Polymer-layered silicate nanocomposites

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Materials Science and Engineering, 28 (2000) 1±63

Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials Michael Alexandre, Philippe Dubois*

Laboratory of Polymeric and Composite Materials, University of Mons-Hainaut, 20 Place du Parc, B-7000 Mons, Belgium Accepted 20 March 2000

Abstract This review aims at reporting on very recent developments in syntheses, properties and (future) applications of polymer-layered silicate nanocomposites. This new type of materials, based on smectite clays usually rendered hydrophobic through ionic exchange of the sodium interlayer cation with an onium cation, may be prepared via various synthetic routes comprising exfoliation adsorption, in situ intercalative polymerization and melt intercalation. The whole range of polymer matrices is covered, i.e. thermoplastics, thermosets and elastomers. Two types of structure may be obtained, namely intercalated nanocomposites where the polymer chains are sandwiched in between silicate layers and exfoliated nanocomposites where the separated, individual silicate layers are more or less uniformly dispersed in the polymer matrix. This new family of materials exhibits enhanced properties at very low filler level, usually inferior to 5 wt.%, such as increased Young's modulus and storage modulus, increase in thermal stability and gas barrier properties and good flame retardancy. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Layered silicate nanocomposites; Intercalative polymerization; Melt intercalation; Exfoliation±adsorption; Mechanical properties; Thermal stability

1. Introduction Manufacturers fill polymers with particles in order to improve the stiffness and the toughness of the materials, to enhance their barrier properties, to enhance their resistance to fire and ignition or Abbreviations: AFM, atomic force microscopy; AIBN, N,N0 -azobis(isobutyronitrile); ALA, aminolauric acid; APP, ammonium polyphosphate; BDMA, benzyldimethylamine; BTFA, boron trifluoride monomethylamine; CEC, cation exchange capacity; DGEBA, diglycidyl ether of bisphenol A; DMA, dynamic mechanical analysis; DSC, differential scanning calorimetry; EDX, energy dispersive X-ray; EVA, ethylene vinyl acetate copolymer; FTIR, Fourier transform infrared spectroscopy; HDPE, high density poly(ethylene); HPMC, hydroxypropylmethylcellulose; HRR, heat release rate; MAO, methylaluminoxane; MMT, montmorillonite; NBR, nitrile rubber; NMA, nadic methyl anhydride; PAA, poly(acrylic acid); PAN, poly(acrylonitrile); PANI, poly(aniline); PBD, poly(butadiene); PCL, poly(e-caprolactone); PDDA, poly(dimethyldiallylammonium); PDMS, poly(dimethylsiloxane); PEO, poly(ethylene oxide); PFT, polymerization-filling technique; PI, poly(imide); PLA, poly(lactide); PP, poly(propylene); PP-MA, maleic anhydride modified poly(propylene); PP-OH, hydroxyl modified poly(propylene); PPV, poly(p-phenylenevinylene); PS, poly(styrene); PS3Br, poly(3-bromostyrene); PVA, poly(vinyl acetate); PVCH, poly(vinylcyclohexane); PVOH, poly(vinyl alcohol); PVP, poly(2-vinyl pyridine); PVPyr, poly(vinylpyrrolidone); PXDMS, poly(p-xylenylene dimethylsulfonium bromide); SBS, symmetric(styrene±butadiene±styrene) block copolymer; SEC, size exclusion chromatography; TEM, transmission electron microscopy; TEOS, tetraethylorthosilicate; THF, tetrahydrofuran; XRD, X-ray diffraction * Corresponding author. Tel.: ‡32-65-373481; fax: ‡32-65-373484. E-mail address: [email protected] (P. Dubois) 0927-796X/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 7 - 7 9 6 X ( 0 0 ) 0 0 0 1 2 - 7

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Table 1 Example of layered host crystals susceptible to intercalation by a polymer Chemical nature

Examples

Element Metal chalcogenides Carbon oxides Metal phosphates Clays and layered silicates

