Assembling Vanadium(V) Oxide and Gelatin into Novel

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398 Chem. Mater. 2010, 22, 398–408 DOI:10.1021/cm902836g

Assembling Vanadium(V) Oxide and Gelatin into Novel Bionanocomposites with Unexpected Rubber-like Properties Florent Carn,† Olivier Durupthy,† Bruno Fayolle,‡ Thibaud Coradin,*,† Gervaise Mosser,† Marc Schmutz,§ Jocelyne Maquet,† Jacques Livage,† and Nathalie Steunou*,† Laboratoire Chimie de la Mati ere Condens ee de Paris - UMR CNRS 7574, UPMC, Univ Paris 06, Coll ege de France, 11 place Marcelin Berthelot 75231 Paris cedex 05, France, ‡Laboratoire Ing enierie des Mat eriaux, UMR CNRS 8006, ENSAM, 151 Bld de l’H^ opital 75013 Paris, France, and §Institut Charles Sadron, Campus CNRS, 23 rue du LOESS, BP 84047, 67034 Strasbourg Cedex 2, France †

Received September 10, 2009. Revised Manuscript Received November 27, 2009

A new kind of bionanocomposites is prepared for the first time in soft conditions by a complex coacervation process and consists of an assembly of decavanadate polyanions and gelatin chains in triple helices and coils conformation. Before drying, well-defined homogeneous monoliths with a striking rubber-like behavior were obtained. These mechanical properties were thoroughly characterized by tensile measurements at large strain revealing a complex behavior (strain hardening, large hysteresis during cycling experiments, Mullins effect) associated with characteristics (E=0.27 MPa, λbreak =8.6, and σbreak=1.4 MPa) that were never reported for gelatin based materials. These results were discussed in the frame of classical biopolymer-based materials and in the emerging field of rubbery polyelectrolyte hydrogels. Compared to gelatin based composites which depict a ductile behavior, the improved strain properties of decavanadate-gelatin composite may be attributed to an adequate gelatin triple helices/coils ratio which depends strongly on gelatin concentration, aging time, temperature, and also on the charge matching between decavanadate and gelatin. Upon aging, the nucleation and growth of V2O5 ribbon like particles occurs in situ in the gelatin matrix that exerts a significant control on vanadium condensation from a kinetics and structural point of view. Introduction Bionanocomposites represent a new generation of nanostructured hybrid materials that combine inorganic nanoparticles with natural, usually abundant, and lowcost biodegradable polymers. They exhibit structural and functional properties which are close to those of nanocomposites derived from synthetic polymers (mechanical properties, thermal stability, gas-barrier properties) and/ or inherent to biopolymers (biocompatibility, biodegradability). Therefore, these composites are of great interest for different potential applications, including heterogeneous catalysis, biosensors, bioelectrochemistry, regenerative medicine, drug vectorization, and so on.1-5 Furthermore, promising perspectives in the design of new architectures are expected by taking into account the *Address correspondence to either author. Phone: þ33(0)144271527. Fax: þ33(0)144271504. E-mail: [email protected] (T.C.); nathalie.steunou@ upmc.fr (N.S.).

(1) Darder, M.; Aranda, P.; Ruiz-Hitzky, E. Adv. Mater. 2007, 19, 1309. (2) Ruiz-Hitzky, E; Darder, M.; Aranda, P. J. Mater. Chem. 2005, 15, 3650. (3) (a) Walcarius, A.; Mandler, D.; Cox, J. A.; Collinson, M.; Lev, O. J. Mater.Chem. 2005, 15, 3663. (b) Mousty, C. Appl. Clay Sci. 2004, 27, 159. (4) Avnir, D.; Coradin, T.; Lev, O.; Livage, J. J. Mater. Chem. 2006, 16, 1013. (5) (a) Molvinger, K.; Quignard, F.; Brunel, D.; Boissiere, M.; Devoisselle, J.-M. Chem. Mater. 2004, 16, 3367. (b) El Kadib, A.; Molvinger, K.; Guimon, C.; Quignard, F.; Brunel, D. Chem. Mater. 2008, 20, 2198.

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self-organization properties of biopolymers and the possible synergy between the inorganic and biological components.6 These materials can be prepared in soft conditions as solid monoliths, films, or hydrogels as recently reviewed by Ruiz-Hitzky et al.1,2,7,8 usually via the simple mixing in a common solvent of the inorganic species (molecular precursors or preformed nanoparticles) with the biomacromolecules. However the development of novel biohybrid materials in the wet state (hydrogel) that display a homogeneous distribution of inorganic entities as well as exhibit multiple functionalities remains a main challenge. First steps in this area were dedicated to the development of bionanocomposites that could find applications in biomedical or environmental fields. Therefore, they were based on biogenic inorganic components with well-known biocompatibility such as silica, clay minerals, iron oxide, calcium carbonate, or hydroxyapatite.1,2,8-11 (6) (a) Sanchez, C.; Arribart, H.; Giraud-Guille, M.-M. Nat. Mater. 2005, 4, 277. (b) Stupp, S. I.; Donners, J. J. J. M.; Li, L.; Mata, A. MRS Bull. 2005, 30, 864. (7) Ruiz, A. I.; Darder, M.; Aranda, P.; Jimenez, R.; Van Damme, H.; Ruiz-Hitzky, E. J. Nanosci. Nanotechnol. 2006, 6, 1602. (8) Darder, M.; Ruiz, A. I.; Aranda, P.; Van Damme, H.; Ruiz-Hitzky, E. Curr. Nanosci. 2006, 2, 231. (9) Zhang, C. P.; Hu, L.; Luo, Y. P.; Deng, W.; Gao, X.; Qiu, X. Y.; Shen, D. Acta Mater. Comp. Sin. 2002, 19, 54. (10) Forano, C.; Vial, S.; Mousty, C. Curr. Nanosci. 2006, 2, 283. (11) (a) Coradin, T.; Mercey, E.; Lisnard, L.; Livage, J. Chem. Commun. 2001, 2496. (b) Benmouhoub, N.; Simmonet, N.; Agoudjil, N.; Coradin, T. Green Chem. 2008, 10, 957.

