**2. Hybrid polymer materials**

In polymer science, we can define a *hybrid polymer material* as a combination of two or more materials mixed at the nanometer level, or sometimes at the molecular level (0.1 – 100 nm) in a predetermined structural configuration, covering a specific engineering purpose. The term *hybrid material* is used to distinguish them from the conventionally known *composites* that are referred as simple mixtures of two or more materials at micro-scale level (> 1 µm).

**Figure 1.** Some examples of structural configurations of hybrids of the composite type: (a) sandwich, (b) concentric cylindrical shells, (c) honeycomb, (d) chopped fibers, (e) particulate, and (f) amorphous blend.

An ideal hybrid polymer material requires an accurate molecular design or structural con‐ trol of its components in order to obtain synergistic properties. As structural configuration of components moves away from its ideal configuration, the material properties will range from an arithmetic average value (average of the properties of each component) to below of that arithmetic value [5]. Thus, the shape and structural configuration of the components in a hybrid polymer material play a key role in determining its properties. Figure 1 shows a scheme of hybrids materials composed by two components, in which one of them is ar‐ ranged so that synergistic properties can be achieved.

The hybrid polymer materials can be classified depending of the nature of interactions be‐ tween their components. In particular, when structural materials in the form of particles, flakes or fibers are incorporated into polymer matrices, this type of hybrid polymer materi‐ als can be classified in (i) *class I* hybrid materials, which show weak interactions between the two components, such as van der Waals, hydrogen bonding or weak electrostatic interac‐ tions, and (ii) *class II* hybrid materials, which show covalent interactions between both com‐ ponents such that there is no tendency for the components to separate at their interfaces when the hybrid material is loaded [6].

Hybrid polymer materials containing CNTs have attracted considerable attention due to the unique atomic structure, high surface area-to-volume ratio and excellent electronic, mechanical and thermal properties of carbon nanotubes. Although the incorporation of CNTs to polymer matrices have significantly improved the mechanical, electrical and morphological properties of polymers, there is plenty of room for controlling the structur‐ al configuration of the hybrid polymer material, thus, different efforts have been focused in the preparation methods.
