Water Adsorption in Wood Microfibril-Hemicellulose System: Role of

Article pubs.acs.org/Biomac

Water Adsorption in Wood Microfibril-Hemicellulose System: Role of the Crystalline−Amorphous Interface Karol Kulasinski,†,‡ Robert Guyer,§,⊥ Dominique Derome,‡ and Jan Carmeliet*,†,‡ †

Chair of Building Physics, Swiss Federal University of Technology Zurich, Stefano-Franscini-Platz 5, 8093 Zürich, Switzerland Laboratory for Multiscale Studies in Building Physics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland § Solid Earth Geophysics Group, Los Alamos National Laboratory, MS D446, Los Alamos, New Mexico 87545, United States ⊥ Department of Physics, University of Nevada, 1664 North Virginia Street, Reno, Nevada 89557, United States

Downloaded by UNIV OF CAMBRIDGE on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 2, 2015 | doi: 10.1021/acs.biomac.5b00878

‡

ABSTRACT: A two-phase model of a wood microfibril consisting of crystalline cellulose and amorphous hemicellulose is investigated with molecular dynamics in full range of sorption to understand the molecular origin of swelling and weakening of wood. Water is adsorbed in hemicellulose, and an excess of sorption is found at the interface, while no sorption occurs within cellulose. Water molecules adsorbed on the interface push away polymer chains, forcing the two phases to separate and causing breaking of h-bonds, particularly pronounced on the interface. Existence of two different regions in moisture response is demonstrated. At low moisture content, water is uniformly adsorbed within hemicellulose, breaking a small amount of hydrogen bonds. Microfibril does not swell, and the porosity does not change. As moisture content increases, water is adsorbed preferentially at the interface, which leads to additional swelling and porosity increase at the interface. Young’s and shear moduli decrease importantly due to breaking of h-bonds and screening of the long-range interactions.

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INTRODUCTION Wood, a material known for thousands of years, has extraordinary properties such as high stiffness-to-density ratio, easy machining ability, low cost, etc.1 Some of these properties are results of its hierarchical structure.1−3 At the microscopic scale, wood cell walls are composed of different layers of varying thickness, structural arrangement, and chemical composition.1,4−7 The largest by mass is the S2 layer, reaching 80% of the mass of a tracheid.8 The very high stiffness of the S2 layer, in longitudinal direction around 80 GPa, is the result of the parallel-stacking of the cellulose microfibrils.9−12 Because of their small size, wood microfibrils are mostly studied with X-ray10,13,14 scattering, NMR,10,15 atomic force microscopy,12,16 as well as infrared spectroscopy;17,18 these reveal approximate geometry and arrangement as well as chemical composition. The microfibrils show a high aspect ratio with 3−5 nm in cross-section ranging from hundreds of nanometers to micrometers in length.9,15,19−22 They are typically composed of crystalline and noncrystalline (amorphous) cellulose phases that are separated by a noncrystalline hemicellulose layer.9,17,18 The presence and role of amorphous regions have yet to be better understood: existing at the surface of crystalline phase, they are known to be a strong water absorbent.23 The noncrystalline phases present in the local neighborhood of a wood microfibril are anisotropic and are found to display a colinearity with crystalline cellulose chains.17,18 © 2015 American Chemical Society

Wood is a strong water absorber, and its properties can drastically change as its water content increases.1,4,24−30 Namely, during adsorption, wood tissue undergoes an important decrease of elastic moduli that is accompanied by deformation/swelling of the tissue.29,30 An increase in adsorbed water content leads also to an increase in diffusion coefficient by increasing the distance between microfibrils and increasing the amount of nonhydrating water.31 Since water adsorption is an atomistic process, its kinetics can be studied with molecular simulations. We note that crystalline cellulose and some hemicelluloses have been extensively studied with molecular dynamics (MD).32−40 However, although MD studies of cellulose microfibrils attract more interest nowadays,41,42 there is still a lot to be understood in terms of interaction of water with microfibrils. The aim of this research is to investigate the influence of the adsorbed water on physical properties of a microfibril. We note that the influence of water layer on cellulose nanocrystals and an impact on mechanical properties have been previously investigated.43,44 Crystalline cellulose is known to adsorb water molecules not within its inner structure, but only at some of its exposed surfaces, that is, (110) and (010).35,38 Water molecules are adsorbed in the disordered amorphous phase in between Received: July 1, 2015 Revised: August 20, 2015 Published: August 27, 2015 2972

