G. Haugstad, B. Coggio
University of Minnesota,
Keywords: Characterization, Silicone, Silica, Composites
Summary:We describe multimodal imaging and nanomechanical investigations of the structure and properties of silicone elastomers consisting of 5-25 wt% of pyrogenic silicone dioxide (?fumed silica?) nanoparticles dispersed in silicone elastomer matrices. The silicone matrices (MPa-regime modulus) are found to include strongly modified interphase regions within nanometers to tens of nanometers of the silica nanoparticles. AFM phase imaging shows that these regions exhibit distinctly elevated dissipative response compared to either the nanoparticles themselves or domains of pure polymer located farther from the silica nanoparticles. So-called "height" images are actually complex convolutions of topography and nanomechanical response. By exploring imaging parameters and modalities we found that both "true" and incremental degrees of "false" topography can be controllably generated as images, revealing a more 3D distribution of nanoparticles (i.e., depth dependence). Thus AFM results are further compared to TEM imaging (i.e., 2D projections of 3D nanoparticle distributions) to enhance understandings of each imaging modality. We also find dramatically different apparent topography in amplitude-modulation dynamic (AC/?tapping?) mode compared to fast force curve mapping (?peak force QNM?). AC-mode phase imaging is further complemented by adhesion and stiffness imaging in fast force curve mapping.  The multiphase morphology was found to be strongly dependent on compositional and processing parameters. AFM results enabled an assessment of the influences of particle size and surface chemical treatment, as well as particle concentration and dispersion, on the multiphase morphology, including the degree of percolation of dissipative domains. We further correlate these observations to the macroscale mechanical properties of the elastomers engineered under variable composition and processing.