Exfoliated nanocomposite systems

Most nanocomposite researchers obdurately believe that the preparation of a completely exfoliated structure is the ultimate target for better overall properties. However, these significant improvements are not observed in every nanocomposite system, including systems where the silicate layers are near to exfoliated [29]. While, from the barrier property standpoint, the development of exfoliated nanocomposites is always preferred, Nylon 6-based nanocomposite systems are completely different from other nanocomposite systems, as discussed [3,8]. 

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 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. 

Reinforcement of the epoxy-clay nanocomposites was also dependent on the clay loading as shown in Fig. 29. Thus for epoxy CH3(CH2)17NH3+-MMT nanocomposite system, the tensile strength and modulus increased nearly linearly with clay loading. More than a tenfold increase in strength and modulus could be realized by the addition of ca. 15 wt % of the exfoliated organoclay. 

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]. 

Bisphenol-A-based epoxy with a poly(amido amine) hardener system cured Mesuaferrea L. seed oil-based hyperbranched polyurethane (HBPU)/ clay nanocomposites obtained by an ex situ solution technique, was also reported. The partially exfoliated nanocomposites showed a two-fold improvement in adhesive strength and scratch hardness, 10 MPa increments in tensile strength and thermostability at 112°C with little effect on impact resistance, bending and elongation at break compared to a pristine epoxy-modified HBPU system. However, similar epoxy-cured Mesua ferrea L. seed oil-based HBPU/clay nanocomposites exhibited a two-fold increase in tensile strength, a 6°C increase in melting point and thermostability at 111°C after nanocomposite formation using an in situ technique. An excellent shape recovery of about 96-99% was observed for the nanocomposites. The above observations confirm that the performance characteristics of nanocomposites are influenced by their preparation technique. 

The ultimate clay nanocomposite is formed when individual clay plates are completely dispersed into a polymer matrix. This type of composite yields the maximum improvement in properties. This complete dispersion is normally referred to as full exfoliation. In many cases the composites reported in the literature are intercalated or partially exfoliated. Intercalated systems are characterized by insertion of polymer between plates of clay with retention of well-defined spacing distance between the plates. This spacing is called the gallery spacing and is determined... 

The key question regarding the structure of a thermoset/clay nanocomposite system is whether a true nanocomposite has formed or not. If not, the material is comparable to a conventional filled microcomposite. Generally, wide angle X-ray diffraction (WAXD) analysis and transmission electron microscopy (TEM) are used to elucidate the structure of a nanocomposite. Due to its easiness and availability, WAXD is most commonly used to establish the nanocomposite structure [30, 31]. By monitoring the position, shape, and intensity of the basal reflections from the distributed silicate layers, the nanocomposite structure (intercalated or exfoliated) may be identified. The X-ray technique is often applied to identify nanocomposite structures through Bragg s relation, which is given below ... 

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