DOI: 10.1021/acs.biomac.5b00878 Biomacromolecules 2015, 16, 2972−2978

Article


Biomacromolecules the crystalline cellulose fibers. We therefore propose a twophase MD model of a wood (softwood) microfibril that consists of a crystalline cellulose core and amorphous hemicellulose phase (galactoglucomannan) that joins the crystalline cellulose cores. Although amorphous hemicellulose is naturally isotropic, the hemicellulose confined between the cellulose crystals reveals some sort of anisotropy as its chains are arranged in a colinear way with those of crystalline cellulose. As will be shown further, because of the MD model construction process, some crystalline cellulose at the interface amorphizes and is actually found to have a less ordered structure than the crystalline cellulose core. To avoid finite-size effect, we apply fully periodic boundary conditions that can mimic a “bundle” of microfibrils, also known as fibril aggregates.4 Studying one such unit embedded in copies of itself aims to avoid surface effects. The constructed model is then studied at several moisture content levels, from dry to saturated state. Unlike previous works, our approach has the advantage of (1) taking into account hemicellulose that makes an interface with crystalline cellulose that can amorphize and (2) studying the system at different moisture content while (3) keeping the realistic density and geometry. A detailed molecular investigation allows us to understand the mechanism of water adsorption in wood causing anisotropic swelling and mechanical weakening.

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Figure 1. Structure of a cellulose microfibril interlayered with hemicellulose in periodic boundary conditions. View (a) across the chains and (b) along the chains.

used in previous studies.33,34 Every successful insertion is followed a 20 ps equilibration. The number of adsorbed molecules, N, can be linked to moisture content, m, by m=

MATERIALS AND METHODS

NM H2O Mdry

(1)

where M stands for mass of water molecule (H2O) or system in dry conditions (dry). The full saturation amounts to roughly m = 25%. When a specific moisture content is reached, at 11 different equally spaced m from dry to fully saturated, the system is equilibrated for 2 ns. The determination of chemical potential, required for relative humidity (RH) determination, is carried out by One Step Perturbation method51 and has been used previously.33 The swelling strain ϵ is determined in reference to the dry state:

The MD simulations are carried out with Gromacs 5.0.445 and Gromos 53a6 force field,46,47 leapfrog algorithm for integrating Newton’s equations of motion, Nosé−Hoover thermostat,48 and Parrinello−Rahman anisotropic barostat.49 We employ long-range Coulomb interactions with particle-mesh Ewald summation and full periodic boundary conditions. The crystalline cellulose chains are constructed using X-ray diffraction data50 and have been studied previously as a separate phase.32 The hemicellulose composition is based on experimental measurements and has been investigated under different moisture content.34 The cellulose chains are covalently linked to themselves to reduce the finite-size effect. The crystalline cellulose phase consists of 36 chains, each 10 units long, arranged in a rectangle form of 6 by 6 chains. The rectangular shape, the number of chains, and size are suggested by experiments.9,10,17 To preserve the experimental cellulose-to-hemicellulose ratio, which amounts to 3.3:1,2,8 the cellulose phase is surrounded by 10 hemicellulose chains, each 10 units long. The chemical composition and branching differ among the hemicellulose chains, but on average they reflect the experimental values.34 The hemicellulose chains are randomly inserted around the cellulose in a way that preserves the colinearity18 (Figure 1). The resulting periodic box size is therefore 4.0 × 4.3 nm2 in crosssection and 5.3 nm in longitudinal direction that corresponds to 93 nm3 of dry volume and the density of 1.35 g cm−3 (Figure 1). This heterogeneous structure is first energy-minimized by steepest descend and then conjugate gradient method. This is followed by constant volume and temperature ensemble for 2 ns with thermostat set to 450 K. Next, without changing the thermostat, the barostat is set to 0 pressure, and the atoms are simulated for 10 ns. Finally, with kept barostat, the structure is relaxed at 300 K for 10 ns. An increased temperature typically accelerates the simulation and in this case helps hemicellulose to find its equilibrium position. However, to not disorder the crystalline cellulose, its atoms are position-restrained when the system is at T = 450 K. Although the amorphous cellulose is not explicitly put into the system, the crystalline cellulose that lays at the interface with amorphous hemicellulose can amorphize and is actually found to have less ordered structure than the cellulose chains that are not exposed to a contact with hemicellulose. Once the dry system is equilibrated, single point charge water molecules are inserted randomly one by one in the noncrystalline phase of the microfibril-hemicellulose system, this method having been

