Polymer-layered silica nanocomposites

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

Based on their dimensions, which are in the nanometre range, three types of fillers can be distinguished. Isodimensional nanoparticles (NP), such as spherical silica NP have three nanometric dimensions. Nanotubes or whiskers are stretched constructions in which two dimensions are in the nanometre range and the other dimension is larger. When only one dimension is in the nanometre range, the composites are termed polymer-layered crystal nanocomposites, and are obtained by the complete intercalation of the polymer inside the galleries of layered host crystals [2]. 

Broadband dielectric spectroscopy is a powerful tool to investigate polymeric systems (see [38]) including polymer-based nanocomposites with different nanofillers like silica [39], polyhedral oligomeric silsesquioxane (POSS) [40-42], and layered silica systems [43-47] just to mention a few. Recently, this method was applied to study the behavior of nanocomposites based on polyethylene and Al-Mg LDH (AlMg-LDH) [48]. The properties of nanocomposites are related to the small size of the filler and its dispersion on the nanometer scale. Besides this, the interfacial area between the nanoparticles and the matrix is crucial for the properties of nanocomposites. Because of the high surface-to-volume ratio of the nanoparticles, the volume fraction of the interfacial area is high. For polyolefin systems, this interfacial area might be accessible by dielectric spectroscopy because polyolefins are nonpolar and, therefore, the polymeric matrix is dielectrically invisible [48]. 

For NR nanocomposite filled with silica, it has always been known that the hydrophilicity-hydrophobicity issue is a challenge since silica is hydrophilic and NR is hydrophobic. The usual method to overcome this issue is by adding coupling agent. In 1987 Wu and coworkers introduced admicellar polymerization where a thin polymeric film will be formed on the silica s surface. This process yields a thin film of polymer on the silica which can further enhance the adhesion between the surfaces of silica and rubber. The steps involved in admicellar polymerization are outlined in Scheme 7.7. In principle, a bilayer of surfactant, i.e. the admicelle, is first formed on the surface of the silica. Monomer will then penetrate the admicelle, i.e. the adsolubilization of monomer. Upon addition of initiator to the reaction system, in situ polymerization occurs in the admicelles. Finally, the surfactant is removed by washing with water and an ultrathin polymer layer is formed on the surface of the silica. The polymerization of the monomer in the admicelles can be induced by thermal process, chemical initiators or radiation. ... 

Ke, Y.C., and Stroeve, P. (2005) Polymer-Layered Silicate and Silica Nanocomposites, Elsevier Sdence, Amsterdam, Holland. 

Based on this same general idea, colloidal dispersions of nanocomposite particles made from silica cores and polymeric overlayers have been successfully prepared using appropriate cationic radical initiators, as described in a recent Japanese patent [100]. Recently, Luna-Xavier et al. also demonstrated the successful formation of nanosize siHca/PMMA composite colloids using AIB A as cationic initiator and a nonionic polyoxyethylenic surfactant (NP30) [63,91,101]. Composite particles made from silica beads surrounded by small heterocoagulated PMMA latexes or a thin polymer layer were produced, depending on the size of the silica beads (Fig. 4.11). 

In the last few years, a lot of research in academic and industrial laboratories has focused on polyolefin nanocomposites because of their high potential as materials with novel properties [96]. The properties of the nanocomposites are not only influenced by the kind of filler but also by the microstructure of the polyolefins and the preparation process. Nanofillers commonly used are metal oxides, sulfides, silica and layered silica as well as fibers such as carbon nanotubes (CNT), carbon nanofibers (CNF), and polymer fibers [107-111],... 

Ke YC, Stroeve P. Polymer-layered silicate and silica nanocomposites. 1st ed. Amsterdam Elsevier B.V 2005. 

There is a wide variety of both synthetic and natural crystalline fillers that are able, under specific conditions, to influence the properties of PP. In PP nanocomposites, particles are dispersed on the nano-scale. " The incorporation of one-, two- and three-dimensional nanoparticles, e.g. layered clays, nanotubes, nanofibres, metal-containing nanoparticles, carbon black, etc. is used to prepare nanocomposite fibres. However, the preparation of nanofilled fibres offers several possibilities, such as the creation of nanocomposite fibres by dispersing of nanoparticles into polymer solutions, the polymer melt blending of nanoparticles, in situ prepared nanoparticles within a polymeric substrate (e.g. PP/silica nanocomposites prepared in situ via sol-gel reaction), " the intercalative polymerization of the monomer. 

Ke, Y.C., Stroeve, P. Polymer-Layered Sihcates tmd Silica Nanocomposites, 1st edn. Elsevier, Amsterdam (2005)... 

Lin and co-workers explained the superior mechanical properties of the PAN-Na-MMT-Si02 nanocomposites as follows The Na-MMT was exfoliated in the PAN nanocomposites (see Figure 11.7), and the enhancement of the storage modulus results from the delamination of the silicates in the PAN matrix and the strong interactions between the polymer chains and the Na-MMT layers. However, the reinforcement could be anisotropic because of the layer shape of the exfoliated Na-MMT layers. Cracks along the direction of the Na-MMT layers may not be resisted. However, in PAN-clay-silica nanocomposites, the well-dispersed Si02 particles could bridge the cracks that are not stopped by the Na-MMT layers. Therefore, the coexistence of the... 

PDMS nanocomposites with layered mica-type silicates were also reported.374 A two-step sol-gel process of the in situ precipitation of silica led to the development of siloxane-based nanocomposites with particularly high transparencies.3 5 Some unusual nanocomposites prepared by threading polymer chains through zeolites, mesoporous silica, or silica nanotubes were reviewed.3 6 Poly(4-vinylpyridine) nanocross-linked by octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane was reported.377... 

In the previous several years, various nanoparticles have been assembled into pairs to fabricate polymer nanocomposites, such as clay/silica (45), clay/carbon black (43), CNTs/clay (41,42), and CNTs/Titanium (38). Polymer/CNTs/clay ternary composite is one of most important multiphase systems with interesting synergistic effect, where sodium based montmorillonite (MMT) are the most commonly used layered clay. In this chapter, we will select some typical examples to demonstrate the importance and synergies of using CNTs and clay together in the preparation of polymer nanocomposites. 

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