Thermochromic Artificial Nacre Based on Montmorillonite - ACS

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Thermochromic Artificial Nacre based on Montmorillonite Jingsong Peng, Yiren Cheng, Antoni P. Tomsia, Lei Jiang, and Qunfeng Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07953 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Thermochromic Artificial Nacre based on Montmorillonite Jingsong Peng,a† Yiren Cheng,a† Antoni P. Tomsia,b Lei Jianga and Qunfeng Chenga*

a

Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing 100191, P. R. China b

Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

KEYWORDS: robust, thermochromic, montmorillonite, nanocomposite, artificial nacre

ABSTRACT: Nacre-inspired nanocomposites have attracted a great deal of attention in recent years due to their special mechanical properties and universality of the underlying principles of materials engineering. The ability to respond to external stimuli will augment the high toughness and high strength of artificial nacre-like composites and open new technological horizons for these materials. Herein, we fabricated robust artificial nacre based on montmorillonite (MMT) that combines robustness with reversible thermochromism. Our artificial nacre shows great potential in various fields such as aerospace, sensors and also opens an avenue to fabricate artificial nacre responsive to other external stimuli in the future.

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1. INTRODUCTION Nacre integrates high strength with excellent toughness. Extensive research has revealed that its outstanding mechanical properties mostly stem from i) the hierarchical layered structure and ii) the synergistic interfacial interactions.1-5 Inspired by nacre, a large amount of research on layered nanocomposites has been performed using different building blocks, such as montmorillonite (MMT),6-16 layered double hydroxides,17 carbon nanotubes,18 aluminum oxide (Al2O3) platelets,19 graphene,1-2,20-24 and calcium carbonate,25-26 To the best of our knowledge, the thermally responsive nacre-like nanocomposites with ability of reacting to the external stimuli, however, are rarely investigated. Thermochromic materials show promise for a number of application in various fields.27-31 Polydiacetylenes are a class of conjugated polymers with excellent thermochromic performance. The thermochromism of polydiacetylenes mainly attributes to the reversible conformational change of ene-yne units of the backbone.29 Since polydiacetylene was first synthesized by Wegner in 1969,32 significant progress on polydiacetylene-based materials has been achieved recently.29,33-40 In particular, Park et al. reported a thermo- and hydro-chromic films utilized for human sweat pore mapping.36 Peng et al. also fabricated reversible electrochromic carbon nanotube/polydiacetylene composite fibers.41 Other responsive materials based on color transition can be also utilized to discriminate solvents such as methanol/ethanol.42 Herein, we demonstrate a robust thermochromic MMT-based artificial nacre with reversible color transition from purple to orange between 20 °C and 70 °C and a high tensile 2 ACS Paragon Plus Environment

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strength of 101.8 ± 2.6 MPa. The fabrication of this artificial nacre can easily be scaled up. Introduction of nacre-like structure contributes to a free-standing feature compared with traditional polydiacetylene-based thermochromic materials.43 Our study opens a door for fabricating artificial nacre responsive to various external stimuli in the future. 2. EXPERIMENTAL SECTION 2.1 Materials MMT, which is Na+-type montmorillonite, was purchased from Zhejiang Fenghong Clay Co. Ltd. 10,12-Pentacosadiynoicacid (PCDA) was purchased from Sigma-Aldrich. (3-aminopropyl) triethoxysilane (APTES) was purchased from J&K. Scientific Ltd. 2.2 Fabrication of the artificial nacre MMT nanosheets were dispersed into 500 ml deionized water and stirred for about a week. Centrifugation was conducted at 2800 rpm for 20 min to yield a homogeneous MMT solution. The as-prepared MMT solution is transparent with a concentration of 3.1 mg·mL-1. The PCDA solution (10 mg·mL-1) was obtained through mixing PCDA powder with sodium hydroxide solution (0.1 mol·L-1) followed by heating at 70 °C. The MMT and PCDA solutions were mixed in different ratios and irradiated under 254 nm UV-light to obtained thermochromic MMT-polyPCDA solution. Then the thermochromic MMT-polyPCDA nanocomposites were obtained through assembling the MMT-polyPCDA hybrid building blocks via vacuum-assisted filtration. After dipping into APTES liquid for covalent cross-linking with different dipping time, the artificial nacre MMT-polyPCDA-APTES was obtained.

