graphene oxide film with ultra-flexible, gas barrier and self-clean properties

Journal Pre-proofs Robust galactomannan/graphene oxide film with ultra-flexible, gas barrier and self-clean properties Chen Huang, Guigan Fang, Yongjun Deng, Samarthya Bhagia, Xianzhi Meng, Yuheng Tao, Qiang Yong, Arthur J. Ragauskas PII: DOI: Reference:

S1359-835X(20)30018-X https://doi.org/10.1016/j.compositesa.2020.105780 JCOMA 105780

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

19 August 2019 9 January 2020 17 January 2020

Please cite this article as: Huang, C., Fang, G., Deng, Y., Bhagia, S., Meng, X., Tao, Y., Yong, Q., Ragauskas, A.J., Robust galactomannan/graphene oxide film with ultra-flexible, gas barrier and self-clean properties, Composites: Part A (2020), doi: https://doi.org/10.1016/j.compositesa.2020.105780

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Robust galactomannan/graphene oxide film with ultra-flexible, gas barrier and self-clean properties Chen Huanga,b,c, Guigan Fanga,b*, Yongjun Denga, Samarthya Bhagiac,e, Xianzhi Mengc, Yuheng Taob, Qiang Yongb, Arthur J. Ragauskasc,d,e* aInstitute

of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042,

China bCo-Innovation

Center for Efficient Processing and Utilization of Forest Resources, Nanjing

Forestry University, Nanjing 210037, China cDepartment

of Chemical and Biomolecular Engineering, University of Tennessee Knoxville,

Knoxville, TN 37996, USA dDepartment

of Forestry, Wildlife, and Fisheries, Center for Renewable Carbon, The University of

Tennessee Institute of Agriculture, Knoxville, TN 37996, USA eUTK-ORNL

Joint Institute for Biological Science, Biosciences Division, Oak Ridge National

Laboratory, Oak Ridge, TN 37831, USA

Corresponding authors: *E-mail: [email protected]; *E-mail: [email protected]

Abstract This study examines a facile technology to manufacture food packaging alternatives with superior mechanical, ultra-flexible, gas barrier and self-clean properties, based on green and benign starting materials. A novel galactomannan (GM) was isolated from the sesbania cannabina seeds, which was used as matrix for the fabrication of GM-based films. Inspired by the brick-and-mortar structure of natural nacre, a facile method was adopted to fabricate an artificial nacre based on the 1

self-assembly of GM and borate crosslinked graphene oxide (GO). These GM/GO composites are ultra-flexibile, which can be folded into various shapes. The tensile strength reached 135.54 MPa which is 2.4 times that of the pure GM film. In addition, after coating with the poly(dimethylsiloxane) (PDMS), these films became hydrophobic (WCA around 120°) with self-cleaning properties. Our study further revealed that the oxygen and water vapor permeabilities were improved with the introduction of GO and PDMS coating. Keywords: galactomannan, graphene oxide, natural nacre, food packaging

1. Introduction Petroleum-based synthetic plastics such as polyethylene and polyvinyl chloride exhibit many useful and convenient attributes which currently dominate materials in food packaging[1]. Despite its convenience and low cost, these plastics have several associated environmental issues, one of which is the so-called “white pollution”, based upon the non-biodegradable nature of petroleum-derived polymers. The rising environmental awareness of pollution from plastic incineration together with the plastic accumulations in landfills and aquatic systems have encouraged society to seek alternative and biodegradable materials from renewable resources[2]. Among these, the utilization of biomass as the starting source for manufacturing various materials has attracted increasing attention due to their benefits of abundance, biodegradability, renewability, and low environmental threat[3]. Hemicelluloses are the most abundant polysaccharides other than cellulose in terrestrial plants and have long been of high interest as a resource to be refined into various materials due to their biodegradability, low cost, and no competition with food production[4]. Hemicelluloses are a typical class of heteropolysaccharides with branched structure and are mainly composed of polysaccharides 2

