Surface Modification of Sepiolite in Aqueous Gels by Using

Feb 28, 2011 - Tolsa S.A. R&D Department, Ctra Vallecas-Mejorada del Campo, 28031 Madrid, Spain. bS Supporting Information. 'INTRODUCTION. Inorganic ...
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Surface Modification of Sepiolite in Aqueous Gels by Using Methoxysilanes and Its Impact on the Nanofiber Dispersion Ability Nuria García,† Julio Guzman,† Esperanza Benito,† Antonio Esteban-Cubillo,‡ Eduardo Aguilar,‡ Julio Santaren,‡ and Pilar Tiemblo*,† † ‡

Instituto de Ciencia y Tecnología de Polímeros, C.S.I.C., Juan de la Cierva 3, 28006 Madrid, Spain Tolsa S.A. R&D Department, Ctra Vallecas-Mejorada del Campo, 28031 Madrid, Spain

bS Supporting Information ABSTRACT: Surface modification reactions on needle-like sepiolite using alkyl and functional silanes have been carried out in the form of aqueous gels. In contrast with modifications in organic solvents, reactions in water make it possible to modify the surface of almost-individual sepiolite fibers and produce either a continuous coating or a nanotexturization of the sepiolite fiber surface, depending on the reaction conditions. This clean procedure substitutes advantageously organic solvent surface modifications and allows the tuning of surface properties such as specific surface area, wetting behavior, and chemical functionalization. A consequence of such tuning is, for example, the excellent dispersion of modified sepiolite nanofibers in a great variety of polymers by routine compounding and processing techniques.

’ INTRODUCTION Inorganic particles with nanometric dimensions manifest surface properties which are very different from their micrometric analogues. This circumstance has many implications being one of them the formation of aggregates which may be extremely difficult to break down. However, to take full advantage of the nanoparticle properties, aggregates must be divided into the smallest possible entities. Sepiolite is a naturally occurring fibrous phyllosilicate which exists in the form of micrometric aggregates with a typical length of 2-10 μm.1 The elemental sepiolite fiber is about 10-12 nm thick,2 and therefore if disaggregated, its aspect ratio is large, about 200-300. Because of its natural abundance, the dimensions of the isolated fiber, and the silanol-based chemistry of the surface, sepiolite is an excellent candidate for many technological and nanotechnological applications such as photocatalysis,3 heavy metal adsorption,4,5 metallic nanoparticle support with biocide6 or plasmonic properties,7 vaccine support,8 or electrical conductors9 not to mention its use as reinforcement10 or thermal stabilizer11 agent in polymer-based nanocomposites. Most of these applications would profit of the availability of isolated or individualized sepiolite fibers, a material unreported for the moment. By physical means it is possible to reduce the size of the aggregates from the macrodimensions with which it occurs in nature to dimensions in the micrometer scale, but not any further. Attempts to reduce even more the aggregation state require the chemical modification of the particle surface. This panorama is common to almost all inorganic nanoparticles profusely forming part of composite and nanocomposite materials. In the case of sepiolite, a number of surface chemical r 2011 American Chemical Society

modification procedures have been published. After pioneering work by Ruiz-Hitzky on the organic modification of sepiolite,12 most of the work13 has been done in either modifications with quaternary ammonium salts14 or with organosilanes, most frequently in toluene4,5,15-17 but also in a mixture isopropanol/ HCl,18,19 in water,20 or by direct impregnation.21 Silanes, either halo or alkoxy, are a good choice for modifying the surface of sepiolite, as it possesses accessible silanol groups both at the corners and along the edges of the fibers. In addition, the covalent bonding formed between the silane and the sepiolite surface is very stable. The incorporation of silanes is in fact one of the most popular surface modification reactions for silica, which also has surface silanols. However, silane chemistry faces, for large-scale production, the drawback of being performed in organic solvents, very frequently, toluene. Additionally, a very strong limitation of all the silane modification processes, impregnation or suspension in organic solvents, is that the surface modification is not produced on the surface of individualized sepiolite fibers but on the external surface of the sepiolite aggregates, since highly hydrophilic sepiolite is not finely dispersed in these solvents. As a consequence, the distribution of the modification in these conditions is strongly heterogeneous. Both limitations, the use of organic solvents and uneven surface modification of aggregates, can be eliminated by a simple and effective experimental procedure based on the reaction of the sepiolite in the form of an aqueous gel, where almost-isolated sepiolite fibers are stabilized by the formation of the gel network. Any chemical modification produced in the gel form occurs thus Received: November 8, 2010 Revised: February 1, 2011 Published: February 28, 2011 3952

