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Article Cite This: Chem. Mater. 2018, 30, 3648−3655

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Enhanced Water Retention Maintains Energy Dissipation in Dehydrated Metal-Coordinate Polymer Networks: Another Role for Fe-Catechol Cross-Links? Sungjin Kim,† Amy M. Peterson,‡ and Niels Holten-Andersen*,† †

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Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02142, United States of America ‡ Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, United States of America S Supporting Information *

ABSTRACT: The change from wet and soft to dry and hard is a viscoelastic to solid material transition widely displayed in nature, in particular in materials rich in metal-coordinate cross-linking. How metal-coordinate cross-link dynamics contribute to macromolecular material mechanics upon solidification by dehydration remains an open question. Using mussel-inspired Fe-catechol cross-linked polymer hydrogels, we address this question. In addition to a nearly 2-fold increase in stiffness, we find that the presence of Fe-catechol coordination bonds in a dehydrated polymer gel also provides the bulk network with a significantly increased energy dissipation with over three times higher loss factor. We present evidence to suggest that small amounts (∼4 wt %) of locally bound water maintain the dynamic nature of Fe-catechol coordinate cross-links in a dehydrated polymer network. The dehydration-induced polymer material mechanics presented here may provide deeper insights on the biological utilization of metal-coordinate cross-link dynamics as well as inspire new ideas on sustainable materials engineering. the strong adhesion of the plaque that firmly tethers each thread to the substrate,32,33 both material properties which are partially attributed to the presence of Fe-catechol coordinate cross-links.34−38 Inspired by their apparent role in the robust mussel thread mechanics, Fe-catechol coordinate cross-link dynamics have been explored as an energy-dissipative crosslinking mechanism in the engineering of tough hydrogels.39,40 However, the contribution of metal-coordinate cross-link dynamics to the mechanics of mussel threads, or musselinspired polymer hydrogels, upon dehydration, has not yet been explored.29,30,41 Using 4-arm-PEG-catechol (4cPEG) hydrogels,42,43 herein we investigate if Fe-catechol cross-link dynamics continue to impact network mechanics upon gel dehydration. We present evidence to suggest that, in addition to their well-known dynamic mechanical contributions to polymer networks swollen in water, Fe-catechol coordinate cross-links also significantly increase energy dissipation of dehydrated polymer networks. To explain these observations, we present additional evidence to support the hypothesis that

1. INTRODUCTION Understanding the process by which biological materials are transformed from wet, compliant and soft into dry, stiff and hard materials is not only intriguing from a fundamental scientific perspective but also important from the viewpoint of bioinspired sustainable materials engineering. Though bones and shells are well-studied materials resulting from one such process (i.e., biomineralization1−7), wood,4,8−11 the outer layer of horns,12 fingernails,13 marine worm jaws,14−17 insect cuticles,18−21 and squid beaks18,22−24 are examples of materials solidified via a synergistic process of macromolecular crosslinking and dehydration, in which little to no mineralization occurs.19,24,25 The resulting mechanical properties of some of these unmineralized materials are comparable to biomineralized tissues such as bone or dentin, but with lower density.17,18 Nature also displays examples of loadbearing materials that experience repeated dehydration−rehydration cycles throughout their functional lifetime. For example, the byssal threads of intertidal mussels experience dehydrating−rehydrating dynamic conditions daily due to tidal cycles.26−28 Yet, to our knowledge, only a few studies29,30 have explored the effect of dehydration on mussel thread mechanical properties. In general, the mechanical robustness of the mussel holdfast is caused by the high energy dissipation of its threads31 and by © 2018 American Chemical Society

