Nanocomposites Derived from Montmorillonite and

Letter pubs.acs.org/Langmuir

Nanocomposites Derived from Montmorillonite and Metallosupramolecular Polyelectrolytes: Modular Compounds for Electrorheological Fluids Matthias F. Geist,† Kevin Boussois,‡ Agnes Smith,‡ Claire S. Peyratout,‡ and Dirk G. Kurth*,†,§ †

Chemical Technology of Advanced Materials, University of Würzburg, Röntgenring 11, 97070 Würzburg, Germany Groupe d’Etude des Matériaux Hétérogènes, Ecole Nationale Supérieure de Céramique Industrielle, 12 rue Atlantis, 87068 Limoges Cedex, France § Fraunhofer-Institut für Silicatforschung ISC, Neunerplatz 1, 97082 Würzburg, Germany ‡

S Supporting Information *

ABSTRACT: Nanocomposites made from Na-montmorillonite and metallo-supramolecular polyelectrolytes (MEPE) based on nickel and ditopic bis-terpyridine ligands are prepared by an aqueous synthesis. Intercalation is confirmed by IR-spectroscopy, thermogravimetric analysis, and X-ray powder diffraction. The rheological response in the presence of an electric field of the dispersed nanocomposites in silicone oil is measured with a rheometer. The nanocomposites show a distinct electrorheological effect depending on the concentration and the kind of intercalated species. The effect occurs with a low content of active material while only very small currents are observed.

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INTRODUCTION Small particles dispersed in a nonconducting fluid can change their viscoelastic properties in the presence of an electric field.1 This so-called electrorheological effect (ER-effect) was discovered by Winslow in 1947.2 The electric field causes the particles to form structured aggregates, thus increasing the viscosity of the fluid. There is a wide field of applications of electrorheological fluids (ERFs), for instance, in intelligent dampers or power transmission,3 The giant electrorheological effect was reported in 2003,4 which results from additional interactions of urea coated particles through hydrogen bonds.5 Electrorheological devices are an interesting alternative to magnetorheological ones. The response of ERFs is fast and operation requires little power, thus reducing electrical gear and weight. The effect is known for a wide range of organic or inorganic materials such as aluminosilicates6 and clays7 as well as clay composites.8,9 In contrast to metal−organic frameworks, which are recently known for their ER properties,10 metallosupramolecular polyelectrolytes (MEPEs) have not been studied until now. MEPEs are readily prepared in aqueous solution by metal ion induced self-assembly of ditopic ligands (bis-terpyridine). Due to their positive charge, they can interact through electrostatic interactions with the silicate surface.11 Montmorillonite (MMT) is a layered aluminosilicate with negatively charged layers compensated by cations such as Na+ (Na-montmorillonite). The cations can be exchanged by suitable ions or polymers.12 Until now, only few studies on the intercalation of coordinative polymers or terpyridine © 2013 American Chemical Society

complexes into silicates have been described in the literature.13−15 Recently, we described the assembly of MEPE in mesoporous silicate.16 Based on these results, we report here the intercalation of MEPE into a layered silicate (for a schematic illustration, see Scheme 1). For the preparation of the composite, we chose Nanofil 116, a commercially available Na-montmorillonite due to its high exchange capacity.

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EXPERIMENTAL SECTION

All chemicals were purchased by Sigma Aldrich and used as received. Nanofil 116, a sodium intercalated montmorillonite, was a gift by Rockwood Additives and was used as received. Silicone oil (AK-10) was purchased from Wacker. IR spectra were recorded on KBr pellets in transmission using a Jasco FT/IR-4100 instrument. Thermal properties were analyzed from 20 to 900 °C (heating rate 10 °C min−1) with a thermal gravimetric analyzer under ambient atmosphere (Netzsch STA 429). X-ray diffraction patterns were recorded in transmission from 2−45° (2θ) on a STOE Stadi P instrument. The STOE Stadi P instrument uses Kα1 radiation from a Cu anode (λ = 1.541 Å) and a curved Ge (111) monochromator. For data collection, a wire detector (Linear PSD) was used. Microscopy images were taken by using a Leica DMRB microscope. The microscopy slides were equipped with copper band and wires connecting to a power source. DC voltage was applied with a DC power supply (modified FuG Elektronik HCP 14-12500). The Received: November 22, 2012 Revised: January 21, 2013 Published: January 24, 2013 1743