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However, these mineral systems represent only a small proportion of the wide range of different inorganic materials that can be prepared at the nanoscale. As a result, the restriction of bionanocomposite formation to biogenic inorganic materials narrows the range of available properties, such as optical, fluorescence, or conductivity, that can be highly desirable in the design of functional materials. In this context, recent results have shown the great interest of interfacing a biological entity with nonbiogenic metal oxides or metal nanoparticles. Such assemblies were performed with the aim of improving electrochemical systems or designing new nanotechnological devices. As an example, a few DNA biochips based on DNA grafting on semiconductor oxides were used for the electrical detection of DNA hybridization.12 It was reported that bionanocomposites of gelatin and Ca2Nb3O10 perovskite exhibit a higher increase of the dielectric permittivity than the dielectric loss in the samples, suggesting potential applications in the microwave industry and high-frequency devices.7 Even superconducting nanowires could be formed with the help of alginate.13 Indeed, in these systems, the biocompatibility of the inorganic component is not an issue because the materials are not designed to function in vivo or in the presence of a living system. In this case, the main advantage of using macromolecules of natural origin is related to its chemical complexity and self-assembly properties, to which no synthetic equivalent is usually available, together with its large abundance and nonfossil origin, two key aspects for the development of “green” materials. In the present study, we show that the assembly of vanadium oxide and gelatin through a complex coacervation process leads to a homogeneous functional bionanocomposite displaying a striking rubber-like behavior at large strain. Vanadium oxide materials have been extensively investigated for a broad range of electronic, magnetic and catalytic properties and are currently used as positive electrodes in rechargeable lithium batteries, catalytic materials, electrical switching devices, and thermochromic windows.14 Among protein biopolymers, gelatin is widely used in food, pharmaceutical, and photographic industries due to its ability to form transparent and elastic gels that can be described as an entangled assembly of rigid triple helices connected by flexible links.15 Recent studies on the association of gelatin with calcium phosphate, hydroxyapatite, clay minerals, silica, layered perovskites, and metallic nanoparticles suggest that this protein can both efficiently control the growth of (12) (a) Stambouli, V.; Zebda, A.; Appert, E.; Guiducci, C.; Labeau, M.; Diard, J.-P.; Le Gorrec, B.; Brack, N.; Pigram, P. J. Electrochim. Acta 2006, 51, 5206. (b) Zebda, A.; Stambouli, V.; Labeau, M.; Guiducci, C.; Diard, J.-P.; Le Gorrec, B. Biosens. Bioelectron. 2006, 22, 178. (13) Schnepp, Z. A. C.; Wimbush, S. C.; Mann, S.; Hall, S. R. Adv. Mater. 2008, 20, 1782. (14) (a) Livage, J. Chem. Mater. 1991, 3, 578. (b) Chirayil, T.; Zavalij, P. Y.; Whittingham, M. S. Chem. Mater. 1998, 10, 2629. (c) Wang, Y.; Cao, G. Chem. Mater. 2006, 18, 2787. (15) (a) Joly-Duhamel, C.; Hellio, D.; Ajdari, A.; Djabourov, M. Langmuir 2002, 18, 7158. (b) Joly-Duhamel, C.; Hellio, D.; Djabourov, M. Langmuir 2002, 18, 7208.

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inorganic materials and form optically transparent matrix in which inorganic particles are homogeneously dispersed, leading to the formation of thin films or hybrid particles that exhibit optic, photochromic, sensing properties, or potential use in biomedical applications.7,8,16 Up to now, the association between vanadium oxides and biopolymers has not been widely explored17 despite the fact that the interactions between these components at the molecular level may lead to complementary and synergetic properties. Recently, we have established that electrostatic interactions between decavanadate anionic clusters ([H2V10O28]4-) and positively charged gelatin chains could induce a two-step liquid-liquid phase separation process fully consistent with a complex coacervation behavior.18,19 Up to now, such a process involving proteins and organic polyelectrolytes is currently used as a microencapsulation tool20 in the pharmaceutical, cosmetics, and food industries or as a method for protein purification.21 In contrast, hybrid coacervates involving inorganic nanoparticles has been rarely reported in the literature and consist mainly of microsphere assemblies of silica or gold nanoparticles, CdS/CdSe quantum dots with synthetic block copolypeptides.22 Recently, magnetic microspheres, core-shell aggregates,23 and nanorods24 were obtained by assembling superparamagnetic iron oxide nanoparticles with polyelectrolytes and cationic-neutral copolymers. While detailed studies of the decavanadate/gelatin interactions in solution were previously reported, we show here how this complex coacervation behavior can be used to design well-defined materials, in which the weak mechanical properties of gelatin hydrogels are converted into a striking rubber-like behavior. We propose a detailed description of these unexpected rubbery properties at large strain and study the influence of the decavanadate clusters on these mechanical properties. Furthermore, we reveal that the interactions between (16) (a) Coradin, T.; Marchal, A.; Abdoul-Aribi, N.; Livage, J. Colloids Surf., B 2005, 44, 191. (b) Allouche, J.; Boissiere, M.; Helary, C.; Livage, J.; Coradin, T. J. Mater. Chem. 2006, 16, 3120. (c) Sasai, R.; Itoh, H.; Shindachi, I.; Shichi, T.; Takagi, K. Chem. Mater. 2001, 13, 2012. (d) Bigi, A.; Bracci, B.; Panzavolta, S. Biomaterials 2004, 25, 2893. (e) Furuichi, K.; Oaki, Y.; Imai, H. Chem. Mater. 2006, 18, 229. (f) Yang, Z.; Jiang, Y.; Yu, L. X.; Wen, B.; Li, F.; Sun, S.; Hou, T. J. Mater. Chem. 2005, 15, 1807. (17) Lutta, S. T.; Dong, H.; Zavalij, P. Y.; Whittingham, M. S. Mater. Res. Bull. 2005, 40, 383. (18) Carn, F.; Steunou, N.; Djabourov, M.; Coradin, T.; Ribot, F.; Livage, J. Soft Matter 2008, 4, 735. (19) Carn, F.; Djabourov, M.; Coradin, T.; Livage, J.; Steunou, N. J. Phys. Chem. B. 2008, 112, 12596. (20) (a) Luzzi, L. A.; Gerraughty, R. J. J. Pharm. Sci. 1967, 56, 634. (b) Madan, P. L.; Madan, D. K.; Price, J. C. J. Pharm. Sci. 1976, 65, 1476. (c) Cooper, C. L.; Dubin, P. L.; Kayitmazer, A. B.; Turksen, S. Curr. Opin. Colloid Interface Sci. 2005, 10, 52. (21) Wang, Y.-F.; Gao, J. Y.; Dubin, P. L. Biotechnol. Prog. 1996, 12, 356. (22) (a) Cha, J. N.; Birkedal, H.; Euliss, L. E.; Bartl, M. H.; Wong, M. S.; Deming, T. J.; Stucky, G. D. J. Am. Chem. Soc. 2003, 125, 8285. (b) Cha, J. N.; Bartl, M. H.; Wong, M. S.; Popitsch, A.; Deming, T. J.; Stucky, G. D. Nano Lett. 2003, 3(7), 907. (23) (a) Berret, J-F; Schonbeck, N.; Gazeau, F.; El Kharrat, D.; Sandre, O.; Vacher, A.; Airiau, M. J. Am. Chem. Soc. 2006, 128, 1755. (b) Toprak, M. S.; McKenna, B. J.; Mikhaylova, M.; Waite, J. H.; Stucky, G. D. Adv. Mater. 2007, 19, 1362. (24) Fresnais, J.; Berret, J.-F.; Frka-Petesic, B.; Sandre, O.; Perzynski, R. Adv. Mater. 2008, 20, 3877.