Graphite [8] (PbS)1.18(TiS2)2 [9], MoS2 [10] Graphite oxide [11,12] Zr(HPO4) [13] Montmorillonite, hectorite, saponite, fluoromica, fluorohectorite, vermiculite, kaolinite, magadiite, . . . M6Al2(OH)16CO3nH2O; MˆMg [14], Zn [15]

Layered double hydroxides

simply to reduce cost. Addition of particulate fillers sometimes imparts drawbacks to the resulting composites such as brittleness or opacity. Nanocomposites are a new class of composites, that are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. One can distinguish three types of nanocomposites, depending on how many dimensions of the dispersed particles are in the nanometer range. When the three dimensions are in the order of nanometers, we are dealing with isodimensional nanoparticles, such as spherical silica nanoparticles obtained by in situ sol±gel methods [1,2] or by polymerization promoted directly from their surface [3], but also can include semiconductor nanoclusters [4] and others [2]. When two dimensions are in the nanometer scale and the third is larger, forming an elongated structure, we speak about nanotubes or whiskers as, for example, carbon nanotubes [5] or cellulose whiskers [6,7] which are extensively studied as reinforcing nanofillers yielding materials with exceptional properties. The third type of nanocomposites is characterized by only one dimension in the nanometer range. In this case the filler is present in the form of sheets of one to a few nanometer thick to hundreds to thousands nanometers long. This family of composites can be gathered under the name of polymer-layered crystal nanocomposites, and their study will constitute the main object of this contribution. These materials are almost exclusively obtained by the intercalation of the polymer (or a monomer subsequently polymerized) inside the galleries of layered host crystals. There is a wide variety of both synthetic and natural crystalline fillers that are able, under specific conditions, to intercalate a polymer. Table 1 presents a non-exhaustive list of possible layered host crystals. Amongst all the potential nanocomposite precursors, those based on clay and layered silicates have been more widely investigated probably because the starting clay materials are easily available and because their intercalation chemistry has been studied for a long time [16,17]. Owing to the nanometer-size particles obtained by dispersion, these nanocomposites exhibit markedly improved mechanical, thermal, optical and physico-chemical properties when compared with the pure polymer or conventional (microscale) composites as firstly demonstrated by Kojima and coworkers [18] for nylon±clay nanocomposites. Improvements can include, for example, increased moduli, strength and heat resistance, decreased gas permeability and flammability. The aim of this report is to review the different techniques used to obtain polymer-layered silicates nanocomposites and the improved properties that those materials can display. 2. Generalities 2.1. Structure of layered silicates The layered silicates commonly used in nanocomposites belong to the structural family known as the 2:1 phyllosilicates. Their crystal lattice consists of two-dimensional layers where a central

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Fig. 1. Structure of 2:1 phyllosilicates (reproduced from [19] with permission).

octahedral sheet of alumina or magnesia is fused to two external silica tetrahedron by the tip so that the oxygen ions of the octahedral sheet do also belong to the tetrahedral sheets. The layer thickness Ê to several microns is around 1 nm and the lateral dimensions of these layers may vary from 300 A and even larger depending on the particular silicate. These layers organize themselves to form stacks with a regular van der Walls gap in between them called the interlayer or the gallery. Isomorphic substitution within the layers (for example, Al3‡ replaced by Mg2‡ or by Fe2‡, or Mg2‡ replaced by Li‡) generates negative charges that are counterbalanced by alkali or alkaline earth cations situated in the interlayer. As the forces that hold the stacks together are relatively weak, the intercalation of small molecules between the layers is easy [16]. In order to render these hydrophilic phyllosilicates more organophilic, the hydrated cations of the interlayer can be exchanged with cationic surfactants such as alkylammonium or alkylphosphonium (onium). The modified clay (or organoclay) being organophilic, its surface energy is lowered and is more compatible with organic polymers. These polymers may be able to intercalate within the galleries, under well defined experimental conditions as will be reported about in Section 3. Montmorillonite, hectorite and saponite are the most commonly used layered silicates. Their structure is given in Fig. 1 [19] and their chemical formula are shown in Table 2. This type of clay is characterized by a moderate negative surface charge (known as the cation exchange capacity, CEC and expressed in meq/100 g). The charge of the layer is not locally constant as it varies from layer to layer and must rather be considered as an average value over the whole crystal. Proportionally, even if a small part of the charge balancing cations is located on the external crystallite surface, the majority of these exchangeable cations is located inside the galleries. When the hydrated cations are ion-exchanged with organic cations such as more bulky alkyammoniums, it usually results in a larger interlayer spacing. Table 2 Chemical structure of commonly used 2:1 phyllosilicatesa 2:1 Phyllosilicate

