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Article Cite This: ACS Omega 2018, 3, 1564−1571
Decoupling Arrest Origins in Hydrogels of Cellulose Nanofibrils Hua-Neng Xu*,†,‡ and Ying-Hao Li‡ †
State Key Laboratory of Food Science and Technology and ‡School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu 214122, People’s Republic of China S Supporting Information *
ABSTRACT: Colloidal gels with various architectures and different types of interactions provide a unique opportunity to shed light on the interplay between microscopic structures and mechanical properties of soft glassy materials. Here, we prepare acetylated cellulose nanofibrils with 2 degrees of substitution and make a structural and rheological characterization of their hydrogels. Two-step yielding processes are observed in the shear experiments, which allow us to deduce more precise knowledge regarding localized structural changes of the fibrils. We separate the viscoelastic response into two contributions: the establishment of cross-linked clusters on a fibril level and the arrested phase separation on a cluster level. We hypothesize that with the addition of salt, the hydrogels exhibit different arrested states that are identified as unable to access the thermodynamic equilibrium. Our results highlight that the coexistence of gelation and glass transitions are experimentally recognized in the hydrogels, with a global gelation driven by a local glasslike arrest during spinodal decomposition.
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INTRODUCTION Cellulose nanoparticles (CNPs), primarily including cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs), have recently attracted increasing attention with regard to their unique structural and mechanical properties and their technological applications.1−3 CNPs can be isolated from many cellulose resources using different preparation methods. In comparison with CNCs, CNFs have a much larger aspect ratio and are viewed as flexible particles that can deform and entangle with each other. It was found that CNFs can form a viscoelastic hydrogel even at a low concentration of 0.125 wt % via the specific interactions that modulate fibril entanglement and bundling.4 The structural flexibility of CNFs makes them attractive building blocks for guided assembly in the hydrogel construction, which is crucial not only to give new insights for the design of CNF-based functional materials but also as model systems for studying arrested states in soft glassy materials that have been observed for different types of systems including granular media, molecular materials, and colloidal suspensions.5−11 CNFs usually possess flocculated nature because of the vast hydroxyl groups in them, which promote the formation of hydrogen bonds among the fibrils. Chemical or physical treatments are often performed to introduce strong anionic or cationic charge on the surface,12−15 which builds up electrostatic repulsion between the fibrils with the same charges and hinder hydrogen bonding and van der Waals interactions. The coexistence of repulsion and attraction controls the degree of entanglement and bundling of the fibrils. Thus, slow and arrested states in CNF gels can have multiple origins. © 2018 American Chemical Society
Rheological characterization has proven to be a promising approach for studying the underlying physical principals in CNF gels.16−21 In general, at least two types of CNF gels should be distinguished depending on their production methods: mechanical fibrillation without enzymatic pretreatment produces suspensions with flocculated structures, whereas mechanical fibrillation with chemical modification produces suspensions with better colloidal stability.16 CNFs are known to have a complex entangled network and the flocculation behavior under shear flow was studied in several works.22−24 During the rheological measurement, the suspension structure was monitored through a transparent outer cylinder with a digital camera. Karppinen et al.22 related different kinetics for the breaking and formation of fibril flocs to the origin of their rheological observation. The flocculated network structure was separated firstly into loose flocs and further into individual spherical flocs with increasing shear rate. At the same time, the flow curves displayed some sudden slope changes, which were related to inhomogeneous flow such as wall slip and shear banding.23 The flow instabilities occur due to depletion of the fibrils near geometry boundaries, which creates a lubrication layer. The flow instabilities will introduce errors and difficulties in the result interpretation. Hence, their influence should be minimized while performing the rheological measurements.24 Until now, most of the studies have focused on the determination of storage and loss moduli under small Received: December 1, 2017 Accepted: January 26, 2018 Published: February 7, 2018 1564
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Figure 1. Stability of 2 wt % hydrogels of acetylated CNFs with different degrees of substitution (CNF0, CNF1.32, and CNF2.01) at different NaCl concentrations (0, 10, and 100 mM) after preparation for 2 weeks. Note that the gels are macroscopically homogeneous over 2 weeks without phase separation, except for the occurrence of visible sedimentation for the CNF0 at high ionic strength.
