Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils

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Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils Composite Hydrogels: Interfacial Dynamics, Energy Dissipation, and Damage Resistance Jun Yang, and Feng Xu Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b00730 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils Composite Hydrogels: Interfacial Dynamics, Energy Dissipation, and Damage Resistance Jun Yang*, Feng Xu Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, 100083, China *

Corresponding author: [email protected]

Tel: 86-10-62337223

ABSTRACT: Engineering reversible cross-links between nanoparticles and polymer matrix is a promising avenue to reinforce the mechanical properties of elastomers and in particular soft hydrogels. In this work, we study a model system of composite hydrogel reinforced with cellulose nanofibrils (CNFs), where the integration of reversible hydrogen bonds into a lightly covalently cross-linked polyacrylamide (PAAm) matrix. This approach yields the dual cross-linked networks with synergistically improved strength, modulus, and toughness. The reversible nature of the hydrogen-bonded cross-links manifests a strong strain rate (έ) dependent dynamics properties. The CNF-PAAm interaction among physically adsorbed chains on the surface of CNF is examined as a function of CNF fraction by sum frequency generation spectroscopy. The results indicate a decrease of the number of free -OH groups on the CNF surface. Moreover, the deformation-resting experiments show a unique interface stiffening mechanism where the polymer chains desorbed from the CNF surface under oscillatory shear become entangled during resting time. The bending micromechanics test reveals that the CNF interfacial slip imparts the capability to strengthen the composites during deformation. The fibril pull-out process activates a series of dissipation mechanisms that increase the crack propagation resistance. These findings advance our understanding the role of interfacial layer in microscopic reinforcement mechanism and provide a constitutive foundation for exploring the deformation behaviors of the cellulosic hydrogels.

1. INTRODUCTION As a typical soft material, hydrogels have shown promising potential in broad applications, such as adsorbents, soft actuators, and biomedical materials.1-3 However, the development of hydrogels as structural functional materials in practical fields is always hampered by their relatively weak mechanical strength, and a large body of research has been focused on improving their mechanical 1

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performance.4-9 Inspired by nature, biological structural materials (such as nacre, spider silk, bonds, and wood) have shown that the rigid reinforcement phase is connected to soft matrix and the remarkable combination of mechanical properties is achieved from the self-assembled structures at different scales. 10-12 In detail, one of critical strategies is to incorporate an efficient energy dissipation mechanism within the hierarchical structure that involving transient sacrificial bonds and hidden lengths.13,14 Indeed, a series of dynamic interactions (e.g. hydrogen bonds,15 metal-ligand complexes,16 hydrophobic associations,17 and host-guest complexes18) have been extensively pursued via facile molecularly engineering approach. Thus, achieving the high defect tolerance of hydrogel materials calls for a fundamental understanding of reversible cross-links mechanics and viscoelastic energy dissipation in these materials.19 One approach to enhance the energy dissipation in polymers, without compromising strength and toughness, is to incorporate nanoparticles into the polymer matrix.20,21 Cellulose nanofibrils (CNFs) are flexible elongated nanoparticles that are generally extracted from wood pulp and plants through mechanical disintegration in aqueous suspension.22,23 The large surface to volume ratio of CNF, rooted from their nanometer diameter, offers a great opportunity to dissipate energy in composites via interfacial interactions.24 Moreover, due to their remarkable intrinsic properties, such as high stiffness and modulus (up to 110 GPa in axial direction), these fibrils can effectively improve strength and stiffness of the composite materials.23 Even though the CNFs show fascinating potential to reinforce composites, this ability is primarily dependent on the specific mechanics of the filler-matrix interface. In general, the improvement of strength and toughness require load transfer between the filler and matrix in an efficient energy dissipation way along the interface.25 The classic shear lag model for traditional fiber-reinforced composites proposed by Cox26 has suggested that the improved energy dissipation of discontinuous fiber reinforced matrix is mainly induced by elastic mismatch between the filler and matrix. Learn from bioinspired strategy to mimic the unique natural structures of tough and strong properties, our and other groups recently attempted to fabricate CNF reinforced composite hydrogels,27-30 based on multiple transient bonds acting as reversible sacrificial bonds to dissipate energy. Since the strength of the CNF is relatively high and the failure of the gels generally occurs at either the CNF-polymer interface or in the bulk of the polymer matrix via fibril pull-out,27 we could specifically focus on the elucidating the role of energy dissipation by CNF pull out from the 2


