Cation-Induced Hydrogels of Cellulose Nanofibrils with Tunable

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Cation-Induced Hydrogels of Cellulose Nanofibrils with Tunable Moduli Hong Dong,*,†,‡ James F. Snyder,† Kristen S. Williams,† and Jan W. Andzelm† †

U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States Bowhead Science and Technology LLC, Belcamp, Maryland 21017, United States

‡

ABSTRACT: Cellulose nanofibrils are biocompatible nanomaterials derived from sustainable natural sources. We report hydrogelation of carboxylated cellulose nanofibrils with divalent or trivalent cations (Ca2+, Zn2+, Cu2+, Al3+, and Fe3+) and subsequent formation of interconnected porous nanofibril networks. The gels were investigated by dynamic viscoelastic measurements. The storage moduli of the gels are strongly related to valency of the metal cations and their binding strength with carboxylate groups on the nanofibrils. Hydrogel moduli may be tuned by appropriate choice of cation. Cation−carboxylate interactions are proposed to initiate gelation by screening of the repulsive charges on the nanofibrils and to dominate gel properties through ionic cross-linking. Binding energies of cations with carboxylate groups were calculated from molecular models developed for nanofibril surfaces to validate the correlation and provide further insight into the cross-linked structures. The cellulose nanofibril-based hydrogels may have a variety of biomedical and other applications, taking advantage of their biocompatibility, high porosity, high surface area, and durability in water and organic solvents.

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INTRODUCTION Hydrogels are a class of materials that are composed of threedimensional hydrophilic polymer networks and large amounts of water. The networks may comprise either synthetic or natural polymers that are cross-linked physically or chemically to prevent dissolution into the aqueous phase.1−4 Hydrogels have wide applicability in fields such as drug delivery, tissue engineering, sorbents, sensors, contact lenses, and purification.1−4 Cellulose is the most abundant and naturally occurring renewable polysaccharide, found as a primary constituent of plants and natural fibers such as cotton. Cellulose hydrogels are typically fabricated either directly from native cellulose via cellulose dissolution in specific solvents, such as ionic liquids and alkali/urea aqueous systems, or from water-soluble cellulose derivatives, such as hydroxypropylmethyl cellulose (HPMC), via physical or chemical cross-linking.3,4 Native wood and plant cellulose can be processed to yield the underlying nanofibrils when specific techniques to facilitate fibrillation are incorporated in the mechanical refining of wood fibers and plant fibers.5 Cellulose nanofibrils (CNFs) have a high aspect ratio (4−20 nm wide, 500−2000 nm in length) and are 100% cellulose.5 Their highly crystalline structure provides the nanofibrils with excellent mechanical and thermal properties, including high Young’s modulus and very low coefficient of thermal expansion. These nanofibrils have rapidly found potential applications as gas barrier films,6 flexible displays,7 composites,8,9 aerogels,10 functional papers,11 templates for nanostructures,12 and so on. Among the techniques of chemistry-assisted nanofibrillation, Isogai et al.13,14 introduced an oxidation pretreatment of cellulose by applying 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO) radicals before mild © 2013 American Chemical Society

mechanical disintegration into nanofibrils in water. The TEMPO-mediated oxidation selectively oxidizes the accessible C6 primary hydroxyls on CNFs producing carboxylate with high densities on the CNF surfaces.13,14 Carboxylate is wellknown to have strong affinity to some metal cations, especially transition metal cations, forming metal−carboxylate complexes.15−17 Structuring CNFs into porous materials such as hydrogels and aerogels can significantly broaden the potential applications of these biobased nanofibrils. Even more useful would be to provide a method for generating hydrogels and aerogels with targeted mechanical properties. Although the aqueous dispersion of carboxylated CNFs at moderate concentrations exhibits “gel-like” character, it can be easily disrupted by high shear rate to form a flowable liquid due to its shear thinning character. On the other hand, the freeze-dried or supercriticaldried aerogels of CNFs have poor durability in water and organic solvents. Water-stable CNF macrostructures have instead been demonstrated by protonating the nanofibril carboxylate groups to form hydrogels, which can further support additives such as cationic dyes.18 In previous work we demonstrated that CNF hydrogels can also be formed using monovalent transition metal ions, using Ag+ as an example, and we compared them with hydrogels formed using alkali metals.19 The Ag+ incorporated in the CNF hydrogels could be spontaneously converted to more functional Ag nanoparticles Received: July 8, 2013 Revised: August 2, 2013 Published: August 6, 2013 3338

