Macromolecular Rapid Communications
Communication
High-Performance TiO2 Nanoparticle/DOPAPolymer Composites Faroha Liaqat, Muhammad Nawaz Tahir,* Eugen Schechtel, Michael Kappl, Günter K. Auernhammer, Kookheon Char, Rudolf Zentel, Hans-Jürgen Butt, Wolfgang Tremel*
Many natural materials are complex composites whose mechanical properties are often outstanding considering the weak constituents from which they are assembled. Nacre, made of inorganic (CaCO3) and organic constituents, is a textbook example because of its strength and toughness, which are related to its hierarchical structure and its well-defined organic–inorganic interface. Emulating the construction principles of nacre using simple inorganic materials and polymers is essential for understanding how chemical composition and structure determine biomaterial functions. A hard multilayered nanocomposite is assembled based on alternating layers of TiO2 nanoparticles and a 3-hydroxytyramine (DOPA) substituted polymer (DOPA-polymer), strongly cemented together by chelation through infiltration of the polymer into the TiO2 mesocrystal. With a Young’s modulus of 17.5 ± 2.5 GPa and a hardness of 1.1 ± 0.3 GPa the resulting material exhibits high resistance against elastic as well as plastic deformation. A key feature leading to the high strength is the strong adhesion of the DOPA-polymer to the TiO2 nanoparticles.
Dr. F. Liaqat, Dr. M. N. Tahir, E. Schechtel, Prof. W. Tremel Institute for Inorganic and Analytical Chemistry Johannes Gutenberg-University Duesbergweg 10-14 , 55099 Mainz, Germany E-mail: [email protected]; [email protected] Dr. M. Kappl, Dr. G. K. Auernhammer, Prof. H.-J. Butt Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Prof. K. Char School of Chemical and Biological Engineering The National Creative Research Initiative Center for Intelligent Hybrids The WCU Program of Chemical Convergence for Energy and Environment Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-744 , South Korea Prof. R. Zentel Institute for Organic Chemistry Johannes Gutenberg-University Duesbergweg 10-14 , 55099 Mainz, Germany Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1. Introduction Nacre[1,2] is a textbook example showing how evolation can lead to high-performance materials by blending intrinsically weak constituents, CaCO3 (aragonite) and collagen-related biopolymers,[3,4] in a composite. It shows a unique integration of remarkable tensile strength toughness,[5] and electrical conductivity[6–8] which supplies a strategy for constructing integrated materials. Materials scientists (especially inspired from nature)[9–14] have adopted this strategy for making technical inorganic/ organic nanocomposites with strong bonds (covalent or iono-covalent) between the organic and inorganic components[15–24] in technical ceramics (e.g., ORMOCERS).[25] In contrast, when weak interactions predominate between the mineral and the organic component “flexible minerals” with enhanced elastic modulus and fracture resistance are formed.[26]
wileyonlinelibrary.com
DOI: 10.1002/marc.201400706
1129
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
Scientists have long sought to duplicate nacre’s strength and lightness in man-made materials.[27–32] In general, the toughness of synthetic composites decreases of with increasing hardness and vice versa. Nacre’s architecture varies at several length scales, from micrometers to nanometers. Replicating all of these scales—each of which contributes to the overall performance of nacre—in a synthetic material remains a challenge. For example, Podsiadlo et al.[16] made nacre-like layered materials through covalent cross-linking nanoclay and poly(vinylalcohol) (PVA) with high tensile strength, but lower toughness. Similarly, Studart and co-workers[18] fabricated tough layered materials with lower tensile strength based on platelets of Al2O3 and chitosan interconnected via hydrogen bonding. Bouville et al.[23] prepared bioinspired ceramics with high stiffness through ice-templating. By virtue of its functional surface groups graphene oxide (GO) turned out to be a promising building block for nanocomposites with high mechanical properties.[33–38] In order to solve the conflict between strength, toughness, and stiffness for integrated materials[39] several points must be addressed. (i) The mineral component should have a large lattice energy to supply sufficient hardness.[40] (ii) Strong interfacial interactions should be present between the building blocks (e.g., by covalent cross-linking). (iii) The covalent cross-links between the building blocks should contain long chains or polymers to supply enough positional entropy for absorbing energy. (iv) Finally, the methodology and overall architecture of the composite will be important.[41–43] The morphology of the inorganic particles, their structural organization, and the chemical bonding between them should provide a physicochemical basis for stiffness and flexibility at multiple length scales, leading to increased robustness against catastrophic materials failure. Nanoparticles (NPs) of metal oxides such as TiO2 or ZrO2 certainly meet the first criterion; in addition, they are cheap and readily available. Dopamine anchor groups with binding constants at the order of 1020 are among the most strongly binding ligands in coordination chemistry,[44–46] which can provide a large amount of interfacial bonding.[47–50] This is indicated by ample reports on the adhesive proteins isolated from the marine mussel Mytilus edulis.[49,51] The DOPA-polymer self-polymerizes (similar as mussel proteins) into long-chain polymers by oxidative chemical cross-linking,[52–54] which greatly improves mechanical strength. We demonstrate in this contribution that blending these DOPA-polymers with a dense, ordered multilayer matrix of metal oxide NPs leads to a nanocomposite with high mechanical strength. We note that the nanocomposite studied here is (i) fully transparent, (ii) the inorganic component can easily be replaced by other metal oxides (e.g., by CeO2,[55] ZrO2[56] or Fe2O3[57]) and (iii) that the metal oxide nanoparticles
1130
contained in these mixed matrices retain their specific surface activity.[58,59]
2. Experimental Section 2.1. Materials Unless noted otherwise, the precursors for the synthesis of nanoparticles and catechol polymer were purchased from Aldrich, Acros, and other commercial suppliers and used as received.
2.2. Polymer Synthesis The poly(active ester) poly(pentafluoro-phenylacrylate) (PFA) was prepared as reported earlier[60] and is described by the reaction scheme (Scheme S1, Supporting Information). This prepolymer was used for the synthesis of multifunctional DOPA-polymer.
