Supramolecular Metallogel That Imparts Self-Healing Properties to

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Supramolecular Metallogel that Impart SelfHealing Properties to Other Gel Networks Tobias Feldner, Marleen Häring, Subhadeep Saha, Jordi Esquena, Rahul Banerjee, and David Diaz Diaz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01144 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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Chemistry of Materials

Supramolecular Metallogel that Impart Self-Healing Properties to Other Gel Networks Tobias Feldner,†,‡ Marleen Häring,†,‡ Subhadeep Saha,# Jordi Esquena,§ Rahul Banerjee# and David Díaz Díaz†,§* †

Institut für Organische Chemie, Universität Regensburg, Universtitätsstr. 31, 93040 Regensburg, Germany

#

Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India § Instituto de Química Avanzada de Cataluña, Consejo Superior de Investigaciones Científicas (IQAC-CSIC), Jordi Girona 18-26, Barcelona 08034, Spain ABSTRACT: A unique proton conductive and moldable supramolecular metallogel (CuA-Ox gel) made upon mixing at room temperature (RT) well-defined stock solutions of Cu(OAc)2·H2O and oxalic acid dihydrate was found to have self-healing properties. Remarkably, the system also displayed an unprecedented ability to impart self-healing properties to other gel networks lacking this capacity. A self-healed CuA-Ox metallogel was found to have essentially the same nanofibrillar morphology, thermal stability, rheological properties and conductivity than the freshly prepared sample. The discovery also allowed the fabrication of self-healing conductive composites containing conductive carbonaceous materials. Overall, this work serves as a proof of concept for the transfer of self-healing properties between completely different gel networks.

INTRODUCTION Over the last few decades, self-healing materials with the ability to repair themselves in response to damage have attracted much attention due to the improvement of their lifetime, reduction of replacement cost and enhancement of product safety.1-8 Indeed, materials capable of autonomous healing have numerous potential applications.9-11 So far, self-healing has been demonstrated in numerous materials including linear polymers,12 dendrimer-clay systems,13 metal ion-polymer systems,14 multicomponent systems15-17 and gel networks.18,19 In general, self-healing mechanisms to restore the strength and function of materials upon damage are either based on the release of healing agents upon damage (in such cases the supply of healing agents is depleted locally and, hence, repetitive healing is usually not possible),20-22 or on the inclusion of latent molecular functionalities that trigger repair via stimuliresponsive reversible interactions including electrostatic interactions,13,23-26 molecular-recognition,14,27-30 hydrogen bonding, 18,19,31-32 metal coordination,13,14,18,19,33-39 hydrophobic40,41 or π– π stacking42,43 association, dynamic chemical bonds,44-50 or molecular diffusion and entanglement.51-53 In the case of supramolecular gel networks, and in particular metallogels,54-57 self-healing has been a property extensively reported in the literature by means of any of the above-mentioned interactions.14,18,19,27,35,48,58-65 However, the use of a supramolecular self-healing gel network to induce self-healing properties in a

completely different second network is very challenge because the delicate interphase balance that define most metastable gels is likely to be altered upon combination of both networks, with a detrimental effect on the functionality of the material. Within this context, we report here a unique supramolecular metallogel with self-healing properties and the unprecedented ability to impart self-healing properties to other gel networks that intrinsically lack this capacity. These gel networks are not only completely different from the structural point of view, but the gelation phenomenon in each case is also driven by different mechanisms. Moreover, this discovery allowed the facile preparation of self-healing conductive composite gels. Despite the remarkable advances in the field of self-healing materials, the fabrication of self-healing hybrid systems by simultaneous growth of a self-healing network (minor component) and a non-self-healing network (major component) has not yet been reported.