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matrix. Indeed, previous studies on the cellulose composite hydrogels’ energy dissipation mechanisms mainly focus on the crack propagation along the soft phase (polymer matrix) interface due to the large discrepancy in mechanical stiffness between cellulose and matrix,28,29 in which the matrix around the fibrils act as the energy dissipation pathways and load distributers. It has been considered that the efficient energy dissipation and good interfacial adhesion are two critical aspects for achieving the synergistic reinforcement in strength and toughness of the composite gels.31 Although significant progress has been achieved in preparing strong cellulosic hydrogels,32,33 the underlying complexity of energy dissipation and interfacial dynamics that involve multiple relaxation dynamic scales are still largely unexplored. To address this challenge, the CNF reinforced polyacrylamide (PAAm) hydrogels are adopted as a model system in current work to explore the influence of interfacial hydrogen bonds as transient cross-links on significant viscoelastic properties of the gels by varying the strain rate in uniaxial deformation. We find that the incorporation of CNF displays a remarkable ability to simultaneously improve extensibility, stiffness, and strength to the dual network at a low volume fraction (0.0125-1%) of the composite gels. The time-dependent dynamics of CNF reinforced hydrogels as characterized by corresponding plateau modulus is also examined. The results indicate that the polymer matrix exhibits the plasticity with a maximum strain of 50% upon mechanical loading, and we attribute such a plastic deformation to the hydrogels hierarchical architecture, in which the polymer chains mediates the rearrangement of CNF in response to external stress. Overall, these findings deepen our understanding of engineering principles for interfacial dynamics and energy dissipation in composite hydrogels, and provide some new design guidelines for developing high performance cellulosic materials where high load-bearing capability is required.

2. EXPERIMETNAL SECTION Extraction of Cellulose Nanofibrils The native bleached softwood Kraft pulp (4 wt%) was mechanically stirred for 4 h at room temperature and then homogeneized using an Ultra-Turrax at 10,000 rpm for 2 h. This primarily refining process was applied to improve cell wall accessibility for the following mechanical shearing treatment. The treated pulp slurry (2.5 wt%) was pumped into a high pressure fluidizer with two different Z-shaped chamber pairs. Firstly, the slurry was 3


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passed into a chamber pair with a diameter of 400 and 200 µm for 14 passes at 120 MPa, then passed through a chamber pair with a diameter of 200 and 75 µm for 10 passes at 160 MPa. Finally, the collected CNF suspension was ultrasonicated at an ice-water bath for 20 min (300 W) and converted to the gel-like suspension with a solid content of 1.2 wt% (dimension distribution by TEM images in Supporting information Figure S1).

Hydrogels Preparation The composite hydrogels with dual cross-links were prepared by free radical polymerization of monomer acrylamide (AAm) with a concentration of 10 wt% at the presence of CNF, as described in our previous work.27 The homogeneously mixed aqueous solution (20 ml) of AAm, 0.25 mol% initiator (potassium persulfate) and 0.4 mol% cross-link (N,N’-methylenebisacrylamide, MBA, both feeding molar ratio relative to AAm concentration), together with CNF (0.02-1.6 wt% relative to aqueous solution) was degassed by N2 bubbling for 10 min and then injected into a PTFE mold under N2 atmosphere for 24 h at room temperature. The attained gels were immersed into excessive water for 4 days to reach the swelling equilibrium and remove the residual chemicals.

