Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties

Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties

Accepted Manuscript Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties Benliang Liang, ...

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Accepted Manuscript Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties Benliang Liang, Yingqi Shu, Pan Wan, Hewei Zhao, Shaohua Dong, Weichang Hao, Penggang Yin PII:

S0266-3538(19)30621-9

DOI:

https://doi.org/10.1016/j.compscitech.2019.107747

Article Number: 107747 Reference:

CSTE 107747

To appear in:

Composites Science and Technology

Received Date: 6 March 2019 Revised Date:

13 July 2019

Accepted Date: 18 July 2019

Please cite this article as: Liang B, Shu Y, Wan P, Zhao H, Dong S, Hao W, Yin P, Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties, Composites Science and Technology (2019), doi: https://doi.org/10.1016/j.compscitech.2019.107747. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Genipin-enhanced nacre-inspired montmorillonite-chitosan film with superior mechanical and UV-blocking properties ‡

Benliang Liang a, b , Yingqi Shu

a,



, Pan Wan a, Hewei Zhao a, Shaohua Dong

*c

, Weichang Hao b,

a.

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Penggang Yin*a Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education,

School of Chemistry, Beihang University, Beijing, 100191, China.

School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China.

c.

Pipeline Technology Research Center, China University of Petroleum-Beijing, Beijing, 102249,

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b.

‡. These authors contributed equally. ABSTRACT:

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China.

Chitosan, as a functional material, has special characteristics: biocompatibility, biodegradability and

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sterilizable properties, suitable for use in many fields. However, the weak mechanical properties of this material limit its applications. Herein, inspired by the relationship between the hierarchical “brick-and-mortar” structure and excellent mechanical properties of natural nacre, monolayer

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montmorillonite sheet was selected as reinforcement nanoplatelet to construct nacre-mimetic layered structure with chitosan together. Genipin, a naturally occurring cross-linking agent, was introduced to

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further strengthen the nacre-mimetics through chemically cross-linking the amino group of chitosan matrix under alkaline condition. The effects of MMT content, GP crosslinking, and pH on the microstructure and interfacial interactions of the hybrid films were studied systematically. It is proved that the synergistic effect originating from hydrogen and covalent bonds improves the mechanical property of the bioinspired composites, generating a tensile strength of 226.3 MPa and a toughness of 5.1 MJ/m3. Meanwhile, the hybrid film has excellent UV-blocking ability (the transmittance is 10.0% at 365 nm) because the spontaneous reaction between GP and CS forms dark blue pigments. Such bioinspired

ACCEPTED MANUSCRIPT hybrid films with improved mechanical properties and excellent UV-blocking ability has promising applications in aerospace, tissue regeneration, and construction. Kewords

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Bioinspiration; Hierarchical structure; Genipin enhanced; Mechanical properties; Nacre-inspired. 1. Introduction

Chitosan (CS), as a functional material, has special characteristics: biocompatibility,

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biodegradability and sterilizable properties. It is also biologically inert, safe for human use, and stable in the natural environment.[1, 2] These properties make CS suitable for use in many

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biomedical applications, including artificial skin, tissue regeneration, and drug delivery systems.[3, 4] However, the weak mechanical property of this material limit its applications. Therefore, improving the mechanical properties of CS is necessary. Cross-linking can be used to enhance the mechanical strength of CS.[5] Genipin (GP) is a

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new naturally occurring biological-based cross-linking agent that can react spontaneously with CS to form dark blue pigments.[6, 7] Compared to conventional glutaraldehyde (GA) cross-linking agent, GP is 5000 to 10000 times less cytotoxic.[8] Recently, Hobbs et al.[9] evaluated the

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genotoxic potential of gardenia blue and genipin. Their result shows that GP in food products does not pose a significant genotoxic concern for humans. Therefore, GP can substitute for

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conventional GA cross-linking agents and has elicited interest from researchers because of its biocompatibility, biodegradability, cell non-toxicity, well defined chemistry, and good operability.[5, 10, 11]

Numerous investigations reveal that the unique mechanical properties of natural materials are attributed to two factors: one is their ordered architecture, and the other is abundant interfacial interactions.[12] For example, natural nacre organizes aragonite platelets and a little proteins into a hierarchical “brick-and-mortar” structure. The proteins glue aragonite platelets together through