General formula

Montmorillonite Hectorite Saponite

Mx(Al4ÿxMgx)Si8O20(OH)4 Mx(Mg6ÿxLix)Si8O20(OH)4 MxMg6(Si8ÿxAlx)O20(OH)4

a

Mˆmonovalent cation; xˆdegree of isomorphous substitution (between 0.5 and 1.3).

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Fig. 2. Alkyl chain aggregation in layered silicates: (a) lateral monolayer; (b) lateral bilayer; (c) paraffin-type monolayer and (d) paraffin-type bilayer (reproduced from [21] with permission).

In order to describe the structure of the interlayer in organoclays, one has to know that, as the negative charge originates in the silicate layer, the cationic head group of the alkylammonium molecule preferentially resides at the layer surface, leaving the organic tail radiating away from the surface. In a given temperature range, two parameters then define the equilibrium layer spacing: the cation exchange capacity of the layered silicate, driving the packing of the chains, and the chain length of organic tail(s). According to X-ray diffraction (XRD) data, the organic chains have been long thought to lie either parallel to the silicate layer, forming mono or bilayers or, depending on the packing density and the chain length, to radiate away from the surface, forming mono or even bimolecular tilted `paraffinic' arrangement [20] as shown in Fig. 2. A more realistic description has been proposed by Vaia et al. [21], based on FTIR experiments. By monitoring frequency shifts of the asymmetric CH2 stretching and bending vibrations, they found that the intercalated chains exist in states with varying degrees of order. In general, as the interlayer packing density or the chain length decreases (or the temperature increases), the intercalated chains adopt a more disordered, liquid-like structure resulting from an increase in the gauche/trans conformer ratio. When the available surface area per molecule is within a certain range, the chains are not completely disordered but retain some orientational order similar to that in the liquid crystalline state (Fig. 3). This interpretation has been recently confirmed by molecular dynamics simulations where a strong layering behavior with a disordered liquid-like arrangement has been found, that can evolve towards a more ordered arrangement by increasing the chain length [22]. As the chain length

Fig. 3. Alkyl chain aggregation models: (a) short alkyl chains: isolated molecules, lateral monolayer; (b) intermediate chain lengths: in-plane disorder and interdigitation to form quasi bilayers and (c) longer chain length: increased interlayer order, liquid crystalline-type environment (reproduced from [21] with permission).

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Fig. 4. Scheme of different types of composite arising from the interaction of layered silicates and polymers: (a) phaseseparated microcomposite; (b) intercalated nanocomposite and (c) exfoliated nanocomposite.

increases, the interlayer structure appears to evolve in a stepwise fashion, from a disordered to more ordered monolayer then `jumping' to a more disordered pseudo-bilayer. 2.2. Nanocomposite structures Depending on the nature of the components used (layered silicate, organic cation and polymer matrix) and the method of preparation, three main types of composites may be obtained when a layered clay is associated with a polymer (Fig. 4). When the polymer is unable to intercalate between the silicate sheets, a phase separated composite (Fig. 4a) is obtained, whose properties stay in the same range as traditional microcomposites. Beyond this classical family of composites, two types of nanocomposites can be recovered. Intercalated structure (Fig. 4b) in which a single (and sometimes more than one) extended polymer chain is intercalated between the silicate layers resulting in a well ordered multilayer morphology built up with alternating polymeric and inorganic layers. When the silicate layers are completely and uniformly dispersed in a continuous polymer matrix, an exfoliated or delaminated structure is obtained (Fig. 4c). Two complementary techniques are used in order to characterize those structures. XRD is used to identify intercalated structures. In such nanocomposites, the repetitive multilayer structure is well preserved, allowing the interlayer spacing to be determined. The intercalation of the polymer chains usually increases the interlayer spacing, in comparison with the spacing of the organoclay used (Fig. 5), leading to a shift of the diffraction peak towards lower angle values (angle and layer spacing values being related through the Bragg's relation: lˆ2d sin y, where l corresponds to the wave length of the X-ray radiation used in the diffraction experiment, d the spacing between diffractional lattice planes and y is the measured diffraction angle or glancing angle). As far as exfoliated structure is concerned, no more diffraction peaks are visible in the XRD diffractograms either because of a much too large spacing between the layers (i.e. exceeding 8 nm in the case of ordered exfoliated structure) or because the nanocomposite does not present ordering