like xylan, arabinan, mannan and so forth, which differ in structure according to plant species[5]. For example, xylan is the backbone of hemicellulose in hardwoods and grasses, which is substituted with side groups like uronic acid, acetyl group, arabinosyl group, and others, whereas mannan is the main hemicellulose component in softwood and seed endosperm. Currently, hemicelluloses are mainly generated in the pulping industry. However, most of them are either discarded as waste or burned for energy in the cooking liquor, causing an inefficient utilization of the resources[6]. Thus, the exploitation of hemicellulose into value-added materials such as biodegradable or edible films for food packaging can facilitate the valorization of these industrial waste products. Indeed, research in using hemicelluloses as packaging films has been ongoing for many years due to their excellent oxygen barrier properties[7], but their wide applications are limited by some of the hemicelluloses’ intrinsic drawbacks. First, hemicellulose films are mechanically weak because of the branched and amorphous hemicellulose structure[8]. The second challenge is the high hygroscopicity of hemicellulose that makes them sensitive to water and highly permeable to water vapor which is detrimental to many film applications since a packaging material should not dissolve upon contact with water[9]. Also, the hydrophilic nature of hemicellulose could cause its swelling in the presence of moisture, thus leading to impaired mechanical and barrier properties. These problems can be partially remedied with the incorporation of two-dimensional (2D) inorganic additives, such as flattened carbon nanotubes[10], alumina particles[11], and nanoclay[8], creating a three-dimensional (3D) network structure that reduces the mobility and disassociation of the hemicellulose materials, as well as enhancing their mechanical properties. Recently, 2D graphene oxide (GO) has attracted research interests due to its activated surface functional groups that allow for various chemical reactions/interactions with adjacent polymers, compared to graphene particles. Till now, several materials have been reinforced with the 3

incorporation of GO nanosheets, including carboxymethylcellulose[12], sodium alginate[13], chitosan[14], and others, in which the matrices and GO were linked by divalent ions, crosslinking linkages or hydrogen bonding. Inspire by the reinforced intercellular network of the plant cell wall, in this study, borate was used as an enhancing agent to crosslink the GO and galactomannan (GM) matrix which was extracted from the sesbania cannabina seeds. The GM/GO film prepared by self-assembly was subsequently dip coated with poly(dimethylsiloxane) (PDMS), in which all the components are nontoxic and have no threat to human health and environment[15], in an attempt to improve GM’s hydrophilic nature. To our knowledge, although GM has been widely investigated as a packaging film, this is the first research study incorporating borate crosslinked GO in a GM film, yielding a robust and ultra-flexible film with a self-clean property. In all, this research was targeted at offering a facile method to improve the value-added utilization of the hemicellulose waste and exploring its film properties for food packaging application.

2. Experimental section 2.1. Materials The sesbania cannabina seeds were harvested from self-planted sesbania cannabina which is widely planted in coastal beach regions to prevent the sand erosion, and the obtained sesbania cannabina seeds were air-dried at RT. The graphite powder and borate (Na2B4O7·12H2O) was purchased from Sigma-Aldrich (Shanghai, China). Poly(dimethylsiloxane) (PDMS) and curing agent (Sylgard 184, Dow Corning) were commercially available online. All other chemicals are purchased from Sinopharm Chemicals Reagents Co. Ltd and are used as received. 2.2. Preparation of galactomannan (GM) 2.2.1. Water extraction of the raw GM (rGM) 4

The rGM was extracted from the sesbania cannabina seeds using DI water in a bioreactor at 50 °C. The endosperm of sesbania cannabina (ESC) was obtained by initially removing the coats of dry sesbania cannabina seeds after sufficient water swelling and the obtained ESC was then dried at RT. The dried ESC was ground with a lab mill to pass through standard 40 mesh sieve, and then soaked in DI water overnight at a solid to liquid ratio of 1:50 to cause swelling. After that, the slurry was transferred into a 10 L bioreactor which was maintained at 50 °C and agitated at 150 rpm for 48 h. After the water extraction, the mixture was centrifuged at 8000 rpm for 10 min thrice to remove any solid residue, leaving behind the supernatant containing rGM. Afterward, the rGM fraction was precipitated by adding 3 times the volume of the 95 v/v% ethanol to the supernatant, followed by centrifugation at 5000 rpm for 10 min. The rGM precipitation was then washed with ethanol three additional times to remove any impurities. Finally, the dry rGM was obtained by drying the precipitate in an oven at 30 °C. 2.2.2. Preparation of the GMs with a different molecular weight To prepare GMs with different weight-average molecular weight (Mw), the rGM underwent an enzymatic hydrolysis with lab-made mannanase (with an activity of ~4.0 U/mL) to cleave the mannan backbone chains[16]. The enzymatic hydrolysis of rGM was carried in a 10 L bioreactor with a working volume of 5 L. The mannanase was first added at the loading of 20 U/g-rGM, followed by the addition of 1 M citrate buffer to control the pH around 4.8. Finally, DI water was supplemented to set the final solid loading to 10%. The enzymatic hydrolysis system was then heated to 50 °C and agitated at 150 rpm for 48 h. Upon completion of the enzymatic hydrolysis, GMs with different Mw were isolated by gradient ethanol precipitation. First, ethanol (95 v/v% purity) was added to the enzymatic hydrolysis slurry to make the concentration of ethanol in the mixture ~40 v/v% which was then centrifuged, 5