ACCEPTED MANUSCRIPT plenty of hydrogen/ionic bond for improving the interaction of components. The two factors enable natural nacre to have excellent mechanical properties. Natural nacre provides a “gold standard” for biomimicry and attracts wide attention.[13] Many nacre-mimetic layered composite

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materials based on two-dimensional inorganic platelets and polymers, have been reported.[14, 15] The employed two-dimensional inorganic platelets mainly include clay,[16-19] aluminum oxide,[20-23]

graphene

oxide,[24]

layered

double

hydroxides

(LDH),[25-27]

black

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phosphorus,[28], mica[29], MXenes[30, 31] and two-dimensionalBaTiO3 platelets.[32] The employed polymers mainly includes poly(vinyl alcohol),[17, 21, 33] alginate,[26, 34, 35] CS,[19,

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27, 29, 36, 37] polyurethane,[38, 39] or cellulose[18, 40, 41] as organic components. The adopted preparation techniques include bottom-up layer-by-layer assembly,[42, 43] ice template,[44, 45] filtration,[35,

46]

hydrogel

casting,[47,

deposition.[50, 51]

48]

evaporation[18,

49]

and

electrophoretic

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Building blocks as main components are very essential for the mechanical properties of nacre-inspired composites. Montmorillonite (MMT) sheets, which exhibit outstanding mechanical properties (e.g., Young's modulus of up to 270 GPa), light weight, fire retardant ability, high

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aspect ratio (e.g., thickness ca. 1 nm, diameter ca. 50–1000 nm), easy accessibility, and environment friendliness, are an ideal strengthened phase platelet that allows easy hydrogen

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bonding with the polymer matrix.[35, 52], A sixfold increase in elastic modulus has been achieved for the nanocomposite with 60% MMT concentration.[53] Khatun et al[54] prepared genipin crosslinked Chitosan-MMT nanoparticles for controlled delivery of curcumin by ionic gelation method. However, using the cross-linking of CS with GP to fabricate nacre-inspired films has been rarely reported yet. Herein, in this work, to obtain superior mechanical properties of CS-based film, we selected MMT nanosheets as inorganic nanoplatelets to construct nacre-mimetic layered structure, and GP was introduced to strengthen

ACCEPTED MANUSCRIPT the matrix through covalent cross-linking with the amino groups of CS. The effects of the oriented arrangement of MMTs, GP crosslinking of CS, and pH on the mechanical properties and interfacial interactions of the hybrid film were studied. This novel layered film shows not only

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high strength (226.3 MPa) and toughness (5.1 MJ/m3) but also excellent UV-blocking ability (the transmittance is 10.0% at 365nm) through the spontaneous reaction between GP and CS, which forms dark blue pigments. This type of integrated nacre-like MMT-CS nanocomposites may have

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great potential applications in aerospace, tissue regeneration, and construction. 2. Experimental

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2.1 Materials

CS (Chitosan, Degree of deacetylation: >95%, Viscosity 100-200 mPa•s) was purchased from Shanghai Macklin Biochemical Co. Ltd. ALG (sodium alginate, chemically pure, viscosity (1%, 20 °C) ≥ 0.02 Pa•s) was purchased from Guangdong Guanghua Chemical Factory Co. Ltd.

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MMT, which is Na+-type montmorillonite (chemically pure, CEC=100 mmol/100 g) was supplied by Zhejiang Fenghong Clay Co. Ltd. Highly purified GP (98%) was purchased from Linchuan Zhixin Biotechnology Co. Ltd. NaOH was purchased from Shanghai Macklin Biochemical Co.

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Ltd. High-purity deionised water was used in all experiments. All materials were used as received. 2.2 Preparation of MMT-CS-GP-NaOH (OH) nanocomposites.

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Simple evaporation-based self-assembly was employed to fabricate the composites, as shown in Figure 1. (1) MMT (6 g) was dispersed in deionized water (1 L). Exfoliation was performed by thorough stirring for 1 week. After centrifugation, the supernatant solution was collected by removing unexfoliated aggregates from the solution. The concentration of the fine MMT suspension (0.30 wt%) was determined by the oven-drying method, and the MMT suspension was diluted to 0.2 wt%. (2) CS (1 g) was dissolved in an aqueous solution of 2 wt% glacial acetic acid (100 mL) and stirred for 24 h to obtain a protonated CS aqueous solution (1 wt%). The desired