dx.doi.org/10.1021/la304657p | Langmuir 2013, 29, 1743−1747

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Scheme 1. Due to the Dynamic Nature of the Metal Ion Ligand Interaction MEPE readily Intercalates in Na-Montmorillonite without Additional Polymerization Reactions

cm−1 is related to the CO stretching mode of the acetate anion of Ni-MEPE, whereas the other bands correlate with vibrations of the terpyridine ligand.21,22 IR spectrum of NiMEPE intercalated in montmorillonite (Ni-MEPE-MMT) features the characteristic MMT band as well as the NiMEPE bands in the region from 1635 to 1300 cm−1. Most importantly, the prominent CO mode is absent in Ni-MEPEMMT, proving the ion exchange of acetate anions during intercalation (see Scheme 1 and Supporting Information S1). Acetate is replaced by the negative charged montmorillonite. The TGA data of neat Ni-MEPE shows two processes, one at around 150 °C and a second at around 320 °C, while the DTA shows a distinct exothermic peak at around 400 °C (Figure 1).

dielectric spectra were measured at 25 °C on a Novocontrol broadband dielectric spectrometer (Novocontrol GmbH). Electrorheologic properties were studied on a rheometer (Physika MCR 301, Anton Paar) with an ERD cell (Anton Parr) and a parallel plate measuring system (0.5 mm gap, plate diameter of 49.972 mm). A DC voltage was applied with a DC power supply (FuG Elektronik HCP 14-12500; Umax= 12.5 V, Imax= 1 mA). The storage and the loss modulus were measured under a varying electric field E (0, 0.2, 0.4, 0.6, 0.8, 1.0, 0 kV mm−1) with an oscillating configuration. The frequency was 10 s−1, and the strain was 0.1%. Ten points were recorded for each electric field, and an arithmetic mean was calculated. The current density was determined by dividing the current by the area of the plate disk (19.613 cm2). All electrorheological measurements were performed at 20 °C. Synthesis. 1,4-Bis(2,2′:6′,2″-terpyridine-4′-yl)benzene (Btpy) and 4′-phenyl-2,2′:6′,2″-terpyridine (Tpy) were synthesized according to literature procedures.17 In addition to the published procedure, Btpy was isolated as hydrobromide and purified by recrystallization in acetic acid (90%).18 Ni-MEPE was prepared in a 1:1 stoichiometry by conductometric titration of the Btpy ligand with Ni(CH3COO)2·4H2O according to the literature.19 Ni(Tpy)2 in 1:2 stoichiometry was synthesized in the same manner. Preparation of Nancomposite. Ni-MEPE (200 mg, 2.79 mmol) was dissolved in deionized water (200 mL) by ultrasonification. The Ni-MEPE solution was added to Nanofil 116 (500 mg) dispersed in deionized water (200 mL). The reaction mixture was stirred for 19 h. The mixture was then centrifuged, and the nanocomposite was washed several times with water. The nanocomposite was dried overnight at 40 °C. Preparation of ERF Samples. All MEPE-montmorillonite samples (15 wt %) were dried before measurement in a vacuum oven at 50 °C overnight to minimize the water content. The samples were crushed with a mortar and suspended in 2.15 mL (2.00 g) of silicone oil (Wacker AK-10). To get a homogeneous suspension, a disperser (IKA T18) was used for 5 min. ER fluids based on NiMEPE-MMT were prepared with 2, 3, 5, and 10 wt % ERF with NiMEPE, and Ni(Tpy)2 were prepared in the same way, but with 0.5, 1.1, and 1.6 wt %.

