**3. Polymer-CNTs hybrid materials**

### **3.1. Structural configuration**

"*Green*" chemistry is based on the use of a set of principles that reduces or eliminates the use of hazardous reagents and solvents in the design, preparation and application of materials [4]. In this context, functionalization of CNTs using microwaves, plasma, and ultrasound waves are strategies very promising for greener production of hybrid polymer materials,

The focus of this chapter will be on the microwaves, ultrasound and plasma assisted func‐ tionalization of CNTs as greener chemistry methods to produce hybrid polymer materials. After a brief overview on preparation of hybrid polymer materials containing CNTs, we will present the physical principles, mechanisms and processing conditions involved in the func‐ tionalization of CNTs for each of these "*Green*" chemistry methods, and then present our point of view on challenges and opportunities in both the immediate and long-term future.

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

**Figure 1.** Some examples of structural configurations of hybrids of the composite type: (a) sandwich, (b) concentric

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

cylindrical shells, (c) honeycomb, (d) chopped fibers, (e) particulate, and (f) amorphous blend.

referred as simple mixtures of two or more materials at micro-scale level (> 1 µm).

due to shorter reaction times, reduced energy consumption, and better yields.

168 Syntheses and Applications of Carbon Nanotubes and Their Composites

**2. Hybrid polymer materials**

Since the first ever materials based on polymer-CNTs were reported in 1994 by Ajayan *et al*. [7], several processing methods have been developed for fabricating polymer-CNTs hy‐ brid materials. These methods mainly include solution mixing, *in-situ* polymerization, and melt blending [8].

Because the unique mechanical properties of CNTs, such as the high modulus, tensile strength and strain to fracture, there have been numerous efforts to obtain hybrid materials with im‐ proved mechanical properties [2]. Within the structural configurations for this specific applica‐ tion, the "*chopped fibers*" configuration, as seen in Figure 1(d), has been the most desired.

On the other hand, for other unique properties of CNTs such as high electrical and thermal conductivity, the obtaining of multiphase polymer amorphous blends, as seen in Figure 1(f), offers a much higher potential for the development of conductive composites containing CNTs. The selective localization of the CNTs either in one of the blend phases or at the inter‐ face of an immiscible co-continuous blend can form an ordered network of conductive phase, creating the so-called segregated systems [9]. In such systems, considerably lower value of per‐ colation threshold compared to "*chopped fibers*" structural configuration can be achieved.

The building of polymer-CNTs hybrid materials with desired structural configurations is po‐ tentially promising to develop advanced hybrid materials; however, the full exploitation of properties of CNTs by means the manufacturing of those desired structural configurations has been severely limited, because difficulties associated with dispersion of the entangled CNTs during processing and their poor interfacial interaction with some polymer matrices.

Physical functionalization method is based in the formation of non-covalent interactions be‐ tween molecules and CNTs. These methods include the wrapping of polymer around the CNTs, the physical adsorption of surfactants and the endohedral method (see Figure 2). In the latter, molecules are stored in the inner cavity of CNTs through the capillary effect, where the insertion often takes place at defect sites localized at the ends or on the sidewalls.

Toward Greener Chemistry Methods for Preparation of Hybrid Polymer Materials Based on Carbon Nanotubes

http://dx.doi.org/10.5772/51257

171

In particular, the covalent functionalization of CNTs has been one of the most preferred methods since it allows an efficient interaction between polymer-CNT interface through the functional moieties of the CNTs surface and the available functional groups of the polymer. However, these methods involve rough acid treatment conditions during functionalization which damage the nanotube framework and decrease the electrical conductivity of the hy‐ brid material. In addition, the use acids and organic solvents as the reaction media represent

In this context, the global trend of seeking for "*Green*" chemistry methods is demanding to researchers in the field to develop environment-friendly methods to functionalize CNTs.

