Bioinspired Interface Engineering for Moisture ... - ACS Publications

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Bioinspired Interface Engineering for Moisture Resistance in NacreMimetic Cellulose Nanofibrils/Clay Nanocomposites Kun Yao,†,‡ Shu Huang,† Hu Tang,† Yeping Xu,§ Gerd Buntkowsky,§ Lars A. Berglund,‡ and Qi Zhou*,†,‡ †

School of Biotechnology, Royal Institute of Technology, Alba Nova University Centre, SE-106 91 Stockholm, Sweden Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Sweden § Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, 64287 Darmstadt, Germany ACS Appl. Mater. Interfaces 2017.9:20169-20178. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 08/11/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The interfacial adhesion design between “mortar” and “bricks” is essential for mechanical and barrier performance of nanocellulose-based nacre-mimetic nanocomposites, especially at high moisture conditions. To address this fundamental challenge, dopamine (DA) has been conjugated to cellulose nanofibrils (CNFs) and subsequently assembled with montmorillonite (MTM) to generate layered nanocomposite films inspired by the strong adhesion of mussel adhesive proteins to inorganic surfaces under water. The selective formation of catechol/metal ion chelation and hydrogen bonding at the interface between MTM platelets and CNFs bearing DA renders transparent films with strong mechanical properties, particularly at high humidity and in wet state. Increasing the amount of conjugated DA on CNFs results in nanocomposites with increased tensile strength and modulus, up to 57.4 MPa and 1.1 GPa, respectively, after the films are swollen in water. The nanocomposites also show excellent gas barrier properties at high relative humidity (95%), complementing the multifunctional property profile. KEYWORDS: nanocomposites, cellulose nanofibrils, clays, nacre-mimetic materials, hygromechanical properties

1. INTRODUCTION Nacre, a pearly substance that lines the interior of some mollusk shells, is mainly composed of aligned platelet-shaped aragonite (CaCO3 crystals) surrounded by a protein matrix that serves as glue between the platelets. The hard aragonite provides strength, while the soft organic layer dissipates viscoplastic deformation energy. The brick-and-mortar structure and complex hierarchical microarchitecture of nacre render it excellent strength and toughness. To mimic the brick-andmortar structure of nacre, montmorillonite (MTM) platelets,1−6 Al2O3 platelets,7,8 graphene nanosheets,9−13 and layered double hydroxides14 have been used as “bricks”. Various kinds of polymers, such as polyelectrolyte,15,16 poly(vinyl alcohol) (PVA),17−20 polyethylene glycol,21 xyloglucan (XG),22,23 gellan gum,24 chitosan,25,26 cellulose derivatives including carboxymethyl cellulose27−30 and hydroxyethyl cellulose (HEC),6,31 and cellulose nanofibrils (CNFs),32−37 have been used as adhesive “mortar”. The mechanical and functional performances of nacre-mimetic nanocomposites are determined not only by the intrinsic properties of “mortar” and “bricks”,38 but also by the interfacial adhesion between the two phases.39 Methods such as infiltration of covalent and ionic crosslinking agents and introduction of hydrogen bonding motifs have been successfully applied in order to enhance the interfacial interactions.40 Such strong interactions have generally resulted in significant increase in stiffness and strength © 2017 American Chemical Society

of the nacre-like nanocomposites but sacrificing their extensibility and toughness. Although synergistic toughening effect from graphene oxide (GO) and lubrication action of molybdenum disulfide (MoS2) in a ternary artificial nacre with thermoplastic polyurethane as the mortar has been successfully demonstrated previously,41 simultaneously improving the strength and toughness of nacre-mimetic layered nanocomposites is still a grand challenge. In addition, the effect of moisture content on the mechanical properties is significant since the soft polymer “mortar” is often hygroscopic. It has been demonstrated that a nacre-inspired PVA/nanoclay composite shows a brittle-to-ductile transition due to a hydration-induced glass-to-rubber transition in the 2D-nanoconfined PVA layers by increasing relative humidity.19 Similar effect of relative humidity on the mechanical properties of nacre-mimetic nanocomposites from graphene oxide (GO) and poly(acrylic acid) has also been found.42 Thus, preserving the strength at high moisture contents is essential for utilizing the full potential of the nacre-mimetic nanocomposites. To increase the wet-strength of water-borne nacre-mimetic materials, ionic supramolecular interactions have been successfully implemented by exchanging monovalent counterion Na+ with Received: February 14, 2017 Accepted: May 22, 2017 Published: May 22, 2017 20169