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Fig. 5. XRD patterns of: (a) phase separated microcomposite (organo-modified fluorohectorite in a HDPE matrix); (b) intercalated nanocomposite (same organomodified fluorohectorite in a PS matrix) and (c) exfoliated nanocomposite (the same organo-modified fluorohectorite in a silicone rubber matrix) (reproduced from [19] with permission).

anymore. In the latter case, transmission electronic spectroscopy (TEM) is used to characterize the nanocomposite morphology. Fig. 6 shows the TEM micrographs obtained for an intercalated and an exfoliated nanocomposite. Besides these two well defined structures, other intermediate organizations can exist presenting both intercalation and exfoliation. In this case, a broadening of the diffraction peak is often observed and one must rely on TEM observation to define the overall structure.

Fig. 6. TEM micrographs of poly(styrene)-based nanocomposites: (a) intercalated nanocomposite (reproduced from [60] with permission) and (b) exfoliated nanocomposite (reproduced from [61] with permission).

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3. Nanocomposite preparation Several strategies have been considered to prepare polymer-layered silicate nanocomposites. They include four main processes [23]:  Exfoliation±adsorption: The layered silicate is exfoliated into single layers using a solvent in which the polymer (or a prepolymer in case of insoluble polymers such as polyimide) is soluble. It is well known that such layered silicates, owing to the weak forces that stack the layers together can be easily dispersed in an adequate solvent. The polymer then adsorbs onto the delaminated sheets and when the solvent is evaporated (or the mixture precipitated), the sheets reassemble, sandwiching the polymer to form, in the best case, an ordered multilayer structure. Under this process are also gathered the nanocomposites obtained through emulsion polymerization where the layered silicate is dispersed in the aqueous phase.  In situ intercalative polymerization: In this technique, the layered silicate is swollen within the liquid monomer (or a monomer solution) so as the polymer formation can occur in between the intercalated sheets. Polymerization can be initiated either by heat or radiation, by the diffusion of a suitable initiator or by an organic initiator or catalyst fixed through cationic exchange inside the interlayer before the swelling step by the monomer.  Melt intercalation: The layered silicate is mixed with the polymer matrix in the molten state. Under these conditions and if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer space and form either an intercalated or an exfoliated nanocomposite. In this technique, no solvent is required.  Template synthesis: This technique, where the silicates are formed in situ in an aqueous solution containing the polymer and the silicate building blocks has been widely used for the synthesis of double-layer hydroxide-based nanocomposites [14,15] but is far less developed for layered silicates. In this technique, based on self-assembly forces, the polymer aids the nucleation and growth of the inorganic host crystals and gets trapped within the layers as they grow. The following sections review the four aforementioned preparation techniques, that will be illustrated with representative examples. 3.1. Exfoliation±adsorption 3.1.1. Exfoliation±adsorption from polymers in solution This technique has been widely used with water-soluble polymers to produce intercalated nanocomposites [24,25] based on poly(vinyl alcohol) (PVOH) [26,27], poly(ethylene oxide) (PEO) [27±31], poly(vinylpyrrolidone) (PVPyr) [32] or poly(acrylic acid) (PAA) [31]. When polymeric aqueous solutions are added to dispersions of fully delaminated sodium layered silicates, the strong interaction existing between the hydrosoluble macromolecules and the silicate layers often trigger the reaggregation of the layers as it occurs for PVPyr [32] or PEO [27]. In the presence of PVOH, the layers remain in colloidal distribution [27]. In the wet state or after mild drying (air drying), the silicate layers are distributed and embedded in the so-obtained PVOH gel. This state actually corresponds to a true nanocomposite hybrid material. However, more intense drying of the PVOH gel in vaccuo causes part of the silicate layers to reaggregate and intercalated species are formed. This is indicated by a basal spacing of 1.36 nm, corresponding to the intercalation of a polymeric monolayer in between the silicate layers. In fact, sterical constraints from the PVOH matrix impede reaggregation of all the silicate layers and some of them remain exfoliated [33]. Interestingly, polymers intercalation using the so-called exfoliation±adsorption technique can also be performed in