Characterization Mechanical tests Uniaxial tensile test was conducted using a Zwick Roell Testing Machine (Z005) with a silicon oil immersion clamp accessory to minimize water evaporation during deformation. The rectangular specimens (40 mm in length × 8 mm in width× 5 mm in thickness) were stretched in a broad range of rate (έ) from 1.5×10-4 to 0.5 s-1 at 25 ℃ with an initial gauge length of 30 mm. The nominal stress (σ) and nominal strain (ε) were calculated from the stretching force divided by the cross-section of the specimen (8 × 5 mm2) before deformation and the deformed length relative to the original length (30 mm), respectively. The Young’s modulus (E) was estimated by fitting the initial slope of the stress-strain curves within a strain range of 10-40%, and the toughness was calculated by the area under the curves until fracture point. For cyclic loading-unloading tests, the specimen was deformed at a constant rate, and the residual strain ratio was defined as (l-lo)/lo×100, where lo and l were the length of the specimen before and after stretching, respectively. For accuracy, four specimens for each sample were measured and the average values were provided. The nanoindentation was conducted using a Hysitron Triboscope 4


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nanomechanical testing system, where the specimen was mounted to a custom designed micromechanical tester. Upon loading, the fibril around the notch tip was stressed, and the polymer chains on its surface between the adjacent fibrils were stretched to form polymer strands. The bending measurement was conducted in the middle of the polymer strand using three-side pyramidal diamond indenter (Berkovich tip) with nominal radius of curvature of 20 nm to examine the deformation behavior of the polymer. During loading stage, the indenter approached the polymer strand at a constant velocity of 2 nN/s (loading), and the bending force was applied to the strand until the deflection attained the preset threshold, then withdrew the indenter at the same velocity (unloading).

Structural characterization The fracture specimens were coated with gold film (~ 15 nm thickness) before the fractography observation with a scanning electron microscopy (SEM, Hitachi S-3500) operating at 6 kV. The transmission electron microscope (TEM, JEOL 2010) worked at 200 kV were applied to perform in situ bending measurements on individual fibrils. The microtomed specimens (～ 100 nm) were randomly scattered onto the TEM grid covered with a thin carbon thin film. The electron beam from the microscope illuminated the thin film to induce shrinkage, applying bending force on the tangled fibrils. The small-angle neutron scattering (SANS) were performed at the National Institute of Standard and Technology Center for Neutron Research (NCNR), where the data were collected using neutrons with a wavelength of 6 Å and a detector distance of 30 m. The hydrogels were prepared by using D2O to enable adequate contrast between the hydrogen-rich gel and the deuterated solvent.

Rheological Measurements Rheology tests were conducted on a strain-controlled rheometer (TA AR2000) equipped with 25-mm diameter parallel plate geometry. The large amplitude oscillatory shear measurements were conducted at a frequency of 1 rad/s. The time dependence of the shear stiffening was performed at a fixing frequency and allowing the specimens rested with different waiting time (10-104 s), and the moduli were recorded with evolution of resting time under periodic strains (from 50-400%).

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3. RESULTS AND DISCUSSION The combination of transient (physical) cross-links into a permanent (covalent) network enables to effectively tune the network mechanical properties in both linear and nonlinear deformation regimes.19 In this work, we present the cross-link dynamics and energy dissipation of composite gels with dual networks, which contain a trace amount of covalent cross-links in PAAm matrix and interface dynamic hydrogen bonds between CNF and the matrix (Figure 1). One can note that the gels form an interconnected porous fibrillar network, which is similar to our previous metal-mediated gels but with a relatively low CNF fractions (0.21 vol%).27 We chose PAAm for polymeric matrix because it is known that PAAm can interact with the hydroxyl groups on CNF surface via hydrogen bonding.32,33 This attractive interaction favors physical adsorption of polymer chains to CNF surface, allowing us to explore the reinforcing mechanism at the interface. It is noted that the CNF concentration should be kept at a relatively low regime (0.02-1 vol%) to avoid coagulation before polymerization, which may impede the dispersion and stress transfer in the composites. Besides, we found that only a very weak gel can be attained without the involving of MBA, implying there was no generation of active sites that vinyl monomers can be covalently bonded on CNF backbones. This result is different from previous cellulose nanocrystals (CNCs) composite gels where the polyelectrolyte in situ covalent grafted from CNCs by sulfuric acid hydrolysis.34 For this dual network, the covalent cross-links serve to maintain the conformation of the network and impart it with elasticity. The transient hydrogen bonds between CNF and the matrix, on the other hand, contribute to the following mechanical properties: (1) enhance fracture resistance through reversible dissociation of hydrogen bonds, i.e., serve as sacrificial bonds to toughen the overall network; (2) improve self-recovery through dynamic hydrogen bonds, and (3) enhance energy dissipation through dynamic dissociation-reassociation of hydrogen bonds.