EXPERIMENTAL SECTION Materials. Unless otherwise stated, all reagents and starting materials were commercially available and used as received without further purification. All solvents were dried according to common procedures. Characterization methods. Fourier transform infrared (FTIR) spectroscopy: Infrared spectra were recorded on a Bio-Rad Excalibur FTS 3000 MX

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spectrophotometer equipped with a Specac Golden Gate Diamond ATR. Rheological measurements: Rheological properties were determined by using an Advanced Rheometer AR 2000 from TA Instruments equipped with a Julabo C cooling system. The oscillation measurements were carried out with a 20 mm plain-plate (cone certificate) with solvent trap at 25 °C. Samples were kept at room temperature for at least 1 day for equilibration before measurement. The total gel volume used for each measurement was 1 mL and the temperature was kept constant at 25 °C. The following three experiments were carried out for each sample: (a) Dynamic Strain Sweep (DSS): Plot of storage modulus (G') and loss modulus (G'') with strain. DSS experiments were initially performed to estimate the smallest strains at which reasonable torque values could be obtained. The linear viscoelastic region of the gels was determined by increasing the amplitude of deformation from 0.1 to 100% at 1 Hz. The material is considered a gel if G' > G''. (b) Dynamic Frequency Sweep (DFS): Plot of G' and G'' with frequency (i.e., 0.1 to 100 Hz) at 0.1% strain, performed to make sure that the frequency used in DTS is within the linear viscoelastic regime. (c) Dynamic Time Sweep (DTS): Plot of G' and G'' with time. In this experiment the strain (0.1%) and the frequency (1 Hz) were kept constant. Thermogravimetric analysis (TGA): TGA was carried out on a Mettler Toledo STARe system in an aluminum jar. Scanning electron microscopy (SEM): SEM pictures were taken on a Nova NanoSEM 230 (FEI) Scanning Electron Microscope, operated at high vacuum mode. Samples, except gels doped with MWNTs, were sputter coated with a thin film of gold. The xerogels were prepared by freeze (liquid nitrogen bath)-drying (high vacuum) the corresponding gels (total gel volume = 2 mL). Estimation of gel-to-sol transition temperature (Tgel) or gel destruction temperature (Td): A sealed vial containing 1 mL of gel was placed into a heatable aluminum block and heated up at 2 ºC min−1. Herein, Tgel and Td were defined as the temperature at which the bulk gel collapsed when the vial was turned upside-down. The method was previously validated by DSC measurements.66-68 The estimated error was ± 2 ºC after several heating-cooling cycles. Conductivity measurements: For proton conductivity measurements, CuA-Ox-based gel (13 mm diameter × 5 mm thickness) was prepared inside a Teflon® cylindrical cell having two stainless steel stoppers at both ends (electrodes) and placed inside a glass chamber. Resistance data derived from impedance measurements were fit into a three-component model consisting of a resistor connected in series with a circuit formed by a resistor and a constant phase element in parallel.69 Voltage amplitude was set up at 30 mV. The model was previously validated by adequate fitting of the Nyquist semicircle at different temperatures (i.e., σ-value increased with increasing temperature as resistance decreased).69 CuA-Ox gel did not show significant electrical conductivity and, hence, the σvalue was ascribed to proton conductivity. Ionic transport activation energy was obtained by means of the Arrhenius equation. The ionic transport may occur through a Grotthusstype mechanism and species such as unreacted oxalic acid, ammonium ions and acetic acid could contribute to the formation of additional H-bonded networks that facilitate the transport. Measurements were validated using a solid conductivity measurement cell (EQ-STC 10 mm, MTI) with stainless steel electrodes. Conductivity of PVDF-based gels was measured