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Fig. 7. X-ray diffractograms of modified montmorillonite, HDPE and nitrile copolymer composite systems (* Ð a high degree of crystallinity of the HDPE is evident) (reproduced from [35] with permission).

organic solvents. PEO has been successfully intercalated in sodium montmorillonite and sodium hectorite by dispersion in acetonitrile [34], allowing to stoichiometrically incorporate one or two polymer chains in between the silicate layers and increasing the intersheet spacing from 0.98 to 1.36 and 1.71 nm, respectively. Study of the chain conformation using two-dimensional double-quantum NMR on 13 C enriched PEO intercalated in sodium hectorite [10] reveals that the conformation of the `±OC±CO±' bonds of PEO is 905% gauche, inducing constraints on the chain conformation in the interlayer. Jeon and coworkers [35] have investigated this technique in attempts to produce nanocomposites with nitrile-based copolymer (Barex 210 E) and polyethylene-based polymer. These nanocomposites were filled with sodium montmorillonite previously modified by a protonated dodecylamine as the organic cation. Upon treatment, the interlayer spacing increased from 11.8 to Ê , attesting for effective cation exchange. In order to produce the nitrile copolymer-based 16.5 A nanocomposite, the copolymer was dissolved in dimethylformamide in the presence of 15 wt.% modified clay. After solvent evaporation in a vacuum oven at 808C for 24 h the film recovered was characterized by both XRD and TEM. XRD reveals a broad diffraction peak that has been shifted Ê , see Fig. 7). The large broadening of the peak may towards a higher interlayer spacing (21.5 A indicate that partial exfoliation has occurred, as corroborated by TEM analysis (Fig. 8) where both stacked (intercalated) and isolated (exfoliated) silicate layers can be observed. High density polyethylene (HDPE)-based nanocomposite has been produced by using a similar technique where the polyolefinic chains were dissolved in a mix of xylene and benzonitrile (80:20 wt.%) with 20 wt.% modified clay dispersed within. The composite material was then recovered by precipitation from tetrahydrofuran (THF) followed by several washings with THF. As seen in Fig. 7, the small increase in the interlayer spacing could account for some intercalation even if the TEM observation let only show small stacks of flake-like particles. Even if conducted under similar experimental conditions, these two syntheses indicate that the exfoliation±adsorption technique can provide quite different results much depending upon the polymer matrix. In other words, it does mean that for every type of polymer, one has to find the right layered clay, organic modifier and solvent(s). Ogata et al. applied the exfoliation±adsorption method for the production of poly(lactide) (PLA) [36] and poly(e-caprolactone) (PCL) biodegradable nanocomposites [37] using montmorillonite modified with distearyldimethylammonium cations. The composites were prepared by dissolving either PLA or PCL in hot chloroform in presence of a given amount of the modified clay, then

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Fig. 8. TEM micrograph of the nitrile copolymer filled with 15 wt.% of organo-modified montmorillonite. Iˆindividual silicate layer and Sˆstacked silicate layers (reproduced from [35] with permission).