2.3. Synthesis of DOPA-Polymer The poly(active ester) PFA (700 mg, 2.94 mmol repeating units) was dissolved in a mixture of 12 mL of dry dimethylformamide (DMF) and 0.5 mL of triethylamine. The next step was adding 3-hydroxytyramine hydrochloride (555 mg) dissolved in 3 mL DMF and 0.5 mL triethylamine to the mixture and the final contents were stirred for 6 h at 50 °C. The solvent was evaporated using rotary evaporator and product was resolved in about 3 mL methanol:aceton (3:1) and precipitated in water. The precipitated polymer was redissolved in a mixture of aceton (3 mL) and methanol (1 mL) and again precipitated in excess of cold ethyl ether. The precipitated polymer was centrifuged (9000 rpm, 10 min, and room temperature) and the solvent was decanted. The process was repeated until 800 mg of colorless solid polymer was obtained on drying. The structure of the polymer is shown in Scheme S2, Supporting Information.
2.4. Synthesis of TiO2 Nanoparticles TiO2 nanoparticles were synthesized following a procedure described in detail by Do and co-workers.[61] In brief, oleic acid (OLEA, Acros Organics)/oleylamine (OAM, Acros Organics)capped anatase TiO2 nanoparticles were synthesized by to a solvothermal method using titanium butoxide as precursor and water vapor as hydrolysis agent. The resulting dark brown mixture was cooled to room temperature and precipitated from the solution using ethanol. The TiO2 nanoparticles thus produced have an average size of ≈8 nm and are monodisperse, as confirmed by transmission electron microscopy (TEM).
2.5. Fabrication of Multilayers The multilayers were coated on a glass substrate or silicon wafer which has been precleaned in acidic piranha solution of concentrated sulphuric acid (H2SO4) and hydrogen peroxide (H2O2) in a volume ratio of 2:1. The multilayers were assembled by consecutive spin coating of DOPA-polymer solution in
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
dimethylacetamide (DMA) and TiO2 NPs dispersion in hexane, starting from the DOPA-polymer layer due to its high affinity to the substrate and adhesive character. The thickness of the layers is controlled by optimizing the concentration of the solutions to be spin-casted while the spin-coating parameters are fixed at the following values, speed: 5000 rpm, acceleration: 5040 rpm s−1, and time: 10 s. Each spin-coated layer was subjected to heat treatment at 120 °C for 15 min. The use of orthogonal solvents and an optimized heating rate is important to sustain the multilayered structure. The final multilayered structure of 12 alternating layers of polymer and TiO2 NPs was used for structural characterization and nanoindentation experiments. The characterization was carried out on a number of samples to ensure reproducibility of results. The multilayers show structural colors (Figure S1, Supporting Information) on the addition of bilayers due to multilayer interference.[62–64]
3. Results and Discussion
2.6. Nanoindentation The Young’s modulus (E) and hardness (H) of the multilayers of DOPA-polymer/TiO2 nanoparticles were measured by nanoindentation. The method involves applying a load to the indenter, which is in contact with the sample. The depth of indentation is measured as the load is applied and the area of contact (A) is calculated based on the shape of the indenter and depth of the indentation. In this experiment, each series of indentations is done on a grid of 2 × 3 indents on a 90 × 90 μm2 area at three different positions on the sample. This means that a total of 18 indents per series were carried out on the multilayers coated on glass substrate or Si wafer. The Young’s moduli and hardness were calculated by fitting the indentation curves according to the Oliver–Pharr method[65] using the analysis software of the nanoindenter. The Young’s modulus was obtained from the slope of the onset of the unloading part in the force versus indentation curve. The hardness of the material is obtained as the ratio of maximum applied load (Pmax) divided by the indenter contact area at that load and it is expressed by H =
Pmax . A
A substrate effect was observed for maximum applied loads higher than 100 μN (represented by the black data points in Figure 4). To eliminate any contribution from the underlying substrate, only maximum applied forces under than 100 μN are considered.
2.7. Physical Characterization The DOPA-polymer/TiO2 hybrid multilayers were built up on glass substrates using a Laurell WS-400-6NPP-LITE spin-coater. The size of the metal oxide nanoparticles was determined by TEM on a Phillips EM-420 equipped with a slow scan CCD detector (1 × 1 k2) and a LaB6 electron gun operated with an acceleration voltage of 120 kV. The TEM images were processed with the Gnu Image Manipulation Program GIMP Version 2.6.8 or with ImageJ Version 1.43u. The TEM imaging of the hybrid multilayers of DOPA-polymer/TiO2 was performed by preparing a lamella using focused ion beam (FIB) (FEI Nova 600 Nanolab FIB instrument). The lamella was fixed on TEM grid and eventually analyzed by electron energy-dispersive X-ray analysis (EDX) using a transmission electron microscope (Technai G2
www.MaterialsViews.com
F20; FEI/Philips) equipped with an EDX detector. A Horiba Jobin Y LabRAM HR Spectrometer with a frequency doubled Nd:YAGlaser (632 nm) was used for performing Raman spectroscopy of the multilayered films. The sample was prepared by cutting a small portion (1 × 1 cm2) of the layered assemblies prepared on glass substrate. The AFM images of the multilayers were taken using a commercial AFM instrument (Multimode Nanoscope IIIa controller, Veeco, California, USA) in tapping mode. A piezoelectric scanner allowed a maximum x, y-scan size of 17 μm and a maximum z-extension of 3.9 μm. All topography and phase contrast images were taken at room temperature under ambient conditions. The Young's modulus and hardness of the multilayer films was determined by nanoindentation using a MFP Nanoindenter (Asylum Research, Santa Barbara, CA) equipped with a diamond Berkovich indenter.