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by dielectric spectroscopy as previously reported.70 Preliminary volume resistivity and resistance measurements of CNMbased composites were carried out by Dr. Kara at Trakya University. The samples were measured under 10 V DC potential by alternating polarity method every 20−30 s with a Keithley Model 6517A electrometer, which employs the ASTM D-257 measurement standard. General procedures. Preparation of CuA-Ox gel: Cu(OAc)2·H2O (0.36 g, 1.82 mmol) was dissolved in dry DMF (5 mL) at room temperature (RT) affording a 0.36 M deep-green solution (sol I). Oxalic acid dihydrate (0.42 g, 3.30 mmol) was dissolved in dry DMF (5 mL) at RT affording a 0.66 M transparent solution (sol II). Sol I (1 mL) was added rapidly at RT to sol II (1 mL) placed into a glass screw cap vial (4 cm length × 1 cm diameter, 1 mm wall thickness), to form immediately a bluish-green inhomogeneous partial gel. The mixture was stirred vigorously and subsequently sonicated (ultrasound bath USC200TH, VWR™) for 10 min at RT to form a blue homogeneous colloidal suspension, which evolved into a stable gel within 6 h under undisturbed conditions. For the preparation of monoliths, the above procedure was followed using a syringe with the tip cut off instead of a glass vial. Note: Previously, we have demonstrated that hydration water molecules of the metal salt and ligand components are not necessary for the gelation phenomenon.69 However, their use results adequate because they allow for the preparation of more concentrated stock solutions increasing the gelation kinetics. Preparation of DACBA gel: DACBA gelator (20 mg, 0.04 mmol) was dissolved in dry DMF (2 mL) by gently heating with a heat gun. For the preparation of the monolith, the resulting isotropic solution was transferred to a syringe where the tip was cut off. The mixture was allowed to cool down spontaneously to RT affording the corresponding organogel. Typical procedure for the preparation of CuA-Ox/DACBA hybrid gels: DACBA gelator was dissolved in dry DMF according to its minimum gelation concentration (i.e., 10 g⋅L-1) by heating and sonication. The mixture was allowed to cool down and the CuA-Ox precursor solutions (sol I and sol II, 1:1 v/v) were added to the isotropic DACBA solution before its gelation. The volumes were adjusted according to the desired final volume of the monolith. Composition of hybrid-0.8 (for 1 mL total volume): DACBA (10 mg), DMF (0.8 mL), stock sol I (0.1 mL), stock sol II (0.1 mL). Composition of hyrid-0.9 (for 1 mL total volume): DACBA (10 mg), DMF (0.9 mL), stock sol I (0.05 mL), stock sol II (0.05 mL). Typical procedure for the preparation of CuA-Ox/CNM composite gels: For the preparation of 2 mL monolithic composite, the desired amount of CNM was weight into a screw cap vial and 1 mL of sol II (stock solution of oxalic acid dihydrate, 0.66 M in DMF) was added. After ca. 1 min of sonication at RT the dispersion was transferred to a 3 mL syringe where the tip was cut off. Quick addition of 1 mL of sol I (stock solution of Cu(OAc)2 monohydrate, 0.36 M in DMF) and stirring with a spatula resulted in a colloidal suspension. The mixture was sonicated for ca. 1 min at RT. Complete gelation was achieved within 6 h of resting. The sonication time could be extended to 5−10 min depending on the CNM used. MWCNs used in these experiments: 10−20 nm diameter, 5−15 µm length; SWNTs used in these experiments: 2 nm diameter, 5−15 µm length As mentioned in the main text, the maximum amount of CNM that could be incorporated into the composite gel depended on the nature of the CNM used. For


