**1. Introduction**

Polymeric composites are all around us. The addition of nanofillers to polymers has opened pathways for designing materials with improved functionalities. The synergistic effects of each component can result in better mechanical properties, thermal or electrical conductivity, and enhanced optical properties. The wide range of applications, spanning diverse fields such as civil engineering (building and constructions), electronics, biomedical materials, etc.,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

renders this kind of materials essential for our daily life. The simplest structure of a composite is the one consisting of a polymeric matrix in which a solid filler is dispersed. The logic behind this combination is the ability to manipulate the composite properties as function of those of the individual components and the interfacial bonding [1]. However, even at this very simple level, the system can be considered as quite complex. In particular, the geometrical shape of the filler as fibers, platelets, and particles (respectively approached as 1-, 2-, and 3-dimensional objects) is pivotal in determining the final properties. Many studies have been dedicated in the past to understand these features and to develop physical models that are able to predict the properties as a function of the structure [2–8].

Furthermore, the same kind of reversible chemistry can be also applied in solution [20–22] where the crosslinking yields gels of different nature finding applications in very popular fields such as biomedical products. It must also be stressed that very recently particular attention has been paid to a new research field, still in its infancy, dealing with the use of supramolecular polymer in composites [23, 24]. The overall concept, stemming from the use of reversible interaction for the polymer backbone, nicely fits in the thermally reversible context although with even more remarkable loss of properties at relatively high temperatures. In the present chapter, we focus on the use of the Diels-Alder (DA) reaction as a modification toolbox for nanocomposites. While presenting a general overview of the recent literature, we aim to highlight the strategic chemical modification routes and, whenever possible, the added value of such strategy for the properties of the final composite. Other kinds of "click"

Thermoreversible Polymeric Nanocomposites http://dx.doi.org/10.5772/intechopen.80218 147

reactions are also possible, the reader being referred to recent works in the field [25].

The use of Diels-Alder chemistry as a modification tool for composite materials stems from the peculiar characteristics of this reaction. The choice to proceed with or without catalyst at relatively mild temperatures as well as the intimate connection between the forward (DA) and reverse, i.e., retro Diels-Alder (rDA) reactions and the structure of the diene and dienophile render this reaction ideal in terms of being able to control its decrosslinking as a function of temperature. As a consequence, many efforts have been reported in the use of DA to decorate various types of fillers including carbon nanotubes (CNT) [20, 26] and silica [27, 28]. In particular, CNT represent a popular choice (**Figure 2**) in view of their increasing availability and the kaleidoscopic variety of properties they potentially endow the composite with (electrical

Functionalization of CNT improves their dispersion in polymer matrices and also improves the processability of the nanocomposites. The DA reaction has a clear effect on the debundling of CNT [29] and also on their dispersion in polymeric matrixes [33]. The mechanical property enhancement is due to two mechanisms: the fillers act as (additional) crosslink points while also serving as stress-transfer points, distributing the stress to the polymer matrix uniformly. In both cases, the key feature is the improved adhesion at the interface between filler and matrix [34]. It is also noteworthy how CNT can act as both diene and dienophile in the DA reaction, respectively, with maleimide and furan groups [30]. As these reactions are mainly aimed at introducing functional groups on the CNT surface, this testifies the versatility of the approach even if in some cases side reactions might be present [19] and there is no mention of the reversibility or its use [31]. Some of the reported approaches towards incorporation of

In the present case, functionalized CNT and furfuryl amine are simply mixed with di-glycidylbisphenol A, yielding in one step, a multifunctional monomer that can be subsequently crosslinked via addition of a bis-maleimide. In this case, the self-healing behavior can be induced by near-infrared (NIR) irradiation (**Figure 4**). Scratch healing was demonstrated in the vicinity of the

CNT via DA clearly indicate relatively easy synthetic routes [35] (**Figure 3**).

**2. Chemical strategies for the insertion of DA groups in** 

**nanocomposites**

conductivity, strength, and shape memory).

One of the challenges in nanocomposites is the nanoscale dispersion and distribution of the filler, which can be achieved by chemical modification of the filler surface. During the last three decades, many works have been focusing on a relatively novel concept, i.e., on the use of a covalent, yet thermally reversible, linkage between the filler and the matrix. On the one hand, this fits the general idea that (thermally) reversible linkages between the components could significantly improve some mechanical characteristics, such as impact properties. This can be achieved also by using relatively weaker interaction forces, such as hydrogen bonding. On the other hand, the use of covalent, and yet reversible bonds, might significantly help in achieving the same goal while avoiding any significant compromise on the strength of the final material [9, 10]. Indeed, among all possible thermoreversible interactions, only covalent bonds can be defined as relatively strong (more than a few kcal/mol) and metal free (**Figure 1**).

Hydrogen bonding [11–14] and ionic interactions [9] have been often used as thermally reversible interactions for composites of different kinds. One of the main driving forces behind the use of such a strategy is the presence of special end properties. For example, the dynamic nature of thermally reversible interactions (as a function of temperature) endows composites with self-healing behavior [11, 13, 15–17] and shape-memory characteristics [18, 19].

**Figure 1.** Schematic representation of different kinds of thermally reversible interactions.

Furthermore, the same kind of reversible chemistry can be also applied in solution [20–22] where the crosslinking yields gels of different nature finding applications in very popular fields such as biomedical products. It must also be stressed that very recently particular attention has been paid to a new research field, still in its infancy, dealing with the use of supramolecular polymer in composites [23, 24]. The overall concept, stemming from the use of reversible interaction for the polymer backbone, nicely fits in the thermally reversible context although with even more remarkable loss of properties at relatively high temperatures. In the present chapter, we focus on the use of the Diels-Alder (DA) reaction as a modification toolbox for nanocomposites. While presenting a general overview of the recent literature, we aim to highlight the strategic chemical modification routes and, whenever possible, the added value of such strategy for the properties of the final composite. Other kinds of "click" reactions are also possible, the reader being referred to recent works in the field [25].