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2.3 Characterization Mechanical properties were tested using a Shimadzu AGS-X Tester equipped with a 100 N load cell. The loading rate was about 1 mm·min-1. The samples were cut into strips with the width of 3 mm and length of 10 mm, and the results for each sample are based on the average value of 3~5 specimens. All measurements were conducted at room temperature and the thickness of all samples was calculated by scanning electron microscopy (SEM). The Young's modulus of all samples was determined by the slope of the linear region of the stress-strain curves. The toughness was calculated by the area under the stress-strain curves. SEM images were obtained by the HITACHI S-4800, conducting at 1–1.5 kV after sputtering a thin Pd/Au coating. Atomic force microscopy (AFM) was conducted by a Leica TCS SP5. the well-dispersed by diluting about 100 times with pure water MMT or MMT-polyPCDA solution was prepared for the test. the diluted dispersion was dropped to the freshly cleaved mica and dried at room temperature. The Fourier transform infrared (FTIR) spectra were obtained by a Thermo Nicolet nexus-470 FTIR instrument with the mode of attenuated total reflection. Every specimen was tested at 3 points. The thermogravimetric analysis (TGA) was performed on a TG/DTA6300, NSK, with a temperature rising rate of 10 K·min-1 under nitrogen. The UV-vis spectroscopy under different temperatures was analyzed via a PERSEE TU-1901 refitted by Material & Industrial Technology Research Institute Beijing. The films were placed on a glass cuvette and equilibrated at each temperature for about 3 minutes. The UV-vis spectra were subtracted from a baseline of the spectra of pure MMT films. X-ray diffraction (XRD) profiles were taken though a Rigaku D/max 2550 with Cu-Kα radiation 4 ACS Paragon Plus Environment

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(λ=1.54Å). The measurement was conducted with the voltage of 50.0 kV and the current of 200.0 mA. 3. RESULTS AND DISCUSSION The fabrication process of thermochromic MMT-based artificial nacre is illustrated in Figure 1a. First, the 10,12-pentacosadiynoic acid (PCDA) monomers were coated on MMT nanosheets via hydrogen bonding and self-polymerized into polyPCDA in solution under 254 nm UV. The atomic force microscope (AFM) image indicates increased thickness of polyPCDA-coated

MMT

nanosheets

(Figures

1b-c

and

Figure

S1).

Next,

the

polyPCDA-MMT solution was filtrated into a layered MMT-polyPCDA nanocomposite. The hydrogen bonding can be evidenced by the red-shifted peaks of carboxylate groups on Fourier transform infrared (FTIR) spectra (Figure S2a).44-45 A series of MMT-polyPCDA nanocomposites with different MMT contents were designated as MMT-polyPCDA-I~V. The exact MMT contents were measured via thermogravimetric analysis (TGA) (Figures S3a-b), as

shown

in

Table

S1.

Finally,

MMT

was

covalently

cross-linked

by

(3-aminopropyl)triethoxysilane (APTES) through the condensation reaction, confirmed by new FTIR characteristic absorption at 3275 cm-1 of amino groups (Figure S2b).46-47 In addition, covalent cross-linking can be demonstrated by the stability under water (Figure S4). MMT-polyPCDA-APTES-I~IV nanocomposites were fabricated with different contents of APTES as measured by TGA (Figures S3c-d, Table S2). The obtained artificial nacre shows a typical nacre-like layered structure (Figure 1d) and purple color at room temperature (Figure 1e). 5 ACS Paragon Plus Environment

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Figure 1. Schematic illustration of the fabricating process of MMT-polyPCDA-APTES artificial nacre. a) The PCDA and MMT solutions were mixed, and followed by UV irradiation, and then the MMT-polyPCDA hybrid building blocks solution was obtained. Vacuum-assistant filtration was utilized to assemble the MMT-polyPCDA hybrid building blocks into layered nanocomposites. Finally, the MMT-polyPCDA nanocomposites were dipped into APTES to achieve thermochromic MMT-polyPCDA-APTES artificial nacre. AFM images of b) MMT nanosheets and c) polyPCDA coated MMT nanosheets. d) Cross-section and e) digital photograph of the MMT-polyPCDA-APTES-I artificial nacre.

