Properties of Nanocomposites

Dimensional stabiUty is critical in many applications. For example, if the layers of a microelectronic chip have different thermal or environmental dimensional stabiUties, then residual stresses can develop and cause premature failure. Poor dimensional stabiUty can also cause warping or other changes in shape that affect the function of a material. Nanocomposites provide methods for improving both thermal and environmental dimensional stability. The possible mechanism by which nanofillers can affect the coefficient of thermal expansion (CTE) of a polymer has also been observed in traditional fillers. 

14 PS and PS/clay nanocomposites after dimension stability test. Clay loading is5wt.%forall nanocomposites. 

Jin and co-workers investigated thermal property of polymer-elay nanocomposites by TGA and cone calorimetry. The thermal stabiUty of the nanoeomposite is enhanced relative to that of virgin polystyrene and this is shown in Fig. 5.15. Typically, the onset temperature of the degradation is about 50 C higher for the nanocomposites than for virgin polystyrene. 

One invariably finds that nanocomposites have a much lower peak heat release rate (PHRR) than the virgin polymer. The peak heat release rate for polystyrene and the three nanocomposites are also shown graphically in Fig. 5.16. P16-3 means that the nanocompoite was formed using 3% of P16 clay with polystyrene. The peak heat release rate falls as the amount of clay was increased. The suggested mechanism by which clay nanocomposites function involves the formation of a char that serves as a barrier to both mass and energy transport. It is reasonable that as the fraction of clay increases, the amount of char that can be formed increases and the rate at which heat is released is decreased. There has 

16 Peak heat release rates for polystyrene and the three nanocomposites.  

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T. Czujko, R.A. Vatin, Z. Wronski, Z. Zaranski, T. Durejko, Synthesis and hydrogen desorption properties of nanocomposite magnesium hydride with sodium borohydride (MgH + NaBH ) , J. Alloys Compd. All (2007) 291-299. 

The necessity of dispersion of nanofillers for enhancement of properties of nanocomposites has been well-documented. Literature describes various techniques used for preparation of the rubber-based nanocomposites [5]. These processes have their inherent pros and cons, but seldom have these issues been quantitatively documented or the processes combined together to synergize and overcome the failings of a technique. 

Carbon materials provide electrical conduction through the pi bonding system that exists between adjacent carbon atoms in the graphite structure [182]. Electrical properties of nanocomposites based on conducting nanofillers such as EG [183-187], CNTs [188-190], and CNFs [191], dispersed in insulating polymer matrix have found widespread applications in industrial sectors. 

Chauve et al. [253] utilized the same technique to examine the reinforcing effects of cellulose whiskers in EVA copolymer nanocomposites. It was shown that larger energy is needed to separate polar EVA copolymers from cellulose than for the nonpolar ethylene homopolymer. The elastomeric properties in the presence of spherical nanoparticles were studied by Sen et al. [254] utilizing Monte Carlo simulations on polypropylene matrix. They found that the presence of the nanofillers, due to their effect on chain conformation, significantly affected the elastomeric properties of nanocomposites. 

This kinetically dependent mechanism provides a means to develop a nanocomposite microstructure with the particles (or the majority of the particles) occluded within the matrix grains. On the other hand, occlusion can be prevented, for the most part, if glass-forming impurity elements are not introduced into the material during the processing stage. As we will see in the next section, the position of the particles (i.e. occluded or at grain boundaries) can influence the microstructurally dependent properties of nanocomposites. 

Alternative processing methods also offer the potential to control the microstructure and final properties of nanocomposites. Both self-propagating high-temperature sintering and spark plasma sintering offer means to obtain metastable yet dense nanocomposites. Subsequent heat treatments can then be used to approach equilibrium microstructures, where the properties will be a function of the heat treatment temperature and time. In this way a variety of microstructures, and thus variations of the composite properties, can become available. 

The galvanomagnetic properties of nanocomposites and their conductivity, in particular, near the percolation transition can be described within the two-component model developed for the case by Efros and Shklovskii [73] on the basis of Dykhne theory [74]. This theory was developed just for the description of materials containing two different components with sharp distinction for conductivity values (Dykhne media) and describes well the concentration dependence of the effective conductivity in the case of the classical grain sizes and so in the absence of quantum effects. However, even if quantum effects do not play an essential role, the adequate description of the conductivity dependence on temperature has not been elaborated till now. The reason is that numerous experimental results for granules, with the metal contents x[Pg.612]

The properties of nanocomposite systems, whose microstructures aim at reproducing real systems, have been examined in various numerical modelling studies [127, 128], In general, the essential features of the hysteresis cycles may be satisfactory reproduced. In particular, soft layer reversal is quantitatively accounted for, which is expected for reversible phenomena. By contrast, the calculated high-field irreversible reversal of the hard phase magnetization is not reproduced in general. Such discrepancy illustrates the already mentioned difficulty to describe irreversible processes. 

