Nanocomposites mechanical behavior, factors

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Figure 6.5 Dynamic mechanical behavior of polyurethane nanocomposite in the presence of various types of nanoparticles, (a and b) Storage modulus and damping factor in the presence of alumina [84], (c and d) storage modulus and damping factor in the presence of CNTs [86],...

Surface modification of the nanofiller will be a challenge in the preparation of new types of rubber nanocomposites. Furthermore, the modification of various nanofillers using other nanofiUer systems will be a key to obtaining materials with designed properties. The interactions at the interface between the nanofillers and the matrix are one of the most important factors connected with the production of the new improved polymeric nanocomposites. Understanding the modification of the nanofiller in the polymer matrix, as well as the mechanical behavior in dynamic mode, leads to the possibility of producing new rubber nanocomposites, for example, for tire applications, where enhanced rolling resistance would improve traction [17]. 

All these aspects have been overviewed in the following sections, preceded by a summary of the main factors affecting the mechanical behavior of nanocomposites and some modeling approaches that have been proposed. 

Compared to microcomposites, where the mechanical behavior is mainly a function of the characteristics of filler and matrix and their respective concentrations, generally speaking, two major characteristics define the mechanical performance of polymer nanocomposites (8) the nanoscale size and geometry of the fillers (aspect ratio) and their dispersion into the polymer matrix, and the interaction between the polymer chains and said nanofillers (interphase). This section highlights the influence of these two factors on the mechanical behavior of polymer nanocomposites. 

Factors Affecting the Mechanical Behavior of Nanocomposites 1119 Fiber... 

However, when compared with pure copolymer, the highly stretched nanocomposite exhibited a higher amount of unoriented crystals, a lower degree of crystal orientation, and a higher amount of 7-crystals. This behavior indicated that polymer crystals in the filled nanocomposite experienced a reduced load, suggesting an effective load transfer from the matrix to MCNF. At elevated temperatures, the presence of MCNF resulted in a thermally stable physically cross-linked network, which facilitated strain-induced crystallization and led to a remarkable improvement in the mechanical properties. For example, the toughness of the 10 wt% nanocomposite was found to increase by a factor of 150 times at 55°C. Although nanofillers... 

The group of Gu Z. in the 2009, reported the behavior of styrene butadiene/ rubber/organo-bentonite nanocomposite prepared from latex dispersion, content was lower than 12 mass%. The results showed were that presence of organo-bentonite in the nanocoposite affects direct in the thermo stability, mechanical properties and swelling behavior, which was attribute to the good barrier properties of the dispersed nanoparticles. The dispersion is an important factor that can affect various properties such as thermal stability [81]. 

Thus, according to works reported in the literature about PHA/CNT nanocomposites, we can conclude that the introduction of CNT in the PHA matrix can change and improve the nanocomposite properties. Thermal behavior, electrical conductivity, mechanical properties, and mainly the crystallization of PHA were the most affected properties by the addition CNT. Obviously, the properties of PHA/ CNT depends on several factors, such as the characteristics of PHA matrix and CNT, amount of CNT in the nanocomposite, method for producing nanocomposite, and functionahzation of CNT. The choice and control of all these factors should be made through the choice of properties to be reached. 

As for the behavior of viscosity, this parameter not only characterizes the physical viscosity of the film, but also determines the effective viscosity that considers all the other mechanisms of losses (scattering on inhomogeneities of the nanocomposite film, electric losses, contact losses, etc.). The diversity of all these factors hinders the unambiguous interpretation of the behavior of the effective viscosity coefficients in the materials under study. 

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