washed and finally dried to obtain the solid GM (labeled as GM40). After that, the supernatant was further diluted with additional ethanol to set the ethanol concentration ~50 v/v%, and the solid phase was obtained following the same procedure as described above and was labeled as GM50. Finally, the last addition of ethanol was introduced into the supernatant (ethanol concentration reached 65 v/v% after the last addition) to generate GM65. The whole process of preparing the GMs with different molecular weight is shown in Fig. 1. 2.3. Preparation of GM/GO films The solutions of rGM, GM40, GM50, and GM65 were obtained by dissolving different GMs in DI water with constant mechanical stirring for 12 h, yielding homogeneous solutions with concentration of 10 g/L. The GO was prepared with modification to Hummers method[17]. The prepared GO was then added into DI water (2.5 g/L), followed by ultrasound irradiation at the power of 400 W (VCX500, Sonics & Materials Inc., USA) for 30 min three times to sufficiently exfoliate the GO nanosheets. The exfoliated GO suspension was then centrifuged at 5000 rpm for 10 min three times to remove any unexfoliated GO aggregates. To prepare the GM/GO films, a GO suspension with a concentration of 2.35 g/L was slowly added to the GM solution at different ratios (solid weight ratios) ranging from 0% to 7 w/w% with continuous stirring for 8 h, followed by the introduction of 0.5% (based on the dry weight) borate (in a solution of 10 g/L) to the GM/GO mixture to induce the crosslinking reaction to occur. The mixture was then vigorously stirred for another 8 h at RT and then poured into a plastic Petri dish (with a diameter of 9 cm) and dried in 35 °C oven for 48 h. Upon drying, the borate crosslinked GM/GO film was further cured at 105 °C overnight (Note: A distinct color change from gray to

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black took place at this stage). It should be noted that the dry weight of all the prepared films was set at 0.20 g (with thickness ranging from 26.3 to 36.7 μm). 2.4. Fabrication of hydrophobic GM/GO film To overcome the hygroscopicity of the GM/GO film, which is detrimental for food packaging application, the dry film was coated with polydimethylsiloxane (PDMS) according to a previous report[18]. Briefly, the PDMS and the curing agent (Sylgard 184, Dow Corning) were dissolved in ethyl acetate at the weight ratio of 10:1:100, in which the curing agent acts as the PDMS solidification agent. The GM/GO film was then immersed in the diluted PDMS solution for 1 h, followed by curing at 105 °C for 1 h. 2.5. Analytical methods Chemical compositions of the rGM were determined with a two-stage sulfuric acid hydrolysis process according to the NREL standard procedure[19]. Monomeric sugar from the acid hydrolysis was measured with high performance anion-exchange chromatography (Dionex-3000, Thermo Fisher Scientific, Waltham, MA, USA) with a Carbopac PA10 column at 30 °C. Weight-average molecular (Mw) weight and number-average molecular weight (Mn) of the GMs were analyzed with gel permeation chromatography (GPC, Agilent 1200 series, Agilent Technologies, Santa Clara, CA, USA) equipped with a refractive index detector using Ultrahydrogen 120 and Ultrahydrogel 250 columns connected in series at 55 °C. The samples for GPC analysis were accomplished in water solutions of ~4 g/L. Cross-section morphology of the films containing different amount of GO was imaged with a SEM (scanning electron microscope, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) equipped with the energy-dispersive X-ray spectroscopy. The morphology and size of the exfoliated GO particles were studied with the atomic force microscopy (AFM, Dimension Edge, 7

Bruker, Germany) working in contact and tapping mode with a Si cantilever at RT. X-ray photoelectron spectroscopy (XPS) was tested on a Shimadzu AXIS UltraDLD instrument (Shimadzu, Japan) with Al Kα X-ray radiation as the X-ray source. FTIR spectra of the films were recorded with a Bruker VERTEX 80v FTIR spectrometer (Bruker, Germany) over the range of 4000-500 cm-1 at a resolution of 4 cm-1. Raman spectra was recorded with a DXR532 Raman Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a 514.5 nm laser in the scanning range of 600-2000 cm-1. The hydrophobicity of the films was tested using an optical water contact angle (WCA) system (SL200KS, KINO Industry, USA) at three different places of one sample with a water droplet of 10 μL. The tensile fragile test was performed with a universal mechanical tester (Shimadzu AGS-X, Japan) equipped with a 500 N load cell working at RT. The sample for the mechanical test was stored in a desiccator before the testing, and the results were averaged on at least five replicates. The self-cleaning properties of the film was performed by removing the contaminants with water flow, according to a previous literature[18]. In detail, a certain amount of garden soil was placed on the PDMS coated film on which water was then injected through a spritz bottle at the distance of 1-2 cm over the film. The testing process was recorded with a Canon camera (Canon EOS 600D, Japan). Oxygen permeability (OP) of the films was analyzed with a VAC-V1 permeability analyzer working at RT in a relative humidity of 45%. The water vapor permeability (WVP) was determined according to the E96-05 procedure. Briefly, films with known thickness were sealed as patches onto acrylic permeation cells (with a diameter of 2.4 cm) which contained 2.00 mL of DI water. The cells were then placed in a desiccator and the test samples were periodically weighted 10 times over a testing time of 7 d. The WVP was calculated by the following equation: 8