Received: December 19, 2017 Revised: May 8, 2018 Published: May 10, 2018 3648

DOI: 10.1021/acs.chemmater.7b05246 Chem. Mater. 2018, 30, 3648−3655

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Chemistry of Materials

where E is the Young’s modulus, σ is the stress, and ε is the strain. Experiments were performed in triplicate. We also note that tensile tests and other characterization under ambient conditions were performed in dry weather when the indoor R.H. was also measured to be ∼16%. 2.7. Dynamic Mechanical Analysis (DMA). To assess compressive storage and loss moduli (K′ and K″, respectively) and loss factor (tan δ = K″/K′) of the specimens, a DMA 242 E Artemis (NETZSCH-Gerätebau) was used. An individual hydrogel sample was predehydrated for at least 1 h within the DMA furnace (25 °C, 0% R.H., air atmosphere) before measurement to prevent damage of the initially soft sample by the load applied by the compression geometry. The isothermal analysis was followed for 9 or more hours at 1 Hz frequency (max amplitude, 240 μm; proportional factor, 1.1) at the same humidity and temperature in the furnace (i.e., 25 °C, 0% R.H., air atmosphere). Data were collected for 1 h after a total of 9 h of dehydration after the moduli had reached plateau values. Experiments were performed in triplicate. 2.8. Dehydration Test. To compare the dehydration profiles of Fe-catechol gels (i.e., 4cPEG, Fe (low pH), Fe (high pH)), samples (17.5 μL) were dehydrated in a desiccator with constant R.H. of 16% at 20 °C and weighed using a balance AG 204 (Mettler Toledo) at different time points during up to 50 h of dehydration. The weight was measured instantly after taking the sample out of the desiccator to minimize possible reabsorption of moisture. The mass loss, i.e., the ratio between the mass of lost water and the mass of the initial hydrogel (Δm/mi, wt %) was calculated using the following equation:

the Fe-catechol cross-links remain dynamic upon bulk network dehydration due to local water binding.

2. EXPERIMENTAL SECTION 2.1. Materials. 10 kDa 4-arm-PEG-NHS (4aPEG) was purchased from JenKem Technology USA, Inc. Iron(III) chloride hexahydrate (FeCl3·6H2O) was purchased from Fluka. All other chemicals were purchased from Sigma-Aldrich unless indicated otherwise. 2.2. Synthesis of 4cPEG. 4-arm-PEG-catechol (4cPEG) was synthesized with some modifications to a procedure reported elsewhere.44 In short, to substitute −NHS to −catechols, 2 g of 4aPEG was mixed with dopamine hydrochloride (dopa-HCl) (1.5× molar relative to -NHS) in a round-bottom flask, dissolved by adding 10 mL of anhydrous N,N-dimethylformamide (DMF) whereafter the reaction was started by adding triethylamine (TEA) (2.5× molar equivalent relative to −NHS). The reaction was protected with N2 gas and allowed to proceed for 12 h at 55 °C using a silicone oil bath. The reaction was stopped via cooling to room temperature. Then, 20 mL of chloroform and 20 mL of water was added so that salts in the aqueous phase and polymer product in the organic phase could be separated. After the organic phase was extracted from the mixture, Na2SO4 was added to this organic solution to remove remnant water. The organic solvent was then vaporized by rotary evaporation. The product was then purified via precipitating in diethyl ether, followed by redissolving in 5 mL of dichloromethane (DCM). After this cycle was repeated three or more times, the precipitate was dried under vacuum. Finally, the precipitate was redissolved in Milli-Q (DI) water, frozen under −80 °C, then lyophilized to obtain the final product. 2.3. Formation of Fe-4cPEG Hydrogels. Fe-4cPEG hydrogels were formed by mixing aqueous solutions of 4cPEG (200 mg/mL) and FeCl3·6H2O (400 mM) in a [catechol]:[Fe3+] ratio of 1:1. Dark green hydrogels instantly formed via Fe3+-oxidation-induced crosslinking of catechols, and after adjusting the gel pH to 3 by adding NaOH (200 mM) we denoted these gels as “Fe (low pH)”. To generate “Fe (high pH)” gels, the pH was instead increased to 12 by adding NaOH (400 mM) quickly after mixing 4cPEG and Fe3+, which immediately produced dark red gels upon further mixing. The final concentrations of NaOH in the hydrogels were 142 mM for Fe (low pH) and 571 mM for Fe (high pH). The final concentrations of 4cPEG and Fe3+ were 142.86 mg/mL and 57.1 mM, respectively, for all types of Fe-4cPEG hydrogels. 2.4. Formation of NaIO4-Induced Covalently Cross-Linked 4cPEG Hydrogels. To form the covalent-only cross-linked 4cPEG network, 4cPEG solution (200 mg/mL) was mixed with an equal volume of the oxidant NaIO4 (40 mM) in a [catechol]:[IO4−] ratio of 2:1. The mixture immediately developed an orange color, and at least 6 h of curing time was allowed for covalent network cross-linking before further characterization. The resulting gels were denoted as “NaIO4” gels. 2.5. UV−vis Spectroscopy. The UV-spectroscopic measurement was performed on hydrogels using a DS-11 Spectrophotometer/ Fluorometer (DeNovix) in microvolume measurement mode. Approximately 1 μL of hydrogel specimen was placed onto the lower stage surface; then the top arm was lowered for the top stage surface to contact the sample, followed by the measurement. DI water, loaded in the same way, was used to set the baseline of spectra. The UV−vis absorbance spectra were obtained between 250 and 650 nm. 2.6. Tensile Test. To obtain Young’s moduli (E) of dehydrated gels, tensile tests were performed using a tensile tester Z2.5 (Zwick/ Roell) with a 20 N load cell. The initially wet specimens were dehydrated for 48 h in dog bone- or bar-shaped acrylic molds under 16% R.H. at 20 °C in a desiccator unless specified otherwise. The measurement was performed under constant strain rate of 10 mm/min (i.e., ∼0.11 s−1 for the 1.5 mm measured sample length) and data were collected in the range of 0.1% maximum strain. Young’s moduli were calculated at 0.06% strain in the linear region. The modulus was calculated by the following equation: σ E= (1) ε

