Design of Cellulose Nanocrystals Template-Assisted Composite

Feb 13, 2015 - Hydrogels: Insights from Static to Dynamic Alignment. Jun Yang,* Xue-Ming ... weight and recover from >85% tensile compression. Analysi...
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Design of Cellulose Nanocrystals Template-Assisted Composite Hydrogels: Insights from Static to Dynamic Alignment Jun Yang,* Xue-Ming Zhang, and Feng Xu Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, 100083, China S Supporting Information *

ABSTRACT: The incorporation of nanoparticles into polymer represents a fundamental pathway in the development of reinforced composites. Inspired by rigid cellulose nanocrystals (CNCs), a simple and robust approach for the fabrication of tough and stretchable hydrogels was proposed that based on the incorporation of a three-dimensional CNC template. By mimicking the hierarchical cork structure, the obtained composite hydrogels with a binary network can sustain structural integrity under a load of 12 000-time of the template weight and recover from >85% tensile compression. Analysis of toughening mechanisms reveals two strategies, template pullout and polymer chain sacrificial bonds, that synergistically corroborate the materials integrity to large deformations. First, the embedded CNC template splayed among neighboring fibrils and bridged crack continuously, which entailed the efficient energy dissipation via strong hydrogen interactions between packed CNC walls without comprising the template integrity. Second, the hydrophobic coiled micelles unraveled and acted as sacrificial bonds to dissipate enormous amount of energy upon being stretched. This versatile approach opens up a new opportunity to highlight the multifunctionality of CNC as an indicator for designing of advanced cellulosic nanocomposites with improved mechanical properties from incompatible components.

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INTRODUCTION In the field of functional materials preparation, the improved mechanical performance and versatility are pursued and often be attained through hybrid structures. Gratifying, nature has found a unique procedure to develop a series of strong and high-performance composites with exceptional multifunctionality via synergistically combining models.1 For example, seashell nacre with hierarchical structure and precise inorganic−organic interface serves as an example for its super toughness and hardness that complemented by aragonite platelets and a small fraction of biopolymers (typically chitin and silk protein in the range of 1−5 wt %).2−4 Currently, it is considered that the improved toughness of these materials partly stems from the rapture of sacrificial bonds in the coiled organic domains, allowing the hidden segment to dissipate significant amount of energy and maintain the backbone integrity to a large strain.3,4 Continuing effect has been witnessed to duplicate the natural composites and various nanoparticles including silicon,5 clay,6,7 carbon nanotube,8,9 and graphene10,11 are applied as fillers to disperse into polymer matrixes for the pursuit of artificial composites with high toughness and stiffness. Collectively, the combining of a hard phase as reinforcing material with soft phase in natural robust composites unravels the energy dissipating and crack deflecting principle, which ignites the inspiration for the optimizing mechanical properties of composites through synergistic interactions among building blocks.1,12 © XXXX American Chemical Society

Hydrogels as a typical soft-and-wet elastomer with the threedimensional polymer network attract a great deal of research interest due to their promising properties in tissue engineering and consumer products.13,14 While most synthetic hydrogels suffer from notorious brittleness and low toughness that seriously limit their load-bearing applications, enhancing the mechanical properties of hydrogels has been a pivotal topic in the field of gel science.15,16 Recent progress manifests significant improvements in flexibility, toughness, and recoverability through intricate molecular arrangements with finely customized and multifaceted characteristics, such as slide-ring hydrogels,17 tetra-PEG hydrogels,18 nanocomposite (NC) hydrogels,19 double-network hydrogels (DN hydrogels),20,21 and macromolecular microsphere composite (MMC) hydrogels.22 Among these efforts, tetra-PEG and slide-ring hydrogels are advanced to possess homogeneous network structures where stress can be distributed at a maximum extent. The DN hydrogels developed by Gong et al. consisting of two crosslinked networks with asymmetric conformation have demonstrated improved strength and toughness, where the first network acted as sacrificial phase to dissipate energy. Other well-known strategies, including NC, MMC, as well as inorganic nanocomposites hydrogels (e.g., silicate,23 oxide Received: December 30, 2014 Revised: January 29, 2015

