Use of a Polyhedral Oligomeric Silsesquioxane (POSS)-Imidazolium

Chemistry Department, U.S. Naval Academy, Annapolis, Maryland 21402, Office ...... Wilkie, C., Bourbigot, S., Eds.; Royal Society of Chemistry: Cambri...
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Langmuir 2007, 23, 7707-7714

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Use of a Polyhedral Oligomeric Silsesquioxane (POSS)-Imidazolium Cation as an Organic Modifier for Montmorillonite Douglas M. Fox,*,† Paul H. Maupin,‡ Richard H. Harris, Jr.,§ Jeffrey W. Gilman,§ Donald V. Eldred,| Dimi Katsoulis,| Paul C. Trulove,† and Hugh C. De Long⊥ Chemistry Department, U.S. NaVal Academy, Annapolis, Maryland 21402, Office of Basic Energy Sciences, Office of Sciences, U.S. Department of Energy, Washington, D.C. 20585, Fire Research DiVision, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Dow Corning Corporation, Midland, Michigan 48686, and Directorate of Chemistry and Life Sciences, Air Force Office of Scientific Research, Arlington, Virginia 22203 ReceiVed December 20, 2006. In Final Form: March 15, 2007 Recent studies on organically modified clays (OMCs) have reported enhanced thermal stabilities when using imidazolium-based surfactants over the typical ammonium-based surfactants. Other studies have shown that polyhedral oligomeric silsesquioxanes (POSS) also improve the thermal properties of composites containing these macromers. In an attempt to utilize the beneficial properties of both imidazolium surfactants and POSS macromers, a dual nanocomposite approach to prepare OMCs was used. In this study, the preparation of a new POSS-imidazolium surfactant and its use as an organic modifier for montmorillonite are reported. The purity, solubility, and thermal characteristics of the POSS-imidazolium chloride were evaluated. In addition, several OMCs were prepared by exchanging the Na+ with POSS imidazolium cations equivalent to 100%, 95%, 40%, 20%, and 5% of the cation exchange capacity of the clay. The subsequent OMCs were characterized using thermal analysis techniques (DSC, SDT, and TGA) as well as 29Si NMR to determine the POSS content in the clay interlayer both before and after thermal oxidation degradation. Results indicate the following: (1) the solvent choice changes the efficiency of the ion-exchange reaction of the clay; (2) self-assembled crystalline POSS domains are present in the clay interlayer; (3) the d-spacing of the exchanged clay is large (3.6 nm), accommodating a bilayer structure of the POSS-imidazolium; and (4) the prepared POSS-imidazolium exchanged clays exhibit higher thermal stabilities than any previously prepared imidazolium or ammonium exchanged montmorillonite.

Introduction Polymer-layered silicate nanocomposites (PLSNs) are attractive due to their enhanced thermal stability and improved flame retardant, mechanical, and barrier properties.1-4 The use of imidazolium-based room-temperature ionic liquids (RTILs) as organic modifiers for PLSNs has gained significant attention, because they exhibit superior processing characteristics over conventional surfactants such as ammonium and phosphonium salts.5-7 The incorporation of polyhedral oligomeric silsesquioxanes (POSS) into polymers has received much attention in recent years.8-15 These materials have many desirable properties over * Current address for Douglas Fox: Department of Chemistry, American University, 4400 Massachusetts Ave., NW, Washington, DC 20016, [email protected]. † U.S. Naval Academy. ‡ U.S. Department of Energy. § National Institute of Standards and Technology. | Dow Corning Corporation. ⊥ Air Force Office of Scientific Research. (1) Alexandre, M.; Dubois, P. Mater. Sci. Eng., R: 2000, 28, 1. (2) Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R. H.; Manias, E.; Giannelis, E. P.; Wuthernow, M.; Hilton, D.; Phillips, S. Chem. Mater. 2000, 12, 1866. (3) Wang, L. Q.; Liu, J.; Exarhos, G. J.; Flanigan, K. Y.; Bordia, B. J. Phys. Chem. B 2000, 104, 2810. (4) Zhao, J.; Morgan, A. B.; Harris, J. D. Polymer 2005, 46, 8641. (5) Gilman, J. W.; Morgan, A. B.; Harris, R. H., Jr.; Trulove, P. C.; De Long, H. C.; Sutto, T. E. Polym. Mater. Sci. Eng. 2000, 83, 59. (6) Gilman, J. W.; Awad, W. H.; Davis, R. D.; Shields, J.; Harris, R. H., Jr.; Davis, C.; Morgan, A. B.; Sutto, T. E.; Callahan, J.; Trulove, P. C.; De Long, H. C. Chem. Mater. 2002, 14, 3776. (7) Awad, W. H.; Gilman, J. W.; Nyden, M.; Harris, R. H., Jr.; Sutto, T. E.; Callahan, J.; Trulove, P. C.; De Long, H. C.; Fox, D. M. Thermochim. Acta 2004, 409, 3.

