Puncture-Resistant Hydrogel: Placing Molecular Complexes Along

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Applications of Polymer, Composite, and Coating Materials

Puncture-Resistant Hydrogel: Placing Molecular Complexes Along Phase Boundaries Xueqi Zhao, Meixiang Wang, Yong Mei Chen, Ziguang Chen, Tao Suo, Wen Qian, Jian Hu, Xiaoping Song, Wai-Ning Mei, Renat F. Sabirianov, and Li Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02328 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Materials & Interfaces

Puncture-Resistant Hydrogel: Placing Molecular Complexes Along Phase Boundaries Xueqi Zhao1, Meixiang Wang1,2, Yongmei Chen1,3*, Ziguang Chen4, Tao Suo5, Wen Qian2,6, Jian Hu1, Xiaoping Song1*, Wai-Ning Mei7, Renat Sabirianov7, and Li Tan2,8* 1School

of Science, State Key Laboratory for Strength and Vibration of Mechanical Structures,

International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an, Shannxi, 710049, China. 2Department of Mechanical & Materials Engineering, University of Nebraska, Lincoln, NE. 3College of Bioresource Chemicals and Materials Engineering, Shaanxi University of Science and Technology, Xi’an, Shannxi, 710021, China. 4Department

of Mechanics, Huazhong University of Science and Technology, Wuhan, 430074,

China. 5School of Aeronautics, Northwestern Polytechnical University, 127 Youyi West Road, Xi'an, Shaanxi, 710072, China. 6Nano-Engineering Research Core Facility (NERCF), University of Nebraska, Lincoln, NE. 7Department of Physics, University of Nebraska at Omaha, Omaha, NE. 8Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE. Correspondence and requests for materials should be addressed to L.T., Y.C., or X.S. KEYWORDS. hydrogel, puncture resistant, molecular complexes, phase boundaries ABSTRACT. Trendy advances in electric cars and wearable electronics triggered growing awareness in device lethality/survivability from accidents. A divergent design in protection calls for high stress resistance, large ductility, as well as efficient energy dissipation, all from the device itself while keeping the weight-specific device performance to its premium. Unfortunately, the 1

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polymer electrolyte or the ductile elastomer lacked a mechanistic design to resist puncture or tear at a high stress level. Here we designed molecular complexes along phase boundaries to mitigate the damages, by placing those mechanically strong complexes along the phase boundaries or between two immiscible polymers. This puncture-resistant gel, dubbed as gel-nacre, is able to survive a few challenging incidents, including a 400-MPa-puncture from a sharp nail, a 1-cm steel ball traveling at 540 km/h, and attempted rupture on stitched samples.

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1. INTRODUCTION State-of-the-art soft materials1-3 in energy storage devices and transparent elastomers4-6 for flexible electronics are not puncture resistant. Puncture resistance requests the soft material simultaneously having a high compressive stress and a large ductility, with the former to stop the penetration of a sharp object and the latter to trigger immense amount of micrometer scale defects through the membrane without necessarily fracturing it into small pieces. While fascinating, the way to design and make a strong-and-stretchy material (or polymer) for those devices is daunting, even the most stretchable material like natural rubber7 or soft tissue8 (cartilage/ligament) can only bear a stress below the 100-MPa level. Fortunately, nacre9-11 (mother of pearl) is hard and resilient, constructed by following a brick-and-mortar motif, where biomolecules (chitin) form the mortar and inorganic crystallites (aragonite) form the brick12-14. Brick sliding over mortar surfaces is energy costly (with a high toughness), similar to those shear induced impact absorption by aluminum grains. There are numerous efforts investigating the nacre formation with inorganic crystallites11 or using layered polymer stacks9,15 or graphene16 to form nacre-like materials. However, these nanostructured constructions have no spatial controls; they are hardly transplantable to the formation of “mortar” inside a bulk polymer. There are works not using structural controls at the grain or interface level, instead they use single polymer17-19 or interpenetrated polymeric network20 to regulate energy dissipation, either via chain-chain disentanglement or through a bond breaking.