This MMT-based artificial nacre displays a fast reversible thermochromism with a response time of about 0.3 s as shown in a Movie S1. For example, the MMT-polyPCDA-APTES-I artificial nacre gradually turns from purple to orange from 20 °C to 70 °C and reversibly switches to purple when cooled to 20 °C (Figure 2a). Under heating, the decreased UV-vis absorption peak at 620 nm corresponds to the blue phase of polyPCDA, while the peak at 530 6 ACS Paragon Plus Environment

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nm was caused by the red phase (Figure 2b), indicating the transition from blue phase to the red.48 The colorimetric response (CR) value was calculated to further clarify the extent of thermochromism49 using the equation: CR=(PB0-PBf)/PB0 × 100%, where PB is the percentage of blue defined by the equation: PB=A620nm/(A620nm+A530nm) and the subscripts 0 and f represent the initial and final states, respectively. As shown in Figure 2c, the CR of our artificial nacre increases to a maximum value of 84.1% as the temperature rises to 70 °C, illustrating the gradual color change with temperature.

Figure 2. The thermochromic properties of MMT-polyPCDA-APTES-I artificial nacre. a) The reversible thermochromism between purple (20 °C ) and orange (70 °C ). b) UV-vis spectra between 20 °C and 70 °C. c) The CR values between 20 °C and 70 °C.

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The reversibility of the color transition was clarified by UV-vis spectroscopy. When the artificial nacre is cooled to 20 °C from 50 °C, the peak intensity of 620 nm and 530 nm recovers (Figure 3a), indicating good reversibility of thermochromism. It should be noted that the reversibility highly relies on the external temperature. For instance, the CR value could easily recover below 50 °C while reversibility is partial at 70 °C (Figure S5), mainly due to the partly reversible conformational deformation of polyPCDA at high temperature.48 To test its long-term reversibility, thermal cycle testing between 20 °C and 50 °C was conducted (Figure 3b). Even after 39 cycles, the CR value can recover to 18.5%, indicating repeatable and stable thermochromism. All the UV-vis spectra of MMT-polyPCDA-APTES-I artificial nacre, MMT-polyPCDA-I nanocomposite, polyPCDA-MMT solution are shown in Figures S5-7.

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Figure 3. The reversibility of thermochromic MMT-polyPCDA-APTES-I artificial nacre. a) UV-vis spectra with 1 cycle of thermal cycle testing between 20 °C and 50 °C. b) The CR values with 39 cycles of thermal cycle testing between 20 °C and 50 °C. c) Illustration of thermochromism. When the artificial nacre is heated, the thermal motion of side chains leads to the shortening of conjugation length of polymer chains.

X-ray diffraction (XRD) was conducted to probe the thermochromism. As shown in Figure S8, the reversible shift of the peak position within a thermal cycle testing between 20 °C and 50 °C can be caused by reversible dehydration and shows the stability of the MMT lamellar structure under heating.50 Furthermore, the diffraction peak located at ~1.4° demonstrates the multimolecular structure of polyPCDA with an interlamellar distance of ~6.3 nm,36,43 as shown in Figure S9. The disappearance and reappearance of the diffraction peak within thermal cycle testing illustrate a reversible disturbance of ordered lamellar structure of polyPCDA under heating. The proposed thermochromism is illustrated in the Figure 3c. The polyPCDA polymer chains are assembled into multimolecular films between two adjacent MMT nanosheets due to the hydrophobic-hydrophobic interaction.36,43 With the increase in temperature, the motional freedom of side chains of polyPCDA is enhanced. The temperature-dependent FTIR spectra (Figure S10) show that the peaks for CH2 asymmetric vibration shift from 2919 cm-1 to 2921 cm-1 under heating confirming the more disordered alkyl side chains.48 The ene-yne backbone of polyPCDA is thus disturbed resulting in a more disordered and less coplanar conformation with decreased characteristic conjugation length and leading to the color transition.36,43 The conformation transition of the ene-yne backbone 9 ACS Paragon Plus Environment

