Xyloglucan Bionanocomposites: A Chemical

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Nacre-mimetic clay/xyloglucan bionanocomposites – a chemical modification route for hygromechanical performance at high humidity Joby Jose Kochumalayil, Seira Morimune, Takashi Nishino, Olli Ikkala, Andreas Walther, and Lars A. Berglund Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/bm400883e • Publication Date (Web): 01 Oct 2013 Downloaded from http://pubs.acs.org on October 22, 2013

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Nacre-mimetic clay/xyloglucan bionanocomposites – a chemical modification route for hygromechanical performance at high humidity Joby J Kochumalayil†, Seira Morimune‡, Takashi Nishino‡, Olli Ikkala₸, Andreas Walther§, and Lars A Berglund†,#* †

Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44

Stockholm, Sweden, ‡Department of Chemical Science and Engineering, Kobe University, Kobe 657-8501, Japan, ₸Molecular Materials, Department of Applied Physics, Helsinki University of Technology/Aalto University, FIN-00076 Finland, §DWI at the RWTH Aachen University – Institute for Interactive Materials Research, D-52056 Aachen, Germany, #Wallenberg Wood Science Centre, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

ABSTRACT Nacre-mimetic bionanocomposites of high montmorillonite (MTM) clay content, prepared from hydrocolloidal suspensions, suffer from reduced strength and stiffness at high relative humidity. We address this problem by chemical modification of xyloglucan in (XG)/MTM nacre-mimetic nanocomposites, by subjecting the XG to regioselective periodate oxidation of side chains to enable it to form covalent crosslinks to hydroxyl groups in neighbouring XG molecules or to the

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MTM surface. The resulting materials are analysed by FTIR spectroscopy, thermogravimetric analysis, carbohydrate analysis, calorimetry, X-ray diffraction, scanning electron microscopy, tensile tests and oxygen barrier properties. We compare the resulting mechanical properties at low and high relative humidity. The periodate oxidation leads to a strong increase in modulus and strength of the materials. A modulus of 30 GPa for cross-linked composite at 50% relative humidity compared with 13.7 GPa for neat XG/MTM demonstrates that periodate oxidation of the XG side chains leads to crucially improved stress transfer at the XG/MTM interface, possibly through covalent bond formation. This enhanced interfacial adhesion and internal crosslinking of the matrix moreover preserves the mechanical properties at high humidity condition and leads to a Young’s modulus of 21 GPa at 90 %RH.

Key words: Xyloglucan, oxygen barrier, biocomposite, montmorillonite, nanocomposite

INTRODUCTION Nanostructured clay composite materials have developed strongly since its inception in 1990s by Toyota researchers.1-3 Though earlier nanocomposites were based on petroleum derived polymers, nanocomposites based on bio-based polymers have been developed during the last decade.4-6 Similarly, the materials design has also changed significantly. The original polymer nanocomposites were based on smaller volume fraction of layered montmorillonite (MTM) added to a matrix polymer, improving the mechanical properties. Difficulties in dispersion and unfavorable interaction between polymer and inorganic phases are the main challenges in these composites.7 An interesting class of bio-inspired nanocomposites was introduced by Tang et.al. in the form of ‘artificial nacre’ with high inorganic content (> 50 vol%).8 Clay platelets were combined with