Figure 2. TEM images of acetylated CNFs with different degrees of substitution (CNF0, CNF1.32, and CNF2.01). The CNF0 looks very straight and can organize into bundles, whereas the CNF1.32 and CNF2.01 form cross-linked clusters.
amplitude oscillatory shear and viscosity under shear flow.16 The highly complex and heterogeneous CNF networks make it difficult to identify the deformation modes and the dominating length scales. Recently, the importance of the contribution of nonlinear behavior under large amplitude oscillatory shear (LAOS), which is very sensitive to the formation of shearinduced microstructures, has been recognized in understanding the stress−strain relation of colloidal suspensions.25 The nature of the arrested dynamics transition can vary from bonded gels with heterogeneous fractal structures to caged glasses with homogeneous amorphous structures. In this context, the complex rheological properties of CNF gels may be rationalized in terms of their structure, inhomogeneous character, and local density. It is intriguing to consider whether the evolution of their structural and dynamical properties is fundamentally related based on a common set of physical processes (gelation and glass transitions). However, no attempt has been performed so far to test the relevance of the scenario for CNF gels, which have the potential to integrate the different experimental results of CNF systems. Here, we develop an efficient approach to design stable water-dispersible CNFs by surface acetylation combined with high-pressure homogenization. The structural variability of the networks is achieved not only by a significant change in fibril morphology but also by the extent of fibril entanglement driven by both electrostatic and hydrophobic interactions within and between the fibrils. We are interested in exploring a structure− mechanics relationship of the CNF gels using characterization techniques operating at different length scales such as rheology, confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). We probe the changes in internal structures and mechanical properties of the gels as a function of acetylation degree and salt concentration. We thereby obtain more precise knowledge regarding the change in hierarchical structures and
separate their contributions to the mechanical response of the gels. Our multiscale study sheds light on the role of dynamical arrest in the construction of the CNF gels, which is critical to provide new insights for the design of CNF biomaterials.
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RESULTS AND DISCUSSION
Above a critical gelation concentration, CNFs can entangle and bundle to form self-supporting networks with the ability to trap water, that is, to form hydrogels. The hydrogels prepared from the different acetylated CNFs (CNF0, CNF1.32, and CNF2.01) according to their degrees of substitution (DS) at a concentration of 2 wt % but increasing the NaCl concentration from 0 to 100 mM are presented in the Figure 1. The gels are macroscopically homogeneous over 2 weeks without phase separation, except for the occurrence of visible sedimentation for the CNF0 gel at high ionic strength. It is obvious that the concentration is large enough to enable the formation of a stable percolating network throughout the volume of the vial. The gels are observed to become less turbid with increasing DS. The different network stability of the CNF gels is expected to depend on the structural characteristics of the fibrils. We suggest that the long fibrils are kinetically trapped in a network structure before the sedimentation takes place. The examined CNFs dispersed in water exhibit a negative surface charge (Supporting Information, Table S1), which may be acquired because of preferential adsorption of hydroxyl ions.27 The coexistence of electrostatic repulsion and hydrophobic attraction controls the aggregate degree of the fibrils. The surface charge is screened, and the zeta potential decreases with increasing NaCl concentration, and hence, the electrostatic repulsion between the fibrils becomes less intense. The sedimentation of the CNF0 at high ionic strength may be initiated by an extremely high coacervation process. 1565
