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Langmuir 2007, 23, 9808-9815
Surface Modification of Nanoclays by Catalytically Active Transition Metal Ions Pranav Nawani,† Mikhail Y. Gelfer,† Benjamin S. Hsiao,*,† Anatoly Frenkel,‡ Jeffrey W. Gilman,§ and Syed Khalid⊥ Chemistry Department, Stony Brook UniVersity, Stony Brook, New York 11794-3400, Physics Department, YeshiVa UniVersity, New York, New York 10016, Fire Research DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8665, and National Synchrotron Light Source, BrookhaVen National Laboratory, Upton, New York 11973-5000 ReceiVed March 29, 2007. In Final Form: May 20, 2007 A unique class of nanoclays was prepared by modification of pristine clays or organoclays (Cloisite C20A) with transition metal ions (TMIs). The composition, structure, morphology and thermal properties of TMI-modified nanoclays were investigated by atomic absorption spectroscopy (AAS), elemental analysis (EA), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray absorption near-edge structure (XANES) spectroscopy. The content of TMIs in modified clays was found to be close to the limiting value of ion exchange capacity. SEM and X-ray results confirmed that TMIs were located between the mineral layers instead of being adsorbed on the surface of clay particles. TGA results indicated that the TMI treatment of organoclays could significantly increase the thermal stability, which was more pronounced in air than in nitrogen. Temperature-resolved SAXS measurements revealed that the presence of TMIs increased the onset temperature of structural degradation. The higher thermal stability of TMI-modified organoclays can be attributed to the change in the thermal degradation mechanism, resulting in a decrease in the yield of volatile products and the formation of char facilitated by the presence of catalytically active TMIs.
Introduction The development of new polymeric materials with enhanced thermal stability and fire retardation property has been an active area of research to improve public safety. The common approach to prepare fire retardant (FR) polymers is not to make them noncombustible, but to render them less likely to ignite. If ignited FR-treated polymers would burn, they should burn less efficiently with self-extinguishing ability. Conventional FR additives such as halogenated organic compounds, organophosphorous compounds, aluminum, and magnesium hydroxides usually have an adverse effect on the mechanical properties of treated polymers. This is because these additives must be used at high loads in order to be effective, which can significantly degrade the mechanical properties. In addition, many of the conventional FR additives raise toxicity1-3 concerns. Thus, it is highly desirable to improve both flame retardation and mechanical properties simultaneously, that is, to design mechanical-reinforcing fillers that can also act as nontoxic FR agents. The family of composites consisting of organically modified layered silicates (organoclays) and polymers may offer such an opportunity. Polymer-organoclay systems (so-called organoclay nanocomposites) have attracted a great deal of attention from both academic and industrial communities.4,5 With organoclays, * To whom correspondence should be addressed. Telephone: 631-6327793. Fax: 631-632-6518. E-mail:
[email protected]. † Stony Brook University. ‡ Yeshiva University. § National Institute of Standards and Technology. ⊥ Brookhaven National Laboratory. (1) Lomakin, S. M.; Zaikov, G. E. Ecological Aspects of Flame Retardancy; VSP International Science Publishers: Zeist, Netherlands, 1999; p 170. (2) Halpern, Y.; Mott, D. M.; Niswander, R. H. Ind. Eng. Chem. Prod. Res. DeV. 1984, 23, 233-238. (3) Rose, S. A new mineral flame retardant with increased thermal stability. Nabaltec GMBH, p 1-10.
improvement of the FR property5 of polymers can be achieved at relatively low filler loads (w < 10 wt %) when compared to the higher filler loads (w > 40%) needed for conventional FR additives, and these nanocomposites can exhibit a higher modulus than neat polymers without sacrificing the toughness.5 However, for these nanocomposites, a drastic reduction in flammability would require substantial filler loading exceeding 10 wt %, which may not be economically viable. Very often, even the most efficient organoclays are not sufficiently active to replace conventional FR additives.6 The major reason for the insufficient activity of organoclays is their low thermal stability, related to the degradation of organic surfactants in organoclays. Usually, above 170 °C, the organic components in organoclays (e.g., quaternary amine surfactant) start to decompose via the Hoffman elimination process,7 releasing combustible alkenes. At the present time, the mechanism of FR activity in organoclays is not completely understood. It has been suggested that the FR mechanism is mostly of a physical nature. Indeed, improved FR properties observed for a variety of polymer-organoclay nanocomposites can be attributed to the presence of nonflammable inorganic particles, higher melt viscosity, reduced gas permeability, and increased char formation, which minimize the flow of burning polymer melt and suppress combustion via reduced fuel feed rates.8-10 The combustion residue, formed during the burning of clay-filled nanocomposites, is comprised of multi(4) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. AdV. Polym. Sci. 1999, 138, 107-147. (5) Alexandre, M.; Dubois, P. Mater. Sci. Eng., R 2000, 28, 1-63. (6) Gilman, J. W.; Jackson, C. L.; Morgan, A. B.; Harris, R.; Manias, E.; Giannelis, E. P.; Wuthenow, M.; Hilton, D.; Phillips, S. H. Chem. Mater. 2000, 12, 1866-1873. (7) Xie, W.; Gao, Z. M.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Chem. Mater. 2001, 13, 2979-2990. (8) Balogh, M.; Laszlo, P. Organic Chemistry Using Clays; Springer: New York, 1993. (9) Gilman, J. W.; Kashiwagi, T.; Lomakin, S.; Giannelis, E.; Manias, E.; Lichtenhan, J.; Jones, P. R. Soc. Chem. (Cambridge) 1998, 203-221.
