Polymer clay nanocomposites formation

Y. Kim and J. L. White. Modeling of polymer-clay nanocomposite formation. Journal of Applied Polymer Science, 101 (2006), 1657-1663. 

Incorporation of modified clays into thermosetting resins, and particularly in epoxy35 or unsaturated polyester resins, in order to improve thermal stability or flame retardancy, has been reported.36 A thermogravimetric study of polyester-clay nanocomposites has shown that addition of nanoclays lowers the decomposition temperature and thermal stability of a standard resin up to 600°C. But, above this temperature, the trend is reversed in a region where a charring residue is formed. Char formation seems not as important as compared with other polymer-clay nanocomposite structures. Nazare et al.37 have studied the combination of APP and ammonium-modified MMT (Cloisite 10A, 15A, 25A, and 30B). The diluent used for polyester resin was methyl methacrylate (MMA). The... 

As pointed out before, the polymer may be functionalized with polar groups to enhance compatibility with the modified clay. Bellucci et al. [92] have reported that the formation of polymer-clay nanocomposites depends mostly on the polymer properties, clay characteristics, and type of organic modifier. 

Polymer/clay nanocomposites (PCN)are a new class of nanocomposite that makes use of clay materials, which are cheap and well known fillers for polymer materials. The research on polymer-clay intercalation has been reported before 1980s [10]. However, these works were not taken in the history of polymer/clay nanocomposites as these did not result in a dramatic improvement in the physical and engineering properties of the polymers. The researchers at Toyota, Japan demonstrated for the first time that clay (so called filler) can do miracles in 1993 [11,12]. While searching for a lightweight material for automotive applications they successfully developed a nylon-6/clay nanocomposite, which resnlts in a dramatic improvement in properties compared to the pristine polymer. Subseqnently, the technique was extended to thermoset resins leading to the formation of thermoset nanocomposites. 

Figure 5.1 The proposed mechanisms for the formation of polymer/clay nanocomposites by the driving force concept, (a) Sodium-type smectite clays, (b) Intercalation of catalyst/ initiator and modified agent leads to inter-layer spacing expansion, (c) Monomers/ oligomers were driven by the catalyst/initiator to swell into the gallery of clay lamellar.

Figure 5.14 Formation of a tortuous path in polymer-clay nanocomposites.

As presented schematically in Figure 16, a second way to prepare polymer-clay nanocomposites via in situ polymerization consists of intercalation of the monomer (or a precursor of the monoma-) in the form of a cation and then later addition of an initiator to induce/polymaization. Thus, the direct exchange of the interlayer cations of smectites by anilinium cations, followed by oxidation with (NH4)2S20g, could be an alternative procedure to reach the formation of PANI/clay nanocomposites. In this case, the expaimental conditions allow the direct formation of PANl as a conducting ema-aldine salt (152). 

Nano-plates are generally naturally occurring layered materials such as layered silicates (montmorillonite plates, a type of clay) which is dispersed within polymers for nanocomposite formation (Hackman and Hollaway, 2006 Tran et al. (2006) initially man-made materials such as silicate acids were used (Wang et al., 1996). The objective in a nanocomposite produced from plate-like fillers is to disperse the latter in a polymer to take advantage of... 

In general, the dispersion of clay particles in a polymer matrix can result in the formation of three general types of composite materials (Figure 1). Conventional composites contain clay tactoids with the layers aggregated in unintercalated face - face form. The clay tactoids are simply dispersed as a segregated phase. Intercalated clay composites are intercalation compounds of definite structure formed by the insertion of one or more molecular layers of polymer into the clay host galleries and the properties usually resemble those of the ceramic host. In contrast, exfoliated polymer-clay nanocomposites have a low clay content, a monolithic structure, a separation between layers that depends on the polymer content of the composite, and properties that reflect those of the nano-confmed polymer. 

The model illustrated in Figure 2 summarizes the overall mechanism for formation of epoxy polymer - clay nanocomposites. Upon solvation of the organoclay by the epoxide monomers, the gallery cations reorient from their initial monolayer, lateral bilayer, or inclined paraffin structure to a perpendicular orientation with epoxy molecules inserted between the onium ions. A related reorientation of alkylammonium ions has been observed previously for e-caprolactam intercalated clay intermediates formed in the synthesis of Nylon-6 -exfoliated clay nanocomposites (9). Thus, the ability of the onium ion chains to reorient into a vertical position in order to optimize solvation interactions with the monomer may be a general prerequisite for pre-loading the clay galleries with sufficient monomer to achieve layer exfoliation upon intragallery polymerization. 

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%. Montmorillonite was used in a two stage process of nanocomposite formation. In the first step, montmorillonite was intercalated with vinyl monomer and then used in the second step to insert polystyrene by in situ polymerization. 

It seems that further research on polymer-clay nanocomposites would focus on the synthesis of organic modifiers with enhanced thermal stability and providing additional functionalities for formation of interfacial bonds and interaction through, for example, graft polymerization. 

It appears that the flame retardant mechanism for the polyamide/clay nanocomposites discussed in this chapter relates to the formation of a continuous protective carbonaceous char layer that acts as a heat shield. The mechanism is similar to that for other kinds of polymer/clay nanocomposites. 

It should be pointed out that many polymer/clay nanocomposite materials finally result in the formation of a mixture of exfoliated and intercalated structures [45], The above types of nanocomposites are schematically compared with an immiscible system in Fig. 3. 

A wide variety of polymer/clay nanocomposites can be synthesized by in situ coordinadmi polymerization methods, which gives the advantage of controlled molecular weight of the polymer nanocomposite. In the case of late transition metal-based coordination polymerization, the process is tolerant to the polar groups or to a little moisture in the clay or catalytic system. This moisture sensitivity can also be overcomed by treating the excess MAO or by in situ formation of MAO using TEA, TMA, or TIBA on the surface of clay, clay-MgCl2, or clay-silica hybrid material. 

---