ϵX (m) =

X(m) − X(0) X(0)

(2)

with X being either x, y (cross-sectional directions), z, (longitudinal direction), or V (total volume). The porosity, ϕ, is determined as the free volume at a given moisture content divided by the reference (total) volume in dry state: ϕ(m) =

Vfree(m) V (m) V (m) V (m) = free = α(m) V (0) V (m) V (0) V (0)

(3)

The free volume at a given moisture content is determined as a product of total volume and the acceptance ratio α. To obtain the acceptance ratio, the water molecules are removed from the structure and the remaining atoms fully position-restrained. Next, probe water molecules are inserted with random position and orientation. For a large number of insertions, 106 per structure, a ratio between successfully inserted molecules and the number of attempts converges to the acceptance ratio. Mechanical weakening is evaluated by determining its elastic moduli. We choose the Young’s modulus in z direction, along the cellulose chains, Ez. The choice of this particular direction is motivated by the number of experimental measurements and numerical simulations as well the importance of longitudinal stiffness in the growth higher plants and wood applications.1,52−55 We determine the shear modulus in the direction perpendicular to cellulose chains, Gxy, and choose this plane to observe changes in shear stiffness because it is characterized by the smallest dry shear modulus, and therefore we expect the changes to be the most pronounced. To estimate a modulus at given moisture content, the corresponding strain is determined at two states: at zero stress and at a chosen uniaxial tensile stress σ (and shear stress τ) chosen in such way that the system remains in linear elastic regime; σ = 100 MPa and τ = 50 MPa: 2973


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Biomacromolecules ⎛ z(m , σ ) − z(m , 0) ⎞−1 Ez(m) = σ ⎜ ⎟ ; z(m , 0) ⎝ ⎠


⎛ xy(m , τ ) − xy(m , 0) ⎞−1 Gxy(m) = τ ⎜ ⎟ y(m , 0) ⎝ ⎠

(4)

During a mechanical loading, the number of water molecules remains constant as the simulations are carried out in NPT ensemble. This implies that the measured moduli are in undrained conditions. The existence of a hydrogen bond between a polymer sorption site and a water molecule or another OH group is determined by the distance between a donor and an acceptor that has to be smaller than 0.35 nm and the angle between hydrogen, donor, and acceptor that has to remain below 30°. In this study, we distinguish and measure three types of hydrogen bonds, namely (1) within the cellulose chains that are at the crystal surface, (2) within the hemicellulose chains, and (3) at the interface, that is, a donor belongs to cellulose, and an acceptor belongs to hemicellulose or vice versa (see Figure 1). In this paper, we will make use of the mixture rule, used often for cellulosic materials,7,56,57 to analyze the difference in sorption and swelling behavior between the microfibril-hemicellulose system with interface and its constituting components. The mixture rule that applies to ξ being moisture content, porosity, or swelling strain, can be expressed as ξ=

MHCξHC + MCCξCC ≈ 23%ξHC + 77%ξCC MHC + MCC

(5)

where MHC and MCC are the dry masses of hemicellulose (HC) and cellulose (CC), respectively.

