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Langmuir 2004, 20, 1564-1571
Aqueous Dispersions of Silane-Functionalized Laponite Clay Platelets. A First Step toward the Elaboration of Water-Based Polymer/Clay Nanocomposites Norma Negrete Herrera,† Jean-Marie Letoffe,‡ Jean-Luc Putaux,§ Laurent David,| and Elodie Bourgeat-Lami*,† Laboratoire de Chimie et Proce´ de´ s de Polyme´ risation - UMR 140 CNRS/CPE, Baˆ t. 308F, 43, Bd. du 11 Nov. 1918, BP 2077, 69616 Villeurbanne Cedex, France, Laboratoire des Multimate´ riaux et Interfaces - UMR CNRS 5615 - Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France, Centre de Recherches sur les Macromole´ cules Ve´ ge´ tales UPR 5301 CNRS, BP 53, F-38041 Grenoble Cedex 9, France, and Laboratoire des Mate´ riaux Polyme` res et Biomate´ riaux - UMR CNRS 5627 IMP, Baˆ t. ISTIL, Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, France Received May 28, 2003. In Final Form: December 3, 2003 Mono- and trifunctional organo alcoxysilane derivatives carrying a terminal reactive methacryloyl group have been used as reagents for the chemical modification of synthetic Laponite clay platelets in toluene. Qualitative evidence of the presence of chemically attached silane molecules was provided by Fourier transform infrared and 29Si and 13C solid-state NMR spectroscopies. Quantitative data (grafted amount and grafting yield) were also obtained by means of elemental and thermogravimetric analysis. While the trifunctional coupling agent was grafted on the clay edges in the form of oligomers pillaring the clay stacks, the monofunctional derivative selectively attached to the individual clay sheets as confirmed by X-ray diffraction and Brunauer-Emmett-Teller measurements. In agreement with these findings, only the clay stacks grafted using the monofunctional coupling agent could be satisfactorily redispersed into water. The aqueous suspensions of the grafted colloidal disks were characterized by small-angle X-ray scattering, dynamic light scattering, and cryogenic transmission electron microscopy. Emulsion copolymer latexes, the surface of which was decorated by individual Laponite platelets, were finally produced using the grafted clay particles as seeds. This new method provides an efficient way for constructing water-based polymer/exfoliated clay nanocomposites.
I. Introduction During the past 50 years, there has been an increased interest in the synthesis of nanocomposite materials by embedding nanosized inorganic particles into polymers.1,2 Since the optical, thermal, rheological, or mechanical properties of these materials strongly depend on the techniques used for their elaboration, a variety of synthesis strategies have been reported worldwide with the aim to control the dispersion of the inorganic component within the polymer matrix at the nanoscale. Among the wide range of nanostructured materials, effort has more recently focused on the elaboration of polymer/layered silicate nanocomposites using natural and synthetic clay minerals.3-5 Clays exhibit many interesting structural features: active sites such as hydroxyl groups, Lewis and Bro¨nsted acidity, and exchangeable interlayer cations.6,7 In addition, the high aspect ratio of clay minerals and the * To whom correspondence should be addressed. † Laboratoire de Chimie et Proce ´ de´s de Polyme´risation - UMR 140 CNRS/CPE. ‡ Laboratoire des Multimate ´ riaux et Interfaces - UMR CNRS 5615 - Universite´ Claude Bernard Lyon 1. § Centre de Recherches sur les Macromole ´ cules Ve´ge´tales - UPR 5301 CNRS. | Laboratoire des Mate ´ riaux Polyme`res et Biomate´riaux - UMR CNRS 5627 IMP, Universite´ Claude Bernard Lyon 1. (1) Kickelbick, G.; Schubert, U. In Synthesis, Functionalization and Surface Treatment of Nanoparticles; Baraton, M.-I., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2002; Chapter 6, p 1. (2) Bourgeat-Lami, E. J. Nanosci. Nanotechnol. 2002, 2, 1. (3) For a review, see: Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2001, 28, 1. (4) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064. (5) Wang, D.; Zu, J.; Yao, Q.; Wilkie, C. A. Chem. Mater. 2002, 14, 3837.