Mass Loss (wt %) =

m −m Δm × 100 = i × 100 mi mi

(2)

where Δm is the mass of lost water, mi is the initial mass of the hydrogel, and m is the mass of the gel at the selected time point. Note that the full dehydration profile for covalent hydrogels produced by NaIO4-oxidation is omitted, because their initial composition and gelation process is different from all other samples, which prevented meaningful comparison. Experiments were performed in triplicate. 2.9. Thermogravimetric Analysis (TGA). A TGA Q500 (TA Instruments) was used to obtain the dehydration profiles upon temperature increase of samples prepared as described above. Samples were heated from 23 to 300 °C at a rate of 5 °C/min in the furnace (0% R.H.) with an N2 atmosphere unless indicated otherwise. The mass loss was calculated by eq 2. Derivative thermogravimetric analysis (DTG) was performed using the TA Analysis 2000 Software (TA Instruments) to examine the rate of mass loss (%) as a function of temperature (dm/dT, %/°C) and time (dm/dt, %/min). 2.10. Dissolution Test. Dissolution tests were performed by immersing gels in over 50× volume of DI water for over 40 h. After the excess aqueous solution was carefully removed, the mass fraction (Φ, %) of the permanent (Φp) and transient (Φt) fractions of the gels were determined by the following equations:

Φp =

md × 100 m0

(3)

Φt =

m0 − md × 100 m0

(4)

where m0 is the dehydrated mass of a sample before dissolution and md is the dehydrated mass after any dissolution. The dehydrated mass (md) was obtained by dehydrating the wet, swollen gel using the standard dehydration (20 °C, 16% R.H. for 48 h). After dissolution tests, the samples were subjected to additional TGA testing with the same experimental conditions as described before. Experiments were performed in triplicate. 2.11. Raman Spectroscopy. Raman spectra were obtained using a Raman Spectrometer-LabRAM Raman confocal microscope (HORIBA Jobin Yvon). A 785 nm near-IR laser excitation was used in combination with a 10× lens. Samples were loaded on a glass substrate, which was placed on a Märzhäuser stage (Märzhäuser Wetzlar), followed by positioning under the microscope lens. The laser 3649

DOI: 10.1021/acs.chemmater.7b05246 Chem. Mater. 2018, 30, 3648−3655

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Chemistry of Materials power was 30 mW. The grating was 600 g/mm, and the filter was adjusted to 10%. The integration time was 0.5 s (or longer, up to 20 s, to increase the signal intensity) with an accumulation of 60 times. The samples were dehydrated for 24 h at 16% R.H. and 20 °C before the measurement. Measurements and data collection were performed using LabSpec 6 (Horiba Scientific) software.