ACCEPTED MANUSCRIPT amount of CS solution (1 wt%) was gradually added to the stirred MMT dispersion. After rapid stirring for approximately 3 h to accomplish maximum CS adsorption and fine dispersion of the stabilized clay platelets (Figure 1c), (3) the required amount of GP solution (1 wt%) was

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gradually added to the mixed dispersion, which was then continually stirred for approximately 3 h to allow the GP to sufficiently cross-link with CS (Figure 1d). The required amount of NaOH solution was gradually instilled into the stirred mixed dispersion to adjust the pH of the solution

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and continually stirred for approximately 3 h to further promote GP cross-linking with CS. (4) After evaporation under ambient conditions, MMT-CS-GP-OH nanocomposite films with

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hierarchically layered structures were obtained. To study the effect of MMT content, GP crosslinking of CS, and pH on the mechanical properties and interfacial interactions of the hybrid films, we prepared a series of nacre-inspired MMT-CS-GP-OH nanocomposites (Table S1). MMT-CS and CS-GP-OH nanocomposites were obtained through steps (1), (2), and (4), whereas

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CS-GP-OH nanocomposites were obtained through steps (1), (3), and (4). ALG and ALG-GP film were also obtained by evaporating process. 2.3 Characterization.

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Scanning electron microscopy (SEM) was performed using a JEOL JSM7500F instrument. An acceleration voltage of 20 kV was applied for energy-dispersive X-ray analysis measurements.

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Fourier transform infrared (FTIR) spectra were collected on an iN10MX FTIR instrument in attenuated total reflection mode. Thermogravimetric analysis (TGA) was performed using a TG/DSC NETZSCH STA 449F3 instrument under a N2 atmosphere with a temperature increase rate of 10 °C/min. UV–vis spectroscopy was performed using a UV–vis–NIR spectrophotometer (UV-3600, Shimadzu). X-ray photoelectron spectroscopy (XPS) was performed in a Thermo Escalab 250XI instrument using a monochromatic Al Kα X-ray source (hv = 1486.6 eV). Measurements were conducted with a power of 150 W and beam size of 500 µm. Mechanical

ACCEPTED MANUSCRIPT behavior tests were conducted using a Shimadzu AGS-X setup at a loading speed of 1 mm/min with a gauge length of 5 mm. All of the samples tested were cut into the same size (20 mm in length and 3 mm in width). X-ray diffraction (XRD) profiles were observed on a Shimadzu

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XRD-6000 diffractometer under the following conditions: 40 kV, 40 mA, and Cu Kα radiation. Atomic force microscope (AFM) images were carried out by Bruker Dimension Icon. A freshly cleaved mica slide was used as the substrate for the AFM measurement.

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The UV-shielding performance of the MMT-CS-PG-OH film was evaluated by the photocatalytic degradation of an aqueous solution of rhodamine B in the presence of TiO2

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nanoparticles using a 400 W high-pressure Hg lamp as the irradiation source. The experiments were conducted as follows: A suspension consisting of 30 mL of rhodamine B aqueous solution (0.01 g/L) with 20 mg of TiO2 nanoparticles was stirred in the dark for 1 h to ensure complete dispersion of the nanoparticles and adsorption/desorption equilibrium. The suspension was then

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charged into a single-necked flask covered by a layer of Al 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. Approximately 1 mL of the suspension was sampled every 4

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min and centrifuged to remove TiO2 nanoparticles. The absorbance of the solution at 552 nm was then determined. The nacre-like MMT-CS-GP-OH film served as the control.

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3. Result and Discussion

3.1. Interfacial interactions

A simple evaporation-based self-assembly method was employed to fabricate the bioinspired MMT-CS-GP-OH hybrid films, as shown in Figure 1. The exfoliated MMT platelet has a thickness of about 1.1 nm, as measured by AFM (Figure S1). Figure 1f shows that the resultant hybrid film can be folded without fracture, indicating its good flexibility. Figure 1g shows the SEM image of the cross section of hybrid film. It is observed that the film has an ordered

ACCEPTED MANUSCRIPT multilayer structure, similar to the brick-and-mortar structure of nacre (Figures 4d and S6–S8). Elemental mapping indicates that the CS and MMT nanosheets are homogeneously distributed within the film (Figure S2). Figure 1h shows a sketch map of the covalent cross-linking network

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formed by GP and CS. The cross-linking reaction occurs through the following mechanisms:[55, 56] Nucleophilic attack by the amino groups of CS occurs on the olefinic carbon atom at C-3, followed by opening of the dihydropyran ring and attack by the secondary amino group on the

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newly formed aldehyde group. In summary, the short chains of condensed GP act as crosslinking bridges.