However, none of these works

considered bulk polymer as a boundary- or defect-rich solid. If we can engineer the weak spots between two different polymer domains (or phases) with mechanically strong molecular complexes, then a high toughness or puncture resistance material is possible. In this report, we designed a novel soft material (dubbed as gel-nacre) that is not only composed of two different 3



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hydrogels but also uniquely combined the brick-and-mortar motif from nacre. Irregular shaped “bricks” dominate a high compressive strength and a thin network of molecular complexes along phase boundaries serves as “mortar” for energy damping. As a result, both the fracture tensile stress and toughness can be tuned more than 100 times, converting an easily crushable sample to a membrane having more than 700-MPa of compressive strength (over 10 times of the state-ofthe-art double-network hydrogel21). In the following sections, we will explain how a gel-nacre was made and reveal the molecular reason behind the phase separated microstructures and their resulting properties. 2. EXPERIMENTAL SECTION 2.1 Synthesis of Precursor and Gel-nacre. PVA (3 g, Mw = 205,000) was dispersed in deionized water (70 g) at 95 °C for 6 h. The viscous but clear solution was then cooled down to room temperature (20-25 °C), followed by mixing with acrylic acid (AA) monomer (27 g, 0.3746 mole), and α-ketoglutaric acid (photo-initiator, 0.5 wt% as to AA monomers). This mixture was later kept in a vacuum chamber for 10 minutes to remove trapped air, followed by a transfer to a glass cell (15 × 15 × 0.3 cm3) that was separated with a 3 mm-thick silicone spacer. Subsequently, the entire package was allowed to cure under UV (365 nm, 40 W) for 6 h at room temperature (20-25 °C) to deliver the precursor. Finally, this soft precursor was converted to gel-nacre by immersing in a large container that was pre-filled with 1 liter of LiCl solution (> 12 M). 2.2 Synthesis of Gel-nacre Nanocomposite. The processes to prepare gel-nacre nanocomposite (silica) were not much different from a process above, all started with a homogenous solution. For instance, silica nanobeads (30-nm, 3 g, 3 wt%) were directly used or diluted to specific concentrations, followed by stirring for 4 h before being poured together with AA and initiator

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into the viscous and clear PVA solution (PVA 3 g, water 70 mL). 2.3 Static Puncture Test. We fixed the flat bottom of a steel nail (4.5 mm in diameter and 80 mm in length; Model T45, China) on a solid block and placed the block on a digital balance by leaving the nail tip upward. Then we held two edges of a rectangular gel-nacre membrane (10 × 10 × 0.3 cm3) and pressed the center of the membrane downward against the nail tip. A contact radius of 0.315 mm for the nail tip / membrane was estimated by pressing the same nail against a transparent plastic film and the dent was measured with a microscope (Nikon AV100). A maximum force of 12.98 kg was added over the nail tip, causing a compressive pressure of 400 MPa and 30 MPa of tensile stress (true stress, Supplementary Figure S1). 2.4 Dynamic Puncture Test. A piece of gel-nacre membrane (15 × 15 × 0.15 cm3) was fixed on an iron stand (10 × 10 × 0.2 cm3). To mimic a small piece of flying object (rock or nail; 2 g) that could have a top speed of 150 mph (or 240 km/h) on highway, a steel ball of 1 cm in diameter (4.1 g) was chosen. As the steel ball had a large contact area with the gel-nacre membrane, a top speed of 540 km/h (or 147.35 m/s) was adjusted, giving 8 times of kinetic energy than that of a sharp flying object. After the steel ball was propelled toward the membrane, gel-nacre remained unpenetrated and recovered to its initial shape right after the dynamic puncture (Supplementary Figure S2). 2.5 Atomic Force Microscopy (AFM). A nanoIR2 (Anasys Instruments, Inc.) was used to collect both AFM topography images and localized nanoIR spectra, as well as chemical IR imaging at a constant wavelength. Contact mode nIR2 probes (Model: PR-EX-nIR2, Anasys Instruments) with a resonance frequency of 13 ± 4 kHz and a spring constant of 0.07−0.4 N/m were used. The AFMnanoIR technique is accomplished by coupling a pulsed tunable IR source with an AFM. The IR