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assembly processes that occur at larger length scales. Figure 4 shows representative images of 2 wt % CNF networks using CLSM with varying NaCl concentrations from 0 to 100 mM. At 0 mM NaCl, it is more probable to form random cross-linking points between two fibrils, leading to small loose clusters with irregularity in their structure as evidenced in the CLSM images. With increasing NaCl concentration, more tightly bound clusters become visible in an otherwise largely heterogeneous network. It is apparent that the salt addition triggers an abrupt density fluctuation in the gels, leading to dense cluster regions by spinodal decomposition. The above structural changes certainly affect rheological properties of the CNF gels. The dependence of steady shear viscosity on the shear rate at 25 °C for 2 wt % CNF (CNF1.32 and CNF2.01) gels is shown in Figure 5. At 0 mM NaCl, the viscosities monotonically decrease as the shear rate increases over the whole investigated ranges, exhibiting a typical singleregion shear-thinning behavior (Figure 5a). The viscosities of the gels with the addition of NaCl are significantly different from those at 0 mM NaCl, with the occurrence of a small extent of shear thickening at a shear rate of about 10 s−1 (Figure 5b,c). The shear thickening may be attributed to a cluster formation with stress overshooting which produces increased association between the fibrils. Finally, at higher shear rates, the densified clusters are eroded and separated into individual fibrils. The oscillatory sweep measurements were performed, and the storage modulus (G′) and loss modulus (G″) are recorded as a function of strain amplitude (γ) or frequency ( f). Figure 6 shows the dependence of G′ and G″ over a wide range of strain amplitudes for the 2 wt % CNF (CNF1.32 and CNF2.01) gels. This strain sweep test allows us to probe the transition from linear to nonlinear viscoelastic behavior. At 0 mM NaCl, the G′ and G″ of the gels are approximately constant at low strains, and then, both moduli decrease rapidly with G′ dropping below G″ at high strains (Figure 6a), implying that the gels have been yielded to flowing liquids. Specifically, at sufficiently high stress amplitude, the gels exhibit a single and relatively smooth yielding transition from a linear viscoelastic regime. The decrease in moduli with increasing strain is likely due to the progressive breakdown of the entangled fibril network. The curves of strain sweeps for the CNF1.32 and CNF2.01 gels have a similar slope but are shifted to a lower elasticity and linear viscoelastic range with the increase of DS. Figure 6b shows viscoelastic behavior of the gels at 10 mM NaCl. The yielding transition is remarkably different from that at 0 mM NaCl, where broader nonlinear regimes and two shoulders in G″ could be clearly seen. The local maximum in G″ is related to some dissipation processes induced by internal rearrangements of the network structure under shear flow. The similar so-called two-step yielding behavior is also observed in a number of disordered systems such as colloidal gels and glasses, where the energy dissipation is generally identified by bond and cage breaking, respectively.28−32 The increase in both G′ and G″ with increasing NaCl concentration also indicates that screening of electrostatic repulsion between the fibrils can contribute to the formation of more dense gels. Further increasing NaCl concentration to 100 mM appears to have a negligible effect on the rheological properties, as evidenced by the similar moduli for the gels (Figure 6c). The G′ and G″ measured during the frequency sweep experiments in the linear domain are plotted in Figure 7. Consistent with the expected solidlike properties of the gels, we
To gain insight into how the acetylation can influence the fibril arrangement, TEM was employed to image local fibril morphology. As shown in Figure 2, very few single fibrils are found, and the fibrils always appear to be connected to neighboring counterparts. The micrographs indicate that the CNF0 can organize into very straight bundles with an average fibril diameter of 9.02 nm, whereas the CNF1.32 and CNF2.01 form cross-linked clusters with average fibril diameters of 7 and 4.34 nm, respectively. It seems that the CNF0 is strongly interacted and behaves effectively as a thicker unit with the fibrils glued together. The OH-group-mediated hydrogen bonds on the fibril surface may play a significant role in the lateral alignment along the long axis within the bundles. The decrease in the diameter of the fibrils with increasing DS may be responsible for reducing their stiffness and affecting their cross-linking density by association of the hydrophobic groups. The internal buildup of the fibrils was also analyzed by SAXS, and the log−log plots of scattered intensity (I) versus scattering vector (q) is shown in Figure 3. In the high q range, the
Figure 3. SAXS intensity profiles for 2 wt % hydrogels of acetylated CNFs. (a) CNF0, CNF1.32, and CNF2.01 at 0 mM NaCl. (b) CNF1.32 at different NaCl concentrations (0, 10, and 100 mM). The fractal dimension of the clusters is obtained from the slope of the plot.