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the decavanadate clusters and the gelatin chains strongly affect the V2O5 network growth by slowing down the condensation process and preventing the regular V2O5 layers from stacking in the material.

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All chemicals were commercially available and used without further purification. Preparation of Vanadate Solutions. Polyoxovanadates have been prepared by the acidification of aqueous solution of NaVO3 (Sigma-Aldrich, >99%), of [V] concentration between 0.1 and 0.5 mol 3 L-1 with a proton exchange resin (DOWEX 50WX 4-100 mesh) according to a procedure used for the synthesis of V2O5 3 nH2O gels.25 A clear yellow solution (pH 1) was obtained after full proton exchange that progressively turns into a red gel after about 12 h. Preparation of Gelatin Solutions. Commercial gelatin extracted from porcine skin (type A with a bloom of ∼175 g corresponding to an average molecular weight of 40 000 g 3 mol-1 and an isoelectric point (IEP) close to 8 (according to the supplier) was purchased from Sigma-Aldrich. Gelatin solutions were prepared by swelling the gelatin granules in an aqueous solution during a minimum of 3 h at 5 °C. Gelatin was then dissolved at 50 °C using a magnetic stirrer for 30 min at 300 rpm. A viscosity of 9.9  10-3 Pa 3 s has been measured for a 10 wt.% pure gelatin solution at 45 °C. Preparation of Gelatin Simple Coacervates. When gelatin is dissolved, the pH is adjusted to pH 8 ≈ IEP with an aqueous NaOH solution (1 M) and the temperature is fixed at 40 °C. Then 750 mL of an ethanol solution at 40 °C is progressively added to 150 mL of the gelatin solution ([G] = 10 wt.%) until the macroscopic phase separation. The as-prepared biphasic mixtures are finally aged during 20 min at 40 °C. Preparation of OVP. When gelatin is dissolved, the pH is adjusted to pH 1 with an aqueous HCl solution (2 N) and the temperature is fixed at 40 °C. Then 150 mL of the vanadate solution ([V] = 0.5 M) at 40 °C is progressively added to 150 mL of the gelatin solution ([G] = 10 wt.%) until the macroscopic phase separation. The as-prepared biphasic mixtures are finally aged during 20 min at 40 °C. Formulation of the Coacervate Tablets. After macroscopic phase separation, the simple and complex coacervates were heated with their supernatant solution at 70 °C for 15 min and then 90 °C for 5 min before being poured without their supernatant solution into rectangular molds (length = 6 cm, width= 9 cm) thermalized at 70 °C. In order to avoid the formation of a brittle surface due to drying, which could not be reversibly reswollen, the molds are immediately capped in an almost hermetic manner. The rubbery coacervate “thick” tablets (Figure 5a) were finally formed by cooling at room temperature during 15 min and then kept at 2 °C for one day. Tensile Test Measurements. In order to perform tensile measurements at a given relative weight loss of solvent, the drying kinetic of the nanocomposite samples was measured in the experimental conditions used for the mechanical test at a temperature (T) of 23 °C and a humidity rate (H.R.) of 54%. (See the Supporting Information (SI)). The drying kinetic is notably higher in the case of simple gelatin coacervates probably due to the higher proportion of solvent present in these samples

and to the more volatile nature of ethanol. The mechanical investigations have been mainly performed on “wet” samples displaying a relative weight loss of 0.04. Nevertheless some measurements have been performed at Δm/m0 =0.07 and 0.20 in order to characterize the evolution of the sample mechanical behavior during drying. Tensile measurements performed on wet samples (Δm/m0 = 0.04) were carried out after 5 min of drying for simple gelatin coacervates and after 10 min for OVP. Taking into account the very short duration of a typical tensile experiment (t ≈ 30 s), we consider that the effects of drying are negligible during the course of our measurements. The mechanical tests were carried out on an INSTRON 4302 tensile testing machine using a 0.1 kN load cell. The instrument resided in a temperature and humidity rate (H.R.) controlled room with T = 23 °C and H.R. = 54%. Dumbbell shaped specimens, having a calibrated length of 15 mm and a width of 2 mm, were cut from the films with an MTS H4 stamp. These samples were held on the machine between pneumatic clamps altered with wood strips to better grip the rubbery materials. It is worth noticing that gelatin hydrogels with gelatin concentration up to 15 wt.% simply prepared through dissolution broke systematically into the clamps during the clamp tightening. Tensile measurements were performed at a constant crosshead displacement rate of 100 mm 3 min-1. Cyclic tests were performed according to a four steps trial composed of an initial loading ramp performed at a constant crosshead displacement rate of 100 mm 3 min-1 up to a maximum drawing ratio (λmax), followed by a relaxation plateau during 90 s that removes the viscous response. At least, an unloading ramp at a constant crosshead displacement rate of 100 mm 3 min-1 is applied followed by a final loading ramp performed at the same displacement rate up to failure. Characterization. Vanadium oxide-gelatin samples were characterized by XRD (PW1830, Philips) and TGA (TA SDT 2960 Instrument). The relative amount of V(IV) in solids was measured by UV-visible spectroscopy, according to a method previously reported.26 51V MAS NMR spectra were recorded at room temperature at 79.0 MHz on a Bruker Avance 300 spectrometer. It was acquired with a rotor synchronized echo sequence (θ-τ-2θ-τ-acq. with θ = π/16, τ = 1/νr, where νr is the spinning frequency) and with power levels corresponding to π/2 lengths for the liquid standard (NH4VO3) of approximately 2.5 μs. A spectral width of 1 MHz and 0.5 s of recycle time were used. The solid sample was spun at 14 kHz and 14 400 transients were accumulated. Isotropic chemical shifts were reported to VOCl3 using a solution of 0.1 mol 3 L-1 NH4VO3 (δ = -578 ppm) as secondary reference. X-ray photoelectron spectroscopy (XPS) analyses were performed with an ESCALAB 220XL spectrometer. The Alk-R monochromatized line (1486.6 eV) was used for excitation with a 200 W applied power, giving a 800 μm spot diameter on the sample. The spectrometer was operated in a constant pass energy mode (Epass = 40 eV) for high resolution spectra recording by using the electromagnetic lens mode. A 6-eV electron flood gun source was applied to the samples to compensate the charge effect during analysis. Binding energies were referenced to the (VOx) core level taken at 530.0 eV. Indeed, Medialdua et al.27 have shown that the difference in binding energy between the O1s and V2p3/2 level should be used to determine the oxidation state of vanadium. During the experiment the residual pressure was less than 10-7 Pa.