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Figure 1. Sepiolite fibers in an aqueous gel (4 wt % solid content) as seen by SEM.

on the surface of individualized fibers, and upon drying their reaggregation may become hindered. In this work, it will be shown that when employing trimethoxysilanes as reagents, this chemical modification in aqueous gel produces either a nanotexturization of the fiber surface or a conventional coating/ grafting surface modification. The former or the latter will be obtained depending on the chemical structure of the trimethoxysilane used and its concentration in the reaction medium.

’ EXPERIMENTAL PART Materials. Sepiolite (Pangel S9), provided by TOLSA, S.A (Spain), is a rheological grade sepiolite product obtained using a wet micronization process22 that produces a disagglomeration of the sepiolite fiber bundles. This product is more easily dispersed in water using mechanical mixing than untreated sepiolite. Trimethoxysilanes: methyl (MTMS, Aldrich, 98%), vinyl (VTMS, Aldrich, 97%), 3-aminopropyl (APTS, Aldrich, 97%), and 3-glycidyloxypropyl (GLYTMS, Aldrich 97%); the polymers and resins: low density polyethylene (LDPE, Alcudia 003 from Repsol), polystyrene (PS, 143E from BASF), polypropylene (PP, Moplen HP501 L from LyondellBasell), poly(methyl methacrylate) (PMMA, Mw = 120 kDa from Aldrich), epoxy system (EPO, Araldite LY556 and hardener XB 3473 from Huntsman), and polyester resin (PST, Crystic 406 NT from Scott Bader) and the photoinitiator: 2, 2-dimethoxy-2-phenylacetophenone (DMPA, Aldrich, 99%) were all used as received. Surface Modification Reactions. Reactions in water were carried out by dispersing 3 g of raw sepiolite in a mixture of 72 mL of water (4 wt % of solids) and the desired amount of alkoxysilane (or mixture of alkoxysilanes) by means of a lab dissolver (Dispermat LC2) operating at 12 000 rpm for 20 min. Unlike smectite clays, sepiolite does not swell and exfoliate spontaneously in water. Sepiolite fiber dispersion requires using mechanical mixing at high shear rate for a relatively long time and at relatively high sepiolite concentrations as the higher density of fiber bundles produces an internal shear and friction that drags the individual particles from the bundles and produces an extensive disentanglement of the fibers. Under these conditions the suspension of sepiolite in water gives rise to a nonfluid, stable-in-time aqueous gel, characterized by the formation of a 3D network of sepiolite fibers.23 The SEM micrographs in Figure 1 and Figure S1 in the Supporting Information show the structure of this gel where the fibers appear disaggregated as almost individualized units. It is worth mentioning that the formation of this suprastructure is conditioned by the shear applied and the solid content: high diluted suspensions and/or low shear do not lead to the gel formation. In the case of two steps reactions in water, the second reactive was afterward added to the mixture, and the mixing protocol was repeated. Then, all the products were cured in an oven at 100 °C overnight. The resulting sepiolites were purified by several cycles of washing in dichloromethane and, finally, dried at 100 °C for 12 h.