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example, the optimal toughness of the gel (based on rheological data) for G-composites was obtained with 25 g⋅L-1. However, the use of higher amounts of G until 70 g⋅L-1 also provided stable gels, albeit with lower stiffness. Preparation of PVDF gels and CuA-Ox/PVDF composites: NaI (0.19 g, 1.27 mmol) was added slowly to dry DMF (2 mL) placed into into a screw cap vial and vigorously stirred for 1 h. PVDF (1.25 g; Aldrich, Mw 180,000, average Mn 71,000) was subsequently added to the previous solution and the mixture stirred until all solid was dissolved (polymer:solvent ratio = 2:3). The solution was transferred to a 3 mL syringe where the tip was cut off and allowed to rest overnight for the formation of corresponding monolithic PVDF gel (i.e., O/Na = 20). For the preparation of the composite, the precursor solutions of CuA-Ox gel (i.e., sol I and sol II, 0.1 mL each) were mixed with the previous clear PVDF/NaI/DMF solution and the mixture stirred vigorously for complete homogenization. The corresponding monolithic composite was received after 6 h of resting at RT. Note: O/Na = [(mass of solvent)/(mass of salt)] × [(MW salt)/(MW solvent)] × n, where n = number of active oxygen. General self-healing experiment: The corresponding monolithic gel was cut in several blocks of approximately same dimensions (e.g., 1 cm length). The pieces where put back in contact and slightly pressed against each other. The reassembled monolith was transferred back to the original syringe and sonicated for ca. 2 min. The monolith was kept undisturbed for 6 h before taking it out from the syringe for further tests.

RESULTS AND DISCUSSION

the self-healing property of the bulk material (Figure S2). So prepared monolithic gels showed self-supporting properties (Figures 1f-g), remarkable load-bearing capacity (Figure S3) and good malleability. This allowed cutting the monoliths into several 0.5 cm blocks without losing liquid. When alternating blocks (i.e., dye-doped/non-doped) were put back in contact together, they were fused within 5 h without the need of any external healing agent (Figures 1h-i). The resting time to achieve recombination could be drastically reduced to 10 min when sonication was applied for ca. 2 min to the reassembled gel blocks placed inside the syringe. The short ultrasound treatment not only provided more stability to the rebuilt monolith but also eliminated completely the cutting sides initially observed at the interfaces between the fused gel blocks (Figure S4). Distribution of the dye through the entire gel structure was gradually observed over time, indicating a good restoration of the diffusion properties at the interfaces of the cut blocks. Thus, we could reconstruct a monolith bridge that supported a loading equivalent to its own weight while being held horizontally between two vials (Figure j). A close look to the interfaces upon collapse of the monolith showed significant roughness suggesting a self-healing mechanism instead of purely adhesive behavior. Moreover, the mechanical stress at the solid-liquid interface upon cutting a freshly prepared monolithic gel alters the liquid environment of the surface facilitating restoration upon resting. As a matter of fact, no recombination was achieved when two unaltered smooth monoliths were put in contact (Figure S5). a

An oxalic acid-based metallogel (CuA-Ox gel) monolith was prepared by mixing in a syringe and at RT equivolumetric amounts of two well-defined DMF stock solutions containing Cu(OAc)2·H2O (sol I, 0.36 M) and oxalic acid dihydrate (sol II, 0.66 M) (Figures 1a-c).71 The initial colloidal suspension formed immediately after mixing the two solutions was sonicated for 10 min to afford a homogeneous metallogel after 6 h (Figure 1d). Complete absence of flow upon inversion of the syringe and oscillatory rheological characterization (Figure S1) confirmed the robustness of the metallogel (i.e., G’ = 4.8 x 104 Pa). As we demonstrated in our previous report,71 the gelation mechanism involve the spontaneous formation of short ribbon-like 1D coordination oligomers (i.e., n < 7, [OxnCun+1]) where each Cu2+ ion is surrounded by 4 oxygen atoms of 2 oxalate anions by weak electrostatic interactions in a square-planar geometry. Further intermolecular interactions between oxygen and copper centers, as well as hydrogen bonding between oxalic acid units, play also a key role in the overall entanglement of the gel network trapping the solvent molecules in its interstices. In contrast to other metallogels based on preformed coordination species,72 the in situ formation of the multicomponent CuA-Ox gel may provide some additional degree of flexibility and reversibility to the network. In addition, mechanical stress applied to freshly prepared CuA-Ox gel may cause significant alteration of the solvated structure due to the solid-liquid interface susceptibility to mechanical stress.74 On the basis of this reasoning, we hypothesized that the bulk material could undergo automatic healing after being cut. For a better visualization of the self-healing tests, a dyedoped monolithic gel (Figure 1e) was also prepared in a similar way by adding a dye (i.e., rhodamine B, c = 2 g⋅L-1) into the mixture of the two precursor solutions. It is worth mentioning that the incorporation of different dyes did not affect

