**5. Properties and potential applications**

Polymer nanocomposites have advantages: (1) they are lighter than conventional composites because high degrees of stiffness and strength are realized with far less high-density material, (2) their barrier properties are improved compared with the neat polymer, (3) their mechanical and thermal properties are potentially superior and (4) exhibit excellent flammability properties and increased biodegradability of biodegradable polymers [106].

There are many applications of polymer/clay nanocomposites and an increasing number of commercial products available on the market. Notable examples are automotive parts [16, 20], packaging [2, 8, 107], construction materials [36], biotechnology [108], medical devices, etc. [3, 36, 108]. One important property observed due to the incorporation of clay particles in polymers is a significant level of flame retardancy. This property also provides avenues for applications in many other areas, such as building materials, computer housings and car interiors [36]. Moreover, balanced mechanical properties, functionalities and biocompatibility of bionanocomposites provide an exciting platform for the design and fabrication of new materials for biomedical applications [36]. Another area of interest for nanocomposites is the packaging industry. The impermeable clay layers mandate a tortuous pathway, which difficult the diffusion of molecules throughout the matrix [109]. Improving food quality and shelf life, while reducing plastic waste, has stimulated the development of biodegradable polymer-based PCNs as advanced and smart packaging materials [109].

There are also several applications with carbon nanotubes and graphene. An area of notable application of these materials is in the optoelectronic industry [110, 111]. There is a wide range of applications which include fiber lasers, supercapacitor, field emission devices and photovoltaics, where the combination of tunable optoelectronic properties as well as structural and chemical stability, high surface area and low mass density of nanofillers with the processability of polymers offers a new class of materials [110, 111].

Nanocomposites of an organic-modified MMT and PA6 with a residual monomer were once produced by melt blending in a torque rheometer [112]. By WAXD, intercalated structures were observed in the nanocomposites with 3 and 5 wt% of MMT; on the other hand, when 7 wt% of MMT was added, an exfoliated structure was obtained due to the predominant linking reactions between the residual monomer and the polar organic surfactant. Solutions of these nanocomposites in formic acid were prepared, and the 3 and 5 wt% nanocomposites were successfully electrospun; however, electrospinning of the 7 wt% nanocomposite was not possible. WAXD, SEM and TEM results showed that the 3 and 5 wt% nanofibers with average diameter between 80 and 250 nm had exfoliated structures. These results indicate that the high elongational forces developed during the electrospinning process changed the initial intercalated/exfoliated structure of the nanocomposites to an exfoliated one [112].

The use of an aqueous dispersion of polyethylene copolymer with a relatively high content of acrylic acid as a compatibilizer and as an alternative medium to obtain polyethylene NFC nanocomposites was a matter of recent study [113]. The NFC content was varied from 1 to 90 wt%, and the appearance, optical, thermal, mechanical and rheological properties, as well the morphology of the films, were evaluated. The PE/NFC films were transparent up to 20 wt% of NFC indicating a good dispersion of NFC, with PE-rich and NFC-rich regions observed by SEM. Improved mechanical properties were achieved with an increase in the Young's modulus. The rheological behavior indicated good melt processability [113].

Water suspensions of NFC with xylan, xyloglucan and pectin were studied for foaming and structural properties as a new means for food structuring [114]. They were analyzed by rheometry, microscopy and optical coherence tomography (OCT). A combination of xylan with TEMPO-oxidized NFC produced a mixture with well-dispersed air bubbles, while the addition of pectin improved the elastic modulus, hardness and toughness of the structures. Shear flow caused NFC to form plate-like flocs in the suspension that accumulated near bubble interfaces. This tendency could be affected by adding laccase to the dispersion. Xyloglucan interacted strongly with TEMPO-oxidized NFC (high storage modulus) [114].