small dimensions of the individual layers render them particularly attractive in several areas of material science. Although there has been much work in the field of polymer/layered silicate nanocomposites since the 1980s, surprisingly only few studies report on the elaboration of exfoliated clay/polymer nanostructures through emulsion polymerization.5,8-11 Emulsion polymers, however, are widely used in industry and can find applications in a variety of domains including water-borne adhesives, paints, and coating formulations. In addition, the latex route obviously offers interesting perspectives to produce homogeneous dispersions of clay minerals into polymer matrixes by taking advantage of the naturally occurring swelling behavior of clay platelets in water. Among the various swelling clay materials, Laponite, a synthetic hectorite made of discoid platelets with a thickness of about 1 nm, a diameter of about 25 nm, and a negative surface charge density of 0.014 e-/Å, is of particular interest because of its high purity and lateral crystal size of the order of magnitude of latex particle diameter.12 However, as far as is known, no study has been performed on the incorporation of Laponite platelets into polymer latexes through in situ emulsion polymerization, although Laponite has gained importance in numerous fields of applications.13 (6) van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1997. (7) Lee, D. C.; Jang, L. W. J. Appl. Polym. Sci. 1996, 61, 1117. (8) Noh, M. W.; Lee, D. C. Polym. Bull. 1999, 42, 619. (9) Noh, M. W.; Jang, L. W.; Lee, D. C. J. Appl. Polym. Sci. 1999, 74, 179. (10) Noh, M. W.; Lee, D. C. J. Appl. Polym. Sci. 1999, 74, 2811. (11) Kim, Y. K.; Choi, Y. S.; Wang, K. H.; Chung, I. J. Chem. Mater. 2002, 14, 4990. (12) Laponite Technical Bulletin L104/90/A; Laporte Industries Ltd.: 1990; p 1.
10.1021/la0349267 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/04/2004
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In this work, we report the chemical modification of Laponite clay minerals using monofunctional γ-methacryloxy propyl dimethyl ethoxysilane (γ-MPDES) and trifunctional γ-methacryloxy propyl trimethoxysilane (γMPTMS) molecules, respectively. The functionalized clay particles were dispersed into water and engaged further in a free radical emulsion polymerization process. By using silane coupling agents, we aim to produce colloidal nanocomposites with covalent grafting between the exfoliated clay sheets and the polymeric matrix. Moreover, polymer attachment on the inorganic surface since the beginning of the emulsion polymerization process ensures polymer growth from the seed surface. This feature is essential for the elaboration of the nanocomposite colloid and the development of interesting mechanical properties in the clay-filled polymeric material.14-16
4. Characterizations. 29Si and 13C solid-state NMR were performed on a Bruker DSX-300 spectrometer operating at 59.63 and 75.47 MHz, respectively, by use of cross-polarization from proton. The contact time was 5 ms, the recycle delay 1 s, and the spinning rate 10 kHz. The 29Si and 13C chemical shifts were referenced to tetramethylsilane (TMS). Infrared spectra were recorded using a Nicolet FTIR 460 spectrometer on powder-pressed KBr pellets. A JEOL JCXA 733 electron microprobe analyzer (EPMA) was used to determine the carbon content of the bare and functionalized clay platelets. The grafted amount (expressed in mequiv of grafted silane per g of bare Laponite) was determined from the difference ∆C (wt %) of carbon content after and before grafting as follows:17
II. Experimental Section
where NC and M (g/mol/) designate the number of carbon atoms and the molecular weight of the grafted silane molecule, respectively (NC ) 7 and M ) 206 for MPTMS, while NC ) 9 and M ) 202 for MPDES). Thermogravimetric analysis (TGA) of the treated clays was performed on a Mettler TG 50/TA 3000 thermobalance, controlled by a TC10A microprocessor. Samples were heated at a rate of 10 °C/min under a nitrogen flow (150 mL/min). The grafted amount was determined using eq 2 from the weight loss, W200-600, between 200 and 600 °C corresponding to silane degradation.
1. Materials. The clay particles used in this work were synthetic Laponite RD (Rockwood Additives Ltd. U.K.). The trifunctional (γ-MPTMS, C9H20O5Si, structure 1) and the monofunctional (γ-MPDES, C10H22O3Si, structure 2) silylating agents from Gelest Inc. were used without further purification. Toluene from Aldrich was of synthetic grade and used as received. The monomers, styrene (Styr) and butyl acrylate (BuA) from Aldrich, were purified upon distillation under reduced pressure before use. The initiator, potassium persulfate (KPS), and the surfactant, sodium dodecyl sulfate (SDS) from Across Organics, were used as supplied.
grafted amount (mequiv/g) )
103 ∆C (1200NC - ∆C(M - 1))
grafted amount (mequiv/g) )
103 W200-600 (100 - W200-600)M
(1)
(2)
where M (g/mol) is the molecular weight of the grafted silane molecules. The grafting yield, which corresponds to the percentage of silane molecules which effectively participated in the coupling reaction, was calculated as follows:
grafting yield (%) ) graft density × 100/[silane]
2. Functionalization. The grafting reaction was carried out in toluene. Laponite (6 g) was first suspended in the organic medium (200 mL), and the required amount of silane, either γ-MPDES (3.9 g) or γ-MPTMS (4.4 g), corresponding to around 3 mequiv of coupling agent per g of Laponite, was introduced in the reaction flask and stirred for several days at ambient temperature. The grafted Laponite was extensively washed with toluene in order to remove the silane in excess and dried at 40 °C in a vacuum oven before use. Unless stated otherwise, aqueous suspensions of grafted and ungrafted clay platelets were prepared by dispersing 10 g L-1 of the corresponding Laponite powder in deionized water and stirring for 1 h. 3. Emulsion Polymerization. The nanocomposite latexes were synthesized using the functionalized Laponite as seed particles. The polymerizations were carried out in batch at 70 °C for up to 24 h under a nitrogen atmosphere. The reactor was charged with 100 g of the aqueous MPDES- or MPTMS-grafted Laponite suspensions (10 g L-1) containing the surfactant (SDS, 2 g L-1). After degassing, the monomers, a mixture of styrene (3 g) and butyl acrylate (7 g), and the initiator (KPS, 0.1 g) were successively introduced at 70 °C under stirring. (13) Coche-Gue´rente, L.; Desprez, V.; Labbe´, P. J. Electroanal. Chem. 1998, 458, 73. (14) Bourgeat-Lami, E.; Espiard, P.; Guyot, A.; Gauthier, C.; David, L.; Vigier, G. Angew. Makromol. Chem. 1996, 242, 105. (15) Corcos, F.; Bourgeat-Lami, E.; Novat, C.; Lang, J. Colloid Polym. Sci. 1999, 277, 1142. (16) Bourgeat-Lami, E.; Lang, J. Macromol. Symp. 2000, 151, 377.