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Macromolecules graphene,24 and hematite25) are designed to increase multifunctional cross-linkers within the networks. Recently, Suo group reported a partially recoverable hybrid hydrogels with high elongation and fracture energy via forming interpenetrated ionically cross-linked Ca2+−alginate and covalently cross-linked polyacrylamide.26 Notably, according to the generalized route for the design of strong hydrogels,15 the efficient dissipation of stress energy upon crack propagation is a critical principle, where the gels achieve re-establishment process through dissociation of dynamic and reversible interactions and potentially adsorb a substantial amount of fracture energy around process zone through a variety of integrated energy dissipation mechanisms. Cellulose is a polydisperse linear polymer of β-(1,4)-Dglucose and one of the most promising features of cellulose is its repeated crystalline hierarchical structure which in turn dictates the mechanical properties of cellulosic composites.27,28 The remaining crystalline domains of cellulose nanocrystals (CNCs) after the isolation of noncrystalline or paracrystalline region exhibit exceptional high strength (up to 140 GPa in modulus).29,30 Since the first report of using cellulose nanowhiskers as reinforcement phase by Favier,31,32 cellulosic materials have received increasing interest, in addition to being wide availability and environmental compatibility. In previous studies,33−35 we have demonstrated that strong physical crosslinking stemming from CNC-matrix hydrogen bondings as well as the entanglements between polymer chains and network could significantly increase the mechanical properties based on the concept of constitutional dynamic chemistry.36 To date, considerable effort has been devoted to maximize the reinforcing efficiency of cellulose and its derivatives in polymeric matrix. For example, the surface PEG-grafted cellulose nanofibrils exhibited the achievement of nanodispersion states in the poly(L-lactide) (PLLA) and polystyrene (PS), where the modified CNCs acted as multifunctional crosslinks to anchor polymer chains and remarkably improved the mechanical properties therey.37,38 Alternatively, pristine CNCs prepared by sulfuric acid hydrolysis were introduced into polyurethane (PU) to synthesize strong elastomers through creating strong filler−matrix interfacial interaction.39 Moreover, Fox et al. reported the supermolecular nanocomposites can afford homogeneously dispersed films and largely improved mechanical properties with CNCs less than 10 wt %.40 However, it is noteworthy that in spite of these efforts, some fundamental issues of CNC mechanical reinforcements are still challenging. For example, to optimize its dispersion and the interfacial adhesion, surface chemical modification is generally required before the full benefit of stress transfer from yielding phase to hard reinforcing phase can be achieved.41 Besides, even though the reinforcement of cellulosic composites has been widely documented, the subtle micromechanics involved mechanisms that demystify the fracture dissipating process for improved strength have been much less interpreted in the literature.35,42 In this article, we aim to report a new approach to construct the strong hydrogels with recoverability by introducing CNC as a template that (1) departs from the general surfactant-mediated or surface functionalized selfaggregation processes, and (2) takes the advantage of sacrificial bond rupture to dissipate enormous amount of energy via unraveling of the coiled polymer chains. We hypothesize that under the proper conditions in such fibrillar template reinforced hydrogels, the sacrificial bonds and hidden polymer lengths are expected to be overcome without damage to the

integrity of the template. Overall, the present work is expected to (1) expand upon the notion of multiscale strategy for the design of strong hydrogels by evaluating mechanical properties across a range of hysteresis behaviors, and (2) highlight the role of sacrificial bonds in suppressing catastrophic crack growth near the crack tips and facilitating stress distribution to a large volume.

