Experimental And Theoretical Investigations Of Size-Dependency In The Mechanical Properties Of 1-Dimensional Nanostructures

With the rapid advancement of nanotechnology there is considerable interest in the measurement and modeling of the mechanical properties of nanostructures. Therefore, this dissertation is concerned with the experimental measurements and computational modeling of the elastic modulus of 1-dimensional nanostructures, with a special emphasis on the size-dependent phenomenon. The nanostructure studied in my research consists of moderately-fine polymeric nanofibers with diameters between 100 and 1200 nm, and ultra-fine metallic nanowires with diameters up to 10 nm. Macro fibers have diameters exceeding 1200nm and they do not exhibit the size-dependent trend.In the experimental measurements, a series of tests involving a single-strand, electrospun PCL nanofiber is conducted. Three kinds of test are carried out; uniaxial tensile, static bending and dynamic bending. Further, since the cross-section of the nanofiber is generally ellipsoidal in shape, it is necessary to correct for the lack of circularity. The analytical correction is performed using data from vertical and horizontal diameters provided by the AFM and SEM measurements, respectively. To study the size-dependent response, measurements on nanofibers of varying diameters that range from 100-1200 nm are carried out. The results on the elastic modulus clearly depict an inverse size-dependent trend with the fiber diameter; the smaller the diameter, the higher the elastic modulus is elevated. This is particularly pronounced in the results of the moderately-fine nanofibers for both the bending tests, but appears to be statistically unchanged for the tensile test.To analytically investigate size-dependency in moderately-fine nanofibers, a strain-gradient (SG) model based on a dynamics formulation is developed. Application of the SG model to the moderately fine nanofibers produces a size-dependent behavior under bending loads and an absence of size-dependency under tensile loads. These predictions are consistent with the results of the experimental tests on the moderately-fine nanofibers. Further, our SG model shows a size-dependent trend in the natural frequency of the nanofibers.To study size-dependency in ultra-fine nanowires an atomic model consisting of lattice cells is developed. Our lattice model predicts a smaller-scale size-dependent trend in ultra-fine nanowires under tension. This result seems to contradict that of the tensile-loaded, moderately-fine nanofibers that do not exhibit size-dependency. Our explanation of this apparent contradiction is that at the ultra-fine scale, the nanowires are a few atom layers thick, making the surface energy predominate over the core energy. Our lattice model size-dependent predictions are checked against those of another atomic model - that of the molecular dynamics simulation, and the two results are consistent with each other. Therefore, we feel confident that ultra-fine nanowires do exhibit size-dependency, albeit at a much small scale compared to bending size-dependency in the moderately-fine nanofibers.