other composites and polymers. Unlike other silica-based fillers, each POSS molecule contains organic substituents on its outer surface that make them compatible with polymers, biological systems, and surfaces.9,11,16 These organic substituents can be modified to make the POSS nanostructure either reactive or nonreactive. Unlike traditional organic compounds, POSS chemicals release no volatile organic compounds below 300 °C and are therefore odorless and potentially environmentally friendly.11,15 In addition, the POSS-polymer nanocomposites have increased thermal stability, oxidative resistance, surface hardening, and improved mechanical properties along with decreased flammability and viscosity during processing.8,9,11 Furthermore, POSS has been successfully incorporated into both thermoplastics and a few thermosets without dramatically changing the processing.11 To date, the majority of research has been devoted to the applications in nanoreinforced polymeric materials, where the POSS molecules are incorporated in the polymer structure using copolymerization, grafting, or blending.4 If POSS were incor(8) Mather, P. T.; Jeon, H. G.; Romo-Uribe, A.; Haddad, T. S.; Lichtenhan, J. D. Macromolecules 1999, 32, 1194. (9) Jeon, H. G.; Mather, P. T.; Haddad, T. S. Polym. Int. 2000, 49, 453. (10) Fu, B. X.; Hsiao, B. S.; Pagola, S.; Stephens, P.; White, H.; Rafailovich, M.; Sokolov, J.; Mather, P. T.; Jeon, H. G.; Phillips, S.; Lichtenhan, J.; Schwab, J. Polymer 2001, 42, 599. (11) Li, G.; Wang, L.; Ni, H.; Pittman, C. U., Jr. J. Inorg. Organomet. Polym. 2001, 11, 123. (12) Leu, C. M.; Chang, Y. T.; Wei, K. H. Macromolecules 2003, 36, 9122. (13) Cardoen, G.; Coughlin, E. B. Macromolecules 2004, 37, 5123. (14) Hanssen, R. W. J. M.; van Santen, R. A.; Abbenhuis, H. C. L. Eur. J. Inorg. Chem. 2004, 67. (15) do Carmo, D. R.; Guinesi, L. S.; Dias, N. L.; Stradiotto, N. R. Appl. Surf. Sci. 2004, 235, 449. (16) Zheng, L.; Waddon, A. J.; Farris, R. J.; Coughlin, E. B. Macromolecules 2002, 35, 2375.

10.1021/la0636863 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

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Figure 1. Chemical formula for DMIPOSS.

porated into layered silicates, the high thermal stability and large size of the POSS molecules should produce OMCs with large d-spacings and wide thermal processing windows. The large organic surface area should facilitate preparation of highly dispersed PLSNs with a variety of polymers, including those requiring high processing temperatures, such as polyamide-6 (PA-6) and poly(ethylene teraphthalate) (PET). Furthermore, since POSS are known to self-assemble into lamellar structures,8,9,12,17-20 the use of a POSS-tethered imidazolium surfactant as an organic modifer for clays could provide a dual reinforcement mechanism and other synergistic benefits. In this study, 1,2-dimethyl-3-(benzyl ethyl isobutyl polyhedral oligomeric silsesquioxane)imidazolium chloride (DMIPOSSCl, cf Figure 1) and DMIPOSS+ modified montmorillonite (DMIPOSS-MMT) have been synthesized. The thermal stabilities of the starting material, chlorobenzyl ethyl isobutyl POSS (POSS-Cl), DMIPOSS-Cl, and DMIPOSS-MMT were determined using thermal gravimetric analysis (TGA) and simultaneous DSC and TGA (SDT). POSS content of the clays before and after thermal oxidation was analyzed using solid-state 29Si NMR. The POSS cage crystallinity and interlayer spacing of modified montmorillonite were examined using X-ray diffraction (XRD). The goal of our work presented here was to prepare a POSS clay useful for melt-processing, since extrusion is a more common and industry-preferred method for nanocomposite preparation. An investigation of the use of this OMC in polymer nanocomposites, including the effects of partial loading of the clay on the extent of exfoliation in polymer nanocomposites, is currently under investigation by these authors. Experimental Section A. Preparation of DMIPOSS-Cl. Chlorobenzyl ethyl isobutyl POSS (10 g, Hybrid Plastics) and a 30 mol % excess of 1,2-dimethylimidazole (Sigma-Aldrich) were added to THF (25 mL, Fisher Scientific, HPLC grade). After dissolution, the solution was heated to a gentle reflux for 3 days. The solution was cooled, dissolved in hot ethyl acetate, and precipitated with addition of acetonitrile. The solid product (10.4 g) was washed with acetonitrile and dried under vacuum overnight. B. Preparation of Organically Modified Clays. The organically modified clay was prepared by cation exchange procedures. Two solvent systems were used to examine the effects of clay swelling on the exchange efficiency of the surfactant. THF/H2O Method. DMIPOSS-Cl (3 g) was dissolved in THF (50 g, Fisher Scientific, HPLC grade); then, 3 g sodium montmorillonite (Na-MMT, Southern Clay Products) was added to form (17) Fu, B. X.; Hsiao, B. S.; Pagola, S.; Stephens, P.; White, H.; Rafailovich, M.; Sokolov, J.; Mather, P. T.; Jeon, H. G.; Phillips, S.; Lichtenhan, J.; Schwab, J. Polymer 2001, 42, 599. (18) Waddon, A. J.; Zheng, L.; Farris, R. J.; Coughlin, E. B. Nano Lett. 2002, 2, 1149. (19) Zheng, L.; Hong, S.; Cardeon, G.; Burgaz, E.; Gido, S. P.; Coughlin, E. B. Macromolecules 2004, 37, 8606. (20) Matejka, L.; Strachota, A.; Plestil, J.; Whelan, P.; Steinhart, M.; Slouf, M. Macromolecules 2004, 37, 9449.