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a charged, water-soluble polymer in an elaborate, yet sequential layer-by-layer (LbL) deposition process. A high degree of nanostructural control was achieved with in-plane platelet orientation. The concept was used in subsequent material preparations using a variety of polymers.9, 10 Apart from improvement in stiffness and strength, the highly oriented platelet assembly also increases the gas barrier properties. 11, 12 A related platelet biocomposite concept has been presented where stiffness and strength was combined with substantial ductility.13 However, the processing routes in these studies are time-consuming and most likely difficult to extend from laboratory practice to large scale industrial processing. Recently Walther et al. presented a major progress to prepare nacre-mimetic layered nanocomposites with excellent mechanical and barrier properties by using a simple and industrially scalable water-based processing approach akin to paper-making.14, 15 The approach is based on fabricating core/shell particles, which upon water removal self-order into the desired layered mesostructure. For the preparation of the core/shell colloids, the MTM in hydrocolloid form was added to an aqueous solution of polyvinyl alcohol (PVOH)14 or polyelectrolyte15, and the polymer adsorbs as molecular layer onto the exfoliated clay platelets. Excess polymer was removed and the polymer-coated platelets subjected to filtration, resulting in an oriented nanocomposite structure of nacre-like “brick-and-mortar” organization at high inorganic content (70 wt%). In contrast to nanocomposites fabricated by LbL approach, here the inorganic platelets were individually separated. In a subsequent study, Yao et al. applied the same procedure, but used a chitosan biopolymer as polymer matrix.16 Other works include the combination of clay with nanofibrillated cellulose (NFC) to prepare clay nanopaper.17-19 All these material designs are classified as ‘nacre-mimic’ owing to the high inorganic content.


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The polymer adsorption on the clay surface is an irreversible process and the driving mechanism for adsorption depends on the component characteristics.20,

21

Naturally occurring

MTM is negatively charged on the surface with a number of hydroxyl groups present at the rims and also on the basal planes, stemming from element substitution of the perfect MTM crystal structure in the natural formation process. For a cationic polymer, the driving mechanism can be expected to be of electrostatic nature, whereas for neutral polymers such as PVOH or polysaccharides, the entropic contribution is relevant in water environment.20 When the polymer is adsorbed, the mechanical or gas diffusion properties of the final material depends significantly on the orientation and degree of exfoliation of clay platelets, as well as clay-polymer interfacial adhesion. Since most polymers used for fabrication are water-soluble, the mechanical and barrier properties will depend on the relative humidity of the environment. Multilayer coatings are among the suggested approaches to address the problem,22 although a single nanocomposite coating would be preferable. Very few scientific investigations have addressed this problem for nacre-mimetic materials. In hydrophilic polymer-coated inorganic platelets, the polymer-polymer interaction is also significant for the properties under humid conditions. The mechanical and barrier properties of such composites will be reduced at high humidity due to moisture adsorption.17 Under hydrated conditions, the mechanical properties of biocomposites can well be preserved by crosslinking as exemplified for squid beaks23 and insect cuticle24. Podsiadlo et. al. successfully used gluteraldehyde molecules to cross-link PVOH/MTM system to improve the mechanical strength and stiffness.9 The cross-linking also preserves the mechanical properties in humid conditions.9 It is likely that covalent cross-links are formed between clay and polymer.


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The gluteraldehyde needs to diffuse into the structure, and as a consequence, the method was inefficient for post crosslinking of thicker structures.14 Cross-linking is therefore one method to preserve properties of composite materials based on hydrophilic polymers in humid conditions. In the present work, we use an approach to create cross-linking sites on the polymer itself followed by adsorption to MTM platelets. A biopolymer – xyloglucan (XG) – derived from the tamarind seed waste product is used as the polymer matrix.25, 26 Xyloglucans, or galactoxyloglucans, are β (1→4) linked glucan polysaccharides with the main chain identical to cellulose.25, 27 Up to 75 % of the glucose residues are substituted with α (1→6) linked xylose residues, where some of these xylose chains have a β (1→2) linked galactose residue. It was observed that, similar to PVOH, xyloglucan is strongly adsorbed to MTM surface in an aqueous environment.28 In the present study, the side groups of XG are selectively oxidized to aldehyde groups and subsequently adsorbed on the clay platelet surfaces. The polymer/clay hybrid material is then self-assembled in a vacuum filtration pathway. Periodate oxidized polysaccharides can form covalent cross-links between the aldehydes and hydroxyl groups.29, 30 Since MTM has many hydroxyl groups present, it is believed that part of these hydroxyl groups can also form covalent bonds with aldehyde groups from oxidized xyloglucan.

