Hectorite polymer-clay nanocomposites

Figure 16.24 shows the schematic representation of dispersed clay particles in a polymer matrix. Conventionally dispersed clay has aggregated layers in face-to-face form. Intercalated clay composites have one or more layers of polymer inserted into the clay host gallery. Exfoliated polymer/clay nanocomposites have low clay content (lower than intercalated clay composites which have clay content -50%). It was found that 1 wt% exfoliated clay such as hectorite, montmorillonite, or fluorohectorite increases the tensile modulus of epoxy resin by 50-65%. ... 

Therefore, polymer/clay nanocomposites can be deflned as a new class of composites with polymer matrices in which the dispersed phase is the silicate constituted by particles that have at least one of the dimensions at nanometer level. One of the components is the matrix, in which the particles of the second material are dispersed. The most used mineral particles in these nanocomposites are smectitic clays (montmorillonite, saponite, and hectorite), having their particles lamellae morphology with sides at micrometer level and thickness around one nanometer (Alexandre and Dubois, 2000 Esteves, Barros-Timmons, and Trindade, 2004). 

Nanoclay fillers are categorized as platelet-like nanoclays or layered silicates and tubular nanoclays in terms of filler shape. With the configuration of two tetrahedral sheets of silicate and a sheet layer of octahedral alumina, platelet-like nanoclays or phyllosilicates are formed, which include smectite, mica, vermiculite, and chlorite. In particular, smectite clays are widely employed with further subcategories of MMT, saponite, hectorite, and nontronite. The typical MMT clays are regarded as one of the most effective nanofillers used in polymer/clay nanocomposites due to their low material cost and easy intercalation and modification (Triantafillidis et al., 2002). On the other hand, the fundamental structure of tubular nanoclays contains an aluminum hydroxide layer and a silicate hydroxide layer. They are also known as dio-ctahedral minerals with two different types of halloysite nanotubes (HNTs) and imo-golite nanotubes (INTs). Notwithstanding their material role as clay minerals, these two types of tubular nanoclays resemble the hollow tubular structure of carbon nanotubes (CNTs). In this section, three different types of clay nanofillers, namely MMTs, HNTs, and INTs are reviewed in detail along with the development of clay modification. 

Generally, polymer-clay nanocomposites use smectic-type clays as fillers, such as hectorite, MMT, kaolin or synthetic mica, aU minerals with a layered structure They are of great industrial value because of their high... 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

This system does not increase the carbon monoxide or soot produced during the combustion, as many commercial FRs do [233]. Other polymer silicate nanocomposites based on a variety of polymers, such as polystyrene, epoxy and polyesters, have been prepared recently by melt intercalation [236]. A direct synthesis of PVA-clay (hectorite) complexes in water solution (hydrothermal crystallization) was reported [237]. It was assumed that the driving force of this phenomenon, at least kinetically, can be described in terms of a simple diffusion reaction of polymers/monomers into clay-layered structures. 

Xiong, L., Hu, X., Liu, X., Tong, Z., 2008. Network chain density and relaxation of in situ synthesized polyaciylamide/hectorite clay nanocomposite hydrogels with ultrahigh tensi-hility. Polymer 49, 5064—5071. 

A sixth article on modulus performance of nylon 6-clay nanocomposites was also published in 1995 in the Journal of Applied Polymer Science [25] by Toyota Research. The Young s modulus of nylon 6-clay nanocomposites was prepared with montmorillonite, saponite, a synthetic hectorite (Laponite RD), and synthetic mica. The in situ polymerization procedure with 12-aminolauric acid exchanged onto the minerals was utilized to prepare the nylon 6 nanocomposites. The montmorillonite polymer nanocomposite has the largest modulus, followed by mica, saponite, and, the lowest value, Laponite RD. 

On the other hand, Bhattacharya et al. have reported the plasticization effect of organically modified layered silicates on dynamic mechanical properties [13]. In this work, nanocomposites of SBR have been prepared using various nanofillers like modified and unmodified montmorillonite, SP, hectorite etc. It has been observed that the Tg shifts to lower temperature in all the nanocomposites, except for systems from hectorite and NA. This is due to the fact that clay layers form capillaries parallel to each other as they become oriented in a particular direction. Due to wall slippage of the unattached polymer through these capillaries, the Tg is lowered, which could be even more in the absence of organo-modifiers [13]. A similar type of plasticization effect is also noted in the case of the low... 

Moncada, E., Quijada, R., and Retuert, J. 2007. Comparative effect of metallocene and Ziegler-Natta polypropylene on the exfoliation of montmorillonite and hectorite clays to obtain nanocomposites. Journal of Applied Polymer Science 103 698-706. 

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

Montmorillonite, hectorite, and saponite are the most widely investigated clays. Their structure and formula are shown in Figure 9.1 and Table 9.1, respectively. Montmorillonite is a very popular choice for nanocomposites because of its small particle size (<2 pm) and hence the easy diffusion of polymer into the clay particles. They also have high aspect ratios (10-2000) and high swelling capacity, which are essential for efficient intercalation of the polymer. The physical mixture of a polymer and a clay may not form a nanocomposite. This situation is analogous to that in a polymer blend, and separation into discrete phases takes place in most cases. Pristine clay usually contains hydrated Na+ or K+ ions. Obviously,... 

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