Bioinspired and Highly Oriented Clay Nanocomposites with a

Nov 30, 2012 - School of Biotechnology, Royal Institute of Technology, SE-106 91, .... Multifunctional Nanoclay Hybrids of High Toughness, Thermal, an...
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Bioinspired and Highly Oriented Clay Nanocomposites with a Xyloglucan Biopolymer Matrix: Extending the Range of Mechanical and Barrier Properties Joby J Kochumalayil,† Malin Bergenstråhle-Wohlert,†,‡ Simon Utsel,† Lars Wågberg,†,‡ Qi Zhou,‡,§ and Lars A Berglund*,†,‡ †

Department of Fibre and Polymer Technology and ‡Wallenberg Wood Science Centre, Royal Institute of Technology, SE-100 44, Stockholm, Sweden § School of Biotechnology, Royal Institute of Technology, SE-106 91, Stockholm, Sweden S Supporting Information *

ABSTRACT: The development of clay bionanocomposites requires processing routes with nanostructural control. Moreover, moisture durability is a concern with water-soluble biopolymers. Here, oriented bionanocomposite coatings with strong in-plane orientation of clay platelets are for the first time prepared by continuous water-based processing. Montmorillonite (MTM) and a “new” unmodified biological polymer (xyloglucan (XG)) are combined. The resulting nanocomposites are characterized by FE-SEM, TEM, and XRD. XG adsorption on MTM is measured by quartz crystal microbalance analysis. Mechanical and gas barrier properties are measured, also at high relative humidity. The reinforcement effects are modeled. XG dimensions in composites are estimated using atomistic simulations. The nanostructure shows highly oriented and intercalated clay platelets. The reinforcement efficiency and effects on barrier properties are remarkable and are likely to be due to highly oriented and well-dispersed MTM and strong XG−MTM interactions. Properties are well preserved in humid conditions and the reasons for this are discussed.

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INTRODUCTION The development of nanostructured clay composites by Toyota researchers was a ground-breaking achievement in the field of polymer composites.1 Since then, the field of clay nanocomposites has grown dramatically and inspired development of new classes of materials.2−4 Montmorillonite (MTM) bionanocomposites, the subject of the present work, have also been widely studied,5−7 including the use of waterprocessed starch, chitosan, and pectin biopolymer matrices.8,9 However, the results in terms of nanocomposite properties are disappointing. The reasons are poor dispersion of MTM, lack of nanostructural order, and an MTM content of typically only 5 wt % or lower. These problems are not limited to this material category alone, the preparation of structures with welldispersed inorganic nanoparticles is a considerable challenge for the whole spectrum of polymer nanocomposites.10 Furthermore, lack of suitable processing methods, which can provide nanostructural control, is also a reason why technology transfer from research to large-scale industrial products has been slow. In biological structures, most load-bearing materials are indeed nanostructured composites. Several contain inorganic particles and examples such as bone, dentin and nacre have sophisticated nanoscale organization.11 One may also note that biological materials are highly hydrated and yet have substantial mechanical performance in moist state. The difficulties encountered with dispersion and nanostructural control in man-made nanocomposites have motivated attempts to prepare bioinspired composites. The work by Tang et al.12 on artificial © 2012 American Chemical Society

nacre was thus a seminal study in that the layered and parallel orientation of inorganic platelets in nacre was mimicked and it was possible to reach high inorganic content (Vf ≈ 50%), an elastic modulus of 11 GPa, and a tensile strength of 100 MPa. MTM clay platelets were combined with a charged, watersoluble polymer in an elaborate layer-by-layer (LbL) deposition process. However, this process is time-consuming and most likely difficult to extend from laboratory practice to large scale industrial processing. The work by Walther et al. therefore meant major progress because nacre-mimicking oriented composites with excellent properties were produced using a simple and industrially scalable water-based processing approach akin to papermaking.13,14 This method was subsequently extended to other composites based on biopolymer matrices such as chitosan15 and nanofibrillated cellulose.16−18 This resulted in nacre− mimetic bionanocomposites of high MTM volume fraction. Moisture durability is a problem for the mentioned nacre− mimetic composites based on polyelectrolytes.12 Polymer−clay interfaces where ionic interactions contribute to interfacial adhesion (i.e., synthetic polyelectrolytes, chitosan) are sensitive to moist environments as a result of a large number of counterions present in the system. At the top of the wish list is, therefore, a water-soluble neutral biopolymer, which interacts Received: September 2, 2012 Revised: November 30, 2012 Published: November 30, 2012 84