Where W (g) is the weight of water permeating the films; T (mm) is the thickness of the samples; A (m2) is the area of the testing cells; t (d) is the testing time; and ∆P (kPa) is the difference of vapor pressure between the two sides of the samples. Thus, the unit for the WVP is g·mm·m-2·d-1·kPa-1. 3. Results and discussion 3.1. Characterization of the prepared GMs While hemicelluloses in hardwoods and grasses are mainly composed of xylan, glucan and arabinan which are not an ideal candidate for film preparation due to the intricate extraction process, low molecular weight and branched structure[20], herein, we prepared a novel hemicellulose of GM from ESC with a simple water extraction method at 50 °C (labeled as rGM). Moreover, the rGM was treated with an enzymatic hydrolysis step to produce a series of GMs with different molecular weight. The GM content and the corresponding molecular weight of the prepared GMs are shown in Table 1. As shown, the rGM has the highest GM content of 93.40% in which the mannan to galactan molar ratio was determined to be 2.16. In addition, it can be found that the Mw of the rGM reached 1.42×106 g/mol, which is higher than that of most hemicelluloses in hardwood, softwood, and grass[4]. After the enzymatic hydrolysis, three GMs of GM40, GM50, and GM65 were obtained by the precipitation with different ethanol concentrations, ranging from 40% to 65%. The GPC results revealed that the GM chains were cleaved by the mannanase during the enzymatic hydrolysis as the weight-average molecular weight of the GMs significantly decreased from 1.42×106 to 1.47×104, 7.90×103 and 4.89×103 g/mol, respectively. The residual carbohydrates in the slurry after the ethanol precipitation was mainly composed of galactomannan-oligosaccharides, which is a family of prebiotics[21]. On the contrary, the high molecular GMs cannot be digested by animals, but they are

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promising candidates in materials fabrication due to their excellent film forming ability and advantages of abundance, renewability, and biocompatibility[22]. 3.2. Fabrication of the borate crosslinked rGM/GO film The rGM with an Mw of 1.42×106 g/mol showed excellent film formation ability which could easily generate an intact film by casting the aqueous rGM solution in a conventional oven. However, the pure rGM film is fragile that makes it challenging to satisfy a wide range of industrial applications. Following the basic principles of natural nacre (mother of pearl) which possesses extraordinary mechanical property due to the multilayered and brick-and-mortar structure[23], in this study, we fabricated a nacre-like rGM/GO composite by self-assembling the rGM with GO, followed by a borate strengthening process. The preparation steps of our borate crosslinked rGM/GO film is displayed in Fig. 2a, along with the hypothetical interaction mechanism in which the rGM inserted into the GO layers and linked the GO with the borate crosslinking and hydrogen bonds. This dual binding interaction system significantly strengthened the composite properties. The exfoliated GO nanosheets were analyzed with atomic force microscopy (AFM), and a typical image is shown in Fig. S1. It was found that the GO was successfully exfoliated with ultrasound irradiation as shown in the AMF image and the diameter of the GO nanosheets ranged from dozens of nanometers to several microns, and the thickness is a few-layers of the GO monolayers, which is consistent with the literature[24]. A digital photo in Fig. 1b displayed a large-area rGM/GO film, which is swarthy with a little bit of metallic luster, and the composite can be cast into desired shapes and sizes by using different molds. Also, the rGM/GO film can be folded into various shapes, just like the regular tissue paper and a large rGM/GO plane is exhibited in Fig. 1c. Moreover, after rolling out the rGM/GO plane, the film could recover its original flatten shape with no cracks on (as shown in Fig. S2), this result demonstrated the ultra-flexibility of our rGM/GO composite which is 10