Fox et al. a slurry. The slurry was heated to reflux, deionized water (25 mL, 18.3 MΩ) was slowly added, and the mixture was removed from the heat. After stirring for 2 days, the solution was stored for 3 days. The slurry was filtered and washed with 3 aliquots of hot THF (50 mL). The exchanged clay was washed for 4 h in a Soxhlet extractor with 3:2 (v/v) THF/H2O, leached with hot 1:1 ethanol/water, washed for 4 h in a Soxhlet extractor with 1:1 (v/v) EtOH/H2O, and filtered. The clay was air-dried, finely ground, and dried at 100 °C for 1 h. Because the THF was uninhibited, the procedure was performed in an Airless-ware system under dry nitrogen. A dyed clay, (5% NB + 95% DMIPOSS)-MMT, was also prepared using this procedure, by dissolving DMIPOSS-Cl (0.85 g) and Nile Blue A perchlorate (0.02 g) in THF (50 mL) prior to adding Na-MMT (0.82 g). EtOH/H2O Method. DMIPOSS-MMT was also prepared using a cation exchange procedure in 1:1 (v/v) EtOH/H2O. DMIPOSSCl (1.5 g) was added to hot 1:1 (v/v) EtOH/H2O (100 mL), and the mixture was stirred until DMIPOSS-MMT completely dissolved. Na-MMT (1.5 g) was added to form a slurry. The slurry was stirred for 2 days at 65 °C and stored for 3 days. The slurry was filtered and washed with 3 aliquots of 1:1 (v/v) EtOH/H2O (50 mL) and washed with EtOH/H2O in a Soxhlet extractor overnight. A portion of the extracted clay was also washed overnight using 3:2 (v/v) THF/H2O in a Soxhlet extractor. The clays were air-dried, finely ground, and dried at 100 °C in air for 1 h. The same procedure was used to prepare 4 dyed clays with 5% CEC Nile Blue A perchlorate and 95%, 40%, 20%, and 5% CEC DMIPOSS-Cl. The 1,2-dimethyl-3-hexadecylimidazolium exchanged montmorillonite (DMHdI-MMT), (5% NB + 95% DMHdI)-MMT, and N,Ndimethyl-N,N-dioctadecylammonium exchanged montmorillonite (DMDOdAmm-MMT) were prepared using standard exchange procedures as described previously.6,7,21 The exchange efficiency of the clays will be discussed in section D of Results and Discussion. C. Product Purities. Gravimetric analysis indicated a 97% product yield, and 1H NMR suggested the prepared DMIPOSS-Cl was close to 99% pure. The predominant substitution pattern on the aromatic ring was found to be the para substitution. Duplicate elemental analysis performed by Galbraith Laboratories [obsd (calcd)]: C, 46.55% (47.3%); H, 7.86% (7.7%); N, 2.45% (2.6%). 1H NMR (CDCl3, 300 MHz) (data not shown) was characterized by the disappearance of the chloromethylene resonance from the starting material, δ ) 4.5 ppm (2H, s) and the appearance of new resonances at δ ) 2.7 ppm (3H, s), 3.9 ppm (3H, s), 5.4 ppm (2H, s), 7.5 ppm (1H, d), and 7.7 ppm (1H, d) corresponding to the imidazolium 2-methyl, the imidazolium 1-methyl, the imidazolium 3-benzylic methylene, and the 4- and 5-position imidazolium hydrogens, respectively. The 3.9 ppm peak was phase-inverted. Assignments were made from comparison with Aldrich spectra of some simple imidazole and imidazolium compounds. D. Instrumentation. TGA. Thermal stabilities were measured using a TA Instruments Q-500 Thermogravimetric Analyzer. 5.0 ( 0.2 mg samples were placed in open platinum pans and heated at a scan rate of 10 °C/min while purging with 100 mL/min N2 or compressed air. The means of three replicate measurements are reported. The temperature of both the onset (5% mass fraction loss) and the peak mass loss rate have an uncertainty of σ ) (2 °C. All samples were held at 90 °C for 1 h prior to each scan to remove any residual water and, in the case of the thermal stability in nitrogen, to remove any residual oxygen from the furnace. SDT. Simultaneous DSC and TGA measurements were carried out using a TA Instruments SDT600 Simultaneous DSC/TGA. 5.0 ( 0.2 mg samples were placed in open alumina pans and heated and cooled at a scan rate of 5 °C/min while purging with 100 mL/min of ultrapure nitrogen. Data were collected during the third consecutive scan. XRD. Powder X-ray diffraction experiments were performed on a Phillips Powder diffractometer. The d-spacing was calculated from peak positions using Bragg’s Law (Cu KR radiation, λ ) 0.154 18 (21) Maupin, P. H.; Gilman, J. W.; Harris, R. H., Jr.; Bellayer, S.; Bur, A. J.; Roth, S. C.; Murariu, M.; Morgan, A. B.; Harris, J. D. Macromol. Rapid Commun. 2004, 25, 788.