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Scheme 1. Schematic Representation of Xyloglucan/MTM Nanocomposite Preparation and Film Formation

tamarind seed xyloglucan (weight average molecular mass, 1.5−2.0 MDa, Innovassynth 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. The MTM concentrations in XG−MTM suspensions were 1.0, 2.5, 5.0, 10.0, and 20.0% (w/w) and mixed with Ultra-Turrax at 13500 rpm for 15 min and kept under magnetic stirring for overnight. The resulting solutions were centrifuged at 4000 rpm for 20 min to remove microbubbles and any clay aggregates remain. The final solid contents of different nanocomposite solutions were in the range of 4 to 5% after rotoevaporation to desired viscosity. The resulting solutions were evenly spread over a Teflon mold and dried under constrained condition in an oven at 40 °C overnight (Scheme.1). The films were peeled off from the Teflon surface for further characterizations. The thickness of the films was in the range of 20 to 30 μm. Different nanocomposite suspensions were coated on an oriented polyester film (OPET) in a comma coater (Hirano Tecseed Co., Ltd., Japan) where the OPET film was rolling at a speed of 0.5 m/min. (see Scheme 1). The film with wet coating was immediately dried in a heating chamber kept at 120 °C. The thicknesses of the wet coatings were adjusted in such a way that the final thickness of the dried films was 4 μm, as measured with a thickness meter of the peeled-off coatings. Quartz Crystal and Dual Polarization Analysis. A quartz crystal microbalance (QCM) E4 from Q-Sense AB (Västra Frölunda, Sweden) was used to study xyloglucan adsorption on clay model surfaces with a continuous flow of 100 μL/min.24 Native XG, enzymatically modified XG (40% galactose removed), and a synthetic polymer PVOH were studied. All samples had a concentration of 100 mg/L. The QCM crystals used were AT-cut quartz crystals (5 MHz resonant frequency) with an active surface of sputtered silica, which were plasma-treated using an air plasma cleaner (Model PDC 002, Harrick Scientific Corporation, NY, U.S.A.) under reduced air pressure for 120 s at high effect (30 W). The samples were adsorbed on model surface and the change in frequency in quartz crystal was recorded. To convert a change in frequency into an adsorbed mass per unit area, the Sauerbrey model was used:25

strongly with the MTM surface also in an environment of high relative humidity. The primary plant cell wall is hydrated and may provide inspiration to find such a polymer. The cell wall needs to support turgor pressure (pressure generated by liquid) and must resist external loads. The load-bearing function is carried by cellulose microfibrils linked by xyloglucan (XG) polysaccharide chains,19 which are most likely physisorbed to cellulose. The nonelectrostatic cellulose−xyloglucan interaction was used in preparation of cellulose nanocrystal/XG multilayers.20 Xyloglucans, or galactoxyloglucans, are polysaccharides with the main chain identical to that of cellulose, that is, β(1→ 4)-linked glucan. Up to 75% of the glucose residues are substituted with β(1→6)-linked xylose, where some of these xylose side chains have an additional β(1→2)-linked galactose residue, see Figure S1 in Supporting Information, SI.21,22 XG extracted from tamarind seed was recently reported to have excellent film-forming, thermo- and hygromechanical properties.22,23 XG was also found to have good oxygen barrier performance in our lab trials. In the present study, a green concept for high-performance clay-biopolymer nanocomposites based on highly oriented clay platelets is developed. The processing concept is continuous in order to facilitate scaling up, and we aim for optical transparency as well as strongly improved mechanical and gas barrier properties, also under moist conditions. The strategy to achieve this with a water-soluble biological polymer is to rely on nonelectrostatic interactions between the clay and the XG biopolymer. Because brittleness is a problem for nacre− mimetic nanocomposites of high MTM volume fraction, we focus on volume fractions up to 0.1 to provide potential for high ductility (strain-to-failure). To the best of our knowledge, mechanical and barrier properties are superior to data previously reported for clay/biopolymer nanocomposites. One of the major application areas for such nanocomposites would be food packaging, where a candidate material should meet critical requirements such as mechanical and oxygen barrier performance, also in humid conditions.