superior to the other carbohydrate based films like carboxymethyl cellulose (CMC), nanocellulose, and arabinoxylan[25]. Cross-section morphology of the films with different amount of GO addition is shown in Fig. S3, and Fig. 1d. It can be seen that the pure rGM film (Fig. S3a) possessed a smooth and compact structure. While with the addition of 0.5% GO, a layered structure started to appear (Fig. S3b), and finally a nacre-like multilayered structure was clearly observed with the introduction of 3% GO (Fig. 1d). This result can be ascribed to the fact that the GO layers could stack together with hydrogen bonding and borate crosslinking, thus contributing to the successful self-assembly reaction between the rGM and GO nanosheets[26]. However, further increasing the GO content to 5%, some GO aggregates were seen inserted into the layers (Fig. S3e), suggesting that the GO could not be dispersed very well in the composite at a high GO addition[12]. Furthermore, we performed EDS mapping on the film cross-section to confirm the presence of the boron element (Fig. 1e), and results showed a uniform distribution of boron element in the composite, suggesting that the GO and rGM are crosslinked evenly with the borate. 3.3. Chemical structure analysis of the borate crosslinked rGM/GO composites. Chemical structure of the pure rGM, GO, and the composite film were analyzed with FTIR, XPS, and Raman spectra, and results are shown in Fig. 3. In Fig. 3a, the FTIR spectra of pure rGM depicted the bands at 2920 cm-1, 1635 cm-1, and 1161 cm-1, corresponding to the C-O-C stretching, C-H stretching of adsorbed water and C-O bending vibration[27]. Combined stretching in the region of 1000-1200 cm-1 showed the presence of the C-O-C and C-O groups of typical carbohydrates. The peaks at 812 cm-1 and 869 cm-1 were associated with the anomeric configuration (α and β conformers) of the glycosidic linkages[22],[28]. As to the GO, the broad and intense peak of O-H groups was centered at 3000-3500 cm-1, and the C-O (epoxy and alkoxy groups) stretching peaks 11

could be centered at 1047 cm-1 and 1229 cm-1, respectively. While characteristic peaks appearing at 1618 cm-1 and 1735 cm-1 were attributed to the C=C skeletal stretching and the C=O stretching of the carboxyl groups situated at the edges of the GO sheets[29], respectively. Interesting, when the rGM was assembled with the GO nanosheets, the signals of GO at 1047 cm-1, 1229 cm-1, and 1618 cm-1 were not present anymore, indicating the crosslinking reaction between the rGM and the oxygen-containing groups of GO, even without borate. Moreover, when the rGM/GO composite was further crosslinked with borate (regardless of curing at 105 °C or not), the GO peak at 1735 cm-1 attributed to the C=O was almost disappeared which is consistent with a previous report, indicating there was double binding-opening reaction of the O-C=O during the reaction[14]. In addition, some new peaks appeared between 1200-1450 cm-1 that can be attributed to the B-O stretching (including asymmetric stretching and terminal asymmetric stretching[12],[30]. Meanwhile, stretching frequencies of the rGM shifted slightly in the composite after crosslinking (e.g., 1642 cm-1 shifted to 1635 cm-1, and 869 cm-1 shifted to 873 cm-1), which was because of the strong hydrogen bonds between GO and rGM. A distinct color change can also be observed when curing the rGM/GO/borate at 105 °C, further indicating that the GO was reduced at high temperature (Fig. S4)[26]. The borate crosslinking reaction was also investigated with XPS spectroscopy, and results are shown in Fig. 3b and 3c, corresponding to the C1s XPS survey spectra for the uncrosslinked rGM/GO composite, and the borate crosslinked rGM/GO composite. The peaks at 284.7, 286.2, 287.6 and 288.5 eV in the uncrosslinked sample are attributed to C-C, C-O-C, C=O and O-C=O, respectively[31]. It can be found that signals of O-C=O were absent in the crosslinked composite, and this agreed the FTIR result that the carboxylic acid groups in the GO played an important role in the borate crosslinking. Also, the peaks of C-C, C-O-C, and C=O shifted to the higher binding 12