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Sepiolite reactions in toluene were performed following a previously reported protocol for silica nanoparticles.24,25 Contrary to what happens in water, sepiolite aggregates are not broken down in toluene, where a nonstable-in-time suspension is formed. Characterization. Solid State NMR. The NMR spectra of the organically modified sepiolite fibers were performed in a Bruker Avance 400 spectrometer (Bruker Analytik GmbH Karlsrube, Germany) equipped with a Bruker Ultrashield 9.4 T (13C and 29Si frequencies of 100.62 and 79.49 MHz, respectively), 8.9 cm vertical-bore superconducting magnet. The spectra were recorded following the previously reported experimental protocol.26 Elemental Analysis. The carbon and nitrogen contents in the organically modified sepiolite fibers were obtained by a LECO CHNS-932 equipment. Specific Surface Area. BET surface area measurements were made by single-point nitrogen adsorption, using a Micromeritics Flowsorb 2300. Microscopic Analysis. The morphology of neat and modified sepiolite fibers and their dispersion in different polymeric matrixes were evaluated by scanning electron (SEM, Philips XL30ESEM and Hitachi SU-8000) and transmission electron microscopies (TEM, Phillips TECNAI 20). Nanocomposites Preparation. Sepiolite fibers modified with several concentrations of MTMS, VTMS, APTS, and GLYTMS have been used to prepare nanocomposites according to the procedures described below. Melt Compounding. PP, LDPE, and PS composites were prepared in a Haake MiniLab extruder. 5.23 g of polymer pellets and 0.28 g of modified sepiolite fibers were directly introduced into the machine. The processing temperature, shear rate, and residence time for PP-based materials were 190 °C, 80 rpm, and 20 min, respectively, whereas for LDPE these experimental parameters were 140 °C, 80 rpm, and 20 min, and for PS were 160 °C, 120 rpm, and 20 min. In this latter case, the compounding was subjected to a second 10 min extrusion step at the same temperature and shear rate. Casting. PMMA and modified sepiolite (5 wt %) composites were prepared by a casting procedure previously reported.27 Thermosets Curing. Modified sepiolite fibers (5 wt % in the final material) were dispersed in the epoxy resin and in the polyester resin by means of a lab dissolver (Dispermat LC2) operating at 12 000 rpm for 20 min. In the case of the epoxy system, the suitable amount of hardener was then added, and the whole mixture was stirred for an additional 5 min. A temperature curing procedure consisting of 2 h at 80 °C and 8 h at 140 °C was used to obtain the final material. DMPA (0.5 wt %) was added to the polyester-sepiolite mixture, and the whole system was photocured in an UV cross-linker (UPV, CL-1000-L) for 10 min. A 1 h postcuring step at 100 °C was finally carried out.

’ RESULTS AND DISCUSSION The modification of sepiolite fibers in the form of an aqueous gel has been performed at different reagent concentrations (ranging from 0.9 to 24 mmol of reagent per gram of raw sepiolite) with MTMS and with functional silanes such as VTMS, APTMS, or GLYTMS. The dispersion of 4 wt % of sepiolite in water under such conditions efficiently breaks down the silicate aggregation, separating fibers or small fiber bundles from one another and forming a tixotropic gel23 as explained in the Experimental Part. Almost-isolated sepiolite fibers are stabilized by the formation of this aqueous gel network, making it possible that chemical modifications produced in this form occur on the surface of individualized fibers. For the sake of comparison, modifications reactions have also been conducted in toluene following a previously reported protocol24,25 and using MTMS and VTMS as reagents at a concentration of 5.3 mmol per gram of sepiolite. As before 3953