DOI: 10.1021/acsami.7b02177 ACS Appl. Mater. Interfaces 2017, 9, 20169−20178

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ACS Applied Materials & Interfaces divalent Cu2+ in the carboxymethylcellulose (CMC)/MTM nanocomposites in ordered to compensate the loss of hydrogen bounding ability at higher relative humidity (RH).29 The covalent hemiacetal bond formation at the XG/MTM interface via regioselective periodate oxidation of side chains of XG has also resulted in nanocomposites with well-preserved mechanical properties at 90% RH.23 The covalent cross-linking with glutaraldehyde has also significantly increased interfacial interaction and structural stability of MTM/HEC artificial nacre at wet state.31 Among the polymeric materials, CNFs with high strength, elastic modulus, and thermal stability have great potential in application as the “mortar” phase. Nacre-mimetic hybrid composites from CNFs and MTM have been extensively studied owing to the unique combination of mechanical performance and excellent oxygen barrier properties for applications in flexible packaging.20,32−36 To increase the interaction between CNFs and nanoclay and to obtain composites with high toughness, several different methods, such as the addition of chitosan as a flocculation agent,35 the introduction of cationic CNFs for ionic interactions,36 the addition of PVA for full exfoliation of MTM and as the adhesive polymer between MTM and CNFs,20 have been successfully demonstrated. However, the mechanical properties of these CNFs/MTM nanocomposites at high RH and hydration state were not addressed and reported although CNFs are highly moisture sensitive. Besides the interfacial design for enhanced adhesion, the orientation of cellulose molecule and cellulose microfibrils is essential for the mechanical properties of nanocellulose-based materials.43−45 Previously, it has been found that the post-translationally modified amino acid L-3,4-dihydroxyphenylalanine (DOPA) in adhesive mussel foot proteins is responsible for the versatile adhesive capabilities of the mussels under water.46−48 DOPA is derived from hydroxylation of tyrosine residues in these adhesive proteins, and the catechol group in DOPA is capable of forming hydrogen bonds, metal−ligand complexes and quinhydrone charge-transfer complexes with various substrates such as mica and SiO2.49 It has been demonstrated that musselinspired synthetic peptides such as random copolymers of DOPA and N′-(2-hydroxyethyl)-L-glutaminecan can mimic the ability of mussel byssus and exhibit strong adhesion to TiO2 and mica.50 The dopamine (DA), containing both the functional catechol group and the amine group, can also react with GO sheets and self-polymerize into long chain polymers of polydopamine (PDA), which improves the adhesion between GO sheets and mechanical properties underwater.9,10 PDAnickel ion (Ni2+) chelation strategy has also been applied in preparation of graphene-based nanocomposites with high tensile strength and toughness.51 Clay platelets have also been modified with DA-derived polydopamine or block copolymer of DOPA-Lys-PEG (Lys = lysine; PEG = polyethylene glycol) and subsequently cross-linked by Fe3+ ions.52,53 Recently, there have also been a number of efforts to functionalize chitosan with catecholic moieties via carbodiimide chemistry54,55 or reductive amination.56 The catechol-conjugated chitosan were specifically used to promote coordination interactions with inorganic nanoparticles or transition metal ions to form composites or hydrogels with robust interfacial bonds and unique mechanical properties.55,56 Inspired by the mussel adhesive proteins, herein we report the fabrication of nacre-mimetic MTM/CNFs nanocomposites with enhanced interfacial interactions by using dopamine-

conjugated CNFs (DA-CNF), which adhere to MTM through the formation of catechol/metal ion chelation and hydrogen bonding, as illustrated in Scheme 1. The DA-CNFs are Scheme 1. Illustration for the Preparation of DA-Conjugated CNFs from Pulp Fibers and Subsequent Assembly of DACNF with MTM to Prepare Transparent Nanocompositea

a

The photograph of a free standing MTM/DA-CNF-3 nanocomposite film is presented.

prepared by mechanical disintegration of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized wood pulp fibers that were partially modified with dopamine hydrochloride through carbodiimide chemistry. The anionically charged residual sodium C6-carboxyl groups on the DA-CNF surfaces stabilize the colloidal suspension of anionically charged MTM in water, and the catechol groups of the conjugated DA molecules impart remarkable adhesive strength to the MTM/ DA-CNF interfaces and significantly enhance the toughness and barrier properties of the nacre-like composites, particularly after swelling in water and at higher RH, respectively.

2. EXPERIMENTAL SECTION Materials. A commercial never-dried softwood sulphite pulp provided by Nordic Paper was used as native cellulose fibers. Sodium montmorillonite (MTM, Cloisite Na+, a natural bentonite, density of 2.86 g/cm3) was supplied by BYK Additives & Instruments (former Rockwood Additives and Southern Clay Products Inc.). The average size of the MTM platelets is 120 ± 75 nm, as measured by Atomic Force Microscopy (Figure S1, Supporting Information). All other chemicals were purchased from Sigma-Aldrich and used without further purification. Preparation of DA-CNFs. Carboxylated wood pulp fibers were prepared by TEMPO-mediated oxidation as described in our previous 20170


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reported as average for at least five specimens in vicinity. Modulus was determined from the slope of the initial low strain region of the stress−strain curve. Toughness, defined as work to fracture, was calculated as the area under the stress−strain curve. Field-emission scanning electron microscopy (FE-SEM) of the tensile fractured surfaces of the composites was observed with a Hitachi S-4800 FESEM, which was operated at 1 kV. The tensile fractured samples were coated with a 5 nm layer of gold−palladium using an Agar HR sputter coater. Oxygen transmission rate (OTR) of the composites was determined using a Mocon OX-TRAN TWIN 2/20 equipped with a coulometric oxygen sensor according to ASTM D 3985-06 at 23 °C and a relative humidity of 50% and 95%. Film samples with a thickness of 35 ± 0.8 μm were partially covered by a tight aluminum foil that has an adhesive on its surface and a circular exposure area of 50 cm2. All the samples were conditioned for 4 h in the cell prior to measurement.

