Local and effective mechanical properties of polymer nanocomposites

Polymer nanocomposites represent a growing class of composite materials due to their potential for improved and multifunctional properties of common epoxies. In this dissertation, the local mechanics of nanocomposite systems were investigated by existing and novel methods of experimentation to shed light into the local mechanical and interfacial behavior of polymer-nanoparticle systems. Specifically, three material systems were investigated: polymer matrix composites with micron and submicron platelets, a polymer epoxy with silica nanospheres and the system of a single nanofiber embedded in an epoxy.;The epoxy-graphite nanoplatelet composites were employed to explore the validity of local nanoindentation tools in obtaining local, effective and in-depth profiles of the mechanical properties of composites with small aspect ratio and platelet-like particles. Nanoindentation measurements provided valid effective material properties for composite systems with small aspect ratio platelets whose dimensions were at least one order of magnitude smaller than the indenter tip radius. On the contrary, the nanoindentation-derived properties of composites with high aspect ratio (>15) platelets were not consistent with the effective quasi-static and dynamic composite properties measured by uniaxial tension and Dynamic Mechanical Analysis (DMA) experiments. Similarly, the utility of nanoindentation for depth sensing of mechanical properties in randomly distributed particulate composite was not found to be valid as the stiffness of the particles at the top surface of a sample determines the contact stiffness.;The determination of local matrix properties was accomplished by the development of an apparatus for in situ loading of nanocomposite samples under an Atomic Force Microscope (AFM) and an optical microscope for concurrent measurement of local and effective (larger than the material representative volume element (RVE)) material deformations. To limit the effect of the randomness in the spatial particle orientation, the selected test samples were epoxy matrix composites with embedded silica nanospheres. This new experimental apparatus was used to investigate the effects of nanoscale filler weight fraction on the local deformation and fracture properties of the particular composites. In terms of effective properties, large silica particles (O100 nm) promoted matrix stiffening at small weight fractions, which decreased at larger weight fractions and converged to that of small silica nanoparticles (O12 nm). The elastic modulus of the composites with 12-nm silica particles increased by as much as 30% for 15 wt.% silica loading. Irrespective of the particle weight fraction and the particle size, the composite strength was the same as that of the control epoxy. This implies that failure always initiated in the matrix, a conclusion that was supported by fracture surface images, where very limited particle debonding was observed.;The study on the effect of nanospheres on the mechanical property improvement of epoxies pointed to the fact that nanostructured materials provide improvements mainly on the toughness of an epoxy. In this regard, local information of the interfacial properties of embedded nanoparticles can be the key in developing calibrated hierarchical damage models. To this effect, a new methodology was developed to quantify the average interfacial shear strength (IFSS) of individual vapor grown carbon nanofibers (VGCNFs) embedded in an aerospace grade epoxy. The IFSS from several experiments with as-grown and non-functionalized VGCNFs was 111+/-32 MPa. This value is up to 24% higher than that of the IFSS of multi-wall carbon nanotubes in epoxy matrices and twice the IFSS of high temperature heat-treated VGCNFs. The measured IFSS values were independent of the nanofiber embedded length. Post-mortem images of the embedded section of the nanofibers after the pull-out experiments showed no traces of the matrix on the fiber surface, which indicates that cracks initiated and propagated along the fiber-matrix interface causing adhesive failure, rather than shearing of the epoxy. (Abstract shortened by UMI.)...