energy of 284.8, 286.4 and 287.8 eV after the borate crosslinking, this result indicated that there were strong hydrogen bonding linkages between borate, rGM, and GO[32]. To further investigate the behavior of GO in the borate crosslinking, Raman spectroscopy was employed (Fig. 3d). The Raman spectra of the GO in rGM/GO film, RT crosslinked film and high temperature treated rGM/GO film exhibited two peaks of D band at 1350 cm-1 and G band at 1583 cm-1 which were attributed to the first-order scattering of the E2g phonon of the sp2 carbon atoms and the size of the in-plane sp2 domains, respectively[33]. The intensity ratio of the D and G bands (ID/IG) can be seen as the degree of structural disorder in the GO sp2-based carbons[34]. The ID/IG value of the pristine GO was calculated to be 0.76, close to that (ID/IG=0.74) of the mixture of rGM and GO crosslinked with borate at RT. However, the ID/IG significantly decreased to 0.65 when the sample was crosslinked by treating at 105 °C which is due to the partial reduction of the GO to graphene during the heat treatment, indicating the desorption of the oxygen-containing groups and the reordering of the graphene basal plane[35]. This phenomenon is similar to the previously reported GO reduction by combustion or microwave treatment and is believed to beneficial to the film mechanical properties[36],[37]. 3.4. Mechanical property of the rGM/GO films. In natural plants, borate ions are ubiquitous and are essential to the healthy growth of many plant species which could significantly enhance the mechanical properties of plant tissue at very low concentrations by forming covalent bonds with oxygen-containing functional groups[38]. In this study, the borate ions were introduced to crosslink the rGM and GO. The mechanical strength of the prepared composites is shown in Fig. 4. As shown in Fig. 4a, the tensile strain and strength of the pure rGM film was only 6.30% and 56.59 MPa, respectively. It can be found that the mechanical strength dramatically increased with the addition of GO nanosheets, which showed a tensile strength of 86.29 MPa with the addition of 3% GO (rGM/GO film without borate crosslinking). This is 13

because that the brick-and-motor structure of the rGM/GO film with deflected microcrack could form a large amount of cooperative force such as hydrogen bondings and other intermolecular interactions on their interfaces[39]. Moreover, when the rGM/GO composite was crosslinked with borate, the tensile strength further increased to 96.48 MPa and 135.54 MPa, respectively, with treatment at different temperature of RT and 105 °C. Interestingly, borate crosslinking also increased the elastic deformation, which was 8.82% and 9.62%, respectively, based on the treatment temperature. It should be noted that the tensile strength of the hybrid film was about 2.40 times that of the pure rGM composite, revealing a synergy strengthening of both the GO reduction and the borate crosslinking. It is believed that the interlocks between rGM and GO were reinforced by forming covalently linkages with the borate crosslinking[40]. As to the reason of the strengthening at 105 °C treatment, partial GO reduction at high temperature could decrease the interlayer distance between graphene nanosheet, which is caused by forming van der Waals forces between graphene and rGM (different from the repulsive interaction between the raw GO and rGM), contributing to the increased mechanical strength of the composite[14, 41]. Different from other composites, which have a compromise between the hardness and the ductility with the addition of inorganic additives [42], our GM/GO film is mechanically strong with improved flexibility (i.e. higher tensile strain), and is an ideal candidate for food packaging. To better understand the synergistic strengthening effects, a possible fracture model is proposed, as shown in Fig. S5. First, when the composite was subjected to the tension stress, hydrogen bondings between the GM molecules were first destroyed, accompanied by the slippage between adjacent GM molecules[43]. However, the physical and chemical linkages between borate, GO nanosheets and GM resisted the sliding, making the force uniformly dispersed in the composite. With increase of the stress, the GM molecules are separated from the surface of GO with destroying 14

the covalent bonds after a further stretching[44]. This cycling of crack initiation-propagation-deflection continued until the final fracture of the film, which resulted in adsorbing more energy and higher strength, compared to the pure GM film[45],[46]. As a result, this dynamic strong property benefits the mechanical and ductility of our GM/GO composite, making it an ideal substitute to plastics for food packaging. The mechanical behavior of the composites also shows a strong dependence on the adding amount of the GO, as shown in Fig. 4b. In detail, the tensile strength increased from 56.59 to 78.95, 90.00, 106.48 and 135.54 MPa, respectively, when the GO content was increased from 0% to 0.5%, 1%, 2% and 3%. Furthermore, in our experiments, when the GO contents in the composite was introduced beyond 3% to 5% and 7%, the tensile strength of the obtained films was reduced to 112.44 MPa and 106.65 MPa, respectively, which could be ascribed to the aggregation of GO particles at a high content, as shown in Fig. S3e. The same phenomenon has also been reported by other researchers who reported that the 0.7% GO addition is the optimized amount in the CMC/GO composite[12]. It is well known that pure GO film is mechanically weak, indicating that the rGM also played a significant role in strengthening the GM/GO composite. As widely reported, the soft biopolymer in nacre plays a significant role in the strengthening and toughening of nacre [47]. Xu and Li [48] proposed a coiled-spring model in which the biopolymer has the capability to strengthen itself during deformation by a deformation-strengthening mechanism that may help to better understand the role of the rGM in strengthening our nacre-like composite. Furthermore, different GMs (i.e., rGM, GM40, GM50, and GM65) with varied Mw were used as matrices to prepare the GM/GO composites in order to investigate the effect of GM Mw on the generated nacre-like composites. As shown in Fig. S6, the tensile strength of the GM/GO films significantly decreased from 135.54 to 64.25 and 50.34 MPa when the GM Mw was decreased from 15