Organic–inorganic hybrid mesocrystals were assembled by consecutive spin-coating sols of titania nanoparticles and polymer solutions in dimethylacetamide (Figure 1). The polymer was synthesized by a reactive ester polymer approach. Poly(pentafluorophenyl acrylate), which specifically reacts with primary amines, was used as starting material. The polymer poly(pentafluorophenyl acrylate) and catechol polymer was characterized using 1H and 19F NMR spectroscopy (Figures S2–S4, Supporting Information). The molecular weight and the PDI of the polymer were ≈18 900 and ≈1.49, respectively. The infiltration of the resulting lamellar TiO2 (the average size of TiO2 nanoparticles ≈8 nm and phase pure anatase) mesocrystal[29] with a mussel-mimetic catechol-containing “sticky” polymer followed by curing through thermal and metal oxidation leads to stiff and tough nanocomposites. Multilayers were assembled by consecutive spincoating of a catechol-polymer solution in DMA and TiO2 nanoparticle dispersion in hexane due to the fact that nanoparticles are stabilized with oleic acid and oleylamine. We started with a polymer layer due to its high affinity to the substrate and the adhesive character on the silicon wafer; the wafer was cleaned in acidic piranha solution, a mixture of concentrated sulfuric acid, H2SO4, and hydrogen peroxide, H2O2, volume ratio of 2:1. The thickness of the layers was controlled by optimizing the concentration of the solutions, while the spin-coating parameters were fixed at a speed of 5000 rpm, an acceleration of 5040 rpm s−1, and a time of 10 s. Each spin-coated layer was annealed at 120 °C for 15 min. The decomposition temperature and glass transition temperature (Tg) of polymer was much higher than 120 °C as verified using thermogravimetric analysis (TGA) (Figure S5, Supporting Information) and differential scanning calorimetry (DSC) (Figure S6, Supporting Information). The use of immiscible (orthogonal) solvents and an optimized heating
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1131
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
Figure 1. Fabrication of multilayers of DOPA polymer/titania nanoparticles. A silicon substrate (A) is coated with DOPA polymer as the first layer (B), followed by annealing at 120 °C for 15 min. The TiO2 nanoparticles are coated in the next step (C) with a catechol-polymer that infiltrates into the inorganic oxide layer. Sixteen spin-coating and heating cycles lead to a 8-bilayer stack of DOPA polymer/TiO2 (D).
rate are important for sustaining the multilayered structure.[63–65] The final structure containing eight alternating layers of polymer and TiO2 nanoparticles was used for structural characterization and nanoindentation experiments. Each characterization step was carried out on at least three samples to ensure reproducibility of results. The multilayers show structural colors upon addition of TiO2/polymer bilayers due to multilayer interference (Figure S1, Supporting Information).[63–65] The polymer strongly binds to the TiO2 nanoparticles, due to the presence of under-coordinated Ti atoms at the surfaces or the partial oxidation of the catechol moiety due to the redox-active behavior of the 1,2-dioxolene groups appears to be an important factor for the strong binding of the catechol ligands of the polymer to the Ti4+ surface sites.[46,48] A hierarchical composite structure was assembled by consecutive spin-coating of a dilute polymer solutions (2% polymer solutions in DMA and NP dispersions (600 mg in 8 mL). Figure 2A depicts the TEM image of as synthesized TiO2 nanoparticles. Eight (polymer/TiO2) bilayers (8BLs) were assembled in this manner, and the resulting (polymer/TiO2)8 nanocomposites were used in all subsequent tests of mechanical properties (vide infra). Figure 2B,C shows a representative cross-sectional scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. The thickness of the polymer layer was significantly smaller than the nominal thickness after spin-coating due to the infiltration of the polymer into the inorganic layer, i.e., during
1132
the spin-coating cycles, the polymer penetrates through the vacancies of the close-packed TiO2 nanoparticle layers and forms a dense cross-linked hybrid network. However, it is worth mentioning that the thickness of a single BL was with ≈50 nm almost identical and showed a linear trend (Figure S7, Supporting Information) with a smooth surface (Figure S8, Supporting Information). The annealing step leads to catechol oxidation and ultimately to the enhanced cross-linking of the catechol-polymer/ TiO2 composite. UV–vis spectra showed a broad peak at 320 nm in addition to the peak of nonoxidized catechol around 280 nm, indicating oxidative cross-linking of the polymer (Figure S9, Supporting Information).[49,66] The Raman spectrum of the as-prepared polymer (Figure 2D) displayed several peaks that are assigned to catechol ring modes. In-plane bending and stretching ring modes appear at 1611, 1297, and 1328 cm−1. The band at 1611 cm−1 is assigned to the in-plane ring stretching mode. Moreover, bands at 874, 826, 786 cm−1 are assigned to the C O groups of trisubstituted aromatic rings and the C C ring stretch appears at 1491 cm−1.[67] Raman studies of the nanocomposite showed the possible surface binding of the catechol groups to TiO2 nanoparticles surfaces that imparts strength to the nanocomposite. A strong band at 591 cm−1 indicates the binding of phenolic oxygen (C3 of catechol group) to the under-coordinated surface Ti atoms in a monodentate fashon.[68–72] However, close inspection of the spectrum (Figure S10, Supporting Information) shows two
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
Figure 2. A) Transmission electron microscopy (TEM) image of as synthesized nanoparticles. B) SEM micrograph showing eight bilayers of titania/catechol-polymer nanoparticles. C) AFM phase image of cross-section of the multilayered (polymer/TiO2)8 films. The organic layers appear narrow as dark region bands with a thickness of ≈4 nm, while the inorganic layers appear as bright regions with a thickness of ≈46 nm. All films were coated on silicon substrates except for samples used for the measurement for Raman, FT-IR and UV–vis. D) Raman spectrum of the polymer (black) and catechol-polymer/TiO2 nanocomposite (red). The most prominent bands at 470–800 and 1100–1500 cm−1 can be assigned to the catechol–iron coordination and to the vibrations of the carbon atoms of the catechol ring, respectively.
very weak bands with almost equal intensity at 562 and 632 cm−1. The band at 632 cm−1 together with that at 591 cm−1 represents the extent of bidentate chelation. The band at 562 cm−1 with an intensity similar to 632 cm−1 indicates charge transfer due to chelation. The Raman data confirms that the polymer binds to the TiO2 nanoparticles surfaces via the phenolic oxygen atoms in a monodentate and to some extent in bidentate manner. However, we point out that the Raman spectra in refs. [68–73] were measured on discrete metal–catecholate complexes. Here, the symmetric bands have a much higher intensity due to well-defined geometry of complexes, whereas it is more difficult to provide an unequivocal picture of the binding of the polymer binding to the surface layer of the TiO2 nanoparticles (which constitutes only a small fraction of the total sample). Possible interactions are illustrated in Figure 3.
www.MaterialsViews.com
Figure 3. Sketch of the connectivity of the TiO2 nanoparticles through the polymer.