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by in situ “green” reduction using CNF surface hydroxyl groups. Building on our previous work with monovalent cations, we describe here hydrogelation of CNFs with high valency cations. We report the formation of a variety of stable gelled networks by cross-linking surface carboxylate groups with selected divalent and trivalent metal cations, where the moduli of the gels strongly correlate to both the electronic structures of metal cations and their binding strengths with carboxylate groups. A mechanism of gel formation has been proposed based on experimental observations. We have also developed molecular models of CNF intrafibril and interfibril bonding, and calculated binding energies with different cations to validate the proposed mechanism and to gain insightful information about the network structures.

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were solvent-exchanged stepwise with acetone/H2O (50:50), acetone/ H2O (75:25), and then with acetone for several times to completely remove water. The acetone gels were then supercritical CO2-dried. FESEM images were taken on the dried specimens that were sputtercoated with gold−palladium. Fourier transform infrared (FT-IR) spectra of the dried samples of CNFs and the CNFs with metal cations were collected on a Thermo Nicolet NEXUS 870 spectrophotometer. The samples were obtained by freeze-drying the hydrogels. Sample pellets were prepared by grinding and pressing KBr powder with the dried samples. The spectra were acquired in transmission mode on the sample pellets at a spectral range of 4000−500 cm−1. The metal contents in the dried samples of the CNF dispersion and the CNFMn+ hydrogels were analyzed using inductively coupled plasma (ICP) or inductively coupled plasma mass spectrometry (ICP-MS). The CNF-Mn+ hydrogels were thoroughly soaked and rinsed with DI water to remove unattached metal cations, followed by freeze-drying. The ICP or ICP-MS analysis was performed by NSL Analytical Services Inc. (Cleveland, OH). Model Development. A model of the CNF surface was built by cleaving the Iβ crystal structure of cellulose20 along the [1−10] direction to create a two-chain surface unit cell. Functionalization of the CNF surface was modeled by replacing one −CH2−OH group per cellobiose unit with a carboxylate anion, COO−, which can then interact with neighboring cations. Molecular models were then constructed from the unit cell, in which the surface of a single fibril is represented by two cellobiose units (from two individual chains) with passivated chain ends. The interaction between 2 or 3 fibrils was modeled by sandwiching cations between 4 or 6 cellulose chains. Geometry optimizations were performed on all molecular models. Binding energies were calculated by subtracting the sum of metal cation and cellulose two-chain energies from the total CNF complex energy. The computational method employed was Density Functional Theory (DFT) using the Perdew−Burke−Ernzerhof (PBE) exchangecorrelation functional approximation. All calculations were performed with the DMol3 program, including solvation effects through an implicit conductor-like screening model (COSMO), as implemented in the Materials Studio suite of modeling programs.21−25