Figure 1. Schematic structure of dual cross-linked gels with covalent cross-links and reversible 6


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H-bonds, and the TEM images of the gels at different magnification scales (CNF volume fraction of 0.6 %), where uniformly dispersed CNFs entangle with the polymer chains at surface.

Interface dynamics The dynamic mechanical behaviors of the gels were firstly examined under low strain conditions by shear test (Figure 2(a, b)), and the oscillatory frequency were measured by plotting the storage moduli (G’) and loss moduli (G’’) versus frequency at a series of CNF volume fraction (φ= 0.0125-1, corresponding to 0.02-1.6 wt %). The result indicates that the plateau modulus Gp, defined as the storage modulus at infinite frequency, increases with increasing CNF volume fraction, implying the formation of rigid network. We further quantitatively examine the influence of φ on the hydrogels’ mechanical properties, and the results show that the Gp varies over several orders of magnitude over the moderate and high volume fraction range with a power law. Indeed, if the Gp is extracted by fitting a Maxwell model, one can note that a single modulus Gp describes the majority of the rheological properties, enabling us to consider the relaxation time and the corresponding modulus in a consistent method. Take a close look at the behavior of Gp above the threshold, we find a strong correlation between CNF fraction and Gp (Gp～φ2.1, Figure 2(c)), reminiscent of elastic regimes for an entangled polymer network.27 Although the understanding of the hydrogels’ transient bonding present in our system is still preliminary at this time, the experimental data are consistent with a collection of elastic chains in a semidilute solution connected by transient cross-links.20 Whereas for the low values of φ (＜ 0.08%), the dependence deviates from the curve with the moduli several orders of magnitude lower than those at higher φ. This sudden loss of rigidity may be related to the connectivity percolation threshold (φ*), below which the nanoparticle is too sparse to build a reinforcement-spanning structure.24

Figure 2. Linear rheological frequency sweep of (a) storage modulus and (b) loss modulus of the 7


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gels with different CNF volume fraction (φ), and (c) the plateau modulus displays a power-law dependence with φ above the CNF overlap volume fraction.

To elucidate the elastic behaviors of the gels under a strain amplitude that greater than the limit of the linear viscoelasticity, the gels were subjected to periodical sinusoidal strain (γ(t) = γ0 sin(ωt)) that increased from zero to γ0 and then decreased to zero in another half of the period time. It has been proposed that the interfacial transient bonds between nanoparticles and polymer chains can be broken and occur chain desorption once subjected to high strains, and these chains could readsorb when the stress is removed.20 One can note that when the specimens were sheared at a 200% strain with resting time increasing from 101 to 104 s between each deformation cycle, the storage modulus increased with CNF fraction as well as with extension of resting time (Figure 3(a)). This result reflects the increased adsorption and entanglement of free matrix chains after nonlinear shear deformation, leading to a denser adsorbed polymer layer and a strengthening effect at the interface.35 Accordingly, the loss factor (tan δ, G″/G′) decreased with resting time, displaying a more significant solid-like behavior (Figure 3(b)). While it should be emphasized that this deformation induced self-strengthening of the network only occurs in the chain relaxation process during resting stage, and the stiffening process is slowed down once the chain relaxation process is disrupted by employing an intermediate strain amplitude (Figure S2).

Figure 3. (a) Storage modulus and (b) loss tangent of gels with periodic resting time in nonlinear deformation regime (γo= 200%, frequency = 1 rad/s).