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1133
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
The Raman spectrum of single-phase anatase TiO2 nanoparticles (Figure S11, Supporting Information) confirms that the TiO2 bands do not interfere with those of the composite. For comparison, the FT-IR spectrum (Figure S12, Supporting Information) of the polymer/TiO2 composite shows almost all prominent bands, except for the C–O ring vibrations, which are slightly shifted to lower wave numbers, and the bands assigned to the phenolic OH groups (broad band extending up to 3600 and 1320 cm−1) that are absent. A strong band centered at 664 cm−1 confirms the complexation of Ti4+ by the catechol. The polymer/TiO2 composite showed extraordinary mechanical strength with a Young’s modulus of E = 17.5 ± 2.5 GPa and a hardness of H = 1.1 ± 0.3 GPa. The estimation of E requires the knowledge of the Poisson ratio ν, the value of which is assumed to be ν = 1/3. Young’s modulus and hardness were measured by nanoindentation using a MFP nanoindenter (Asylum Research, Santa Barbara, CA) equipped with a diamond Berkovich indenter. The indentations were done on a 90 × 90 μm2 area at three different positions on the sample, with increasing maximum force (Figure 4). The Young's moduli and hardness were calculated by fitting the indentation curves according to the Oliver–Pharr method[65] using the analysis software of the nanoindenter. The Young's modulus was obtained from the slope of the onset of the unloading curve as the elastic response of the sample upon unloading. The hardness of the composite was determined as the ratio of maximum applied load divided by the indenter contact area at that load, and it indicates the resistance of the polymer/TiO2 composite against plastic deformation. To
Figure 4. Young’s modulus for the multilayers of catechol polymer/TiO2 nanoparticles obtained by nanoindentation as a function of different applied force. The result is independent of the applied maximum force as long as the maximum force is below ≈200 μN. For higher forces, a substrate effect is clearly visible. The black and red points are results of slightly different fitting procedures to the same force curves. On averaging the values between 20 and 200 μN for the red circles, the Young’s modulus gives a value of 17.5 ± 2.3 GPa.
1134
exclude any contribution from the silicon substrate only indentation forces of less than 200 μN were used. When increasing the load gradually up to 1000 μN, we observed an increase of the apparent Young's modulus due to the substrate for forces larger than 200 μN (Figure 3). The protein-based adhesives encountered in the marine mussel Mytilus edulis give an important hint into the strength of the polymer/TiO2 composite.[16,49,74] The catechol groups of the adhesive foot proteins act as “glue” which is responsible for the solidification via metal coordination[44,45] or oxidative chemical cross-linking with catechols of neighboring the polymer chains.[49,52–54,66] Catechols are easily oxidized to the quinones at higher temperature (>90 °C) that can undergo a Michael-type addition with the catechol groups.[49,66,75] This is indicated by measuring the solubility of the polymer after annealing under a similar set of conditions. The polymer is insoluble in DMA after heating even after extended ultrasonication (Figure S13, Supporting Information). However, it is difficult to extract the exact interaction (covalent or hydrogen bonding) by MAS 13C NMR (Figure S14, Supporting Information), because such addition leads to the formation of a quaternary carbon atom (4 °C) with a chemical shift (δ = 144 ppm), which overlapping with the signal of another already existing C4 of the aromatic ring. At ambient temperature catechols do not exhibit strong chemical reactivity, and lower E-moduli are observed.[76] Metal–catecholate interactions are among the strongest chemical bonds as indicated by the large formation constants of metal–catecholate complexes.[77] The dense and well-ordered packing of the nanoparticles in the nanocomposites and smoothness (as confirmed by the SEM images in Figure 2B) is a second important structural prerequisite for the high mechanical strength. In the nanocomposite, the TiO2 nanoparticles are approximately close packed (Figure 2C), and the individual polymer chains can directly cross-link neighboring particles. For a particle diameter of 8 nm, each TiO2 particle has a surface area of ≈20.000 Å2. Assuming for simplicity a full surface coverage of the stabilizing ligand on the TiO2 nanoparticles (dihydroxycinnamic acid), a surface segment of 25 Å2 per catechol anchor group, a 100% side group substitution of the polymer and full surface coverage, each particle will be bound to ≈800 catechol groups of different polymer chains. This maximizes the number of metal–polymer bonds. The stiffness, i.e., the resistance against plastic deformation, is strongly enhanced through the crosslinking of nanoparticles by the polymer, because many strong Ti-catechol bonds (or the backbone of the polymer chain itself) have to be broken to allow for plastic deformation as the indenter penetrates into the surface of the hybrid nanocomposite. The Young’s modulus of E = 12 ± 2.7 GPa and the hardness of H = 1.1 ± 0.4 GPa measured for the pure annealed
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
catechol polymer show that the plastic deformation (contributing to H) is determined mainly by the polymer, whereas the elastic (E) modulus, which measures the stiffness of the material under the effects of unidirectional loading (which tends to stretch or compress it elastically and where the plastic deformation does not contribute) increased considerably. The metal–catecholate coordination plays a critical role in defining the performance of the nonmineralized composite. The complexation constants of the 3d metals indicate that the near-covalent stiffness and strength depends on the coordination state and type of metals. Sanchez and co-workers[78] suggested that relevant factors determining the mechanical properties of inorganic–organic hybrids are: (i) the choice of solvent, (ii) the number of anchor groups, (iii) the nature of the interaction and (iv) the annealing temperature. Indeed, multidentate binding of the polymer and different nanoparticles within a layer is crucial for the mechanical properties of the composite, because the TiO2 nanoparticles with diameters
Communication
High-Performance TiO2 Nanoparticle/DOPAPolymer Composites Faroha Liaqat, Muhammad Nawaz Tahir,* Eugen Schechtel, Michael Kappl, Günter K. Auernhammer, Kookheon Char, Rudolf Zentel, Hans-Jürgen Butt, Wolfgang Tremel*
Many natural materials are complex composites whose mechanical properties are often outstanding considering the weak constituents from which they are assembled. Nacre, made of inorganic (CaCO3) and organic constituents, is a textbook example because of its strength and toughness, which are related to its hierarchical structure and its well-defined organic–inorganic interface. Emulating the construction principles of nacre using simple inorganic materials and polymers is essential for understanding how chemical composition and structure determine biomaterial functions. A hard multilayered nanocomposite is assembled based on alternating layers of TiO2 nanoparticles and a 3-hydroxytyramine (DOPA) substituted polymer (DOPA-polymer), strongly cemented together by chelation through infiltration of the polymer into the TiO2 mesocrystal. With a Young’s modulus of 17.5 ± 2.5 GPa and a hardness of 1.1 ± 0.3 GPa the resulting material exhibits high resistance against elastic as well as plastic deformation. A key feature leading to the high strength is the strong adhesion of the DOPA-polymer to the TiO2 nanoparticles.