EXPERIMENTAL SECTION

Materials. Deionized water with resistance ∼ 18.2 MΩ·m was used in all experiments. Calcium nitrate tetrahydrate, zinc nitrate hexahydrate, copper(II) nitrate trihydrate, aluminum nitrate nonahydrate, and iron(III) nitrate nonahydrate were all purchased from Sigma-Aldrich and used as received. Aqueous dispersion of carboxylated cellulose nanofibrils (balanced with sodium ions) was provided courtesy of the USDA Forest Products Laboratory (Madison, Wisconsin). The cellulose nanofibrils were produced from wood pulp using the TEMPO oxidation technique and sodium hypochlorite as the terminal oxidant.14 The dispersion used in this study has a concentration of 1.27% and a surface carboxylate content of 1.3 mmol/g. The nanofibrils were measured to be ∼5 nm in average diameter from TEM images. Hydrogelation. CNF hydrogels were produced by addition of a metal salt solution to the top of the CNF aqueous dispersion without stirring. To ensure formation of a uniform hydrogel with a smooth surface, the thixotropic CNF dispersion was first stirred to form a low viscosity liquid, followed by degassing and transferring to a glass container. An equal weight of a 50 mM aqueous solution of metal salt, such as Ca(NO3)2, Zn(NO3)2, Cu(NO3)2, Al(NO3)3, and Fe(NO3)3, was added dropwise along the wall of the container into the CNF dispersion without stirring. Gelation occurred rapidly upon the addition of any of the metal salt solutions. After standing for overnight, the metal salt solution on the top was decanted and the resulting hydrogel was soaked and rinsed with water several times to remove unbounded metal ions. For the hydrogel generated with Fe(NO3)3, a yellow gel formed after addition of 50 mM Fe(NO3)3. The yellow color of the gel turned brown slowly after it was rinsed with DI water, probably due to formation of iron-based clusters at a neutral pH. To reduce it, the gel of CNF-Fe3+ was rinsed with water of pH 3 before rinsing with neutral water. Rheological Tests. To measure mechanical properties of the hydrogels, rheological tests were carried out with a rheometer Physica MCR 501 operating in 25 mm parallel-plate configuration and 1 mm gap distance. A 1 mL aliquot of CNF dispersion was dispensed onto the bottom plate and gelled with 1 mL of 50 mM metal salt solution for 10 min before loading the top plate. The experiment was performed at 25 °C in a controlled-temperature chamber. To minimize solvent evaporation, water-saturated tissue was placed around the gel on the plate. A time sweep was performed first for 2 h with an angular frequency of 6.28 rad/s and a strain rate of 0.5% to determine gelation. The dynamic modulus of the hydrogel was then measured as a function of frequency with a strain rate of 0.5%, where the frequency sweep was carried out between 0.1 and 100 rad/s. A strain sweep was performed at a frequency of 6.28 rad/s to ensure the measurements were made in the linear viscoelastic regime. The measurements were repeated three times to ensure reproducibility. Instrumental Characterizations. The networked nanofibril structures derived from the hydrogels were examined using a Hitachi S-4700 field emission scanning electron microscope (FESEM) at an accelerating voltage of 5 kV. To prepare the specimens, the hydrogels

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RESULTS AND DISCUSSION Hydrogel Formation and Properties. CNF gels were produced by diffusing metal cations into the CNF dispersion and utilizing binding affinity of metal cations with carboxylate groups to initiate the gelation process. Addition of a 50 mM salt solution of divalent alkaline earth cation Ca2+ or transition metal cations Zn2+ or Cu2+ to the CNF dispersion triggered rapid gelation of CNFs, as demonstrated by inverting the vials with the gels. Unlike the preceding CNF aqueous dispersion, the hydrogels had no fluidity and maintained shape even with shaking. Similar gelation phenomena were also observed using 50 mM salt solutions of trivalent cations Al3+ or Fe3+. The trivalent gels were accompanied by some shrinkage in volumes of about 7% (CNF-Al3+) and about 12% (CNF- Fe3+). Minimal shrinkage was detected using divalent cations. All gels prepared in this way were macroscopically homogeneous and had slightly less clarity than the starting material. UV−vis spectroscopy was used to compare the transmittance of the CNF dispersion and the colorless gels of CNF-Ca2+ and CNF-Al3+. At 550 nm, the transmittance of the CNF dispersion is around 91%, while the gels of CNF-Ca2+ and CNF-Al3+ have respective lower transmittances of 88 and 85% that suggest coalescence of nanofibrils. The gels were mechanically robust to be free-standing and could be handled without permanent deformation. Figure 1 shows self-supporting forms of the gels. The blue and yellow colors of the CNF-Cu2+ and CNF-Fe3+ gels are typical for complexes with these ions. The CNF gels incorporating Ca2+, 3339

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complexes. Solvent exchange with organic solvents such as acetone did not change the macroscopic shape of the sample. Addition of stronger ligands than the carboxylate ions, however, caused collapse of the gels that was attributed to dissociation of the cations from the carboxylate groups on the nanofibril surfaces. For example, addition of tetrasodium ethylenediaminetetraacetate (Na4EDTA), a chelating ligand that forms metal complexes with high stability constants, to the gel of CNFCu2+, CNF-Zn2+, or CNF-Fe3+ was observed to disintegrate the bulk gel into a microgel suspension in water. The hydrogels were solvent-exchanged with acetone and then supercritical-dried with CO2 in order to investigate the structures. The FESEM images in Figure 2 reveal interconnected nanofibrils that form a three-dimensional highly porous network. The networks appear to primarily comprise fine threads that are interconnected with some thicker strands and junctions of overlapping strands. After deducting the thickness of surface metal alloy coating layer from sputter-coating, the diameters of the underlying nanofibrils for the fine threads are measured to be in the range of 4−7 nm using ImageJ software, which are close to the diameter of single CNF fibrils. Thicknesses at the junctions were found to be larger. Significant differences in the network structures are not noted between gels using different metal ions. The variation in supercritical

Figure 1. Photos of the CNF aqueous dispersion and the free-standing gels formed by addition of metal salt solutions to the carboxylated CNF dispersions.