work.57 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC, 0.58 g, 3.0 mmol) was first added to 250 g of 1 wt % carboxylated pulp fibers/water slurry and stirred for 30 min. Nhydroxysuccinimide (NHS, 0.53 g, 4.5 mmol) and dopamine hydrochloride were then added and stirred for 24 h at 22 °C under nitrogen atmosphere. The product was filtrated and washed thoroughly with deionized water. The obtained slurry of DA-grafted pulp fibers were dispersed in water at a concentration of 0.2 wt % and disintegrated by high-speed kitchen blender (Vita-Prep 3 model, VitaMix Corp., U.S.A.) for 10 min to produce DA-conjugated CNFs sample. By varying the added amount of DA in the reaction (1.5, 3.0, and 6.0 mmol), three samples were prepared and designated as DACNF-1, DA-CNF-2, and DA-CNF-3, respectively. The TEMPOoxidized cellulose nanofibrils (TO-CNF) were disintegrated directly from carboxylated wood pulp fibers using the same procedure. Preparation of MTM/DA-CNF Nanocomposite Films. Typically, a desired amount of 0.15 wt % DA-CNF water suspension was slowly added to a 0.2 wt % MTM water dispersion. The mixed dispersion was then magnetically stirred for 24 h followed by a vigorous stirring at 15 000 rpm for 2 min using a homogenizer (Ultra Turrax mixer, IKA). After degassing, the obtained MTM/DA-CNF suspension was vacuum filtered to form a hydrogel on a 0.22 μm Durapore membrane filter. After being air-dried at room temperature, a freestanding film was peeled off from the filter membrane. The control MTM/TO-CNF sample was prepared using the same procedure. Characterizations. Atomic force microscopy (AFM) height images of MTM platelets were recorded using a MultiMode 8 (Bruker, Santa Barbara, U.S.A.) with a RTESP cantilever having a nominal tip radius of 8 nm and a spring constant of 5 N/m (Bruker, U.S.A.). The dispersion of MTM platelets in water were dried on a silica wafer and scanned using the tapping method in air. The conductometric titration was measured with a conductometric station (SevenCompact, Mettler-Toledo). FTIR was performed on a PerkinElmer Spectrum 2000 FTIR equipped with a MKII Golden Gate, Single Reflection ATR system from Specac Ltd., London, U.K. The ATR crystal was a MKII heated diamond 45° ATR top plate. The measurement spectral range was 600−4000 cm−1 with a resolution of 4 cm−1. UV spectra were obtained on a Varian Cary 50 Bio UV−visible spectrophotometer at room temperature. The colorimetric essay for the quantification of DA content was performed according to the procedure reported by Waite and Tanzer.58 Solid-state single pulse MAS 13C NMR spectra were recorded at 75.47 MHz on a Bruker Avance III spectrometer with a broadband double-resonance 4 mm probe. All spectra were accumulated with a recycle delay of 60 s, pulse width of 2 μs as 45° pulse, and a spinning rate of 6 kHz. The 13C chemical shifts were referenced to TMS at 0 ppm. Transmission electron microscopy (TEM) images were obtained using the Hitachi Model HT7700 transmission electron microscope operated in highresolution mode at 100 kV. A drop of the dilute water suspension of cellulose nanofibrils was deposited on a carbon-coated grid and treated with 1% uranyl acetate negative stain. The X-ray diffraction (XRD) diffractograms were recorded with a Philips X’Pert Pro diffractometer (model PW 3040/60) in the reflection mode (1−35° 2θ angular range, steps of 0.05°). The CuKα radiation (λ = 1.5418 Å) was generated at 45 kV and 40 mA and monochromatized using a 20 μm Ni filter. Diffractograms were recorded from rotating specimens using a position sensitive detector. Thermogravimetric analysis (TGA) was measured on a Mettler Toledo TGA/DSC 1 STARe System using nitrogen as the purge gas at a flow rate of 50 mL min−1. Composite samples of about 7.0 mg were heated from 30 to 700 °C at a rate of 10 °C min−1. The tensile mechanical properties of the composites were measured using a universal material testing machine (Instron 5944, U.K.). The samples were conditioned at the relative humidity (RH) of 50% and 100% for at least 3 days, or presoaked in water for 2 h. The thickness of the nanocomposites after swelling in water was measured with a micrometer caliper by placing the film between two overhead projector transparency films. The thickness values of the swollen samples measured as such were verified by optical microscopy. At least seven specimens were tested from each sample and results are