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EXPERIMENTAL SECTION

Materials. A never-dried native bleached pulp (poplar wood) obtained from Donghua Pulp Factory, China, was used as a source of cellulose. Pluronic F127 (PEO99−PPO65−PEO99) and photoinitiator Irgacure 2595 were purchased from Sigma-Aldrich. The sulfuric acid, acryloyl chloride, dichloromethane, and tetrahydrofuran (THF) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All chemicals were used as received without further purification and deionized water was used in all of the experiments. Extraction of Cellulose Nanocrystals (CNCs) from Pulp Fiber. The CNCs were prepared by sulfuric acid hydrolysis of native pulp fiber according to previous literature with minor modifications.35 Briefly, the fiber was milled and extracted with 4 wt % NaOH solution (10 g fibers for 1 L solution) for 4 h at 60 °C to remove residual additives and rinse with water. Then H2SO4 hydrolysis (50 wt %) was performed at 55 °C for 2 h under mechanical stirring (10 g fibers for 0.5 L acid solution). The resulting suspension was cooled in an ice bath and washed until neutral pH by successive centrifugations. Finally, the suspension was sonicated and few drops of chloroform were added to avoid degradation. The concentration of CNC in the final dispersion was measured gravimetrically to be ∼25 mg/mL and the anionic charge (−OSO3−) density was determined (85 mmol OSO3−/kg) by conductometric titration with 0.01 M NaOH. Preparation of CNC Organic Gels. The CNC organic gels were fabricated from aqueous CNC suspension via a solvent-exchange process where gelation was induced by addition of water miscible organic solvent into the CNC aqueous suspension. For example, the aqueous CNC suspension (100 mL, 25 mg/mL) was sonicated for 30 min in an ice bath, then THF (500 mL) was gently added into the above CNC aqueous suspension. The mixture was left overnight without stirring. The supernatant organic layer was exchange 2−3 times daily until the bottom cellulose assembled into a coherent nanofiber organic gel in THF. Synthesis of Pluronic F127 Diacrylate. The acrylate endfunctionalized Pluronic F127 were synthesized according to the well established procedures.43 Briefly, 10 g of Pluronic F127 was dissolved in 100 mL of anhydrous dichloromethane in a three-neck flask under nitrogen atmosphere, then 8-fold molar excess triethylamine and acryloyl chloride were sequentially dropped into the flask. Then mixture was stirred at 25 °C for 24 h under nitrogen, and filtrated to remove the precipitates (triethylammonium chloride). The Pluronic F127 diacrylate was precipitated by adding the above filtrate into excess anhydrous diethyl ether, and dried under vacuum at 25 °C for 24 h. Preparation of CNC Template-Assisted Hydrogels. The desired amount of CNC gels and Pluronic F127 diacrylate were added in THF to form a translucent suspension (15 min was adequate for two-phase diffusion and homogeneously mixing). The photoinitiator Irgacure 2595 (0.2% w/v) was then added to make a precursor suspension, mixed and exposed to ultraviolet (UV) light with a wavelength 365 nm for 40 min. For tensile and compressive tests, the hydrogel samples were synthesized inside rectangular and cylindrical silicone mold (squeezed between two glass coverslips), respectively. The cured hybrid gels were removed from the mold and washed with excess THF three times to purified unreacted materials. The obtained gels were immersed in water, which was replaced several times until swelling equilibrium was reached. The weight fractions of all the composition were determined from the starting composition of Pluronic F127 diacrylate and cellulose, and the details of the reactants are listed in Table 1. To prepare freeze-dried samples, the purified hydrogels were transferred into liquid nitrogen for 10 min and freezeB

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an accuracy of 0.1 mg. Swelling ratio (Q) was defined by Q = Ws/Wd, where Ws and Wd were the weight of swollen sample and that of corresponding dried sample, respectively. Scanning electron microscopy (SEM) observation was conducted using a Hitachi S-3500 at an accelerating voltage of 5 kV and a working distance of 6.5 mm, where the cryogenically fractured surfaces of the composites were stacked on the sample holder using a carbon pad and coated with gold for 30 s. Thin slice (∼100 nm) of the freeze-dried samples were cut using an ultramicrotome and deposited on a copper grid with a carbon-hole film, then observed for transmission electron microscopy (TEM) analysis using a JEM 1010 (JEOL) at 80 kV.