c

b i) mixing, RT

+

Cu(CH3COO)2 · H 2O 0.36 M in DMF (sol I)

ii) sonication 10 min, RT O OH

HO O

· 2 H 2O

0.66 M in DMF (sol II)

6 h, RT d CuA-Ox gel

dye-doped undoped monolithic gel monolithic gel e

f

g

h

i

j

Figure 1. a-d) Synthesis of 2 mL monolithic CuA-Ox gel; e-g) self-supporting properties of monoliths. Rhodamine B was used as a dye to stain one monolith; h-j) self-healing and load-bearing properties of a monolithic gel made by fusion of alternate dyedoped and undoped block gels.

In addition, replacement of DMF by other polar aprotic solvents such as dimethylacetamide (DMAC) or Nmethylpyrrolidone (NMPD) had a significant impact on the



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kinetics of the self-healing process (i.e., self-healing kinetics: DMF > NMDP >> DMA; Figure S6). From the mechanistic point of view, the high binding constant exhibited by Cu(II) oxalate (i.e., log k ≈ 6.3) makes its dissociation unlikely even at acidic pH. Although the elucidation of the exact self-healing mechanism in this case is very difficult due to the large number of molecular species involved, acting probably in a synergistic manner, it seems reasonable to think that the underlying physics of stress at solid-liquid interfaces in soft materials¡ Error!Marcador no definido. could be behind this interesting phenomenon. Rapid desolvation/resolvation of copper oxalatebased 1D coordination oligomers at the interface may be induced by physical stress and subsequent relaxation, allowing for a rapid restoration of the multicomponent supramolecular network without necessarily disassembling the coordination complexes. Importantly, a self-healed CuA-Ox metallogel was found to have essentially the same entwined nanofibrillar morphology (i.e., fibers width ~ 20−50 nm), thermal stability (i.e., degradation of the coordination oligomer ~ 300 ºC), rheological properties (i.e., high G' values ~ 105 Pa, low frequency dependence G' ~ ω0.05; tan δ ~ 0.12) and proton conductivity (i.e., derived from the Nyquist plot, σ ~ 1.4 × 10-4 S⋅cm-1 at 27 ºC under dehumidified conditions) than the freshly prepared monolithic sample (Figure S7). We have described in detail all these properties in our previous report.71 Similarly to the original gel,¡ Error!Marcador no definido.71 the rebuilt monolithic gel survived to the immersion in some organic solvents (e.g., DMAC, MeCN, Et2O, MeOH). CuA-Ox in DMF Cu(CH3COO)2 · H 2O a

DACBA in DMF NHCOR

O

+

OH

HO O

NHCOR R = -C11H 23-n

· 2 H 2O

CuA-Ox gel

b connection of alternate pieces 12 h, RT

DACBA gel

c

d i) cut into pieces

monolithic hybrid gel in DMF

ii) reconnection 12 h, RT

DACBA:CuA-Ox 0.8:0.2 (v/v gel) e

f

g

Figure 2. a-b) Preparation of a self-supporting monolithic bridge by reconnection of alternate block gels made of CuA-Ox and DACBA; c-d) preparation of self-supporting and self-healing hybrid-0.8 (i.e., DACBA:CuA-Ox = 0.8:02 v/v) by simultaneous formation of CuA-Ox and DACBA gel networks; e) hybrid-0.9; f) hybrid-0.95; g) spontaneous collapse of the gel made only of DACBA. Pictures d-f) correspond to the material obtained after cutting/reconnecting the gel (4 pieces). Total volume in each case = 2 mL.

