HEP molecules would facilitate the formation of a hydrogen bond network at the organic− inorganic interface, which led the O−H bands of the film to become broader.40 In the spectrum of the HEP/LDH film compared with that of heparin sodium salt (Figure 2d), a few characteristic bands of the pristine HEP were also observed, such as the stretching vibration of CO symmetric stretching (1421 cm−1), SO antisymmetric stretching vibration (1230 cm−1), and symmetric stretching vibration (1045 cm−1).41,42 Moreover, the Δν (νas − νs = 185 cm−1) value for the HEP/ LDH film was close to that obtained from the spectrum of heparin sodium salt (Δν = 195 cm−1), which implied that HEP interacted like a free-like anion with the LDH nanosheets to form strong electrostatic interactions and a hydrogen bond network at the HEP−LDH interface.43 Thermogravimetric analysis (TGA) (Figure 3) revealed exceptionally high loading of the inorganic material as in nacre (87.5 wt % LDH as determined by TGA). The resulting composites revealed a favorable majority of inorganic material. The residual contents of HEP and HEP/LDH at 700 °C in air were 39 and 60%, respectively. The weight fraction of LDH in HEP/LDH multiplied by 63% plus the weight fraction of HEP in HEP/LDH multiplied by 39% equals 60%. Therefore, the weight fraction of LDH was calculated to be about 87.5%. The contents of adsorbed HEP molecules was estimated by TGA to be 12.5 wt %. The low organic constituent content of the obtained nacre-like HEP/LDH bionanocomposite films was similar to that of the natural nacre. 3.2. Multifunctionalities of Hybrid HEP/LDH Films. 3.2.1. Mechanical Properties. To demonstrate the full nanomechanical characterization of HEP/LDH film, a novel partial unloading (PUL) technique was used to obtain correct results. PUL enabled depth profiling by performing multiple partial unloads followed by reloading to a greater maximum load during a single test, which was used to test the ultrathin film (HEP/LDH)n by a bottom-up layer-by-layer (LBL) technique in our previous report,36 and a similar technique was also used by Li44 to test the mechanical characterization of a shell. Prior to testing, the specimen was imaged in situ using the nanoindentation tip in scanning probe microscopy (SPM) to ensure that the location for the indentation was clean and relatively flat. Once the desired position was defined (green mark in Figure 4a), the PUL pattern was used to obtain the mechanical properties of the HEP/LDH film. Two representative 15 μm, two-dimensional (2-D) topographical in situ SPM images were obtained on the film before and after the PUL test (Figure 4a). No cracks were found around the indentation from the residual SPM image. In Figure 4b, the unloading segments of each indentation were then analyzed by using the Oliver and Pharr approach45 to determine the hardness and the reduced modulus. Similar to analysis of the single-cycle nanoindentation data, depth profiling nanoindentation results could be ranked according to measured mechanical properties. Figure 4c,d

and Cu Kα radiation. Fourier transform infrared (FTIR) spectra were measured on a Nicolet NEXUS-470 infrared spectrophotometer using the KBr pellet technique. The UV/vis transmittance spectra of the film were collected in the range from 200 to 800 nm on a Shimadzu-3600 spectrophotometer. Scanning electron microscopy (SEM) was performed on a JEOL JSM7500FA field emission microscope, typically operated at 3.0 kV. The acceleration voltage was increased to 10 kV for energy-dispersive X-ray analysis (EDX) measurements. Loading of LDH inside the free-standing film was determined with a thermogravimetric analyzer (TGA) TA SDT Q600, with a temperature ramp-up rate of 10 °C/min while being purged with air at a flow rate of 100 mL/min. The mechanical properties of the film were tested using the HYSITRON TI-950 TriboIndenter. A Berkovich shape indenter was used. The hardness and Young’s modulus were calculated and recorded.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of Hybrid HEP/LDH Films. Heparin molecules are very easily coated onto exfoliated LDH nanosheets to yield hybrid building blocks by strong electrostatic and hydrogen-bonding interactions. The aligned self-assemblies were maintained throughout the whole sample despite the rapidness of the process. A photograph of the obtained HEP/LDH film is shown in Figure 1a. The film had

Figure 1. (a) Photograph of the obtained free-standing HEP/LDH hybrid film, with the printed “Great Wall” logo placed behind. (b) Low- and high-resolution (inset of (b)) SEM images of the nacre-like hybrid film clearly revealing the strong orientation of the LDH nanoplatelets paralleled.