Table 1. Compositions of Reactant for the Hydrogels sample codea

F127 diacrylate (g)

irgacure 2959 (mg)

CNC (g)

H2O (mL)

CNCmF F

2.5 2.5

5.0 5.0

0.046−6.86 −

10 10

a

m stands for the CNC template volume fraction in hydrogels at the hydrated state.

dried at −50 °C under a vacuum of 0.1 Pa for 36 h to remove the water molecules. For the cross-section observation, the freeze-dried samples were immersed into liquid nitrogen for 5 min and mechanically fractured before sticking to the sample holder. Characterization. Mechanical tests were conducted at 25 °C using a Zwick Z005 Universal Testing Machine equipped with a 5000 N load cell. The samples for tensile measurements were rectangular in shape (30 mm in length ×8 mm in width ×4 mm in thickness) and distance between two clamps was 20 mm at a crosshead speed of 40 mm/min. For compression tests, the cylindrical samples with dimensions of 20 mm in diameter ×45 mm in height were measured at a strain rate of 15% min−1. It is noted that water was sprayed on the surface of hydrogels to prevent water evaporation. The compressive strength was defined by the stress at deformation of 85%. Raw date were recorded as force versus displacement and converted to stress versus strain with respect to the initial sample dimensions. For accuracy and repeatability, five parallel specimens were applied and the averages were obtained with standard deviation for each data. The rheological tests were conducted by using a TA AR2000 rheometer in the frequency sweep mode with the parallel plate setup. Swelling experiments were performed by immersing as-prepared cubic samples (1 cm × 1 cm × 1 cm) in large amount of water at 25 °C. The weight of swollen hydrogels at specific times was determined with a balance at

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RESULTS AND DISCUSSION The processing scheme of fibrillar template strategy is shown in Figure 1 where native CNCs build an interconnected network through a self-aggregation process, then the template was immersed into precursor aqueous solution with a polymer of choice to perform the consequent in situ polymerization and fabricated the CNC template-assisted composite hydrogels. The first procedure is the preparation of a nanofibrillated template through the sol−gel process, where sulfuric acid hydrolysis obtained CNCs formed homogeneous suspension (TEM image of CNCs in Supporting Information, Figure S1) and transferred to gel via a water-miscible tetrahydrofuran (THF) solvent exchange process. Interestingly, CNCs in this form of interconnected cellulose fibrils display gel-like behavior in THF suspension even at the content as low as 5 wt % (Figure S2). The CNC content in the final composite hydrogels can be tailored by the concentration of initial CNC aqueous suspension and the volume of solvent (typically 0.2−

Figure 1. CNC template-assisted composite hydrogels. (a) Self-aggregation of gelled fibrillar template via solvent exchange procedure (not to scale). (b) Schematic illustration of the synthesis of Pluronic F127 diacrylate. (c) Synthesis and swelling of the nanocomposite hydrogels. C

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Macromolecules 30% discussed here). To explore the microstructure and properties of CNC gel that derived from solvent exchange, it was converted to a cellulose aerogel by supercritical CO2 (Figure 2a). Previous studies have demonstrated that as the