(3)
where [silane] (mequiv/g) designates the initial silane concentration. X-ray powder diffraction patterns were obtained using a Siemens D500 diffractometer (Ni-filtered Cu KR radiation, λ ) 1.5405 Å). The d001 basal spacings were calculated from the 2θ values using the EVA software. Nitrogen adsorption measurements were performed using a Quantachrome Autosorb-1-MP automated gas adsorption system. The samples (either the bare or the functionalized clay platelets) were degassed at 200 °C for 16 h in a vacuum furnace before analysis. The adsorption isotherms were of type II, and the Brunauer-Emmett-Teller (BET) method was used for determination of the specific surface area (Sspec, m2 g-1).18 Particle size was determined by dynamic light scattering (DLS, Brookhaven instrument). Surface tension measurements were carried out by using a Kruss K-12 tensiometer with a DuNoy ring. The samples were prepared by adding known amounts of the clay platelets to water solutions. Extreme care was taken to prevent any contamination of the samples. Small-angle X-ray scattering (SAXS) studies were carried out at the ESRF BM2-D2AM synchrotron beamline. Measurements were made on dilute (10 g L-1) aqueous suspensions of bare and functionalized Laponite using low-density polyethylene 0.5 mL insulin syringes. A 3 mm diameter hole is drilled close to the basis and perpendicular to the long axis of each syringe and then sealed by an adhesive polyimide Kapton film. The beam passes through the Kapton windows and Laponite suspensions after positioning the syringe in suitable sample holders. The transmission is measured for each sample with two additional Kapton foils oriented at 45° with respect to incident beam and photo(17) Berendsen, G. E.; De Galan, R. J. J. Liq. Chromatogr. 1978, 1, 561. (18) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.
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Chart 1. Idealized Structure of Laponite Clay Mineral
multipliers. The empty cell is subtracted from each scattering curve. The data were collected in point collimation mode at 16 keV, with a CCD detector (Roper Scientific). Radial averages were performed with the software developed by J. F. Be´rar on the D2AM beam line, accounting for dark field response, flat field correction, hot pixel filtering, distortion of the camera, and normalization to the transmission of the samples.19 Cryogenic transmission electron microscopy (cryo-TEM) experiments were carried out following a method described elsewhere.20-23 Thin liquid films of 0.1 wt % suspensions formed onto commercial “lacey” carbon membranes (Pelco, USA) were quench-frozen into liquid ethane (-171 °C). The samples were mounted in a Gatan 626 specimen holder cooled with liquid nitrogen and subsequently observed at -180 °C, under low dose illumination, using a Philips CM200 Cryo microscope operated at 80 kV. The micrographs were recorded on Kodak SO163 films.
