Novel Multifunctional Graphene Based Nanocomposites for Technological Applications

Abstract

This work aims at exploring modified reduced graphene oxide (RGO) for active packaging or energy storage applications. To fulfill this aim, surface modification of RGO with two types of modifiers, hyperbranched polyester (PES) for the first application and silver/copper oxide nanoparticles, has been carried out. The purpose of surface functionalization of RGO with PES, is not only to improve the dispersion of graphene nanosheets and hence their properties but also to impart new characteristics to a hosting biodegradable matrix (we choose polycaprolactone (PCL)). Meanwhile, the surface decoration of RGO with metal/metal oxide nanoparticles goes to enhance the electrochemical properties of supercapacitor electrodes. In more details, RGO was firstly prepared by Hummer method and then functionalized with PES using in-situ and ex-situ approaches, in order to determine the best technique that provides improved properties. The successful preparation of graphene and its modified forms was confirmed with different characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, infrared spectroscopy, Raman spectroscopy, transmission electron microscope, and atomic force microscope. The biological activity against deleterious pathogens was also discussed and the results showed improved biocidal activity in presence of PES. Moreover, the impact of RGO during the in-situ polymerization step was investigated using proton magnetic resonance and gel permeation chromatography for structural analysis of unattached PES obtained in presence of RGO compared with pure PES that prepared in absence of RGO. The results confirmed that, in presence of RGO, the polymerization was occurred and the PES was formed with its well-known chemical structure. Nevertheless, the degree of branching was found lower than in case of pure PES. Finally, the prepared RGO and its in-situ and ex-situ modified forms were added with different ratios to the PCL films utilizing two comparable techniques; casting and melting. In the latter process, the filler or its modified form was blended with the commercial PCL under thermal conditions in the Barbender and then thermal pressing to obtain free standing films. Moreover, these nanocomposite films were also obtained using casting technique. The thermal and mechanical properties were studied for all produced films. The results obtained were different and depending on the method of film fabrication. Particularly, the films prepared with casting technique exhibited improvement in the degradation and crystallization temperatures of PCL film, with the incorporation of fillers, without significant changing in the Tg. On contrast, inclusion of such fillers under melting conditions showed increasing in the decomposition and glass transition temperatures without changing in the crystallinity of the PCL matrix. Mechanically, addition of the prepared fillers to melt PCL matrix led to enhancement of stiffness, mechanical strength, tensile strength, and elongation at break. Nevertheless, the surface modification of RGO with PES could be an effective approach to fine tune the stiffness and elasticity of the PCL matrix prepared with casting method. The improvement of surface wettability was checked with contact angle measurements. The results emphasized that the inclusion of modified fillers decreased the values of contact angle whatever the method of film fabrication. The rate of water permeability decreased to ~ one third of PCL film after the reinforcement with modified filler either with casting or with melting process. However, further improvement ~ 20 % against the permeability of CO2 and H2O was observed for the nanocomposite blends prepared with melting process. Meanwhile, the casted films did not exhibit a significant enhancement either in water or gas permeability. The antimicrobial performance against Gram-positive and Gram-negative bacteria was debated in details. Specifically, the PCL films incorporated with the prepared fillers are selective to Gram-positive bacteria regardless the way of film preparation. Lastly, the biodegradation test indicated to the successful decay of the PCL and its nanocomposite films in soil. However, utilizing fungal based method, blends reinforced with modified graphene showed a weight loss three-fold higher than the neat matrix. Generally, the results indicate the strong workability of the modified RGO incorporated in the PCL matrix particularly under melting process which simulates the industrial conditions.