Exfoliation nanoclays

Paul and Robeson et al. [139] have published an extremely informative review on the properties of exfoliated nanoclay-based nanocomposites. These have dominated the polymer literature, but there are a large number of other significant areas of current and emerging interest. This review details the technology involved with exfoliated clay-based nanocomposites and also includes other important areas, such as barrier properties, flammability resistance, biomedical applications, electrical/electronic/optoelectronic applications, and fuel cell interests. The important question of the nanoeffect of nanoparticles or fiber inclusion relative to their large-scale counterparts is addressed relative to crystallization and glass transition behavior. Other polymer (and composite)-based properties derive benefits from the nanoscale filler or fiber addition, and fhese questions are addressed. 

Das et al., demonstrates, an approach of compatibilization between polychloroprene (CR) and ethylene propylene diene monomer rubber (EPDM) by using nanoclay (NC) as a compatibilizer and, simultaneously, as a very strong reinforcing nano-fiUer. With the incorporation of less than 9 wt.% nanoclay, the dynamic storage modulus above the glass transition region of such a blend increases from 2 MPa to 54 MPa. This tremendous reinforcing as weU as the compatibilization effect of the nanoclay was understood by thermodynamically driven preferential framework-like accumulation of exfoliated nanoclay platelets in the phase... 

The experimental data utilizing the measured aspect ratio should be the basis for comparison. An example of this type of composite is data reported by Zhong et al. [4] on a nanocomposite of ethylene vinyl acetate (EVA) with montmorillonite. Eigure 4.7 contains the experimental relative permeabilities for this composite compared to the curves predicted by the tortuous path model utilizing aspect ratios of 200 and 50. The 200 aspect ratio represents the fully exfoliated nanoclay and the 50 represents... 

TGA also provides indirect information about the amount of exfoliated nanoclay in the nanocomposite [59, 61]. 

Subambient thermal volatilization analysis can also be used to probe the effects of physical fillers in silicone materials. In 2008 Lewicki et al. [51] studied the degradation profiles and product speciation of a series of montmorillonite clay filled silicone elastomers which had been characterized using SATVA. Shown in Figure 13.17 are a series of TVA thermal degradation profiles for the non-oxidative degradation of a bimodal-condensation-cured silicone matrix, filled with 0-8 wt% of organically modified montmorillonite (0-MMT) exfoliated nanoclay platelets. 

The results suggest that the thermal stability improves with higher loading till 6 phr of nanoclay and this improvement is attributed to the barrier effect of the exfoliated and the intercalated nanoclay particles. 

When a solvent diffuses across a neat polymer, it must travel the thickness of the sample (do). When the same solvent diffuses through a nanocomposite film with nanoclays, its path length is increased by the distance it must travel around each clay layer it strikes. According to Lan et al. [99] the path length of a gas molecule diffusing through an exfoliated nanocomposite is... 

This is a highly polar polymer and crystalline due to the presence of amide linkages. To achieve effective intercalation and exfoliation, the nanoclay has to be modified with some functional polar group. Most commonly, amino acid treatment is done for the nanoclays. Nanocomposites have been prepared using in situ polymerization [85] and melt-intercalation methods [113-117]. Crystallization behavior [118-122], mechanical [123,124], thermal, and barrier properties, and kinetic study [125,126] have been carried out. Nylon-based nanocomposites are now being produced commercially. 

In subsequent discussion, we will demonstrate the use and interpretation of some of these techniques. Figure 2a shows typical XRD traces of nanocomposite systems of styrene butadiene rubber (SBR) containing unmodified and modified nanoclay, describing an exfoliated and intercalated nanocomposite [5]. photographs of these systems are also given in the same figure (Fig. 2b, c). In the present case, the information obtained from both the techniques is complimentary. 

The same group carried out ATRP of EA in bulk at 90°C in the presence of organically modified nanoclay as an additive. They found remarkable enhancement in the rate of polymerization as compared with the ATRP of EA without nanoclay. Interestingly, the resulting nanocomposites had exfoliated clay particles, as evident from WAXD and studies [80]. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

The lowering of die swell values has a direct consequence on the improvement of processability. It is apparent that the processability improves with the incorporation of the unmodified and the modified nanofillers. Figure lOa-c show the SEM micrographs of the surface of the extrudates at a particular shear rate of 61.2 s 1 for the unfilled and the nanoclay-filled 23SBR systems. The surface smoothness increases on addition of the unmodified filler, and further improves with the incorporation of the modified filler. This has been again attributed to the improved rubber-clay interaction in the exfoliated nanocomposites. 

The X-ray diffraction peaks observed in the range of 3°-10° for the modified clays disappear in the rubber nanocomposites. photographs show predominantly exfoliation of the clays in the range of 12 4 nm in the BIMS. Consequently, excellent improvement in mechanical properties like tensile strength, elongation at break, and modulus is observed by the incorporation of the nanoclays in the BIMS. Maiti and Bhowmick have also studied the effect of solution concentration (5, 10, 15, 20, and 25 wt%) on the properties of fluorocarbon clay nanocomposites [64]. They noticed that optimum properties are achieved at 20 wt% solution. At the optimized solution concentration, they also prepared rubber/clay nanocomposites by a solution mixing process using fluoroelastomer and different nanoclays (namely NA, 10A, 20A, and 30B) and the effect of these nanoclays on the mechanical properties of the nanocomposites has been reported, as shown in Table 4 [93]. 

It is a common phenomenon that the intercalated-exfoliated clay coexists in the bulk and in the interface of a blend. Previous studies of polymer blend-clay systems usually show that the clay resides either at the interface [81] or in the bulk [82]. The simultaneous existence of clay layers in the interface and bulk allows two functions to be attributed to the nanoclay particles one as a compatibilizer because the clays are being accumulated at the interface, and the other as a nanofiller that can reinforce the rubber polymer and subsequently improve the mechanical properties of the compound. The firm existence of the exfoliated clay layers and an interconnected chain-like structure at the interface of CR and EPDM (as evident from Fig. 42a, b) surely affects the interfacial energy between CR and EPDM, and these arrangements seem to enhance the compatibility between the two rubbers. 

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