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source has a pulse length of 10 ns and can cover a broad range of the mid-IR. The light from this source was focused onto the tip−sample contact area. When the pulsed light was absorbed by the sample, a rapid heating/expansion in sample creats an impulse onto the AFM cantilever, which induces an oscillation. Through the resonance enhanced mode, it enabled IR measurement at less than 20 nm in length scale. Prior to imaging, gel-nacre or its precursor was dried with supercritical CO2. As this technique did not interrupt the structure of the sample but removed all the water molecules, images received reflected the real microstructure in gel-nacre or the precursor. 2.6 Extreme Heat Insulation. An alcohol burner was placed under the center of a ring holder (5.2 mm inner diameter). Then the ring was capped with one single layer of gel-nacre membrane (6.5 × 6.5 × 0.1 cm3) and one gel-nacre insulation envelope (vacumm pumped or air filled), with top of the envelope supporting a small cube of butter (0.5 g). Height of the ring holder was adjusted later to keep the bottom membrane at the rim of the outer flame (840 ~ 850 oC; measured by a thermocouple with Ni/Cr as the anode and Ni/Si as the cathode). Time for the butter being completely melted was recorded. Details in supercritical CO2 drying, assemble gel-nacre into thermal insulation envelope, vacuumpumped thermal insulation, and tensile & compression test are all included in Supporting Information. 3. RESULTS AND DISCUSSION Figure 1 presents several property highlights of our gel-nacre, respectively showing the process to stop a 400-MPa-puncture from a sharp nail (1a, Movie S1), bouncing a 1-cm steel ball traveling at 540 km/h without fragmention (1b, Movie S2), and resistance to attempted rupture on stitched samples (1c, Movie S3). We will then illustrate the gel-nacre fabrication mechanism with a 6


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scheme in Figure 2a. First, photo-initiators, monomers (for polymer B), and polymer A will be mixed in a good solvent, where some of these monomers will form molecular complexes with polymer A.

Then, photo-initiators are allowed to trigger a radical polymerization, either

producing freely moving polymer B or as polymer A-bound-polymer B. Subsequently, this hydrogel-like precursor will be transferred to another container with a bad solvent. Once the good solvent is leached out, polymer B from the soft gel will precipitate out first, manifested as a domain of their own and keeping the rest of polymer A uniformly inside the gel body. Meanwhile, volume of the hydrogel precursor will shrink after polymer chains being contracted. Later on, further removal of the good solvent will convert polymer A and polymer A bound complexes into coils, next to those earlier precipitated polymer B domains. When there is no more volume reduction and mass change is observed, a gel-nacre is then received (Figure 2a). The prerequisites to form a tight boundary in gel-nacre are to have two polymeric constituents with different solubilities and to have some sort of bonding in between. For example, hydrogen bond between acid and alcohol can be utilized as a bonding or complexation mechanism (Supplementary Figures S3-S4). "Bricks" can be polymerized acrylic acid (PAA) and the "mortar" be polyvinyl alcohol (PVA). Full potential of the complexation between PAA and PVA can be promoted when this mixture is exposed to a concentrated lithium solution. Figure 2b shows a simple modeling of this, where PVA forms strong hydrogen bonds with PAA (binding energy ~ 2.08 eV), but PAA forms another even stronger bonding with Li+ (binding energy ~ 2.56 eV). This difference in bonding preference suggested us a controlled phase boundary formation, either through PAA-Li or PAA-PVA complexes. Figure 2c shows the exact steps to fabricate the gel-nacre. First, we loaded a glass cell with a mixture of polyvinyl alcohol (PVA), acrylic acid (AA) monomer, α-ketoglutaric acid, and water 7



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(details see Methods). Once AA was allowed to polymerize under UV light, a soft and transparent hydrogel (precursor) having two different polymers was formed. Later, this brittle precursor was carefully pulled out and transferred to another container having a large amount of saline (concentrated lithium salt), where the Li+ ions (> 12 M) were allowed to diffuse into the hydrogel matrix. Gradually, the precursor shrunk and turned cloudy after 20 min but became clear again after 4 h (Figure 2d). After 24 h, the volume and weight was found with no further change, this rendered us the puncture resistant gel-nacre membrane (Supplementary Figures S5-S7). We show the key signatures from the gel-nacre with two differnet samples, one before phase separation (precursor) and the other after (gel-nacre). Both samples were cut into thin slices (~100 μm), followed by a supercritical CO2 drying to remove water without interrupting microstructures4 (see Methods). When these two samples were imaged under scanning electron microscope (SEM), the precursor revealed a rather smooth or featureless texture; in contrast, the gel-nacre showed a dendrite like trunk-branch feature (Figure 3a). While these SEM images hinted some level of texture evolution before and after, this comparison did not indicate any molecular variation in composition or phase. Atomic force microscopy (AFM) with an infrared (nanoIR) capability, on the other hand, clearly differentiated both samples. Particularly, there is a submicrometer scale phase separation of PAA (-COOH; yellow color) from PVA (-OH; blue color). In contrast, the precursor showed no clear difference in nanoIR mapping, implying a uniform mixing. Additional evidence was found in nanoIR peaks (Figures 3a), where carbonyl groups (C=O) of PAA in precursor showed two strong vibrations, one in 1750 cm-1 and another in 1680 cm-1, respectively indicating some PAA in a PAA-rich region and other in a PVA-rich environment2223.