3. RESULTS AND DISCUSSION 3.1. Structure of Cellulose Nanofibirls Bearing Dopamine. Previously, Karabulut et al.59 pioneered the covalent conjugation of DA to carboxymethylated cellulose nanofibrils (CNFC) by using the carbodiimide activation technique in a very dilute aqueous suspension of CNFs (0.14 wt %). To scale up the production of DA-conjugated CNFs, chemical modification of TEMPO-oxidized wood pulp fibers (1 wt %) with DA was in the present study carried out before mechanical disintegration into cellulose nanofibrils, as illustrated in Scheme 1. The successful conjugation of DA onto cellulose nanofibrils was confirmed by FTIR and UV−vis spectroscopy. The FTIR spectra of TO-CNF and DA-CNF samples are aligned and compared in Figure 1a. For all samples, the absorption band at 1159 cm−1 arises from the asymmetrical stretching of glyosidic bridge COC. For the TO-CNF sample dried from water suspension at pH 3, the absorption band located at 1723 cm−1 is ascribed to the stretching vibration of carboxyl groups in the acid forms, and the band at 1632 cm−1 corresponds to water associated with cellulose. At pH 7, the carboxyl groups are in the salt forms and the absorption band is shifted to 1601 cm−1. After DA conjugation, the amide bond formation between the amine group of DA and the carboxyl group which was oxidized from the C6 primary hydroxyl on cellulose microfibril surfaces is evidenced by the detection of the NH bending (amide II) at 1523 cm−1, and the absorbance increases with the increasing amount of conjugated DA. The absorption bands of the carbonyl stretching (amide I) at 1700 and 1597 cm−1 overlap with the asymmetric stretching vibration of free carboxyl groups in salt form at 1601 cm−1, resulting in a significant broadening of the peak. A new absorption band at 1284 cm−1 corresponding to the asymmetric phenolic CO stretching vibration is also detected in all DA-CNF samples. Figure 1b shows the UV−vis spectra of 0.062 mM DA, and the water suspensions of TOCNF and DA-CNF samples at pH 7. The DA molecules absorb UV radiation at 220 and 281 nm, which are ascribed to π−π* and La−Lb coincident transition, respectively. By contrast, no similar absorption bands are observed in the TO-CNF dispersion. DA-CNF suspensions also show UV absorption at 220 and 281 nm with the absorbance that is proportional to the content of conjugated DA, in support of successful grafting of DA molecules to CNF nanofibrils. The absence of additional peaks at wavelengths longer than 300 nm suggests that the catechol groups of DA were not oxidized or the degree of oxidation was very low and could be neglected. The conjugation of DA onto CNFs was also confirmed by Solidstate 13C single pulse MAS NMR (Figure 2). The TO-CNF 20171


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Figure 2. Solid-state single pulse MAS 13C NMR spectra of TO-CNF and DA-CNF samples.

carboxylate content of TO-CNF and the content of residual carboxyl groups in DA-CNF samples were determined by conductometric titration. Typical titration curves showed presence of strong and weak acid groups (Figure S2). The amount of strong acid corresponded with the added HCl, and that of weak acid corresponded with the carboxyl content. The carboxylate content of TO-CNF is 1.25 mmol/g, and it decreases with the increasing conjugated amount of DA in the DA-CNF samples. As compared to the DA content, there were carboxyl groups that were consumed by the side reactions due to the formation of undesired N-acylurea in all DA-CNF samples.57 The crystal structure of the TO-CNF and DA-CNF samples was investigated by X-ray diffraction. All samples show a typical diffraction pattern of cellulose I (Figure S3). The elementary crystallite size was estimated from the peak corresponding to the (200) plane of cellulose Iβ using Scherrer’s equation62 and summarized in Table S1. The crystallite size of DA-CNF is similar to that of TO-CNF, indicating that the covalent conjugation of DA mainly occurred on the surface of cellulose nanofibrils. The crystallinity index (CrI) is calculated from the height ratio between the intensity of the crystalline peak (I200 − IAM) and total intensity (I200) after subtraction of the background signal measured without cellulose. The crystallinity index (CrI) value of DA-CNF samples slightly decreases with the increasing amount of conjugated DA (Table S1). The morphology of TO-CNF and DA-CNF samples was characterized by transmission electron microscopy (TEM) as shown in Figure 3. Wood cellulose nanofibrils have a very thin and uniform width of a few nanometers. The measured width values are, however, largely affected by the measurement method used.56 In this work, for the purpose of comparison

Figure 1. (a) FTIR spectra in the range of 1800−1100 cm−1 for TOCNF and DA-CNF samples with different conjugated amount of DA that were dried from water suspensions at pH 7, as compared to a TOCNF sample dried at pH 3. (b) UV−vis spectra of 0.062 mM DA and 0.1 mg/mL water suspensions of TO-CNF and DA-CNF samples at pH 7.