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RESULTS Adsorption at the Interface. The model of microfibrilhemicellulose system is subjected to moisture adsorption, and for each moisture content value, the corresponding RH is determined. The obtained sorption data points are in good agreement with experimentally obtained data58 on cellulose whiskers and microfibrillated cellulose, which demonstrates the validity of the applied approach. During the simulation, for any value of moisture content, water molecules did not enter the crystalline region, which agrees with experimental observations. We present in Figure 2 the time-averaged density profiles of water molecules (blue) and solid matrix (black) for three different moisture content values. The darker blue denotes more adsorbed amount, and therefore one can clearly see, in particular in Figure 2, panels b and c, that water molecules prefer to occupy the interface between crystalline cellulose and hemicellulose. Assuming symmetry, the density profiles of adsorbed water and polymer matrix are evaluated across the interface in Figure 2, panels d and e, where x = 0 is the geometric center of the interface. The moisture profiles evolve with moisture content. As the adsorbed amount increases, the number of water molecules adsorbed between hemicellulose and cellulose increases, as related to the water adsorbed in hemicellulose (center line, Figure 2d). Simultaneously, the average density of hemicellulose (near the center line, Figure 2e) decreases, which means the hemicellulose swells and is less confined between the cellulose crystals. The water adsorption is accompanied by interface broadening (Figure 2). Preferential adsorption on crystal surfaces and broadening of the interfaces have also been observed experimentally for cellulose microfibrils.31 To better understand the nature of the water adsorbed at the interface, in Figure 2, panel f, we present the average dipole moment projected on y axis, that is, along interface, parallel to crystal surface. We find that the water molecules are ordered

Figure 2. Density profiles over 2 ns trajectory of the microfibrilhemicellulose (gray) and adsorbed water (blue). Adsorption shown at three stages: (a) m = 2%, (b) m = 14%, (c) m = 24%. Average density profiles across interface, calculated for (d) adsorbed H2O and (e) solid matrix (cellulose and hemicellulose), at three different moisture contents. (f) Dipole orientation of water molecules at the interface for three different moisture contents.

near the crystal surface, the ordering reaching 0 at the center of the interface. Also, because of stronger interactions at low moisture content, the ordering seems to increase with decreasing moisture content. Similar results of ordering of adsorbed water have been observed for mica surfaces.59 Moreover, the orientation of dipoles depends on the orientation of the crystal surface. We note that although the ordering is clear, the maximum average dipole moment that was obtained (μ = 0.38 D) is still much smaller than the dipole moment of a single water molecule (for SPC water, μ = 2.35 D). To quantify the nonhomogenous water distribution, we apply the mixture rule for the determination of the adsorbed amount by the composite. We assume that the pure crystalline structure does not adsorb moisture in its inner structure. Water can be sorbed at the surface of cellulose as well as within the hemicellulose (the interface region). The adsorption of moisture by pure hemicellulose was determined by MD simulations in a previous study.34 The resulting data points are presented in Figure 3 as hollow circles. For RH < 0.1, the data points are overlapping, which means that up to this value of RH, the water molecules are adsorbed only in hemicellulose (see Figure 2a). As the RH increases, the difference between measured m and the one determined by mixture rule increases. We attribute this difference to the extra sorption in the interface region. The amount of water molecules adsorbed in the space between crystalline and amorphous chains, being initially zero, 2974