Furthermore, the shear-induced stiffening mechanism was examined as a function of strain amplitude, in which the specimens were subjected to increasing strain from 50-400 % at a fixing rest time (1200 s, sufficient time to observe stiffening) between each cycle. For the gels with up to 8


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φ of 1%, there appeared a critical strain amplitude around 200% where the plateau modulus was attained (Figure 4(a)). The tensile stress-strain curves of the gels after periodic shear demonstrate a remarkable increase in fracture strength (Figure 4(b, c)), which is consistent with the result of interfacial strengthening. Besides, the small-angle neutron scattering (SANS) experiments indicated no pronounced changes in the curves after deformation (Figure 4(d)), this lack of architecture divergence suggests the strain stiffening is related to the reptation of chains during rest time rather than particle aggregation.20 In fact, these relaxation modes facilitate the rearrangement of polymer chains around the CNF and conformal to CNF surface roughness, which avoids stress accumulation through mechanical interlocking during sliding (discuss below). Thus, we speculate that the PAAm chains realignment under the large strain amplitudes lead to the formation of a dense entangled layer at CNF surface, where the entanglement of bulk chains with surface adsorbed chains is enhanced under nonlinear deformation.36

Figure 4. (a) Normalized modulus of the gels (sample symbol the same as in Figure 3) as a function of strain, (b) elastic stress-strain curves of gels (φ= 0.6%, strain rate 0.1 s-1) after different resting time and (c) normalized fracture stress (σ) and Young’s modulus (E), and (d) SANS profiles of the gels before and after 1200 s resting time.

Previous work of dynamics of nanocomposites with attractive interfacial interactions has 9


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proposed that the enthalpy gained in favorable particle-polymer matrix interaction is balanced by the loss in entropy of the chain,20 leading to the formation of adsorbed polymer layers on particle surface. From this view, the attractive interaction of the adsorbed layer on CNF surface can potentially change the mechanical properties of the composites as shown in Figure 4(b). Such formation of denser interfacial layer effectively reduces stress concentration and promotes energy dissipation through plastic deformation in each fibril.37 Thus, the adsorbed polymer chain on the CNF surface serves as lubricant to mediate stress transfer, rendering the fibril a unique plastic deformation characteristic. To unveil the effect of electron beam irradiation on the adsorbed layer, 60-second irradiation was applied to bring knock-on damage on the selected interfacial area via in situ TEM observation (Figure 5). Previous studies have revealed that radiolysis may stimulate the atom moving in a short distance under low transferred energy (electron beam irradiation energy calculation in SI),38 and this short range chain diffusion helps to form a more thermodynamic stable phase.

Figure 5. In situ TEM observation of polymer coated fibrils (φ= 0.6%) with the increasing bending strain. The rightmost HRTEM image shows the parallel oriented fibrils.

The surface sensitive technique, vibrational sum frequency generation (SFG), a second-order nonlinear vibrational spectroscopy that is sensitive to the detection of –OH,39 was further 10


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performed to detect the change of CNF surface free –OH (detailed experimental protocol in SI). The adsorption at 3650 cm-1 represents non-interacting (free) –OH vibration at the CNF surface and the amplitudes for this characteristic band is scaled by estimating the number of density on the CNF surface (Figure 6(a)). Given a correlation between the examined SFG intensity and the group number density, the amplitude of –OH band evaluated from the fit of the spectra implies that the low number of the free –OH binding site on the surface of CNF corresponds to a high CNF fraction (Figure 6(b)). This systematic change in SFG spectroscopy indicates a significant increase in the number of interaction sites between the CNF and PAAm with increasing CNF fraction. Generally, the gels with a higher CNF fraction attract more polymer chains to pack on its surface, leading to the increase of entropic contribution associated with the increase in chain length.18 Indeed, for a higher filler fraction in a polymer matrix, the surface adsorbed chains tend to consist of small loops,40 and this type of interfacial conformation is consistent with our results where an increase in number of the hydrogen bonds between CNF and PAAm with increasing CNF fraction. Besides, these small loops can spread over neighboring anchor sites and essentially screen other occupied binds sites, yielding a decrease in the number of free-OH observed from the SFG spectra. Thus, polymer chains occupying more binding sites should be predominately packed tightly and it is consistent with the result of interface region stiffening mechanism in composites relative to the pristine matrix.20 In addition, the SFG result demonstrates that the number of hydrogen bond in interfacial layer is related to the CNF fraction and the tightly tailed chains on CNF surface remain in the hydrated state well above Tg. It has been proposed that the chains trapped on the CNF surface may exist in two states: in non-equilibrium state that requires a long time to relax or in equilibrium state that the loss in entropy via chain packing is compensated by the gain from enthalpy due to the attractive interfacial interaction.27 Although it is immature to determine the specific situation in this work, the chains should pack much easier on the surface at a high CNF fraction. Therefore, these anchoring sites that are not occupied at the initial adsorption process would be occupied by the chains that diffuse from the bulk during following stress deformation, leading to a slightly higher polymer density near the interface than the bulk due to the attractive interactions.20