3. RESULTS 3.1. Cross-Linking of PEG-Catechol Polymer Networks. We follow a PEG-catechol hydrogel assembly method inspired by the proposed mussel thread processing pathway (Figure S1),45−48 with an initial low pH mixing step at a 1:1 molar ratio between Fe3+ and catechol and subsequent Fecatechol coordinate cross-link formation induced by a pH jump.48 For comparison, we also assembled PEG-catechol hydrogels with only covalent cross-links, using NaIO4 as the oxidizer and without pH jump.42 The two different hydrogels are labeled “Fe (high pH)”, for the mussel-inspired hybrid network produced from the Fe-mediated cross-linking resulting in both covalent and coordination cross-links, and “NaIO4”, for the only covalently cross-linked network formed by NaIO4induced oxidative cross-linking (Figure 1a). Please see Experimental Section for details of the hydrogel processing methods. The two distinct hydrogel cross-linking processes were confirmed by UV spectroscopy (Figure 2, S1). An UV absorbance maximum ∼480 nm is evidence of the Fe-catechol charge transfer complex,43,49 which is clearly seen in the Fe (high pH) hydrogel, but is absent in the NaIO4 hydrogel. This

Figure 2. UV−vis absorbance spectra of hydrogel samples before dehydration; Fe (high pH) (red), Fe (low pH) (green), NaIO4 (orange) and a pure solution of 4cPEG (navy). The insets are photographs of the corresponding hydrogels.

observation is in agreement with a high pH-induced triscatechol-Fe coordination complex. The catechol absorbance peak ∼280 nm,49,50 observed in the Fe (high pH) gel, and a pure 4cPEG control, shifts to a slightly shorter wavelength ∼266 nm in the NaIO4 hydrogel spectrum, which is in agreement with the oxidative covalent coupling of catechol.49 The additional shoulder at ∼395 nm in the NaIO4 spectrum likely corresponds to quinone groups,50 another indicator of oxidation-induced covalent cross-linking of catechols. The peak at ∼320 nm in a hydrogel control sample with a 1:1 Fe:catechol ratio but no pH jump (labeled Fe (low pH)), also supports oxidation-induced covalent cross-linking of catechols via DOPA-indole derivatives or α,β-dehydro-DOPA as reported in melanin formation or insect cuticle sclerotization, respectively.42 Although it should be noted that the absorbance spectra of catechol compounds in the UV range have not been fully characterized, these results support covalent catecholbased cross-linking in both Fe (high pH) and NaIO4 gels. 3.2. Mechanical Properties of the Dehydrated CrossLinked Networks. To compare the mechanical properties of the Fe (high pH) and NaIO4 gels after dehydration, we utilized tensile testing and dynamic mechanical analysis (DMA) in compression (Figure 1b, S2). Tensile tests were performed on samples after 48 h of dehydration at 20 °C, 16% R.H. (see Experimental Section for details on dehydration conditions). Fe (high pH) networks display a Young’s modulus of ∼1.8 GPa, approximately twice that of NaIO4. In DMA, the Fe (high pH) similarly show higher compressive storage modulus as well as higher loss modulus (Figure S3a) and loss factor (Figure 1b, S3b). In fact, with a loss factor above 0.7, the dehydrated Fe (high pH) gels display good mechanical damping properties.51 These results demonstrate that the dehydrated Fe (high pH) network is not only stiffer under both tension and compression but also significantly more energy-dissipative than a dehydrated covalent-only network (NaIO4). 3.3. Water Retention by Fe-Catechol Coordination. To better understand the enhanced energy-dissipative properties of the dehydrated Fe (high pH) network, we investigated its dehydration profile in comparison to the Fe (low pH)

Figure 1. (a) The polymer cross-links formed via mussel-inspired Fe3+-mediated network cross-linking45−48 -Fe (high pH) (left) and via catechol oxidation using NaIO4 (right). Note that the illustration is conceptual and not meant to represent the detailed chemical structure of the network. For more details, please see Figure S1. (b) Young’s moduli (E, left axis, solid columns) and loss factors (tan δ, right axis, dotted columns) of the Fe (high pH) (red) and NaIO4 (orange) hydrogels after dehydration obtained using tensile testing and compressive DMA, respectively. 3650