Dr. F. Liaqat, Dr. M. N. Tahir, E. Schechtel, Prof. W. Tremel Institute for Inorganic and Analytical Chemistry Johannes Gutenberg-University Duesbergweg 10-14 , 55099 Mainz, Germany E-mail: [email protected]; [email protected] Dr. M. Kappl, Dr. G. K. Auernhammer, Prof. H.-J. Butt Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Prof. K. Char School of Chemical and Biological Engineering The National Creative Research Initiative Center for Intelligent Hybrids The WCU Program of Chemical Convergence for Energy and Environment Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-744 , South Korea Prof. R. Zentel Institute for Organic Chemistry Johannes Gutenberg-University Duesbergweg 10-14 , 55099 Mainz, Germany Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1. Introduction Nacre[1,2] is a textbook example showing how evolation can lead to high-performance materials by blending intrinsically weak constituents, CaCO3 (aragonite) and collagen-related biopolymers,[3,4] in a composite. It shows a unique integration of remarkable tensile strength toughness,[5] and electrical conductivity[6–8] which supplies a strategy for constructing integrated materials. Materials scientists (especially inspired from nature)[9–14] have adopted this strategy for making technical inorganic/ organic nanocomposites with strong bonds (covalent or iono-covalent) between the organic and inorganic components[15–24] in technical ceramics (e.g., ORMOCERS).[25] In contrast, when weak interactions predominate between the mineral and the organic component “flexible minerals” with enhanced elastic modulus and fracture resistance are formed.[26]
wileyonlinelibrary.com
DOI: 10.1002/marc.201400706
1129
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
Scientists have long sought to duplicate nacre’s strength and lightness in man-made materials.[27–32] In general, the toughness of synthetic composites decreases of with increasing hardness and vice versa. Nacre’s architecture varies at several length scales, from micrometers to nanometers. Replicating all of these scales—each of which contributes to the overall performance of nacre—in a synthetic material remains a challenge. For example, Podsiadlo et al.[16] made nacre-like layered materials through covalent cross-linking nanoclay and poly(vinylalcohol) (PVA) with high tensile strength, but lower toughness. Similarly, Studart and co-workers[18] fabricated tough layered materials with lower tensile strength based on platelets of Al2O3 and chitosan interconnected via hydrogen bonding. Bouville et al.[23] prepared bioinspired ceramics with high stiffness through ice-templating. By virtue of its functional surface groups graphene oxide (GO) turned out to be a promising building block for nanocomposites with high mechanical properties.[33–38] In order to solve the conflict between strength, toughness, and stiffness for integrated materials[39] several points must be addressed. (i) The mineral component should have a large lattice energy to supply sufficient hardness.[40] (ii) Strong interfacial interactions should be present between the building blocks (e.g., by covalent cross-linking). (iii) The covalent cross-links between the building blocks should contain long chains or polymers to supply enough positional entropy for absorbing energy. (iv) Finally, the methodology and overall architecture of the composite will be important.[41–43] The morphology of the inorganic particles, their structural organization, and the chemical bonding between them should provide a physicochemical basis for stiffness and flexibility at multiple length scales, leading to increased robustness against catastrophic materials failure. Nanoparticles (NPs) of metal oxides such as TiO2 or ZrO2 certainly meet the first criterion; in addition, they are cheap and readily available. Dopamine anchor groups with binding constants at the order of 1020 are among the most strongly binding ligands in coordination chemistry,[44–46] which can provide a large amount of interfacial bonding.[47–50] This is indicated by ample reports on the adhesive proteins isolated from the marine mussel Mytilus edulis.[49,51] The DOPA-polymer self-polymerizes (similar as mussel proteins) into long-chain polymers by oxidative chemical cross-linking,[52–54] which greatly improves mechanical strength. We demonstrate in this contribution that blending these DOPA-polymers with a dense, ordered multilayer matrix of metal oxide NPs leads to a nanocomposite with high mechanical strength. We note that the nanocomposite studied here is (i) fully transparent, (ii) the inorganic component can easily be replaced by other metal oxides (e.g., by CeO2,[55] ZrO2[56] or Fe2O3[57]) and (iii) that the metal oxide nanoparticles
1130
contained in these mixed matrices retain their specific surface activity.[58,59]
2. Experimental Section 2.1. Materials Unless noted otherwise, the precursors for the synthesis of nanoparticles and catechol polymer were purchased from Aldrich, Acros, and other commercial suppliers and used as received.
2.2. Polymer Synthesis The poly(active ester) poly(pentafluoro-phenylacrylate) (PFA) was prepared as reported earlier[60] and is described by the reaction scheme (Scheme S1, Supporting Information). This prepolymer was used for the synthesis of multifunctional DOPA-polymer.