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The uniqueness of this metallogel lies in its ability to impart self-healing properties to other physical gel networks that inherently lack this capacity. As a proof of concept, we employed first a well-known and very reliable low molecular weight (LMW) gelator based on diaminocyclohexane bis(amide) (i.e., DACBA).75 DACBA forms soft thermoreversible gels at relatively low concentrations (i.e., c = 2−44 g⋅L-1) in a variety of organic solvents, including DMF, upon standard heating-cooling protocol (see SI for details). However, upon cutting a monolithic DACBA gel prepared in DMF (i.e., c = 10 g⋅L-1 in DMF = critical gelation concentration) into several pieces, the blocks are unable to fused each other and reform a self-standing monolithic gel (Figure S16). Thus, we were delighted to observe that a stable hybrid material could be formed by placing in direct contact CuA-Ox and DACBA pre-cut monolithic blocks prepared in DMF (Figures 2a-b). After 12 h of equilibration, the new monolith exhibited remarkable self-standing properties and could form a stable bridge between two vials. A tendency to unify the color of the hybrid monolith was also evident to the naked eye, suggesting the internal dynamic nature of the composite material and the formation of interpenetrated networks (vide infra). Even more remarkable was the possibility to prepare (DACBA)-(CuA-Ox) hybrid gels with self-healing properties by simultaneous formation of both gel networks (Figures 2c-d) (see SI for details). Initial optimization studies established a gel ratio of 0.8:0.2 (DACBA:CuA-Ox, v/v) as the lowest ratio necessary for the preparation of a self-healing monolithic hybrid gel based on non-healable DACBA gel (Figures 2e-g). Therefore, it was clear that small amounts of the CuA-Ox system could impart self-healing properties to the hybrid material. In order to ascertain the effectiveness of the incorporation of self-healing properties into hybrid materials, different specimens were compared by means of rheology, electron microscopy, thermal stability and FTIR spectroscopy. The results of stress-strain rheological measurements showed crossover points at ca. 1% and 5.5% strain for DACBA and CuA-Ox gels respectively, which was in agreement with the higher mechanical stability of the latter. On the other hand, hybridization of DACBA with small amounts of CuA-Ox afforded a hybrid (i.e., hybrid-0.8) with a critical strain (γc ≈ 1.2% strain) at the end of the linear viscoelastic region, above which non-linear response is obtained. The average storage modulus G’ of the gels DACBA, CuA-Ox and hybrid-0.8 were 2.1 × 104 Pa, 7.5 × 105 Pa and 4.8 × 104 Pa, respectively (Figure S9). The results indicate an increase in the stiffness of pristine DACBA gel caused by the inclusion of CuA-Ox network. In general, cutting and reassembling the fresh monoliths provided materials with virtually same properties. In contrast to the fibrillar porous CuA-Ox gel network (Figure 3a), DACBA self-assembles through antiparallel hydrogen bonding resulting in the formation of asymmetric tape-like superstructures that are interlocked through van der Waals interactions affording helical fiber-like aggregates that trap the solvent molecules in the interstices (Figure 3c).75 Therefore, we expected to observe a significant alteration of the nanostructure if both DACBA and CuA-Ox networks are grown concurrently caused by the interpenetration of the two networks. This was evidenced by scanning electron microscopy (SEM) of the corresponding xerogels (Figure 3d). Even the hybrid material obtained upon connection of CuA-Ox and DACBA gel blocks (Figure 2b), showed morphological features of the two networks (Figure 3b). Coexistence of both networks was also evident by FTIR spectroscopy (Figure