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can be proved by the Ramam spectra which shows a significant shift of peaks for C=C and C≡C under heating (Figure S11).37,48 The enhanced fluorescence emission with the rise of temperature (Figure S12) also illustrates the conformation transition from non-fluorescent blue phase to fluorescent red phase.29,40 Compared with pure polyPCDA, the reversibility of MMT-polyPCDA-APTES-I is enhanced. The retained high CR value of cooled polyPCDA (Figure S13) indicates an irreversible color transition.

Figure 4. a) Stress-strain curves of MMT films, MMT-polyPCDA-I nanocomposites and MMT-polyPCDA-APTES-I artificial nacres. b) The tensile strength and toughness of MMT-polyPCDA-APTES artificial nacres with different contents of APTES. c) Front fracture morphology of MMT-polyPCDA-APTES-I artificial nacre.

Besides the color transition, this artificial nacre also shows high mechanical performance. MMT-polyPCDA-I with a MMT content of 94.2 wt.% reaches maximum value of tensile

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strength and toughness. But compared with pure MMT film, only incremental improvement was achieved (Figure 4a) due to weak hydrogen bonding between MMT and polyPCDA. To further improve the mechanical properties, APTES was introduced to covalently cross-link MMT nanosheets. After covalent cross-linking, the mechanical properties dramatically improved (Figure 4a). For MMT-polyPCDA-APTES-I, the tensile strength and toughness reach up to 101.8 ± 2.6 MPa and 0.65 ± 0.03 MJ/m3, which are 2.4 times and 2.6 times higher than those of pure MMT film (42.1 ± 2.0 MPa and 0.25 ± 0.04 MJ/m3). Higher APTES content, however, leads to poorer mechanical properties (Figure 4b) probably due to the disordered layered structure caused by the excess APTES.51 This can be confirmed through the XRD pattern (Figure S8), which shows a decrease of peak intensity with the insertion of APTES. The Young’s modulus after cross-liking is also improved (Table S3). All the mechanical properties of MMT-polyPCDA and MMT-polyPCDA-APTES nanocomposites are listed in Table S3. The hydrogen bonding from polyPCDA, dissipating large amount of loading energy and covalent bonding from APTES, resisting high tensile strength, work together to enhance the toughness and tensile strength simultaneously. The front fracture morphology of MMT-polyPCDA-APTES-I demonstrates distinct pull-out of MMT nanosheets (Figure 4c). Abundant edge curling of MMT nanosheets demonstrates the breaking of strong covalent bonding. No edge curling of MMT nanosheets has been observed in pure MMT films or MMT-polyPCDA nanocomposites (Figure S14).