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Figure 6. (a) Typical SFG spectra for gels collected from quartz substrate and their fits (solid curves), and (b) correlation between amplitude of the free hydrogen group stretching mode with CNF fraction in polymer matrix.

Since the CNF-polymer chain interfacial dynamics is related to the conformation of the chains around the CNF surface,18 we continued to explore the deformation induced denser interfacial layer via AFM technique in tapping model, where a bend test in the middle of polymer strands was performed between two adjacent fibrils. From the typical force-deflection curves of the strand (Figure 7), one can note that the indenter loading stage contains two distinct segments: the initial segment is relatively smooth, reflecting the elastic deformation of the polymer chains under the stress from tip, whereas the second segment appears rather serrated behavior, manifests inelastic behavior. Besides, the slope of the second segment (75.1 nN/nm) is apparently higher than that in first segment (24.5 nN/nm), denoting the increased stiffness of the chains with increased loading. This kind of deformation induced stiffening mechanism is consistent with the above oscillatory shear experiment, implying more free chains become entangled with CNF and form denser adsorbed polymer layer. In addition, the unloading curve did not overlap with the loading one, which suggests the plastic deformation of the chains and significant energy dissipation during bending stage.27

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Figure 7. Typical bending force-deflection curves of gels (φ= 0.6%).

Energy dissipation Uniaxial tensile tests were performed to investigate the nonlinear and viscoelastic behavior of the dual networks. It is interesting to find that a small amount of CNF (φ= 0.0125 %) and a trace amount of covalent cross-link (0.4 mol% of monomer), along with an overall mass concentration of only 10 wt%, could collectively contribute the dual network with remarkable stretchability and strength. The typical stress-strain curves of the gels in Figure 8(a) show that the composite gels (φ=1%) with dual cross-linking network exhibit high toughness (~ 47 kJ/m3) and Young’s modulus (0.13 MPa) at a strain rate of 0.5 s-1, which is related to the presence of reversible H bonds between rigid CNF and flexible PAAm. As schemed in Figure 1, at the initial deformation, both H-bonds and chemical bonds are largely intact and their total amount determines the Young’s modulus. With further increase of deformation, the H-bonds gradually dissociate and reassociate with neighboring regime to relax stress, while the covalent cross-links remain strained. In the cyclic loading-unloading test within a small strain (ε=50 %), the hydrogels exhibited significant hysteresis and gradually recovered the original dimension with a little residual strain (~ 4 %) after unloading (Figure 8(b)). This large hysteresis loop is related to the energy dissipation through the dissociation of H-bonds. After resting in silicone oil for 5 min at 25 ℃, the gels recover their initial state as evidenced by the almost overlapped loading-unloading profiles. Moreover, this recovery process can be accelerated as the specimens were annealed at 35 ℃ for 1 min, which is consistent with the higher temperature facilitates the dissociation of H-bond.27 The curves in Figure 8(c) show the viscoelastic properties of the gels that stretched to a larger strain of 500 % 13


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with more remarkable hysteresis loops in the following unloading process, which validate the elasticity of covalent network and reversibility of interfacial H-bonded cross-links between CNF and PAAm.37 Together, these results indicate that during unloading process, the stress stored in the strained covalent PAAm network is released and promote the transient H-bond to recover the initial stress-free conformation. Thus, the both types of cross-links act distinct roles in mechanical properties, where the covalent cross-links maintain the elasticity and integrity of the network at large deformation and reversible H-bond dissociation-reassociation survives external stress at a high level and provides toughness thereby.

