Polymer-clay nanocomposites characteristics

Thermogravimetric analyses have confirmed the general enhancement in the stability of the various polymer-clay nanocomposites relative to the base polymers and also provided useful information on the extent of the polymer loading in such composites. The thermogravimetric characteristics of several typical systems are highlighted below. 

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. 

In the past decade, clay-based polymer nanocomposites have attracted considerable attention from the research field and in various applications. This is due to the capacity of clay to improve nanocomposite properties and the strong synergistic effects between the polymer and the silicate platelets on both a molecular and nanometric scale [2,3], Polymer-clay nanocomposites have several advantages (a) they are lighter in weight than the same polymers filled with other types of fillers (b) they have enhanced flame retardance and thermal stability and (c) they exhibit enhanced barrier properties. This chapter focuses on the polymer clay-based nanocomposites, their background, specific characteristics, synthesis, applications and advantages over the other composites. 

Unlike polymer-clay nanocomposites, in rubber-clay nanocomposites complete exfoliation of clay layers results in disappearance of the diffraction maxima in their XRD patterns. However, this can also occur due to other reasons, like extremely low concentration of clay materials in the composites, crystal defects, etc. The majority of the reports on rubber-clay nanocomposites display the intercalated or swollen nature of the clay structures. The presence of the basal reflections in the XRD patterns of such type of nanocomposites indicates that the clay crystal structure is not destroyed completely. But, shifting of their positions to lower 26 values is interpreted as an expansion of the interlayer region by the macromolecular rubber chains. Besides, broadening of the characteristic reflections in nanocomposites is often related to the defects in the crystal layer stacking caused by the interlayer polymeric species. 

Over the last two decades, the polymer-clay nanocomposites have been widely investigated as materials exhibiting superior properties, such as high modulus, increased thermal stability, and good gas-barrier characteristics [18-20]. Their development started from the nylon/clay hybrid found by Kamigaito et al. [21, 22] and has extended to various combinations of monomer/nanofiller using more... 

Several organic compounds, especially imidazolium, pyridinium, and phosphonium salts, were shown to have noticeably higher thermal stability, and they could be applied as compatibilizers for polymer/clay nanocomposites, providing a good level of dispersion and improved properties. Additional functionalization of surfactants is most often aimed at improvement of miscibility or synergistic enhancement of nanocomposite properties however, any change in the chemical constitution of an organomodifier involves simultaneous alteration of its thermal characteristic. 

The improved mechanical properties in polymer/clay nanocomposites are associated with particle geometries of high aspect ratio and the resulting high interfacial area per unit volume. In this work, an elastic Finite Element analysis of idealized clay platelet configurations was carried out identifying which platelet characteristics are important in producing the property enhancement. 

Ren, H.-Y., Zhu, M., Haraguchi, K., 2011. Characteristic swelling-deswelling of polymer/clay nanocomposite gels. Macromolecules 44, 8516—8526. 

Kim, J.W. Jang, L.W. Choi, H.J. Jhon, M.S. Physical and electroresponsive characteristics of the intercalated styrene-acrylonitrile copolymer/ clay nanocomposite under applied electric fields. J. Appl. Polym. Sci. 2003, 89, 821-827. 

The rheological behavior of these materials is still far from being fully understood but relationships between their rheology and the degree of exfoliation of the nanoparticles have been reported [73]. An increase in the steady shear flow viscosity with the clay content has been reported for most systems [62, 74], while in some cases, viscosity decreases with low clay loading [46, 75]. Another important characteristic of exfoliated nanocomposites is the loss of the complex viscosity Newtonian plateau in oscillatory shear flow [76-80]. Transient experiments have also been used to study the rheological response of polymer nanocomposites. The degree of exfoliation is associated with the amplitude of stress overshoots in start-up experiment [81]. Two main modes of relaxation have been observed in the stress relaxation (step shear) test, namely, a fast mode associated with the polymer matrix and a slow mode associated with the polymer-clay network [60]. The presence of a clay-polymer network has also been evidenced by Cole-Cole plots [82]. 

H. Sh. Xia, M. Song, Characteristic length of dynamic glass transition based on polymer/clay intercalated nanocomposites. Thermo. Acta 2005, 429, 1. 

Layered silicates have positive ions present on the surface which makes them hydrophilic and therefore incompatible with many polymers as they are generally hydrophobic [22]. To make bio-nanocomposites, it is necessary to modify the layered silicates by replacing the interlayer cations with cations bearing long alkyl chains, such as alkylphosphonium or alkylammonium [23]. Alkyl ammonium cations are employed to lower the surface energy and enhance the wetting characteristics with the polymer [24]. By incorporating polymers or monomers into the interlayer, this can lead to the formation of a bio-nanocomposite. Different structures can exist for polymer/clay bio-nanocomposites and they are as follows (see Fig. 2) ... 

Certain polymers of interest for potential use as ion-conductors, such as PEO, largely studied in layered clay nanocomposites [229, 232-235, 237], could also be assembled with sepiolite. Thus, PEO can be assembled with sepiolite either from solution in acetonitrile or from the melt by microwave irradiation, giving rise to nanocomposites in which a partial penetration of the polymer chains takes place [17, 237]. Further attempts recently reported focus on the modification of the physical characteristics of PEO-sepiolite composites by controlled modulation of the silicate interphase afforded by the incorporation of polyethylene glycol and other additives [246]. However, to our knowledge, no studies to date have addressed the use of PEO-fibrous clays as ion-conducting systems. 

J Polym Sci, Part B Polym Phys 46(10) 979-987 Chen B, Evans JRGG (2005) Thermoplastic starch-clay nanocomposites and their characteristics. Carbohydr Polym 61(4) 455 63... 

---