EXPERIMENTAL SECTION

Nanocomposite preparation: A 0.2 % MTM suspension (Cloisite Na+, density of 2.86 g / cc, Southern Clay Products, Inc.) was prepared by using Ultra Turrax® blender (IKA, DI25 Basic) at 25000 rpm for 15 min followed by sonication using Vibra-Cell (Sonics & Materials, Inc.) ultrasonic processor at ambient temperature. It was repeated three times in succession and the resultant suspension was kept undisturbed for one week and any clay aggregates were removed.


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The industrially available xyloglucan (weight average molecular mass, 1.5 MDa, Innovasynth Technologies Ltd., India) was purified by centrifugation (4000 rpm for 45 min) of 0.5 wt% dilute solution and freeze dried to obtain pure xyloglucan. In order to partially oxidize the side groups present on XG, sodium meta-periodate (Sigma Aldrich) was added to 200 ml of 0.5 wt% XG solution, corresponding to nominal molar amounts of 10%, and 20% for complete oxidation of the carbohydrate rings. These dialdehyde XGs are designated as oxidXG10, and oxidXG20 corresponding to the amount of periodate added. The reaction was allowed to continue for 13 hours in dark conditions under magnetic stirring at room temperature. In order to remove the salts, the solutions were dialyzed under running deionized water for three days. An aqueous suspension containing 200 g each of 0.5 wt% XG or oxidized XG (oxidXG) solution and 0.2 wt% MTM suspension were mixed by slow addition of the MTM suspension into polymer solution. The excess polymer was removed by centrifugation and subsequent washing steps as described by Walther et.al.14 The XG-coated MTM platelets were re-dispersed in 100 ml water and magnetically stirred overnight. The resulting suspensions were filtered through a vacuum filter set up using a 0.65 µm PTFE membrane filter to form self-assembled nanocomposite films. The films were dried at 92 °C for half-an-hour under vacuum condition. The resulting composite films were kept under vacuum condition at 50 °C overnight. Carbonyl content: The amount of aldehydes formed was determined by addition of hydroxylamine hydrochloride, using a method similar to the one described for polyaldehyde dextran.31 Oxidised XG solution containing 0.1g solid material was added to 25 ml of hydroxylamine hydrochloride solution at pH 4.0 and stirred for 2 h. The carbonyl content was determined by titration back to pH 4.0 using 0.1 M sodium hydroxide solution.


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Carbohydrate analysis: This was to determine the sugar composition in XG and oxidized XG samples. After acid hydrolysis to individual sugar molecules using 70% sulphuric acid32, the hydrolyzates were analyzed using high performance anion exchange chromatography equipped with Pulsed Amperometric Detector (HPAEC-PAD, Dionex ICS-3000). Standard solutions of glucose, xylose and galactose were used for calibration. Fourier Transform Infrared Spectroscopy (FTIR): FTIR was performed on a Perkin-Elmer Spectrum 2000 FTIR equipped with a MKII Golden Gate, Single Reflection ATR system from Specac Ltd, London, U.K. The spectral range was 600 to 4000 cm-1, and resolution 4 cm-1. The spectra were normalized, allowing a comparison between the spectra. A Realtime IR was performed at 92 °C for 30 min. in order to see the progression of cross-linking by monitoring the intensity difference of Si-O-C bond formation in oxidized XG/MTM composite suspension compared to Si-O-Si in MTM suspension during drying. Thermo gravimetric Analysis (TGA): Thermogravimetric analysis (TGA) was conducted on a Mettler Toledo TGA/SDTA851 instrument. The samples were heated from 25 to 800 °C, using a heating rate of 10 °C/min, in an O2 / N2 flow of 50 mL / min. The thermogram recorded in O2 atmosphere was used to find the inorganic content in the nanocomposites whereas thermogram recorded in N2 atmosphere was used to study the degradation behavior of nanocomposites. Differential scanning calorimentry analysis (DSC): DSC analysis was conducted on a Mettler Toledo DSC instrument. The samples were heated from room temperature to 120 °C, followed by quenching to -40 °C at a heating/cooling rate of 5 °C / min, in a N2 flow of 50 mL / min. This is followed by another temperature scan from -40 °C to 360 °C at the same condition and this scan data is used for comparative analysis.