DOI: 10.1021/acs.chemmater.7b05246 Chem. Mater. 2018, 30, 3648−3655

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Figure 3. (a) Dehydration profiles of Fe (high pH) (red), Fe (low pH) (green) hydrogels and a 4cPEG aqueous solution (blue) with equivalent polymer concentration at 20 °C, 16% R.H.. The mass loss (wt %) is estimated as the ratio between the lost mass and the initial mass of the hydrogel (eq 2 in the Experimental Section). The horizontal dashed lines indicate the maximum possible water mass loss from each sample calculated from the composition of water and nonwater components of the initial hydrogel. The inset shows the dehydration profile from 10 to 50 h. (b) Mass loss measured by TGA. (c) Raman spectra of the given samples after dehydration. Please see Table 1 for peak assignments.

hydrogel, as well as pure 4cPEG (Figure 3, S4, S5). We used Fe (low pH) in lieu of NaIO4 as a comparison to the dehydration kinetics of a covalent network, to keep the gelation time and the material composition similar during the dehydration. The Fe (low pH) and pure 4cPEG display a mass loss equivalent to the original amount of water present in the hydrated gels within 24 h of dehydration, whereas Fe (high pH) gels show slower and lower mass loss, which suggests increased water retention. We note that the NaIO4 gel also reached its maximum possible water mass loss within 4 h (Figure S4a). TGA further confirms the increased water retention of Fe (high pH) upon heating to 300 °C (Figure 3b). To investigate the role of Fe-catechol coordination in this water-retention effect, we performed a control experiment on a sample of nonfunctionalized 4-armPEG (4aPEG), i.e., without catechol modification, prepared identically to Fe (high pH) (Figure S4). This specimen (4aPEG Fe (high pH)) displayed faster and greater dehydration than the Fe (high pH) gel under both ambient conditions and in a heating scan (Figure S4a and S4b, respectively), ultimately reaching its maximum possible water mass loss. DTG analyses (Figure S5) further confirmed that the Fe (high pH) gel shows

the slowest dehydration rate upon heating. These results provide further evidence of the enhanced affinity to water of the Fe (high pH) network. They also indicate that the interaction between water and Fe3+ coordinated with catechols is stronger than the possible water-retaining effect of salts in the Fecontaining networks. To explore the possible molecular mechanism underlying the increased water retention of Fe (high pH) networks, we performed Raman spectroscopic analyses on dehydrated gels of Fe (high pH), Fe (low pH), and dried out 4cPEG (Figure 3c). The measured vibration bands for catechols agree well with previous studies43,50 (Table 1): (i) phenyl ring vibrations from ∼1250 to ∼1480 cm−1, (ii) distinct coordination peaks at ∼590 and ∼635 cm−1 from the interactions between the oxygen of catechols and Fe3+, as well as a peak at ∼530 cm−1 from the charge transfer interaction from bidentate chelates clearly seen in Fe (high pH), whereas (iii) a broad peak from ∼500 to ∼650 cm−1 of Fe (low pH) indicates weak coordination between catechol and Fe3+. In addition, a vibration band at ∼315 cm−1 is observed uniquely in the Fe (high pH) network, which correlates with previous peak assignments of Fe−H2O 3651

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Chemistry of Materials Table 1. Assignment of the Observed Raman Peaks in Figure 3ca Approx. Raman Shift (cm−1) 315 530

590, 635

1250−1480

Assignment O−Fe−O stretching from Fe−H2O interaction charge transfer interaction from bidentate chelate interaction between the oxygens of catechols and Fe3+ phenyl ring vibrations

Present in

Note

Fe (high pH) Fe (high pH), Fe (low pH)*

* broad, undistinguished peak

Fe (high pH), Fe (low pH)* Fe (high pH), Fe (low pH), 4cPEG

The vibration band from the O−Fe−O deformation at ∼315 cm−1 attributed to the Fe−H2O interaction52−54 is only present in the Fe (high pH) sample. a