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Langmuir mentioned, toluene is not able to break down the sepiolite aggregates into small bundles or isolated fibers; additionally, only traces of water from sepiolite are present in the toluene suspensions, and hence hydrolysis and condensation of the silanes are much slower and less complete.28 Obviously, the product of water modification and toluene modification must be very different. In what follows, the reaction products are nominated with the silane used, its concentration per gram of raw sepiolite, and the reaction medium (“w” or “t” stands for water or toluene, respectively). TEM micrographs of sample MTMS-3-w, which corresponds to sepiolite fibers after aqueous reaction with MTMS at a concentration of 3 mmol of reagent per gram of raw sepiolite, are collected in Figure 2 (additional TEM images are included in Figure S2 of the Supporting Information). Under these reaction conditions, the surface of the fibers becomes covered with condensed MTMS discrete spheres of about 10 nm diameter. The condensation of MTMS forming spherical clusters, which shape and size depend on the experimental conditions, has been already reported in the synthesis of silica aerogels in a medium of

Figure 2. TEM images of sepiolite fibers after aqueous reaction with MTMS (sample name: MTMS-3-w).

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water/methanol and ammonia as catalyst.29 In our system, the distribution of the spheres on the fiber surface not being strictly periodic is anyway surprisingly regular. This type of surface modification will be named nanotexturization hereafter, for it may not only introduce chemical functionalities on the sepiolite surface but also produces a specific surface texture. It is easy to imagine that this nanotexturization reduces strongly the contact surface between fibers, making reaggregation difficult upon drying. As will be shown along this section, the nanotexturization of the surface occurs only under very specific reaction conditions, as it depends strongly on the reagent used and its concentration in the reaction medium. The yield of the modification reactions performed with the different silanes used is shown in Figure 3a as a function of the silane concentration. The silane molar concentration was determined from the modified sepiolite carbon content obtained from elemental analysis, assuming that total hydrolysis of methoxy groups occurs and therefore that the measured carbon arises solely from the silane substituent. This is true for the organosepiolites obtained from aqueous reactions in which no signal assigned to methoxy groups was detected in 13C CP/MAS solid state NMR spectra.26 The lower hydrolysis rate of alkoxysilanes in toluene compared to aqueous medium makes that the sepiolite obtained from reactions in toluene presented a significant contribution of methoxy groups, which results in a slight overestimation of the reaction yield presented in Figure 3a. In any case, the reaction yield was roughly over 80% regardless of the reaction conditions. This silane loading must be covalently attached, i.e., grafted onto the sepiolite surface since the reaction products were thoroughly washed before analysis. The specific surface of the products as a function of the molar silane load is depicted in Figure 3b. The BET value is reflecting the sepiolite surface free of organic coating which remains in each sample. Contrary to what happens with the reaction yield, the BET specific surface strongly depends on the silane type and the reaction medium. Per mole of silane incorporated, VTMS in water is more efficient in reducing the specific surface than MTMS, though the most efficient of the reagents in water is APTMS. The products obtained from reactions in toluene have, on their turn, less specific surface than analogue water-modified ones. These results directly affect the wetting behavior of the

Figure 3. (a) Reaction yield expressed as mmol of silane grafted as a function of mmol of reagent per gram of raw sepiolite. (b) Specific surface of modified sepiolite as a function of the silane load (lines are merely guides for the eyes). 3954

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Figure 4. (a) 29Si CP/MAS solid state NMR spectra of sepiolites modified with MTMS in water and toluene, VTMS in water, and APTMS in water at similar molar concentration. The figure includes chemical structures of the T region signals (R stands for the silane chain). (b) TEM images of the corresponding sepiolites, scale bars being 200 nm long. (c) High-resolution TEM images of the same sepiolites, scale bars being 100 nm long.