Zn2+, and Al3+ did not exhibit color as these ions do not contain partly filled d-orbitals from which color typically arises in ligand

Figure 2. FESEM images show that the CNF hydrogels prepared by addition of metal cations have porous and interconnected network structures. (a, b) CNF-Ca2+, (c) CNF-Zn2+, (d) CNF-Cu2+, (e) CNF-Al3+, and (f) CNF-Fe3+. 3340

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cm−1 to 1731 cm−1 for CNF-Fe3+, CNF-Al3+, and CNF-Cu2+. Considering that solutions of Fe(NO3)3, Al(NO3)3, and Cu(NO3)2 have pH values of 2, 4, and 5, respectively, due to hydrolysis of the metal salts, while the pH values of Ca(NO3)2 and Zn(NO3)2 are close to neutral, this additional band is attributed to uncomplexed carboxylate groups that exist as carboxylic acid form.18 The band at 1727 cm−1 of CNF-Fe3+ was split from the band at 1615 cm−1, and the areas of the two bands were integrated. Using this technique, it was determined that in CNF-Fe3+ about 76% of the carboxylate groups are bound to cations and 24% are “free” carboxylic acid. To demonstrate the valence of each metal ion within the CNF environment, the contents of cations in the freeze-dried CNF-Mn+ samples were analyzed using ICP or ICP-MS, shown in Table 2. The content of Fe3+ is not presented in the table

CO2 drying process and the small imaging area may make the difference in pore structure and fibril bundling less noticeable in these high magnification (50 K) FESEM images. FT-IR was used to investigate chemical compositions of the samples prepared from freeze-drying of the CNF dispersion and the CNF-Mn+ hydrogels (Figure 3). The vibration

Table 2. Metal Contents, Primary Stability Constants with CH3COO−, and Mechanical Properties of the CNF Dispersion (CNF-Na+) and the CNF-Mn+ Hydrogelsa

n+

Figure 3. FT-IR spectra of the dried CNF dispersion and CNF-M hydrogels: (a) CNF-Na+, (b) CNF-Ca2+, (c) CNF-Zn2+, (d) CNFCu2+, (e) CNF-Al3+, and (f) CNF-Fe3+.

Table 1. IR Frequencies (cm−1) for the CNF Dispersion (CNF-Na+) and the CNF-Mn+ Hydrogels nanofibrils +

CNF-Na CNF-Ca2+ CNF-Zn2+ CNF-Cu2+ CNF-Al3+ CNF-Fe3+

υOH (free)

υOH (Hbonded)

υas OCO

υs OCO

3735 3734 3730 3732 3735 3736

3347 3349 3354 3354 3351 3347

1614 1607 1621 1617 1620 1615

1417 1426 1424 1424 1426 1428

cations (50 mM)

Fe3+

Al3+

Cu2+

Zn2+

Ca2+

CNFNa+

cations in gels (mmol/g) log K1* (CH3COO−) G′ of gel at 0.1 rad/s (kPa) G″ of gel at 0.1 rad/s (kPa) tan δ at 0.1 rad/s

n/a

0.37

0.65

0.59

0.61

1.37

4.00

3.13

2.22

1.49

0.44

−0.07

31.6

23.4

11.7

5.00

3.39

0.0021

2.68

3.05

1.35

0.475

0.239

0.0016

0.085

0.130

0.115

0.095

0.071

0.739

Note: the primary stability constants of cations with CH3COO− were referenced from 29. a

υCO

because it is quite challenging to completely remove unbounded Fe3+, thus, complicating quantitative evaluation. The original dispersed nanofibrils have a sodium content of 1.37 mmol/g, consistent with the carboxylate content of 1.3 mmol/g for CNFs. The sodium contents in all the analyzed samples of CNF-Mn+ hydrogels are minimal at tens ppm or less, indicating near complete ion exchanges of sodium with divalent or trivalent cations. The molar quantity of divalent cations is close to half of that of sodium in pristine CNFs and the molar quantity of trivalent cation Al3+ is close to one-third, further indicating near complete ion exchange of monovalent sodium as well as confirming that no excess salt appears to be present. The dynamic rheological properties of the original CNF dispersion and the CNF-Mn+ hydrogels were investigated at 25 °C, shown in Figure 4a,b and Table 2. The difference between storage modulus (G′) and loss modulus (G″) of the CNF dispersion is small, and both G′ and G″ demonstrate a clear frequency dependence typically associated with liquids and dispersions because the time scale of experimental oscillations at high frequencies exceeds that of molecular relaxation.26 An equilibrium modulus is not observed at very low frequencies for the CNF dispersion, suggesting a lack of interfibril cross-linklike interactions. However, G′ is greater than G″ over the entire angular frequency range, a characteristic more typically associated with gels. The elastic behavior of this viscous liquid is attributed to the coupling of nanofibrils with cohesive forces such as weak hydrogen bonding and van der Waals interactions. Entanglements are otherwise not expected to occur between the charged nanofibrils.