2.3. Synthesis of DOPA-Polymer The poly(active ester) PFA (700 mg, 2.94 mmol repeating units) was dissolved in a mixture of 12 mL of dry dimethylformamide (DMF) and 0.5 mL of triethylamine. The next step was adding 3-hydroxytyramine hydrochloride (555 mg) dissolved in 3 mL DMF and 0.5 mL triethylamine to the mixture and the final contents were stirred for 6 h at 50 °C. The solvent was evaporated using rotary evaporator and product was resolved in about 3 mL methanol:aceton (3:1) and precipitated in water. The precipitated polymer was redissolved in a mixture of aceton (3 mL) and methanol (1 mL) and again precipitated in excess of cold ethyl ether. The precipitated polymer was centrifuged (9000 rpm, 10 min, and room temperature) and the solvent was decanted. The process was repeated until 800 mg of colorless solid polymer was obtained on drying. The structure of the polymer is shown in Scheme S2, Supporting Information.
2.4. Synthesis of TiO2 Nanoparticles TiO2 nanoparticles were synthesized following a procedure described in detail by Do and co-workers.[61] In brief, oleic acid (OLEA, Acros Organics)/oleylamine (OAM, Acros Organics)capped anatase TiO2 nanoparticles were synthesized by to a solvothermal method using titanium butoxide as precursor and water vapor as hydrolysis agent. The resulting dark brown mixture was cooled to room temperature and precipitated from the solution using ethanol. The TiO2 nanoparticles thus produced have an average size of ≈8 nm and are monodisperse, as confirmed by transmission electron microscopy (TEM).
2.5. Fabrication of Multilayers The multilayers were coated on a glass substrate or silicon wafer which has been precleaned in acidic piranha solution of concentrated sulphuric acid (H2SO4) and hydrogen peroxide (H2O2) in a volume ratio of 2:1. The multilayers were assembled by consecutive spin coating of DOPA-polymer solution in
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
dimethylacetamide (DMA) and TiO2 NPs dispersion in hexane, starting from the DOPA-polymer layer due to its high affinity to the substrate and adhesive character. The thickness of the layers is controlled by optimizing the concentration of the solutions to be spin-casted while the spin-coating parameters are fixed at the following values, speed: 5000 rpm, acceleration: 5040 rpm s−1, and time: 10 s. Each spin-coated layer was subjected to heat treatment at 120 °C for 15 min. The use of orthogonal solvents and an optimized heating rate is important to sustain the multilayered structure. The final multilayered structure of 12 alternating layers of polymer and TiO2 NPs was used for structural characterization and nanoindentation experiments. The characterization was carried out on a number of samples to ensure reproducibility of results. The multilayers show structural colors (Figure S1, Supporting Information) on the addition of bilayers due to multilayer interference.[62–64]
3. Results and Discussion
2.6. Nanoindentation The Young’s modulus (E) and hardness (H) of the multilayers of DOPA-polymer/TiO2 nanoparticles were measured by nanoindentation. The method involves applying a load to the indenter, which is in contact with the sample. The depth of indentation is measured as the load is applied and the area of contact (A) is calculated based on the shape of the indenter and depth of the indentation. In this experiment, each series of indentations is done on a grid of 2 × 3 indents on a 90 × 90 μm2 area at three different positions on the sample. This means that a total of 18 indents per series were carried out on the multilayers coated on glass substrate or Si wafer. The Young’s moduli and hardness were calculated by fitting the indentation curves according to the Oliver–Pharr method[65] using the analysis software of the nanoindenter. The Young’s modulus was obtained from the slope of the onset of the unloading part in the force versus indentation curve. The hardness of the material is obtained as the ratio of maximum applied load (Pmax) divided by the indenter contact area at that load and it is expressed by H =
Pmax . A
A substrate effect was observed for maximum applied loads higher than 100 μN (represented by the black data points in Figure 4). To eliminate any contribution from the underlying substrate, only maximum applied forces under than 100 μN are considered.
2.7. Physical Characterization The DOPA-polymer/TiO2 hybrid multilayers were built up on glass substrates using a Laurell WS-400-6NPP-LITE spin-coater. The size of the metal oxide nanoparticles was determined by TEM on a Phillips EM-420 equipped with a slow scan CCD detector (1 × 1 k2) and a LaB6 electron gun operated with an acceleration voltage of 120 kV. The TEM images were processed with the Gnu Image Manipulation Program GIMP Version 2.6.8 or with ImageJ Version 1.43u. The TEM imaging of the hybrid multilayers of DOPA-polymer/TiO2 was performed by preparing a lamella using focused ion beam (FIB) (FEI Nova 600 Nanolab FIB instrument). The lamella was fixed on TEM grid and eventually analyzed by electron energy-dispersive X-ray analysis (EDX) using a transmission electron microscope (Technai G2
www.MaterialsViews.com
F20; FEI/Philips) equipped with an EDX detector. A Horiba Jobin Y LabRAM HR Spectrometer with a frequency doubled Nd:YAGlaser (632 nm) was used for performing Raman spectroscopy of the multilayered films. The sample was prepared by cutting a small portion (1 × 1 cm2) of the layered assemblies prepared on glass substrate. The AFM images of the multilayers were taken using a commercial AFM instrument (Multimode Nanoscope IIIa controller, Veeco, California, USA) in tapping mode. A piezoelectric scanner allowed a maximum x, y-scan size of 17 μm and a maximum z-extension of 3.9 μm. All topography and phase contrast images were taken at room temperature under ambient conditions. The Young's modulus and hardness of the multilayer films was determined by nanoindentation using a MFP Nanoindenter (Asylum Research, Santa Barbara, CA) equipped with a diamond Berkovich indenter.