This alerted us, when precursor was phase separated to form the gel-nacre, PAA could have

been left out from the original mixture, by having carbonyls mostly stayed together in a PAA-rich 8


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domain (1750 cm-1). A consequence of this was the disappearance or diminish of the higher wavenumber peak. While all these suggested a domain segregation or phase separation between the PAA and PVA polymers, potential hydrogen bonding or a tight interface between those domains at the molecular level cannot be excluded. According to the XRD in Figure 3b, the precursor had three broad bumps at 2Θ angles of 8.82, 30.35, and 39.27°, respectively corresponding to a d-spacing of 10.02, 2.94 and 2.29 Å. After the precursor was transformed into gel-nacre, the 2.94 and 2.29 Å peaks representing hydrogen bond between free moving water molecules24 almost disappeared by leaving 10.02 Å peak dominating the spectra of gel-nacre. Apparently, if the 10.02 Å peak represented some form of ordering, it occurred long before the salting or brining process. As this peak position was different from the signature of crystalized PAA22-23 or PVA24, we ascribe this peak being the interface complex between the PAA and PVA molecules. Not surprisingly, a slight shift of this peak from the precursor to the final gel-nacre indicated certain amount of interfacial shrinkage. Since this dspacing is twice the typical van der Waals distance (5.0 Å)25 between two organic molecules, we assign distance between PVA-bound-PAA complexes the leading cause. As we illustrated earlier back in Figure 2a, polar functional groups (-COOH) from the AA monomers are expected to compete with water molecules to form hydrogen bonds with PVA chains26. As a result, later polymerization made PVA bearing some dangling PAA, with distance between these PAA chains the source of the 10.02 Å XRD peak. Alternatively, radical transfer from the initiator to hydroxyl groups of PVA could graft PAA directly on PVA as side chains 27, making PVA-g-PAA (three different complexes, see Scheme S1). Either way, PVA could act as molecular templates in making PVA-bound-PAA complexes, similar to the way where liquid crystal polymer being proposed as rigid molecular templates in another hydrogel synthesis28.

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If we consider PAA as the brick and PVA the mortar, the extremely tough gel-nacre is then the output of an optimal distribution of two domains, echoing the illustration back in Figure 2a. Indeed, this optimal layout in microstructures made them remarkably better comparing to their parents domains, i.e., pure PAA or PVA, with the best perfomance coming from PVA/PAA = 1 : 9 (Figure 3c). We found the fracture strength in this sample reached 11.3 ± 2.1 MPa, with a stretchability over 1300%, toughness of 48.0 ± 3.1 MJ/m3 and fracture energy of 54.9 kJ/m2 (Supplementary Figure S8). If we encourge the complexation between PVA and PAA even more, either by lowering the pH value in the starting material or by having less initiators, higher fracture strength (14.9 or 21.0 MPa) and specific toughness (54.2 or 77.6 MJ/m3) were respectively received (Supplementary Figure S9). However, if we discourage the complexation between PAA and PVA by using dry powders of them and later hand mixed, followed by heat pressing29, then this anhydrous composite would break at a strain of less than 100%. While adding rigid nanoscale elements in a soft hydrogel8 can make it stronger, our material is composed of two different polymers that are phase separated. To make it stronger and stretchable, the elements are better incorporated in the “brick” domain (PAA), by leaving the “mortar” domain (PVA) untouched. Indeed, when 30-nm silica beads were dispersed into the precursor following our initial recipe and later phase separated, AFM images revealed almost uniformly distributed particles (yellow dots; Figure 3d-inset). Because dimension of these dots (inset-inset: 500 nm × 500 nm) was in the 200-nm regime, approximately 6 or 7 particles were aggregated together. We hypothesize, during the phase separation process, these particles had much higher possiblity to precipitate out than those PVA molecules. As a consequence, there is a large likelihood that 6 or 7 of them were sitting together and buried within PAA domains. Later mechanical test did reveal fracture strength of 32.3 ± 2.1 MPa and a super high fracture energy of 125.3 kJ/m2 (or specific