obtained curves seemingly superimpose, but in the low q range, the scattering intensity decreases with increasing DS (Figure 3a). The fractal dimension of the clusters is obtained from the slope of the plot. The fractal dimensions are 1.73, 1.64, and 1.29 for CNF0, CNF1.32, and CNF2.01, respectively, which confirms that the propensity of the CNFs to form a bundle is decreased with increasing DS. Moreover, the SAXS curves are almost superimposed with increasing NaCl concentration (Figure 3b), showing that the networks are formed with the same building blocks. The result suggests that the formation of clusters at the nanoscale remains unaffected regardless of the NaCl concentration. It also highlights the importance of the 1566
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Figure 4. Comparison of pattern evolution observed in CLSM images for 2 wt % hydrogels of acetylated CNFs. (a) CNF1.32, 0 mM NaCl. (b) CNF1.32, 10 mM NaCl. (c) CNF1.32, 100 mM NaCl. (d) CNF2.01, 0 mM NaCl. (e) CNF2.01, 10 mM NaCl. (f) CNF2.01, 100 mM NaCl. The salt addition triggers an abrupt density fluctuation in the gels leading to dense cluster regions by spinodal decomposition.
find that G′ dominates over G″ in the measured frequency range. It is important to note that G′ and G″ are almost independent of frequency, and no crossover is observed for them. The CNF1.32 and CNF2.01 gels exhibit qualitatively similar relaxation spectra. We now turn to the dependence of the gel strength on the CNF concentration (c) investigated. The gel strength can be characterized by the elastic modulus plateau (G′0), measured during the strain sweep experiments in the linear domain, and a power-law scaling of G′0 with c is assessed to determine the changes in the gels formed as G0′ ≈ cn. Figure 8 shows the concentration dependence of G0′ for the CNF1.32 and CNF2.01 gels, respectively, at different NaCl concentrations. We find that at 0 mM NaCl the scaling exponent is 6.311 and 7.438 for the CNF1.32 and CNF2.01 gels, respectively. The values are considerably larger than those proposed by models for semiflexible biopolymers,33,34 which yield scaling exponents of 2−2.5. More recently, a new scaling theory with the powerlaw exponent of 11/3 to 7 was proposed for the CNF suspensions with increasing cellulose volume fraction, where the elastic energy of cellulose fibers is attributed to bending.35 One possible explanation for the discrepancies is that the different ability to form physically entangled structures and the different type of fibril entanglements may dictate the measured strength. The CNF2.01 gel is composed of more flexible fibrils that can entangle more readily than the stiffer fibrils present in the CNF1.32 gel, resulting in a higher entanglement density. However, at 10 and 100 mM NaCl, the scaling exponent is changed to 1.981 and 3.135 for the CNF1.32 and CNF2.01 gels, respectively. The different exponent shows further evidence for distinct gelation routes of the CNF gels with and without salt addition. In the presence of NaCl, the large clusters observed can act as a single building block contributing to the elasticity of the gels. The slow relaxation of coupled clusters contributes to the gel strength in different ways and leads probably to a lower exponent. Remarkably, the same scaling behavior is observed for the CNF gels, regardless the NaCl concentration. We are thus motivated to identify the link between the local structure and dynamical arrest with increasing DS and NaCl
concentration. Gelation requires the formation of a solidified space-spanning network, which can be interpreted as a consequence of the interplay between long-range repulsion and short-range attraction. Here, the interplay gives rise to the formation of finite-sized aggregates with defined morphology spanning the gap between nano- and microscale (Figures 2 and 4). The gels can be formed through cross-link percolation if the fractal clusters become sufficiently large to result in a percolated network. The salt addition is proposed to initiate gelation by screening of the repulsive charges on the fibrils and to dominate gel properties through entanglement and cross-linking.36 The increase in gel strength with increasing NaCl concentration is caused by the reduction in the Debye length and stronger screening of electrostatic repulsion between the fibrils, thereby making van der Waals interactions dominant forces to facilitate the promotion of interfibril association as well as the onset of gelation. Moreover, the salt addition can trigger an abrupt density fluctuation in the suspensions accompanied