Exfoliation-adsorption

Scheme of the preparation of an exfoliated nanocomposite by the "exfoliation-adsorption" mechanism. 

Figure 5 illustrates schematically the exfoliation-adsorption method. Layered host sheets are delaminated into single-sheet particles and then dispersed as a colloidal suspension. In the 1980s, the layered metal disulfides M0S2 and WS2 were observed to form such single-sheet colloidal suspensions when their lithiated forms were rapidly hydrolyzed [87-89]. More recently, a layered sheet surface-... 

FIGURE 5 Schematic representation of the exfoliation/adsorption method. 

Until 2003, Chen s [28], Qu s [29-31], and Hu s [32] groups independently reported nanocomposites with polymeric matrices for the first time the. In Hsueh and Chen s work, exfoUated polyimide/LDH was prepared by in situ polymerization of a mixture of aminobenzoate-modified Mg-Al LDH and polyamic acid (polyimide precursor) in N,N-dimethylactamide [28]. In other work, Chen and Qu successfully synthesized exfoliated polyethylene-g-maleic anhydride (PE-g-MA)/LDH nanocomposites by refluxing in a nonpolar xylene solution of PE-g-MA [29,30]. Then, Li et al. prepared polyfmethyl methacrylate) (PMMA)/MgAl LDH by exfoliation/adsorption with acetone as cosolvent [32]. Since then, polymer/LDH nanocomposites have attracted extensive interest. The wide variety of polymers used for nanocomposite preparation include polyethylene (PE) [29, 30, 33 9], polystyrene (PS) [48, 50-58], poly(propylene carbonate) [59], poly(3-hydroxybutyrate) [60-62], poly(vinyl chloride) [63], syndiotactic polystyrene [64], polyurethane [65], poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] [66], polypropylene (PP) [48, 67-70], nylon 6 [9,71,72], ethylene vinyl acetate copolymer (EVA) [73-77], poly(L-lactide) [78], poly(ethylene terephthalate) [79, 80], poly(caprolactone) [81], poly(p-dioxanone) [82], poly(vinyl alcohol) [83], PMMA [32,47, 48, 57, 84-93], poly(2-hydroxyethyl methacrylate) [94], poly(styrene-co-methyl methacrylate) [95], polyimide [28], and epoxy [96-98]. These nanocomposites often exhibit enhanced mechanical, thermal, optical, and electrical properties and flame retardancy. Among them, the thermal properties and flame retardancy are the most interesting and will be discussed in the following sections. 

These materials, unlike the other nanophase materials described in this chapter, are nano-sized in only one dimension and thereby act as nanoplatelets that sandwich polymer chains in composites. Mont-morillonite (MMT) is a well-characterized layered silicate that can be made hydrophobic through either ionic exchange or modification with organic surfactant molecules to aid in dispersion [5,23]. Polymer-layered silicates may be synthesized by exfoliation adsorption, in situ intercalative polymerization, and melt intercalation to yield three general types of polymer/clay nanocomposites. Intercalated structures are characterized as alternating polymer and siHcate layers in an ordered pattern with a periodic space between layers of a few nanometers [13], ExfoHated or delaminated structure occurs when silicate layers are uniformly distributed throughout the polymer matrix. In some cases, the polymer does not intercalate... 

Epoxy-clay nanocomposites are prepared by in situ polymerization with organo-modified clays. In this technique, the prepolymer intercalates between the layered crystals during swelling, a process in which heat, stirring, and/or sonication is applied to favor prepolymer diffusion into the clay gallery. Then polymerization can be initiated between the intercalated sheets either by heat, radiation, or a suitable initiator. A variant of in situ polymerization is the exfoliation-adsorption technique, in which the organo-modified clay is first dispersed in a solvent before starting the standard in situ polymerization. 

Epoxy-clay nanocomposites were studied extensively, but only ordered exfoliated nanocomposites were reported with in situ polymerization. Two recent works showed that disordered and highly exfoliated epoxy-clay nanocomposites can be prepared using an exfoliation-adsorption process." In this method the organo-modified clay is first dispersed in a solvent. It is well known that due to the weak forces that stack the layers together, layered silicates can easily be swelled and eventually delaminated in an adequate solvent. The polymer then adsorbs onto the delaminated sheets, and when the solvent is evaporated, a highly disordered structure is obtained before starting the standard in situ polymerization. 

FIGURE 9.6 XRD pattern of a clay-epoxy system (epoxy/DM-clay) before and after curing prepared by the exfoliation-adsorption process, and for comparison, bis(2-hydrox-yethyl)methyl tallow alkylammonium-modified MMT (Epoxy/clay-30B). (From Ref. 43, copyright 2004, American Chemical Society, with permission.)... 

Very effective flame retardant data were obtained with styrene-butyl acrylate copolymer/graphite oxide (St-BA/GO) nanocomposites. "- The GO was prepared by oxidation of expandable graphite, and the St-BA/GO nanocomposites (GO content of up to 4% mass fraction) were synthesized by exfoliation- adsorption of monomer followed by in situ emulsion polymerization. The distribution of the GO particles was examined by XRD, TEM, and electron diffraction exfoliated GO layers in crystalline structures were observed. The thermo-gravimetric analysis (TGA) data show a slight increase in thermal stability (up to 15°C with a 3% mass fraction of GO). Significant reduction in heat release rate by increasing GO content has been reported all nanocomposites reduced about 40% of total heat released compared with that of St-BA, as shown in Figures 10.14 and 10.15. 

Table 3 Potential Applications for Several Nanocomposites with LMC Hosts Prepared by the Exfoliation/Adsorption Method...

Three main methods have been developed to prepare polymer-clay nanocomposites.The first is exfoliation-adsorption the layered clay is exfoliated into a single layer in solvent in which the polymer can dissolve. Owing to the weak forces between the clay layers, the polymer enters between the delaminated layers and, when the solvent is evaporated, the nanocomposites are formed. The second is melt intercalation the polymer, in the molten state, is incorporated with the layered clay. The polymer then crawls into the interlayer space, causing the layers to separate and form nanocomposites. These two methods are physical and no chemical reaction occurs. The third method in situ intercalative polymerization the modified layered clay absorbs the liquid monomer and polymerization can be carried out between the intercalated sheets as shown in Figure 8.2. So far, many polymer-clay nanocomposites have been prepared through in situ intercalative polymerization initiated by chemical agents but very few by irradiation. ... 

The four main strategic processes for preparing polymer/layered silicate nanocomposites are exfoliation-adsorption, in situ intercalative polymerization, melt intercalation and template synthesis. ... 

Exfoliation-adsorption in emulsion with Na -montmorillonite, known to readily delaminate clay in water has been studied to promote intercalation of water insoluble polymers. Studies by Lee et al showed that only intercalated nanocomposites were obtained in systems PMMA, PS, ° and... 

Polyamide/layered silicate nanocomposites with improved mechanical properties can in general be prepared in three ways (i) in situ polymerization (ii) mixing in melt or (iii) exfoliation-adsorption. A successful research study initiated during the 1980s by Toyota Co. focused on a polymerization of ehydrolytic mechanism in the presence of an organically modified montmoriUonite (MMT) sodium ions present in the interlayers of MMT were substituted by pro-tonated co-aminolauric acid [90, 91]. e-Caprolactam, e-aminocaproic acid and H3PO4 were then added to a dispersion of MMT in water such that, when the polymerization had been carried out, a fully exfohated morphology of MMT was produced. 

---