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toughness of 99.8 ± 3.1 MJ/m3) due to a high strength and large stretchability (> 800%) from this nanocomposite (Figure 3d). Networked phase boundaries, even in a small amount, could confine the deformation of polymer domains. The overwhelming occurrence of fibrous bundles in Figure 3e supported this argument, where an extremely large compressive load of 729 ± 69 MPa (10 times the value of state-of-theart DN gel) was needed to fracture the gel-nacre at 85% in strain (Note that dense but amorphous polymer will not form fiber bundles during a compression test). The unusual micrographs in Figure 3e suggest two things at the same time: 1) there are micro-phased domains sliding under shear stress; and 2) lithium salts in PVA acted as plasticizers or lubricants, triggering neighboring PAA domains to shear or slide under a large compressive load (ions in PAA are trapped, Figure 2b). As expected, there were trails of elements of C and O on fiber surfaces, but the Cl (from LiCl) was more or less evenly distributed, including those gaps between the fibers. Overall, these experiments confirmed that PVA-bound-PAA complexes were crucial to the observed resilience in gel-nacre. Now let us translate above qualitative interpretation into a peridynamics mechanical model30 in Figure 4a. First, in an effort to fully mimic the precipitation or seed growth of polymer B (PAA) in hydrogel, we laid 100 seeds randomly in the plane and later they all grew into polygon shaped, closed cells by following the well-known Voronoi tessellation as in graphene studies25. From left to right, three different cases were presented, where the percentage of polymer B over the entire sample was respectively 75.4, 83.1, and 91.4%. In Case 1, all the polymer B domains were fully separated by polymer A (PVA), with no chemical or non-covalent bonds between the two polymers (Movie S4). In Case 2, some boundaries were so thin, a strong interface layer (yellow color; polymer A-bound-polymer B) formed in between (Movie S5). In Case 3, all the boundaries

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were occupied by those interface layers, with the yellow region forming a continuous network (Movie S6). We then chose the mechanical parameters as follows (E for modulus and for fracture energy, all estimated from experimental data in Figure 4a): (polymer B) E = 1.0 MPa, G0 = 54.9 kJ/m2; (polymer A) E = 20.0 kPa, G0 = 0.1 kJ/m2; (polymer A-bound-polymer B) E = 1.0 MPa, G0 = 220.0 kJ/m2. When a small crack was placed in the middle of the sample and when a uniaxial tensile load was applied, the imposed boundary displacements increased by 0.4 mm at each loading step. This value was selected for computational efficiency and based on a convergence study that showed similar crack path compared to that produced by value of 0.8 or 0.2 mm. When external load was increased, above Voronoi features allowed us to track the structural changes in sample and we highlight the main results here (analytical model see Supplementary Note 1). Briefly, once a crack was initiated in Case 1, it propagated immediately through the polymer A network and fully fractured the sample into two pieces. In this process, only the polymer A along the crack path dissipated energies. When the domains were partially bonded by interface layers (Case 2), the initiated crack was confined by the network of interface layer. Before the sample was fully separated, several polymer A sites (distributed through the whole sample) were broken into voids, with a large amount of energies being dissipated. As no free polymer A existed as in Case 3, we obtained a highly elastic sample, with little energy dissipation during the deformation process. Note that as controls, pristine PAA and PVA samples with the same geometry were also modeled, their results agreed with experimental data. Among all these cases, PAA percentage (83.1%) from Case 2 was closest to the feeding ratio of two repeating units (AA vs. VA) in the best performed gel-nacre (84.6% in mole). Crack propagation in Case 2 did match the observed resilience from Figure 1 as well, with additional evidences found from segmented loadings as shown in Figure 4b. In this new comparison, gel-

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nacre showed extensive energy loss after stretching, where a pronounced hysteresis was observed. If we split the loading curve into three segments (differentiated by three distinct slopes) that spanned among 4 different states, the structural deformation can be assigned to three consecutive events: (i) expansion of individual PVA and PAA domains, possibly due to straightening of loosely entangled polymer chains (Figure 4b, ab); (ii) extensive void formation or stretching in PVA domain (the only place capable of large strain) (bc); and (iii) crack penetration through the interface layer (breaking hydrogen bonds) (cd). It is interesting to note that, voidgeneration26,28 has long been confirmed as an efficient means in promoting toughness in polycarbonates. From this angle, the gel-nacre behaved like a tough thermoplastic, but with an unmatched ductility and compressive strength (Table 1). Likely, the “mortar” domain in gel-nacre governed the stretchability and energy dissipation, the “brick” domain resisted higher load, and the inteface layer determined the final fracture stress. Combination of all three then delivered us a material that was able to resist the puncture from a sharp nail or a traveling bullet back in Figure 1. Another feature from molecular complexes is its potential to deliver a self-healable system and this is verified by Figure 4c and Movie S7. To successfully weld two broken pieces back together, molecular entities inside the gel-nacre must have sufficient mobility. In our case, this is achieved by using a relatively low molecular weight PAA and hydrogen bonded two functional groups (COOH and OH). When gel-nacre was fractured into two pieces, by raising the temperature from ambient (20 ~ 25 °C) to 60 °C, a “hot resting” made the self-healed gel-nacre highly stretchable, more than 600% in maximum strain after 8 h (Figure 4d). Even though the fracture stress seemed a little low (1.8 MPa), it is yet descent comparing to many other self-healable materials31. Finally, hydrated PVA has a relatively low glass-transition temperature32 and the binding