1.42×106 g/mol (rGM) to 1.47×104 g/mol (GM40) and 7.90×103 g/mol (GM50), respectively. This result indicated that the high Mw polymer matrix could contribute to the mechanical strength of the resulting composite which can be explained by the fact that higher Mw GM could cause more interlinks with the GO nanosheets compared to the low Mw samples. In the case of the GM65/GO sample, it cracked during casting (Fig. S6b) compared to the intact and flexible rGM/GO film (Fig. S4b), which is very brittle and cannot be handled for the mechanical testing. In sum, the rGM based film showed better mechanical and flexible properties than that using other GM matrices, due to its high molecular weight, which may help to alleviate the compromised hardness and ductility with the addition of other 2D additives [49]. In addition to the mechanical property, the borate crosslinking also improved the water stability of the rGM/GO film. A remarkable difference of the rGM/GO films with and without borate ions crosslinking are shown in Fig. 5. As shown, the rGM/GO film without borate ions started to disintegrate after 10 min of immersion in the water, and finally completely disintegrated in 1 h. In contrast, the hybrid with borate introduction was stable in the water, and it can maintain its shape for more than even 1 week. In summary, a water stable nacre-like film was prepared based on the utilization of water-soluble starting materials. 3.5. Fabrication of the hydrophobic film with self-clean property As a prerequisite for the potential application in food packaging, the material should be hydrophobic and water-resistant in order to sustain the extremely humid conditions. However, both the rGM and GO are hydrophilic and tend to lose their original robustness with the presence of little water vapor[8]. The hydrophobic surface coating provides an effective point to protect the synthesized rGM/GO films due to its water-repellent and self-cleaning properties. In this study, the borate crosslinked rGM/GO film was coated with PDMS layer by a simple dipping and curing 16

procedure, and the results are shown in Fig. 6. It can be found that the water droplets attached to the rGM/GO film (without PDMS coating) and soon caused its shrinkage (Fig. 6a). In contrast, the PDMS coating could resist the water, and the water flow automatically rolled away at a small slanting angle, enabling the film to remain dry (Fig. 6b and Movie S1). Such a water-proof feature could also help to deice and clean the contaminants on the film. The self-clean property of the PDMS coated film was tested by placing the garden soil on its surface, which was then water injected with a syringe, and the results are shown in Fig. 6c and Movie S2. It can be seen that the water flow picked up the contaminants and removed them from the film surface. After water washing, the film became very clean without any containments, indicating that the hygroscopicity nature of the material was successfully overcome with the PDMS coating. Besides, the water contact angle (WCA) of the PDMS coated film at different pH values were shown in Fig. 6d. The WCA was all around 120° with independence of the pH value, which revealed the stable water repellency under highly corrosive conditions. These results suggested that our nacre-like film coated with PDMS has potential applications in food packaging due to overcoming the hygroscopic issue of the pristine hemicellulose film. 3.6. Gas-barrier property of the films The permeability of gases through the films, including water vapor and oxygen, should be as low as possible to broaden their application in food packaging. GO is believed to be an impermeable material for all gases, which makes it attractive for exploitation as a barrier film. The barrier property of our nacre-like composite with different GO addition was tested, and the results are shown in Table 2. As shown, the GO introduction could significantly decrease the gas permeability of both water vapor and oxygen. Specifically, the water vapor permeability (WVP) decreased from 5.82 (without GO) to 5.08, 4.36, 3.74, 3.49 and 3.33 g·mm·m-2·d-1·kPa-1 with the addition 0.5%, 1%, 17

2%, 3%, 5% GO nanosheets, respectively. The same trend was also observed for oxygen permeability (OP), which decreased from 0.39 to 0.25, 0.23, 0.16, 0.15, and 0.11 mL·μm·m-2·d-1·kPa-1 with increasing GO concentrations. Besides, the WVP significantly decreased to 0.13 g·mm·m-2·d-1·kPa-1 when the film was coated with PDMS. Furthermore, oxygen cannot penetrate the film when it was coated with PDMS. It should be noted that the barrier property of our PDMS/rGM/GO composite was close to some commercial materials such as polyvinyl alcohol, polyethylene, and polystyrene[50], showing promising packaging applications. This improved barrier property is likely because of the presence of GO and the hydrophobic PDMS coating that creates a tortuous pathway and physical obstacle, hindering the absorption and diffusion of water vapor and oxygen. 4. Conclusion A novel galactomannan (GM) was water extracted and used as a matrix for the fabrication of the hemicellulose composite. We successfully prepared the nacre-like GM-based hybrid by the in situ reduction and crosslinking of GM, graphene oxide (GO) and borate. These nacre-like films showed improved tensile strength of 135.54 MPa (3% GO addition), 2.40 times that of the original GM film, which is due to the brick-and-mortar structure and borate crosslinking. To further overcome its hygroscopicity, GM/GO composite was then dip coated with a PDMS layer, after which the composite became hydrophobic with self-clean property. Additionally, the water vapor permeability (WVP) and oxygen permeability (OP) were dramatically decreased to 3.33 g·mm·m-2·d-1·kPa-1 and 0.11 mL·μm·m-2·d-1·kPa-1 with the addition of 5% GO. Moreover, the WVP and OP were only 0.13 g·mm·m-2·d-1·kPa-1 and 0 mL·μm·m-2·d-1·kPa-1 with the coating of PDMS. Our strategies provided a facile method to overcome the inherent drawbacks associated with natural