interactions around 315−332 cm−1 originating from O−Fe−O stretching.52−54 In another catechol-Fe system, Schmitt et al.54 suggested that the peak at this Raman shift is caused by water molecules filling positions in the first shell of the catechol-Fe coordination sphere. To further support the hypothesis that this peak corresponds to unique Fe−H2O interactions in the catechol-Fe coordination sphere, we performed two additional experiments. First, the Fe (high pH) sample was prepared in D2O instead of H2O, which caused a slight downshift of the peak as expected if this peak correlates to water-binding (Figure S6).55 Second, the Fe (high pH) sample was lyophilized, which resulted in an additional ∼1% water removal (Figure S7a) accompanied by a decrease in the relative peak intensity at ∼315 cm−1 (Figure S7b). Finally, to confirm that the proposed enhanced water-binding capability of Fe-catechol coordination complexes is thermodynamic rather than kinetic in nature, we verified that the water retention of Fe (high pH) is indeed independent of temperature increase rates (Figure S8). Because both the increased mechanical damping and water retention correlate with the presence of Fe-catechol coordinate cross-links in the dehydrated network, we tested whether removing the Fe-catechol transient network fraction in the Fe (high pH) gel would decrease the water binding capacity upon dehydration of the remaining covalently cross-linked network. By using mass lost by dissolution as a measure of the transient fraction Φt of the network, and the remaining undissolved mass as a measure of the permanent covalent fraction Φp, we found Φt in Fe (high pH) gels to be ∼67%, whereas in NaIO4 gels Φt ∼ 0% (i.e., Φp ∼ 100%) (Figure 4a,b). After removal of the Fecatechol transient network fraction, the water binding ability of the remaining covalent fraction of the Fe (high pH) network decreases and becomes similar to that of a NaIO4 network with only covalent bonds (Figure 4c). This observation further supports that the water retained in the dehydrated Fe (high pH) network is preferentially bound to the transient Fecatechol cross-links.

Figure 4. (a) Fe (high pH) (left) and NaIO4 (right) immersed in DI water for 40+ hours (upper row) and after the removal of water (lower row). (b) Comparison between the covalently cross-linked permanent fraction Φp (solid) and the Fe-catechol coordinate cross-linked transient fraction Φt (patterned) of the two networks calculated by eqs 3 and 4 in the Experimental Section, respectively. (c) TGA of Fe (high pH) hydrogels before (solid red) or after (dashed red) dissolution of the Φt and of a NaIO4 hydrogel (dotted orange).

dissipation of a similar hybrid network compared to a covalentonly network.48 However, that study was performed with hydrogels, whereas we have demonstrated for the first time that these mechanical differences persist even after dehydration. The network shrinkage during dehydration will induce significant physical interactions between the transient and permanent network fractions of the hybrid gels, even if, as suggested by the data in Figure 4b, the two fractions are covalently unconnected (see Figure S1a for further details). Because Fe-catechol bonds are stronger than other types of noncovalent interactions,56 the combined coordinate and covalent cross-links will increase the overall effective crosslink density and thereby increase the elastic modulus of this dehydrated hybrid network compared to the covalent-only network. Observations similar to this synthetic hybrid polymer network demonstration have been documented in the cuticles of the mussel thread and the nereis jaw; in both materials, metal-coordinate cross-linking has been reported to significantly increase stiffness compared to the chelated metal-free covalently cross-linked material.17,54,57 Importantly, in addition to the increased stiffness compared to covalent-only networks, the synthetic hybrid networks also retain an enhanced energy dissipation upon dehydration. We speculate that the proposed water retention helps to preserve a more dynamic energy-dissipative state of the metal coordinate bonds in the hybrid networks after dehydration. Interestingly, Fe-catechol coordinate bonds were also recently demonstrated to maintain their kinetically labile state and dramatically

4. DISCUSSION In this work, we report that a hybrid covalent and Fe-catechol coordinate cross-linked PEG network, initially established as a hydrogel via a mussel-inspired two-step cross-linking process, after dehydration displays considerably enhanced stiffness and energy dissipation when compared to a dehydrated NaIO4induced covalently cross-linked network. A previous study by Barrett et al. likewise reported increased stiffness and energy 3652

