Nanomechanical studies of thin film porosity and graphene reinforcement in thermoplastic polymers

A study of the nanomechanical properties of graphene reinforced polymers and thin film polymers of varying porosity was conducted. Nanoindentation was used to characterize the nanoscale properties of injection molded polymers reinforced with graphene nano flakes (GNFs) and porous and densified electro-sprayed polystyrene (PS). Reduced modulus was used as the comparative property between each of these materials. Each polymer was tested with a conical indenter tip to ensure the comparability of the tests while considering the experimental restrictions of the most sensitive material.The graphene reinforced polymers were made with two varieties of matrices, nylon PA-66 and PEEK. Each matrix was reinforced with graphite at 35 wt% that was exfoliated into graphene over a multicycles process during injection molding. The number of exfoliation cycles was varied in the PEEK specimens to test the relationship between reduced modulus and graphene exfoliation. The pure PA-66 was found to have an average reduced modulus of 1.48 +/- 0.08 GPa while the PA-66 with GNFs was found to have an average reduced modulus of 1.26 +/- 0.11 GPa. The reduced modulus decrease may be attributed processing disparities or surface morphology variance. The PURE PEEK exhibited a reduced modulus of 2.90 +/- 0.31 GPa which was the lowest of the specimens tested with PEEK matrices. The PEEK with GNFs exfoliated for one (1) cycle and for five (5) cycles demonstrated higher moduli than the PEEK, with values of 3.88 +/- 0.30 GPa and 4.81 +/- 0.68 GPa, respectively. Each PEEK specimen had a decreasing trend in reduced modulus as depth increased. The electro sprayed polystyrene films were tested using a conical indenter tip. The porous films were taken from the sprayer with no post processing, while the dense films were densified via high temperature process from porous films. Each of these films was tested to 10% of their film thickness to avoid substrate effects. The porous PS was found to have an average reduced modulus of 0.136 +/- 0.036 GPa, while the dense PS was found to have an average reduced modulus of 6.92 +/- 0.80 GPa. A study of the absorbed and dissipated energy during indentation was conducted on each specimen to further compare the materials. All GNF reinforced composites demonstrated a power law trend between work of indentation and max depth, with the PEEK, PEEK+GNFs (1 cycle), and PEEK+GNFs (5 cycles) showing increasing work of indentation as the exfoliation increased. Dense PS displayed a power law trend, while porous PS demonstrated a linear relationship between work of indentation and max depth. The latter suggests that local architecture plays an important role in the response of electro-sprayed PS films.M.S.Includes bibliographical reference