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S10). In terms of thermal stability, the destruction temperature (Td) of non-thermoreversible CuA-Ox gel was established in 150 ± 2 °C, which is around the boiling point of DMF (153 ºC). At the Td irreversible precipitation of copper oxalate is observed.71 In contrast, thermoreversible DACBA gel showed a gel-to-sol transition temperature (Tgel) of only 41 ± 2 °C in agreement with its weaker network. Remarkably, hybrid-0.9 and hybrid-0.8 showed Td values of 70 ± 2 °C and 120 ± 2 °C, respectively. Interestingly, preliminary experiments demonstrated that the hybridization strategy was not limited to physical gels but also chemical gels could be modified to display self-healing properties to some extent upon combination with CuA-Ox gel precursor solutions before the bulk gelation. We exemplified this using a known polymer gel electrolyte based on a mixture of polyvinylidene difluoride (PVDF) and NaI in DMF (Figure S11).76 a

some bulk materials into a desired shape without major losses of fluid (Figure 4e). a

c

b 500 nm e

d

(1)

(2) (3)

b 400 nm

3 µm c

3 µm d

500 nm

5 µm

Figure 3. Representative SEM images of different xerogels obtained by freeze-drying the corresponding organogels: a) CuA-Ox xerogel; b) CuA-Ox xerogel after being damaged and selfrepaired; c) DBCBA xerogel (c = 10 g⋅L-1 in DMF); d) hybrid-0.8 xerogel. See SI for images at different magnifications.

Finally, considering the composition of CuA-Ox gel in DMF and the fact that this solvent is an effective dispersant for carbonaceous materials (CNMs),77 we decided to investigate the possibility to prepare different self-healing conductive composite gels.78-84 Thus, we used the precursor stock solution of oxalic acid dihydrate in DMF (sol II, 0.66 M) to disperse first different CNMs, namely graphite (G), graphene oxide (GO), multi-walled carbon nanotubes (MWNTs) and singlewalled carbon nanotubes (SWNTs). Subsequent addition of the Cu(OAc)2·H2O stock solution (sol I, 0.36 M) resulted in a colloidal suspension, which was sonicated at RT for 2 min. Complete gelation occurred after 6 h of resting, affording homogeneous and self-supporting black gel composites containing large amounts of CNMs (i.e., G = 25 g⋅L-1; GO = 20 g⋅L-1; MWNTs = 12.5 g⋅L-1; SWNTs = 5 g⋅L-1) (Figures 4a-b). The values are significantly higher than others previously reported in the literature using different supramolecular gel scaffolds.85 The embedment of CNMs into the gel matrix was also evidenced by SEM imaging of the corresponding xerogels indicating good compatibility between the materials (Figures 4c-d and Figure S12). Furthermore, the remarkable malleability, self-supporting property and ability of CuA-Ox-based gels to host external additives allowed high-scale synthesis of monolithic gels as well as molding, cutting and reassembling

(4)

(5)

Figure 4. a-b) Freestanding composites made by combining CuAOx precursor solutions and CNMs: a) from left to right, G (25 g⋅L1 ), MWNTs (12.5 g⋅L-1) and SWNTs (5 g⋅L-1), b) GO (10 g⋅L-1); c-d) SEM images of MWNTs embedded in the CuA-Ox gel matrix; e) “LEGO-car” fabricated by using CuA-Ox gel prepared as described above (1); CuA-Ox gel doped with rhodamine B (2); CuA-Ox gel doped with lanasol (3); DBCBA gel (0.02 M in DMF) (4) and composite gel containing MWNTs (1 wt.%) (5).