by the formation of locally dense clusters and voids by spinodal decomposition. The formation of the stable clusters as building blocks seems a precursor to the gelation. Moreover, it has been suggested that the LAOS behavior, which reflects both the intensity of mechanical constrains and the deformation length scale, can be effectively used as a tool for classifying the colloid gels and glasses.28−32 On the basis of the relaxation of density fluctuations as probed by rheology, it seems that two characteristic length scales directly emerge from this scenario: the first yield point is due to the breaking of bonds between interconnected clusters, while the second one is attributed to the breaking of clusters into smaller constituents. We hypothesize that in this situation the fibrils localized in clusters are load-bearing because of excluded volume effects, and the dense clusters confined to a finite volume are developed into an attractive glass, which can be formulated in terms of a cage where each cluster is trapped within the potential landscape set up by the neighboring clusters.37,38 Thus, the origin of the dynamical arrest can be qualitatively diverged into simultaneous development of a fractal structure of cross-linked fibrils from percolation and of a larger scale structure of domains resulting from spinodal decomposition (Figure 9). It should be noted 1567
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Figure 6. Storage modulus G′ (solid symbols) and loss modulus G″ (open symbols) as a function of strain amplitude for 2 wt % hydrogels of CNF1.32 and CNF2.01 at NaCl concentrations of (a) 0, (b) 10, and (c) 100 mM.
Figure 5. Steady shear viscosity as a function of shear rate from 0.1 to 100 s−1 for 2 wt % hydrogels of CNF1.32 and CNF2.01 at NaCl concentrations of (a) 0, (b) 10, and (c) 100 mM.
system provides a potential framework to understand and generalize the gelation and glass transition to elucidate the nature of the yielding of hydrogels.
that the continuity between the gelation and glass transition is recognized in the same experimental systems. This result is consistent with a very recent theoretical speculation that attractive particles may undergo a fluid to solid transition because of jamming.39 It demonstrated that a phase diagram can be used to unify the description of a wide variety of transitions from fluidlike to solidlike behavior by means of the so-called jamming transition.
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EXPERIMENTAL SECTION Preparation of Acetylated CNFs and Their Hydrogels. Cotton microcrystalline cellulose (Aldrich Chemistry, Shanghai, China) was dispersed into a mixture of acetic acid (1:10, w/ v) and sulfuric acid (5%, w/w cellulose), and then a desired amount of acetic anhydride was added to obtain different acetylated samples. The reaction mixture was stirred vigorously at 50 °C for 1 h. After acetylation, 10-fold deionized water was added, and the resulting sample was centrifuged at 4000 rpm for 10 min to remove excess acid. Then, the sample was collected and passed 10 times through a high-pressure homogenizer (AH-2010, ATS Engineering Inc., Germany) under value pressure of 1000 bar to obtain acetylated CNF samples. For further purification, the acetylated CNF was dialyzed with membranes against deionized water for several days until the pH of the water from successive washes stayed constant. The acetylation was confirmed by Fourier transform
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CONCLUSIONS The viscoelastic response of the CNF hydrogels can be separated into two contributions including the establishment of cross-linked clusters on a fibril level and the arrested spinodal decomposition on a cluster level. The arrested spinodal decomposition produces the gels composed of glassy clusters not commonly observed in soft matter systems. This implies that the gelation and glass transitions are dynamically coupled on different length scales, with dense attractive glasses as building blocks for the gel formation. Because gels and glasses are observed in the same experimental systems, they have been linked within unified pictures of jamming transition. The model 1568
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Figure 9. Schematic illustration of the link between the local structure and dynamical arrest for CNF gels. The gels can be formed through cross-link percolation and arrested spinodal decomposition. The coexistence of gelation and glass transitions are recognized with a global gelation driven by a local glasslike arrest during spinodal decomposition.
