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J. Phys. Chem. C 2009, 113, 21308–21313
From Layered Double Hydroxides to Layered Double Hydroxide-Based NanocompositessA Solid-State NMR Study Anastasia Vyalikh,† Francis Reny Costa,‡,† Udo Wagenknecht,† Gert Heinrich,† Dominique Massiot,§ and Ulrich Scheler*,† Leibniz Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany, and CEMHTI UPR3079 CNRS, UniVersite´ d’Orle´ans, 1D AVenue de la Recherche Scientifique, 45071 Orle´ans Cedex 2, France ReceiVed: July 22, 2009; ReVised Manuscript ReceiVed: NoVember 3, 2009
The local structure of pristine and surfactant-modified aluminum layered double hydroxides (LDH) has been characterized by 27Al and 1H solid-state NMR. Values for the 27Al quadrupole coupling constants and the isotropic chemical shifts obtained from 27Al triple-quantum (3Q)MAS NMR have been applied to fit the one-dimensional 27Al MAS spectra and to characterize the structural changes in the different stages of LDH modification by the regeneration method quantitatively. Six-coordinated (octahedral) aluminum is found in all LDHs studied, indicating that the LDH structure is composed of octahedra formed by six hydroxyl groups. Two octahedral Al sites with different local environments have been distinguished in the 27Al 3QMAS NMR spectra. The calcined and subsequently surfactant-treated LDHs show additionally a considerable fraction of 4-fold coordinated (tetrahedral) aluminum. The quantitative analysis shows that ca. 29% of aluminum has been converted from octahedral to tetrahedral sites in bis(2-ethylhexyl)phosphate (BEHP)-modified LDH, while modification with sodium dodecyl benzenesulfonate (SDBS) leads to formation of only ca. 17% tetrahedral Al sites. The presence of tetrahedral aluminum is attributed to calcination products, which are not converted after rehydration. Differences in the relative contents of AlO4 in surfactant-modified LDH is explained by the presence of interlayer water molecules in SDBS-LDH that is confirmed by 1H MAS NMR. 1. Introduction Incorporation of particulate fillers into the polymer matrix has proven to be an effective way to enhance the overall properties of composite materials.1 Particularly, polymer composites functionalized with nanosized fillers show significant improvement of their properties, even at low filler content due to a very high surface-to-volume ratio of the nanofiller particles resulting in a much larger filler-matrix interface. An example of nanofiller with layered crystalline structure is layered double hydroxide (LDH), which belongs to a class of anionic clay minerals. LDH-based nanocomposites have proven to show reduced flammability at very low filler content.2-5 The flame inhibition during combustion results from increasing melt viscosity of the polymer-LDH nanocomposite and, consequently, from reducing the oxygen supply to the bulk phase under the burning surface.6 However, to maximize the positive effect of nanofiller incorporation on the flame-retardant property of LDH-based nanocomposite while retaining the mechanical properties intact and maintaining the lightweight advantage of polymer materials, a high degree of dispersion of LDH throughout the polymer matrix is required. The structure of LDH is composed of divalent and trivalent cations that are coordinated by six OH groups (hydroxide), forming octahedral sheets in a layered environment. The magnesium- and aluminum-based LDHs are well-known examples of this type of compounds. The interlayer space is * Corresponding author. E-mail:
[email protected]. † Leibniz Institute of Polymer Research Dresden. ‡ Present address: Borealis Polyolefine GmbH, St.-Peter-Str., A-4021 Linz, Austria. § Universite´ d’Orle´ans.
