Crystalline morphology of electrospun polymeric nanofibers strongly influences the mechanical properties. In this study, electrospun polycaprolactone nanofibers with smaller diameters that are produced from dilute polymer solutions have a higher degree of molecular orientation, crystallinity, stiffness, and strength, but lower ductility. Nanofiber diameter and the resulting crystalline morphology is influenced by whether complete crystallization of polymer chains took place before or after the electrospinning jet has reached the collector. The former would result in the formation of smaller fibers with fibrillar structure and aligned lamellae, whereas, the latter would result in the formation of a misaligned lamellar structure.

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The nanostructural and elastic properties of a single polymeric nanofiber extracted from a nanofibrous scaffold are investigated using atomic force microscopy (AFM). AFM imaging of the nanofibers reveals a “shish-kebab” structure. A portion of the nanofiber is suspended over a microscale groove etched on a silicon wafer. A nanoscale three-point bend test is performed to obtain the elastic modulus. This elastic modulus is found to be 1.0±0.2 GPa for fibers less than 350 nm but decrease with increase in fiber diameter in excess of 350 nm. This is due to the significance of shear deformation as the length to diameter ratio decreases.

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Biodegradable polymeric nanofibres produced by electrospinning have been used as scaffolds for tissue engineering. Before these nanofibrous scaffolds can be implanted into the human body, it is important to know if the individual nanofibres are strong enough to withstand the forces exerted by the cells as they grow and migrate on the scaffold. However, due to the small size of the nanofibres, it is a challenge to characterize the mechanical properties of individual nanofibres. Therefore, we aim to mechanically characterize a single nanofibre using both a tensile test and a nanoscale three-point bend test. As some scaffolds may be heat-treated by annealing to enhance the stiffness and strength of the nanofibres, we also investigate the effects of annealing on the structural and mechanical properties of single nanofibres. The material properties of as-spun and annealed nanofibres were studied using differential scanning calorimetry and atomic force microscopy. Annealing was found to increase the Young's modulus of the nanofibre mainly due to the increase in crystallinity and the change in morphology from a purely fibrillar structure to a mixture of fibrillar and nano-granular structure with enhanced interfibrillar bonding.

In this study, an approach using an atomic force microscope (AFM) tip to stretch a single electrospun polyethylene oxide (PEO) nanofiber is demonstrated. One end of the nanofiber is attached to a movable optical microscope stage and the other end of the nanofiber to a piezoresistive AFM cantilever tip. The nanofiber is stretched by moving the microscope stage and the force is measured via the deflection of the cantilever. The elastic modulus of PEO nanofiber is found to be about 45MPa.

The authors investigate the crystallinity and surface effects on Young’s modulus of cupric oxide (CuO) nanowires by performing three-point bend test using atomic force microscopy. Young’s modulus of the nanowires obtained ranges from 70to300GPa and is dependent on two factors. Firstly, it depends on whether the nanowire is mono- or polycrystalline, as indicated by the absence or presence of an amorphous surface layer. Second, the modulus increases with decreasing diameter for both types of nanowires. Combined with transmission electron microscopy and computational simulation studies, the nanostructure-mechanical property relationship of CuO nanowires is elucidated.

Nanoindentation study of a single poly(L-lactic acid) nanofiber produced by the phase separation method was performed using an atomic force microscope (AFM) cantilever tip. Issues concerning the use of AFM for nanoindentation of polymer nanofibers were discussed. The Hertz theory of contact mechanics was used to analyze the indentation results. It was found that the elastic modulus was comparable to that obtained from the nanoscale three-point bend test done in our previous study, after roughness correction was made.