**5. Summary and final remarks**

The properties of CNTs and its application in the preparation of polymer hybrid materials have been very active research fields over the last decade. The preparation of polymer-CNTs hybrid materials faces considerable research challenges because their performance depends on the dispersion of CNTs in the polymer matrix and interfacial interactions between the CNTs and the polymer. In order to achieve the maximum performance, the surface modifi‐ cation of CNTs through several functionalization methods is one of the most used strategies to achieve this goal. Nowadays, the global environmental trends provide convincing reasons for exploring greener chemistry methods for functionalization of CNTs, which represent ad‐ ditional research challenges.

Microwave, ultrasound and plasma technology have emerged as promising green ap‐ proaches since they have demonstrated to reduce the energy consumption, shorter reaction times and increase yields. These greener methods offer a range of energies that are not avail‐ able from other conventional sources such as thermal, so leave open the possibility to ex‐ plore more efficient functionalization routes, taking into account the 12 principles of "*Green*" chemistry as a framework.

Despite the progress described within this chapter, there are still considerable research chal‐ lenges within this field that remain to be addressed. The successful functionalization of CNTs depends on availability of active sites on CNTs surface; however, the mechanisms that generate these sites remain unclear. Therefore, an in-depth understanding of the mecha‐ nisms of interaction between microwave/ultrasound/plasma-CNTs is required. For satisfy‐ ing this need, further studies on correlation between the irradiation conditions and the level of the functionalization are suggested, in order to establish the most efficient and greener conditions for each particular system.

In addition, the incorporation of a dispersion stage of CNTs in gas phase previous to irradia‐ tion is highly recommended, because the surface area exposed to irradiation is increased and therefore the interactions with the matter are highly promoted.

As a general statement, the "*Green*" chemistry methods based on microwave, ultrasound and plasma energy can be easily incorporated to emerging fields of nanotechnology, in par‐ ticular into strategies for preparation polymer-CNTs hybrid materials. However, there are still a number of important research challenges to study.