Diverse definitions of "*Green*" chemistry can be found in the literature. According to EPA (Environment Protection Agency) "*Green*" chemistry philosophy speaks of chemicals and chemical processes designed to reduce or eliminate negative environmental impacts, where the use and production of these chemicals may involve reduced waste products, non-toxic components, and improved efficiency. Anastas and Warner [12], who are considered the founders of this field that born in 1990s, define "*Green*" chemistry as the utilization of a set of principles that reduce or eliminates the use or generation of hazardous substances in the

The 12 Principles of "*Green*" chemistry (defined by Anastas and Warner) help us think about

**1.** *Prevention*. It is better to prevent waste to treat or clean up waste after it has been created.

**2.** *Atom Economy*. Synthetic methods should be designed to maximize the incorporation of

**3.** *Less Hazardous Chemical Synthesis*. Synthetic methods should be designed to use and generate substances that possess little or no toxicity to people or the environment.

**4.** *Designing Safer Chemicals*. Chemical products should be designed to affect their desired

**5.** *Safer Solvents and Auxiliaries*. The use of auxiliary substances should be made unnecessa‐

problems of environmental pollution and health hazard.

**4.1. "***Green***" chemistry: definition and principles**

design, manufacture and application of chemical products.

how to prevent pollution when creating new chemicals and materials:

all materials used in the process into the final product.

function while minimizing their toxicity

ry whenever possible and innocuous when used.

**4. Greener production of polymer-CNTs hybrid materials**

#### **3.2. Chemical and physical functionalization of CNTs**

The efficient exploitation of the unique properties associated with CNTs depends on its uni‐ form and stable dispersion in the host polymer matrix, as well as the nature of the interfacial interactions with the polymer. Thus, obtaining of polymer-CNTs hybrid materials with de‐ sired properties has represented a great challenge, because CNTs exhibit strong inter-tube van der Waals' forces of attraction that impede its uniform and stable dispersion in the ma‐ trix, in addition to certain properties of the polymer matrix like wetting, polarity, crystallini‐ ty, melt viscosity, among others [2, 10].

Surface modification of CNTs has been one of the most used strategies in order to improve its affinity with the polymer matrix, and therefore to achieve a better uniform dispersion. These methods have been conveniently divided into chemical functionalization and physical functionalization [3, 11].

**Figure 2.** Strategies for chemical and physical functionalization of CNTs: a) covalent sidewall functionalization, b) co‐ valent defect sidewall functionalization, c) non-covalent adsorption of surfactants, d) wrapping of polymers, and e) endohedral functionalization (case for C60).

Chemical functionalization method is based on the covalent linkage of functional groups such as –COOH or –OH on the surface of CNTs. These methods can be also divided in side‐ wall functionalization and defect functionalization (see Figure 2). The reaction mechanisms that take place at their sidewall include fluorination and derivate reactions, hydrogenation, cycloaddition, and radical (R•) attachment; whilst the reaction mechanisms by amidation, esterification, thiolation, silanization, and polymer grafting (*grafting to* and *grafting from*) takes advantages of chemical transformation of defect sites on CNTs.

Physical functionalization method is based in the formation of non-covalent interactions be‐ tween molecules and CNTs. These methods include the wrapping of polymer around the CNTs, the physical adsorption of surfactants and the endohedral method (see Figure 2). In the latter, molecules are stored in the inner cavity of CNTs through the capillary effect, where the insertion often takes place at defect sites localized at the ends or on the sidewalls.

In particular, the covalent functionalization of CNTs has been one of the most preferred methods since it allows an efficient interaction between polymer-CNT interface through the functional moieties of the CNTs surface and the available functional groups of the polymer. However, these methods involve rough acid treatment conditions during functionalization which damage the nanotube framework and decrease the electrical conductivity of the hy‐ brid material. In addition, the use acids and organic solvents as the reaction media represent problems of environmental pollution and health hazard.

In this context, the global trend of seeking for "*Green*" chemistry methods is demanding to researchers in the field to develop environment-friendly methods to functionalize CNTs.