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Tensile properties: Tensile testing was performed on a Deben microtester with a load cell of 200 N. The films were cut in rectangular strips of dimensions 5 mm wide and 20 mm length and conditioned at 23 ºC and 50 %RH / 90 %RH for one week. The gauge length was 10 mm and the extension rate was 0.1 mm/min. Thickness of each sample was measured precisely using a thickness meter and was approx. 30 µm. At least six specimens were tested from each sample and results are reported as average for at least three specimens in vicinity. Digital speckle photography (DSP) was used to determine strain (details in Supporting Information, SI).Young’s modulus (E) was determined from the slope of the low strain part in the initial elastic region. The volume fractions of the inorganic content in composite films were determined from weight fraction obtained from thermogravimetric analysis in O2 atmosphere. The MTM volume fraction is calculated as: where, (2.86 g/cm3),

= Volume fraction of clay, and

1

= density of XG (1.5 g/cm3),

= density of clay

are weight fraction of MTM and XG respectively. It is assumed that

modified XGs have similar density in the composite film. It is also assumed that the composites have negligible porosity and swelling in humid conditions. Oxygen Permeability: The oxygen transmission rate (OTR) measurements were performed with Oxygen Permeation Analyser (Systech 8001, Systech Instruments Ltd., UK) at 23 °C using 100 % oxygen as test gas. Tests were done in dry condition and at 80 %RH in both N2 and O2 flow path. The active area of measurement was 5 cm2 by using a steel mask. X-ray Diffraction: X-ray diffraction images were obtained by using an X-ray diffractometer (Rigaku, RINT 2100) equipped with Ni filtered Cu Kα radiation. The X-ray beam was operated at 40 kV and 20 mA. The samples were irradiated in parallel (cross-sectional) direction of the


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composite films of XG/MTM and oxidXG20/MTM. From azimuthal intensity distribution profiles for the (060) equatorial reflection, the degree of orientation (Π), and Herman’s orientation parameter (f), were calculated according to equations 2−3:

Degree of orientation, Π Π

(180-FWHM) 2 180

Herman’s orientation factor, f

cos ϕ 3

∑

cos ϕ 2

∑

/ /

I ϕ cos ϕ I ϕ sinϕ 1

3

4

where FWHM is the full width at half maximum of the peak in the azimuthal profile of MTM, ϕ represents the azimuthal angle and I(ϕ) is the intensity along the Debye−Scherrer ring. f = 1 corresponds to a maximum orientation of the platelets in the plane of the film, whereas f = 0 indicates random orientation of the platelets and -1/2 corresponds to perpendicular orientation. Filed Emission-Scanning Electron Microscopy: A high resolution FE-SEM (Hitachi S-4800) employing a semi-in-lens design and a cold field emission electron source is used for crosssectional analysis. The cross-sectional samples were prepared by microtome-cutting. Prior to SEM observation, samples were vacuum dried. In order to suppress specimen charging during analysis, the specimen samples were coated with gold/palladium (3 nm thickness) using an Agar HR sputter coater.


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RESULTS AND DISCUSSION In order to activate the XG chemically for covalent binding to the MTM surface (and within the XG matrix), we subjected the purified XG to periodate oxidation to form dialdehyde XG, see Figure 1.