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crystalline cellulose (as water molecules do not enter its lattice), the swelling of the microfibril-hemicellulose system can be estimated by the mixture rule. In Figure 4, panel a, we observe the swelling estimated with mixture rule to be smaller than directly determined by MD, the difference increasing with moisture content. The difference of the estimated swelling is attributed to the extra swelling in the interface region. The water molecules, after having partially filled up the pores of hemicellulose,34 gather preferentially in the interface layer between hemicellulose and cellulose. We note that the size or absolute volume of this layer is hard to determine; however, we observe that it increases with moisture content as more molecules enter it, which causes the additional swelling. Initially, until low moisture content, m ≈ 0.05, there is little swelling as the water molecules first fill the existing pores. This was also observed and discussed in more detail for amorphous cellulose and hemicellulose separately.33 With increasing moisture content, swelling is linear with m, as was also observed for amorphous cellulose.33 The moisture swelling is strictly related to the porosity available for water molecules to reside. Measurement of the total porosity is presented in Figure 4, panel b as filled circles. Similarly to the swelling strain, for m < 0.05, the porosity does not change, and above this value, it increases linearly. The total porosity includes as well the crystalline cellulose porosity that is not accessible for the water molecules. The measured porosity of the crystalline cellulose amounts to 0.05 and does not vary with moisture content. We note that the presented porosity values are calculated with respect to the total volume. To determine the interface porosity, we employ the rule of mixtures, using the porosity data from ref 34, and we plot the results in Figure 4, panel b as hollow circles. We note that the obtained porosity does not display any threshold value and is much below the measured porosity. We therefore subtract the porosity of the crystalline cellulose and hemicellulose from the total porosity for each m and obtain the interface porosity (Figure 4c). We observe the threshold at m = 0.05 and linearity above this value. This confirms our earlier statement that up to m = 0.05, the water molecules do not adsorb preferentially within the interface. However, with further increase in m, the interface porosity increases, reaching about 0.2 at maximum moisture content, which amounts to roughly half of the total porosity, the other half being “volumetric” porosity of the hemicellulose. Determination of the slope of swelling strain (porosity) versus moisture content gives dϵ/dm ≈ 1.2 (Figure 4a) and dϕ/dm ≈ 0.9 (Figure 4c) assuming


Figure 3. (a) Adsorption isotherm of cellulose microfibril as measured experimentally58 (triangles), by MD (filled circles), and using mixture rule (hollow circles). The difference between the adsorbed amount of moisture in the model and predicted by the mixture rule has to be attributed to the interface. (b) Sorption at the interface.

starts to increase at RH ≈ 0.1, reaching at RH ≈ 1 around 2/3 of the total adsorbed amount in the studied system (Figures 2b,c and 3). Further, in this paper, we investigate the role of the interface in terms of moisture adsorption and how its presence might affect other properties such as the stiffness of the microfibril. Interface Porosity. We measure the geometrical expansion of the microfibril-hemicellulose system with increasing moisture content and present the swelling strains in Figure 4, panel a. We

Figure 4. Moisture (a) swelling and (b) porosity increase, as referred to the dry volume. Note the m = 0.05 threshold in both cases. Swelling and increase in porosity are much larger than predicted by the mixture rule, which implies that the interface does not behave like the bulk. (c) Estimated porosity of the interface. (d) Increase in volume is linear with porosity.

ϵ = (1 − ϕ0)ϵs + ϕ − ϕ0

and therefore note that the moisture swelling is anisotropic, as the strain along crystalline cellulose chains is almost zero. Therefore, the volumetric strain equals the strain in lateral directions. The anisotropy of the studied system moduli is not only the result of the Young’s modulus anisotropy of crystalline cellulose itself, but also it is an effect of geometrical arrangement of cellulose and hemicellulose (parallel chains), as measured by MD and in experiments.32,55 The strong covalent bonds restrain swelling much more than the weaker van der Waals or even hydrogen bonds. By knowing the swelling coefficient of hemicellulose determined by MD,34 and by assuming no swelling of the

dϵ dϕ dϵ = (1 − ϕ0) s + dm dm dm

gives the solid strain dϵ/dm ≈ 0.33. The swelling versus porosity, plotted in Figure 4, panel d, follows a linear relation characterized by a slope dϵ/dϕ ≈ 1.38, which means for every 1 nm3 of an increase in pore volume, the corresponding increase in the volume occupied by solid is 1.38 nm3. This value being larger than 1 means that the swelling is not only due to an increase in porosity. According to poromechanics,60 the missing strain has to be attributed to the swelling of the solid part. In this case, swelling can be understood as an increase in effective 2975