(25) (a) Hazel, J. F.; McNabb, W. M.; Santini, R. J. Phys. Chem. 1953, 57, 681. (b) Lemerle, J.; Nejem, L.; Lefebvre, J. J. Inorg. Nucl. Chem. 1980, 42, 17.

(26) Durupthy, O.; Steunou, N.; Coradin, T.; Maquet, J.; Bonhomme, C.; Livage, J. J. Mater. Chem. 2005, 15, 1090. (27) Mendialdua, J.; Casanova, R.; Barbaux, Y. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 249.

Experimental Section

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Figure 1. (a) and (b), Stability domains of the vanadium oxide-gelatin compounds obtained at room temperature and pH 1, after 24 h (a) or after 1 month (b) as a function of vanadium concentration (mol 3 L-1) and weight percent of gelatin (wt%). (c) and (d), Photographs of (c) OVP under stretching and (d) OG obtained after 24 h at room temperature.

Experimental quantification and spectral simulation were obtained by using the Eclipse software provided by VG Scientific. The surface composition of the samples was estimated from XPS peak areas and corrected by the difference in cross section according to Scofield data.28 Freeze Fracture and Transmission Electron Microscopic Studies. The samples were placed between two copper holders and rapidly frozen by plunging them into liquid nitrogen. The samples were kept frozen and transferred into a freeze-fracture apparatus developed by Dr. J.-C. Homo. The samples were cryocleaved and platinum was evaporated onto the sample under 45° angle, and then carbon under a 90° angle respective to the surface. The sample was warmed to room temperature, and the replica was rinsed with acid acetic 500 mM and deposited on 400 mesh grids. Transmission electron microscopy (TEM) was performed with a Philips CM12 microscope operating at 120 kV. Images were recorded on image films SO163 that were developed 12 mn in pure D19 Kodak developer. The negatives were scanned on an Epson Perfection 1400 photo scanner at 1200 dpi.

Results and Discussion Synthesis of Vanadium Oxide-Gelatin Hybrids. Vanadium oxide-gelatin hybrids were synthesized by mixing decavanadate and gelatin aqueous solutions at pH 1. The experiments were performed at 40 °C since the physical gels of gelatin are thermoreversible: they become liquid by increasing temperature above 30 °C due to the reverse conformational change from helix to coil. Depending on the vanadium concentration (0.1-0.5 mol. L-1) and gelatin content (1-10 wt %) of the starting solutions, three different situations were encountered after one day (Figure 1a). For a low [V]/[G] ratio, a yellow solution is obtained that turns to a yellow gel (YG) after one day at room temperature. In contrast, for larger [V]/[G] ratios, addition of the vanadate solution to the gelatin sol gives immediately rise to a phase separation between an orange viscous phase (OVP) and a yellow transparent solution. In the wet state and at room temperature, this OVP exhibits a striking rubber-like behavior. Indeed, a photograph of Figure 1c shows that OVP can easily be processed in different forms including thin membranes obtained by stretching. Upon drying, OVP evolves to an orange glassy compound (OG) due to the vitreous transition of the biopolymer (Figure 1d). According to (28) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.

Figure 2. (a) 51V MAS NMR spectrum of the OG compound. (b) Polyhedral drawing of [H2V10O28]4-.

thermogravimetric and chemical analysis, freshly obtained OVP contains 37.5 wt % of gelatin, 50 wt % of water, and 12.5 wt % of vanadium oxide independently of the initial precursor concentrations, suggesting that OVP has a well-defined stoichiometry. Upon aging, OVP and OG turn to brown compounds indicating a partial reduction of vanadium(V) to (IV). A V(IV) rate of 8% is typically obtained for OVP that increases to 12% for OG aged for a few days. Moreover when left in the supernatant orange solution, OVP evolves into a dark red precipitate (RP). The kinetics of this transformation depends on the [V]/[G] ratios: for the highest ratios, this reaction occurs only after one day whereas it needs about one month for solutions with lower [V]/[G] ratios (Figure 1b). However, when OVP is withdrawn from the supernatant solution, this transformation does not occur. Characterization of Vanadium Oxide-Gelatin Hybrids. The powder X-ray diffraction pattern of OG (see SI) displays only two very broad peaks close to 2θ = 10° and 25° indicating that OG is amorphous. No hkl reflections characteristic of the double layered structure of V2O5 are observed. The experimental 51V MAS NMR spectrum of OG presented in Figure 2a consists of superimposed patterns of spinning sidebands extending over a spectral width of about 1 MHz. Two broad peaks of the isotropic part of the spectrum can be observed at -520 and -450 ppm. The broadening of the signals prevents the determination of the chemical shift anisotropy and quadrupolar parameters. However, the isotropic chemical shifts agree fairly well with those of the cesium

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Figure 3. X-ray diffraction patterns in reflection geometry of RP compound with a gelatin rate between 0.5 and 5 wt % (pH 1, room temperature, [V] = 0.5 mol. L-1).