Figure 1. Oxidation of XG and formation of composite film by vacuum filtration of oxidized XG coated MTM hydrocolloid suspension. Schematic representation of a possible cross-linking reaction of dialdehyde XG and hydroxyl groups on XG chains or MTM surface is shown. The FTIR spectra of the oxidized XGs show distinct carbonyl absorption peak at 1720 cm-1, see Figure S2(A), demonstrating the successful oxidation. The carbonyl estimation showed that oxidized XG samples have 1.6 mmol and 1.8 mmol carbonyls per g of sample for oxidXG10 and oxidXG20 samples, respectively. The carbohydrate analysis of the sugars present in XG and oxidized XGs is presented in Table 1. The glucose content is similar for all samples, which means that the cellulose backbone is preserved during the oxidation. The periodate oxidation of XG is therefore regio-selective for the present oxidation condition so that only the side chains are


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oxidized. XG is a unique polysaccharide, in contrast to other linear polysaccharides, since it is possible to selectively oxidize large amounts of sugar units attached to the backbone, while keeping the backbone structure rather unaffected.33 The side chains, galactose and xylose residues, are increasingly oxidized as the amount of periodate was increased. This can be deduced from the strong reduction in galactose content (outer carbohydrate residue on the side chain), but also from the reduction in xylose (inner carbohydrate residue on the side chain).

Table 1. Carbohydrate composition of XG and oxidized XG samples. Data are reported as mmols of carbohydrates / 100 mg of sample. Designations oxidXG10, and oxidXG20 refer to the stoichiometric amount of periodate used during oxidation of XG. Samples

Galactose

Glucose

Xylose

XG

0.086

0.256

0.183

oxidXG10

0.042

0.269

0.180

oxidXG20

0.017

0.265

0.159

Colloidal suspensions were prepared with XG adsorbed MTM platelets (reference material) and also suspensions with MTM platelets to which oxidized XG had been adsorbed. The coated MTM suspensions were vacuum filtered and dried to form nanocomposite films, according to a previously developed procedure.14 The crosslinking reaction demonstrated in Figure 1 is likely to take place during drying, and covalent bonds are formed between the aldehyde groups on oxidized XG and hydroxyls present at XG chains or MTM. 29, 30 The realtime FTIR study reveals


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that the intensity of the absorption peak at around 1140 cm-1 increases for the composite with oxidised XG sample (SI Figure S3) and can be associated with the formation of Si-O-C bonds.9 One important feature of the XG biopolymer is its strong adsorption to the MTM surface.28 The present study reveals that modified XGs show similar adsorption behavior. The TGA results (SI Figure S3) show that approx. 40 wt% of XG is adsorbed to MTM, and the amount of adsorption is similar for oxidized XGs. Thus oxidation of XG in the present condition did not affect the affinity of XG chains to the MTM surface.

Nanocomposite film characterization Characterization of the nacre-mimetic nanocomposite cross-sections with scanning electron microscopy shows a layered architecture with strong in-plane orientation of the MTM platelets (see Figure 2). This is conceptually similar to the hybrid materials fabricated by layer-by-layer assembly.9

Figure 2. Scanning electron microscopy images of the cross-section of nacre-mimetic XG/MTM nanocomposites.


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Figure 3. (A) and (B) Cross-sectional X-ray diffraction images of nacre-mimetic XG/MTM and oxidXG20/MTM nanocomposites respectively. The corresponding azimuthal intensity distribution profiles of the equatorial reflection (060) of MTM is presented in (C) and (D). Designation oxidXG20 refers to the stoichiometric amount of periodate used during oxidation of XG. In the cross-sectional plane, in-plane orientation of MTM platelets is observed, see XRD images in Figure 3 (A) and (B) for XG/MTM and oxidXG/MTM respectively. Interestingly, the


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degree of orientation for nacre-mimetic XG/MTM and oxidXG/MTM are in the same range and as high as 82%, see Table 2. Furthermore Herman’s orientation factor (f) is calculated to be as high as 0.4, confirming the high degree of in-plane orientation of platelets. The processing method adopted in the present case results in a more oriented platelet distribution in the composites compared to solution-cast films, where the degree of orientation was 74%.28 The molecular rigidity of XG is related to its cellulose backbone and may be significant for the inplane orientation of MTM platelets. The highly oriented MTM platelets have important implications for mechanical and barrier properties of the nanocomposites. Table 2. Degree of orientation (Π) and Herman’s orientation parameter (f) of nacre-mimetic XG/MTM nanocomposite samples. Sample