On the microscale, each CNC makes a high aspect ratio consecutive skeleton and contributes to the flexibility. Since the stiff and straight CNCs are prone to form hydrogen attractions as a result of their parallel conformations,44,46 the obtained CNC template is found to be stable and could retain its structural integrity. Upon immersing into toluene as a representative solvent, prolonged ultrasonication (>4 h) did not dismantle the templates. Besides, the template sustains large strain deformations and resists structural fatigue under cyclic compressive conditions in air. Uniaxial compressive measurements at different densities (ρ = 8.2, 10.5, 12.1, 14.6 mg/cm3) exhibit reproducible results where the curves include an initial linear elastic region at ε < 30% and a plateau with gradually increasing slope until strains up to 60% (Figure 3a). The large compression on the template stems from squeezing of internanofiber pores, and the SEM images do not reveal much morphological difference between the initial and recovered CNC templates. Moreover, the hysteresis loop in loading−unloading curves reveals a notable energy dissipation due to the friction between air and CNC skeleton (Figure 3(c)), and no apparent decrease in term of work at 85% strain is noted after continuous compressive loading−unloading cycles (Figure 3d), suggesting the highly elastic performance of CNC monolithic template. In this natural inspired cork-like hierarchical structure, cellular nanofibrils in cell wall are comprised of tightly packed face-to-face layered structure, which maximizes the elasticity. As shown in compressive deformations, the formation of an ordered, multilayered structure would facilitate the hydrogen bonding to be overcome by elastic energy in the nearly parallel manner, allowing the template to bounce back to its original size. In the following step, we demonstrate the template approach advantages by imbibing CNC organogel (45 wt %) into a cosolvent (THF) of PEO99−PPO65−PEO99 (Pluronic F127) diacrylate and afford the ability to produce gels with cellulose volume fraction up to 30%. The obtained hydrogels demonstrate swelling equilibrium rather than dissolve in large excesses of water (Figure S4), inferring the formation of a stable network composed of polymer chains and CNC template, without conventional cross-linker (e.g., N,N′methylenebis(acrylamide)). The most striking effect of the hydrogels is the significant enhancement of the tensile fracture strength by over 18 times from 27 kPa for the pristine polymer to 496 kPa for the CNC25F containing 25 vol % cellulose template. For a reference, the specimen were also prepared by mixing randomly dispersed CNCs in THF solution of Pluronic F127 diacrylate at the same filler content, and the results displayed notable phase separation and the tensile strength was as low as 84 kPa (Figure S5), revealing the template strategy provides a simple procedure to synthesize tough elastomers from immiscible components. The compressive loading− unloading cycles also indicated that the hydrogel could retain its reinforcement at a large strain compression (up to 85% compressive strain) after successive deformation (Figure S6). Indeed, the pristine Pluronic F127 materials were broken into many small pieces whereas the CNC template reinforced hydrogels appeared just a single crack after the compressive test (Figure S7), suggesting the porous and interconnected template improved the loading dispersion and energy dissipation. Indeed, the TEM analysis (Figure S6) has indicated that the template skeleton consists packed cellulose wall in a porous manner, which promotes intimate interactions and local stress passing through the interconnected binary network.

Figure 2. Light and porous CNC templates. (a) Sponge-like CNC monolithic template at a bulk density of 8.2 mg/cm3. (b) Top-view and cross-sectional SEM images of the template showing the continuous porous morphology and overlapped CNCs, resembling honeycomb-like cellular structure of natural cork. The insert HRTEM analysis reveals the nearly parallel oriented CNC in cellular layer wall.

fraction of nanoparticle is higher than the percolation threshold, the entrapped nanoparticles could build a continuous and porous network.7,8 Here we are particularly interested in exploring how the rod-like CNCs interact with each other via supercritical extraction without interference of surfactant additives and yielding the manner of porous structure on micrometer scales. SEM observation reveals that the spongelike bulk aerogel at a density of 8.2 mg/cm3 contains CNCs self-aggregation into an interconnected, robust, three-dimensional skeleton (Figure 2b, with more SEM images for CNC template at different densities in Figure S3). The template exhibits a porous cellular structure with pore size dimension in the order of tens of micrometers. The high-resolution transmission electron microscopy (HRTEM) analysis indicates that the rod-like CNCs are well oriented in an almost parallel arrangement and yields a highly ordered layer-assembly structure. In fact, this packed alignment is similar to biological cellular structure observed in the directional flow, a geometry has been argued to be beneficial to delivery elastic modulus.44 Besides, no obvious difference in the morphology and distribution was noted from the top surface and side-walls, suggesting the spatial uniformity and three-dimensionally isotropic structure of the CNC aerogel. Indeed, compression along three directions (x, y, z direction) at the same strain (ε = 85%) indicates isotropic response in the stress−strain curves (Figure 3b), where the maximum stress along the thickness (direction z, 4.7 kPa) is close to that of in other two orthogonal directions (4.52, 4.34 kPa). It is assumed that the structural integrity under large deformation is ascribed to the interconnected CNCs in a three dimensionally isotropic configuration, which can sustain splitting between CNCs along the deformation direction.40 D