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The FTIR spectroscopy results are shown in Figure 2a. Pure CS exhibited O–H stretching vibrations in the wavenumber range of 3000–3600 cm-1, and the signals of the amide II group (N–H bending vibrations) peaked at approximately 1545 cm-1. The absorption band at 1643 cm-1 is characteristic of amide absorption. The absorption bands between 1000 and 1100 cm−1 were

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attributed to C–O and C–C–N bending vibrations.[11, 57] In the MMT-CS hybrid film, the O–H stretching vibrations were suppressed due to the formation of a hydrogen bonding network between MMT and CS.[17, 52] The absorption bands between 1000 and 1100 cm−1 deformation

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(Figure 2a, red dashed frame) are also due to the formation of a hydrogen bonding network between MMT and CS.

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After crosslinking with GP, the peak at 1643 cm−1 could be associated with the characteristic absorption of amide groups of CS, and the adsorption of CS-GP-OH nanocomposites gradually blue-shifted to 1650 cm−1. The characteristic of absorption of amides in MMT-CS at 1646 cm−1 was blue-shifted to 1654 cm−1. The adsorption at 1545 cm−1 could be associated with the N–H (amide I) bending vibrations of CS (MMT-CS), and the absorption peak of the CS-GP-OH (MMT-CS-GP-OH) nanocomposites gradually blue-shifted to 1554 cm−1. These results confirm that GP was cross-linked with CS. Covalent bonding between GP and CS could also be confirmed

ACCEPTED MANUSCRIPT by the XPS (Figures 2b–2c) and TGA (Figure 2d) results. The color of the artificial nacre changed with increasing addition of GP, as shown in Figure 5a. The XPS results of the MMT-CS-GP-OH nanocomposite (Figure 2c) show that the peak intensity of C–N increased whereas that of amine

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groups of MMT-CS-GP-OH decreased in comparison with those of the MMT-CS nanocomposites (Figure 2b). The peak of C(O)O and amide appear in the MMT-CS-GP-OH comparison with MMT-CS nanocomposites, further confirming that amine groups were transformed into amide

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groups after the chemical crosslinking reaction in the MMT-CS-GP-OH nanocomposite.[58] This result could be attributed to the formation of amides and the absorption of the NH2 group due to

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the reaction between the amino groups on CS and ester groups of GP.

The thermal stability of the CS particles was measured by TGA. Changes in weight of the artificial nacre with increasing temperature are shown in Figure 2d. In all samples, a weight loss of up to 100 °C was found due to elimination of adsorbed water. The CS-GP-OH and

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MMT-CS-GP-OH films showed lower weight loss compared with the pure CS and MMT-CS, respectively, which indicates that the thermal stability of the samples increased by the crosslinking between GP and CS. CS was thermally stable up to 250 °C and showed remarkable

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weight loss from 270 °C to 500 °C. This decomposition step can be attributed to the complex dehydration of saccharide rings, depolymerization, and pyrolytic decomposition of the

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polysaccharide structure.[11, 59] 3.2. Mechanical properties

Uniaxial tensile tests were conducted to study the mechanical behavior of the MMT-CS-GP-OH artificial nacre. To reveal the strengthening effect of GP, we prepared composites without GP crosslinking for comparison. The stress–strain curves are displayed in Figure 3a. The MMT-CS-GP-OH films exhibited a combination of high strength and high toughness. The tensile strength and toughness are improved remarkably from 141.3 MPa and 1.7

ACCEPTED MANUSCRIPT MJ/m3 for MMT-CS film to 226.3 MPa and 5.1 MJ/m3 for MMT-CS-GP-OH artificial nacre, indicating the important role of GP crosslinking. The tensile strength of MMT-CS-GP-OH artificial nacre is improved by 255%, compared with that of CS component (88.6 MPa). For

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natural nacre, the tensile strength is about 80 MPa, 1.6 times higher than its protein component (50 MPa). Such comparison proves that the “brick-and-mortar” structure and GP crosslinking effectively improves the mechanical property of CS. The superior mechanical properties of

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MMT-CS-GP-OH artificial nacre could be attributed to the synergistic interactions of hydrogen and covalent bonding. CS molecules are easily coated onto the exfoliated MMT nanosheets to