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m=C

Δf n

(1)

where m = adsorbed mass per unit area [mg/m2], C = sensitivity constant, −0.177 [mg/(m2·Hz)], Δf = change in resonant frequency [Hz], and n = overtone number Preparation of Clay Model Surface. Clay model surfaces were prepared in situ by consecutively adsorbing three bilayers of poly(diallyl dimethyl ammonium chloride) (pDADMAC) and exfoliated MTM using the LbL technique. Both samples had a concentration of 100 mg/L. Preparation of Modified Xyloglucan. XG was enzymatically treated to partially remove galactose by using β-galactosidase (from Aspergillus oryzae, Sigma Aldrich).22 The details of the modification procedure are given in SI. The extent of galactose removal was around 40% in 100 mg of material as compared to the amount present in native XG, obtained from carbohydrate analysis using high perform-

EXPERIMENTAL SECTION

Preparation of Xyloglucan−MTM (XG/MTM) Nanocomposites. A total of 1 wt % MTM (Cloisite Na+, density of 2.86 g/cm3, Southern Clay Products, Inc.) solution was prepared by using Ultra Turrax mixer (IKA, DI25 Basic) at 25000 rpm for 15 min, followed by sonication using Vibra-Cell (Sonics and Materials, Inc.) ultrasonic processor. It was repeated several times and the resultant solution was kept undisturbed for one week and the supernatant MTM suspension was used for composite preparation. The industrially available 85

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Figure 1. (A) Amount of xyloglucan and modified xyloglucan adsorbed on MTM model surface in comparison with PVOH. Model surface was prepared by consecutive deposition of three bilayers of MTM and polyDADMAC. The amounts were calculated from the frequency difference in Quartz crystal microbalance (QCM) analysis (B) schematic representation of adsorption of xyloglucan chain segment on MTM platelets. ance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Molecular Dynamics Simulation of Xyloglucan Oligosaccharides. To estimate the occupied volume and structure of xyloglucan, molecular dynamics (MD) simulations were conducted for small systems containing atomistic models of xyloglucan fragments (XXXG, XXLG, and XXXG−XXXG−XXXG−XXXG) in water. All simulations were performed with the GROMACS 4.0 simulation software package26 and the GROMOS G45a4 carbohydrate force field27 was applied for the xyloglucan fragments together with SPC water.28 The details of oligosaccharide nomenclature and simulation steps are given in SI. Oxygen Permeability. Oxygen transmission rate of film coatings was measured using a Mocon Ox-tran 2/21 (Modern Controls Inc., Minneapolis, U.S.A.) with an oxygen sensor that conforms to ASTM D-3985 standard. Tests were done in dry conditions, 50% RH and 80% RH in both N2 and O2 flow path. The coating side was exposed to the O2 flow. Light Transmittance. Light transmittance of the coatings over OPET films was measured from 400 to 600 nm using a Hitachi U3010 spectrophotometer, and was correlated based on the film thicknesses using the Lambert−Beer’s law. X-ray Diffraction. Diffractograms were recorded in reflection mode in the angular range of 0.5−12° (2θ). The measurements were done with an X’Pert Pro diffractometer (model PW 3040/60). The Cu Kα radiation (1.5418 Å) generated with a tension of 45 kV and current 35 mA was monochromatized using a 20 μm Ni filter. An increment step of 0.05° and a rate of 1 step per 10 s were used. Samples were dried prior to experiment. From the angle of diffraction, the d-spacing corresponding to the (001) lattice spacing of MTM was calculated using the Bragg’s equation:

nλ = 2d sin θ

Π=

180 − fwhm 100 180

(3)