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Our artificial nacre is comparable to previously reported MMT-chitosan nacre-like nanocomposites.7 Podsiadlo et al. reported strong MMT-based layered nanocomposites cross-linked with glutaraldehyde (GA).6 The short chain of GA, however, may cause the small ultimate strain (0.33%). Recently, Zhu et al. reported high-performance MMT-based layered nanocomposites with self-healing polymer.8 But the MMT content is only 50 wt.%, lower than nacre.1-2 Our artificial nacre retains high mechanical properties during thermochromism. For example, the tensile strength is as high as 90.0 ± 5.9 MPa after 8 cycles of thermal cycle testing (Table S3). On the other hand, the use of APTES shows no impairment on the reversible color transition as illustrated by similar MMT-polyPCDA-I and MMT-polyPCDA-APTES-I thermochromic properties (Figures S5-6). The fabrication of our robust thermochromic artificial nacre is easily scaled up. The MMT-polyPCDA solution was sprayed with a patterned template on different substrates (Figure 5a), such as paper, glass, steel, plastic, etc. After drying, APTES was sprayed on the surface of these patterned shapes. Then artificial nacres with different shapes of rose, leaf, and flower bud were obtained for temperature sensors as shown in Figures 5b-e. This method can work well even on a curved surface of a plastic cup (Figure 5e). The cross-section morphology (Figure S15) indicates a well-formed nacre-like layered structure. Meanwhile, this patterned artificial nacre shows reversible thermochromism illustrated by the color-changed flower bud on the cup as pouring hot water (Figure 5e). In addition, this artificial nacre functions well under hot water. (Figure S16). Compared with other conventional methods33,36,40 and recently inkjet printing on paper35 or preparing composite 12 ACS Paragon Plus Environment

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crayon,37 the spray coating shows versatility and simplicity without any limitation on the size and substrate.

Figure 5. a) Illustration of the spray coating via a spray gun. Different patterns of artificial nacre can be obtained by spraying the thermochromic solution with the assistance of a template.

The artificial nacres with shapes of b) rose and leaf, c) flower bud and d) rose

patterned on the b) paper, c) glass slide and d) steel plate. All these artificial nacres show reversible thermochromism between 20 °C and 50 °C. e) Artificial nacre with a shape of a flower bud on the a curved plastic cup via spray coating. The color quickly transforms from purple to orange with poring hot water. Scale bar: 1cm.

4. CONCLUSION In summary, we fabricated a novel thermochromic MMT-based artificial nacre with several advantages: i) reversible thermochromism; ii) retained mechanical properties; iii) easy process for scaling up. Combination of good mechanical properties and reversible 13 ACS Paragon Plus Environment

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thermochromism enables the artificial nacre to be utilized in many fields, including sensors, armor, and aerospace. This study also provides a novel strategy for fabricating artificial nacres that are responsive to several external stimuli in the future. ASSOCIATED CONTENT Supporting Information. FTIR spectra of pure MMT films, MMT-polyPCDA nanocomposites, MMT-polyPCDA-APTES artificial nacre, and polyPCDA. Waterproof tests of MMT-polyPCDA nanocomposites and MMT-polyPCDA-APTES artificial nacre. Detailed UV-vis

spectra

of

MMT-polyPCDA-APTES

artificial

nacre,

MMT-polyPCDA

nanocomposites, MMT-polyPCDA solution, pure polyPCDA. XRD data of pure MMT film, MMT-polyPCDA

nanocomposite

and

MMT-polyPCDA-APTES

artificial.

Temperature-dependent FTIR. Raman spectra. Fluorescent spectra. TGA curves, SEM images of fracture morphologies of pure MMT film and MMT-polyPCDA nanocomposite. SEM images of thermochromic coating. Presentation of thermochromism under water of artificial nacre via spray coating. The exact MMT content of MMT-polyPCDA films and MMT-polyPCDA-APTES films. The mechanical properties of MMT-polyPCDA films with different MMT contents and MMT-polyPCDA-APTES films with different APTES contents. (PDF) The movie of thermochromism. (AVI)

AUTHOR INFORMATION Corresponding Author *Correspondence should be addressed to Qunfeng Cheng, E-mail: [email protected].

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †These two authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the Excellent Young Scientist Foundation of NSFC (51522301), the National Natural Science Foundation of China (21273017, 51103004), the Program for New Century Excellent Talents in University (NCET-12-0034), the Fok Ying-Tong Education Foundation (141045), the 111 Project (B14009), the Aeronautical Science Foundation of China (20145251035, 2015ZF21009), State Key Laboratory of Organic-Inorganic

Composites,

Beijing

University

of

Chemical

Technology

(oic-201701007), and the Fundamental Research Funds for the Central Universities (YWF-16-BJ-J-09, YWF-17-BJ-J-33).

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