decavanadate Cs4[H2V10O28] 3 4H2O phase,29 suggesting that both OG and OVP contain decavanadate [H2V10O28]4- polyanions (see Figure 2b). Three chemically different vanadium sites can be distinguished in the decavanadate polyanion (see Figure 2b). The chemical shift at -450 ppm is typical of the central distorted VO6 site present in the decavanadate anion (type 3 in Figure 2b).29,30 The signal at -520 ppm can be assigned to vanadium sites of type 1 and 2. Moreover, the 51V NMR spectrum of YG (not shown), was found to be identical to that of OVP, confirming that decavanadate anions are polyoxovanadate species that interact strongly with positive gelatin chains. This assumption is consistent with our previous characterization of colloidal or dilute vanadium oxide-gelatin solutions by liquid 51V NMR, dynamic light scattering, rheology, and calorimetry.18,19 Red precipitates (RP) prepared with an initial gelatin content between 0.5 and 5 wt % were characterized by powder X-ray diffraction in reflection geometry (Figure 3). The X-ray diffraction pattern of RP for 0.5 wt % of gelatin is similar to that of a V2O5. 1.8H2O xerogel. This diagram displays the series of 00l reflections but the basal distance (d00l = 12.6 A˚) is larger compared to that of V2O5 3 1.8H2O xerogel (d00l = 11.5 A˚), showing that gelatin chains are intercalated between the layers. This basal distance increases with the amount of gelatin in the initial solution. For 5 wt % of gelatin, 00l reflections are no longer observed, suggesting a possible exfoliation of V2O5 ribbons due to their association with gelatin (see Figure 3). Such a delamination process has already been observed by combining gelatin with layered inorganic solids such as clay minerals.8 In contrast to V2O5 3 1.8 H2O xerogels, the XRD pattern of RP displays also the hk0 set of reflections corresponding to the two-dimensional structure of the layers. Such a series of diffraction peaks is generally not observed for V2O5 3 1.8 H2O xerogels in reflection geometry due to the preferential orientation of the ribbons on flat surfaces. These results suggest that strong interactions between gelatin chains and decavanadate (29) (a) Durupthy, O.; Jaber, M.; Steunou, N.; Maquet, J.; Chandrappa, G. T.; Livage, J. Chem. Mater. 2005, 17, 6395. (b) Durupthy, O.; Maquet, J.; Bonhomme, C.; Coradin, T.; Livage, J.; Steunou, N. J. Mater. Chem. 2008, 18, 3702. (30) Day, V. W.; Klemperer, W. G.; Maltbie, D. J. J. Am. Chem. Soc. 1987, 109, 2991.

Figure 4. (a), (b), and (c), Freeze-fracture electron micrographs. (a) Cryofractured samples of 2 wt % pure gelatin solutions reveal organized lamellar textures. The lamellar texture is revealed by the oriented steps (open arrow heads) while the inner organization of gelatin is revealed by the regular fine stripes (arrows along the direction of the stripes). Locally nonoriented areas are seen which show small globular entities (15 nm, white arrowhead). (b) Cryofractured samples of pure V2O5.nH2O gels reveal fine ribbons that can be locally oriented. Stacks of the ribbons are also visualized here and there (black arrow heads). (c) Cryofractured samples obtained after aging for 4 days mixtures of acidified vanadate solutions ([V] = 0.25 mol. L-1) and gelatin (2 wt %). These samples reveal lamellar textures (open arrowhead) with small compact ribbon like entities (black arrowhead) that coexist with small globular entities (15 nm, white arrowhead). A fine underlying organization of the ribbons is observed which orientation is indicated by the white arrow. (d) SEM image of fractured samples obtained after aging for 7 days mixtures of acidified vanadate solutions ([V] = 0.25 mol 3 L-1) and gelatin (2 wt %).

Figure 5. Photograph of OVP at T = 23.0 °C and Δm/m0 = 0.04 in the form of a rectangular tablet (a) in which dumbbell shaped specimens (b), having a calibrated length of 15 mm and a width of 2 mm, were cut for the mechanical tests.

anions induce an extensive perturbation of the stacking order in the material and favor a disorganized assembly of V2O5 layers and gelatin chains, as it was similarly reported for V2O5-dipeptide phases.31 The presence of V2O5 in RP could be confirmed by X-ray photoelectron spectroscopy (see SI). The results corresponding to the spectral data of RP were compared to those of a V2O5 3 1.8H2O reference xerogel. The V5þ 2p binding energies of RP are close to those of the xerogel. Additional signals in the RP spectrum were observed and could be attributed to small amounts of V4þ. The O1s ionization energy of RP is composed of different sets that correspond mainly to the oxo ligands of the V2O5 network and carbonyl groups of gelatin. In particular, the intense component at 531.2 eV can be assigned to the carbonyl groups of the gelatin backbone.32 (31) Durupthy, O.; Steunou, N.; Coradin, T.; Livage, J. J. Phys. Chem. Solids 2006, 67, 944. (32) (a) Monti, S.; Carravetta, V.; Battocchio, C.; Iucci, G.; Polzonetti, G. Langmuir 2008, 24, 3205. (b) Jingrun, G.; Jin, W.; Hong, S.; Nan, H. Appl. Surf. Sci. 2008, 255, 263.

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The C1s region is fitted with four components that can be assigned to C;H, C;N, and CdO, respectively. Finally, the N1s region is fitted with two components that correspond to the terminal amine and amide (peptide bond and amide groups of lateral chains) nitrogen atoms of gelatin.32 These data confirm that RP is made of an assembly of gelatin chains and slightly reduced V2O5 ribbons. SEM images were recorded on fractured samples obtained after aging OVP in the supernatant vanadate solution for about 1, 7, and 14 days (see Figure 4d and the SI). Both samples exhibit the same homogeneous and smooth texture which is certainly mainly imparted by the biopolymer. The samples aged 15 days displays a granular microstructure typical of a precipitate, showing the progressive transformation of OVP into RP upon aging. The OVP was characterized by TEM on cryofractured samples in order to reveal the inner surfaces of the samples (see Figure 4c). This technique was particularly used for the analysis of soft matter systems33 and therefore appears suitable to investigate these bionanocomposites. This compound was compared to pure gelatin (2 wt %) and V2O5 3 nH2O xerogel (see Figure 4a and b). In order to visualize the growth of V2O5 ribbons in the gelatin matrix, the OVP aged in the supernatant solution for 4 days was extracted from the solution and cryofractured. Samples of 2 wt % gelatin exhibit organized lamellar textures as revealed by the oriented steps (open arrowhead in Figure 4a). The TEM image of cryofractured V2O5 3 nH2O gels is fully consistent with those previously reported on V2O5 gels directly deposited on EM grids,14a showing that these gels can be described by the entanglement of ribbon like particles that are about 10 nm wide and a few μm long. As observed in Figure 4c, OVP exhibits a lamellar structure which is certainly imparted by the structured gelatin. However, in contrast to pure gelatin, a large amount of globular entities (white arrowhead) is present and may be assigned to nonstructured gelatin. Upon aging OVP in the supernatant solution, small ribbons (black arrowhead) can clearly be observed in the gelatin matrix and may be attributed to the V2O5 particles. The large amount of globular entities and the dramatic morphological change of the V2O5 ribbons upon mixing both components indicate a strong interaction between the inorganic and the biological components. Tensile Tests until Failure of the OVP Rubber. We investigate the striking rubber-like behavior (Figure 1c) of the undried OVP at large strain by performing tensile tests until failure, and tensile tests in cycling at different strain amplitudes. Quantitative measurements were obtained at room temperature by considering well-defined geometry test samples (Figure 5b) cut to the required dimensions from homogeneous thick tablets (Figure 5a) prepared via a simple handling procedure taking profit from the gelatin sol-gel (33) (a) Alexeev, V. L.; Ilekti, P.; Persello, J.; Lambard, J.; Gulik, T.; Cabane, B. Langmuir 1996, 12, 2392. (b) Bardelang, D.; Camerel, F.; Margeson, J. C.; Lee, D. M.; Schmutz, M.; Badruz Zaman, M.; Yu, K.; Soldatov, D. V.; Ziessel, R.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 2008, 130, 3313. (c) Bartelang, D.; Camerel, F.; Ziessel, R.; Schmutz, M.; Hannon, M. J. J. Mater. Chem. 2008, 18, 489.