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enhance energy dissipation in a mussel-inspired dehydrated elastomer.58,59 We are currently further investigating the relationship between bond dynamics and the proposed water retention capability of metal-coordinate cross-links. Using the Fe (high pH) data in Figure 3a, we can crudely estimate the number of water molecules retained in our dehydrated hybrid networks to be ∼50-fold higher than the equimolar number of Fe3+ or catechols (see SI for details on calculations). While attempted explanations of how this water is bound within the hybrid network remain speculative, our Raman data (Figure 3c) and dehydration control tests (Figure S4, S5) suggest that catechol-Fe coordination complexes are the main source of the water binding.54 Hence, although the details of these proposed water-Fe-catechol coordinate complex interactions are not yet understood, our estimates would suggest multiple layers of hydration shells per coordination complex, which is in agreement with the numbers reported for biomacromolecules in general.60,61 It is possible that part of the population of Fe-catechol complexes are of the bis-type form, wherein water molecules are bound extra tight in the first coordination shell by serving as ligands. It is generally known that the potential energy of water molecules can be significantly lower near the polar or dipolar region of a macromolecule due to their nonrandom average orientation compared to that in bulk water.62 In addition, if the water molecules interact favorably with the macromolecule surface (e.g., via H-bonds, dipole−dipole interactions, etc.), the water close to the surface becomes very difficult to remove.62,63 Likewise, we infer that the water molecules near a Fe-catechol complex can be strongly bound, possibly via the coordination we propose, effectively resisting external perturbations such as heating (Figure 3b, S4b) or freezing (Figure S7). We also note that in a recent study of another catechol-functionalized polymer material, Ca-catechol ionic complexes were suggested to aggregate upon dehydration to form concentrated metal ion-clusters with a high water uptake.64 The water retention of Fe-catechol coordination complexes presented in our study could plausibly be enhanced by a similar cluster formation induced upon gel dehydration, a mechanism which will be further investigated in future studies. Furthermore, we note that a Raman peak at ∼315 cm−1, possibly associated with water-Fe-catechol coordinate complex interactions as we propose here, was also observed in the dehydrated Fe-catechol-cross-linked elastomer mentioned above,58 as well as in Fe-treated mussel thread cuticle.37 Hence, the proposed water binding by Fe-catechol coordination complexes may be affecting cross-link bond dynamics, and thereby possibly dehydrated material mechanics, of both native mussel threads and their bioinspired derivatives.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05246. Calculation of remnant water molecules; cross-linking pathways; pictures of gels upon dehydration; stress-strain curves; DMA profiles; additional dehydration, TGA, DTG profiles and Raman spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*N. Holten-Andersen. E-mail: [email protected]. ORCID

Sungjin Kim: 0000-0002-2068-9189 Amy M. Peterson: 0000-0002-4612-0062 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by the Office of Naval Research (ONR) under the Young Investigators Program Grant ONR.N00014-15-1-2763 is gratefully acknowledged. This work was supported in part by the MRSEC Program of the National Science Foundation under Award DMR-1419807. The first author thankfully acknowledges financial support by the 12th Samsung Scholarship Program and helps by Hyunwoo Yuk from Prof. Xuanhe Zhao’s Group (Department of Mechanical Engineering, MIT) for tensile tests and Dr. Eugene N. Cho from Prof. Jeffrey C. Grossman’s Group (Department of Materials Science and Engineering, MIT) for TGA. He also thanks Xuejian Lyu from Prof. Amy M. Peterson’s Group (Department of Chemical Engineering, WPI) for his assistance in using DMA. Finally, we thank Prof. Herbert Waite (Department of Molecular, Cellular, and Developmental Biology, UCSB) and Dr. Emmanouela Filippidi (Department of Mechanical Engineering, UCSB) for constructive discussion.

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REFERENCES

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5. CONCLUSION Using the pH-controlled two-step macromolecular cross-linking mechanism inspired by the mussel holdfast, we have presented evidence to suggest that Fe-catechol coordinate cross-linking can increase the energy dissipation in largely dehydrated polymer networks by more than 3-fold through enhanced water retention. The possible interplay between transient crosslink bond dynamics and microscopic water binding of Fecatechol, and perhaps other metal−ligand coordinate complexes, may allow energy dissipative mechanical properties to be retained during dehydration-processed viscoelastic-to-solid transitions of existing biological and future bioinspired macromolecular materials. 3653

DOI: 10.1021/acs.chemmater.7b05246 Chem. Mater. 2018, 30, 3648−3655

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DOI: 10.1021/acs.chemmater.7b05246 Chem. Mater. 2018, 30, 3648−3655