FWHM (deg)

Degree of orientation Hermans orientation (Π), % parameter (f)

XG/MTM

31.8

82.3

0.39

oxidXG20/MTM

31.7

82.4

0.41

Xyloglucan is an uncharged polymer, but still interacts strongly with the negatively charged MTM surface.28 For neutral polymers, an entropy-driven adsorption mechanism is the most widely held view,20 although the mechanism is likely to be more complex. Thermal analysis of the nanocomposites shows an influence of the MTM surface on the behavior of the composites. The DSC thermogram (Figure 4) for native XG shows a glass transition temperature at about 270 °C. The oxidized XG does not show any glass transition. Since we expect a cross-linked structure, this most likely increases Tg above the temperature for onset of thermal degradation. In nacre-mimetic XG/MTM composites, there is distinct change in thermal behavior due to the presence of the MTM. The DSC curves show thermal changes at around 196 °C for nacre-


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mimetic XG/MTM, whereas for oxidXG/MTM, thermal transitions are apparent at even lower temperature. Moreover, there are two different thermal transitions for oxidXG/MTM samples. This indicates two chemically different environments in the composite. One is associated with the interaction of sugar residues with MTM and the other possibly arises from covalent crosslinks between aldehyde groups and hydroxyl groups, as previously discussed. The TGA results (Figure S5 in SI) show that thermal degradation starts at around 200 °C for composites. The results are in good agreement with DSC results. Naturally occurring montmorillonite is a complex material with many different ions present, including Fe3+ and Mg2+ ions.34 These ions can form weak complexes with polysaccharides and polyols35,

36

and reduce thermal stability.

The phenomenon was noticed with other high clay content nanocomposites fabricated from polyvinyl alcohol.9

Figure 4. (A) Differential scanning calorimetry analysis of XG, oxidized XG and corresponding nacre-mimetic nanocomposites with montmorillonite (MTM). (B) Closer view of the thermal changes associated with nacre-mimetic nanocomposites made from native XG and oxidXG samples. Designation oxidXG10, and oxidXG20 refer to the stoichiometric amount of periodate used during oxidation of XG.


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The negative effect of MTM on thermal stability of the XG polymers is in support of a large specific surface area of the MTM platelets, which is exposed to the XG matrix. This is expected for a structure where monolayers of XG chains are adsorbed on MTM surfaces as “coatings”. Clay aggregation would possibly improve the thermal stability of the nanocomposite.17 However, a thermal stability of the order of 200 °C is sufficient for most engineering applications of polymer composites.

Nanocomposite film properties The tensile properties of nanocomposites at 50 %RH and 23 °C are reported in SI Table S1 and typical stress-strain curves are presented in Figure 5(A). The stiffness and strength of nacremimetic XG/MTM is increased significantly compared to native XG. Interestingly, most nacremimetic nanocomposites previously reported show mechanical properties in the same range elastic modulus of ~10 GPa, and tensile strength ~ 100 MPa at 50 %RH and 23 °C.8, 9, 16 The tensile strength and modulus at high clay content are typically observed to be lower than theoretical predictions based on simple rule-of-mixtures for composite materials.37 Walther et.al. reported a modulus as high as 27 GPa for non-cross-linked PVOH/MTM nanocomposites with more than 50 vol% of clay although the relative humidity or moisture content was not reported. Chitosan adsorbed MTM composites with approx. 50 vol% inorganic content showed a maximum tensile strength of 76 MPa and an elastic modulus 10.7 GPa at 50 %RH and 23 °C.16 Meanwhile, the elastic modulus of the present XG/MTM nanocomposite is 13.7 GPa and tensile strength is ~ 100 MPa. Differences at similar MTM content are often due to differences in MTM aspect ratio, orientation distribution and MTM-polymer interfacial adhesion. For oxidXG/MTM, the elastic modulus is twice as high at 50 %RH and is 30 GPa, whereas the tensile strength is increased to 147.5 MPa. Interestingly, in the nominal 50 %RH state, the effect of oxidation on


