Preparation and characterization of electrically conductive foams made of Polyethermide (PEI) filled with graphene nanoparticles

Abstract

The aim of this study is to develop a novel multi-functional polymer nanocomposite for potential usage in aerospace. It is based on facile approach to produce microcellular PEI based foams reinforced with graphene nanoplatlets (GnP) with around 3 times reduction in weight compared to solid material using water vapor induced phase separation process. In order to find optimal composition of the material with maximum advancement of properties the foams were prepared with different concentrations of polymer in solvent (25 and 30 wt %) and loadings of the GnP (1, 2, 5 and 10 wt %) which affect the properties of the nanocomposite material. Additionally, the effect of adding a fraction of PAI and different thicknesses have been studied. Regarding the morphology of the material, PEI-GnP foams displayed a quite regular closed-cell microcellular structure with high degree of interconnectivity. The average cell size achieved was in the interval 7.5 - 13.5 Êm, and the cell density value in 6.9 x 108 - 3.4 x 109 cell/cm3. The average cell size increased slightly and as a consequence cell density decreased with rising the concentration of GnP and the thickness of the foam. For PEI nanocomposite foams the flow generated during cellular formation allows the graphene to place on cell walls whereas for PAI/PEI nanocomposite foams with smaller cell size, due to noticeable difference between the mean size of graphene particles and the cell size, the material forms a different structure where the graphene induces a relatively larger cells around itself. The thermal stability of the foams was analyzed by means of TGA. A two-step thermal decomposition mechanism similar to solid PEI was identified, where the first step was related to degradation of non-aromatic part of PEI and the second step was correspondence to simultaneous decomposition of the aromatic part of PEI and graphene. Higher thermal stability was obtained in PEI-GnP foams compare to PEI foams as a consequence of the barrier effect of graphene nanoplatlets located in the cell walls. Decomposition of the material seemed to be driven by two compatible features, the thermal conductivity and barrier effect, both induced by the filler. For different concentrations the material behaved differently. At concentration of 2 wt% GnP the material seemed to reach its optimal thermal stability compared to 5 and 10 wt% GnP where foams displayed a less homogeneous cellular morphology with more significant presence of interconnected cells and GnP aggregates. With regard to dynamic mechanical properties, it was found that the storage modulus (E ) was increased remarkably with both addition of GnP and density of the foam. Moreover, a reduction of tan  in samples with higher fraction of filler indicates the improvement in stiffness of the material for same reasons. Slight variations of Tg were observed, while there was a direct influence of the relative density, a dual trend with regard to the GnP concentration was found. For low GnP loadings (0-2 wt%), the Tg did not change or slightly increased while for more than 2wt% it decreased notably. Although the electrical conductivity measurements show a steep increase for the nanocomposite foam with 2 vol% of GnP, pointing out that for this certain concentration, the conductive network established by the GnP overcomes the insulative properties of the polymer and presenting that for lower concentrations, the particles were too far apart to conduct a network, a tunnel like approach found to fit the experimental data, also explaining the existence of conduction for lower GnP loadings than expected based on percolation.