Organic–inorganic hybrid mesocrystals were assembled by consecutive spin-coating sols of titania nanoparticles and polymer solutions in dimethylacetamide (Figure 1). The polymer was synthesized by a reactive ester polymer approach. Poly(pentafluorophenyl acrylate), which specifically reacts with primary amines, was used as starting material. The polymer poly(pentafluorophenyl acrylate) and catechol polymer was characterized using 1H and 19F NMR spectroscopy (Figures S2–S4, Supporting Information). The molecular weight and the PDI of the polymer were ≈18 900 and ≈1.49, respectively. The infiltration of the resulting lamellar TiO2 (the average size of TiO2 nanoparticles ≈8 nm and phase pure anatase) mesocrystal[29] with a mussel-mimetic catechol-containing “sticky” polymer followed by curing through thermal and metal oxidation leads to stiff and tough nanocomposites. Multilayers were assembled by consecutive spincoating of a catechol-polymer solution in DMA and TiO2 nanoparticle dispersion in hexane due to the fact that nanoparticles are stabilized with oleic acid and oleylamine. We started with a polymer layer due to its high affinity to the substrate and the adhesive character on the silicon wafer; the wafer was cleaned in acidic piranha solution, a mixture of concentrated sulfuric acid, H2SO4, and hydrogen peroxide, H2O2, volume ratio of 2:1. The thickness of the layers was controlled by optimizing the concentration of the solutions, while the spin-coating parameters were fixed at a speed of 5000 rpm, an acceleration of 5040 rpm s−1, and a time of 10 s. Each spin-coated layer was annealed at 120 °C for 15 min. The decomposition temperature and glass transition temperature (Tg) of polymer was much higher than 120 °C as verified using thermogravimetric analysis (TGA) (Figure S5, Supporting Information) and differential scanning calorimetry (DSC) (Figure S6, Supporting Information). The use of immiscible (orthogonal) solvents and an optimized heating
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1131
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
Figure 1. Fabrication of multilayers of DOPA polymer/titania nanoparticles. A silicon substrate (A) is coated with DOPA polymer as the first layer (B), followed by annealing at 120 °C for 15 min. The TiO2 nanoparticles are coated in the next step (C) with a catechol-polymer that infiltrates into the inorganic oxide layer. Sixteen spin-coating and heating cycles lead to a 8-bilayer stack of DOPA polymer/TiO2 (D).
rate are important for sustaining the multilayered structure.[63–65] The final structure containing eight alternating layers of polymer and TiO2 nanoparticles was used for structural characterization and nanoindentation experiments. Each characterization step was carried out on at least three samples to ensure reproducibility of results. The multilayers show structural colors upon addition of TiO2/polymer bilayers due to multilayer interference (Figure S1, Supporting Information).[63–65] The polymer strongly binds to the TiO2 nanoparticles, due to the presence of under-coordinated Ti atoms at the surfaces or the partial oxidation of the catechol moiety due to the redox-active behavior of the 1,2-dioxolene groups appears to be an important factor for the strong binding of the catechol ligands of the polymer to the Ti4+ surface sites.[46,48] A hierarchical composite structure was assembled by consecutive spin-coating of a dilute polymer solutions (2% polymer solutions in DMA and NP dispersions (600 mg in 8 mL). Figure 2A depicts the TEM image of as synthesized TiO2 nanoparticles. Eight (polymer/TiO2) bilayers (8BLs) were assembled in this manner, and the resulting (polymer/TiO2)8 nanocomposites were used in all subsequent tests of mechanical properties (vide infra). Figure 2B,C shows a representative cross-sectional scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. The thickness of the polymer layer was significantly smaller than the nominal thickness after spin-coating due to the infiltration of the polymer into the inorganic layer, i.e., during
1132
the spin-coating cycles, the polymer penetrates through the vacancies of the close-packed TiO2 nanoparticle layers and forms a dense cross-linked hybrid network. However, it is worth mentioning that the thickness of a single BL was with ≈50 nm almost identical and showed a linear trend (Figure S7, Supporting Information) with a smooth surface (Figure S8, Supporting Information). The annealing step leads to catechol oxidation and ultimately to the enhanced cross-linking of the catechol-polymer/ TiO2 composite. UV–vis spectra showed a broad peak at 320 nm in addition to the peak of nonoxidized catechol around 280 nm, indicating oxidative cross-linking of the polymer (Figure S9, Supporting Information).[49,66] The Raman spectrum of the as-prepared polymer (Figure 2D) displayed several peaks that are assigned to catechol ring modes. In-plane bending and stretching ring modes appear at 1611, 1297, and 1328 cm−1. The band at 1611 cm−1 is assigned to the in-plane ring stretching mode. Moreover, bands at 874, 826, 786 cm−1 are assigned to the C O groups of trisubstituted aromatic rings and the C C ring stretch appears at 1491 cm−1.[67] Raman studies of the nanocomposite showed the possible surface binding of the catechol groups to TiO2 nanoparticles surfaces that imparts strength to the nanocomposite. A strong band at 591 cm−1 indicates the binding of phenolic oxygen (C3 of catechol group) to the under-coordinated surface Ti atoms in a monodentate fashon.[68–72] However, close inspection of the spectrum (Figure S10, Supporting Information) shows two
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
Figure 2. A) Transmission electron microscopy (TEM) image of as synthesized nanoparticles. B) SEM micrograph showing eight bilayers of titania/catechol-polymer nanoparticles. C) AFM phase image of cross-section of the multilayered (polymer/TiO2)8 films. The organic layers appear narrow as dark region bands with a thickness of ≈4 nm, while the inorganic layers appear as bright regions with a thickness of ≈46 nm. All films were coated on silicon substrates except for samples used for the measurement for Raman, FT-IR and UV–vis. D) Raman spectrum of the polymer (black) and catechol-polymer/TiO2 nanocomposite (red). The most prominent bands at 470–800 and 1100–1500 cm−1 can be assigned to the catechol–iron coordination and to the vibrations of the carbon atoms of the catechol ring, respectively.
very weak bands with almost equal intensity at 562 and 632 cm−1. The band at 632 cm−1 together with that at 591 cm−1 represents the extent of bidentate chelation. The band at 562 cm−1 with an intensity similar to 632 cm−1 indicates charge transfer due to chelation. The Raman data confirms that the polymer binds to the TiO2 nanoparticles surfaces via the phenolic oxygen atoms in a monodentate and to some extent in bidentate manner. However, we point out that the Raman spectra in refs. [68–73] were measured on discrete metal–catecholate complexes. Here, the symmetric bands have a much higher intensity due to well-defined geometry of complexes, whereas it is more difficult to provide an unequivocal picture of the binding of the polymer binding to the surface layer of the TiO2 nanoparticles (which constitutes only a small fraction of the total sample). Possible interactions are illustrated in Figure 3.
www.MaterialsViews.com
Figure 3. Sketch of the connectivity of the TiO2 nanoparticles through the polymer.