high optical quality (translucency) and glossiness. Scanning electron microscopy (SEM) shows the structure with a strikingly strong alignment of the LDH platelets (Figure 1b) and a homogeneous component distribution as was concluded from the energy-dispersive X-ray analysis (EDX) element maps (Supporting Information, Figure S1). The HEP/LDH hybrid building blocks were stacked together to form a densely oriented lamellar microstructure, which was reminiscent of the brick-and-mortar structure of nacre. The ordered stacking of HEP/LDH thin film was confirmed by XRD measurement (Figure 2a), from which a so-called superlattice reflection of the inorganic−organic repeating nanostructure was observed. The peak at ca. 5.5° corresponded to the d003 = 1.5 nm of the hybrid films, demonstrating the film had long-range stacking order. Allowing for a thickness of about 0.48 nm for the LDH nanosheets,36 the layer height of HEP along the film normal was estimated to be about 1.0 nm (Figure 2b). From the FTIR spectra (Figure 2c), the spectrum of pristine NiAl−NO3 LDH showed some peaks: (i) the broad 3822

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Figure 2. (a) XRD patterns of the LDH/HEP film. (b) Structure model of the HEP/Ni−Al LDH system. (c) FTIR spectra of NiAl−NO3, HEP, and HEP/LDH film, respectively. (d) Close-up of the spectra in (c).

chemical reactions, weathering of polymers, fading of certain coloring, and even eye and skin damage. For these reasons, UV light blocking is one of the properties in great demand in multifunctional materials. The HEP/LDH film had a blue-green hue due to the color of the Ni−Al LDH. This would have an impact on the result of the hybrid film exhibiting permselectivity in the narrow wavelength range of the different colors in the visible region (Figure 5a). The extraordinary visible-light transparency of hybrid films was attributed to the flat and uniform orientation of the LDH platelets in the hybrid films. The optical properties of the HEP/LDH hybrid film were revealed by UV−vis transmittance spectra. In Figure 5b, HEP exhibited a high transmittance the UV and visible regions. LDH nanosheets showed partial blocking in the UV region because nanoscaled laminates of LDH sheets (60−70 nm) reflected and scattered UV radiation. This phenomenon could be ascribed to two possible aspects. On one hand, the HEP/LDH film exhibited a complete blocking to the passage of UVC radiation. Relevant regions for protection are the UVB and UVA regions, where the maximum transmittance value of the film is just 20%, yielding a very efficient blocking effect. The scattering of the laminates of LDH nanosheets and the layered interfaces between the HEP and LDH contributed to the good UVshielding properties of the HEP/LDH film. On the other hand, the strong electrostatic interaction and hydrogen bond network at the organic−inorganic interface between the LDH nanosheets and HEP molecules played an important role in shielding of UV irradiation. The HEP/LDH film blocked radiation partially in the UVA region and totally in the UVB and UVC regions, indicating its potential application in special window materials protecting objects from being damaged by UV light. The UV-shielding ability of the nacre-like HEP/LDH film was examined by the photocatalytic degradation of Rhodamine B aqueous solution in the presence of TiO2 nanoparticles (Figure 5c).37 For the sake of comparison, the photocatalytic degradation behavior of unprotected Rhodamine B aqueous solutions was measured under the same conditions. As shown

Figure 3. TGA results for (a) LDH powder, (b) HEP polymer, and (c) HEP/LDH film.

shows the typical hardness−contact depth curve and reduced elastic modulus−contact depth curve for the film. Both the hardness (H) and reduced elastic modulus (Er) of the film at first increased sharply to a maximum value and then became a constant value with increasing indentation depth, with H of ∼0.27 GPa and Er of ∼23.4 GPa. The PUL tests were also done in different parts of the HEP/LDH film, which had consistent results (Supporting Information, Figure S2). The values were consistent with those for our previous (HEP/LDH)n ultrathin film by LBL.36 We suggested that a good performance in mechanical properties mostly related to well-interlocked assemblies by HEP and LDHs. The high loading amount of ordered LDH building blocks, combined with strong electrostatic interaction and hydrogen binding, led to effective load transfer between LDHs and the HEP. Considering the fast and easy processing method together with the lower energy requirements, these biomimetic materials can arouse broad interest. We foresaw these advanced biomimetic HEP/LDH films, valuable for applications in lightweight construction and coatings. 3.2.2. UV-Blocking Properties. Ultraviolet radiation (generally classified in three zones: UVC (200−280 nm), UVB (280−315 nm), and UVA (315−400 nm)) accounts for only 3% of the total radiation that reaches the earth, but it causes 3823