III. Results and Discussion 1. Structure and Physical Properties of Laponite RD. Laponite RD is generally described as being made of nanosized disk-shaped crystalline lamellae that readily disperse into water.24,25 Laponite has the same structural characteristics as the well-known Hectorite and can be seen as a two-dimensional inorganic polymer with six octahedral magnesium ions sandwiched between two layers of four tetrahedral silicon atoms (Chart 1). The silicon and magnesium atoms are balanced by 20 oxygen atoms and 4 hydroxyl groups. In practice, some magnesium ions are substituted by lithium ions, and a negative charge is developed within the interlayer gallery space, which is compensated by exchangeable sodium ions. The edge of the disk is similar to a hydrous oxide and is characterized by the presence of amphoteric hydroxyl groups and small positive charges mostly located at the broken edges of the crystal. Laponite also contains intralayer MgOH groups, but these groups are believed to hardly be accessible to the coupling agent molecules.26 Details on chemical composition and physicochemical properties of Laponite RD can be found in the Supporting Information. 2. Modification of Laponite RD. While Laponite sheets expand in water by osmotic swelling and hydration of interlayer cations which balance the negative electrical charge, the clay platelets have a tendency to pile up into tactoids held together by long-range attractive forces when suspended in nonpolar liquids. In the following, we report the chemical modification of such Laponite clay tactoids (19) For more information, visit the web site: http://www.esrf.fr. (20) Dubochet, J.; Adrian, M.; Chang, J.-J.; Homo, J. C.; Lepault, J.; McDowall, A. W.; Schultz, P. Q. Rev. Biophys. 1988, 21, 129. (21) Jackson, C. L.; Chanzy, H. D.; Booy, F.; Drake, B. J.; Tomalia, D. A.; Bauer, B. J.; Amis, E. J. Macromolecules 1998, 31, 6259. (22) Putaux, J.-L.; Bule´on, A.; Borsali R.; Chanzy, H. Int. J. Biol. Macromol. 1999, 26, 145. (23) Chalaye, S.; Bourgeat-Lami, E.; Putaux, J.-L.; Lang, J. Macromol. Symp. 2001, 169, 89. (24) Avery, R. G.; Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 448. (25) Thompson, D. W.; Butterworth, J. T. J. Colloid Interface Sci. 1992, 151, 236. (26) McCabe, R. W. In Inorganic Materials, 2nd ed.; Bruce, D. W., O’Hare, D., Eds.; Wiley: New York, 1996; Chapter 6, p 314.
Figure 1. Infrared spectra of (a) Laponite RD, (b) MPDESgrafted Laponite, and (c) MPTMS-grafted Laponite.
in toluene using either a trifunctional or a monofunctional silane derivative. Qualitative evidence of grafting was provided by Fourier transform infrared (FTIR) and solid-state NMR spectroscopies. The FTIR spectra of bare and functionalized Laponites are reported in Figure 1. The spectra of Figure 1b,c show characteristic vibrations of the carbonyl (νCdO, 1736 cm-1) and the aliphatic CH2 and CH3 groups (νCH, 2850, 2920, and 2980 cm-1; and δCH, 1380 cm-1) of the mono- and the trifunctional silane molecules. The absorption band at 1634 cm-1, corresponding to the stretching vibration of the CdC double bond, overlaps with the δOH deformation band at 1635 cm-1 due to physisorbed water. 29Si NMR and 13C spectroscopies give additional evidence of grafting. The 29Si cross-polarization/magic angle spinning (CP/MAS) NMR spectra of raw and grafted Laponites are shown in Figure 2. The different species are denoted according to the conventional Qn, Tn, and Mn notation where Q, T, and M designate tetra-, tri-, and monofunctional units, respectively, and n is the number of bridging O atoms surrounding the silicon atom. The NMR spectrum of raw Laponite is characterized by two resonances, at -94.7 and -84.8 ppm, which correspond respectively to Q3 trioxo coordinated framework silicon and Q2 sites attributed to isolated silanol groups present at the silicate sheet edges.27 The grafting is evidenced by the appearance of signals assigned to M1 (15 ppm) and T2,3 (-56.9, -66.5 ppm) silicate units derived from monoand trialkoxysilanes, respectively, and by the decrease of the relative intensity of the Q2 signal due to reaction with the SiOH groups of the clay sheets. 13C CP/MAS NMR provides further confirmation of the formation of Si-OSi linkages. The 13C NMR spectra of the functionalized clay samples (not shown) exhibit all the signal characteristics of the coupling agent molecules. Chemical shift assignments of the starting silanes and of the grafted Laponite powders can be found in the Supporting Information. The 1-3 ppm upfield shift of the resonances attributed to the carbonyl (C4) and to the propyl carbon directly attached to the silicon atom (C7) is consistent with the formation of bound siloxane polymer. Next, we investigated whether the silylation reaction had an influence on the physicochemical properties of the clay mineral. Table 1 provides the basal spacings of raw and functionalized Laponites after drying at 40 °C under a vacuum. The initial basal spacing of 12 Å corresponding to sodic Laponite increased to 17 Å for the MPTMS-grafted clay, while the X-ray diffraction (XRD) patterns of the clay powders indicate nearly no change of the interlayer (27) Mandair, A.-P. S.; Michael, P. J.; McWhinnie, W. R. Polyhedron 1990, 9, 517.
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Figure 3. TGA degradation profiles of (a) bare Laponite, (b) MPDES-grafted Laponite, and (c) MPTMS-grafted Laponite. Chart 2. Schematic Representation of the Coupling Reaction of the Monofunctional and Trifunctional Silane Molecules on the Clay Edges
Figure 2.
29
Si NMR spectra of raw and grafted Laponites.