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the mechanical properties does not depend largely on the amount of oxidized units. Already 10% of oxidation suffices to boost the mechanical properties, as a further oxidation with 20% periodate only leads to moderate improvements. This can be related to the heterogeneous nature of periodate oxidation and the resultant number of carbonyl groups formed on polymer chain. As discussed previously, the carbonyl contents for the two oxidation conditions were not significantly different. Moreover, it is to be noted that not all the carbonyls are involved in crosslinking reaction (FTIR spectra of composites in SI Figure S2(B)). It has been reported for periodate oxidized cellulose that free aldehyde groups will be present even in highly dried samples.38 It depends on the hydration at the interface and the proximity of –OH groups for crosslinking reaction. To our best knowledge, this is the stiffest bionanocomposite reported, see the material chart representation in SI Figure S7. The term bionanocomposite here for the MTM case means that the polymer matrix is a biopolymer, or a derivative. The high mechanical performance of oxidXG/MTM nanocomposites is related to the formation of covalent bonds at the MTM-polymer interface, see Figure 1. The covalent bond formation between XG chains and MTM are more significant for the MTM-polymer load transfer efficiency than the improved interaction between XG chains. In fact, at higher oxidation degree, the mechanical properties of the oxidized XG (oxidXG20) was significantly reduced, presumably due to high inter chain cross-linking of high molar mass XG, see the tensile results in SI (Figure S6 and Table S2). Since the cross-linking sites were pre-formed on the polymer, the cross-linking can be expected to be homogeneous throughout the material. Podsiadlo et.al. added gluteraldehyde to the PVOH/MTM composites for crosslinking purposes.9 In comparison to the simple cross-linking approach used in the present case, this requires a more elaborate processing step of performing gluteraldehyde cross-linking for 30 min after every 10th bilayer deposition. The gluteraldehyde


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needs to diffuse into the structure, and as a consequence, the method was inefficient for thicker films.14 For the present study, one may note that all aldehyde groups may not be involved in hemiacetal bond formation. The extent of hemiacetal linkages formed depends on the drying conditions as well, where removal of water molecules from the aldehyde/hydroxyl interface is critical.30 The mechanical properties of the nanocomposites at 90 %RH and 23 °C are presented in Figure 5(B) and Table S1. XG at this humidity has an elastic modulus of 2.4 GPa.25 With 45 vol% of inorganic content, the modulus is 3.6 GPa for nacre-mimetic XG/MTM nanocomposites at this high humidity, significant reduction from 13.7 GPa at 50 %RH. However, the materials show significantly higher strain-to-failure, reaching up to 5.8%. The influence of moisture is two-fold. First, it reduces the mechanical properties of the matrix XG and secondly, it reduces the XG/MTM interfacial adhesion by competitive hydrogen bonding. This diminishes stress transfer at the interface and explains the reduced modulus.

Figure 5. Typical stress- strain curves of nacre-mimetic XG-MTM films conditioned at 50 % RH (A) and 90 %RH (B) and 23 °C. Designations oxidXG10, and oxidXG20, refer to the stoichiometric amount of periodate used during oxidation of XG.