where fwhm = full width half-maximum in degrees. The azimuthal intensity profile is given in the SI. Transmission Electron Microscopy. The samples for transmission electron microscopy (TEM) study were prepared by embedding in epoxy resin. The cured epoxies containing nanocomposite film strips were microtomed with a LKB Bromma 2088 ultramicrotome into 50−100 nm thickness. These slices were placed on a 200 mesh copper grid and imaged using a Philips Tecnai 10 electron microscope operated at 80 kV. Scanning Electron Microscopy. An ultrahigh resolution FE-SEM (Hitachi S-4800) employing a semi-in-lens design and a cold field emission electron source was used for imaging the cross-section of nanocomposites. The microtomed cross-sectional samples prepared for TEM were used for SEM analysis. Prior to SEM observation, samples were vacuum-dried, and 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. Tensile Testing. 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. The gauge length was 10 mm and the extension rate was 0.5 mm/min. The samples were conditioned at 50% RH and 92% RH prior to testing. Young’s modulus (E) was determined from the slope of the low strain part in the initial elastic region. At least ten specimens were tested from each sample and tensile data were averaged over at least three specimens that do not show premature failure in elastic region. Model for Tensile Modulus. The model predictions for elastic modulus of platelet composites, ECeff, were based on a simplified rule of mixtures for infinite aspect ratio isotropic platelets in polymer matrix:29

(2)

where d and θ are the distance and angle between two consecutive (001) crystal lattice planes in MTM, λ is the wavelength of the irradiated X-ray, and n is the order of diffraction. Stacking of MTM platelets gives a regular gallery distance between the layers and the corresponding d-spacing will be in the vicinity of the thickness of a single MTM platelet, around 1 nm (see the AFM height image of clay suspension in water in Figure S3). Polymer intercalation results in layer expansion and corresponding increase in the gallery height. The resulting d-spacing gives the thickness of a single MTM platelet and polymer layer together. The interlayer distance in nanocomposites was calculated from this distance. Wide angle X-ray diffraction images were obtained by irradiating the sample with Mo Kα radiation (25 mA and 60 kV) in perpendicular (in-plane) and parallel (cross-section) to the surface of the nanocomposite film surface. The azimuthal intensity distribution profile from the cross-sectional image was extracted from the most intense (020) reflections using FIT2D software. The degree of orientation was calculated according to

ECeff = E XGVXG + EMTMVMTM

(4)

where ECeff is composite modulus in-the-plane, EMTM is clay platelet modulus, VMTM is volume fraction of MTM, EXG is modulus of XG, and VXG is volume fraction of XG matrix. The third term in the original version of eq 4 is assumed to be small and is neglected. It is assumed that platelets are isotropic and oriented in the in-plane loading direction, and that interfacial platelet−matrix adhesion is perfect. EMTM is 100 GPa, obtained from an approximate fitting of the model to experimental data. E XG is 4 GPa as determined experimentally. Because the aspect ratio of MTM platelets is very high (>100), effects from finite MTM platelet aspect ratio are neglected. The purpose of the procedure of fitting EMTM to experimental data of ECeff is to use the obtained value for effective EMTM as a measure of reinforcement efficiency. The volume fractions of the inorganic content in composite films were determined from weight fraction obtained from thermogravimetric analysis. The MTM volume fraction is calculated as 86

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Figure 2. (A) Cross-sectional SEM image of a nanocomposite containing 10 wt % MTM in xyloglucan (XG) matrix, X-ray diffractograms of a nanocomposite film containing 20 wt % MTM in parallel (B) and perpendicular (C) to the film surface. (D) X-ray diffraction pattern of the XG− MTM hybrid. (E) TEM image of the cross-section of XG−MTM nanocomposite containing 10 wt % of MTM showing the silicate layers as alternating dark lines. (F) Schematic picture of an XG chain fragment modeled as cylinder. The XG radius is denoted as R. VMTM =