Table 1. Basal Spacings and Specific Surface Areas of Bare and Functionalized Laponites samples
d (Å)a
Sspec (m2/g)b
Laponite RD MPTMS-grafted Laponite MPDES-grafted Laponite
12 17 13
370 212 247
a Determined by wide-angle X-ray scattering. b External surface area calculated using the BET method.
distances for MPDES-grafted Laponite. The increase in the basal spacings from 12 Å for crude Laponite to 17 Å for MPTMS-grafted Laponite suggests that polycondensates are formed in the interlaminate space. Similar results have been reported in the literature for the modification of kanemite and magadiite, two layered polysilicates using various mono-, di-, and trichloro(alkyl)silanes.28,29 However, contrary to these previous works, in the present system, the silane molecules can only react at the “broken” edges of the crystalline sheets, and the formation of intercalation compounds is highly improbable. To explain our data, one must thus consider that the silylation reaction produces siloxane polymers able to penetrate the external part of the interlaminate space and push the clay sheets apart (Chart 2b). This picture was corroborated by measuring the specific surface area before and after (28) Shimojima, A.; Mochizuki, D.; Kuroda, K. Chem. Mater. 2001, 13, 3603. (29) Isoda, K.; Kuroda, K. Chem. Mater. 2000, 12, 1702.
grafting (Table 1). The Laponite specific surface area decreases from 370 m2/g for bare Laponite to 212 m2/g for MPTMS-functionalized clays, supporting the view that the access to internal porosity is blocked by the organic molecule grafted on the border of the clay stacks. To gain more insight into the grafting process, elemental and thermal analyses were used to determine the amount of silane molecules chemically anchored on the clay edges after extensive washing of the nonreacted coupling agent. Figure 3 shows the TGA curves before and after grafting of the silane molecules. The thermograms exhibit three main decomposition regions. Only the region between 200 and 600 °C, which corresponds to the thermal decomposition of the organic molecule, was considered for quantitative determination of the silane coverage. As seen in Table 2, the grafted amount determined by TGA analysis is in good agreement with the silane content determined by microanalysis. Both techniques indicate a lower silane content for the monofunctional than for the trifunctional coupling agent (i.e., 0.35 and 0.75 mequiv/g, respectively). This result, together with the 29Si NMR data described above, is consistent with the grafting of the monofunctional silane by only one Si-OR group while the trifunctional modifier is believed to form oligomers attached to the clay edges by two (T2) or three (T3) Si-OR groups.
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Table 2. Elemental and Thermogravimetric Analyses of Bare and Silane-Functionalized Laponites elemental analysis
Laponite RD MPTMS-grafted Laponite MPDES-grafted Laponite
thermogravimetric analysis
carbon content (wt %)
∆C (%)a
grafted amount (mequiv/g)b
grafting yield (%)c
weight loss (%)d
grafted amount (mequiv/g)e
grafting yield (%)c
0.4 6.0 4.0
0 4.6 3.6
0.77 0.36
26.1 12.1
8.9 6.6
0.75 0.35
25.4 11.8
a Difference in carbon content after and before grafting. b Determined using eq 1. c Determined using eq 3. and 600 °C. e Determined using eq 2.
With this picture in mind, a schematic structural feature of the chemically attached silane molecules can be proposed for both the MPDES- and the MPTMS-grafted clay samples (Chart 2a,b, respectively). Chart 2b is characterized by the formation of a chemical bond between the individual clay platelets giving rise to irreversibly attached clay stacks, while in Chart 2a, cross-linking of the clay platelets is unlikely since the monofunctional silane modifier can react with solely one Si-OH group on the clay edges. 3. Aqueous Suspensions of Functionalized Laponite Clay Platelets. Before being introduced in emulsion polymerization, the functionalized clay platelets need to be dispersed into water. They were first isolated by filtration, extensively washed with toluene, and dried under a vacuum as reported in the experimental part. They were then introduced in deionized water under magnetic stirring. Contrary to raw Laponite which readily dispersed into water giving rise to the formation of a fully transparent colloidal suspension of elementary platelets, MPTMS-grafted Laponite gave large agglomerates discernible to the eyes by an increase in suspension turbidity. An attempt to determine their size by DLS was unsuccessful as could be expected from the previous observation. The dispersion state could be improved by adding anionic surfactant, but the suspension still contained agglomerates settling down on standing. As far as the MPDESgrafted clay platelets are concerned, a significant different behavior was observed. The functionalized clay powder could be satisfactorily suspended into water indicating the predominance of nonaggregated clay platelets in the suspension medium although some small agglomerates were still formed as evidenced by the bluish aspect of the suspension. These aggregates have an average diameter of around 60-70 nm as estimated by DLS. 4. Structural Characterization of the Aqueous Functionalized Clay Suspensions. In the following, cryo-TEM and SAXS experiments were carried out in order to provide more insights into the structure of the functionalized clay suspensions. Cryo-TEM Analysis of Bare and Functionalized Laponite RD Suspensions. Figure 4a is a cryo-TEM image of randomly dispersed individual Laponite platelets embedded in vitreous ice. The contrast of the platelets strongly depends on their orientation with respect to the electron beam. Diffraction contrast is strong when the Laponite nanocrystals are seen with their basal plane parallel to the electron beam. The platelets then appear as dark “filaments” with a length varying from 20 to 40 nm. Depending on the defocus of the objective lens, more or less pronounced Fresnel fringes may appear on each side of the diffracting platelets, artificially increasing the thickness value. Laponite crystals are also seen with their basal plane more or less perpendicular to the electron beam. In that orientation, diffraction contrast is very weak but depending on the image defocus, platelets with an elongated shape can be observed, with a length of 20-40 nm and a width of about 10 nm. As a matter of fact, during a rapid survey of cryo-TEM images of Laponite suspen-
d
Weight loss between 200
Figure 4. Cryo-TEM of 0.1 wt % aqueous suspensions of (a) Laponite RD and (b) MPDES-grafted Laponite. Scale bar: 100 nm.