modified sepiolite as shown in Figure S3, where a picture of vials with water and three different reaction products is included. MTMS-5.3-w forms a fairly stable dispersion in water in clear contrast with MTMS-5.3-t, which is highly hydrophobic (sepiolite stays as a layer at the water-air interface), whereas MTMS-24-w, a sepiolite obtained from an aqueous reaction, also exhibits a hydrophobic behavior. This suggests that a reduction of BET surface below that corresponding to sepiolite interior channels (roughly 200 m2 g-1)30 produces hydrophobic products whenever MTMS or VTMS are used as reagents. (The hydrophilic character of APTMS precludes the preparation of hydrophobic materials from this reagent.) It is noteworthy that MTMS-5.3-w not only forms stable suspensions in water but also in nonpolar organic solvents, such as toluene. This is remarkable, as neat sepiolite does not form stable suspensions in toluene, as was mentioned before. In fact, as will be shown in a forthcoming section, samples with the surface characteristics of MTMS-5.3-w are composed of highly disaggregated nanofibers which are

therefore easy to disperse in a large variety of media, solvents, and polymers. These surface properties must be related to the structure of the organic coverage on the surface. Figure 4 collects TEM images and 29Si CP/MAS solid-state NMR spectra of sepiolites modified with MTMS in water and toluene, VTMS in water, and APTMS in water at similar molar concentration. These images illustrate what happens to the nanotexturization when the reaction conditions are changed from those used in sample MTMS-3-w shown in Figure 2. First, both MTMS and VTMS reagents in aqueous reactions condense in sphere-like structures along the sepiolite fibers, whereas this is undetected when using APTMS. Second, when the reaction is carried out in toluene (MTMS-5.3-t), no nanotexturization of the fiber is seen; condensed MTMS forms clusters which appear mixed with the sepiolite fibers. Third, if instead of MTMS VTMS is used, then the spheres appear more deformed and heterogeneous in size. Fourth, increasing the concentration of MTMS in MTMS-5.3-w produces a structure similar to that of Figure 2, 3955

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Langmuir but where the neat regular spheres observed in MTMS-3-w are being substituted by larger, deformed ones which occasionally come into touch forming a continuous coating. It is worth mentioning that at low concentration of silane reagent, such as in MTMS-0.9-w sample (not shown in the figure), neither spheres nor any other type of coating are visible in TEM images. Additional information about the surface modification is obtained from the 29Si CP/MAS NMR spectra in Figure 4a. These spectra indicate that the intensity ratio T3 to T2 signals (see schematic structures in Figure 4a)24-26 is larger for sepiolites modified in water than for those modified in toluene. Hence, condensation is higher and hydrolysis more complete (no detection of methoxy groups in 13C CP/MAS NMR spectra) in the water-modified samples, which is an expected result.28 In light of the information provided by TEM images and 29Si spectra, it is worthwhile looking back at the BET results in Figure 3b. A sphere is the less efficient form of coating, its surface to volume ratio is the smallest, and hence reagents which organize themselves into spheres are the least efficient in reducing the specific surface area. According both to the TEM images and the BET results, the sphere-forming ability in aqueous solution decreases in the following sense: MTMS > VTMS . APTMS. Discrete spheres grafted onto the surface, such as those formed in samples MTMS-3-w or MTMS-5.3-w, leave the specific surface almost unaffected. When the size and shape of the spheres make them to coalesce and a pseudo-continuous coating is formed, the specific surface is diminished (see BET values for VTMS-X-w samples). APTMS-modified sepiolite shows such a low BET surface that forcedly the reagent must be forming a condensed network (as deduced by the T3/T2 ratio) grafted onto the sepiolite surface which blocks access to sepiolite channels, this grafted coating being undetectable by TEM as it does not produce nanometric spheres. A similar channel-blocking must be occurring in the highly loaded hydrophobic MTMS-24-w as a result of the coalescence of large, deformed condensed MTMS spheres on the sepiolite surface. The BET results and TEM images of the sepiolite modified in toluene are not as straightforwardly understood as those in water. On their own, the silica clusters seen in Figure 4c for MTMS-5.3-t cannot explain the sharp decrease in BET surface or the structure and intensity of the signals in the 29Si spectrum; therefore, the existence of a grafted low-condensed MTMS coating not detected by TEM imaging, in addition to the clusters, seems to be plausible. This coating would be located along the fiber surface and inside or blocking the sepiolite channels, as proposed for APTMS-modified sepiolite. As mentioned before, the 29Si spectra in Figure 4a add information on the structure of the modified sepiolites. The four products in Figure 4a have similar silane loading; however, the intensity ratio of the inorganic sepiolite signals (Q region) to the organic silane signals (T region) is higher in MTMS-5.3-t and APTMS-4.5-w. This is also in agreement with the occurrence of a surface grafted coating in these two samples: the efficiency of the cross-polarization (CP) pulse is higher the closer the silane protons are to the aprotic silicate silicon; therefore, a continuous surface coating will produce a more intense Q signal. Another indication of the presence of a continuous coating grafted to the surface is the reduction in the intensity of the -85 ppm Q signal, assigned to reactive surface silanols.31 This signal is absent in APTMS-4.5-w and significantly reduced in MTMS-5.3-t in comparison with MTMS modified sepiolite in water (shown in Figure S4 of the Supporting Information where a comparative