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Figure 3. Mechanical properties of the CNC templates. (a) Compressive loading−unloading curves at different CNC template density (ρ = 8.2, 10.5, 12.1, 14.6 mg/cm3), and (b) compressing along three orthogonal directions (x, y, and z for length, width, and thickness, respectively). (c) Cyclic stress−strain curves in compressing with time interval of 15 min for each test. The inserts show SEM images for cross-surface of CNC template before and after compressive loading. (d) Toughness and stress at 85% strain for six continuous compressive cycles. The inset shows stress-time curves. The samples were measured for parts b−d at a density of 10.5 mg/cm3.

toughness, defined by the area under the compressive stress− strain curves, was increased from 0.42 to 37 kJ/m3 (Figure 4d). Moreover, we examine whether the large deformation causes a permanent damage to the networks, and the results confirmed that the elastomers could recover the original strength and exhibited almost overlapping hysteresis loops (Figures S9 and S10), an indication of self-recovery upon successive loading with a rest time. We have obtained the elastomers with varied CNC volume fractions (Vc) simply by tailoring the content of initial CNC gel and their mechanical properties are listed in Table S1. Owning to well-organized hierarchical structure, the Young’s modulus (E) of elastomers was found to scale with Vc as E ∼ Vc 1.2 (Figure 4(b)). According to morphology observation (Figure S11), it is noteworthy that rod-like CNCs reveal a wormlike morphology and this self-scrolling fibrous structure helps load transfer and structural integrity.35 As the primary building block in the structural region of plant, the application of CNCs as reinforcing phase is an extension of their natural role and the following factors are generally involved before the expected reinforcement is accommodated.28 First, the individual CNC should sufficiently bind to the matrix with tightly packed arrangement, so that it can achieve maximized resilience and efficiently transfer the loads. Besides, the load must be distributed throughout the matrix so that the outermost layer does not shear off. In this regard, the biomimetic template strategy here leads us to deduce that it is the unique porous

The typical monotonic uniaxial tensile tests showed an ultrastretchablility with a fracture strain of ∼2000%, in comparison to that of 300% for the corresponding pristine Pluronic F127 (Figure 4a). The rheological tests in Figure S8 also exhibited a significant enhancement in storage modulus and loss modulus for hybrids relative to the pristine polymer matrix. Besides, the cyclic tensile loading−unloading measurements revealed a notable hysteresis (Figure S9), suggesting a large amount of energy was dissipated during deformation via the rearrangement of Pluronic F127 micelles (will be discussed below). This result may lead us to hypothesize that the hydrophobic associated groups of Pluronic F127 start to be extensively elongate and are forced to dissociate from initial coiled configuration, which could consume a large amount of mechanical energy and thus increases the resistance against crack propagation.47,48 Interestingly, some minor residual strain (ε < 8%) was noted after unloading at 25 °C, whereas an elevated temperature (e.g., 45 °C) accelerated the restructuration and led to the nearly complete strength recovery (Figure S9). This result further indicates that the nanomicelle domains only disassociate but do not decompose upon stretching within the CNC template, and they could recover the original dimension via chain slippage at an elevated temperature. In fact, this slippage and disentanglement also accounts for the extremely large stretchability. Similarly, compressive tests on the elastomers showed extraordinary strength (1355 kPa) and did not fracture at 85% compressive strain (Figure 4c), and the E

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Figure 4. Mechanical properties of elastomers. (a) Tensile stress−strain curves for the as-prepared hydrogels, displaying high tensibility with elongation at break ∼2000%. (b) Young’s modulus as a function of CNC volume fraction. (c) Compressive stress−strain curves and (d) work of extension for the as-prepared CNC hydrogels, revealing excellent compressive strength with increased CNC content. The inserted SEM images (top view) of elastomers before and after compressive cycles confirm the maintenance of honeycomb-like structure of the elastomers after deformation.