sions, one mostly sees the platelets with an edge-on orientation. This may lead to a false impression of the actual concentration in the specimen. A more careful examination of the same samples using the proper defocus reveals many additional crystals oriented in planar views. A cryo-TEM image of a suspension containing MPDESfunctionalized Laponite platelets is presented in Figure 4b. By comparing this image with that of a homogeneous ungrafted Laponite dispersion (Figure 4a), one can see that the crystals have clearly organized in small groups containing about a dozen units. We assume that these aggregates originate from the development of attractive interactions between the individual clay platelets due to the presence of the coupling agent molecules. Three types of aggregate morphologies can be imagined: (i) a starlike organization (Chart 3a), (ii) a vesicular-like organization (Chart 3b), and (iii) a more disorganized bundle-like structure (Chart 3c). However, considering the irregular shape and polydispersity of individual Laponite platelets
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Chart 3. Possible Morphologies for Functionalized Laponite Platelets in Aqueous Suspension Using the Monofunctional (a-c) or the Trifunctional (d) Coupling Agenta
a
For clarity, only crystals seen “edge-on” are drawn. The grafted part is indicated in black.
Figure 5. Evolution of surface tension as a function of the concentration of raw Laponite (0), MTMS-grafted Laponite (2), and MPDES-grafted Laponite (O) suspensions.
and the complex geometry of the grafting, the formation of regular and monodisperse aggregate structures is unlikely. Looking at many aggregates seen in cryo-TEM images, it appears that case c is indeed the most favorable geometry. The flocculating behavior of the MPTMS-grafted clay suspensions in water makes it impossible to accurately characterize their morphology by cryo-TEM. However, as described above, these suspensions are presumed to contain aggregates with a stacked-like structure, the clay platelets being irreversibly locked together by siloxane bridges bonding the clay edges (Chart 2b). In view of the sedimentation behavior of these suspensions, we suppose that they also contain interconnected clay stacks as illustrated in Chart 3d. To evaluate the amphiphilic character of the clay surface (the clay borders being hydrophobic and the basal planes hydrophilic), we checked whether the functionalized clay particles were surface active. The surface active properties of the clay platelets in water can be easily estimated by measuring the variations of the air/water interfacial tension, σ, with the clay content. The results shown in Figure 5 for both nonfunctionalized and functionalized aqueous clay suspensions indicate that only the clay platelets grafted using the monofunctional silane coupling agent have a significant influence on surface tension. σ continuously decreases with increasing clay content and reaches a pseudoplateau at a concentration around 0.5 g L-1, which is far below the actual clay content of the aqueous suspensions described above (i.e., 10 g L-1). This result clearly attests to the surface active properties of the MPDES-grafted clay platelets. The fact that the MTPMS-grafted clay sample has nearly no influence on surface tension is presumably due to the flocculating behavior reported above. Indeed, since in this case most of the clay platelets have sedimented in the reaction flask, there are nearly no particles remaining in suspension. SAXS Analysis. The SAXS patterns of the Laponite suspensions provide complementary information on the
Figure 6. SAXS q2I(q) profiles of 1% w/w MPDES-grafted (s) and MPTMS-grafted (- - -) Laponite suspensions. The insert shows the corresponding scattering I(q) patterns for comparison.
morphology and state of dispersion of the clay platelets. The small-angle scattering of X-rays by thin platelets of radius R, thickness H, and volume V is given by30
2 I(q) ) R∆F2NV 2 exp(-H 2/12)S(q) (qR)2 where R is a normalization constant, ∆F is the difference in electron density between the platelet and the surrounding medium, q is the scattering vector, N is the number of platelets in the irradiated volume, and S(q) is the structure factor (containing the information about the location of the platelets respectively to the others). As a consequence, to study the structural organization of functionalized Laponite clays in water solution, we use a representation31-33 close to that of Morvan et al. or Saunders et al. by multiplication of the scattered intensity I(q) by q2. Indeed, we do not know a priori the thickness of the platelets and the structure factor calculation needs the evaluation of the exponential (Guinier) term. The results obtained from 1% w/w silane-grafted Laponite suspensions in pure water are shown in Figure 6. The MPDES-grafted Laponite suspension displays a characteristic pattern32,33 dominated at low q values (below 0.07 Å-1) by the structure factor S(q) and at larger q values by the Guinier term exp(-H 2/12). In the q range indicated between the two vertical lines of Figure 6 (between 0.07 and 0.14 Å-1), the Guinier plot yields a platelet thickness close to 13 Å (assuming that S(q) ) 1). In the q range above 0.14 Å-1, the apparent thickness is much lower (close (30) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley and Sons: New York, 1955. (31) Ramsay, J. D. F.; Swanton, S. W.; Bunce, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3919. (32) Morvan, M.; Espinat, D.; Lambard, J.; Zemb, Th. Colloids Surf., A 1994, 82, 193. (33) Saunders, J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B 1999, 103, 9211.