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form hybrid building blocks by hydrogen bonding interactions.[26, 35] As shown in Figure 1h, the terminal aldehyde groups on the polymerized GP undergo a Schiff reaction with the amino groups on CS to form crosslinked networks,[56] which would strengthen the MMT-CS-GP-OH composites. The high-density pulled out MMTs (inset in Figure 3e, red frame) and the obvious

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chain of CS molecules stretched between cracks (inset in Figure 3e, yellow arrow) provide direct evidence of the hydrogen and covalent bonding enhancement effect. A comparison of the mechanical performance of our artificial nacre with those of natural

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nacre and other CS-based nanocomposites is shown in Figure 3e. Our artificial nacre exhibited a good combination of strength and toughness. The respective tensile strength and toughness of our

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MMT-CS-GP-OH composites were 2.2 and 2.8 times higher than those of natural nacre,[60] which exhibited a tensile strength of 80–130 MPa and toughness of 1.8 MJ/m3. The synergistic interactions of hydrogen and covalent bonding are mostly responsible for improving the strength and toughness of the artificial nacre simultaneously. The tensile strength and toughness of MMT-CS-GP-OH are superior to those of other CS-based nanocomposites, such as binary nanocomposites

obtained

through

different

preparation

methods,

including

MMT-CS

(filtration),[19] MTM-CH (LBL),[61] MMT-CS (evaporation),[19] and Alumina-CS (doctor

ACCEPTED MANUSCRIPT blading),[62] and ternary nanocomposites, such as MMT-CS-au,[36] GO-CTS-CS,[37] MMT-NFC-CS,[63] NSC-GO-Zn,[64] and CNFs-CS-MTM.[65] Some artificial nacres achieved very high tensile strength but at the expense of their toughness. For example, although

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Mica-Pei-CS composites[29] showed a high strength of 259 MPa, which is slightly higher than that of our artificial nacre, its toughness is 3.5 MJ/m3 lower than that of our samples. CTS-RGO[66] and Cu-NO3-CS[27] showed much higher toughness but lower strength than our

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samples. The detailed mechanical properties of natural nacre and CS-based nanocomposites are listed in Table S2.

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3.3 Synergistic effect from MMT nanosheets, GP crosslinking, and pH

The mechanical properties of other MMT-CS-GP-OH nanocomposites with different MMT, GP, and pH are shown in Figures 3b–3d. The optimal MMT-CS-GP-OH featured an MMT content of approximately 30 wt%, mGP/CS = 0.06, and pH = 5.5 and showed the best mechanical

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properties. Figure 3b shows the dependence of tensile strength and strain-to-failure on MMT content. With increasing MMT content, the tensile strength of the artificial nacre first increases dramatically, reaches a maximum value at 30 wt% MMT and then decreased. The trend is

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consistent with previous studies.[53, 67] The decrease of tensile strength at high MMT content may be attributed to the local formation of MMT into tactoids. Nacre bricks are comprised of

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millions of nanograins (~30 nm) glued together by a biopolymer,[13, 68] Li et al.[69, 70] demonstrated that inaddition to the crack deflection along the biopolymer interface between aragonite platelets, the nanoparticle-architecture within individual aragonite platelets offers additional crack extension resistance for nacre’s toughness amplication. MMT nanosheets not like nacre bricks could deformable and cracks may invade the bricks, when MMT content is high, the deformation of organics decreases, stress concentration increases, and toughness and strength of samples decrease. The strain of the artificial nacre decreased continuously with increasing MMT.

ACCEPTED MANUSCRIPT The detailed stress–strain curves of MMT-CS-GP-OH composites with different MMT contents are shown in Figure S3. The layered structure of the films became increasingly clear with increasing MMT content, as shown in Figure S6. Figure 3c shows the relationship between tensile

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strength–strain and mGP/CS in the MMT-CS-GP-OH composites. Similar to the variation trend of MMT content, with increasing mGP/CS, the mechanical property of the samples increased at prime tense and then decreased. The crosslinked networks formed by the Schiff reaction between GP

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with CS restrict the free movement of the molecular chain of CS, thereby reducing the degree of hydrogen bonding between CS and the MMT sheets. With increasing mGP/CS, the degree of

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crosslinking between GP with CS increased whereas the degree of hydrogen bonding between CS and the MMT sheets decreased. Thus, the optimal mGP/CS is 0.06, at which point the covalent crosslinking of CS with GP and interfacial hydrogen bonding between MMT and CS are finely balanced.[26] So, at mGP/CS = 0.06, the tensile strength of the samples peaked. However the strain