vaporizing the solvent to obtain homogeneous films. However, under those conditions, it was found that no intercalation took place in the presence whatever polyester. It is worth to point out that the organo-modified clay rather formed a remarkable geometric structure in the filled polymers where tactoids consisting of several silicate monolayers form a superstructure in the thickness direction of the film. Such structural features have been found on one hand to substantially increase the Young's modulus of the PLA-based composites (which is almost doubled with 5 wt.% of organo-modified clay) and on the other hand, to enhance both storage and loss moduli determined by dynamic mechanical analysis (DMA) carried out on the organoclay-filled PCL. 3.1.2. Exfoliation±adsorption from prepolymers in solution Some polymeric materials such as poly(imides) or some conjugated polymers have the particular property of being infusible and insoluble in organic solvents. Therefore, the only possible route to produce nanocomposites with these types of polymers consists in using soluble polymeric precursors that can be intercalated in the layered silicate and then thermally or chemically converted in the desired polymer. This has been successfully achieved by using the exfoliation±adsorption process. The Toyota Research group has been the first to use this method to produce poly(imide) (PI) nanocomposites [38]. The polyimide±montmorillonite nanocomposite has been synthesized by mixing in dimethylacetamide a modified montmorillonite with the poly(imide) precursor, that is a poly(amic acid) obtained from the step polymerization of 4,40 -diaminodiphenyl ether with pyromellitic dianhydride. The organo-modified montmorillonite was prepared by previous intercalation with dodecylammonium hydrochloride. After elimination of the solvent, an organoclay filled poly(amic acid) film was recovered, which was thermally treated up to 3008C in order to trigger the imidization reaction and to produce the poly(imide) nanocomposite. The XRD patterns of these filled PI films do not show any diffraction peak typical of an intercalated morphology leading the authors to conclude to the formation of an exfoliated structure and explaining the excellent gas

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Table 3 Nature, CEC and length of the layered silicates used in the synthesis of polyimide as presented in [39] Layered silicate ‡

Hectorite Na Saponite Na‡ Montmorillonite Na‡ Synthetic mica Na‡ a

CEC (meq/100 g)

Ê )a Length of dispersed particles (A

55 100 119 119

460 1650 2180 12300

Longer particle dimension as determined by TEM observation.

barrier properties of the resulting films (see Section 4.3). This experiment has been extended to other layered silicates (hectorite, saponite and synthetic mica) with different aspect ratios [39] (Table 3). X-ray diffractograms of the obtained polyimide-based nanocomposites again show no noticeable peak indicating an exfoliated structure for the montmorillonite and the synthetic mica. Ê is observed, indicating that For both hectorite and saponite, a broad peak, centered on a value of 15 A for those layered silicates, polymer intercalation occurs probably together with some exfoliation. For saponite, the measured interlayer spacing is even smaller than the value measured for the starting Ê ) suggesting that the organic cation could have been expelled from organically modified clay (18 A the clay interlayer during imidization reaction. The same phenomenon has been observed by Lan et al. [40] when studying the effect of the chain length of the organic cation in the preparation of PI nanocomposites by the same synthetic methodology. Starting from sodium montmorillonite with a CEC of 92 meq/100 g and various protonated linear primary alkylamine, i.e. CH3±(CH2)nÿ1NH3‡ (where nˆ4, 8, 12, 16 and 18), they obtained, after imidization by curing at 3008C, composites Ê , independently of the chain length of the intercalated showing the same interlayer spacing of 13.2 A alkylammonium cation (Fig. 9).

Fig. 9. XRD patterns of polymer/CH3(CH2)nÿ1NH3‡ modified montmorillonite composites (clay loading: 10 wt.%): (A) air-dried poly(amic acid) films and (B) poly(imide) films cured at 3008C (reproduced from [40] with permission).