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to 6 Å), thus showing that an additional contribution to scattering is to be considered. In the lower q range (below 0.07 Å-1), the function q2I(q) exhibits a maximum in good agreement with the shapes of S(q) functions deduced in previous studies for raw Laponite in water.31-33 For the concentration investigated here of 1% w/w, the maximum is close to 0.021 Å-1, thus revealing a correlation distance d* ) 2π/qmax ) 300 Å in good agreement with that of more concentrated suspensions (4% w/w) at a similar pH,33 and associated with the “house of cards” aggregation model. The scattering diagram displayed by the MPTMSgrafted Laponite dispersion is quite different from those of raw Laponite and the MPDES-grafted Laponite suspension, as an increase of intensity is observed at larger q values. This contribution can be due to the presence of tactoids introducing diffraction peaks that are well resolved in the powder diagrams above 0.3 Å-1 (see Table 1). The presence of a few tactoids in MPDES-grafted Laponite suspensions is also possible, thus explaining the values of the low Guinier slopes, in the high q range, with no direct physical meaning. As a result, the MPTMS-grafted Laponite exhibits a low population of exfoliated platelets with a dominant scattering contribution of the tactoids, and the MPDESgrafted Laponite suspension is composed of a large fraction of exfoliated platelets with a much smaller tactoid scattering contribution. The SAXS analysis thus evidences and confirms that MPTMS functionalization “locks” the individual platelets in the tactoids, whereas the monofunctional coupling agent MPDES enables Laponite exfoliation and gel formation by platelet aggregation in a similar way to raw Laponite suspensions. 5. Emulsion Polymerization in the Presence of Functionalized Clay Platelets. Emulsion polymerization offers very interesting perspectives for the elaboration of clay/polymer nanocomposites. In a series of articles, Lee, Jang, and Noh7-10 described for instance the formation of montmorillonite (MMT)/polymer intercalated structures and reported on a substantial improvement of the physical properties of the nanocomposite materials. More recently, Huang and Brittain34 demonstrated that exfoliated structures can be obtained by postaddition of an aqueous dispersion of layered silicates (either MMT or Laponite) into a polymethyl methacrylate latex suspension produced in the presence of suitable cationic compounds (cationic initiator, monomer, or surfactant). Since the latex particles were cationic and the clay platelets anionic, strong electrostatic forces were developed at the polymer/clay interface. Following a similar strategy, Putlitz and co-workers synthesized armored latexes where the surface of polymer particles was covered with nanosized clay plates.35 Herein, we report on a significantly different approach by in situ emulsion polymerization using the MPDES- or the MPTMS-grafted clay particles as seeds. Related strategies have already been described in the literature for a variety of oxide particles including silica,14-16,36 alumina,37 and titanium dioxide,38 but as far as we are aware, no study has been done on clay colloids. With the aim to ultimately produce film materials containing the functionalized clay platelets, we used a mixture of styrene and butyl acrylate monomers. Both the MPDES- and the MPTMS-grafted clay suspensions were engaged in the emulsion polymerization process, and an anionic surfactant was used to stabilize the polymer latex particles. (34) Huang, X.; Brittain, W. J. Macromolecules 2001, 34, 3255. (35) zu Putlitz, B.; Landfester, K.; Fischer, H.; Antonietti, M. Adv. Mater. 2001, 13, 500. (36) Sondi, I.; Fedynyshyn, T. H.; Sinta, R.; Matijevic, E. Langmuir 2000, 16, 9031.
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Figure 7. (a) Postsynthetic mixture of copolymer latex particles and the raw Laponite clay suspension. Scale bar: 100 nm. (b,c) Nanocomposite colloids obtained by emulsion polymerization in the presence of (b) 10 wt % of MPTMS- and (c) 10 wt % of MPDES-functionalized Laponite clay platelets.
Nanocomposite particles frozen in a thin film of vitreous ice and observed by cryo-TEM are shown in Figure 7. Figure 7a corresponds to a sample prepared by simply mixing the Laponite dispersion with the latex suspension. (37) Duguet, E.; Abboud, M.; Morvan, F.; Maheu, P.; Fontanille, M. Macromol. Symp. 2000, 151, 365. (38) Hofman-Caris, C. H. M. New J. Chem. 1994, 18, 1087.