occupied by anions (e.g., carbonate anions) and water molecules. However, the distance between layers is too short (ca. 0.76 nm) to ultimately exfoliate LDH in the resulting polymer nanocomposites. Therefore, prior treatment of the interlayer regions, leading to the expansion of the interlayer distance, on one hand, and improving the compatibility of the LDH materials with organic polymers, on the other hand, is necessary. Different methods to prepare organomodified LDH have been developed. The in situ self-assembly method recently reported is based on introducing the surfactant modifier through a one-step route.7 The regeneration method based on the so-called “memory effect” of LDH, (i.e., ability to recover its original layered structure) involves calcination of LDH followed by dispersion in a surfactant solution.8,9 It has been shown by XRD10 that regeneration of LDH in sodium dodecyl benzenesulfonate (SDBS) solution results in an increase of interlayer distance to 2.96 nm. However, solid-state 27Al NMR analysis carried out in our previous study11 has revealed the essential difference in local LDH structures before and after modification prepared by the regeneration method. Namely, a considerable amount of four-coordinated aluminum was observed in bis(2-ethylhexyl)phosphate (BEHP) modified LDH, whereas unmodified LDH showed the presence of six-coordinated aluminum only, which is an intrinsic structural unit of octahedral LDH sheets. The potential origin of 4-fold aluminum is thought to be the calcination process involved in the regeneration procedure, since high-temperature treatment causes significant changes in chemical composition and the crystal structure,12 which, in principle, might affect the physical properties and rheological behavior of the composite materials. Hence, the detailed knowledge of crystal and chemical structure of LDH and thorough understanding of structural changes occurring at each step during
10.1021/jp9069338 2009 American Chemical Society Published on Web 11/25/2009
Pristine and Modified Al Layered Double Hydroxides regeneration process are crucial from the point of view of materials science. Solid-state 27Al NMR has been widely applied to provide quantitative structural information on crystalline and amorphous aluminum-containing solids,13-16 because the coordination number of aluminum is manifested in characteristic ranges of the 27Al chemical shifts. Additionally, a nonzero 27Al electric quadrupole moment gives rise to quadrupolar interactions, which are very sensitive to the presence of electric field gradients at the nucleus under study. Therefore, the symmetry in aluminum local environment can be characterized by lineshape analysis of a 27Al NMR spectrum. However, in disordered or polycrystalline solids, variations in the local environment of individual sites exist, which lead to the diversity of isotropic chemical shifts and to the distribution of quadrupolar interaction parameters, resulting in a smearing out of the distinct features of the secondorder quadrupolar NMR lineshape. This is referred to as inhomogeneous broadening. These facts along with the heteronuclear dipolar coupling with protons can cause broadening and overlapping of the individual 27Al signals in conventional 27Al MAS NMR. Therefore, to improve the spectral resolution in amorphous materials or other samples with a large distribution of spectral parameters, two-dimensional multiple-quantum (MQMAS) NMR spectroscopy is particularly favored.16-19 This technique allows averaging out of the second-order quadrupolar broadenings, leading to high-resolution projections for halfinteger quadrupolar nuclei and providing information on both quadrupolar interaction and isotropic chemical shift, in twodimensional experiments. MQMAS NMR has been successfully used to resolve sites of the same coordination type in natural and synthetic aluminum hydroxides11,20-22 or to evidence combined distributions of quadrupolar interaction and chemical shift in disordered materials.18,33 The aim of this study is to characterize the local structures of the Al ions in LDH materials being prepared as potential filler for polymer nanocomposites. Therefore, 27Al MAS NMR and triple-quantum 3QMAS NMR have been applied to monitor and characterize quantitatively the structural changes in LDH on each synthesis step during the surfactant modification by the regeneration method. Additionally, we used 1H MAS NMR to examine the role of water molecules in building-up the layered structure. 2. Experimental Section 2.1. Samples. Pristine (unmodified) Mg-Al LDH was synthesized by the method described by Costantino et al.23 Details of the synthesis and characterization by FTIR, TEM, and XRD are given elsewhere.24 Subsequently, LDH was calcined in a muffle furnace at 450 °C for about 3 h in order to convert the material into its oxide form (CLDH). Then a part of the calcined product was dispersed in an aqueous solution of Na2CO3 under ambient condition. The dispersion was stirred continuously for about 24 h and the residue was separated followed by drying at 65 °C until a constant weight was reached. The regeneration of the layered structure was confirmed by XRD analysis and the resulting product was termed regenerated LDH (LDH-R). A part of CLDH was treated with two different anionic surfactants, SDBS and bis(2-ethylhexyl)phosphate (BEHP), according to the procedure described in refs 6 and 10. The products were labeled as SDBS-LDH and BEHP-LDH, respectively. Aqueous solution of each surfactant was prepared for theoretical 100% anion exchange. Finally, the SDBS-LDH was added to low-density polyethylene using maleic anhydride as compatibilizer to prepare LDH-based polymer nanocompos-
J. Phys. Chem. C, Vol. 113, No. 51, 2009 21309
Figure 1. 27Al MAS NMR spectra of the pristine and modified LDH. The strongly magnified amplitudes in the insets show the presence of tetrahedral Al in LDH-R (bottom) as compared to parent LDH (top). On the right-hand side the corresponding quantification diagrams are presented.
ites (LDH-PE). The details of the synthesis are given elsewhere.24 The morphological analysis of the LDH-PE nanocomposite showed that the LDH particles exist in the polymer matrix in different forms, starting from nanoscale-exfoliated fragments to micrometer-sized particle clusters.25 For NMR measurements, six samples of pristine and modified LDH were put into 4- and 3.2-mm o.d. tightly capped zirconium rotors. 2.2. Solid-State NMR Experiments. All NMR measurements were performed on a 11.7 T Bruker Avance 500 spectrometer operating at resonance frequencies of 130.34 MHz for 27Al and 500.13 MHz for 1H. 27Al chemical shifts were referenced externally to 1 M AlCl3 aqueous solution at 0 ppm. One-dimensional single-quantum 27Al MAS NMR spectra were acquired at a spinning frequency of 14 kHz by employing a BL4 HXY 4 mm MAS probehead. A single pulse of 1 µs pulse duration (
