Nanoscale Strain Measurements in Polymer Nanocomposites
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Nanoscale Strain Measurements in Polymer Nanocomposites Qi Chen1, Ioannis Chasiotis1, Chenggang Chen2, and Ajit Roy3 1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 2 University of Dayton Research Institute, Dayton, OH, 45469 3 Air Force Research Laboratory, Wright-Patterson AFB, Dayton, OH, 45469 ABSTRACT A multiscale experimental investigation of the mechanical behavior of polymer nanocomposites with nanoscale fumed silica inclusions is described. The objective is to shed light into the effect of the hard nanoparticles on the quasistatic mechanical behavior of the epoxy matrix and the implications of the latter to the effective composite properties. The main variable in this study was the nanofiller volume fraction while the particle size was either 15 nm or 100 nm. Local strain measurements indicated strain field localization in the vicinity of the nanofillers at strains that macroscopically fall in the linearly elastic regime. The matrix strains were as high as three times the applied far field strain at applied effective strains of ~ 1%. At larger stresses the local strain fields evolved to maxima that were considerably higher than the applied strain, and they were affected by local particle density and distribution. In composites with the largest particle weight fraction, 5 wt.%, 100 nm fillers, neighboring particles located in small proximities behaved as single large particles and often resulted in matrix strain shielding thus decreasing the benefit of the large surface-to-volume ratio and the associated efficiency in load transfer. On the other hand the 15 nm fillers resulted in more uniformly distributed deformation compared to composites with 100 nm particles. INTRODUCTION Polymer nanocomposites have attracted significant interest due to a multitude of potential multifunctional applications with improved thermomechanical and electrical behavior [1,2]. The addition of inorganic nanoscale particles to a polymer matrix may result in enhanced conductivity [3], elastic modulus, and fracture toughness [4-7] without compromising the composite density and its optical properties [8,9]. The composite properties are influenced by the volume/weight fraction, quality of the dispersion, shape and size, as well as the surface functionalization of the nanofiller. These parameters determine the load transfer between the composite constituents and the mechanism and amount of energy dissipation during fracture. Experimentally, the local mechanisms of deformation governed by the aforementioned properties have been investigated only qualitatively by using post-mortem SEM and TEM analyses and macroscale mechanical testing. To establish interrelations between nanoparticle parameters and effective composite properties, it is important to understand the local nanoscale mechanical response of the matrix. While various experimental techniques have been used to measure the effective composite properties [10,11], submicron and nanoscale experimentation are still lacking. To this ef
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