Aqueous Dispersions of Laponite Clay Platelets
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Scheme 1. Hypothetical Nucleation and Growth Mechanism during the Synthesis of Clay/Polymer Nanocomposite Colloids though Emulsion Polymerization Using the MPDES-Grafted Clay Particles as Seeds
Laponite crystals appear as dark filaments and polymer particles as gray nanospheres. No particular interaction could be detected between the clay platelets and the latex spheres. The specimen shown in Figure 7b corresponds to polymerization performed in the presence of the MPTMSgrafted clay sample. The Laponite platelets clearly form flocculating aggregates with again no detectable interaction with the polymer particles. These aggregates display a stacked-like structure in agreement with the above description (Chart 3d). In contrast, when the monofunctional silane molecule was used as the coupling agent, the clay platelets were homogeneously distributed within the latex sample and formed a nanocomposite structure (Figure 7c). There is no evidence that the Laponite platelets are actually incorporated inside the polymer particles as reported in the literature for MPTMS-grafted silica beads.15,16 On the contrary, they can be clearly seen spread on the polymer surface as if they formed a shell. Only the platelets with their basal plane parallel to the electron beam generate a diffraction contrast strong enough to allow their direct detection on the edge of the polymer. However, the heterogeneous aspect of the surface of the particles suggests that clay sheets, with planes more or less normal to the electron beam and a very weak contrast, lie everywhere on the latex surface. In addition, the composite particles are slightly polygonal as the natural rigidity of Laponite crystals generates a faceting of the surface. The analogy between the above morphology, the clay platelets forming a rigid corona around the latex particles, and the vesicular superstructure of Chart 3b, is relevant. This suggests that polymerization is taking place within the clay aggregates since the beginning of the reaction. But one might also argue that the above-depicted morphology simply emerged as the equilibrium morphology of the nucleation/growth process. Unfortunately, in view of the very limited number of experiments which have been done so far, attempts to conclude between these two alternatives would be purely speculative and further investigations are necessary to clarify this issue. Perhaps more pertinent to the present work is the observation that the clay platelets clearly appear to serve as stabilizers for the growing polymer particles. This is presumably due to the anisotropy of grafting and the amphiphilic character of the functionalized clay surface. Indeed, since the beginning of the emulsion polymerization process, two events can take place simultaneously: (i) in situ formation of polymer precursors on the clay edges by reaction of the growing polymer radicals with the attached double bonds and (ii) stabilization of these precursors by the negatively charged surface of the clay platelets. On one hand, the growing polymer chains tend to minimize their contact with the clay surface as much as possible, remaining localized on the edges of the platelets, and on the other hand, the planar faces of the clay sheets keep in contact with water to minimize the overall energy of the system (Scheme 1). As polymerization continues, the primary
particles aggregate into larger particles by association with polymer chains growing on the same clay platelet or on neighboring ones, until they reach their optimal stability and become stable mature particles. The nanocomposite colloid then continues to grow as usual by solubilizing monomer and trapping radicals from the water phase. This results in the formation of latex particles, the surface of which is covered by the exfoliated clay sheets. IV. Conclusion Organic-inorganic nanocomposite colloids containing anisotropic synthetic Laponite clay platelets have been prepared by emulsion polymerization in two steps. The clay mineral was first functionalized using either a monofunctional or a trifunctional silane coupling agent. The functionalized clay particles were then suspended into water in the presence of surfactant. Distinct behaviors were observed depending on the functionality of the silane derivative. While the trifunctional silane was able to chemically lock the individual platelets into irreversibly attached clay stacks, the monofunctional coupling agent was grafted to the clay edges by only one SiOH group as supported by several techniques such as solid-state NMR, XRD, N2-adsorption, elemental analysis, light scattering, SAXS, and cryo-TEM. One important consequence is that only the clay stacks grafted using the monofunctional derivative could be satisfactorily dispersed into water; this feature was essential for successful dispersion of the clay platelets into the latex particles. Nanocomposite colloids were indeed successfully obtained with the MPDES-grafted Laponite sample. The clay platelets were found to cover the latex surface, giving a faceted appearance to the particles. The anisotropy of the grafting process and the amphiphilicity of the functionalized clay surface were suspected to be responsible for this morphology. Work is currently underway in order to introduce polymerizable groups on the basal faces of the clay platelets and encapsulate the individual clay sheets. Acknowledgment. This work was financially supported by the SFERE-CONACYT exchange program. The gift of a sample of Laponite RD by Rockwood Additives is gratefully acknowledged. We are grateful to Cyrille Rochas (ESRF and Laboratoire de Spectroscopie Physique, Universite´ Joseph Fourier, Grenoble, France) for his friendship and local contact help (from beam focusing to file processing) during the SAXS experiments on the D2AM beamline. Supporting Information Available: Physicochemical properties of Laponite RD and13C CP/MAS NMR chemical shifts of the starting and grafted silane molecules. This material is available free of charge via the Internet at http://pubs.acs.org. LA0349267