Figure 5. Cross-section of fracture surface (CNC15F). (a) Hierarchical porous structure contains interconnected pores imparted by CNCs at both micro- and nanometer. (b) Nanopores with filamentous networks are stretched in the crack deflection. Insert of a magnified SEM micrograph of a crack tip region and the void growth between bridged nanofibrils with surrounding matrix indicated by white dash circle.

hierarchical structure of the template enables the gratifying mechanical properties. On the one hand, the cellulose in the cell wall is organized in a parallel arrangement that delivery elastic strength. On the other hand, this face-to-face tightly packed multilayered structure also facilitates the energy dissipation in soft phase, where hydrogen interactions between layers and sacrificial hydrophobic associations allow to be overcome by the elastic energy. Thus, the unique cell with a thickness of 20 nm enables a high resilience along plane direction and accommodates a large deformation, where van

der Waals interaction and friction allow efficient energy dissipation under stretching. The toughening mechanisms of CNC template reinforced hydrogels were investigated by examining crack tip around the cryofractured surfaces using SEM. Figure 5 mirrors the fracture section generally observed in composite systems containing high aspect ratio nanofillers,34,42 where crack deflection are noted with increasing crack propagation (more SEM images of the cross-section of the fracture surface in Figure S12). Reminisced by our previous work,35 the wormlike CNCs bridged neighboring crack planes at the edge of tip region and F

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Figure 6. HRTEM images of crack propagation and CNC template splaying between adjacent fibrils in the template. Region (a) corresponds to an unstretched structure with undistorted CNCs. Then stress arises in the presence of crack that overwhelms the noncovalent interactions and leads to continuous breakage between neighboring CNC-bridged layers (b). As the electron beam continuously concentrates on the above initiated splaying region (c), the growth of crack aligns perpendicular with respect to the CNCs, yielding efficient energy dissipation where external stress can be transferred to the neighboring CNCs (d). Both ends of CNCs that originally embedded in the matrix show the pullout of the bridging fragment (e). Enlarged (e′) view delineates the eventual rapture of unraveled region.

bonding mechanism, revealing a hidden length can span times of length of the initially coiled chains.47,48 Since the resultant deformation opposing separation of associative domains is smaller in magnitude than that bridging CNC (Pluronic F127 micelle size distribution in Figure S13), the significant hidden lengths of coiled polymer chains impart the CNC bridging effect to a much larger range and slide adjacent fibrils under stretching (TEM observation and scheme in Figure S14). Such a pattern contributes to a successive opening of hydrophobic associative intrachains clusters that built from neighboring polymer segments. Thus, this cluster-based dissociation provides an efficient energy dissipation process and a magnified bridging zone to enhance fracture toughness. It is worth noting that even though the attractive interactions are required for significant energy dissipation via CNC bridging, the low activation energy for associative domain splaying still ensures toughening and relaxes the constraints on the CNC−matrix interface.51 As a result, the both CNC-bridging and polymer unraveling account for multiple toughening mechanisms interdependently, which may be tailored by a variety of factors and continued exploration is requested in our near future study. The applications of using cellulose high modulus involve the well-dispersed particles within a polymer matrix and the maintenance of structural integrity upon large deformations. Modern biological structure analysis has confirmed that individual cells in a micrometer size dimension connect to build a honeycomb-like monolith, suggesting the assemble of individual units is pivotal for achieving efficient load transfer and mechanical reinforcement.28,44 Inspired by this bioinspired strategy, the CNC templates possess the ability to transfer striking strength to matrix if they can be hierarchically structured in an efficient manner. In principle, besides the efficient mechanical energy dissipation, the maintenance of elasticity is another significant criterion in designing tough hydrogels.15 In this CNC template-assisted network, the CNCs act the role of analogous cross-links via interfacial hydrogen bonding to increase the cross-linking density, which lead to the hybrid physically + chemically cross-linked system and increase the cross-linking density thereby. According to the timedependent tensile recovery experiments, no permanent plastic deformation is noted after loading−unloading cycles, which is