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interface between hemicellulose and cellulose, as documented earlier. To understand better the mechanical behavior of the microfibril, the elastic constants need to be taken into account. Crystalline cellulose is an anisotropic material that has a modulus of 150 GPa along the chain direction and on average 15 GPa across the chains, as determined by MD and reported in ref 32. On the other hand, hemicellulose is anisotropic, characterized by a modulus of the order of 5 GPa.34 Although the crystalline cellulose can be assumed not to change its mechanical properties with moisture content, the hemicellulose modulus decreases by more than one order of magnitude over the full range of moisture content.34 Breaking of Hydrogen Bonds. We finally investigate the structure of hydrogen bonds (HB) that are key to understand the role of the interface and the weakening phenomenon. The number of hydrogen bonds per unit volume is presented in Figure 6, panel a. The volume taken in each case is the dry


volume of the polymer chains caused mainly by breaking of the hydrogen bonds, which we investigate next. Moisture-Induced Mechanical Weakening. We now turn to the measurements of mechanical stiffness of the microfibril. We determine, for each m, the Young’s modulus in z direction (Figure 5a), Ez, that is, along the crystalline chains,

Figure 5. Weakening effect of moisture adsorption: (a) Young’s modulus in the longitudinal direction, that is, along the cellulose chains, (b) shear modulus in the direction orthogonal to the cellulose chains. The filled circles correspond to a direct determination, whereas hollow circles are the data corrected for swelling.

that is related to the longitudinal direction of the wood tissue. We note that the value of the dry modulus is very close to the experimental measurements,12 which demonstrates the general validity of the constructed model. The modulus decreases overall with moisture content by roughly 30%, such decrease being attributed to the geometric expansion in cross-section. The values of Ez corrected for swelling are presented in Figure 5, panel a as hollow circles. The fluctuation around the constant value of 75 GPa can be attributed to hydrogen bonds breaking. At low moisture content, we however observe a little strengthening at m = 0.03, which is attributed to the formation of additional H2O−polymer hydrogen bonds bridges between the polymer chains and have been also observed experimentally.61 Since large drops in shear stiffness of wood tissue are known to take place due to moisture adsorption, we determine the shear modulus of the microfibril, Gxy, in xy plane that is perpendicular to the chain direction. The value of dry shear modulus is close to that of hemicellulose34 showing that shear deformation mainly occurs in the amorphous phase and not in crystalline phase. The value of G is much lower than Ez due to the fact that the restraining actions of the solid material only involve weak interactions (van der Waals and dispersion forces) and hydrogen bonds.32 This is in contrast to the strong interactions of the crystalline phase building up the stiffness of the microfibril-hemicellulose system in longitudinal direction. Upon moisture content increase, the shear modulus decreases, losing approximately 62% of its dry value. We note that a more pronounced loss of stiffness in shear modulus is observed experimentally.62,63 As for Young’s modulus, we observe a slight increase in shear modulus at around m = 0.03 (Figure 5b). The decrease of modulus is found to follow a power-law decrease.34 As expected, the correction for swelling (Figure 5b, hollow circles) has almost no impact on the shear strain in xy-plane. The reason for smaller loss of stiffness in longitudinal direction is due the presence of covalent bonds in z-direction, which are not affected by adsorbed water molecules. The larger loss of stiffness in xy-plane is due to the absence of covalent bonds, the presence of hydrogen bonds that are broken upon increasing moisture content, and the presence of a moisture-sensitive

Figure 6. (a) Breaking of the hydrogen bonds within interfacial cellulose (filled circles), hemicellulose (squares), and on interface between cellulose and hemicellulose (hollow circles) versus hydrogen bond density. (b) Linear dependence of measured moduli as plotted versus interface hydrogen bonds.