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Table 1. Average Mechanical Parameters of (a) Simple Gelatin Coacervates and (b) OVP at different relative weight losses (Δm/m0) during Drying in Ambient Conditions (T = 23 °C, H.R. = 54%) (a) Δm/m0 E[MPa] λbreak σbreak [MPa]

0.04a 15 ( 3 3.8 ( 0.3 11 ( 2

0.07b 1.6 ( 0.1 3.0 ( 0.4 2.5 ( 0.5

0.2b 170 ( 8 1.1 ( 0.1 27 ( 5

(b) Δm/m0 E[MPa] λbreak σbreak[MPa]

0.04a 0.27 ( 0.03 8.6 ( 0.5 1.4 ( 0.1

0.07b 2.5 ( 0.1 1.8 ( 0.2 3.8 ( 0.2

0.2b (27 ( 4).101 1.1 ( 0.1 (5 ( 1).101

a E, λbreak and σbreak parameters correspond to average values obtained from ten different samples. bE, λbreak and σbreak parameters correspond to average values obtained from three different samples.

transition (see the Experimental Section). As already reported,18 OVP is prepared by a complex coacervation process. Therefore, in order to better distinguish the respective role played by the coacervation process and by the decavanadate clusters on the mechanical properties of the composite materials, we have studied “simple” gelatin coacervate obtained upon ethanol addition34 ([gelatin] = 45.0 wt.%, [water] = 55.0 wt.%) and OVP (“complex” gelatin-decavanadate coacervates) ([gelatin] = 37.5 wt.%, [vanadates] = 12.5 wt.%, [water] = 50.0 wt.%). Obviously, the stoichiometry of these samples is fixed by charge matching conditions and cannot be modified for a given system. First of all, the mechanical behavior of the different samples has been characterized by using simple tensile experiments until failure. Table 1 summarizes the average mechanical parameters determined from ten different samples for both kinds of coacervates upon drying. In Figure 6, a typical Cauchy Stress σ (with σ = F/S0, where F is the force measured at a given strain and S0 is the initial section area of the sample) is plotted as a function of stretch (λ). Simple gelatin coacervates are characterized by an initial linear elastic region for λ e 1.1 ( 0.1 (see Table 1a) followed by a nonlinear behavior associated to a plastic deformation (see below) until failure for λbreak= 3.8 ( 0.3 and σ break=11 ( 2 MPa (Table 1a). According to a linear fit of the low strain elastic response, a Young modulus E = 15 ( 3 MPa can be extracted. In contrast to pure gelatin samples, OVP exhibits a complex behavior characterized by an initial nonlinear part for λ e 4.0 (Figure 6b) followed by an up-turn phenomenon up to failure for λbreak = 8.6 ( 0.5 and σ break = 1.4 ( 0.1 MPa (Table 1a). It is noteworthy that this curve (σ = f(λ)) exhibits the same shape as rubber-like materials.35 In order to determine the Young modulus (E) below the up-turn and to check if this behavior can be described by the neo-hookean behavior relationship36 (σ = E/3 3 (λ λ-2)) we have plotted σ as a function of λ - λ-2. As shown (34) (a) Bohidar, H. B.; Mohanty, B. Phys. Rev. E 2004, 69, 021902. (b) Gupta, A.; Bohidar, H. B. Phys. Rev. E 2005, 72, 011507. (35) Treloar, L. R. G. The Physics of Rubber Elasticity; Clarendon: Oxford, 1975. (36) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, 1953.

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Figure 6. Typical representation of a Cauchy stress (σ = F/S0) as a function of stretch (λ) measured in the same experimental conditions (T = 23 °C, H.R. = 54%, Δm/m0 = 0.04) for (a) simple gelatin coacervate and (b) OVP. The red curve of (b) is a fit corresponding to a neo-hookean behavior with a Young modulus (E) of 0.27 MPa determined from the slope of the curve σ = f (λ- λ-2) (inset of (b)).

in the inset of Figure 6b, this basic model fits fairly well with the experimental data in the λ range under study confirming the rubbery character of OVP. According to this fit, a value of Young modulus: E = 0.27 ( 0.03 MPa (Table 1a) can be extracted. This modulus value is close to those found for unfilled elastomers.35 However, in order to assess the rubbery character of this behavior, in particular the partial reversibility behavior, additional experiments should be performed as described below. From a general point of view, if the simple gelatin coacervate behavior agrees fairly well with the usual behavior of gelatin gels prepared through dissolution,37,42-47 the Young modulus and the failure characteristics, λbreak and σ break of OVP are significantly enhanced. In a first approach, this result can be understood by considering that the simple coacervation approach enables the preparation of highly concentrated samples ([gelatin] = 45.0 wt.%) in contrast to a classical dissolution approach since the gelatin solubility at 40.0 °C is close to 30.0 wt.%. Moreover, in both types of coacervates, the Young modulus E and σ break increase markedly while λbreak decreases with increasing the drying rate leading to glassy materials (Figure 1d) that display a classical brittle failure (Table 1). This observation is (37) Groot, R. D.; Bot, A.; Agterof, W. G. M. J. Chem. Phys. 1996, 104, 9202. (38) (a) Coppola, M.; Djabourov, M.; Ferrand, M. Macromol. Symp. 2008, 273, 56. (b) Yakimets, I.; Wellner, N.; Smith, A. C.; Wilson, R. H.; Farhat, I.; Mitchell, J. Polymer 2005, 46, 12577.