value of 3, which can be determined by a titration method.26 According to their degrees of substitution, three CNF samples are obtained as CNF0, CNF1.32, and CNF2.01, respectively. The CNF suspension was concentrated by immersing the dialysis tube in polyethylene glycol 20 000 and then diluted with deionized water in the concentration range of 0.5−2.6 wt %. For studies on the influence of salt addition, NaCl was added to the sample with different concentrations of 0, 10, and 100 mM. Characterization of Acetylated CNFs and Their Hydrogels. The morphology of the CNF samples was analyzed using a Philips Tecnai 12 transmission electron microscope at 120 kV acceleration voltage. Drops of the diluted CNF samples were deposited on a TEM grid and dried under ambient conditions. To improve the contrast, the acety-CNF suspension was negatively stained with phosphomolybdic acid. ImageJ software was used to measure the physical dimensions of the CNFs. The microstructure of the CNF samples was characterized by the NanoSTAR small angle X-ray scattering (Bruker-AXS, Germany). The samples were sealed in quartz capillaries, and a two-dimension charge-coupled detector (Vantec-2000, USA) was used to record the scattering intensity I. The distance of the sample detector was set to 1070 mm, and the X-ray wavelength λ = 1.54 Å, giving an accessible scattering vector qrange of 0.007−0.23 Å−1. Scattering data recorded from a cell filled with water were subtracted as background. The microstructure observation of the CNF samples was carried out with a CLSM system (Zeiss LSM 710, Carl Zeiss Inc., Braunschweig, Germany). For fluorescent staining, a drop of 1% calcofluor-white dissolved in acetone was added to 1 mL of the samples. The rheology measurements were performed on a stresscontrolled rotational rheometer (DHR-3, TA Instruments) using a plate−plate geometry (plate diameter of 40 mm and gap of 1 mm). Steady-shear experiments and large-amplitude oscillatory experiments were carried out at a constant temperature of 25 °C only for samples with gel-like properties. A steady-shear rate flow curve was obtained covering the range from 0.1 to 100 s−1. An oscillatory strain amplitude sweep was done in the strain range from 0.01 to 5000% at a fixed frequency of 1 Hz, and a frequency sweep was done over a range from 0.1 to 10 Hz at 0.2% strain (within the linear
Figure 7. Storage modulus G′ (solid symbols) and loss modulus G″ (open symbols) as a function of frequency for 2 wt % hydrogels of (a) CNF1.32 and (b) CNF2.01 at different NaCl concentrations (0, 10, and 100 mM).
Figure 8. G0′ as a function of CNF concentration (c) for 2 wt % hydrogels of (a) CNF1.32 and (b) CNF2.01 at different NaCl concentrations (0, 10, and 100 mM). Solid line is obtained using a power-law scaling of G′0 with c.
infrared spectroscopy with the characteristic vibrations of the grafted acetyl groups easily identified (Supporting Information, Figure S1). The DS defined as the average number of acetyl groups substituted on a glucose unit with a maximum possible 1569
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viscoelastic domain, as determined from the strain amplitude sweep). As a result, the mechanical spectra for storage modulus (G′) and loss modulus (G″) have been recorded to determine the viscoelastic properties of the gels.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01905. Characterization of the CNFs using FTIR spectra; distribution of hydrodynamic diameters; and hydrodynamic diameters, zeta potentials, and average widths of the CNF samples (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hua-Neng Xu: 0000-0002-1714-5105 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (no. 21576116). REFERENCES
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