ρXG WMTM ρMTM WXG + ρXG WMTM

conformation is indicated schematically in Figure 1B. In a thermodynamic sense, PVOH is also more soluble in water than XG. These differences are part of the answer to the complex question of why XG shows higher QCM-adsorption values compared to PVOH. Synthetic PVOH has been used previously to assemble nacre−mimetic composites,13,31 so the present adsorption data motivates the use of XG and modified XG to make structurally related bionanocomposites. The adsorption of uncharged polymers to clays is largely driven by the entropy gained by the desorption of water molecules from the clay surface.32 XG is considered watersoluble, but the individual chains are not fully hydrated, showing an amphiphilic character, even in dilute solutions;33 modified XG has even lower water solubility as a result of increased self-association.34 The corresponding QCM data includes polymer and associated water and show slight decrease in adsorption for modified XG compared to XG. If this difference in associated water is taken into account, the driving mechanism for the adsorption of XG and modified XG is likely to be partly of hydrophobic interaction with MTM surface that ultimately increase the entropy of the system. Molecular dynamics simulations (MD) have demonstrated that van der Waals interactions are of substantial importance for the strong cellulose−XG interactions in water environment.35 This is interesting, because one hypothesis is then that MTM− XG nanocomposites may also preserve mechanical performance under moist conditions, in case substantial van der Waals interaction contributes to interfacial adhesion for this system. Films and coatings with MTM content as high as 20% by dry weight (approximately 12 vol %) were prepared successfully

(5)

where VMTM = volume fraction of clay, ρXG = density of XG (1.5 g/ cm3), ρMTM = density of MTM (2.86 g/cm3), and WMTM and WXG are weight fraction of MTM and XG, respectively. Dynamic Mechanical Thermal Analysis (DMTA). DMTA measurements were performed on a dynamic mechanical analyzer (TA Instruments Q800) operating in tensile mode. Typical sample dimensions were 15 mm long and 5 mm wide. The measurement frequency was 1 Hz. At a nominal strain of 0.02%, temperature scan was made in the range 25−300 °C at a heating rate of 3 °C min−1 under an air atmosphere.

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RESULTS AND DISCUSSION To evaluate the feasibility of favorable XG−MTM interaction, MTM model surfaces were prepared, and adsorption of XG and modified XG from water solution was studied. Modified XG was pretreated with a β-galactosidase enzyme in order to partially remove the galactose residues. Quartz crystal microbalance (QCM) data for XG and modified XG were compared with polyvinyl alcohol (PVOH), a synthetic polymer widely used in clay/polymer nanocomposites and packaging applications. QCM measurements (see Figure 1A, Figure S4 in SI) show that the amount of XG adsorbed on MTM is 6.5 mg/m2, which is higher than that for modified-XG (5.1 mg/m2) and significantly higher than the amount of adsorbed PVOH (2.8 mg/m2). Note that these adsorption values include immobilized water: see SI. Even for a polysaccharide, XG has a very stiff chain with high conformational rigidity,30 whereas the conformation of PVOH is more coiled. The adsorbed XG 87

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Figure 3. (A) Typical stress−strain curves of XG−MTM films conditioned at 50% RH and 23 °C. (B) Modulus of XG/MTM nanocomposites as a function of MTM added in comparison to model, ECeff = 100VMTM + 4Vm.29 (C) Material charts adopted from Ashby and Wegst37 using Grant CES EduPack2011. The data for nacre−mimic nanocomposites were from refs 12 and 31 and bionanocomposites and polymer composites were derived from the previous works depicted in Table S2 in SI. (D) Storage modulus and loss modulus (inset) of xyloglucan nanocomposites as a function of temperature.

from MTM−XG aqueous suspensions. SEM image, Figure 2A, of the cross-section of a sample with 10 wt % MTM reveals strong in-plane orientation of the platelets. Figure 2 presents Xray diffraction patterns in parallel (Figure 2B) and perpendicular (Figure 2C) to the film surface. In the cross-sectional plane, strong orientation of MTM platelets can be observed as shown by strong reflections from (020) planes (Figure 2B), whereas the orientation in the plane of the film is completely random, Figure 2C. The degree of orientation is estimated (see SI) as high as 74% in the cross-sectional plane. This reveals a layered structure conceptually similar to the high clay content multilayered structures prepared by LbL assembly12 or papermaking approaches.13,16 These observations of strong in-plane orientations are unexpected for solvent-cast, freestanding films and different from what is typical for bionanocomposites.8 The molecular rigidity of XG is related to its cellulose backbone and may be significant for the in-plane orientation of MTM platelets. The X-ray diffraction (XRD) spectra, Figure 2D, indicate that for MTM addition of 1 and 2.5 wt %, the MTM platelets are completely exfoliated in the XG matrix polymer, the XRD peak associated with d001 = 9.8 Å disappears completely. At 5 wt % MTM or more, intercalated MTM structures are obtained as interpreted from the XRD data. A lattice plane diffraction peak corresponding to d001 = 36 Å is obtained, and this corresponds to an average interlayer spacing of 26 Å. Interestingly, the interlayer gallery spacing for the MTM−XG nanocomposites was independent of MTM content. This is consistent with a nanocomposite formation mechanism based on “deck-of-card” assembly of XG-coated MTM platelets during water evaporation. TEM micrographs of a representative nanocomposite film with 10 wt % MTM are presented in Figure 2E and in SI, Figure S6. Intercalated MTM platelets with parallel orientation