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Figure 7. Cauchy stress (σ = F/S0) as a function of stretch (λ) measured in the same experimental conditions (T = 23 °C, H.R. = 54%, Δm/m0= 0.04) for (a) simple gelatin coacervate and (b) OVP during cycling tests at different maximum strain. The intermediate region between the loading and the unloading curve corresponds to a relaxation plateau of 90 s in each case (see the Experimental Section). The red curve presented in (b) is a fit corresponding to a neo-hookean behavior as already shown in Figure 6b. The inset of (b) represents the evolution of the hysteresis energy as a function of the maximum strain applied during a cycle for OVP. The red curve of this inset is a simple exponential fit of the curve: Ehysteresis= f (λmax).

presumably due to an increase of the glass transition temperature (TG) of gelatin with dehydration.38 The presence of decavanadate clusters does not strongly affect this evolution and the final properties of the dried nanocomposites despite the fact that the drying kinetics are slowed down in OVP (see SI). At this point, it is worth recalling that the striking rubber-like behavior is restricted to undried samples. At the same time, it is expected that the triple helix concentration does not drastically increase during one hour of drying at 23 °C after 24 h of aging at 2 °C. As a consequence, we propose to mainly ascribe this mechanical evolution upon drying to an increase of gelatin TG with decreasing water content. In order to assess the rubbery character of OVP materials, it is necessary to study their ability to recover their initial shape after deformation (i.e., reversibility behavior), via cycling experiments. Figure 7 shows the behavior of different freshly prepared samples submitted to one loading-unloading cycle (see the Experimental Section) for different values of the maximum stretch (λmax). First of all, in both samples an important hysteresis appears at all strains and increases with increasing

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λmax. However, in the absence of decavanadate, one can observe the absence of shape recovery for λmax=2 and 2.5 (Figure 7a). This poorly reversible behavior shows that the nonlinear deformation is mainly related to a plastic deformation in agreement with the ductile character evidenced during the first set of measurements (vide supra). In contrast, OVP restores a significant amount of energy during shape recovery even if the main fraction of energy is dissipated in the hysteresis between the loadingunloading curves. The irreversible part, which can be followed by the residual extension (permanent set),39 increases with λmax. The energy dissipated during loading/unloading cycle (Ehysteresis) can be calculated according to the following R R equation: Ehysteresis ¼ loading σ:dλ - unloading σ 3 dλ. According to the inset in Figure 7.b representing the evolution of Ehysteresis with λmax, the dissipated energy increases with the maximum achieved strain during loading. Below the up-turn (λup-turn ≈ 4.0), this evolution is linear and becomes exponential at larger strain. This result agrees with the usual picture of an irreversible reorganization in the material induced by a strain-hardening phenomenon.35-37 In a last stage, we have performed tensile measurements until failure immediately after one loading-unloading cycle. Figure 8 shows the tensile behavior of OVP for various initial λmax. It appears clearly that if the residual extension increases with λmax, λbreak is only slightly dependent on the value of λmax applied for the cycle. In each case, when the second extension exceeds the maximum “first” extension previously applied, the stress-strain response of the material returns via the same path as that of the monotonous uniaxial tensile test (Figure 8), after a short transition which increases with the amount of initial stretch (first loading). In contrast, for stretches lower or equal to λmax, we observe a softening of the second stress-strain response in comparison to the first loading on the same range of stretches. All these observations concerning OVP are consistent with the Mullins effect40 which is characteristic of rubber-like materials filled with carbon black, inorganic nanoparticles, and/or containing crystalline regions.39 This fact is quite surprising since Mullins effects are generally observed with chemically cross-linked networks where most of the polymer chains are above the glass transition temperature. In contrast, in the complex decavanadate-gelatin coacervate, crosslinking is provided through weak physical interactions such as hydrogen bonds, electrostatic interactions and gelatin chains entanglement, as discussed below. Mechanical Properties of Gelatin-Based Coacervates. From a structural point of view, the undried OVP can be described at room temperature as a nanocomposite hydrogel consisting of a three-dimensional network of entangled gelatin chains physically reticulated both by triple helices (hydrogen bonds) and decavanadate electrostatic bridging.18,19 The mechanical behavior of a system in which the physical cross-links can be broken (39) Diani, J.; Fayolle, B.; Gilormini, P. Eur. Polym. J. 2009, 45, 601. (40) Mullins, L. Rubber Chem. Technol. 1969, 42, 339. (41) (a) Hellio-Serughetti, D.; Djabourov, M. Langmuir 2006, 22, 8509. (b) Hellio-Serughetti, D.; Djabourov, M. Langmuir 2006, 22, 8516.

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Figure 8. Cauchy stress (σ = F/S0) as a function of the drawing ratio (λ) for OVP during cycling tests composed of an initial loading ramp (blue squares) followed by an unloading ramp (green triangles) and a final loading ramp until failure (red circles). The initial loading ramps (blue squares) have been performed for different maximum strain: (a) λmax= 3, (b) λmax = 5 and (c) λmax = 7.

and recovered have been thoroughly studied at low strain, 15,19,41 but less attention has been devoted to their mechanical properties at large strain until very recently. The peculiar case of gelatin gels prepared by gelatin powder dissolution has been investigated experimentally at large strain by local or macroscopic shear deformation,37,42,43 compression,42 crack dynamic,44 or simple tension.45-47 However, a systematic description of this kind of gel is still needed at large strain in order to compare results (42) Bot, A.; Van Amerongen, I. A.; Groot, R. D.; Hoekstra, L. L.; Agterof, W. G. M. Polym. Gels Networks 1996, 4, 189. (43) Wilking, J. N.; Mason, T. G. Phys. Rev. E 2008, 77, 055101. (44) (a) Baumberger, T.; Caroli, C.; Martina, D. Eur. Phys. J. E 2006, 21, 81. (b) Baumberger, T.; Caroli, C.; Martina, D. Nat. Mater. 2006, 5, 552. (45) (a) McEvoy, H.; Ross-Murphy, S. B.; Clark, A. H. Polymer 1985, 26, 1483. (b) McEvoy, H.; Ross-Murphy, S. B.; Clark, A. H. Polymer 1985, 26, 1493. (46) (a) Normand, V.; Plucknett, K. P.; Pomfret, S. J.; Ferdinando, D.; Norton, I. T. J. Appl. Polym. Sci. 2000, 82, 124. (b) Plucknett, K. P.; Normand, V.; Pomfret, S. J.; Ferdinando, D. P. Polymer 2000, 41, 2319. (47) Zheng, J. P.; Li, P.; Ma, Y. L.; Yao, K. D. J. Appl. Polym. Sci. 2002, 86, 1189.