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Table 1. Qualitative Solubility Characteristics of POSS-Cl & DMIPOSS-Cl solvent

classificationa

dielectric constant

D (Debye units)

POSS-Cl solubility

hexane cyclohexene toluene triethylamine i-propanol dichloromethane ethanol methanol water tetrahydrofuran ethyl acetate bromohexane acetone triethyl phosphate dimethyl formamide acetonitrile dimethyl sulfoxide

aprotic, inert aprotic, inert aprotic, π-donor protophilic protic, neutral aprotic, inert protic, neutral protic, neutral protic, neutral dipolar aprotic dipolar aprotic aprotic, inert dipolar aprotic dipolar aprotic protophilic dipolar aprotic dipolar aprotic

1.89 2.22* 2.39 2.42* 18.3* 9.14 25.00 33.62 80.20 7.58 6.11 5.82* 20.71* 13.43(15) 38.3 37.5 48.9

0 0.28 0.45 0.9(B*) 1.58 1.60 1.69 1.70 1.85 1.75(B*) 1.78 2.06 2.88 3.08(B*) 3.82(B*) 3.92 3.9(B*)

S S S S I S I I I S S S S I I I I

DMIPOSS-Cl solubility I SS S SS SS S SS SS I S S SS SS I I I I

a Solvent classification follows Kolthoff’s modification of Bronsted’s classification of solvents.22 Dielectric and dipole moments were measured at 20 °C, except (*), which was measured at 25 °C and (B), which was measured in benzene solution.23 Soluble (S) - dissolution occurs almost immediately; sparingly soluble (SS) - dissolution occurs after several minutes of stirring; relatively insoluble (I) - none appeared to dissolve after 30 min of stirring.

Figure 2. SDT scan at 5 °C/min of DMIPOSS-Cl under ultrapure nitrogen. The heat flow has been corrected for the empty pan.

Figure 3. XRD of chlorobenzyl ethyl iso-butyl-POSS, DMIPOSSCl, and DMIPOSS-MMT. Plots have been shifted vertically for clarity.

nm). Clay samples were ground to a fine powder prior to loading. Standard X-ray measurements were performed over a 2θ range of 2.0-12° using a step size of 0.04° and 3 s dwell time. 29Si MAS NMR. Solid-state 29Si NMR spectra were collected at Dow Corning Corporation using an Inova 400 MHz spectrometer quipped with a 7 mm OD Jacobson Style MAS probe. Samples were packed into ZrO rotors and spun at 5000 Hz during acquisition. Spectra were acquired with 90° pulses, signal averaged for 1024 scans, decoupled using CW decoupling during the acquisition period, and a 60 s relaxation period was used. Spectra are referenced externally to TMS at 0 ppm. Accuracy and precision have not been established for this data.

Results and Discussion A. Solubility and Melting Characterization. Table 1 lists the solubility characteristics of POSS-Cl and DMIPOSS-Cl in a representative number of solvents, covering an extensive polarity range. The results indicate that POSS-Cl is solvated primarily by van der Waals and dipole-dipole interactions, while DMIPOSS-Cl is solvated primarily through ion-dipole and π-π interactions. The inability of highly polar solvents to solvate either POSS-Cl or DMIPOSS-Cl illustrates the strong repulsive forces between the hydrocarbon groups surrounding the POSS cage and the localized charges on the solvents. The increase in

Figure 4. Powder X-ray diffraction for several clays. Intensities have been shifted upward for clarity.