sample shows a signal at 173 ppm owing to the sodium carboxylate carbons. The C1, C4, C2,3,5, and C6 signals of cellulose I are in the range of 60−110 ppm. The chemical shift and pattern of these signals are essentially unchanged even after the conjugation with DA, but new signals at (A) 41 ppm, (B) 116 ppm, (C) 120 ppm, (D) 128 ppm, and (E) 143 ppm are detected and their assignments are shown in the representative chemical structure of DA-CNF. The relative intensity of these new signals is increased with the increasing amount of conjugated DA in the DA-CNF samples. The DA content of the DA-CNF samples was determined from a colorimetric assay58,60 using the classical Arnow’s stain61 for the detection of catechol, and the results are shown in Table S1. The grafted amounts of DA on cellulose nanofibril surfaces were 0.11, 0.37, and 0.46 mmol/g cellulose for the DA-CNF-1, DA-CNF-2, and DA-CNF-3 samples, respectively, as determined by A500 nm of the water suspensions of DA-CNF samples treated with the Arnow’s stain versus a standard curve derived from a series of DA solutions with known concentration. The 20172


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Figure 3. Transmission electron microscopy images of (a) TO-CNF, (b) DA-CNF-1, (c) DA-CNF-2, and (d) DA-CNF-3 nanofibrils airdried from dilute water suspension and stained with 1% uranyl acetate.

Figure 4. FE-SEM images for the cross sections of the fractured surfaces of (a) MTM/DA-CNF-3 and (c) MTM/TO-CNF nanocomposites obtained by uniaxial tensile testes at 50% relative humidity, and (b) and (d) are the corresponding images at higher magnification, respectively.

between TO-CNF and DA-CNF, the width and length of the nanofibrils were measured directly from negatively stained TEM images. The TO-CNF, DA-CNF-1, DA-CNF-2, and DACNF-3 samples appear as completely individualized nanofibrils with a homogeneous width of 4.5 ± 0.6, 5.0 ± 0.7, 4.1 ± 0.7, and 4.3 ± 0.6 nm, and a length of 794 ± 368, 738 ± 402, 857 ± 629, and 1000 ± 701 nm, respectively. The conjugation of DA was expected to cause aggregation of cellulose nanofibrils, however the residual carboxyl groups facilitated the disintegration of DA modified pulp fibers into nanofibrils and maintained the colloidal stability of DA-CNF fibrils in water. The water suspensions of TO-CNF and DA-CNF samples were optically transparent and homogeneous without any visible precipitation of nanofibril aggregates. When the DA content was higher than 0.60 mmol/g cellulose, the water suspension of DA-CNF was cloudy with fibril aggregates due to the hydrophobicity of DA molecules. 3.2. Structure of the MTM/CNF Nanocomposites. The MTM/DA-CNF nanocomposites with 50 wt % MTM and 50 wt % DA-CNF samples were fabricated by vacuum-filtration of the water suspensions of MTM and DA-CNFs followed by drying under ambient laboratory condition. The composition of MTM and DA-CNF in the nanocomposite films was verified by TGA analysis as shown in Figure S4. Since significant decrease of protein adhesion energy with increasing pH has been previously observed by surface force examination on an isolated mussel foot protein,63 the pH value of the mixture of MTM and DA-CNF suspensions was adjusted to 5.0. This is to avoid partial oxidation of DA at higher pH because the deprotonated catechol is prone to be oxidized into o-quinone, and the reduced catechol form provides stronger surface binding than the oxidized quinone form.64 Transparent MTM/DA-CNF nanocomposite films were prepared and photograph of a typical sample of MTM/DA-CNF-3 is shown in Scheme 1. The cross sections of the fractured surfaces of MTM/DA-CNF-3 and the control MTM/TO-CNF nanocomposite films obtained by uniaxial tension at 50% RH were observed by field emission scanning electron microscopy (FE-SEM) and the micrographs are also shown in Figure 4. Both MTM/DA-CNF-3 (Figure 4a) and MTM/TO-CNF (Figure 4c) nanocomposites show a lamellar microstructure, resembling the brick-and-mortar structure of nacre. The overall orientational in-plane order of

the MTM platelets is relatively good, but there are also wavy MTM sheets, resulting in micrometer- and submicrometersized defects (voids) between layered structures. The pull-out of CNFs are visible on fracture surfaces, and the MTM platelets are connected by CNFs in both composite samples (Figure 4b,d), indicating the strong interaction between MTM platelets and cellulose nanofibrils at RH 50%. The X-ray diffraction (XRD) data of MTM/DA-CNF nanocomposites with different conjugated amount of DA on CNF show a clear shift in MTM basal space reflection peaks (Figure S5). The neat MTM has a 2θ peak at 7.4°, corresponding to a d001 lattice spacing of 11.9 Å. The MTM/DA-CNF composites and the control MTM/ TO-CNF have MTM basal space reflection peaks at 6.7° and 7.1°, corresponding to d001 lattice spacing values of 13.2 and 12.4 Å, respectively. Thus, there is no sign of intercalation of CNFs between MTM platelets, similar to the previous study in clay nanopaper.32 Since the width of both DA-CNF and TOCNF was around 4−5 nm, a full separation and intercalation for ultrathin clay platelets in a bulk composite was not expected at 50 wt % of MTM. However, the CNFs are located in the space between MTM platelets or tactoids as shown in Figure 4b and 4d. The interaction between MTM and cellulose nanofibrils was further studied by the FTIR spectroscopy. The frequency of vibrational bands at 3610 cm−1 corresponds to hydrogen bonding in MTM, while the frequency of vibrational bands at 3326 cm−1 corresponds to hydrogen bonding in cellulose. For both MTM/DA-CNF and MTM/TO-CNF nanocomposites, the frequency of vibrational bands at 3326 cm−1 shifted toward a higher frequency value of 3336 cm−1 (Figure S6). The carboxyl groups of DA-CNF and TO-CNF are still in the salt form in the composites at pH 5.0. The vibration frequency of carboxyl group also shifted to 1607 cm−1 for both MTM/DACNF and MTM/TO-CNF nanocomposites (Figure 5), from 1597 and 1601 cm−1 for neat DA-CNF and TO-CNF (Figure 1a), respectively. These higher shifts for the vibration frequencies of the carboxyl and hydroxyl groups indicate that interfibril hydrogen bonding in both DA-CNF and TO-CNF is reduced due to the adsorption of CNFs to the silicate platelet surfaces. In addition, a weak shoulder peak at 840 cm−1 20173


