Conclusion

Nanocomposites represent a new prospective branch in the huge field of polymer materials science and technology. Though they have not yet been thoroughly studied, they show promising results in many aspects. To improve their competitiveness in the market, direct mixing of nano-particles and polymers by existing blending techniques proved to be a possible breakthrough as long as the fillers are suitably pretreated. In contrast to the composites filled with microparticles, nanoparticles might permit simultaneous enhancement of the modulus, strength, toughness, and thermal deformation temperature of polymers without affecting the thermoformability under a rather low filler loading [3]. In addition, the rheological performance of the melts of the nanosystem is similar to those of the unfilled versions. As a result, the density, cost, and gloss of the final products are nearly not influenced, providing broad applicability in a variety of processing and manufacturing.

To truly ascertain the present and future potential applications of polymer nanocomposites, considerably more research and development needs to be done. The following issues in particular have to be borne in mind:

• What are the theoretical upper limits of the mechanical properties of a polymer nanocomposite?

So far, the reported increment in strength and toughness of nanocomposites (including intercalated and exfoliated nanocomposites) relative to the properties of the matrices is less than one order of magnitude, regardless of how the composites were prepared. It strongly indicates the necessity of establishing a framework that is able to quantitatively predict the reinforcing effects of nanoparticles as well as of new compounding techniques. These are the prerequisites for further development of the nanocomposites.

• How does the interphase influence the mechanical performance of a polymer nanocomposite?

In fact, this question is closely related to the previous one. When the reinforcing fillers reach a nanoscopic size, the mechanical properties of the nanocomposites should be governed almost entirely by the interface, and the bulk properties play a secondary role [5]. As the nanofillers might have the same size as the segments of the surrounding polymer chains, the classic models are no longer valid.

• What kind of surface treatment is most effective for purposes of separating nanoparticle agglomerates and improving filler/matrix interfacial adhesion?

It seems to be somewhat difficult to simultaneously meet both demands. Actually, no matter what methods have been used to date, a uniform dispersion of nanoparticles on a nanometer scale is hardly obtained throughout the composites when it is done by the direct addition of premade nanoscale fillers to a thermoplastic matrix. Therefore, in addition to a continuous study of novel dispersion techniques and compounding facilities, research interests should also focus on developing surface treatments capable of giving full play to the reinforcing effect of nanoparticulate fillers, even though a homogeneous dispersion in the matrix is not available.

• What is the optimum phase morphology of nano-composites, and how can this morphology be obtained by choosing proper processing conditions (including surface treatments of nanoparticles and mixing variables)?

Self-organization and self-assembly have been successfully adopted in the construction of well-defined discrete nanosystems, with a physical oriented performance toward certain functions [19, 229, 230]. Comparatively speaking, the effect of the spatial distribution of nanoparticles or nanoparticle agglomerates on the mechanical properties of nanocomposites has not yet been understood, to say nothing of the effects of the adjustment of the microstructural arrangement.

• Are there any relationships between the species of the nanoparticles and the improvement in mechanical performance of the nanocomposites?

The study of PP-based nanocomposites demonstrated that SiO2 and CaCO3 have similar strengthening and toughening effects [153]. It seems that a proper interfacial bonding, which facilitates (i) stress transfer between the matrix and the nanoparticles and (ii) plastic deformation of the matrix, is a very important issue. If this is the case, however, it is somewhat difficult to understand the positive synergistic effect provided by ultrafine CaCO3 and talc [231], which cannot perform well when used individually in reinforcing HDPE.

• What are the applications for which only nano-composites are competent?

The superiority of a new material should be expressed by its unique technical importance. For example, Al2O3 is not a suitable filler in the microscale particulate form for wear-resisting composites because of its angularity and thus its propensity to damage the counterface. However, the material in the nanoscale particulate form has much lower angularity and, therefore, is not that abrasive [154].

To have an idea of the prospects of the nanocomposites, a brief survey of the history of the development of intrinsically conducting polymers might be enlightening. Much of the early research on conducting polymers was spurred by their potential application as replacements for existing metals and semiconductors. However, with many years of intensive development activity, it seems that many of the proposed applications for these materials were rather optimistic and have not yet become possible. With growing experience, a number of more realistic applications have now emerged that tend to exploit the novel features associated with conducting polymers rather than those properties that are readily obtainable in traditional materials [232]. Similarly, the future commercial success of polymer nanocomposites might also require a careful consideration of other technological and economic aspects besides their mechanical role.

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