DMIPOSS-Cl solubility in solvents with low polarity and increased π-electron donating ability is likely due to π-π interactions between the π-electrons of the solvent and the aromatic benzyl and imidazole rings of DMIPOSS-Cl. DMIPOSS-Cl is probably solvated as an ion pair in these solvents.22 In an inert atmosphere, POSS macromers generally do not melt and

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Figure 5. Bilayer structure of POSS-imidazolium exchanged montmorillonite. Due to the size of the surfactant, the exchange efficiency is less than 100% as shown by unexchanged sites.

have a propensity toward sublimation.15,24-26 To access the melting characteristics of DMIPOSS-Cl and differentiate the peaks associated with melting from those due to decomposition, simultaneous DSC-TGA scans were measured under N2. The absence of an endotherm in the SDT plot prior to the onset of mass loss indicates that DMIPOSS-Cl also does not melt (cf. Figure 2). The heat flow indicates a small endothermic response at 200 °C followed by large exothermic events between 250 and 400 °C and between 500 and 600 °C. This suggests that some sublimation of the DMIPOSS-Cl begins, but that the presence of the nucleophilic Cl- ion leads to the decomposition of the imidazolium cation prior to complete POSS sublimation. Furthermore, the presence of a significant amount of char after pyrolysis up to 800 °C (8%) suggests that some of the decoupled POSS cages decompose rather than completely sublime. The absence of an endothermic peak prior to the onset of decomposition in the SDT of the organoclay, DMIPOSS-MMT (figure not shown), shows that the DMIPOSS+ crystalline regions do not undergo a melting-like order-disorder transition or randomization of surfactant conformation that often occurs in organically modified clays.3,27-30 B. X-ray Characterization. The XRD spectra of the POSS macromer reactant, imidazolium surfactant, and POSS-imidazolium MMT are shown in Figure 3. The peaks at 8.3° and 11.0° 2θ are characteristic of the hexagonal crystalline structure of POSS.16,17 This indicates that the POSS crystalline cages selfassemble, even after tethering the POSS macromer to the imidazolium cation. The loss of some peaks may be due to the strong repulsive forces between the hydrocarbon groups surrounding the POSS cage and the localized charges on the (22) Popovych, O.; Tomkins, R. P. T. Nonaqueous Solution Chemistry; John Wiley and Sons: New York, 1981. (23) Dean, J. A., Ed. Lange’s Handbook of Chemistry, 14th ed.; McGraw-Hill, Inc.: New York, 1992. (24) Mantz, R. A.; Jones, P. F.; Chaffee, K. P.; Lichtenhan, J. D.; Gilman, J. W; Ismail, I. M. K.; Burmeister, M. J. Chem. Mater. 1996, 8, 1250. (25) Fina, A.; Tabuani, D.; Camino, G.; Frache, A.; Boccaleri, E. In Fire Retardency of Polymers: New Applications of Mineral Fillers; Le Bras, M., Wilkie, C., Bourbigot, S., Eds.; Royal Society of Chemistry: Cambridge, U.K., 2005; p 202. (26) Fina, A.; Tabuani, D.; Carniato, F.; Frache, A.; Boccaleri, E.; Camino, G. Thermochim. Acta 2006, 440, 36. (27) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (28) Varlot, K.; Reynaud, E.; Klopper, M. H.; Vigier, G.; Varlet, J. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1360. (29) Li, Y.; Ishida, H. Langmuir 2003, 19, 2479. (30) Gelfer, M.; Burger, C.; Fadeev, A.; Sics, I.; Chu, B.; Hsiao, B. S.; Heintz, A.; Kojo, K.; Hsu, S. L.; Si, M.; Rafailovich, M. Langmuir 2004, 20, 3746.

imidazolium ring. The presence of the peaks associated with the POSS crystalline structure in the POSS-imidazolium MMT plot suggests that the POSS cages aggregate within the clay interlayer, retaining a crystalline structure. The observed propensity of POSS to self-assemble and retain crystalline domains is not unlike the results for POSS copolymers reported by many others.8,9,12,17-20 A comparison of the XRD for DMIPOSS to previously reported OMCs is shown in Figure 4. The large d-spacing for DMIPOSS+ exchanged MMT (3.60 nm) is indicative of a bilayer structure as shown in Figure 5. It is unclear whether these results agree with the results reported by Yei et al.31 and Lui et.al.32 on POSS modified clays, because their reported XRD data do not go to low enough angles (2θ) to identify a bilayer structure. Regardless, the presence of the (003) peak at 7.6° indicates a greater degree of layered silicate order in the DMIPOSS-MMT than in the alkylammonium POSS MMT reported by Yei and Lui. Furthermore, the peaks at 8.3° and 11.0° (2θ) suggest that the DMIPOSS-MMT has POSS crystalline domains in the interlayer which were not observed in the alkylammonium POSS-MMT reported by Yei and Lui. Evidence of various degrees of surfactant ordering within the clay layers3,27,29 and surfactant melting on the surface of the clay layers28 have been reported previously. However, this is the first evidence to these authors’ knowledge of surfactant crystalline domains within the clay interlayer. C. Thermal Gravimetric Analysis. The thermal stabilities of the organically modified clays (OMCs) were determined using thermal gravimetric analysis (TGA) in air and nitrogen. As shown in Figure 6, DMIPOSS-MMT exhibits a thermal stability (peak decomposition temperature) in nitrogen which is 100 °C higher than 1-hexadecyl-2,3-dimethylimidazolium exchanged clay (DMHdI-MMT) and 200 °C higher than the conventional N,Ndimethyl-N,N-dioctadecylammonium exchanged clay (DMDOdAmm-MMT). The effects of solvent choice on the exchange of DMIPOSS+ was examined using TGA. Figure 7 shows the TGA mass loss rate under N2 flow of DMIPOSS-MMT prepared in THF/H2O, EtOH/H2O, and EtOH/H2O followed by washing in THF/H2O. The exchanged clay using THF/H2O shows a small peak at 200 °C, while the clay using EtOH/H2O shows a large peak at this same temperature. Comparing these peaks to the decomposition of the surfactant, DMIPOSS-Cl, indicates that they are likely due to nucleophilic attack on the exchanged surfactant (31) Yei, D. R.; Kuo, S. W.; Su, Y. C.; Chang, F. C. Polymer 2004, 45, 2633. (32) Liu, H.; Zhang, W.; Zheng, S. Polymer 2005, 46, 157.

