Nanoparticles thermal mobility

Additional factors complicating the matter arise from the chemical and thermal (in)stability of metal nanoparticles as well as from the proposed mobility of surface bound capping agents (usually thiols). The chemical stability or instability of thiol-capped metal nanoparticles towards oxidation (i.e., oxidation of surface-bound thiols in air or in the presence of other oxidants) [70], towards halides [71], and towards alkaline metal ions has been studied by a number of groups [72] using TEM, UV-vis, NMR, as well as X-ray photoelectron spectroscopy (XPS) [73], and this collective work highlights the importance of determining nanoparticle purity. 

Considering, for instance, a system containing 1 nm thick plates, Ipm in diameter, the distance between plates would approach 10 nm at only 7 vol% of plates [217]. The behavior of PNCs can be rationalized as follows. The proliferation of internal inorganic-polymer interfaces means the majority of polymer chains reside near an inorganic surface. Since an interface restricts the conformations that polymer molecules can adopt, and since in PNCs with only a few volume percent of dispersed nanoparticles the entire matrix polymer may be considered as nanoscopically confined interfacial polymer, the restrictions in chain conformations will alter molecular mobility, relaxation behavior, and the consequent thermal transitions such as glass transition temperature of the composites [217]. 

Polymer matrix nanocomposite is the most important type of nanocomposite in which the performance of a polymer matrix can be enhanced by appropriately adding nanoparticulates to it [12] and good dispersion of the filler can be achieved [ 12]. A imiform dispersion of nanoparticles leads to a very large matrix/filler interfacial area, which changes the molecular mobility, the relaxation behavior and the consequent thermal and mechanical properties of the material. A polymer matrix could be reinforced by much stiffer nanoparticles [13,14] of ceramics, clays, or carbon nanotubes, etc. Recent research on thin films (thickness < 50 micrometer) made of polymer nanocomposites has resulted in a new and scalable synthesis technique increasing the facile incorporation of greater nanomaterial quantities [15]. Such advances will enable the future development of multifunctional small scale devices (i.e., sensors, actuators, medical equipment), which rely on polymer nanocomposites. 

Mixing of fillers into a polymer containing polar groups alters mechanical, thermal, and electrical properties of the polymer matrix, to a degree determined primarily by the nature and amount of the filler and the interaction between the two components. Several dielectric studies concentrated on monitoring composition-dependent perturbations in the sidechain (noncooperative) and the segmental (cooperative) relaxation dynamics of the thermoplastic component. The relaxational response of the polymeric matrix could sometimes be modified by a competition between several factors. One of them is the looser packing of the polymer chains, due to the presence of nanoparticles and interactions with them. This factor leads to increased free volume and enhanced molecular mobility. Another factor is the formation of a layer of modified polymer around the nanoparticles, which leads to decreased molecular mobility [e.g., see Mohomed et al. (2005)]. 

Thermal stability of elastomer can be assessed from the weight loss as a function of temperature. TGA thermograms of pure NR and its composites have been shown in Fig. 4 [65]. Conventional carbon black (CB) dispersed in NR caimot increase the thermal stability of NR while CNT dispersed in NR increase the degradation temperature significantly mainly due to thermal barrier of the nanoparticles. Another reason for this improvement might be due to restriction on the mobilization of rubber macromolecules in presence of CNT and carry out heat homogeneously and avoid heat concentration [88]. On the other hand, Falco et al. has shown similar thermal stability of composites to that of pure SBR with the addition of MWCNT [29]. 

The positive effect on thermal stability of polymers due to polymers can be attributed to (i) high surface volume, (ii) improving barrier properties due to the clay contribution to tortuosity path, (iii) reduction of polymer molecular mobility, (iv) low permeability and decrease in the rate of evolution of the formed volatile products, (v) formation of high-performance carbonaceous silicate char on the nanoparticles surface that insulate the underline material and slows the escape of volatile products generated during the decomposition, (vi) absorption of formed gases into clay plates. 

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