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Figure 1. TGA and DTA curves of neat Ni-MEPE (black) and NiMEPE-MMT (gray) in the range of 30−900 °C (ambient atmosphere).

In contrast, the nanocomposite decomposes not until 400 °C. The exothermic DTA peak of the nanocomposite occurs also at higher temperatures compared to neat Ni-MEPE. The peak of the composite is much broader starting at 480 °C and ending at around 800 °C. The thermogravimetric analysis supports our hypothesis that MEPE is intercalated. Intercalation of MEPE shifts the decomposition temperature to higher values, which is a well-documented phenomenon for intercalated composites including mononuclear terpyridine complexes.14 From TGA measurements we estimate the amount of intercalated NiMEPE in montmorillonite to be approximately 15%. To further examine the intercalation of MEPE, we used powder X-ray diffraction (XRD). The diffraction pattern is shown in Figure 2. XRD of Nanofil 116 shows a strong basal (001) reflection at about 7°. Other basal reflections cannot be observed. The higher (002) and (004) reflections at 14° and 28°, respectively, are also observed, whereas the (003) reflection is covered by an asymmetric hk reflex at 20°. According to Bragg’s law, the interlayer distance is determined to be 12.6 Å. The corresponding (001) reflection of Ni-MMT

RESULTS AND DISCUSSION

In this Letter, we present a composite based on silicate and MEPEs. The intercalation is achieved in a straightforward way by self-assembly. Due to their dynamic nature, MEPEs can directly intercalate into and assemble in MMT, making additional chemical polymerization steps unnecessary. Intercalation of MEPE into Nanofil under standardized conditions (SI) is readily confirmed by the color of the composite. Due to scattering of visible radiation, we use IR spectroscopy to confirm intercalation. The IR-spectra of NiMEPE as well as Ni-MEPE-MMT can be found in the Supporting Information. Pure Nanofil shows characteristic broad bands at 3625, 3446, and 1023 cm−1. These bands result from −OH stretching vibrations in the alumina octahedron (3625 and 3446 cm−1) and from Si−O stretching vibration in the silicate layer (1023 cm−1).20 Ni-MEPE shows characteristic bands at 1669, 1611, 1473, and 1400 cm−1. The band at 1669 1744


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Table 1. G′, G″, and Current Density j Values under an Electric Field E (1 kVmm−1) for Different Systems active material