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of the composites continuously increased with increasing mGP/CS. The detailed stress–strain curves of MMT-CS-GP-OH composites with different mGP/CS are shown in Figure S4. The terminal aldehyde groups on the polymerized GP undergo a Schiff reaction with the

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amino groups on CS to form crosslinked networks. Therefore, pH plays an important role in the crosslinking reactions.[56] To some extent, as pH increases, the cross-linking reaction of GP and

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CS is promoted. Thus, we studied the effect of pH on the mechanical properties of the hybrid films. The detailed stress–strain curves of MMT-CS-GP-OH composites with different pH are shown in Figure S5. Figure 3d shows the relationship between the tensile strength–strain and pH of the MMT-CS-GP-OH composites. With increasing pH, the tensile strength and strain of the samples increased at prime tense and then decreased. As the amount of cross-linked CS increased, the layered structure of the sample became increasingly blurred with increasing GP content and pH (Figures S7–8).

ACCEPTED MANUSCRIPT 3.4 Microstructure of MMT-CS-GP-OH hybrid film Detailed structural analysis was performed to understand the intrinsic factors influencing the mechanical strength of the films. The effects of added MMT and crosslinking of GP and CS on

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the microstructures of the composites were examined by XRD (Figure S9) and SEM, as shown in Figures 4a–4d. It is important to use monolayer MMT to build the brick-mortar architecture. Agglomeration of MMTs will lead to the stress concentration of the at the MMT aggregation site,

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and the mechanical properties of the samples will decline seriously. Hyrod prassive[71, 72] and organic hydrogen bending adsorption[19] are effective method to prevent inorganic aggregated.

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Just like Yu's reporting method,[19] we gradually added CS solution to the stirred MMT dispersion. Then rapid stirring to accomplish CS adsorption and fine dispersion of the stabilized clay platelets. Then aligned to a nacre-like lamellar microstructure by water-evaporation-induced self-assembly because of the role that the orientation of the nanosheets and linking of the chitosan

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play. The XRD results show that the d-spacing increased from 1.2 nm in the pure MMT power to 2.6 nm in the MMT-CS nanocomposite; this result demonstrates that CS was successfully adsorbed onto the MMT nanosheets to form a uniform layered nacre-like structure, It is consistent

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with yu's report.[16] We observed that the peak of the MMT-CS-GP-OH nanocomposite broadened and that its d-spacing decreased to 1.5 nm. This result could be attributed to the

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introduction of GP changing the spatial configuration of CS molecule chains, reducing the layer spacing between MMT nanosheets, and blurring the nacre-like lamellar structures, all of which could be further verified by SEM. With the introduction of GP, the fracture morphology of the CS-GP-OH films changed from burr-like (Figure 4a) in the pure CS sample to relatively smooth (Figure 4b), This is because covalent cross-linking by GP limits the free spatial motion of chitosan chains. Compared with that in MMT-CS (Figure 4c), cross-linking of GP caused the nacre-like lamellar microstructures of the MMT-CS-GP-OH-nanocomposites to become vague

ACCEPTED MANUSCRIPT (Figure 4d). In the cross-section of MMT-CS-GP-OH, we clearly observed that the CS adhered between the MMT sheets was stretched and deformed but not broken. Figures 4e–4f show the surface morphology of the MMT-CS-GP-OH film. Many MMT sheets are pulled out due to

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strong interactions between adjacent MMT sheets. The edges of the MMT sheets are curved rather than flat (magnified image in Figure 4h, red frame), indicating that breakage of the covalent bonds between GP and CS

brings about obvious deformation of the MMT sheets, which requires

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additional energy.[58, 73] 3.5 crack mechanism

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A typical fracture mechanism is proposed to illustrate the synergistic toughening effect of the interfacial interactions in the MMT-CS-GP-OH, as shown in Figure 4i. The binding energy of covalent bonding is higher than that of hydrogen bonding;[58] therefore, strong covalent cross-linking forms a primary network, while weak hydrogen bonding forms a sacrificial network

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under loading. The deformation–strengthening mechanism of the biopolymer matrix contributes to the ultrahigh toughness of nacre.[74, 75] Under loading (Stage I), the MMT-CS-GP-OH artificial nacre shows elastic deformation. The hydrogen bonds between MMT nanosheets and CS