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mechanism (hydrogen bonds)33 inside domain boundaries are yet dynamic. Macroscopically not only can we recover a deformed ductile sample, we could also make it stronger. To reveal this possibility, we loaded a pristine gel-nacre to a strain of 300% (in a 21.1 s-duration), unload (in a 21.1 s-duration), and then let it rest for 4 h. Afterwards, this “heavily rested” sample further went through ten successive loading-unloading tests. Indeed, the “rested” piece demonstrated higher maximum stress (2.7 MPa) than the initial run (2.1 MPa), with the stress-strain curves towering over the original sample (Figure 4e). As the re-grouping or re-structuring during this extra 4 h had promoted the mechanical robustness to a better level, this process could not be explained using conventional creep or relaxation theory34 for viscoelastic materials. Rather, this was possible only after the re-crosslinking or re-association of broken bonds. Heavily dissolved salt in PAA domain is another factor worth of discussion. When the gel-nacre was used as thermal insulation media as shown in Figure 4f, these salts acted as a good shield in preventing heat accumulation inside the membrane. For example, a vacuum packed thermal insulation package with a freestanding gel-nacre was able to hold a butter cube without melting for more than 110 s, where the temperature from the burner reached 840-850 °C (see Methods). In this process, solvated salts formed layers of rock (combination of carbon black and LiClO2; Supplementary Figures S12-S13) during the high heat shielding. And this ceramic layer played a stronger role even than the vacuum inside the envelope, where an air-filled reference could still hold the butter for 90 s. Unlike the real nacre or seashell that took platelets as the shape for the bricks or hard domains, a precipitation pathway inside a hydrogel body made the rigid polymers (PAA) as polygon shaped cells, with their boundaries connected by soft polymers (PVA) and a thin layer of the complexes along the phase boundaries (PVA-bound-PAA). As dimension of these individual components ran

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across multiple scales, deformation can occur at the micrometer scale (domain level) and the molecular scale (hydrogen bonded complex), rendering a record-high value in fracture energy or energy dissipation among all artificial tissues (see Table S2) and many engineering materials (see Table 1). 4. CONCLUSIONS Over the past, designing a double-network (DN) hydrogel31, 35 for enhanced toughness has been a focal point in hydrogel research, initially with two independently crosslinked networks (soft & hard) and later improved by using micro crystals36 to replace the hard network. To some extent, both the “mortar” domains and the phase boundaries in gel-nacre mimicked the roles of those two networks, one being responsible for high ductility and another as robust clusters to stop voids in “mortar” domains fully propagating through the whole membrane. The “brick” domains in gelnacre, on the other hand, can be treated as the third component, by promoting a large compressive stress and an efficient domain-specific hardening. Currently, gel-nacre having 1- or 2-mm in thickness is sufficient to replace some of the counterparts in wearable electronics and polymerbased fuel cells; it is however too thick as polymer electrolytes in lithium-ion batteries. More versatile or stronger molecular complexes, however, are abundant in chemistry or materials research37-38. Continued efforts to utilize these complexes for hardening and ductility, plus diversifying the hydrogel system for electrochemical compatibility, could potentially make gelnacre a foreseeable candidate in puncture resistant batteries, fuel cells, and electronics. ASSOCIATED CONTENT Supplementary Information.