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series of 29Si CP/MAS NMR spectra are included). On the other hand, it must be also stressed that the Q spectral region in MTMS-5.3-t presents two single signals in comparison with the three Q signals observed in the rest of the samples. The merging of two of the Q signals has been observed in dehydrated sepiolite,31 where zeolitic water has been removed by thermal treatments similar to those used to dry the samples in this work. This process is, however, reversible in a few days. The hydrophobic character of MTMS-5.3-t makes the rehydration process be slower than in the rest of the samples, and hence only two signals in the Q region appear for several weeks. It has been shown that by controlling the concentration and type of reagent it is possible to produce very different surface structures. The spherical structures shown in Figures 2 and 4 are likely formed on the fibers, as if formed in the medium such small nanospheres would tend to aggregate and not to deposit regularly on the fiber surface. Sepiolite has surface silanols which are acidic sites that could act as nucleating points onto which organosilanes can favorably hydrolyze and condense during the reaction time and the thermal curing step. That a discontinuous nanotexturization in the form of spheres or a continuous coating develops depends on several factors. First, not all trimethoxysilanes are equally able of producing sphere-like structures, as this requires a certain degree of symmetry around silicon in the silane: the larger the organic substitution of the silane, the stronger the steric hindrance and the more difficult the formation of spherelike structures. The size and shape regularity of MTMS spheres compared to less homogeneous VTMS ones can be explained from this reason and also from the fact that solubility of MTMS in water is higher. Second, the results suggest that spheres are more likely formed when silane condensation in the medium is kept under a limit, for the higher the concentration or reactivity of the silane, the larger the amount of silane forming part of a nonspherical 3D network. This is the case of APTMS which produces no spheres and is the silane which appears more strongly condensed and which has reacted more extensively with the surface silanols (disappearance of -85 ppm peak). Besides the consequences in surface properties and wetting behavior detailed so far, the different surface structures shown in Figures 2 and 4 have also implications in the macroscopic and microscopic morphology of these modified sepiolites. This is illustrated in Figure 5, where SEM images at the microscale of several modified sepiolites are included. At the microscale, it is impossible to mistake a modified sepiolite for the raw fibers. Sepiolites modified in water with moderate concentrations of VTMS and MTMS (Figure 5, c and e, respectively) look much fluffier than raw sepiolite (Figure 5a) or sepiolite modified in toluene (Figure 5b); differences between VTMS-sepiolite and MTMS-sepiolite in micrographs at the microscale are subtle. Differences between APTMS-4.5-w (Figure 5f) and MTMS-5.3-w (Figure 5c) are, on the contrary, very clear: the former shows fibers which appear agglomerated and seem to be much thicker than those of VTMS-5.3-w or MTMS-5.3-w. Something similar happens when the concentration of MTMS in the reaction medium is increased (MTMS-24-w, Figure 5d); the sepiolite becomes more agglomerated and the fibers thicker. Bearing in mind the SEM image in Figure 1 and TEM images in Figure 2, depicting the surface structure at the nanoscale, the immediate conclusion is that the thickening of the individual fibers in APTMS-4.5-w or in MTMS-24-w is caused by the chemical bridging of individual sepiolite fiber by the organic 3956

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Figure 5. SEM images at low magnification scales of different products obtained after surface modification reaction of sepiolite fibers: (a) raw sepiolite, (b) MTMS-5.3-t, (c) MTMS-5.3-w, (d) MTMS-24-w, (e) VTMS-5.3-w, and (f) APTMS-4.5-w.