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MTM interfacial interaction (including catechol/metal ion chelation and/or hydrogen bonding of catechol with oxygen on the minerals as shown in Scheme 1) is confirmed through the loss of the phenolic band at 1284 cm−1 in the MTM/DA-CNF nanocomposite samples (Figure 5). 3.3. Hygromechanical and Barrier Properties. The effect of such interface structure on mechanical performance of the nanocomposites was investigated by tensile test. The stress−strain curves of MTM/DA-CNF and MTM/TO-CNF nanocomposites at RH 50%, RH 100%, and wet state are shown in Figure 6, respectively, and their mechanical properties data are summarized in Table S2. At 50% RH, the tensile strength and modulus of the MTM/DA-CNF nanocomposites increases slightly with the increasing amount of conjugated DA. The MTM/DA-CNF-3 sample has a tensile strength of 357.8 ± 10.2 MPa and a modulus of 12.1 ± 0.3 GPa, 25% higher than those for the control MTM/TO-CNF sample. Moisture has a strong impact on the mechanical properties of CNFs nanopaper according to the first thorough work on the topic by Benitez et al.66 Although decay of mechanical properties occurred at higher RH and wet state, significant improvement in mechanical properties was observed for the MTM/DA-CNF nanocomposites as compared to the control MTM/TO-CNF sample. As shown in Figure 6b, at 100% RH, the tensile strength of MTM/DA-CNF-3 sample is 280.1 ± 7.5 MPa, twice as high as that for the MTM/TO-CNF sample. The higher strength is due to higher interfacial adhesion in the presence of moisture at higher RH. When the nanocomposite films were soaked in water, the MTM/TO-CNF sample

Figure 5. FTIR spectra in the range of 1800−700 cm−1 for neat MTM, the MTM/DA-CNF and MTM/TO-CNF nanocomposites prepared at pH 5.0.

corresponding to the vibration of Al−O−C was observed for both MTM/DA-CNF and MTM/TO-CNF nanocomposites (Figure 5), similar to the clay nanopaper prepared from MTM and cellulose nanofibers, in which the hydroxyl groups of CNFs was found interacting with the Al of MTM.32 In general, the formation of catechol chelating metal ions results in the broadening or split of the aryl vibrational band at 1405 cm−1 with a second peak at 1375 cm−1.55,65 Although the aryl band in the DA-CNF samples is overlapped with the absorption band of CH bending in cellulose (Figure 1a), the strong DA-CNF/

Figure 6. Stress−strain curves for MTM/TO-CNF and MTM/DA-CNF nanocomposites at relative humidity of (a) 50% and (b) 100%, and (c) after swelling in water for 2 h. Inserted photograph in (c) shows more than 500 g in weight supported by the MTM/DA-CNF-3 nanocomposite sample under water after swelling for 24 h (initial dry sample with a width of 2.5 mm, a length of 10 mm and a thickness of 35 μm). (d) Water adsorption kinetic curves of the MTM/TO-CNF and MTM/DA-CNF nanocomposites in deionized water. 20174


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and frictional sliding of platelets is much more limited in the hydrated state due to the strong interfacial adhesion in water. The water content in the hydrated MTM/DA-CNF-3 sample is ca. 64 wt %, similar to the hydrogel for tissue engineering applications but with much higher mechanical performance under water. It can hold an overall mass of more than 500 g under water for more than 24 h (Figure 6c). A typical double network polyacrylamide hydrogel with water content of 50−75 wt % has a maximum tensile strength of ca. 4.5 MPa67 and a polyethylene glycol based interpenetrating polymeric network hydrogel with 70.2 wt % water has a tensile strength of only 1.2 MPa.68 The oxygen transmission rate (OTR) values for MTM/DACNF-3 and MTM/TO-CNF samples at 50% RH condition were 0.039 and 0.048 cm3 mm m−2 day−1 atm−1, respectively, similar to those reported previously for MTM/NFC nanopaper.32 The OTR of MTM/DA-CNF-3 sample at 95% RH was 0.533 cm3 mm m−2 day−1 atm−1, much lower than the values for MTM/TO-CNF, MTM/NFC nanopaper, and neat NFC nanopaper (Table 1). This result demonstrates a clear benefit from the enhanced interfacial adhesion between dopamine-conjugated CNFs and MTM especially at high relative humidity.