1731 1717 1727

assignments of the bands are cited in Table 1. The original CNF dispersion was noted as CNF-Na+ here for clarity of description since the carboxylate groups were balanced with sodium ions. In the spectrum of CNF-Na+, a very weak band corresponding to stretching of the “free” hydroxyl was observed at 3735 cm−1, while a strong band corresponding to the hydrogen-bonded hydroxyl was observed at 3347 cm−1. The spectrum of CNF-Na+ also displayed an asymmetric −OCO− stretching band at 1614 cm−1 and a symmetric stretching band at 1417 cm−1, which arise from carboxylate anions. Displacement of Na+ with the other cations studied here did not have significant effects on the bands of “free” hydroxyl and hydrogen-bonded hydroxyl stretching vibrations. Some hydroxyl groups on the CNF surfaces may coordinate with the divalent or trivalent cations, however, the percentage of overall hydroxyl groups participating in these interactions was so small that it did not show significant impact on the absorption bands. The band corresponding to the symmetric stretching vibrations of −OCO− group was shifted from 1417 cm−1 for CNF-Na+ to 1424−1428 cm−1 for CNFs with other cations. This result suggests complexation of −OCO− to the added cations. Interestingly, an additional band appeared in the range of 1717 3341

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Proposed Mechanism. The initial driving force for gelation is proposed to be screening of the interfibril repulsive forces generated by the CNF carboxylate surface charges. It is well-known that surface charges generate a “double-layer” potential providing interfibril repulsion that can be screened more effectively with increasing valence and concentration of counterions.27 This phenomenon is exemplified by the empirical Schulze-Hardy rule that the threshold for screening the double layer potential is proportional to z6, in which z is the counterion valence.28 Sufficient screening enables fibrils to come close enough for attractive cohesive interactions and van der Waals forces to dominate. Gel formation via charge screening has been previously noted for carboxylated cellulose nanofibrils using monovalent counterions such as silver.19 Multivalent ions should promote more rapid gelation due to their greater electrostatic attraction to carboxylate anions and due to the entropic benefit of having a single multivalent ion provide the same electrostatic shielding as multiple monovalent ions. Strong metal-carboxylate bonding is critical to providing a stable screening effect of the repulsive double-layer potentials and ultimately to formation of the final gel structure. To exemplify this point, the stability constants previously calculated for carboxylate complexes of these metal ions are shown in Table 2.29 These values are a useful guide toward metal−ligand bond strength for the metal ion complexes investigated here and show good correlation with the trend in G′ values. The low K1 for Na+ reflects its preference to remain in solution. The high positive K1 values for the other metal ions suggest that they associate more closely with the carboxylate groups, thus, screening those repulsive fibril surface charges much more effectively than Na+. Furthermore, metal ions with higher binding constant are expected to more effectively exclude water molecules that were previously associated with charge stabilization of both carboxylate and metal ions. Deswelling behavior resulting from complexation of metal ions with polymer ionic groups has been noted in previous studies.30 Some evidence of this behavior may be noted here as gels with trivalent metal ions yielded more dense internal structures with lower water contents. Elimination of water from the carboxylate regions may help reduce interfibril distance to facilitate the onset of gelation as well as promote more significant interfibril adhesion. The final structures and properties of the gels are primarily dictated by strength of interfibril interactions and degree of networking. Hydrogen bonding is expected to prevail among the available intermolecular forces, especially considering the large numbers of hydroxyl and carboxylate groups and the dominating role that hydrogen bonding plays in knitting cellulose chains within the fibrils. Hydrogen bonding can be particularly dominant in CNFs at low pH where the carboxylate groups have been protonated to enable interfibril dual hydrogen bonding interactions.17 This factor may complicate analysis of the modulus data given the acidic properties of some metal salt solutions, such as Fe(NO3)3 (pH ∼ 2). As indicated in the FT-IR spectra (Figure 3), a small portion of the CNF carboxylate groups was protonated in the presence of the Fe3+, Al3+, and to a much lesser degree Cu2+ salt solutions. To determine the maximum extent in which dual hydrogen bonding might contribute to modulus, we tested the hydrogel formed by complete protonation of CNFs with 100 mM HCl. The storage modulus at 0.1 rad/s was measured to be 11.8 kPa, substantially lower than that of Fe3+ and Al3+. Because no more