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hemicellulose and may broaden the use of the hemicellulose-based materials in the food packaging applications. Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the Jiangsu Province Key Laboratory of Biomass Energy and Materials (JSBEM-S-202004), the Innovative Ability Enhancement Project for R&D Team of ICIFP, CAF (LHSXKQ8) and the Fundamental Research Funds of CAF “Enhancing Project in the Field of Wood Composite Materials and Chemical Resources Utilization” (CAFYBB2017ZX003-08). Reference: [1] Pereira PH, Waldron KW, Wilson DR, Cunha AP, Brito ES, Rodrigues TH, et al. Wheat straw hemicelluloses added with cellulose nanocrystals and citric acid. Effect on film physical properties. Carbohydr Polym 2017;164:317-24. [2] Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Charles A. Eckert, et al. The path forward for biofuels and biomaterials. Science. 2006;311:484-489. [3] Nypelo T, Laine C, Colson J, Henniges U, Tammelin T. Submicron hierarchy of cellulose nanofibril films with etherified hemicelluloses. Carbohydr Polym 2017;177:126-34. [4] Mikkonen KS, Tenkanen M. Sustainable food-packaging materials based on future biorefinery products: Xylans and mannans. Trends Food Sci Technol 2012;28(2):90-102. [5] Svard A, Brannvall E, Edlund U. Rapeseed straw as a renewable source of hemicelluloses: Extraction, characterization and film formation. Carbohydr Polym. 2015;133:179-86. [6] Farhata W, Vendittia, R,

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Table 1 Galactomannan content and the molecular weight of different GMs. Galactomannan Mw Mn Polydispersity Index (%) (g/mol) (g/mol) 6 rGM 93.40 1.42×10 0.91×106 1.56 4 4 GM40 87.42 1.47×10 1.20×10 1.23 3 3 GM50 84.31 7.90×10 7.24×10 1.09 3 3 GM65 82.59 4.89×10 4.38×10 1.12

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Table 2 Oxygen permeability and water vapor permeability of the films. WVP OP Samples (g·mm·m-2·d-1·kPa-1) (mL·μm·m-2·d-1·kPa-1) GM 5.82±0.05 0.39±0.02 GM/0.5%GO 5.08±0.12 0.25±0.08 GM/1%GO 4.36±0.09 0.23±0.03 GM/2%GO 3.74±0.22 0.16±0.01 GM/3%GO 3.49±0.11 0.15±0.00 GM/5%GO 3.33±0.18 0.11±0.01 GM/3%GO with PDMS coating 0.13±0.01 0±0

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Fig. 1. Schematic diagram of preparing GMs with different molecular weight.

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Fig. 2. Preparation process of the borate crosslinked rGM/GO film(a); Photography of a large-area composite film (b); An airplane made from the composite film (c); Cross-section morphology of the film (GO 3%) (d) and the corresponding SEM-EDS mapping for boron element (e).

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a

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1642 812869

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1047 1229 1618 1735

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Fig. 3. FTIR spectra of the rGM, GO and borate crosslinked rGM/GO composite (a); C1s XPS spectra of the rGM/GO film before (b) and after (c) borate crosslinking; Raman spectra for rGM/GO film without crosslinking and borate crosslinked rGM/GO at RT and 105 °C.

a

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Fig. 4. Typical tensile stress-strain curves for rGM, rGM/borate, rGM/GO, rGM/GO/borate at RT and rGM/GO/borate at 105 °C (a); The tensile stress of the borate crosslinked rGM/GO film (all treated at 105 °C) with different GO contents.

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Fig. 5. Water stability of the rGM/GO films without borate (a) and with borate crosslinking (b).

Fig. 6. Hydrophobic and self-clean properties of the PDMS coated composite films.

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