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Figure 6. Thermal stability of organically modified clays in nitrogen. Figure 8. XRD of MMT partially exchanged with DMIPOSS+. Plots have been shifted vertically for clarity.

Figure 7. Effects of solvent on DMIPOSS exchange of MMT under 100 mL/min N2.

cation. Morgan and Harris33 have shown that Soxhlet extraction of the organically modified clay can improve the properties of clay-polymer nanocomposites, and Davis et al.34 have shown that extraction with just ethanol leads to the incomplete removal of the unexchanged surfactant (in this case, DMIPOSS-Cl) while extraction with just THF leads to the incomplete removal of the exchanged sodium halide (in this case, NaCl). The incomplete removal of the halide ions can lead to nucleophilic attack of halide on the imidazolium cation.7,35 By extracting the clay with first EtOH/H2O, then THF/H2O, we were able to effectively remove all residual halide ions, resulting in an improvement in thermal stability of 50-100 °C. D. Partially Exchanged Clays. In melt-blended polymer/ layered silicate nanocomposites, better exfoliation of the clay layers can be achieved by establishing more favorable polymerclay interactions, which can sometimes be improved through reduced surfactant loading.36,37 A series of OMCs using partial loadings of DMIPOSS were prepared and characterized in an attempt to improve the exfoliation of the clay layers. A small amount (5% CEC of the clay) of a fluorescent dye was added, so that the PLSNs could be characterized using fluorescence techniques.21 XRD of the partially exchanged OMCs (see Figure 8) reveals that lower surfactant loadings slightly change the d-spacing of the exchanged clays (peak at 3.6 nm shifts to 3.2 nm). In the fully exchanged samples, the d-spacing peak (33) Morgan, A. B.; Harris, J. D. Polymer 2003, 44, 2313. (34) Moore, D. M.; Reynolds, R. C., Jr. X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: Oxford, U.K., 1989. (35) Fox, D. M.; Gilman, J. W.; De Long, H. C.; Trulove, P. C. J. Chem. Thermodyn. 2005, 37, 900. (36) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000. (37) Zhao, Z.; Tang, T.; Qin, Y.; Huang, B. Langmuir 2003, 19, 7157.

associated with the unmodified Na-MMT (1.02 nm) disappears, and there are strong signals associated with the (001), (002), and (003) peaks for the d-spacing of the clay interlayer (3.60, 1.80, and 1.21 nm, respectively). The intensities of the three interlayer peaks are related to the composition of the smectite and are similar to those found for montmorillonite in other studies.38-40 As the amount of DMIPOSS+ exchanged decreases, the (001) peak is shifted to slightly lower d-spacings (from 3.6 to 3.2 nm), the (002) and (003) peaks disappear, the peak associated with the POSS crystallinity (8.3° 2θ) disappears, and the peak associated with Na-MMT (7.6° 2θ) appears. Unlike typical surfactants with long floppy alkyl chains, DMIPOSS+ is rather rigid and incompressible. The retained bilayer structure of the POSS surfactant within the clay interlayer is likely due to the strong hydrophobic interactions associated with the POSS cages.18,19,41 However, the loss of the crystallinity peaks suggests that the neighboring POSS cages no longer form extended crystal regimes. This is possibly due to the lower packing density at the lower % CEC loadings. Furthermore, the slight decrease in d-spacing suggests that the molecules now lay parallel to the clay layers, rather than being fully extended across the gallery. This is rather typical of layered organosilicates with less than 100% CEC exchange of the Na+ cations.37,41,42 In addition, the presence of a peak associated with Na-MMT illustrates that the cationic sites are exchanged with DMIPOSS+ to form a heterogeneous clay rather than every layer exhibiting some sites exchanged with DMIPOSS+.20,42 The thermal stabilities of the OMCs in air and nitrogen environments as well as the “apparent exchange efficiencies” are shown in Table 2. The peak temperatures shown in parentheses represent the peak dehydroxylation temperatures. The dehydroxylation of the silicate layers are evident by mass losses between 600 and 800 °C in the TGA.34,43 Balek et al.43 concluded that the dehydroxylation process is catalyzed by protons originating from water coordinated to the exchangeable cation and that the catalytic effect increases with the polarizing power (Lewis acidity) of the cation. This is in agreement with the rise in dehydroxylation temperatures as the amount of Na+ exchanged (38) Mielenz, R. C.; King, M. E. In Clays and Clay Technology; Pask, J. A., Turner, M. D., Eds.; California Division of Mines, Bulletin 169, 1955; p 119. (39) Kohyama, N.; Shimoda, S.; Sudo, T. Clays Clay Miner. 1973, 21, 229. (40) Ferrage, E.; Tournassat, C.; Rinnert, E.; Charlet, L.; Lanson, B. Clays Clay Miner. 2005, 53, 348. (41) Yaron-Marcovich, D.; Chen, Y.; Nir, S.; Prost, R. EnViron. Sci. Technol. 2005, 39, 1231. (42) Lee, S. Y.; Kim, S. J. J. Colloid Interface Sci. 2002, 248, 231. (43) Balek, V.; Malek, Z.; Yariv, S.; Matuschek, G. J. Therm. Anal. Calorim. 1999, 56, 67.