loss modulus G″ [Pa] at 1 kV mm−1

current density j [μAcm−2] at 1 kV mm−1

Ni-MEPE

0.5 1.1 1.6

1014 1632 2278

116 160 191

NiMEPEMMT

2 (0.3)b

136

33

0.01

3 (0.5)b 5 (0.8)b 10 (1.5)b

247 885 2680

57 234 1049

0.01 0.02 0.01

2 (0.3)a

52

6

0.01

3 (0.5)a 5 (0.8)a

131 51

24 6

0.01 0.01

5

76

33

51

10

420

105

51

Figure 2. XRD pattern of Nanofil 116 and Ni-MEPE-MMT. Ni-TpyMMT

appears at a lower angle (4.5°), indicating an enhanced interlayer spacing of 19.8 Å. The increase of the interlayer spacing is most likely caused by the intercalation of Ni-MEPE. Unlike basal reflections, hk reflections are not shifted. The montmorillonite inner sheet structure is obviously not affected by the intercalation. From the interlayer spacing, we propose that the MEPE rods are lying flat on the silicate surface, which would maximize the electrostatic interactions. As a control, we studied the mononuclear complex Ni(Tpy)2. The XRD pattern shows also a shift of the (001) reflex from 7 to 4.6° 2θ (19.1 Å), which indicates the intercalation of Ni(Tpy)2. Thermogravimetric analysis shows an increased decomposition temperature and an amount of 15% of intercalated Ni(Tpy)2. In conclusion, our results confirm that Ni-MEPE as well as Ni(Tpy)2 readily intercalates into montmorillonite forming nanocomposites. For electrorheological measurements of the nanocomposite, different Ni-MEPE-MMT dispersions in silicone oil are prepared. Oscillating experiments are performed with a frequency of 10 s−1 and a strain of 0.1% to determine the storage and the loss modulus (and G′ and G″) under a varying electric field, E. All experiments were performed with a nanocomposite with 15 wt % Ni-MEPE. As seen in Table 1, all samples of Ni-MEPE-MMT show an ER effect. G′ increases by applying an electric field. In contrast, the increase of the loss modulus G″ is not very high. Figure 3 shows the ER effect of four different Ni-MEPE-MMT dispersions. Compared to a 10 wt % dispersion of HClO4-doped polythiophene, which shows a storage modulus of approximately 1000 Pa at 1 kV mm−1,23 Ni-MEPE-MMT shows under the same conditions a storage modulus value of approximately 3000 Pa. ERF based on urea coated nanoparticles show a very high effect (also called giant ER effect), but higher concentrations are necessary and the current density is about 4 μA cm−2.4 We note that using the MEPE-composite leads to a significant enhancement of the storage modulus, in particular if we consider that the amount of active material in the composite is very low. To make sure that the ER effect results from the intercalated MEPE, intercalated Ni(Tpy)2 (Ni-Tpy-MMT) and pure Nanofil 116 are analyzed as well (see Table 1 and Figure 3). Ni-Tpy-MMT shows only a marginal ER-effect. The storage modulus of Ni-Tpy-MMT (5 wt %) increases under an electric field from 22 to 51 Pa, while that of neat Nanofil (5 wt %) increases from 19 to 76 Pa. The Nanofil dispersion with 10 wt % shows also a lower ER-effect than the nanocomposite. While the mechanism responsible for the ER-effect is yet unclear, our

wt %

a

storage modulus G′ [Pa] at 1 kV mm−1

Nanofil 116

35 51 47

a

Active compound in silicone oil. bThe weight percent of pure NiMEPE is shown in parentheses.

data show that intercalation of MEPE is required to achieve the ER-effect. Pure Ni-MEPE dispersions show a higher ER-effect (see Table 1), which is attributed to the concentration of active material. For instance, based on the TGA data, the 5 wt % dispersion of Ni-MEPE-MMT contains approximately 0.8 wt % Ni-MEPE as active material. The dispersion of neat Ni-MEPE contains 1.1 wt % of active material. As a result, the neat NiMEPE sample exhibits a stronger ER-effect. However, we note that the current density of the neat samples is also much higher, limiting their use as ER-material. The polarization of particles in an electric field and the resulting formation of chainlike structures is currently the predominant macroscopic description of the solidification.24 To confirm and to illustrate the ER-effect, pictures of Ni-MEPEMMT in silicone oil were taken under an electric field (see Figure 4). It is clearly seen that Ni-MEPE-MMT particles form chainlike structures. This behavior is consistent with the results presented in the previous paragraph. A higher field results in a stronger fibrillation and, therefore, in a higher storage modulus G′ (see Figure 3), in agreement with our experimental results.25 We examined the dispersions by impedance spectroscopy.26,27 The dielectric loss factor ε″ as a function of frequency of neat Nanofil 116 dispersed in silicone oil (10 wt %) has a broad relaxation peak in the area from 102 to 104 Hz. In contrast, Ni-MEPE-MMT (10 wt %) exhibits a peak at higher frequency ranging from 104 to 106 Hz, which is probably the relaxation of the Ni-MEPE. Further dielectric studies are necessary to explain the relative high relaxation frequency for the composites. The dielectric constant ε′, which is related to the polarizability, of Ni-MEPE-MMT is higher compared to pure Nanofil. These measurements confirm the enhancement of the ER activity by intercalation of Ni-MEPE. Finally, we note that Ni-MEPE-MMT shows a very small current density; at 1 kV mm−1, the current density is 0.02 μA mm−2. Neat montmorillonite exhibits a current density of 51 1745