reached a saturation water absorbance value of ca. 580% (Figure 6d) with an increase of film thickness from 33 to 293 μm after only 1 min due to high hydrophilicity of both MTM and CNF, while the MTM/DA-CNF-3 sample reached the saturation value of only 179% after swelling in water for 2 h and the film thickness increased from 35 to 112 μm. This is owing to the increased hydrophobicity of DA conjugated CNF and the catechol induced covalent bonding between DA-CNF and MTM, which is stable in the presence of water molecules compared to hydrogen bonding. After swelling in water, the tensile strength and modulus of MTM/TO-CNF decreased by almost 2 orders of magnitude compared to RH 50%, and the strain-to-failure increased from 6.3% to 29.0%, displaying large plastic deformation. This is due to disengagement of the hydrogen-bonding at the interface between CNF and MTM and lower nanofibrillar adhesion as water molecules adsorb on the nanofibril surface, which facilitates CNF-MTM debonding. Indeed, FE-SEM imaging of fracture surfaces demonstrates pull-out of MTM platelet tactoids, for the MTM/TO-CNF sample (Figure 7b), while the extent of MTM pull-out is much

Table 1. OTR (cm3 mm m−2 day−1 atm−1) of MTM/TOCNF and MTM/DA-CNF-3 Nanocomposite Films under 0, 50, and 95% Relative Humidity (RH) and 100% Oxygen Conditions, As Compared with the Previously Reported MTM/NFC Clay Nanopaper samples

0% RH

50% RH

95% RH

MTM/DA-CNF-3a MTM/TO-CNFa MTM/NFCb NFCb

c

0.039 0.048 0.045 0.048

0.533 2.536 3.5 17.8

c c c

a This work. bMTM/NFC (50N/50M) clay nanopaper and NFC nanopaper reported previously (ref 32). cOTR at 0% RH 100% O2 for these materials was below detection limit.

4. CONCLUSIONS In summary, the interface between MTM and CNFs has been successfully engineered for increased wet adhesion via partial conjugation of DA molecules onto CNFs and reaction with MTM, inspired by the versatile bioadhesive capabilities of mussel adhesive proteins. To facilitate upscaling, DA modification was performed on porous TEMPO-oxidized pulp fibers, which were then disintegrated into DA-CNF. MTM/DA-CNF nanocomposites with 50 wt % MTM were fabricated via vacuum filtration, a procedure akin to paper making, and resulted in a transparent film with nacre-like brickand-mortar microstructure. The nanocomposites exhibited superior barrier and mechanical properties particularly at higher humidity and in wet state with the increasing conjugated amount of DA on CNFs. The hydrated MTM/DA-CNF-3 sample with a water content of 64 wt % demonstrated a tensile strength, modulus, and toughness of 57.4 ± 2.2 MPa, 1.1 ± 0.1 GPa, and 3.6 ± 0.3 MJ m−3, respectively, much higher than the conventional polymer hydrogels. Such enhancement in hygromechanical and barrier properties is ascribed to the strong interactions between DA-CNF and MTM at the interface, where the two rigid components show strong adhesion by dopamine through catechol/metal ion chelation

Figure 7. FE-SEM images for the freeze-dried cross sections of the fractured surfaces of (a) MTM/DA-CNF-3 and (b) MTM/TO-CNF nanocomposites obtained by uniaxial tension after swelling in water for 2 h.

more limited for the MTM/DA-CNF sample (Figure 7a). As expected, the MTM/DA-CNF nanocomposites show excellent mechanical properties after swelling in water. The modulus and tensile strength of the hydrated MTM/DA-CNF-3 sample are 1.1 ± 0.1 GPa and 57.4 ± 2.2 MPa, an order of magnitude higher than for the hydrated MTM/TO-CNF sample. Such remarkable improvement is owing to the strong interfacial adhesion between CNF and MTM caused by the conjugated DA molecules on CNF fibrils surfaces. CNF-MTM debonding 20175