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Figure 4. PUL test made on HEP/LDH film. (a) A 15 μm, 2-D topographical in situ SPM image of the sample surface (left) before and (right) after the PUL nanoindentation test. (b) Load−displacement profile obtained by the PUL test. (c) Depth profile of H data from the PUL nanoindentation test on the HEP/LDH film. (d) Modulus values of the sample.

photocatalytic degradation test of Rhodamine B also showed good UV-shielding property of the HEP/LDH. A combination film with such transparency and UV-blocking properties might be inspiring for a broad range of fields in optical applications. 3.2.3. Fire-Shielding Properties. Owing to the high content of ordered LDH platelets, good fire-resistance and heat-shield capabilities were also expected for the composites as well as mechanical properties. Similar test techniques were also used by Walther8,13 and Yu32 et al. to test the fire-shielding properties of organic−inorganic hybrid films. Cotton placed behind a HEP/ LDH hybrid film (0.2 mm thick) did not catch fire even upon prolonged exposure (Supporting Information, video 1). When exposed to a high temperature gas flame (ca. 2000 °C), the film first caught fire for a short moment (Figure6a) on account of the small amount of the HEP adsorbed on the LDH nanosheets. Then, the film gradually became black (Figure 6b), which was partly caused by carbonization of the HEP. After the HEP was burned out, the LDH nanosheets did not support any burning and remained inert upon prolonged exposure to a very high temperature flame. Moreover, the film never led to undesired dripping of hot fluids and maintained its shape even with prolonged exposure to the flame, which produced very rapid temperature changes and high local thermal gradients. The excellent flame-retardant properties were mainly due to the formation of the dense, multilayered inorganic micro/nanoporous structures inside the hybrid film, and the majority content of inorganic LDHs played a key role in the barrier. In essence, the composite film could be prepared with simple vacuum-filtration technology, and acted as an efficient thermal and flame shield similar to ceramics. SEM images (Figure 6c,d) exhibit the development of a condensed protective inside structure of the film exposed to the flame, a mesoporous layered network developed in the inside of the film. Obviously, such nacre-like films, which could even be simply fabricated, are extremely valuable for applications in transportation, construction, and insulation.

Figure 5. (a) Hybrid HEP/LDH film (placed on the paper with colored “ABC”) with permselectivity for different colors in the visible region. (b) UV−vis spectra (transmittance) of HEP, LDH, and HEP/ LDH film with a thickness of 0.1 mm. (c) Schematic illustration of UV shielding performance testing of the HEP/LDH film. (d) Photocatalytic degradation profile of Rhodamine B unprotected and protected by HEP/LDH film.

in Figure 5d, after being irradiated for 30 min, the unprotected Rhodamine B showed a loss of 79%, while the Rhodamine B protected by HEP/LDH film only showed a decrease of 20%. The small amount of loss of Rhodamine B protected by HEP/ LDH film suggested good UV shielding. Rhodamine B was still partially degraded when protected by the film, which was probably the result of the incomplete shielding of UV light, which occurred mainly in the range 310−360 nm. This 3824

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Basic Research Program of China (2010CB934700) and the Doctoral Fund of Innovation of Beijing University of Technology.