Figure 8. Mechanical properties of the gels. (a) Uniaxial tensile stress-strain curves up to fracture obtained at different CNF fractions (έ = 0.5 s-1), (b) loading-unloading behaviors recovery at elevated temperature at the έ of 0.5 s-1, (c) loading-unloading cycles at different έ ranging from 0.00015 to 0.5 s-1 (εmax=500%), and (d) effect of strain rate on hysteresis (gels with CNF fraction of 0.6 % were tested in the repeated cycles).

It has been well recognized that (1) the combination of a loose covalent network and a dense physical network enable the dual network to dissipate significant amount of energy upon 14


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deformation,19 and (2) the dissipative property can be characterized by hysteresis loop between the loading-unloading curves, where the loop area gives the energy dissipated per unit volume.37 Hence, it is reasonable to anticipate that the reversible dissociation of H-bonds in the gels exhibit strong dependence on the strain rate (έ). The loading-unloading curves measured as a function of έ from 1.5×10-4 to 0.5 s-1 are exhibited in Figure 8(c), and the results indicate that the hysteresis loop, directly related to the reversible dissociation of H-bond, exhibits significantly strain rate dependence and increases with strain rate at a power relation with a exponent of 0.7 (Figure 8(d)). Cyclic tensile test were further systematically conducted to investigate the self-recovery on account of dynamic feature of hydrogen bonds. The loading-unloading cycles at different strains in Figure 9(a) illustrate quasiplastic deformation curves and the pronounced hysteresis loop, for which the hydrogen bonds play a critical role. This result suggests that the deformation induced rupture of hydrogen bonds acting as sacrificial bonds contribute to the effective energy dissipation, similar to other reported high-toughness hydrogels.4-6 During the unloading process the residual strain gradually decreases with time (Figure 9(b)), which is ascribed to the elasticity arising from the covalent cross-link. A more than sixty-percent hysteresis ratio was noted after 2 min, and the remaining 30% hysteresis gradually recovered in the following 8 min at a much slower rate (Figure 9(c)). This two-stage recovery process is reminiscent of previous polyampholyte gels,41 in which the covalent cross-links dominate the first stage elastic retraction and lead to fast recovery. When the residual strain decreases to a critical value, the re-arrangement of the interfacial chains under the elastic contraction promotes the complete recovery. In fact, this two-stage recovery process can be further validated by the even faster self-recovery at the higher MBA concentration, where no significant difference in hysteresis as a function of resting time between two consecutive cyclic test (Figure S4).

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Figure 9. (a) Loading-unloading cycles with varying strains, (b) recovery behavior by consecutive cyclic tests at a maximum strain of 550%, and (c) effect of resting time on hysteresis ratio and residual strain (gels with CNF fraction of 0.6 % were tested in the repeated cycles at έ of 0.5 s-1). Damage resistance SEM observation was performed to examine the morphology of the hydrogels, and the results indicate that the freeze-dried specimens exist a highly inter-connected porous network with homogeneously dispersed fibrils. Besides, many strip structures that protruding from the fracture surface are observed (Figure 10(a)). This strip structure consists of short fibrils bridged at the tip and coated with PAAm at the bottom (Figure 10(b)), implying a good adhesion between CNF and matrix. The cross-sectional view of fibrils embedded in the matrix exhibits that the circumference of the CNF is noted in close contact with the matrix and no gaps observed at nanometer resolution. In addition, the cross section along the longitudinal direction of CNF in Figure 10(c) show a dimension change in fibril diameter, which is ascribed to the local atomic defects and non-uniformity. In fact, this local non-uniformity along the CNF, including varying diameter and kinks at places as a result of local atomic defects, leads to surface roughness present on CNF and contributes to the interfacial adhesion by mechanical interlocking.42