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Nanoclay Composites with Excellent Gas and Water Vapor Barrier Properties. ACS Appl. Mater. Interfaces 2016, 8 (38), 25535−25543. (5) Makiniemi, R. O.; Das, P.; Honders, D.; Grygiel, K.; Cordella, D.; Detrembleur, C.; Yuan, J. Y.; Walther, A. Conducting, Self-Assembled, Nacre-Mimetic Polymer/Clay Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7 (29), 15681−15685. (6) 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 (15), 7613−7620. (7) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Bioinspired design and assembly of platelet reinforced polymer films. Science 2008, 319 (5866), 1069−1073. (8) Abba, M. T.; Hunger, P. M.; Kalidindi, S. R.; Wegst, U. G. K. Nacre-like hybrid films: Structure, properties, and the effect of relative humidity. J. Mech Behav Biomed 2016, 55, 140−150. (9) Zhong, D.; Yang, Q. L.; Guo, L.; Dou, S. X.; Liu, K. S.; Jiang, L. Fusion of nacre, mussel, and lotus leaf: bio-inspired graphene composite paper with multifunctional integration. Nanoscale 2013, 5 (13), 5758−5764. (10) Cui, W.; Li, M. Z.; Liu, J. Y.; Wang, B.; Zhang, C.; Jiang, L.; Cheng, Q. F. A Strong Integrated Strength and Toughness Artificial Nacre Based on Dopamine Cross-Linked Graphene Oxide. ACS Nano 2014, 8 (9), 9511−9517. (11) Cheng, Q. F.; Duan, J. L.; Zhang, Q.; Jiang, L. Learning from Nature: Constructing Integrated Graphene-Based Artificial Nacre. ACS Nano 2015, 9 (3), 2231−2234. (12) Wan, S. J.; Peng, J. S.; Jiang, L.; Cheng, Q. F. Bioinspired Graphene-Based Nanocomposites and Their Application in Flexible Energy Devices. Adv. Mater. 2016, 28 (36), 7862−7898. (13) Zhang, Y. Y.; Gong, S. S.; Zhang, Q.; Ming, P.; Wan, S. J.; Peng, J. S.; Jiang, L.; Cheng, Q. F. Graphene-based artificial nacre nanocomposites. Chem. Soc. Rev. 2016, 45 (9), 2378−2395. (14) Shu, Y. Q.; Yin, P. G.; Liang, B. L.; Wang, H.; Guo, L. Bioinspired Design and Assembly of Layered Double Hydroxide/ Poly(vinyl alcohol) Film with High Mechanical Performance. ACS Appl. Mater. Interfaces 2014, 6 (17), 15154−15161. (15) 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 (36), 6448− 6453. (16) Martikainen, L.; Walther, A.; Seitsonen, J.; Berglund, L.; Ikkala, O. Deoxyguanosine Phosphate Mediated Sacrificial Bonds Promote Synergistic Mechanical Properties in Nacre-Mimetic Nanocomposites. Biomacromolecules 2013, 14 (8), 2531−2535. (17) Podsiadlo, P.; Kaushik, A. K.; Shim, B. S.; Agarwal, A.; Tang, Z. Y.; Waas, A. M.; Arruda, E. M.; Kotov, N. A. Can Nature’s Design be Improved Upon? High Strength, Transparent Nacre-Like Nanocomposites with Double Network of Sacrificial Cross Links. J. Phys. Chem. B 2008, 112 (46), 14359−14363. (18) 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 (8), 2742−2748. (19) Verho, T.; Karesoja, M.; Das, P.; Martikainen, L.; Lund, R.; Alegria, A.; Walther, A.; Ikkala, O. Hydration and Dynamic State of Nanoconfined Polymer Layers Govern Toughness in Nacre-mimetic Nanocomposites. Adv. Mater. 2013, 25 (36), 5055−5059. (20) Wang, J. F.; Cheng, Q. F.; Lin, L.; Jiang, L. Synergistic Toughening of Bioinspired Poly(vinyl alcohol)-Clay-Nanofibrillar Cellulose Artificial Nacre. ACS Nano 2014, 8 (3), 2739−2745. (21) Shikinaka, K.; Aizawa, K.; Fujii, N.; Osada, Y.; Tokita, M.; Watanabe, J.; Shigehara, K. Flexible, Transparent Nanocomposite Film with a Large Clay Component and Ordered Structure Obtained by a Simple Solution-Casting Method. Langmuir 2010, 26 (15), 12493− 12495. (22) Kochumalayil, J. J.; Bergenstrahle-Wohlert, M.; Utsel, S.; Wagberg, L.; Zhou, Q.; Berglund, L. A. Bioinspired and Highly

and/or hydrogen bonding of catechol with oxygen on the MTM platelets. In contrast with cross-linking strategies in polymer/MTM systems, the interface only is selectively engineered so that CNF network ductility is preserved. By combining mussel and nacre mimicking in designing lightweight high-performance nanocellulose/inorganic nanocomposites, bioinspired interface engineering for strong adhesion enables combination of multiple functions including tunable swelling in water, high transparency, high mechanical performance under water, and enhanced gas barrier properties. Such multifunctional combination renders these nanocomposites interesting for broader applications, such as encapsulation of drugs and tissue engineering, as well as load-bearing applications outdoors. We foresee further development of multifunctional nanocellulose-based nanocomposites from this strategy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02177. Summaries of DA-CNFs properties and mechanical data of the nanocomposites, AFM image of MTM platelets, conductometric titration curve of CNFs, X-ray diffraction data of DA-CNFs and the nanocomposites, and FTIR spectra in full range (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +46 8 790 96 25. E-mail: [email protected] (Q.Z.). ORCID

Hu Tang: 0000-0001-5968-3756 Gerd Buntkowsky: 0000-0003-1304-9762 Qi Zhou: 0000-0001-9832-027X Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the European Union’s Seventh Framework for research, technological development, and demonstration (MultiBioPro, Grant Agreement 311804) and Wallenberg Wood Science Center for supporting this work.

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REFERENCES

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