Modeling the size-dependent elastic properties of polymeric nanofibers.

We present a strain gradient (SG) theory to explain the strongly inverse size dependence between the elastic modulus and fiber diameter in polymeric nanofibers. For centrosymmetric and isotropic materials we showed that the three length-scale parameters can be combined into a single parameter that can be used to predict the onset of the size-dependent trend when the fiber diameter is reduced past its critical size. To address the issue of whether the SG offers a plausible explanation of the size-dependent behavior we conducted a series of uniaxial tensile and static bending tests involving polycaprolactone nanofibers. Since the elastic modulus is highly sensitive to the fiber diameter, it is necessary to correct the experimental data to account for the lack of circularity in the cross-section of the real fiber. Additionally, we applied the SG model to study the size-dependent elastic properties of polypyrrole nanotubes. By approaching the SG theory from a dynamics point of view, our model is able to capture size-dependent effects in the mechanics of fine-scale materials for both static and dynamic responses.