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Details in static & dynamic puncture, rupture, self-healing, and peridynamics modeling were recorded in Movies 1-7. Technical details in dynamic puncture, gel-nacre soaked in different salt concentrations and time, fracture energy measurement, peridynamic modeling, and many others are all included in Supplementary Figures S1-S13. AUTHOR INFORMATION Corresponding Author *Correspondence and requests for materials should be addressed to L.T. (email: [email protected]), Y.M.C. (e-mail: [email protected]), and X.P.S. (e-mail: [email protected]). Author Contributions Y.M.C. and X.Z. conceptualized the work. X.Z. carried out most of the experiments. M.W. and W.Q. performed AFM and XRD. T.S. did the dynamic puncture test and analysis. Z.C. performed the peridynamics modeling. J. H. provided beneficial discussion and equipment support. W.M. and R.S. performed the atomistic molecular dynamics (AMD) simulations. Y.M.C. and L.T. analyzed the data and wrote the first draft of the manuscript. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (Grant No. 11674263), International Science and Technology Cooperation Program by Ministry of Science and Technology of China and Shaanxi Province (2013KW14-02), Fundamental Research Funds 16


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for the Central Universities, the Program for the Key Science and Technology Innovative Team of Shaanxi Province. L.T. gratefully acknowledges the financial support from the National Science Foundation (CMMI 1098652 and IIA 1338988) and the J.A. Woollam Foundation. Z.C. was supported by the Thousand Young Talent Program of China. X.Z. was supported by the State Scholarship Fund by the China Scholarship Council. We thank Professor Zhi Mao Yang, Mr. Yuan He, and Dr. Wen Jiang Zheng for helpful discussions. ABBREVIATIONS AA, acrylic acid; PAA, poly(acrylic acid); PVA, poly(vinyl alcohol). REFERENCES (1) Cheng, X. L.; Pan, J.; Zhao, Y.; Liao, M.; Peng, H. S. Gel Polymer Electrolytes for Electrochemical Energy Storage. Adv. Energy Mater. 2018, 8, 1702184. (2) Li, N. W.; Shi, Y.; Yin, Y. X.; Zeng, X. X.; Li, J. Y.; Li, C. J.; Wan, L. J.; Wen, R.; Guo, Y. G. A Flexible Solid Electrolyte Interphase Layer for Long-Life Lithium Metal Anodes. Angew Chem. Int. Edit. 2018, 57, 1505-1509. (3) Dubal, D. P.; Chodankar, N. R.; Kim, D. H.; Gomez-Romero, P. Towards flexible solid-state supercapacitors for smart and wearable electronics. Chem. Soc. Rev. 2018, 47, 2065-2129. (4) Gao, Y.; Song, J. F.; Li, S. M.; Elowsky, C.; Zhou, Y.; Ducharme, S.; Chen, Y. M.; Zhou, Q.; Tan, L. Hydrogel Microphones for Stealthy Underwater Listening. Nat. Commun. 2016, 7, 12316. (5) Kim, C. C.; Lee, H. H.; Oh, K. H.; Sun, J. Y. Highly Stretchable, Transparent Ionic Touch Panel. Science 2016, 353, 682-687. (6) Liu, N.; Chortos, A.; Lei, T.; Jin, L. H.; Kim, T. R.; Bae, W. G.; Zhu, C. X.; Wang, S. H.; Pfattner, R.; Chen, X. Y.; Sinclair, R.; Bao, Z. A. Ultratransparent and Stretchable Graphene Electrodes. Sci. Adv. 2017, 3, e1700159. (7) South, J. T. Mechanical Properties and Durability of Natural Rubber Compounds and Composites. Ph.D. Thesis Virginia Polytechnic Institute and State University, 2001. (8) Rauner, N.; Meuris, M.; Zoric, M.; Tiller, J. C. Enzymatic Mineralization Generates Ultrastiff and Tough Hydrogels with Tunable Mechanics. Nature 2017, 543, 407-410. (9) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured Artificial Nacre. Nat. Mater. 2003, 2, 413-418. (10) Mayer, G. Rigid Biological Systems as Models for Synthetic Composites. Science 2005, 310, 17



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Figure 1| Snapshots of resilient gel-nacre against external stimuli. (a) Puncture resistance of a 1.5-mm thick sample against a static pressure of 400 MPa; (b) resistance of gel-nacre against a flying steel ball (10 mm in diameter, 4.10 g, speed of 540 km/h) without penetration (inside the yellow circle); and (c) two pieces of gel-nacre (3 × 4 × 0.2 cm3) stitched together resisted rupture against a hard pulling from each other.