volume of the system unit cell. It can be clearly observed that the number of hydrogen bonds decreases with moisture content, mostly due to breaking of hydrogen bonds by geometric expansion (swelling) and by competition of water molecules to form hydrogen bonds with adsorption sites.64−67 The measurements of hydrogen bonds show that the outer surface of cellulose forms three times as many hydrogen bonds as hemicellulose, and there are more hydrogen bonds at the interface than within hemicellulose. This is caused by a wellordered, crystalline structure and the fact that the exposed surface of the crystal has many adsorption sites “sticking out” of the crystalline region that can form hydrogen bonds either with another polymer or with water molecules. The greater amount of the hydrogen bonds at the interface than within hemicellulose does not mean that the interface is stronger, as the main source of stiffness of the hemicellulose comes from the covalent bonds, much stronger than hydrogen bonds. We observe that the density of the hydrogen bonds in the regions affected by moisture decreases with moisture content. The interface hydrogen bonds, that is, those linking the crystalline and the amorphous phase, experience the largest drop (75%). This can be easily understood when taking into account the increasing porosity of the interface and the adsorbed water molecules that break the polymer−polymer HB and replace them by polymer−H2O HB in the interface zone.34 To show that the amount of the interface hydrogen bonds is closely related to the system stiffness, we plot Young’s and shear moduli versus interface hydrogen bonds in Figure 6, 2976


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panel b. A linear correlation holds particularly well for smaller HB density values. For higher values of the moduli, or high HB density, related to low moisture content, there is no linear dependence on the hydrogen bonds. In this range, the porosity does not change, and the few hydrogen bonds that are lost are replaced by the hydrogen bonds between polymer adsorption sites and water molecules.


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSION

In this paper, we studied sorption, swelling, and mechanical behavior of a two-phase cellulose microfibril interlayered with hemicellulose using MD simulations. We paid particular attention to the moisture-dependent behavior of the interface between crystalline cellulose and noncrystalline hemicellulose. We found that the moisture is adsorbed in hemicellulose and an excess of sorption on the interface, while no sorption occurs in the crystalline cellulose. The water molecules, adsorbed on the interface, push away the polymer chains, forcing the two phases to separate. This causes the breaking of hydrogen bonds, particularly pronounced on the interface. Breakage of hydrogen bonds, shielding of the long-range Coulomb forces, and an overall increase in porosity cause a weakening of the structure, manifested by a loss of stiffness in the direction along the chains, as well as shear stiffness across them. To elucidate the process of adsorption, we highlight two “extreme” states. We note that m = 0.05 is the critical moisture content above which the porosity of the interface increases causing a loss of shear stiffness. At low moisture content value (below 0.05), the moisture is uniformly adsorbed within hemicellulose and at the interface, which breaks a small amount of the hydrogen bonds. The microfibril does not swell, and the porosity does not change, whereas the stiffness can even increase due to additional H2O−polymer hydrogen bonds bridges. As the moisture content increases, water molecules are adsorbed not only in the bulk amorphous phase, but also preferentially at the interface, leading to an increase in swelling proportional to the porosity increase. Stiffness, particularly shear, decreases importantly, which is accompanied by breaking of numerous hydrogen bonds. The interface between the crystalline cellulose and amorphous hemicellulose turns out to be a region of preferential water adsorption, in particular at higher moisture content. This can be explained on the free energy basis. Adsorption of a water molecules involves steric repulsion of the polymer chains and therefore has an impact on the existing bonds. The interface is characterized only by hydrogen bonds and weak interactions, whereas most of the energy of the hemicellulose is stored in the much stronger covalent bonds. Stretching of covalent bonds denotes larger energy penalty than breaking of hydrogen bonds. As a result, above the threshold when the existing pores are filled, the water molecules prefer to occupy the smaller-penalty region, that is, the interface. This study helps us to understand better the moisture adsorption in wood and the accompanying phenomena such as swelling and weakening. The existence of an interface appears to play a crucial role in the global moisture-related behavior. By being able to explain why so much moisture prefers to stack at the interface rather than within polymer, a new functionalized material could be designed involving chemically modified wood. This will be a subject of future investigation. 2977


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Water Adsorption in Wood Microfibril-Hemicellulose System: Role of

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