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1133
Macromolecular Rapid Communications
F. Liaqat et al.
www.mrc-journal.de
The Raman spectrum of single-phase anatase TiO2 nanoparticles (Figure S11, Supporting Information) confirms that the TiO2 bands do not interfere with those of the composite. For comparison, the FT-IR spectrum (Figure S12, Supporting Information) of the polymer/TiO2 composite shows almost all prominent bands, except for the C–O ring vibrations, which are slightly shifted to lower wave numbers, and the bands assigned to the phenolic OH groups (broad band extending up to 3600 and 1320 cm−1) that are absent. A strong band centered at 664 cm−1 confirms the complexation of Ti4+ by the catechol. The polymer/TiO2 composite showed extraordinary mechanical strength with a Young’s modulus of E = 17.5 ± 2.5 GPa and a hardness of H = 1.1 ± 0.3 GPa. The estimation of E requires the knowledge of the Poisson ratio ν, the value of which is assumed to be ν = 1/3. Young’s modulus and hardness were measured by nanoindentation using a MFP nanoindenter (Asylum Research, Santa Barbara, CA) equipped with a diamond Berkovich indenter. The indentations were done on a 90 × 90 μm2 area at three different positions on the sample, with increasing maximum force (Figure 4). The Young's moduli and hardness were calculated by fitting the indentation curves according to the Oliver–Pharr method[65] using the analysis software of the nanoindenter. The Young's modulus was obtained from the slope of the onset of the unloading curve as the elastic response of the sample upon unloading. The hardness of the composite was determined as the ratio of maximum applied load divided by the indenter contact area at that load, and it indicates the resistance of the polymer/TiO2 composite against plastic deformation. To
Figure 4. Young’s modulus for the multilayers of catechol polymer/TiO2 nanoparticles obtained by nanoindentation as a function of different applied force. The result is independent of the applied maximum force as long as the maximum force is below ≈200 μN. For higher forces, a substrate effect is clearly visible. The black and red points are results of slightly different fitting procedures to the same force curves. On averaging the values between 20 and 200 μN for the red circles, the Young’s modulus gives a value of 17.5 ± 2.3 GPa.
1134
exclude any contribution from the silicon substrate only indentation forces of less than 200 μN were used. When increasing the load gradually up to 1000 μN, we observed an increase of the apparent Young's modulus due to the substrate for forces larger than 200 μN (Figure 3). The protein-based adhesives encountered in the marine mussel Mytilus edulis give an important hint into the strength of the polymer/TiO2 composite.[16,49,74] The catechol groups of the adhesive foot proteins act as “glue” which is responsible for the solidification via metal coordination[44,45] or oxidative chemical cross-linking with catechols of neighboring the polymer chains.[49,52–54,66] Catechols are easily oxidized to the quinones at higher temperature (>90 °C) that can undergo a Michael-type addition with the catechol groups.[49,66,75] This is indicated by measuring the solubility of the polymer after annealing under a similar set of conditions. The polymer is insoluble in DMA after heating even after extended ultrasonication (Figure S13, Supporting Information). However, it is difficult to extract the exact interaction (covalent or hydrogen bonding) by MAS 13C NMR (Figure S14, Supporting Information), because such addition leads to the formation of a quaternary carbon atom (4 °C) with a chemical shift (δ = 144 ppm), which overlapping with the signal of another already existing C4 of the aromatic ring. At ambient temperature catechols do not exhibit strong chemical reactivity, and lower E-moduli are observed.[76] Metal–catecholate interactions are among the strongest chemical bonds as indicated by the large formation constants of metal–catecholate complexes.[77] The dense and well-ordered packing of the nanoparticles in the nanocomposites and smoothness (as confirmed by the SEM images in Figure 2B) is a second important structural prerequisite for the high mechanical strength. In the nanocomposite, the TiO2 nanoparticles are approximately close packed (Figure 2C), and the individual polymer chains can directly cross-link neighboring particles. For a particle diameter of 8 nm, each TiO2 particle has a surface area of ≈20.000 Å2. Assuming for simplicity a full surface coverage of the stabilizing ligand on the TiO2 nanoparticles (dihydroxycinnamic acid), a surface segment of 25 Å2 per catechol anchor group, a 100% side group substitution of the polymer and full surface coverage, each particle will be bound to ≈800 catechol groups of different polymer chains. This maximizes the number of metal–polymer bonds. The stiffness, i.e., the resistance against plastic deformation, is strongly enhanced through the crosslinking of nanoparticles by the polymer, because many strong Ti-catechol bonds (or the backbone of the polymer chain itself) have to be broken to allow for plastic deformation as the indenter penetrates into the surface of the hybrid nanocomposite. The Young’s modulus of E = 12 ± 2.7 GPa and the hardness of H = 1.1 ± 0.4 GPa measured for the pure annealed
Macromol. Rapid Commun. 2015, 36, 1129−1137 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Macromolecular Rapid Communications
High-Performance TiO2 Nanoparticle/DOPA-Polymer Composites
www.mrc-journal.de
catechol polymer show that the plastic deformation (contributing to H) is determined mainly by the polymer, whereas the elastic (E) modulus, which measures the stiffness of the material under the effects of unidirectional loading (which tends to stretch or compress it elastically and where the plastic deformation does not contribute) increased considerably. The metal–catecholate coordination plays a critical role in defining the performance of the nonmineralized composite. The complexation constants of the 3d metals indicate that the near-covalent stiffness and strength depends on the coordination state and type of metals. Sanchez and co-workers[78] suggested that relevant factors determining the mechanical properties of inorganic–organic hybrids are: (i) the choice of solvent, (ii) the number of anchor groups, (iii) the nature of the interaction and (iv) the annealing temperature. Indeed, multidentate binding of the polymer and different nanoparticles within a layer is crucial for the mechanical properties of the composite, because the TiO2 nanoparticles with diameters
