ABSTRACT
Nanocomposites, that is, multiphase materials resulting from combination of a bulk matrix and one or more nanodimensional phases, play a key role in the development of new materials and technologies (Ajayan et al. 2003, and references therein). In general, nanocomposites can be classi˜ed in three different categories, on the basis of their main bulk phase, the so-called “matrix,” which can be made of metals, ceramics, or polymers. In particular, polymer matrix nanocomposites are showing an ever-increasing extension toward manifold areas of materials science and technology, with applications in materials for transportation, energy, health, packaging, information and communication, etc. In many cases, the interest in these materials is due not only to their structural but also to their functional properties. In nanocomposites, the synergetic combination of bulk phase and nanophase can result in signi˜cant improvements of both chemical and physical properties. According to
8.1 Introduction .................................................................................................. 163 8.2 Nano˜llers..................................................................................................... 168
8.2.1 Gold Nanoparticles as a New Class of Reinforcing Nano˜llers ....... 168 8.2.2 New Perspectives for “Traditional” Reinforcing Fillers ................... 171
8.3 Functional Nanophase and Molecular Moieties ........................................... 172 8.3.1 Inorganic Catalysts for Self-Healing Reactions ............................... 172 8.3.2 Nanoparticles for Magnetic Field-Triggered Self-Healing .............. 179 8.3.3 Nanophases for Electrically Triggered Self-Healing ........................ 183 8.3.4 Nanophases for Light-Triggered Self-Healing .................................. 186
8.4 Conclusions and Perspectives ....................................................................... 192 References .............................................................................................................. 193
the de˜nitions given by Vaia and Wagner (2004), nanophases can mainly operate in two ways. In the simplest and most frequently observed cases, nanophases play just as nanoscale ˜llers for polymer matrices, yielding a direct enhancement of some properties in comparison with the corresponding un˜lled matrices or macrocomposites. Moreover, nanophases can provide novel, unique properties, which can give rise to entirely new materials, just like supramolecular assemblies may exhibit new functionalities with respect to the single molecular moieties which they are made of. In the latter case, nanocomposites are indicated as “polymer/inorganic hybrids” or “molecular composites.” Whatever the type of nanocomposite is, developing strategies to prevent damages or repair them should be considered a mandatory task to accomplish in order to take full advantage of their potential. In this context, the term “damage” is not limited to indicate a physical alteration of the structure, but is more generally extended to all the changes that can irreversibly jeopardize either functionalities or performances of these materials. In general, prevention of detrimental effects can rely on different strategies, which can be classi˜ed, on the basis of the mechanisms involved in the protection process, as passive or active (Fischer 2010). The former strategy makes use of protecting agents or coating layers as passive barriers against chemical, photo-, thermal and mechanical degradation. This route is on/off type, since damages can be only prevented, but not repaired, once they have occurred. Moreover, passive protection can fail when damage-triggering defects are already present in the original materials. This limitation is due to the absence of any feedback systems that can detect damage (which is often much localized) before it reaches a critical extent, and intervene to repair it. Nowadays, passive protection is still the most widely used strategy, due to their universal applicability. On the other hand, systems including mechanisms of self-repair are indicated as “active.” In this case, structural and functional damages can be detected and repaired thanks to different active agents that can trigger self-healing in response either to external stimuli or directly to damage itself. Therefore, self-healing materials are generally distinguished between nonautonomous (or stimuli-assisted) and autonomous systems (Hager et al. 2010). Nonautonomous self-healing can be induced and controlled by various parameters, such as heat, light, mechanical forces, chemical reactions, pH, etc. On the other hand, autonomous self-healing does not need any external intervention, as the damage itself triggers repair processes. Autonomous systems behave as smart, adaptive materials. However, up to now only few examples of autonomic selfhealing materials have been successfully reported, and the self-repairing mechanisms have been addressed only to restoring of mechanical properties. Autonomous self-healing processes can be further distinguished between intrinsic and extrinsic. Intrinsic autonomous self-healing is the ultimate goal for most applications, in particular for mending mechanical failures, since it is based on the formation of either covalent or noncovalent chemical bonds between cracked interfaces. Unfortunately, engineering intrinsic processes is quite hard for most of the materials. Extrinsic autonomous self-healing requires the presence of externally loaded healing agents, which can be triggered by a mechanical damage (White et al. 2001).