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Figure 3. Storage modulus G′ of Ni-MEPE-MMT dispersed in silicone oil with different mass concentrations (left side) and storage moduli G′ of NiMEPE, Ni-MEPE-MMT, Ni-Tpy-MMT, and Nanofil 116 (right side).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49-(0) 931-31-82109. Tel: +49-(0)931-31-82633. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Franziska Brede, Laetitia Carrara, and Youness Fakhor for their help preparing the nanocomposites and performing the microscopy experiments. Special thanks to Dr. Torsten Staab for his help building the ER microscopy slide and Wolfgang Virsik for modifying the DC power supply. Furthermore, we thank Peter Löschke for the dielectric spectroscopy. Rockwood Additives Ltd. supported us by a generous gift of Nanofil 116.

Figure 4. Structure of Ni-MEPE-MMT in silicone oil (10 wt %) without and with electric field. The arrow indicates the direction of the electric field E.

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μA mm−2 for a 5 and 10 wt % dispersion. The Na+ ions of MMT are probably responsible for the current as in the case of zeolites.28 We propose that MMT acts as a macroscopic counter charge for MEPE, thus replacing the mobile counterions through the tight interaction of the oppositely charged species.

(1) Halsey, T. C. Electrorheological Fluids. Science 1992, 258, 761− 766. (2) Winslow, W. M. Method And Means For Translating Electrical Impulses Into Mechanical Force. U.S. Patent 2,417,850, March 25, 1947. (3) Hao, T. Electrorheological Fluids. Adv. Mater. 2001, 13, 1847− 1857. (4) Wen, W.; Huang, X.; Yang, S.; Lu, K.; Sheng, P. The giant electrorheological effect in suspensions of nanoparticles. Nat. Mater. 2003, 2, 727−730. (5) Chen, S.; Huang, X.; van der Vegt, N. F. A.; Wen, W.; Sheng, P. Giant Electrorheological Effect: A Microscopic Mechanism. Phys. Rev. Lett. 2010, 105, 046001. (6) Hao, T. Electrorheological suspensions. Adv. Colloid Interface Sci. 2002, 97, 1−35. (7) Parmar, K. P. S.; Meheust, Y.; Schjelderupsen, B.; Fossum, J. O. Electrorheological Suspensions of Laponite in Oil: Rheometry Studies. Langmuir 2008, 24, 1814−1822. (8) Choi, H. J.; Jhon, M. S. Electrorheology of polymers and nanocomposites. Soft Matter 2009, 5, 1562−1567. (9) Kim, J. W.; Kim, S. G.; Choi, H. J.; Jhon, M. S. Synthesis and electrorheological properties of polyaniline-Na+-montmorillonite suspensions. Macromol. Rapid Commun. 1999, 20, 450−452. (10) Liu, Y. D.; Kim, J.; Ahn, W.-S.; Choi, H. J. Novel electrorheological properties of a metal-organic framework Cu3(BTC)2. Chem. Commun. 2012, 48, 5635−5637. (11) Whittell, G. R.; Manners, I. Metallopolymers: New Multifunctional Materials. Adv. Mater. 2007, 19, 3439−3468. (12) Fei Fang, F.; Jin Choi, H.; Joo, J. Conducting Polymer/Clay Nanocomposites and Their Applications. J. Nanosci. Nanotechnol. 2008, 8, 1559−1581.

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CONCLUSIONS Electrorheological fluids are an interesting alternative to magnetorheological fluids; however, high current densities limit the maximum possible potential and, therefore, the working range of the fluid.3 Also, the ER-effect has a natural limit given by the dielectric constant contrast between the particles and the oil.29 We show that MEPE can be intercalated into the MMT, resulting in a layered composite. The preparation of the nanocomposite is simple and environmentally friendly. Dispersions of the composite exhibit a concentration dependent electrorheological effect. The current densities remain uncritically low in the investigated potential range. While the mechanism is not currently fully understood, the origin of the ER-effect in these composites is clearly attributed to MEPE. Neither pure MMT nor mononuclear complexes show a distinct ER-effect under these conditions.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Comparison of the IR-spectra of Ni-MEPE and Ni-MEPEMMT (S1). This material is available free of charge via the Internet at http://pubs.acs.org. 1746


