Multifunctionality in epoxy/glass fibers composites with graphene interphase
In this project, the synergetic effect of a graphene interphase in epoxy/glass fibers composites was investigated by coating glass fibers (GF) with graphene oxide (GO) and reduced graphene oxide (rGO) nanosheets by an electrophoretic deposition (EPD) technique. Graphite oxide was prepared using modified Hummers method in which raw graphite powder was oxidized using potassium permanganate (KMnO4) in acidic solution. Using ultrasonic technique, the graphite oxide was dispersed homogenously in water to create a stable GO suspension which was used as a bath in the EPD process.
For the coating process, two copper plates were used as electrodes in the EPD process in which GF were placed in front of anode for GO deposition since GO tends to carry negative charges due to oxygen functional groups attached on the graphene structure as produced in the modified Hummers method. The deposition was carried out at different applied fields while maintaining the dispersion concentration and deposition time constant. This process produced GF coated with GO nanosheets, while to obtain GF coated with rGO, GO coated fibers were subjected to chemical reduction process where the fibers were placed in an environment of hydrazine hydrate which reduced the GO coating on GF. Through this step, rGO coated GF were obtained.
The oxidation level of GO and rGO was evaluated using x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron (XPS) spectroscopy techniques which confirmed the successful oxidation of graphite powder into graphite oxide due to liquid chemical oxidation process while the hydrazine reduction method reduced the oxygen amount from 34% to 10% in GO hence converting it into rGO. Scanning electron microscopy analysis of coated fibers revealed uniform coating of GF with GO and rGO where the amount of deposition increased with increased applied field.
The effect of GO or rGO coating on GF obtained by EPD process was first evaluated by determining the adhesion between GF and epoxy matrix. Single fiber fragmentation test was utilized to determine the interfacial shear strength (ISS) between the uncoated or coated fibers and epoxy matrix. Single fiber epoxy composites were prepared by using GO and rGO coated fibers and were tested using a mini-tensile testing machine and monitoring the lengths of fragments of fibers obtained during the tensile test. It was observed that in case of GO coated fibers, the ISS increased by 218% in comparison to uncoated fiber based composite. The increase of interfacial adhesion in this case, it can be attributed to the fact that GO carries oxygen functional groups which creates physical and chemical bonding between both the GF surface and the epoxy matrix.
For investigating the interactions between GF and GO, atomic force microscopy (AFM) was used to determine the interfacial adhesion between them by scratching GO on GF. It was proved that the delamination strength was higher than the ISS, hence proving the efficacy of the selected GO deposition method. On the other hand, single fiber fragmentation tests indicated a 70% increase in ISS for rGO coated GF when embedded in the epoxy matrix as compared to uncoated fibers.
This increment is lower than that observed for GO coated fibers and it has been attributed to the fact that rGO does not possess enough oxygen based functional groups to efficiently interact with the polymer matrix. The observed increase in ISS with respect to uncoated GF is based on the frictional forces offered by the roughness of rGO nanosheets. This confirms that the presence of an interphase (either GO or rGO) creates favorable load transfer mechanism through either chemical or physical bonding or even both depending on the nature of the interphase.
To test further the positive effect of GO based interphase in epoxy/glass composites in terms of mechanical reinforcement, multifiber (uncoated and GO or rGO coated) reinforced epoxy composites were created by hand lay-up method. Laminas of fibers were wetted by epoxy resin and stacked over one another in certain number depending on the thickness of the resultant composite. These composites were subjected to various mechanical tests, such as flexural tests, short-beam shear tests, mode I interlaminar fracture toughness and creep tests which also confirmed that GO and rGO based interphase in epoxy/glass composites increase the performances of the composite with respect to that of the uncoated GF based composites. GO proved to be the best interphase in terms of mechanical properties obtained, as proved before.
The multifunctionality of such interphases based on graphene was analyzed and confirmed using multiple tests on epoxy/glass composites containing uncoated and coated (GO or rGO). In particular, the electrical and thermal conductivity of the composites were tested in which the composites based on rGO interphase showed the highest conductivity which not only confirms that rGO coated fibers in epoxy/glass composites render the composites conductive but also proves the successful chemical reduction process used in this work.
These conductive composites were subjected to piezoresistivity tests in which the applied longitudinal strain in different modes resulted in change in resistance thus showing a possibility of using such composites as strain sensing devices or for structural health monitoring purposes in automotive or aerospace applications.
These conductive composite specimens were also analyzed for their dielectric properties. The tests showed increased permittivity values as compared to both uncoated and GO coated composites thus revealing the possibility to use composites containing rGO coated fibers for electromagnetic interference shielding applications.