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are broken due to fracture, accompanying the coiled CS chains extending along the sliding direction and bridging the MMT nanosheets via the network of covalent cross-linking, The

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elongated CS chains are also attached to the cracks (magnified image in Figure 4h, yellow arrow) the MMT nanosheets begin to slide over each other, leading to the formation of cracks. Crack deflection reduces stress concentration, increases crack propagation path and requires a higher driving force,which results in higher fracture toughness of materials. Seashells have laminated architecture that deflects cracks, leading to high toughness.[76] After further loading (Stage II), the long CS chains are gradually stretched to allow some of the chains to break. At this point, some of the fractured hydrogen bonds reform due to the interlayer sliding of the MMT

ACCEPTED MANUSCRIPT nanosheets. Fracture crack roughens during the crack deflection process, which increases the total fracture surface area and further improves the fracture toughness of materials.[77] The crack deflects to form a bent crack direction, which dissipates more energy, as shown in the Figure 4g.

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As the loading is increased further (Stage III), the covalent bonds are broken and a large amount of energy is dissipated, resulting in curling of the MMT nanosheets (magnified image in Figure 4h, red frame). Mechanical interlocking[78, 79] and enhance interfacial interactions[12] are two

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of the strengthening and toughening mechanisms that work in nacre. Narducci et al[80] demonstrated that the staggered arrangement of interlocking tiles succeeded in deflecting cracks

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around a tortuous path, thus avoiding brittle failure in the most loaded part of the structure. Since the surface of the MMT nanosheet is smooth, there was weak mechanical interlock enhancement mechanism in the samples we prepared, but the synergistic toughening of double networks (hydrogen and covalent bonds) remarkably improves the mechanical properties of the

3.6 UV-Blocking Properties

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MMT-CS-GP-OH films, consistent with previous reports.[46, 49, 81].

UV blocking is a feature that is in high demand in multifunctional materials because UV

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radiation causes chemical reactions, weathering of polymers, discoloration of certain pigments, and even damage to the eyes and skin.[82] The optical properties of the artificial nacre film were

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revealed by UV−vis transmittance spectra. In Figure 5a, the pure CS and MMT-CS film exhibited high transmittance in the UV and visible regions. After introduction of GP, an effective naturally occurring-cross-linking agent that can react spontaneously with CS, dark blue pigments were formed.[6] The UV transmittance of MMT-CS-GP-OH was greatly reduced by 7.3% at 365nm from 79.7% for MMT-CS film. When the pH was changed to 5.5, cross-linking of GP was further promoted and the UV transmittance of the MMT-CS-GP-OH was reduced to 4.2% at 365nm, It shows the smiliar UV blocking properties as the B4C-nanowire/carbon-microfiber composites.[83]

ACCEPTED MANUSCRIPT The macroscopic color change of the artificial nacre can be clearly seen in the film (inset in Figure 5a). In order to investigate the real reason for UV barrier, we compared the UV−vis transmittance

(Figure

S10a)

of

ALG,

ALG-0.06GP(mGP/ALG

=

0.06

)

and

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ALG-0.6GP(mGP/ALG = 0.6), illustrated the main reason for The UV blocking of The MMT-CS-GP-OH is the spontaneous reaction between GP and CS, which forms dark blue pigments. The UV-shielding ability of the nacre-like MMT-CS-GP-OH film was further

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confirmed by the photocatalytic degradation of rhodamine B aqueous solution in the presence of TiO2 nanoparticles (Figure 5c).[84] For comparison, the photocatalytic degradation behavior of

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unprotected rhodamine B aqueous solutions was measured under the same conditions. As shown in Figure 5d, after 40 min of irradiation, the unprotected rhodamine B showed 80% degradation, whereas the rhodamine B solution protected by MMT-CS-GP-OH film showed only 5% degradation. The small amount of loss of rhodamine B protected by MMT-CS-GP-OH film

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suggests the good UV shielding properties of the latter.