are apparent. The basal spacing obtained by XRD and TEM are in good agreement, while the TEM reveals that part of MTM platelets are exfoliated (see SI, Figure S6). Molecular dynamics (MD) simulations of XG oligosaccharides corresponding to XG segments (XXXG, XXLG, and XXXGXXXGXXXGXXXG) in water provided data of XG lateral dimensions. Because XG has a stiff cellulose backbone, the simulation results for lateral dimension of the oligosaccharides could be representative for XG macromolecules. If each molecule is assumed to occupy a cylindrically shaped space of volume, V = π × length × R2 (schematically presented in Figure 2F), an approximate XG radius can be extracted from the volume data. The results are presented in Table S1 in SI. XG chains have an approximate radius R = 5 Å perpendicular to the chain direction and an average XG−MTM hydrogen bonding donor−acceptor distance of D = 1.75 Å is considered. The experimentally measured average gallery spacing of 26 Å corresponds to an average of one layer of XG chains adsorbed to each face of MTM. The constant interlayer spacing from XRD data, and MD simulation results provide support that gallery distance is unaffected in intercalated state. The tensile properties showed remarkable improvements for XG/MTM nanocomposites (see Figure 3A and Table 1). The tensile strength increases from 92 MPa for native XG to 123 MPa with 20 wt % MTM at 50% RH and 23 °C. For most MTM nanocomposites in the literature, excluding nacre− mimetic ones with much higher clay content, high inorganic content leads to reduced strength. There is a 3-fold increase in XG/MTM modulus for the same composition. At less than half the MTM content, the modulus reaches the same level as nanocomposites with more than 50 wt % MTM in PVOH or polyelectrolyte matrices prepared by LbL technique.12,15,31 This shows that the present XG/MTM bionanocomposite has high reinforcement efficiency. The value for effective MTM 88

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GPa for MTM modulus can be compared with the theoretical value of 250 GPa.31 This is the best reinforcement efficiency obtained for an MTM/biopolymer matrix composite. Most likely, the main reason for not reaching even higher reinforcement efficiency (higher EMTM) is nonideal stress transfer at MTM platelet surfaces. This can be related to MTM/XG interfacial adhesion and MTM dispersion. Because the reinforcement efficiency decreases at higher MTM content (see Figure 3B), MTM dispersion may be important. The best reinforcement efficiencies reported in the literature are for nacre−mimetic composites, where cross-linking reactions were used.13,31 Ductility is also important. Note in Table 1 that, even at 20 wt % MTM content, many samples showed higher strain-tofailure (around 2%) than is typically observed for the higher volume fraction nacre-type of materials. The higher matrix content in the present materials improves ductility, and the ductility also indicates good dispersion of the MTM platelets because agglomerates will initiate failure at low strains. The material data chart (Figure 3C) for specific modulus versus specific tensile strength illustrates that XG/MTM is located in the same property space as nacre-mimetic composites of substantially higher clay content. Moreover, the chart reveals that present nanocomposites have far better mechanical performance than conventional bionanocomposites based on starch, PLA, and PCL. Even the nanocomposites tailored with synthetic polymers are inferior to XG/MTM. In Table 1, mechanical properties of samples conditioned at 92% RH and 23 °C are also reported. Even in this rather severe environment, roughly half the strength and modulus or more are preserved. The excellent mechanical properties of XG−clay nanocomposites rely on strong molecular interaction between the matrix polymer and the inorganic reinforcement, even in