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Table 2. Thermal Stability of DMIPOSS and Organically Modified Clays in air

in N2

mass loss %

organically modified MMT

Tonset ((2 °C)

Tpeak ((2 °C)

Tonset ((2 °C)

Tpeak ((2 °C)

exp.

theor.

apparent exchange efficiencya

(5% NB) (5% NB + 95% DMIPOSS) (5% NB + 40% DMIPOSS) (5% NB + 20% DMIPOSS) (5% NB + 5% DMIPOSS)

252 273 264 259 262

624 332 335 334 453

272 291 311 282 285

--- (623) 356 (707) 355 (647) 342 (623) 342 (633)

6.6 32 20 14 7.9

6.6 45 31 20 11

100 71 65 72 71

a Apparent exchange efficiency assumes that neither NB nor DMIPOSS are preferentially exchanged for Na+ and that no surfactant remains in the clay interlayer after thermal oxidation.

Figure 9. 29NMR spectra and the spectrum resulting from spectral subtraction of Na-MMT for (5% NB + 95% DMIPOSS)-MMT (a) prior to heating and (b) after heating.

with DMIPOSS+ increases. (Imidazolium cations are known to be less acidic than Na+.44) The low apparent exchange efficiencies can be explained several ways: less than 100% of the Na+ was exchanged during the preparation of the organically modified clay; some of the DMIPOSS+ cations were trapped in the interlayer during the pyrolysis; or the DMIPOSS+ partially decomposed, leaving some POSS residuals in the interlayer. To investigate the TGA behavior of the exchanged clays, 29Si MAS NMR was performed on the samples prior to and after thermal oxidation in air (cf. Figure 9). The estimated contributions of POSS relative to the clay sample for the pristine and thermally oxidized samples based on spectral subtraction results are provided in Table 3. The NMR results for the pristine clay samples indicate that the exchange efficiency during the preparation of the OMS is 90-95% when using EtOH/water as the solvent and 80-85% when using THF/water. The oxidized samples all reveal high POSS contents relative to the mass lost during heating, suggesting that the (44) Vaughan, J. D.; Mughrabi, Z.; Wu, E. C. J. Org. Chem. 1970, 35, 1141.

DMIPOSS+ cation only partially decomposes during the heating, leaving some POSS residue trapped in the interlayer of the clay. Furthermore, the negative values for the % POSS remaining using TGA data suggest that some of the assumptions used for the tabulated data are invalid and that some of the organic groups remain within the POSS structure after oxidation. There are slight variations in the apparent exchange efficiency due to both the solvent used in the exchange reaction and the amount of DMIPOSS+ relative to the clay CEC added to the reaction. The effects of using different solvents are illustrated by XRD (3.40 nm using EtOH and 3.60 nm using THF), TGA (15% mass lost at 350° and 10% mass lost at 500 °C when using EtOH and 5% mass lost at 350 °C and 15% mass lost at 500 °C when using THF), and Si analysis (0.91 SiPOSS/SiClay using EtOH and 0.75 SiPOSS/SiClay using THF). The tendency for POSS cages to self-assemble could lead to the inclusion of DMIPOSS-Cl ion pairs within the clay interlayer (see Figure 10). Indeed, Davis et al.34 found that EtOH-prepared organoclays often contain