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(1) Li, Y. Q.; Yu, T.; Yang, T. Y.; Zheng, L. X.; Liao, K. Bio-inspired nacre-like composite films based on graphene with superior mechanical, electrical, and biocompatible properties. Adv. Mater. 2012, 24, 3426−3431. (2) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured artificial nacre. Nat. Mater. 2003, 2, 413−418. (3) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J. D.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and stiff layered polymer nanocomposites. Science 2007, 318, 80−83. (4) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Bioinspired design and assembly of platelet reinforced polymer films. Science 2008, 319, 1069−1073. (5) Podsiadlo, P.; Arruda, E. M.; Kheng, E.; Waas, A. M.; Lee, J.; Critchley, K.; Qin, M.; Chuang, E.; Kaushik, A. K.; Kim, H. S.; Qi, Y.; Noh, S. T.; Kotov, N. A. LBL assembled laminates with hierarchical organization from nano- to microscale: high-toughness nanomaterials and deformation imaging. ACS Nano 2009, 3, 1564−1572. (6) Podsiadlo, P.; Michel, M.; Critchley, K.; Srivastava, S.; Qin, M.; Lee, J. W.; Verploegen, E.; Hart, A. J.; Qi, Y.; Kotov, N. A. Diffusional self-organization in exponential layer-by-layer films with micro- and nanoscale periodicity. Angew. Chem., Int. Ed. 2009, 48, 7073−7077. (7) Yao, H. B.; Tan, Z. H.; Fang, H. Y.; Yu, S. H. Artificial nacre-like bionanocomposite films from the self-assembly of chitosan-montmorillonite hybrid building blocks. Angew. Chem., Int. Ed. 2010, 49, 10127−10131. (8) Walther, A.; Bjurhager, I.; Malho, J. M.; Pere, J.; Ruokolainen, J.; Berglund, L. A.; Ikkala, O. Large-area, lightweight and thick biomimetic composites with superior material properties via fast, economic, and green pathways. Nano Lett. 2010, 10, 2742−2748. (9) Wang, J. F.; Lin, L.; Cheng, Q. F.; Jiang, L. A strong bio-inspired layered PNIPAM-clay nanocomposite hydrogel. Angew. Chem., Int. Ed. 2012, 51, 4676−4680. (10) Huang, L.; Li, C.; Yuan, W. J.; Shi, G. Q. Strong composite films with layered structures prepared by casting silk fibroin-graphene oxide hydrogels. Nanoscale 2013, 5, 3780−3786. (11) Wang, J. F.; Cheng, Q. F.; Lin, L.; Chen, L. F.; Jiang, L. Understanding the relationship of performance with nanofiller content in the biomimetic layered nanocomposites. Nanoscale 2013, 5, 6356− 6362. (12) Das, P.; Schipmann, S.; Malho, J. M.; Zhu, B. L.; Klemradt, U.; Walther, A. Facile access to large-scale, self-assembled, nacre-inspired, high-performance materials with tunable nanoscale periodicities. ACS Appl. Mater. Interfaces 2013, 5, 3738−3747. (13) Walther, A.; Bjurhager, I.; Malho, J. M.; Ruokolainen, J.; Berglund, L.; Ikkala, O. Supramolecular control of stiffness and strength in lightweight high-performance nacre-mimetic paper with fire-shielding properties. Angew. Chem., Int. Ed. 2010, 49, 6448−6453. (14) Sehaqui, H.; Kochumalayil, J.; Liu, A. D.; Zimmermann, T.; Berglund, L. A. Multifunctional nanoclay hybrids of high toughness, thermal, and barrier performances. ACS Appl. Mater. Interfaces 2013, 5, 7613−7620. (15) Choudary, B. M.; Madhi, S.; Chowdari, N. S.; Kantam, M. L.; Sreedhar, B. Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes. J. Am. Chem. Soc. 2002, 124, 14127−14136. (16) Liu, P.; Wang, H.; Feng, Z. C.; Ying, P. L.; Li, C. Direct immobilization of self-assembled polyoxometalate catalyst in layered

Figure 6. Flame-shielding property of nacre-like HEP/LDH films. (a, b) Photograph of an initially 0.2 mm thick nacre-mimetic film exposed to a gas burner (at ca. 2000 °C), protecting cotton positioned about 4 mm behind. The film first caught fire for a short moment and then gradually became black. Throughout the whole process the cotton was intact under the protection of the HEP/LDH film. Inset: photographs from another direction, showing the exposed whole cotton. Videos of the flammability tests are provided as Supporting Information. (c, d) SEM images of the nacre-like film before and after flame treatment. A condensed protective inside structure of the film was exposed to the flame, whereas a mesoporous layered network developed inside the film after the treatment.

4. CONCLUSIONS In summary, we demonstrated a high loading amount of inorganic and multifunctional layered composite materials mimicking nacre. Both the modulus and hardness of the HEP/LDH films were remarkably higher than those of other reported polymer/LDH composites. Simultaneously, the hybrid films showed a combination of outstanding properties of UVblocking and fire-resistance properties. Considering that our multifunctional nacre-like films were easily made, we strongly believe that the films are promising for applications such as fireprotective coatings, photoprotective materials, and windows of buildings. In the near future, more research would be needed on the fabrication of functional assemblies using these LDH nanosheets as building blocks. The exploration of their properties and applications based on these functional assemblies is expected to promote great development in multifunctional material design and fabricating, as well as exciting improvement in a broad range of applications.

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Full EDX spectrum of the HEP/LDH film, other PUL results, and fire-shielding videos. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 3825

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dx.doi.org/10.1021/ie404115g | Ind. Eng. Chem. Res. 2014, 53, 3820−3826