Table 1. Tensile Properties of Xyloglucan Nanocomposites samples conditioned at 50% RH and 23 °C sample xyloglucan XG/1 wt % MTM XG/2.5 wt % MTM XG/5 wt % MTM XG/10 wt % MTM XG/20 wt % MTM

tensile strength, MPa

tensile strain at break, %

elastic modulus, GPa

92.9 ± 5.8 89.1 ± 6.9

8.9 ± 2.0 15.5 ± 2.9

4.1 ± 0.15 5.1 ± 0.53

96.2 ± 6.7

12.2 ± 1.7

5.9 ± 0.1

103.9 ± 2.7

6.6 ± 1.9

6.2 ± 0.48

114.3 ± 6.3

3.8 ± 1.2

8.6 ± 0.26

123 ± 7.4

2.1 ± 0.31

11.6 ± 1.7

samples conditioned at 92% RH and 23 °Ca sample XG XG/10 wt % MTM XG/20 wt % MTM a

tensile strength, MPa

tensile strain at break, %

elastic modulus, GPa

56 ± 6.1 63 ± 5.2

9.9 ± 5.2 6.4 ± 2.1

3.0 ± 0.14 4.1 ± 0.14

81 ± 2.3

3.0 ± 0.46

6.8 ± 1.6

Measurements were made at 50% RH and 23 °C.

modulus, EMTM, defined in the Experimental Section, can be used as a measure of reinforcement efficiency. In Figure 3B, elastic modulus, E, is plotted versus volume fraction of MTM. Predictions (“model”) are based on a model of perfectly aligned clay platelets embedded in polymer matrix (rule of mixtures)29 and values for MTM platelet and polymer modulii are assigned 100 (fit to data) and 4 GPa (experimental data), respectively. The predictions are close to the present modulus data for XG− MTM nanocomposites, especially up to 5 vol %. The fit of 100

Figure 4. (A) Schematic representation of xyloglucan nanocomposite coated OPET film with a cross-sectional view of xyloglucan−MTM composites (8.9 wt % MTM) coated over OPET film as observed in SEM. (B) Light transmittance of xyloglucan−MTM nanocomposites coated over OPET film. (C) Relative oxygen permeability for XG/MTM nanocomposites with respect to neat XG in dry condition and 23 °C as a function of MTM content (volume fraction), model predictions, and experimental data. The calculated fits based on L = 100 nm and W = 1 nm, which is a typical dimension of MTM platelets. 89

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be of interest as barrier films or coatings from renewable resources in packaging applications.

the moist state, so that stress can be efficiently transferred from the matrix to the stiffer MTM platelets. The thermomechanical properties of native XG and nanocomposites prepared with MTM are presented in Figure 3D. The storage modulus is increased significantly in the glassy state. The thermal stability of the material is notable and superior to chitosan.36 The loss modulus peak associated with the glass transition increased with MTM addition (about 6 °C increase with 20 wt % MTM compared to native XG). The oxygen transmission rate was measured on coatings of XG/MTM on OPET film. The cross-sectional FE-SEM images (Figures 4A and S7 in SI) reveal that the MTM platelets are predominantly oriented parallel to the substrate film similar to cast films previously discussed. The UV−visible light absorption spectrum of the films showed high transmittance in the visible spectrum range from 400 to 600 nm (Figure 4B). All coatings increase the optical transparency of the OPET film to some extent, probably by decreasing the microroughness on the OPET surface.38 The apparent oxygen permeability of the nanocomposite coatings were calculated from oxygen transmission rate data (given in Table S4 in SI) and presented in Table 2. The average