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(13) Shukla, N.; Thakur, A. K. Ion transport model in exfoliated and intercalated polymer−clay nanocomposites. Solid State Ionics 2010, 181, 921−932. (14) Bora, M.; Ganguli, J. N.; Dutta, D. K. Thermal and spectroscopic studies on the decomposition of [Ni{di(2-aminoethyl)amine}2]- and [Ni(2,2′:6′,2″-terpyridine)2]-Montmorillonite intercalated composites. Thermochim. Acta 2000, 346, 169−175. (15) Yagi, M.; Narita, K. Catalytic O2 Evolution from Water Induced by Adsorption of [(OH2)(Terpy)Mn(μ-O)2Mn(Terpy)(OH2)]3+ Complex onto Clay Compounds. J. Am. Chem. Soc. 2004, 126, 8084−8085. (16) Akcakayiran, D.; Kurth, D. G.; Röhrs, S.; Rupprechter, G.; Findenegg, G. H. Self-Assembly of a Metallosupramolecular Coordination Polyelectrolyte in the Pores of SBA-15 and MCM-41 Silica. Langmuir 2005, 21, 7501−7506. (17) Winter, A.; van den Berg, A. M. J.; Hoogenboom, R.; Kickelbick, G.; Schubert, U. S. A Green and Straightforward Synthesis of 4′Substituted Terpyridines. Synthesis 2006, 2006, 2873−2878. (18) Spahni, W.; Calzagerri, G. Synthese von para-substituierten phenyl-terpyridin liganden. Helv. Chim. Acta 1984, 67, 450−454. (19) Schwarz, G.; Sievers, T. K.; Bodenthin, Y.; Hasslauer, I.; Geue, T.; Koetz, J.; Kurth, D. G. The structure of metallo-supramolecular polyelectrolytes in solution and on surfaces. J. Mater. Chem. 2010, 20, 4142−4148. (20) Madejová, J. FTIR techniques in clay mineral studies. Vib. Spectrosc. 2003, 31, 1−10. (21) Sarkar, A.; Chakravorti, S. Vibrational Spectra of 2,6-Diphenyl Pyridine, 2,2′:6′,2″-Terpyridine and Meta-terphenyl. Spectrosc. Lett. 1994, 27, 305−321. (22) Strathmann, T. J.; Myneni, S. C. B. Speciation of aqueous Ni(II)-carboxylate and Ni(II)-fulvic acid solutions: Combined ATRFTIR and XAFS analysis. Geochim. Cosmochim. Acta 2004, 68, 3441− 3458. (23) Chotpattananont, D.; Sirivat, A.; Jamieson, A. Electrorheological properties of perchloric acid-doped polythiophene suspensions. Colloid Polym. Sci. 2004, 282, 357−365. (24) Sheng, P.; Wen, W. Electrorheology: Statics and dynamics. Solid State Commun. 2010, 150, 1023−1039. (25) Hill, J. C.; Steenkiste, T. H. V. Response times of electrorheological fluids. J. Appl. Phys. 1991, 70, 1207−1211. (26) Wu, J.; Xu, G.; Cheng, Y.; Liu, F.; Guo, J.; Cui, P. The influence of high dielectric constant core on the activity of core−shell structure electrorheological fluid. J. Colloid Interface Sci. 2012, 378, 36−43. (27) Block, H.; Kelly, J. P.; Qin, A.; Watson, T. Materials and mechanisms in electrorheology. Langmuir 1990, 6, 6−14. (28) Uebe, J.; Böse, H.; Reichert Y. Electrorheological fluid comprising organic dopants and use thereof. Ger. Offen. DE 10 2009 048 825 A1, Oct 9, 2009. (29) Wen, W.; Huang, X.; Sheng, P. Electrorheological fluids: structures and mechanisms. Soft Matter 2008, 4, 200−210.

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Nanocomposites Derived from Montmorillonite and

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