If ordinary glass can be endowed with shading performance, at the same time, it also has the function of blocking UV, which will have important significance. In order to further verify that

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the MMT-CS-GP-OH nanocomposite has good UV blocking application special window materials such as architectural and automotive glass. We cast 5ml optimal formula

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MMT-CS-GP-OH solutions in the glass petri dish. After evaporation, a photograph of the MMT-CS-GP-OH film on the glass petri dish shows in Figure S 10c. As shown in the Figure S 10b., it can be seen that the MMT-CS-GP-OH film has good transparency (at 550nm, the transmittance is 78.3%) with excellent UV-blocking versatility (at the 365nm, the transmittance is 10.0%). indicating its potential application in special window materials to protect objects from UV light damage. Conclusions

ACCEPTED MANUSCRIPT In conclusion, A new class of GP-enhanced MMT-CS-GP-OH composites was successfully fabricated via an evaporation-based self-assembly process. The effect of MMT content, GP crosslinking of CS, and pH on the mechanical properties and interfacial interactions of the hybrid

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film were studied. The obtained bioinspired nanocomposites featured synergistic effects originating from hydrogen and covalent bonds and exhibited an excellent tensile strength of 226 MPa and toughness of 5.1 MJ/m3. Given that GP, as a naturally occurring cross-linking agent, can

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react spontaneously with CS to form dark blue pigments, the artificial nacre was endowed with excellent UV-blocking versatility(10.0% at 365nm). Therefore, this type of integrated nacre-like

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MMT-CS nanocomposites may have great potential applications in aerospace, tissue regeneration, and construction. Acknowledgements

This work was supported by the National Natural Science Foundation of China (51874322).

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Supporting Information

Supplementary data to this article can be found online at http:// References

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Figure 1. Illustration of the fabrication process of the artificial nacre. (a)–(e) Preparation process

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of the MMT-CS-PG-OH. (f) The dark blue artificial nacre can be folded without fracturing, thereby indicating its flexibility. (g) SEM image of an MMT-CS-PG-OH composite demonstrating a strongly aligned layered arrangement. (h) Sketch map showing GP crosslinked with CS.

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Figure 2. (a) FTIR spectra of the MMT, CS, CS-GP-OH, MMT-CS, and MMT-CS-GP-OH films. XPS spectra of (b) the MMT-CS film and (c) the MMT-CS-GP-OH film. (d) TGA curves of the

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MMT, CS, CS-GP-OH, MMT-CS, and MMT-CS-GP-OH nanocomposites.

ACCEPTED MANUSCRIPT Figure 3. Mechanical properties of the nacre-inspired films. (a) Tensile stress-strain curves of pure CS, CS-GP-OH, MMT-CS, and MMT-CS-GP-OH. (b) Tensile strength–strain curves of composites with different MMT contents. A maximum strength of ~226 MPa and maximum

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toughness of 5.1 MJ/m3 was achieved in composites with ~30 wt% MMT content. (c) Tensile strength–strain curves of composites with different GP contents. Maximum strength was achieved in composites with mGP/mCS = 0.06. (d) Tensile strength–strain curves of composites at different

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pH. Maximum strength was achieved when pH = 5.5. (e) Summary of the tensile strength and toughness of CS-based papers. The inset in (e) shows that MMTs are pulled out and CS molecules

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are stretched during fracture formation.

Figure 4. Microimages of the cross-sections of the (a) pure CS film, (b) CS-GP-OH, (c) MMT-CS, and (d) MMT-CS-GP-OH nanocomposites revealing orderly laminated structures in the MMT-CS-GP-OH film. (e) – (f) the surface morphology of the fracture MMT-CS-GP-OH film revealing pulled-out MMTs. During fracture formation, the MMTs are pulled out and CS molecules are stretched. (g) shows The crack deflects to form a bent crack direction, which

ACCEPTED MANUSCRIPT dissipates more energy. (f) During fracture formation,MMTs are pulled out and chitosan molecules are stretched. (i) Proposed synergistic mechanism of MMT-CS-GP-OH bioinspired

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nanocomposites.

Figure 5. (a) UV–vis spectra of CS, CS-GP-OH, MMT-CS, MMT-CS-GP, and MMT-CS-GP-OH

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films. The inset in (a) shows a photograph of the MMT-CS, MMT-CS-GP, and MMT-CS-GP-OH films. The color of the artificial nacre changed remarkably with addition of GP. Changing the pH

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to 5.5 promoted cross-linking and turned the sample a darker shade of blue. (b) UV–vis spectra of MMT-CS-GP-OH films with different MMT contents. (c) Schematic illustration of UV-shielding performance testing of the MMT-CS-PG-OH film. (d) Photocatalytic degradation profile of rhodamine B unprotected and protected by the MMT-CS-PG-OH film.