POSS-Imidazolium Cation as Organic Modifier

Langmuir, Vol. 23, No. 14, 2007 7713 Table 3. POSS Content of OMS g DMIPOSS/ g clay

organically modified MMT

solvent

theor.b

Na-MMT 100% DMIPOSS 100% DMIPOSS (5% NB + 95% DMIPOSS) (5% NB + 95% DMIPOSS) (5% NB + 40% DMIPOSS) (5% NB + 20% DMIPOSS)

none THF EtOH THF EtOH EtOH EtOH

0 0.96 0.96 0.91 0.91 0.38 0.19

% POSS lost upon heating

NMRa,c

exchange efficiency (NMR/ theor)

NMRoxid/NMRpristine

TGAd

NMRa,e

0 N/A 0.86 0.74 0.85 0.58 0.17

0 ∼0.8 0.91 0.82 0.93 1.52 0.89

0 N/A 0.37 N/A 0.11 0.41 0.62

0 -2.5 7.5 -19 -2.1 -13 2.7

0 N/A 10 N/A 3.2 22 21

a Indication of POSS in these samples arises from an increase in overall SiO4/2 content relative to SiO3/2(OZ), where Z is a non-silicon atom, most likely H. b Theoretical mass ratio assumes 100% exchange efficiency and that neither NB nor DMIPOSS are preferentially exchanged for Na+. c (Si + + POSS/SiClay) × (4 mol Si/mol clay) × (1031 g/mol DMIPOSS )/(8 mol Si/mol DMIPOSS )/(549 g/mol clay). Note: MMclay used in the calculations actually varies slightly due to the amount of Na+ replaced by the surfactant. d Mass % POSS lost, using the tabulated exchange efficiency and assuming that all organic groups oxidize with remaining POSS existing as H8Si8O12. e (NMRoxid/NMRpristine) × (4 mol Si/mol oxidized clay) × (425 g/mol HPOSS)/(8 mol Si/mol HPOSS)/(519 g/mol oxidized clay) × (mass of oxidized clay)/(mass of pristine clay) × 100%, where HPOSS is the Si-O cage structure with all organic groups oxidized to H+ (H8Si8O12) and oxidized clay is the oxidized organoclay with a molecular weight based on the final weight of Na-MMT at 700 °C.

Figure 10. Proposed bilayer structures of DMIPOSS-MMT. The left diagram illustrates only cation exchange, and the right diagram shows one possible mechanism of DMIPOSS-Cl ion pair inclusion within the clay interlayer.

Figure 11. XRD of DMIPOSS-MMT after thermal oxidation, revealing swelled clay layers due to POSS cage residue remaining in the clay gallery. Plots have been shifted vertically for clarity.

unbound surfactant ion pairs that are “dissolved” between the clay platelets, while THF-prepared organoclays do not. E. POSS Clay Thermal Oxidation Residue. Thermal decomposition studies of POSS macromers have indicated that, while POSS cages generally sublime under inert conditions, there

is a competitive mechanism between the sublimation of the POSS macromer and the oxidation of the organic groups followed by a partial rearrangement of the POSS cages to a more amorphous silica structure.24-26 Our results are in agreement with these previous studies. Figure 11 compares the XRD of DMIPOSSMMT and (5% NB + 95% DMIPOSS)-MMT before and after thermal oxidation (10 °C/min to 700 °C in air). After thermal oxidation, there is still significant spacing between the clay layers (2.16 nm), but the peaks are much broader than the original OMC. This is consistent with the loss of organic groups, the partial rearrangement or decomposition of the POSS cage structure, and the formation of amorphous Si-O-H structures. Unsymmetric POSS structures have been known to lose water upon heating, so the POSS residue could also be due to the condensation product of the original POSS macromer.24 Furthermore, as suggested in the 29Si NMR data, there is a significant amount of POSS residue retained in the clay interlayer, and the clay layers appear to be fused in a separated layer arrangement after thermal oxidation. We have prepared a new surfactant for preparing organically modified montmorillonite based on a POSS macromer and an imidazolium cation. The solubility characteristics of the new surfactant, DMIPOSS-Cl, are dominated by the R-group on the POSS macromer, but the imidazolium cation head group does

7714 Langmuir, Vol. 23, No. 14, 2007

Fox et al.

play a significant role in the solubilities and XRD behavior. The exchange efficiency of the ion-exchange reaction with montmorillonite is dependent on the solvent used in the reaction. At high temperatures, some of the surfactant is lost through sublimation, similarly to other POSS compounds; however, there are large numbers of POSS Si-O residues that remain trapped in the clay interlayer after thermal oxidation. In fact, the clay layer d-spacing remains as high as 2.2 nm after thermal oxidation. The exchanged montmorillonite samples exhibit larger clay interlayer spacing and higher thermal stabilities than previously prepared imidazolium or ammonium exchanged montmorillonite clays. Indeed, the DMIPOSS+ cations appear to form a bilayer structure in the clay interlayer with strong POSS cage crystalline domains. This bilayer arrangement of DMIPOSS is formed even with low surfactant loading levels (