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This value again corresponds to the intercalation of the PI chains adopting a flattened conformation within the galleries, and the removal of the organic cation out of the interlayer. As the same is achieved at much lower temperature (drying at 1008C), this structural change cannot be related to the thermal degradation of the onium ion still stable in this temperature range. Further experimental evidence for the cation eviction out of the clay interlayers comes from the fact that the Ê interlayer spacing does not change when the material is heated up to at 4508C, a temperature 13.2 A where alkylammonium cations are usually degraded. No clear explanation for the expulsion of the alkylammonium ions has been reached. The presence of organically modified layered silicate such as montmorillonite previously modified with protonated p-phenylene diamine has also shown to improve the kinetics of the imidization reaction, allowing for a reduction of both the imidization temperature and reaction time [41]. The activation energy of the imidization reaction (based on a first order kinetics), monitored by FTIR spectroscopy, is shown to drop down by ca. 20% in presence of 7 wt.% of organoclay. A reaction mechanism has been tentatively proposed, involving the modified silicate layers as active partners in the imidization process (Fig. 10). It has to be noted that a complete exfoliated structure has been observed for those nanomaterials by both XRD and TEM. Conjugated polymers are another family of polymers prone to be intercalated through this twostep technique. Oriakhi et al. [42] have elegantly shown that the exfoliation±adsorption method could be explored to prepare nanocomposites with poly(p-phenylenevinylene) (PPV) as the continuous polymeric matrix. The polymer precursor to be intercalated was the poly(p-xylenylene dimethylsulfonium bromide) (PXDMS). The PXMDS-montmorillonite layered nanocomposite was accordingly prepared by reaction of a colloidal dispersion of Na-montmorillonite with an aqueous solution of PXMDS at 08C. As the precursor bears two cationic sites, it readily intercalates between the montmorillonite sheets by cationic exchange. The precursor is then chemically transformed into PPV by a base-mediated elimination of dimethylsulfide and HBr. This is achieved by stirring the crude product with 20% ethanolic NaOH solution at ambient temperature for 48 h. XRD patterns Ê , respectively. before and after chemical conversion give interlayer spacing of 15.1 and 14.6 A Ê ), it indicates a gallery Compared to the initial interlayer spacing of the Na-montmorillonite (9.6 A Ê expansion of, respectively, 5.5 and 5.0 A, consistent with the expected dimensions for a polymer

Fig. 10. A possible reaction mechanism for involving silicate layers in the imidization process (reproduced from [41] with permission).

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monolayer with the phenyl rings oriented perpendicular to the layered silicate surfaces. This gallery expansion together with the absence of any XRD peak related to the interlayer spacing of Namontmorillonite attest for the formation of an intercalated nanocomposite. The presence of a weak absorption bandpeak in the sp3 C±H region of the FTIR spectrum (2890 cmÿ1), however, suggests that the elimination of the dimethylsulfonium groups is not quantitative and that the incorporated polymer could be in fact a copolymer of PXMDS and PPV. 3.1.3. Exfoliation±adsorption by emulsion polymerization Emulsion polymerization has been also studied in order to promote the intercalation of water insoluble polymers within Na-montmorillonite that is well known to readily delaminate in water [43±45]. Poly(methyl methacrylate) (PMMA) was first tested by this method [43]. The emulsion polymerization was thus carried out in water in the presence of various amounts of the layered silicate. The previously distilled methyl methacrylate monomer (MMA) was dispersed in the aqueous phase with the aid of sodium lauryl sulfate as a surfactant. Polymerization was conducted at 708C for 12 h by using potassium persulfate as the free-radical initiator. The obtained latex is then coagulated with an aluminum sulfate solution, filtered and dried under reduced pressure. The obtained composites were extracted with hot toluene for 5 days by means of Soxhlet extraction. Contents of intercalated polymer were determined for both extracted and non-extracted materials and are given in Table 4 together with the molecular weights and polydispersities of the extracted polymers. These results demonstrate that part of the PMMA chains stay immobilized onto and/or inside the layered silicates and cannot be extracted. This is further confirmed by FTIR of the extracted composite that shows the absorption bands typical of PMMA chains. It can be observed that the relative content of clay does not substantially modify the PMMA molecular weights (Mw), the value of which is quite comparable to the Mw of PMMA polymerized in absence of clay (entry 1, Table 4). Clearly, the presence of layered silicates does not seem to perturb the free-radical polymerization. Ê is Intercalation is evidenced by XRD where an increase in the interlayer distance of about 5.5 A observed for both PMMA 10, 20 and 30. This increase relatively well correlates with the thickness of the polymer chain in its extended form. DSC data obtained for the extracted nanocomposites does not show any glass transition, in accordance with what is usually observed for intercalated polymers. Ion±dipole interactions are believed to be the driving force for the immobilization of the organic polymer chains lying flat onto the layered silicate surface. The same methodology has been also applied to produce montmorillonite intercalated with poly(styrene) (PS) [45]. The nanocomposite Table 4 Montmorillonite feed ratios, PMMA contents in non-extracted and extracted composites, average molecular weights and polydispersities of extracted PMMAs Sample PMMA PMMA10 PMMA20 PMMA30 PMMA40 PMMA50 a b