0 50 80

XG 0.02 ± 0.00 0.45 ± 0.00

XG + 4.3 wt % MTM

XG + 8.9 wt % MTM

XG + 20 wt % MTM

0.01 ± 0.00 0.18 ± 0.01 30.1 ± 0.01

0.00 ± 0.00 0.04 ± 0.00 6.03 ± 0.02

0.00 ± 0.00 0.05 ± 0.00 1.44 ± 0.00

CONCLUSIONS

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ASSOCIATED CONTENT

The present MTM/XG bionanocomposites show strong inplane platelet orientation. Compared with earlier studies of clay bionanocomposites, MTM/XG shows higher mechanical properties, better optical transparency, and higher oxygen barrier properties at comparable clay content. The material therefore occupies new property space. In contrast to in-plane oriented MTM/polyelectrolyte nanocomposites, favorable oxygen barrier and mechanical properties are observed at high relative humidity. This improvement relies on the strong physical adsorption of XG to MTM in wet condition. XG is amphiphilic and it is likely that hydrophobic interactions with MTM are important. Agreement with predictions from the Nielsen model, provide support that the increased tortuosity factor partly explains the favorable effects of MTM on oxygen barrier properties. The potential for continuous processing has also been demonstrated. The reinforcement efficiency becomes very high with a modulus of 11.6 GPa and tensile strength of 123 MPa at an MTM volume fraction of only 12%. We have not found any previous clay nanocomposite study with as high modulus at such low MTM content. Together, the strength and modulus data indicate favorable interfacial adhesion and that the extent of MTM aggregation is limited. XG/MTM composition with 12 vol % MTM has an oxygen permeability of only 1.44 cc μm m−2 d−1 kPa−1 indicates that the present bionanocomposite can, as an example, be a candidate material as a “green” and transparent replacement for aluminum gas barrier films in packaging. Also, coated paperboard and PLA film demonstrators are included in SI. It is well-established that XG functions as a physisorbed physical linker of the cellulose microfibrils in the primary plant cell wall. In the cell wall, nonelectrostatic interactions provide adhesion also in wet conditions. The present results indicate that the molecular characteristics of XG are causing strong adsorption of XG to wet inorganic surfaces by similar mechanisms. This result may allow future tailoring of bionanocomposites for improved moisture durability, which is not achievable with charged polymer matrices.

Table 2. Oxygen Permeability of XG/MTM Nanocomposite Films (cc·μm/[m2·day] kPa−1) % RH at 23 °C

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oxygen permeability of free-standing XG films at 50% RH and 23 °C was 0.5−2.0 cc μm m−2 d−1 kPa−1. This is in the range of commercial barrier polymers such as poly (vinyl alcohol; PVOH) and recently reported biopolymers such as wood hemicelluloses (see Table S3 in SI).39−42 There are two main observations from the addition of MTM in barrier properties: increasing MTM content strongly improves barrier properties, and even at high relative humidity, the oxygen permeability was very low for high MTM content composite coatings. The relative permeability of XG/MTM with respect to native XG in the dry state is plotted as a function of MTM content in Figure 4C, as well as predictions based on the Nielsen model (see SI for details).43 The MTM effect in the Nielsen model is based on the more tortuous path which a penetrant gas is forced to diffuse along due to the in-plane oriented MTM platelets. The agreement between predictions and data are very good up to an MTM content of 3% by volume, in support of the tortuosity mechanism. The experimental data reach a plateau value earlier than predicted. However, considering the nature of the model and the data, it is difficult to ascribe this observation to a specific mechanism. An interesting application of XG/MTM films could be as environmentally friendly replacement of aluminum barriers in liquid packaging. The oxygen permeability at 80% RH is then of particular interest because polysaccharides typically fail to perform under these conditions. In Table 2, it is apparent that the XG/MTM composition with 20 wt % MTM has an oxygen permeability of only 1.44 cc μm m−2 d−1 kPa−1. Because inorganic coatings suffer from pin holes and can have higher values, the present data are encouraging, and XG/MTM may

* Supporting Information S

Figure showing XG oligosaccharides, details of enzymatic modification of XG, MD simulation procedure and results, details of quartz crystal and dual polarization interferometer analysis, additional SEM (coating) and TEM images, table presenting the oxygen permeability of polymers, data analysis steps for oxygen permeability, Nielsen model, and coating demonstrators on paper board and PLA film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Ph.: +46 87908118. Fax: +46 8 7908101. Notes

The authors declare no competing financial interest. 90

dx.doi.org/10.1021/bm301382d | Biomacromolecules 2013, 14, 84−91

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Article

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ACKNOWLEDGMENTS J.K. and S.U. are funded by the Biofiber Materials Centre (BiMac Innovation, http://www.bimacinnovation.kth.se/), L.A.B., L.W., M.-B.W., and Q.Z. are funded by Wallenberg Wood Science Center (WWSC, http://wwsc.se/). The authors thank TetraPak for film coating and oxygen transmission analysis.

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