Effect of Nanoscale Fillers on the Local Mechanical Behavior of Polymer Nanocomposites

摘要

In this research program we investigated (a) the effective and local mechanical and fracture behavior of EPON epoxy with nanoscale fumed silica particles, and (b) the mechanical and interfacial properties of individual vapor grown carbon nanofibers (VGCNFs) embedded in EPON epoxy. Local Atomic Force Microscopy/Digital Image Correlation (AFM/DIC) measurements showed strain localization and matrix yielding in the vicinity of 100-nanometer silica particles, which reduced the composite stiffness. The tensile strength of all silica composites was independent of particle size and weight fraction due to strong particle bonding and failure initiation in the matrix, while the critical mode I stress intensity factor of 12-nanometer silica composites increased by as much as 35% for 15 wt.% silica. We also conducted experiments on the mechanical and failure response of VGCNFs by a MEMS mechanical testing platform. Their mechanical strength averaged 2.74 - 3.34 GPa for different fabrication conditions, with as-fabricated VGCNFs having the widest flaw population which was reduced significantly upon heat treatment. The elastic modulus of as received and heat-treated VGCNFs averaged 180 and 250 GPa, respectively, which is significantly higher than reports from previous 'indirect' experiments. The strength of VGCNF-Epon 828 matrix interfaces was also quantified for the first time by novel nanoscale fiber pull-out experiments. The interfacial shear strength (IFSS) of as fabricated VGCNFs was 111 plus or minus 32 MPa and was reduced to 66 plus or minus 10 MPa after heat treatment. Finally, surface functionalization of heat treated VGCNFs restored their IFSS to 214 plus or minus 10 MPa.