led to high ductility of the network. For example, some individual or clustered CNC pullout segments after debonding from the matrix were observed, an indicator of fiber-centric toughening mechanism.45 Figure 5a illustrates the network in the free-standing state and contains undistorted fibrils. At the end crack region where the matrix has not been broken, the CNCs are found to bridge neighboring crack tip and the both ends of bridged CNC appear to be stretched as the crack develops. Figure 5b exhibits the crack propagation region where stress concentrations arise in the crack tip. In this fibrils embedded system, the deformation of template is largely restrained by the neighboring matrix, preventing continuous crack propagation between individual fibrils.49,50 The time-elapsed TEM observation of the CNC template embedded in Pluronic F127 matrix was also performed (Figure 6): as electron beam warps the matrix, the resulting strain leads to splay near the CNCs while the backbone of a splayed CNC is still intact, delineating a sacrificial mechanism. Since the template-matrix interfacial interactions as well as hydrophobic association interactions are both reversible, dynamic debonding can be expected along the direction of crack propagation. Indeed, crack bridging is a well-known reinforcing mechanism in cellulose-based composite systems,51 where the rapture of weak interactions induces splaying breaks of template into multiple segments along the fracture direction. Earlier work has revealed that if the rod-like fillers in path of crack propagation is perpendicular to the crack direction, the both ends of fillers may still be embedded in the matrix as crack propagation.35,50 As a consequence, the bridge-effect dominates crack mechanism and imparts the tensile stress along the stress axis, yielding a dangling nanofiber at the rupture ends.46,50 Given that CNCs serve as physical cross-links in the composites and the improved interfacial interactions promote chain orientation by allowing the chains to slip along the CNC,50 the underlying mechanisms accounting for the enhancement of strength may be a combination of several synergistic effects: bridging at the interface between crack tip, viscoelastic energy dissipation, and maintenance of template integrity after deformation. Except for the above energy dissipation via bridging CNCs near the propagating cracks, the unraveled hydrophobic associative domains may represent another part of sacrificial G

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Macromolecules largely contributed to the interconnected cellular fibrous template assisted dynamic and reversible configuration arrangement. Thus, the synergistic effects of energy dissipation and elasticity maintenance are achieved in this unique template guided strategy. The approach here requires no additional surface treatment since the nanofibrillated cellulose has already formed an interconnected, three-dimensional template, which significantly simplifies the synthesis process of advanced cellulosic composites. By the marriage of hierarchical cellulose templates with sacrificial interactions in polymer matrix lead to the formation of tough and elastic elastomers. At 85% strain, the hybrid could sustain over 12 000 times its own weight without fracture and was able to recover to its original shape when the stress released. Moreover, there was no apparent decrease in terms of maximum stress at 85% and compressive work after continuous loading−unloading cycles, indicating negligible decay within consecutive loadings and an excellent fatigue resistance property of the materials.

Fund for Distinguished Young Scholars (31225005), and Chinese Ministry of Education (113014A).

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CONCLUSIONS In summary, we present a facile approach to fabricate composite hydrogels with excellent mechanical properties by using a porous, mechanically efficient nanofibrillated cellulose template, which is distinct from other randomly filamentous cellulose scaffold. The evidence of sacrificial bonds that contribute to the toughening mechanism is proposed. First, the embedded CNCs dissipate significant amount of energy as the crack propagation. Second, the associative domains could undergo splaying in response to stress by unraveling hidden lengths among matrix. The interconnected CNC template, interfacial compatibility of the template and matrix, as well as the unfolding of tight coiled polymer chains collectively contribute to the efficient energy dissipation process. This sacrificial bonding mechanism in the soft phase + hard phase via biomimetic strategy not only mirrors the crucial effect of the sacrificial bonds in toughening mechanism, but also opens the door to expand the library of versatile cellulosic composites.

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ASSOCIATED CONTENT

* Supporting Information S

Supercritical drying process for CNC aerogel, SEM and TEM images of CNC and elastomers, tensile and compressive stress−strain curves of elastomers, rheological behaviors of the CNC and elastomers, swelling ratio as a function of CNC content, and fracture energy of elastomers. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(J.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Fundamental Research Funds for the Central Universities (TD2011-10), National Natural Science Foundation of China (21404011), Research Fund for the Doctoral Program of Higher Education of China (20120014120006), Program for New Century Excellent Talents in University (NCET-12-0782), National Science H

DOI: 10.1021/ma5026175 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/ma5026175 Macromolecules XXXX, XXX, XXX−XXX