10.1021/la700908m CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
Surface Modification of Nanoclays
layered carbonaceous-silicate structures.7 The buildup of carbonaceous-silicate chars during burning can result in the retention of fuel and reduced gas permeability. In addition, these layered structures can act as a thermal insulator. The silicate inclusion reinforces the residue, preventing it from breaking apart. In a way, the char enriched by silicate platelets forms a protective barrier to retard the spreading of fire. Thus, we hypothesize that the FR activity of organoclays can be improved by increasing their thermal stability as well as their capability of forming hightemperature residues (i.e., char). The ability to promote charring is particularly desirable for an efficient FR additive.9 It has been suggested that the small amount of catalytically active transition metal ions (TMIs) in naturally occurring clays may play a significant role in the charring process of polymerorganoclay nanocomposites. The presence of TMI salts has been shown to improve the FR properties of polymeric materials,11 although the role of TMIs in the polymer combustion and degradation processes has not been fully understood. The poor compatibility between TMI compounds and a polymer matrix is a major concern in the application of TMIs as FR agents for polymers.12 Thus, we suggest that organoclays, which are more compatible with a broad range of polymers, can serve as a conduit for the introduction of TMIs into polymers. Recently, we demonstrated that the incorporation of catalytically active TMIs into ethylene-vinyl acetate copolymers can generate a new class of nanocomposites with improved FR properties.13 The purpose of this study is to understand the relationship between the structure and properties (especially the thermal stability) of organoclays and pristine clays modified with various catalytically active TMIs on the layer surface of each silicate. In this study, we demonstrated that the use of ion exchange capability in organoclays is an effective means for modifications of organoclays with catalytically active TMI. The modification was also applied to pristine clays, which were used as the reference materials. The resulting TMI-modified organoclays/clays were carefully characterized by atomic absorption spectroscopy (AAS), X-ray absorption near-edge structure (XANES) spectroscopy, elemental analysis (EA), scanning electron microscopy (SEM), simultaneous small-angle X-ray scattering (SAXS) and wideangle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. Experimental results confirmed that TMIs can be incorporated within the layer silicate stacks, and the content of TMIs in modified clays is close to the limiting value determined by the ion exchange capacity. The improved FR activity of TMImodified organoclays in the application of nanocomposites has been described earlier.13 Experimental Materials and Preparation. The TMI modification schemes were carried out in two different clay systems: pristine clay (i.e., montmorillonite or Cloisite Na+ from Aldrich Chemicals), which was used as a reference material, and a commercially available organoclay (Cloisite C20A from the Southern Clay Company). The C20A organoclay contained the Wyoming Cloisite (also montmorillonite) and dimethyl dihydrogenated tallow quaternary ammonium chloride (DMDTA) surfactant. The content of the surfactant in C20A determined by TGA was 35 wt %. DMDTA is a blend of surfactants prepared from natural products by Akzo Nobel. The major component (10) Le Baron, P. C.; Wang, Z.; Pinnavaia, T. J. Appl. Clay Sci. 1999, 15, 11-29. (11) Weil, E. D.; Patel, N. G. Polym. Degrad. Stab. 2003, 82, 291-296. (12) Lewin, M.; Endo, M. Polym. AdV. Technol. 2003, 14, 3-11. (13) Nawani, P.; Desai, P.; Lundwall, M.; Gelfer, M. Y.; Hsiao, B. S.; Rafailovich, M.; Frenkel, A.; Tsou, A. H.; Gilman, J. W.; Khalid, S. Polymer 2007, 48 (3), 827-840.
Langmuir, Vol. 23, No. 19, 2007 9809 Table 1. TMI Salts and Solvents Used in the Modification Procedures type of clay
solvent used
TMI salt used
concentration [M]
montmorillonite
water methanol ethanol dioxane
pristine clay copper (II) sulfate copper (II) chloride copper (II) chloride copper (II) nitrate
0.3 0.3 0.3 0.3
Cloisite C20A
methanol ethanol dioxane
organoclay Ni, Cu, Co, and Fe chloride copper (II) chloride copper (II) nitrate
0.3 0.3 0.3
in this blend is dimethyl dioctadecylammonium chloride; minor components include (in the order of decreasing content) dimethyl octadecylhexadecylammonium chloride, dimethyl dihexadecylammonium chloride, and a small (