S. Advani, Processing and Properties of Nanocomposites, World Scientific, 2006. 

To understand the effect of the molecular weight of PLLA on the properties of nanocomposite, Chen et al. prepared different MWCNT-g-PLLA hybrids with varying molecular weights of PLLA, viz., 1000, 3000, 11000, and 15000. For detailed experimental conditions, refer to Chen et al. (64). The extent of PLLA grafting on MWCNTs was examined by both SEM and TEM (Figure 9.3 and Figure 9.4, respectively). TEM studies revealed that the degree... 

Crucially, structure of CNTs and polymers plays a key role on mechanical properties and load-transfer of nanocomposites. Efficient load-transfer is only possible when adequate interfacial bonding strength is available. Interfacial failure may compromise the reinforcement effect and then the full potential of CNTs may not be realized (11). Therefore, it is of great importance to understand the effect of molecular structure, interfacial structure and morphology characteristics on the tensile properties of nanocomposite materials. 

Some physieal-mechanieal properties of nanocomposites produced by in situ method, and also produeed via melt blending polyethyleneterephthalate with organomodified montmorillonite (nalchikit-M), educed from bentonite clay of Gerpegezh field (Russia, KBR) and from eommercial clay bentonite-128. 

Size monitoring and design of optical properties of nanocomposites... 

The work on nanocomposites utilizing novolacs and resols can be considered preliminary and promising. The very subtle differences between novolacs and resols appear to have the potential to contribute to significantly different nanocomposite systems. The distinct separation of the silicate layers may provide the basis for the properties of nanocomposites prepared as novolacs or resols. 

Sadhu, S. Bhowmick, A.K. Preparation and properties of nanocomposites based on acrylonitrile-butadiene rubber, styrene-butadiene rubber, and polybutadiene rubber. J. Polym. Sci. B Polym. Phys. 2004, 42 (9), 1573-1585. 

Electron behavior, optical properties, catalytic properties, conductivity, and magnetic properties of nanocomposites were discussed in an extensive review pa-per. Complementary use of electron paramagnetic resonance and nuclear magnetic resonance helped to understand chain mobility in nanocomposites obtained from poly(ethylene oxide) encapped with triethoxy silicon. This nanocomposite is composed of PEO chains attached to silica clusters. It was found that chain fragments close to the silica clusters have hindered mobility due to the reduction of local free volume. The length of this hindered segment is estimated as three ethylene oxide units. 

Haraguchi K, Takehisa T, Simon F (2002) Effects of clay content on the properties of nanocomposite hydrogels composed of poly (N-isopropylacrlamide) and clay. Macromolecules 35 10162-10171... 

Haraguchi K, Farnworth R, Ohbayashi A, Takehisa T (2003) Compositional effects on mechanical properties of nanocomposite hydrogels composed of poly(N,N -dimethylacrylamide) and clay. Macromolecules 36 5732-5741... 

As the local electric field in the particles is enhanced at the SPR, the metal nonlinear optical response can be amplified as compared to the bulk solid one. Moreover, the intrinsic nonlinear properties of metals may themselves be modified by effects linked with electronic confinement. These interesting features have led an increasing number of people to devote their research to the study of nonlinear optical properties of nanocomposite media for about two decades. Tire third-order nonlinear response known as optical Kerr effect have been particularly investigated, both theoretically and experimentally. It results in the linear variation of both the refraction index and the absorption coefficient as a function of light intensity. These effects are usually measured by techniques employing pulsed lasers. 

In the first part, emphasis will be put on the linear optical properties of dielectric media doped with noble metal nanoparticles. Indeed, the study of the linear response is definitely needed to further explore the nonlinear one. We will then introduce the fundamentals of the theoretical tools required to understand why and how people inquire into the third-order nonlinear properties of nanocomposite materials. In the second part, experimental results will be presented by first examining the different nonlinear optical phenomena which have been observed in these media. We will then focus on the nanoparticle intrinsic nonlinear susceptibility before analysing the influence of the main morphological factors on the nonlinear optical response. The dependence of the latter on laser characteristics will finally be investigated, as well as the crucial role played by different thermal effects. 

As for the linear optical response, different approaches have been proposed to describe the nonlinear optical properties of nanocomposite media. Nevertheless, a few general principles can be identified. First, each component of such a medium possesses its own susceptibility however, as the typical structure size is much smaller than the wavelength, the observable result of light interaction with the medium is different from a simple combination of the individual responses of the separated constituents (again, we do not treat the case of spatially-resolved studies of die optical response). One is then again led to introduce the concept of effective medium, extended to the case of nonlinear optical properties. 

This formulation can be also obtained by other approaches [81, 83, 87, 88]. It is extensively used in the literature to analyse the nonlinear optical properties of nanocomposite materials determined experimentally. 

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