Figure 4. Viscoelastic properties of the CNF dispersion and the CNFMn+ hydrogels: dynamic frequency sweep (25 °C) of the gels at a strain rate of 0.5%; (a) storage modulus plots, (b) loss modulus plots.

In contrast to the original CNF dispersion, the G′ values of CNF-Mn+ hydrogels are an order of magnitude higher than that of G″ and both moduli show little dependence on frequency in the studied range, indicative of a stable gel state. The G′ values of the CNF-Mn+ hydrogels are also 2 to 4 orders of magnitude higher than the corresponding G′ of the CNF dispersion. Because there is no structure change to the CNF polymers or fibrils themselves by addition of metal ions, this greater elastic character of the CNF-Mn+ hydrogels indicates formation of more substantial interfibril cohesive interactions. The tan δ values at low frequency (Table 2) are all very low for the CNFMn+ hydrogels, indicating formation of networks with few “defects” that would contribute to viscous energy loss.26 Rheological defects in a typical polymer system could include loops or dangling chain ends, which would not be expected in the current networks. The CNF-Mn+ hydrogels storage moduli span an order of magnitude, from 32 to 3.4 kPa, and clearly follow the order of Fe3+ > Al3+ > Cu2+ > Zn2+ >Ca2+ (Table 2), providing a method for generating CNF hydrogels with tunable mechanical properties. Because they do not possess covalently derived cross-links and substantial fibril aggregation is neither expected nor evident under these conditions, the differences in moduli are expected to be caused by the strength and frequency of interfibril connections via ionic cross-links, coordination complexes with bridging metal ions, and hydrogen bonding. To better understand the impact of metal cations on hydrogel moduli, it is critical to consider the dominant cohesive forces acting on the fibrils during gelation. 3342

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Scheme 1. Schematic Representation of Proposed CNFs Crosslinking with Metal Cations

screening of the repulsive potentials to enable fibrils to come closer together. It is likely that as interfibril distances are reduced during gelation some fractions of the metal ions are able to effectively form complexes between fibrils and thus form cross-links, also shown in Scheme 1. Trivalent cations may have higher tendency than divalent cations to cross-link between two nanofibrils considering the greater electrostatic attraction and geometrical needs to interact with three surrounding carboxylate groups. In some cases, the higher valence of Fe3+ and Al3+ ions might even bridge three fibrils, although the steric hindrance associated with three fibrils is expected to make this a rare occurrence. Either cross-linking scenario produces higher gel moduli as well as denser gel structures that exclude more water, leading to the volume shrinkage as observed on formation of CNF-Fe3+ and CNF-Al3+ hydrogels. Molecular Modeling of CNF-Cation Gelation. To validate the proposed gelation mechanism and understand the experimentally observed trend of the gel modulus, we have developed molecular models of various cellulose surfaces, calculated binding energies of cations to carboxylate groups constrained on the fibril surfaces, and then related computational trends to the experimental observations. Table 3 contains displaced solvation volume, ΔVsolv (in Å3) and binding energies (in kcal/mol) for all CNF molecular models. ΔVsolv is defined as the amount of fibril surface accessible to solvent that is displaced when multiple chains are bound via intercalating cations. Binding energy was calculated by subtracting the sum