Figure 6. TEM images of PP nanocomposites prepared with (a) 5 wt % MTMS-5.3-w and (b) 5 wt % MTMS-5.3-t.

coating around them. This bridging produces organosepiolite agglomerates. If the bridging produced in those organosepiolites is chemical, then those agglomerates should be almost impossible to break down by physical means. Therefore, though the nanotexturization produced by moderate concentrations of MTMS is the least efficient to reduce the specific surface (less surface coverage), it should be the most efficient to act as a physical damp between sepiolite fibers, without bridging them. Polymer Nanocomposites. As touchstone of the hypothesis put forward so far, and because it is one of our research aims, we have prepared composites with all of the modified sepiolites in a large set of polymer matrices, including PP and LDPE, where dispersing nanoparticles is especially hard because of its low

polarity. Among the organosepiolites prepared, only moderate concentration MTMS- and VTMS-water modified sepiolites do produce nanocomposites with high quality dispersion, where isolated sepiolite fibers or small fiber bundles are the prevailing particle entities. As an example, Figure 6 shows representative TEM images of PP nanocomposites prepared with 7 wt % MTMS-5.3-w (a) and 7 wt % MTMS-5.3-t (b); while in the former sepiolite appears as well dispersed needle-like entities, in the latter large aggregates as a typical feature coexist with some isolated fibers. As a matter of fact, Figure 6b is very similar to the few sepiolite nanocomposite TEM images which can be found in the literature. APTMS, GLYTMS, and large concentrations of MTMS or VTMS produce microcomposites, where, in agreement with the 3957

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Figure 7. SEM images of 5 wt % MTMS-5.3-w dispersed in (a) a PST resin, (b) LDPE, and (c) PS. (d) Picture showing the appearance of films prepared from materials shown in (a), (b), (c), and 5 wt % MTMS-5.3-w dispersed in PMMA and in PP (the thickness of each film is included in the image). (e) High resolution SEM images of 5 wt % MTMS-3-VTMS-1-w dispersed in an epoxy resin. These last images were obtained without sputtering a coating of conductive materials. The inset shows a TEM image of this sepiolite modified with a mixture of silanes.

hypothesis above, organosepiolite agglomerates (formed by the organic bridging of sepiolite fibers) of micrometric size are the prevailing entities. The best nanocomposites have been obtained with MTMS-5.3-w, followed by MTMS-3-w. Apart from that shown in Figure 6a, excellent dispersions of MTMS-5.3-w in several polymer matrixes are also obtained processing by conventional methods, as is shown in Figure 7ad. The included polymer matrixes are thermoplastics processed by casting (PMMA) and extrusion (LDPE and PS) and cured thermosets (PST and EPO resins), i.e., most of the conventional/industrial processing means. The processing of the modified sepiolite with the polymer has been done following conventional procedures without using masterbatches or coupling agents. Figure 7a-c shows the optimum dispersion of MTMS-5.3-w (the sepiolite fibers are seen as white spots in SEM micrographs) in some of these polymers; additionally fibers are not broken, as also observed in Figure 6a, and preserve the aspect ratio typical of the raw sepiolite because processing conditions can be chosen to be mild (no use of masterbatches and thus no need for reprocessing or using high shears). Isolated fibers or small fiber bundles are the typical inorganic entities in all of these nanocomposites; no medium-large micrometric aggregates are seen. The optimal dispersion and homogeneity of these materials can be also observed at the macroscale as shown in the picture in Figure 7d, where five different films of relative high thickness prepared from these