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Figure 10. Fracture morphology of the CNF composite gels (φ= 0.1%). (a) Cross-section morphology of the gels with interconnected porous structure, and (b) local enlarged region shows the CNF pulled into the fibrils at the interface along the stretching direction (marked by arrows). (c) The rod-like CNFs show the non-uniformity in diameter dimension. (d) The filamentous fibrils are stretched at the crack tip region and bridge the surrounding matrix. (e) Cracks occur at the CNF-matrix interface where CNFs pull-out upon crack propagation. (f) The composite gels show a core-shell structure where the dark aggregates (CNFs) are tightly covered by light gray sections (matrix) and allow for an efficient stress transfer.

As shown of fracture surface in Figure 10(a, b), a large number of microscopic inelastic deformation was noted along the stress direction, including deformation twinning, nano-/microcracking, and fragmentation. Under high loading stress, the composite gels are able to form a series of microcrack surrounding the damage zone while maintain the overall network integrity, i.e., this formation of interconnected network microcracks is ascribed to the crack deflection by connection centers.33 Besides, since the propagation of crack need to fracture the CNF-polymer interface through crossing the connected bridges, local stress field can be generated around the vicinity of the connection centers. In addition, a bundle of CNFs being pulled out from the matrix is shown in Figure 10(d, e), and the individual fibrils are aligned along the loading direction, implying the reinforcing effect for the matrix. Close examination of these CNFs by 17


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TEM revealed that their surface are coated by PAAm (Figure 10 (f)), suggesting good wetting of CNF by PAAm and the surface energy favor CNF-PAAm contact. In fact, this ‘fiber pull-out’ has been identified as one of critical mechanisms responsible for the high toughness of fiber reinforced composites.42 Previously experimental analysis based on shear force transfer from a continuous soft polymer matrix to the rigid CNF indicates that the aspect ratio of the CNF acts a key role in determining the strength and toughness of the composites, where the larger aspect ratios promote efficient stress transfer to CNF and thus increase the strength.44

4. CONCLUSIONS The CNF-PAAm composite hydrogels with a loose covalent PAAm network and a transient interfacial H-bond network between CNF-PEG were synthesized, and the viscoelastic properties of the hydrogels under periodic deformation condition were examined. The results indicate that due to the bonding-debonding of chains on CNF surface, the interfacial shear stress is a fundamental parameter controlling the process of stress transfer and hence mechanical properties of the hydrogels. The cyclic large strain amplitude shear leads to a dense adsorbed polymer chain layer at CNF surface due to the attractive interfacial interactions and becomes stiffened during resting stage, during which the relaxation of chains through reversible adsorption-desorption delineates the dynamic strengthening mechanism. The SFG vibrational spectroscopy indicates that the number of free –OH on CNF surface is found to decrease with increasing CNF fraction, providing direct evidence that interfacial interactions influence the chain conformation and dominate the reinforcement thereby. The electron microscopy demonstrates that the pull-out fibril ends are covered by polymer chains, suggesting a strong interfacial adhesion between CNF and the matrix. The dislocation-like connection centers within the hierarchical composites gels are able to activate a series of distinct energy dissipation mechanisms. These findings on the energy dissipation and interfacial dynamics in CNF composite gels provide a guideline toward the design of hydrogels with excellent fracture resistance capacity.

Supporting Information TEM image of CNF and its dimension distribution, storage moduli of the gels subject to small strains during resting time, energy of electron beam irradiation of the specimens, SFG experiment 18


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protocol, and cyclic tests of the gels at a higher chemical cross-linking concentration. ACKNOWLEDGMENTS: This work is financially supported by Fundamental Research Funds for the Central Universities (2017ZY35), and National Natural Science Foundation of China (21404011, 21674013).

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Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils Composite Hydrogels: Interfacial Dynamics, Energy Dissipation, and Damage Resistance Jun Yang*, Feng Xu

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Synergistic Reinforcing Mechanisms in Cellulose Nanofibrils

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