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Polymer-based Nanocomposites: a Brief Overview

By Prof. Michele Laus, Universitŕ degli Studi del Piemonte Orientale, Alessandria, Italy

The use of organic and inorganic fillers is a common practice in the plastics industry to improve the mechanical properties of thermoplastic materials such as heat distortion temperature, hardness, toughness, stiffness and mold shrinkage or to decrease other properties such as gas permeability and often, material price. The filler effect on the composite properties strongly depends on its shape, size, aggregation state, surface characteristics and degree of dispersion. In general, the mechanical properties of a composite material with micron-sized fillers are inferior to the properties of a composite with nano-sized fillers. Physical properties such as surface smoothness and barrier properties cannot be achieved effectively using conventional, micron-sized fillers.

For these basic reasons, systematic work in polymer nanocomposites has exploded over the last few years.¹ This novel material technology, based on work at the molecular level to create large structures endowed with peculiar molecular organizations, is leading to nanoscale materials with properties different from the ones of their macroscale counterparts.

On the other hand, new problems in polymer processing arise because of the inherent characteristics of the nano-sized fillers. In general, the specific features of nanocomposite materials are due mainly to the very high interfacial area and very short distance of the reinforcing particles. The contact surface in such a dispersion between the filler elements and the matrix material grows dramatically with respect to conventional fillers, thus leading to the creation of wide interfacial regions which are of utmost importance in determining the material properties. In addition, the percolation threshold is expected to occur at very low concentration of the filler. However, the smaller the reinforcing composite elements, the larger their internal surface and hence their tendency to agglomerate rather than to disperse homogeneously in a matrix.

On the basis of aspect ratio, nanotubes, spherical nanoparticles and sheet-like particles are the most representative nanofiller geometries. Although nanocomposites based on carbon nanotubes, which so far display the highest values of elastic modulus (~1.7 TPa), or spherical nanoparticles², for example PTFE or calcium carbonate, display interesting mechanical, electrical and also optical properties³ and are the subject of intense studies, the most promising class of nanocomposites includes those made from polymers and layered phylosilicate clay materials, such as montmorillonites.

The montmorillonites are often modified through an ion exchange process in which the interlayer cations are substituted by cationic surfactants such as alkylammonium ions. The intercalation of these small molecules between the layers reduces the forces that hold the stacks together and the resulting modified clays (or organoclays) are more compatible with the organic polymers. In fact, unlike talc and mica, the organoclays can be delaminated and dispersed into individual layers with a thickness in the order of one nanometer and very high aspect ratios (10-100).

Unseparated montmorillonites, after introduction into the polymer, are usually referred to as tactoids. Two types of structure, schematically represented in Figure 1, may be obtained, namely intercalated nanocomposites, where the polymer chains are sandwiched between silicate layers, and exfoliated nanocomposites, where the separated, individual silicate layers are more or less uniformly dispersed in the polymer matrix. As the original montmorillonite is made up of particles nominally 8-10 µm in size, there should be one million or more platelets in each particle.

Figure 1. Schematic representation of nanocomposite structures.

Consequently, the key benefit of montmorillonites in nanocomposites, but the challenge in processing to make the nanocomposite, is to obtain a perfect dispersion of such a great number of individual platelets within the polymer matrix. The attainment of a good dispersion derives from two different aspects: the chemistry of the clay surface, including the compatibility of the surfactant with the polymer matrix and the processing conditions. Given that the surfactant chains are miscible with the polymer matrix, a complete layer separation depends on the establishment of very favorable polymer-surface interactions in order to overcome the penalty of polymer chain confinement. In addition, the help of shear forces during the preparation and the processing of nanocomposite material is often necessary.

In this respect, the extruder configuration⁴ with appropriate screw design appears essential. In fact, shear intensity is required to start the dispersion process, by shearing the particles apart into tactoids or intercalants (Figure 2). Residence time in a low shearing or mildly shearing environment was demonstrated to be required to allow polymers to enter the clay galleries and peel the platelets apart. Considerable work must still be carried out to find the proper balance between the chemistry of the clay surface and processing conditions. In this context, it should be recognized that metastable morphologies can be frozen in by processing if the compatibility between the polymer and the organoclay is too low. Under this condition, substantial aging effects are expected, thus complicating the inherent properties and material dynamics.

Figure 2. Shear intensity is required to start the dispersion process.

Nanocomposites exhibit enhanced mechanical properties at very low filler level, usually less than five percent by weight. Mechanical analysis via stress-strain testing showed a substantial increase in the Young’s modulus, while values for strain at break and yield stress remain nearly at the same level of the pure matrix material. An increase in thermal stability, gas barrier properties and flame retardancy was demonstrated. In addition, provided that an excellent dispersion degree is obtained, these materials are often optically transparent. An enhanced fracture toughness is also claimed for these nanocomposites although it is hard to find any convincing evidence in the open literature.

Concerning possible field applications⁵, these nanocomposite materials have to compete with traditional composites, reinforced by carbon, glass and aramids. Consequently, only in areas where enhanced mechanical properties and transparency are required have these quite complex and far from cheap materials a chance for application today. In addition, in areas in which the characteristic material dimensions are measured in microns, as in the case of glues, fibers, foams and coatings, a prospective use of nanocomposites could be envisioned.

Prof. Michele Laus will present his work with nanocomposite materials at the 2004 International Moldflow User Group Conference, May 17-19 in Frankfurt, Germany.