materials show high transparency (an important property for many of the nanocomposite applications). It has been shown in this work that functional silanes such as APTMS or GLYTMS do not produce by themselves the nanotexturization of the fiber surface but the bridging between fibers, and thus their use as functional surface modifiers precludes adequate fiber disaggregation. If such reagents are to be incorporated on a sepiolite fiber to functionalize its surface, then a different procedure has to be followed, as will be detailed below. Assuming that the damping effect of the condensed MTMS spheres deposited onto the fiber surface is mandatory to avoid reaggregation and that the functional trimethoxysilane concentration has to be kept low to avoid bridging and agglomeration, both reagents have been added either in a two-stage reaction (first MTMS, second the functional reagent) or together in a one-stage reaction. For both approaches to succeed, the reagent ratio chosen is critical: enough MTMS concentration has to be added, so as to produce nanotexturization, and a low concentration of functional reagent has to be used to prevent bridging from occurring. To visualize the structure of the coating on the sepiolite surface, high resolution SEM has been obtained for some selected samples. This is shown in Figure 7e, which corresponds to a 5 wt % of a sepiolite modified in aqueous medium with a mixture of MTMS:VTMS (3:1 mmol per gram of raw sepiolite), namely MTMS-3-VTMS-1-w, dispersed in an epoxy resin. The 3958

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Langmuir high resolution SEM images could be obtained without sputtering the sample with a conductive material layer. Two features are noteworthy in these images: on one hand, the images show the excellent dispersion of the MTMS-3-VTMS-1-w, comparable to that obtained with MTMS-5.3-w, and on the other hand, the resolution of the images and the lack of the additional conductive layer enable the visualization of the condensed spheres on the sepiolite fibers. The nanotexturization of these fibers can be better observed by TEM imaging as shown in the inset of Figure 7e. On the basis of the results presented so far, the nanotexturixation of the surface of MTMS-3-VTMS-1-w must be formed by combined spheres of MTMS and the functional VTMS.

’ CONCLUSIONS In this work it has been shown that it is possible to modify almost-individual sepiolite fibers by carrying out surface modification reactions in the form of aqueous gels, what is not possible by using organic solvents that are unable to separate the sepiolite aggregates into individual fibers. Water as a solvent of the reaction not only can substitute advantageously other organic solvents but also allows a fine-tuning of the surface structure. Changing the reaction conditions as regards reagent type and concentration allows controlling the surface nanotexture, chemical composition, specific surface area, and wetting behavior of the individualized sepiolite nanofibers. For instance, the surface of sepiolite fibers modified in aqueous gels with moderate concentrations of MTMS becomes covered with nanometric MTMS condensed spheres, which reduce surface contact between fibers and preclude strong reaggregation of the silicate. This particular type of modified sepiolite has proved to be easily dispersed in an ample set of polymers by routine polymer compounding and processing techniques. It has also been shown that the introduction of functional groups like amino or glycidyl requires the nanotexturization of the nanofibers by condensation of MTMS nanospheres, as the sole functionalization with such groups produced the bridging between fibers and hence large organosepiolite agglomerates which are impossible to disaggregate a posteriori. ’ ASSOCIATED CONTENT

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Supporting Information. Figures S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected], Tel þ34 91 562 29 00, Fax þ34 91 564 48 53.

’ ACKNOWLEDGMENT Financial support from DOMINO (CENIT-2007-1001) Spanish project is acknowledged. The authors thank Carolina Belver from Instituto de Materiales de Madrid, Jesus Gonzalez from Universidad Rey Juan Carlos, and David Gomez and Carlos García from Instituto de Ciencia y Tecnología de Polímeros for the experimental measurements.

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dx.doi.org/10.1021/la104410r |Langmuir 2011, 27, 3952–3959