As observed with pristine CuA-Ox gel, CNM-containing composite gels also showed remarkable robustness, selfsupporting, load-bearing (Figures 5a-c) and self-healing properties within minutes (Figures 5d-g), allowing even the fusion between blocks of these two materials (Figure S13). The incorporation of CNMs at optimized concentrations resulted in the enhancement of the mechanical strength of pristine CuAOx gel as demonstrated by comparative rheological measurements (Figure S14 and Table S1). For instance, the incorporation of 5−10 g⋅L-1 of GO shifted the critical strain γc to 7.34− 11.12%, respectively (Figure S14). The latter value is almost twice as high as undoped CuA-Ox gel (6%), suggesting good interactions between the GO sheets and the gel matrix. For MWNTs (c = 10 g⋅L-1), SWNTs (c = 5 g⋅L-1) and G (c = 25 g⋅L-1), the maximum yield stress of the composites decreased ca. 10−30% with respect to that of undoped CuA-Ox (Figure S14). Therefore, the amount of CNMs incorporated into the gel can be used to fine-tune above and below its mechanical strength. All composites displayed reasonable high conductance or low resistance (in kilo-ohm order) (Table S2 and Figure S15). The ability to restore conductivity after macroscopic damage was also demonstrated by connecting a monolithic composite in a simple circuit having a light emitting diode (LED) bulb that was lit with a DC voltage of 3.0 V (Figures 5h-k). Cutting the monolith and subsequent selfhealing with the consequent restoration of the conductivity could be repeated several times showing the same efficiency than the first cycle.



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b

a

g

c

induce self-healing properties to a completely different supramolecular gel network.

d

cut into pieces f

reconnect

ASSOCIATED CONTENT Supporting Information Oscillatory rheological data including DSS, DFS and DTS plots, Nyquist plots, FTIR spectra, TGA curves, additional SEM images, resistivity data and additional digital photographs of different gel materials prepared in this work. The Supporting Information is available free of charge on the ACS Publications website.

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i

AUTHOR INFORMATION Corresponding Author

k

* Institut für Organische Chemie, Universität Regensburg, Universtitätsstr. 31, 93040 Regensburg, Germany. Fax: (+) 49-941943-4121, Tel: (+) 49-941-943-4373. E-mail: [email protected]

j

Author Contributions

Figure 5. a-c) CuA-Ox/MWNTs composite gel cylinder (1.1 cm × 2 cm, 1.5 g total weight) before (a), during (b) and after (c) application of a load of 100 g. The monolithic gel showed preservation of its original shape and dimensions; d-g) demonstration of the self-healing process of CuA-Ox/MWNTs composite gel; h-k) demonstration of the restoration of bulk conductivity of the CuAOx/MWNTs composite gel used to bridge an electrical circuit where a LED bulb was lit with a DC voltage. Cutting and reassembling the composite blocks allowed turning the LED off and on multiple times. In this figure the concentration of MWNTs was 10 g⋅L-1 in each case.

In conclusion, a proton conductive, moldable and selfhealing supramolecular metallogel (CuA-Ox gel) can be obtained in situ by combining Cu(OAc)2·H2O and oxalic acid dihydrate stock solutions in DMF. This system has the unprecedented ability to impart self-healing properties to other gel networks lacking this capacity, affording interpenetrated hybrid gels. In addition, self-healing conductive gel composites containing carbonaceous materials can be also fabricated using CuA-Ox gel as semi-solid dispersant medium. To the best of our knowledge this is the first example of a selfhealable supramolecular metallogel network that is proven to

T.F., M.H., S.S. and D.D.D. carried out the experimental work. J.E. carried out electron microscopy studies. D.D.D. and R.B. supervised the work. D.D.D. wrote the manuscript. All authors have contributed to the correction of the manuscript and have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding Sources University of Regensburg, Deutsche Forschungsgemeinschaft (DFG, 9209720), IQAC-CSIC and the Spanish Ministry of Economy and Competitiveness (CTQ2011-23842; CTQ2014-52687C3-1-P).

ACKNOWLEDGMENT Financial from Deutsche Forschungsgemeinschaft (DFG, 9209720), the University of Regensburg, IQAC-CSIC and the Spanish Ministry of Economy and Competitiveness (CTQ201123842; CTQ2014-52687-C3-1-P) are gratefully acknowledged. We thank Susana Vílchez (IQAC) for her assistance with SEM imaging and Dr. Selim Kara (Trakya University) for preliminary resistivity measurements. D.D.D. thanks DFG for the Heisenberg Professorship Award

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