than 24% of surface carboxylate groups are shown by FT-IR to be protonated in the metal cation hydrogels, we do not expect hydrogen bonding through carboxylic acid groups to play the dominant role in their rheology. Furthermore, it is noteworthy that addition of EDTA, which has a high binding constant to the metal cations due to the chelating effect, causes the Mn+CNF hydrogels to collapse. If the gel structures were primarily defined by hydrogen bonding or other intermolecular forces then the chelation of metal cations should not have such a substantial impact on the gel structures. We propose that the dominant interfibril bonds dictating gel structure and properties in the CNF-Mn+ hydrogels are attributed to cross-linking through the metal cations. Such specific interactions dominate other interaction forces, such as hydrogen bonding and van der Waals interactions. The prominence of metal-carboxylate interactions have already been suggested with respect to the role of K1 values (Table 2) on hydrogel modulus. The ICP data (Table 2) confirms that divalent or trivalent cations roughly meet the stoichiometric ratio (1:2 or 1:3) of cations to carboxylate required by charge neutrality. Thus, the high valency of the introduced metal ions may lead to interaction with multiple carboxylate groups to produce association between fibrils. Cellulose nanofibrils each comprise bundles of polymer chains that are aligned and interconnected by substantial hydrogen bonding. Metal−carboxylate bonds are expected to form from cellulose chains either on the surface of the same fibrils or between the fibrils (Scheme 1), providing a dominant force in networking the CNFs. The effectiveness with which the metal ions bridge fibrils depends in a large part on the distribution of ligand or counterion groups on the fibril surfaces. Cellulose nanofibrils consist of linear chains of β(1→ 4) linked D-glucose repeating units. On the surface of the nanofibrils the C6 primary hydroxyl group on every other glucosyl unit is exposed, while the interstitial C6 primary hydroxyl groups interact with internal chains via hydrogen bonding. Almost all of the exposed C6 primary hydroxyl groups are converted to carboxylate by TEMPO-mediated oxidation.31 The distance between two adjacent carboxylate groups in the same molecular chain on one nanofibril surface is ∼1 nm.32 These distances are large enough to be expected to frustrate formation of metal−carboxylate complexes in the same molecular chain of one nanofibril. Considering the shorter distance (0.5−0.6 nm) between the two adjacent surface molecular chains on one fibril,31 the metal cations might crosslink the adjacent chains on the same fibril surface, shown in Scheme 1. These intrafibril interactions should provide stable

Table 3. Displaced Solvation Volume (in Å3) and Binding Energies (in kcal/mol) for the Molecular Models Shown in Figure 5 model

cation

intrafibril two-chain on one fibril surface

Na+ Ca2+ Zn2+ Cu2+ Na+ Ca2+ Zn2+ Cu2+ Al3+ Fe3+ Cu2+ Fe3+

interfibril four-chain from two fibrils

interfibril six-chain from three fibrils

3343

ΔVsolv/ atom

Ebind/ atom

−0.39 −0.49 −0.85 −0.98 −0.99 −1.22 −3.01 −2.79

−0.83 −1.46 −3.54 −4.44 −1.17 −1.94 −3.95 −4.81 −5.97 −5.60 −5.01 −12.03

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Figure 5. Molecular models of cellulose intrafibril (a) and interfibril bonding (b, c) due to intercalated cations. Green spheres are divalent, M2+ = Ca2+, Zn2+, and Cu2+. Purple spheres are Fe3+. Dashed red lines indicate Mn+−O bonds.

trivalent cations displacing more solvation volume as indicated from both the four- and six-chain interfibril models. Negative values of ΔVsolv indicate that the presence of cations draws adjacent fibrils together and prevents water molecules from solvating the CNF surface. The findings from molecular modeling provide strong support to our hypothesis that cation-induced networking of cellulose nanofibrils could occur between two or three fibrils, facilitating the gelation process and leading to improved mechanical properties. A good correlation is obtained between trends of CNF-Mn+ moduli and binding energies of cations to carboxylate groups constrained on fibril surfaces. Some deviation, particularly among the trivalent ions, may indicate unforeseen complexity of the systems being modeled. The trends in calculated binding energies for different metals follow the same order as the primary stability constants included in Table 2. Transition divalent cations have higher binding energies to carboxylate than does the alkaline earth metal Ca2+, and this difference probably arises from the types of bonds, for example, coordination-covalent versus ionic bonds. The stronger binding energies for trivalent cations than divalent cations can be attributed to multiple factors, such as stronger electrostatic attractions, stronger covalent interactions, and geometry of the cellulose surface. More extensive modeling of the CNF surface along with analysis of electrostatic and coordination-covalent interactions between cellulose ligands and cations will be presented elsewhere.

