Nanocomposite flocculated

Layered silicates, single-wall nanotubes (SWNTs), and other extreme aspect ratio, very thin (0.5-2 nm) nanoparticles, exhibit translational symmetry within the powder." Polymer/layered nanocomposites in general can be classified into three diflferent types, namely intercalated nanocomposites, flocculated nanocomposites and exfoliated nanocomposites (see Figure 6.4). ... 

The polymer-clay nanocomposites were synthesized by exfoliating a known amount of clay in water and then mixing this with a very small amount of polymer. Nanocomposite particles flocculated from the solution within 1-2 h. The mixture was stirred for 12-24 h and the nanocomposites were isolated by centrifugation followed by washing and drying for at least 24 h in vacuo. 

By using an exfoliation and flocculation technique, Prasad et al. were able to prepare a polyaniline/NbWOe hybrid material [74]. Dispersed nanosheets of HNbWOe were obtained by treating HNbWOe with tetrabutyl ammonium hydroxide. The suspension of the exfoliated layers was then added to an alcoholic solution of aniline, followed by addition of a few drops of 1M HCl, and overnight sonication in order to induce flocculation of the aniline-NbWOe nanocomposite. The nanocomposite was isolated by centrifugation, and dried. Treatment of the nanocomposite with O2 at 130 °C for several weeks resulted in the formation of intercalated PANI. 

Ray et al. [42] had successfully prepared PLA/MMT nanocomposites by simple melt extrusion of PLA wherein the silicate layers of the MMT were intercalated and randomly distributed in the matrix. They also showed that the incorporation of very small amounts of oligo( -caprolactone) (o-PCL) as compatibUizer in PLA led to a better parallel stacking of the silicate layers and also much stronger flocculation due to the hydroxylated edge-edge interaction of the silicate layers. The PLA/MMT nanocomposites also exhibited remarkable improvement of materials properties in both solid and melt states compared to the matrix without MMT. 

Styrene-butadiene copolymers are extremely important to the rubber industry. They are particularly important in tire manufacture. Styrene-butadiene polymer is produced by emulsion polymerization and solution polymerization. Most of the volume is by emulsion polymerization. This affords the opportunity to prepare polymer nanocomposites by several avenues. One can blend an aqueous dispersion of the nanoparticles with the styrene-butadiene latex before flocculation to produce the rubber crumb, disperse an organically treated nanoparticle in the styrene-butadiene solution polymer before the solvent is stripped from the polymer, disperse the organically treated nanoparticles into the monomers, or prepare the rubber nanocomposite in the traditional compounding approach. One finds all of these approaches in the literature. One also finds functional modifications of the styrene-butadiene polymer in the literature designed to improve the efficiency of the dispersion and interaction of the nanoparticles with the polymer. 

Parker et al. [49] altered the stabilization mechanism for styrene-butadiene latex prepared by emulsion polymerization from anionic to cationic so that they could get a spontaneous flocculation with aqueous montmorillonite slurry and the latex. Evaluation of the rubber nanocomposite prepared in this manner gave dramatic increases in modulus, strength, percent elongation, and decrease in hysteresis. 

Only limited success has been achieved in compounding organomontmoriUonites with styrene—butadiene rubber to prepare rubber nanocomposites [51], Knudson et al. [51] discovered that flocculation of the aqueous blend of styrene-butadiene latex and montmorOlrMiite gives an exfoliated clay-rubber nanocomposite. The approach offers the most convenient and effective method for the preparation of clay-styrene-butadiene rubber nanocomposites. 

Molesa et al. [61] compared compounded styrene-butadiene nanocomposites with polymer nanocomposites that were prepared by blending the latex with an aqueous dispersion of the montmoriUonite. The loading of the dispersed phase was at 10 phr. The initial results are consistent with the information found above. The flocculated rubber nanocomposite from the aqueous blend has superior strength properties when vulcanized and compared with the rubber nanocomposite prepared by compounding. MontmoriUonite that was organically treated demonstrated superior tensile strength when compared with rubber compounded with sUica. 

Zhang et al. [63] prepared styrene-butadiene nanocomposites by dispersing an aqueous dispersion of montmoril-lonite and latex and flocculating the dispersion with acid. The performance of the rubber nanocomposites were compared with clay, carbon black, and silica rubber composites prepared by standard compotmding methods. The montmoriUonite loadings for the rubber nanocomposite were up to 60 phr. The morphology of the rubber nanocomposites by transmission electron microscopy appears to indicate intercalated structures. The mechanical properties of the rubber nanocomposites were superior to all of the other additives up to about 30 phr. However, rebound resistance was inferior to all of the additives except sUica. The state of cure was not evaluated. 

Yaakub et al. [76], prepared natural mbber-clay nanocomposites by blending an aqueous dispersion of the clay with an aqueous dispersion of the mbber and flocculating the dispersion with other additives for cure. Hectorite and montmorillonite were evaluated. Mechanical properties improved as well as an increase in coefficient of friction and abrasion resistance. 

Intercalated and flocculated nanocomposites (Figure 23.6(d)). In this case, polymer chains are intercalated between two opposite layers that are flocculated due to hydroxylated edge-edge interaction . ... 

FIGURE 12.9 Schematic representation of the three broad classes of nanocomposites (2) flocculated, and (3) exfoliated. 

In summary, significant reduction in the peak heat release rate for the PA 6/clay nanocomposites was achieved by the formation of protective floccules on the polymer surface, which shielded the PA 6 from external thermal radiation and feedback from the flame. That is, the carbonaceous floccules acted as thermal insulation. 

A separation between the silicate platelets. The result is a well-ordered multilayer structure of alternating polymeric and inorganic layers, with a repeat distance between them. Sometimes the silicate layers in intercalated nanocomposites are flocculated due to hydroxylated edge-edge interaction of the silicate layers. 

The PLA layered silicate nanocomposites were prepared by adding small amounts of the compatibilizer to form the randomly distributed intercalated silicate layers. Simple melt extmsion of PLA and organically modified montmorillonite lead to better parallel stacking of silicate layers and much stronger flocculation due to hydroxylated edge-edge interactions of silicate layers and consequently improved mechanical and barrier properties, which makes it suitable for food packaging applicatioa Fmther, Bondeson et al. used melt extmsion to fabricate a transparent bio-based nanocomposite of 5 wt% cellulose nanowhiskers (CNW) and cellulose acetate butyrate (CAB), plasticized by triethyl citrate (TEC) (2007). 

A major application is the synthesis of high molecular weight water-soluble polymers (e.g., polymers and copolymers of acrylamide, acrylic acid, and its salts) for flocculants and tertiary oil recovery. Other uses are the synthesis of polyaniline/CdSe quantum dots composites [49], hybrid polyaniline/carbon nanotube nanocomposites [50], polyani-line-montmorillonite nanocomposites [51], or in reversible addition-fragmentation chain-transfer-controlled radical polymerization (RAFT) [52]. 

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