Feed ratio of MMA/clay (g/g)

PMMA content (wt.%)a Non-extracted

Extracted

100/0 100/10 100/20 100/30 100/40 100/50

± 87.4 79.3 60.4 58.6 46.1

± 58.7 49.6 33.4 22.8 18.4

b

As determined by TGA. Composite recovered after Soxhlet extraction in toluene for 5 days.

Mn10ÿ3 (g/mol)

Mw10ÿ3 (g/mol)

Mw/Mn

23 44 60 63 82 38

160 250 200 150 390 290

6.6 5.8 3.4 2.4 4.8 7.6

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Table 5 Montmorillonite feed ratios, PS contents in non-extracted and extracted composites and interlayer distances in PS-based nanocomposites obtained by emulsion polymerization Sample MMT PS5 PS10 PS20 PS30 a b

Feed ratio styrene/MMT (g/g) 0/100 95/5 90/10 80/20 70/30

Ê) Interlayer distance (A

PS content (wt.%)a Non-extracted

Extracted

± 88.7 82.8 75.4 65.9

± 45.6 33.9 28.8 21.7

b

9.8 15.5 14.6 13.8 12.4

As determined by TGA. Composite recovered after Soxhlet extraction in toluene for 5 days.

syntheses were essentially comparable to MMA emulsion polymerization except that the Namontmorillonite was sonicated prior to polymerization. Results are presented in Table 5. Here again, a true intercalated structure is formed with an interlayer spacing that changes with the PS content. The interlayer distance slightly decreases at higher montmorillonite feed ratios. Similarly to MMA polymerization, the molecular weight of the PS recovered fraction does not seem to be affected by the presence of the dispersed clay. DSC thermograms also attest for the intercalation of PS in an extended form as no more glass transition can be observed in the extracted nanocomposites. However, in the case of filled PS, the stabilization of the PS chains in the interlayer cannot be accounted for by ion±dipole interactions anymore. Rather, the authors propose the cooperative formation of ion-induced dipole interactions. Note finally a report on the formation of epoxy-montmorillonite intercalated composites by emulsion polymerization [44]. Again an increase Ê is observed. But contrary to observations achieved for both of the interlayer spacing of about 6 A PMMA and PS, the epoxy content in the extracted composites appears to increase with the montmorillonite content, indicating that the layered silicates possibly participate in the polymerization reaction. 3.2. In situ intercalative polymerization 3.2.1. Thermoplastic nanocomposites Many interlamellar polymerization reactions were studied in the 1960s and the 1970s using layered silicates (see [25,46] and references therein) but it is with the work initiated by the Toyota research team [47,48] that the study of polymer-layered silicate nanocomposites came into vogue about 10 years ago. They studied the ability of Na-montmorillonite organically modified by protonated a,o-aminoacid (‡H3N±(CH2)nÿ1±COOH, with nˆ2, 3, 4, 5, 6, 8, 11, 12, 18) to be swollen by the e-caprolactam monomer (melting temperatureˆ708C) at 1008C and subsequently to initiate its ring opening polymerization to obtain nylon-6-based nanocomposites [48,49]. A clear difference occurs in the swelling behavior between the montmorillonite with relatively short (n