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At similar relative humidity, oxidXG/MTM, behaves drastically different. The mechanical properties are preserved to a major extent and the modulus is as high as 20 GPa, a nearly 6-fold increase compared to native XG/MTM nanocomposites. This indicates that the oxidized XG is covalently linked to the MTM as was reported for nacre mimetic PVOH/MTM nanocomposites cross-linked with gluteraldehyde.9 Oxygen permeability data of nanocomposites are presented in Table 4. Interestingly, the effect of moisture uptake on oxygen transmission is smaller compared to the effect on mechanical properties. In dry state, the OTR values were less than the detectable limit of the instrument, 0.008 cc/[m2-day]. At 80 %RH, the oxygen transmission rate of nanocomposites made from native XG and oxidized XG are nearly in the same range, indicating the importance of in-plane orientation of platelets. Judging from modulus data, XG-MTM interaction is expected to be reduced at elevated moisture content but this does not seem to strongly influence barrier properties. As discussed previously, the degree of in-plane orientation of XG/MTM and oxidized XG/MTM nanocomposites are in the same range. The in-plane orientation of MTM creates a longer tortuous path for the diffusion of oxygen molecules through the membrane.39, 40 In food packaging applications, mechanical and barrier performance in moist environments are critical. The oxygen barrier performance of the present nanocomposites combined with favorable mechanical properties makes these materials a promising “green” alternative to conventional aluminium barriers.


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Biomacromolecules

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Table 4. Oxygen barrier properties of nacre-mimetic XG/MTM nanocomposites. Designations oxidXG10, and oxidXG20 refer to stoichiometric amount of periodate used during oxidation of XG. Sample

Oxygen transmission rate, Oxygen permeability cc/[m2-day] ccµm/[m2-day]kPa (80 %RH, 23 °C) (80 %RH, 23 °C) XG

40.0

11.5

XG/MTM

2.0

0.58

oxidXG10/MTM

2.24

0.72

oxidXG20/MTM

1.86

0.65

CONCLUSIONS The problem of reduced mechanical performance in ‘nacre-mimetic’, high clay content bionanocomposites has been addressed. A xyloglucan (XG) biopolymer is adsorbed to MTM platelets and excess XG is removed, according to a processing concept already described in the literature. In order to address the reduced mechanical performance of nacre-mimetic composites, XG is modified by regioselective periodate oxidation of side chains. Adsorption to MTM is not influenced by the modification. During drying, chemical crosslinking is expected to occur from aldehyde sites by the formation of hemiacetal bonds with hydroxyl groups. The periodate oxidation leads to strong effects on mechanical properties of XG/MTM at 50% relative humidity and ambient temperature. The modulus is increased from 13.7 GPa to 27-30 GPa. The tensile strength increased by 40 - 60 percent up to 140 – 150 MPa. This is interpreted to be due to the formation of covalent bonds between the modified XG and the MTM surfaces. Possibly, moisture is localized at the XG/MTM interface in unmodified XG/MTM so that interfacial adhesion and therefore stress transfer and modulus become reduced. Oxygen barrier properties


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are very favorable for nacre-mimetic XG/MTM composites due to the high clay content and the highly tortuous diffusion path for the gas in highly oriented clay platelet composites. At moist conditions, the oxygen permeability data are still low and independent of chemical XG modification. MTM content and orientation are apparently more important for oxygen barrier performance than moisture effects on XG/MTM interface and XG matrix. The present bionanocomposite is a candidate for “green” replacement of aluminium barrier films in packaging.

ASSOCIATED CONTENT Supporting Information (SI) AFM image of MTM suspension, FTIR of oxidized XG samples and composites, Realtime FTIR profile for MTM reacted with oxidised XG, thermogravimetric analysis (TGA) results for composites in O2 and N2 atmosphere, experimental procedure for Digital Speckle Photography (DSP), tensile properties of oxidized XG samples measured at 50%RH, and material chart (Ashby chart) for XG nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] (L.A.Berglund), Ph: + 4687908118, Fax: +46 8 7908101

ACKNOWLEDGEMENTS J. Kochumalayil is funded by the Biofiber Materials Centre (BiMac Innovation, http://www.bimacinnovation.kth.se/), L.A. Berglund is funded by Wallenberg Wood Science


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Center (WWSC, http://wwsc.se/), A. Walther is funded by the German Ministry of Education and Research and O. Ikkala is funded by Academy of Finland.

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Xyloglucan Bionanocomposites: A Chemical

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