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obtained by different techniques and conditions (aging time, temperature, pH, etc.) and with various gelatin samples (gelatin concentration, gelatin bloom value, etc.). Nevertheless, these studies have generally pointed out that gelatin hydrogels depict a linear elastic behavior at low strain followed by a nonlinear stress-strain relationship before breaking at stretches, λ, that are typically between 1.3 and 2.0.37,42-47 It was reported that more complex nonhookean phenomenological models37,42 are necessary to describe the stress-strain response at large strain. The resulting fitting parameters obtained in this nonlinear domain do not have any intrinsic physical sense as it is impossible to relate them to the molecular structure of the gel. The qualitative molecular interpretation of the nonlinear properties is that the gelatin chains are partially in a rigid crystalline triple helix state (cross-links) and partially in a random coil state (the network bonds), and the more extensive the rigid cross-link regions, the shortest and most stretched the network bond becomes as a result of an externally applied deformation.42 According to these authors, the network bonds behave as anharmonic springs under a very large strain.42 In the case of simple gelatin coacervate, we have evidenced a linear elastic behavior at low strain (λ e 1.1 ( 0.1) followed by a nonlinear stress-strain relationship before breaking, in good agreement with the usual behavior of gelatin gels prepared through dissolution.42 However, the Young modulus and the failure characteristics, λbreak and σ break, are drastically higher than those reported for gelatin gels prepared through dissolution, thereby showing an increase in the stiffness of the material. During cycling, these samples display a poorly-reversible behavior (Figure 7a) in agreement with the ductile character evidenced during the first set of experiments (Figure 6a). A part of the discrepancy existing between simple coacervates and classical gels can be easily understood by considering that the simple gelatin coacervates exhibit very high gelatin concentrations ([gelatin]= 45.0 wt.%) in comparison with gelatin gels prepared through dissolution ([gelatin] < 15 wt.%). In order to assess the impact of the gelatin concentration on the Young modulus, we have reported on the same plot (Figure 9) experimental data extracted from the literature relative to gelatin gels prepared through dissolution42 and data obtained in this study on simple gelatin coacervates and complex gelatin/ vanadate coacervates (OVP). For samples prepared through dissolution, the Young modulus scales with [gelatin]1.8 as already observed in different studies based on shear or compression measurements.42 If we assume that this power law is suitable at high gelatin concentration, the sample prepared through simple coacervation displays a Young modulus larger than the predicted value by this power law. This discrepancy is certainly strongly related to the difference in aging conditions between our samples (24 h at 2 °C) and the samples prepared by A. Bot et al. (24 h at 21 °C),42 which is consistent with the fact that the nucleation and growth of gelatin triple helices are favored at very low temperatures.15 Therefore, here-prepared simple gelatin coacervates presumably exhibit a higher triple helix

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Figure 9. Evolution of the Young modulus as a function of gelatin concentration. The black triangles correspond to experimental data extracted from42 and relative to gelatin samples (Bloom=245 g) prepared through dissolution and aged 24 h at 21 °C before compression experiments. The green line is a linear fit of these data. The blue circle and the red square correspond, respectively, to OVP and the simple gelatin coacervate considered in this study.

concentration than gelatin gels prepared by dissolution due to the very important gelatin concentration and the low aging temperature. In the frame of the description proposed previously by A. Bot et al.,42 the qualitative picture that emerges from these data is that a high gelatin concentration may give rise to a high cross-links (gelatin triple helices) density and consequently to an enhancement of the Young modulus and stress at failure. Actually, the growth of the crystalline regions has the effect that the coils in between two cross-links become shorter thus decreasing the ability of the chains to extend upon deformation of the gel and to behave like harmonic springs. However, strong structural differences may exist between a coacervate phase and a biopolymer gel obtained through dissolution, and their contribution cannot be taken into account in this qualitative interpretation of the mechanical properties. In contrast to simple gelatin coacervates, we have shown that OVP displays a typical rubbery behavior characterized by an initial nonlinear part for λ e 4.0 followed by an up-turn35 up to failure for λbreak =8.6 ( 0.5 and σ break=1.4 ( 0.1 MPa (Figure 6b). These samples restore a significant amount of energy during shape recovery even if the major fraction of energy is dissipated in the hysteresis between the loading-unloading curves. From a structural point of view, complex decavanadategelatin coacervates exhibit a lower gelatin concentration ([G] = 37.5 wt.%) than samples obtained through simple coacervation ([G] = 45.0 wt.%) due to the difference in the charge neutralization process. According to a previous study dedicated to the decavanadate cluster influence on the gelatin rheological properties,19 the presence of inorganic clusters is not expected to strongly influence the elastic modulus in presence of an extended network of gelatin triple helices. Nevertheless, we have previously shown that the weak decavanadate-gelatin cross-links presumably favor the triple helix nucleation in the first stage of the cooling process and the stability of these regions.19 Based on our previous discussion concerning simple coacervates, for comparable aging conditions,

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the complex coacervates should present a lower triplehelix concentration than simple coacervates due to a difference in gelatin concentration. Therefore, complex coacervates are characterized by a lower stress at break than simple coacervates as well as a lower initial Young modulus, which is in fair agreement with the extrapolation of the power law, ER[gelatin]1.8 (see Figure 9). Furthermore, according to a lower triple helix concentration, the coils regions situated in between two cross-links may be longer and more extended during deformation. Moreover, the electrostatic interactions between gelatin chains are possibly screened by decavanadate clusters thus promoting a Gaussian conformation of gelatin chains and a strong degree of their entanglement that may additionally explain the large deformation of the material. In presence of decavanadates the sample failure occurs for deformation well above the up-turn threshold (λup-turn =4.0>λbreak =8.6) suggesting that a great potential of reorganization is stored in the material. According to the weak nature of the decavanadate-gelatin electrostatic interactions, one can suggest that some vanadate cluster/gelatin bonds are broken for relatively low stress (σbreakλmax: the exponential increase of Ehyst could be mainly explained by the formation of new gelatin triple helices under strain48 in agreement with the strain hardening observed in this domain of stretching. Nucleation and Growth of V2O5 Controlled by Gelatin. Once extracted from the acidified supernatant vanadate solution and dried in ambient conditions, OVP is converted into an orange glass (OG) that contains the decavanadate polyanion. However, if OVP is left in the acidified vanadate solution for at least 7 days, a red precipitate (RP) is obtained whose structure can be described by an association of decavanadate, V2O5 ribbon like particles and gelatin chains in triple helices and coils conformation. We observed that V2O5 layers can also be obtained before 7 days by replacing the vanadate solution with a freshly prepared acidified vanadate solution. In fact, it is well-known that an acid vanadate solution of 1