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Figure 2| Schematic illustration and formation of gel-nacre by forming complexes along phase boundaries. (a) Hypothetic mechanism. First, monomers (for polymer B; green dots) and polymer A (purple lines) are mixed in a good solvent. After polymerization, a hydrogel-like precursor having polymer B (green lines) and polymer A-bound-polymer B (yellow sites) is received. Subsequently, a bad solvent is introduced, which precipitates the polymer B but keeps polymer A uniformly dispersed in the bulk. Futher leaching out of good solvent from the gel body triggers the polymer A-bound complexes coiling up and wrapping around the complex-free polymer B domains. (b) The atomistic molecular dynamics model between (1) chains of PAA and PVA and between (2) PAA and Li+. (c) Fabrication steps for gel-nacre. Step 1, polyvinyl alcohol (PVA), acrylic acid (AA) monomer, α-ketoglutaric acid are mixed together as an aqueous solution. Step 2, the solution is poured into a transparent mold. Step 3, the solution is exposed to UV (365 nm). Step 4, resulting hydrogel-like precursor is transferred to a concentrated saline (LiCl). (d) Photographs of a soft hydrogel precursor becoming smaller in Step 4. Yellow squares are used to guide the view on the size and shape of the sample.

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Figure 3| Phase separation (or domain segregation) in hydrogel and their effects on promoting mechanical properties. (a) AFM-nanoIR microscopy confirmed phase separation in gel-nacre, whereras the precursor had a uniform structure. Two different modes were used, including height and chemical imaging. In the chemical imaging mode, yellow region represented surfaces with carboxyl (–COOH) identities and blue region for hydroxyl (–OH). (b) Comparison of X-ray diffraction (XRD) between precursor and gel-nacres prepared at different temperatures. (c) Large fracture stress and high ductility are realized by optimizing the polymer component ratio (PVA vs. PAA) while keeping the total content of PAA and PAA in precursor at 30 wt%. (d) Incorporation of silica nanoparticles in the hard domain (PAA) promotes higher fracture stress and yet a descent elongation over 800%. Collectively, this makes nanocomposite 100% more tougher than gel-nacre. (e) strong confinement of polymer domains in gel-nacre produced fibrous bundles during compressions.

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Figure 4| Molecular reason for the resilience in gel-nacre and its unique potential in selfhealing and fire resistance. (a) Voronoi stacked soft and hard domains in gel-nacre, where PAA percentage and interface layer varied from Case 1 to 3. Two polymers with different solubility are allowed to phase separate. Without the A-B complex at the interface (yellow), material easily fails after an initial crack (Case 1). With small amount of the A-B complex, material becomes resilient to external load (Case 2 & 3). (b) Segmented loadings had different stages in energy absorption, indicating damage and fracture evolutions. Case 2 from (a) suggests sufficient damages in “mortar” domains (PVA-bound) through the sample before the failure of the interface layer. (c) Even after gel-nacre was cut into two halves, they were able to self-heal (60 °C, 24 h) and resist a load of 500 g. (d) Stress-strain curves of self-healed gel-nacre after an extended (4 to 8 h) baking at 60 °C. Those two slices were respectively stained with red (rhodamine B) and blue (methylene blue). (e) Consecutive loading-unloading is senstivite to the relaxation process, where a 4-h rest can beef up its tensile stress. (f) Envelope shaped gel-nacre showed resistance to burning fire (840-850 °C) by keeping the butter cube without melting for more than 110 s. SEM images of fire contact area revealed a layer of porous rock (right two panels).

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Table 1. Comparison of gel-nacre versus other mechanically resilient materials. Materials

Fracture Stress Compressive (MPa)

Tensile (MPa)

60,000-70,000

80-130

70,000

Polycarbonate

Ductility (mm/mm)

Fracture Energy (kJ/m2)

(MJ/m3)

-

0.35-1.24

0.1-1.8 (ref.11, 38)

90

0.5

-

26 (ref.39)

83

72

1

5 (ref.40)

-

Natural rubber

13-30

18

5

10 (ref.7)

-

Double-network hydrogel

20-60

0.06-14

5-22

3-13 (ref.30, 3435, 41)

-

650-820

11.3 ± 2.1

11.5 ± 1.5

54.9

48 ± 3.1

Gel-nacre, SiO2 Nanocomposite

-

32.3 ± 2.3

9.0 ± 1.0

125.3

99.8 ± 3.1

Gel-nacre, 90% less initiator

-

21.0

10.2

-

77.6

Gel-nacre, H2SO4

-

14.5

8.3

-

54.2

Nacre Aluminum

Gel-nacre,

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Table of Contents Graphic

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Puncture-Resistant Hydrogel: Placing Molecular Complexes Along

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