of cellulose ligand and cation energies from the total complex energy; binding energies were then normalized by the number of atoms to allow for comparison between models of different size. Negative binding energy indicates favorable binding of the cation to the fibril surface. The interaction of divalent cations with carboxylate groups from two adjacent chains on one fibril is presented in Figure 5a and is described as an intrafibril twochain cellulose surface model. The structure of cationscarboxylate complexes composed of four cellulose molecular chains from two separate fibrils was defined as a four-chain interfibril model (Figure 5b), and that composed of six chains from three separate fibrils was defined as a six-chain interfibril model (Figure 5c). Looking closely at the binding energies and displaced solvation volumes presented in Table 3, we found that both divalent and trivalent cations have higher binding energies than Na+, which is consistent with nearly complete replacement of Na+ in CNF dispersion with those cations during gelation as noted earlier. In the intrafibril model, divalent transition metals Cu2+ and Zn2+ have significantly higher Ebind/atom than that of the alkaline-earth cation Ca2+, following the order Cu2+ > Zn2+ > Ca2+. The same sequence was also observed in the four-chain interfibril model, with slightly higher Ebind/atom compared with the respective cations in the intrafibril model. This suggests that for divalent cations the interfibril interactions are slightly favored over intrafibril interactions leading to cross-linking between fibrils. In the case of trivalent cations, the binding energy data from both the four- and six-chain interfibril models indicates that trivalent cations have the ability to bind 2 or 3 fibrils more strongly than divalent cations, because trivalent cations can intercalate between COO− groups on adjacent chains as well as multiple COO− groups on opposite fibrils. Moreover, complexes with Fe3+ have a much higher binding energy in the six-chain interfibril model than the four-chain interfibril model, reflecting that trivalent cations have high tendency to cross-link three fibrils within this simplified model. In contrast, the divalent cation Cu2+ has similar values in both interfibril models. These binding interactions contribute to the mechanical properties of the gels, supporting the experimental data that trivalent cations have significantly higher moduli than divalent cations and have the potential for supporting a greater degree of networking. It is interesting to note that the values of ΔVsolv follow the same trend as the binding energies, with

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CONCLUSIONS In the present study, we have shown gelation of carboxylated cellulose nanofibrils and formation of interconnected porous networks by addition of divalent or trivalent cations to the nanofibril aqueous dispersion. The chemical compositions of the hydrogels were analyzed by FT-IR and ICP tests. The storage moduli of the CNF-Mn+ hydrogels follow the order of Fe3+ > Al3+ > Cu2+ > Zn2+ > Ca2+. The driving force of gelation was proposed to be a screening effect of the repulsive potentials on the fibril surfaces by strong metal-carboxylate bonding. The final structures and properties of the gels are primarily dictated by interfibril cation−carboxylate interactions. The binding energies of cations to carboxylate on fibrils calculated from molecular models support the hypothesis. Divalent transition cations have higher binding energies to carboxylate than the 3344

dx.doi.org/10.1021/bm400993f | Biomacromolecules 2013, 14, 3338−3345

Biomacromolecules

Article

alkaline earth metal Ca2+. The stronger binding energies for trivalent cations than divalent cations can be attributed to multiple factors. Our work demonstrates that cellulose hydrogels can be prepared directly from dispersion of cellulose nanofibril extracted from sustainable and renewable resources using a facile method and that the moduli of the hydrogels can be tuned by choice of cations. Because of CNF biocompatibility, biodegradability, and its ability to be cross-linked with multivalent metal cations, we foresee numerous potential applications of CNF-Mn+ hydrogels in biomedical and other fields. A few examples include drug delivery vehicle in pharmaceutical and encapsulations of biomolecular species. The sustainable resources and low-cost production add more advantages into applications over other types of biopolymer materials. The network structures of CNFMn+ derived from hydrogels could also act as high surface area scaffolds for loading functional molecules and particles such as catalysts, taking advantage of its high porosity, high surface area, and especially, durability in water and organic solvents. Additionally, we expect that ionic cross-links, although weak compared with covalent bonds, but more likely demonstrate recoverable energy dissipation that is necessary for fatigue resistance when these CNF networks are applied in polymer composites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 410-306-4398. Fax: 410-306-0676. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors would like to thank Dr. Alan Rudie and Mr. Richard Reiner at USDA Forest Product Laboratory (FPL) for courtesy of providing CNF dispersions. Dr. Joshua Orlicki and Dr. Randy Mrozek at Army Research Laboratory (ARL) and Dr. Alan Rudie at FPL are acknowledged for useful discussions. Mr. Joshua Steele at ARL is acknowledged for help in the supercritical CO2 drying process. K.W. acknowledges support as a Postdoctoral Fellow from the Oak Ridge Institute for Science and Education.

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