Michael Burman

Effect of supramolecular structure on polymer nanofibre elasticity

Polymer materials of reduced size and dimensionality, such as thin films, polymer nanofibres and nanotubes, exhibit exceptional mechanical properties compared with those of their macroscopic counterparts. We discuss here the abrupt increase in Youngs modulus in polymer nanofibres. Using scaling estimation we show that this effect occurs when, in the amorphous (non-crystalline) part of the nanofibres, the transversal size of regions consisting of orientation-correlated macromolecules is comparable to the nanofibre diameter, thereby resulting in confinement of the supramolecular structure. We suggest that in polymer nanofibres the resulting supramolecular microstructure plays a more dominant role in the deformation process than previously thought, challenging the commonly held view that surface effects are most significant. The concept we develop also provides a way to interpret the observed--but not yet understood--temperature dependence of Youngs modulus in nanofibres of different diameters.

Free flight of an oscillated string pendulum as a tool for the mechanical characterization of an individual polymer nanofiber

The physical principles of a method for the mechanical testing of individual nanofibers are presented. A fiber with an attached mass undergoing a test is considered as a string pendulum. In addition to regular oscillations under the elastic force, the suspended bob performs free flight only under gravity which can be easily tracked. Based on a model developed to analyze the resonant frequency dependence of these flights, the Young’s modulus of the nanofiber was determined. The proposed method was verified with testing of individual nanofibers of nylon-66, which demonstrated the increase in the Young’s modulus for fiber diameters below 500nm.

Do surface effects explain the unique elasticity of polymer nanofibers

The elastic modulus of electrospun Polyamide (Nylon-6.6) nanofibers, which sharply increases in nanofibers with diameters below 500 nm, was measured by both bending and tensile deformation modes. Since the nanofiber surface energy contributes significantly to the elastic modulus measured by bending, but is negligible in the modulus obtained by stretching, the contribution of the surface energy to the modulus can be extrapolated from datasets collected after both types of deformation. The results unambiguously show that the abrupt increase in the elastic modulus of the nanofibers cannot be attributed either qualitatively or quantitatively, to a surface energy effect. The presented results contradict earlier claims with regards to the contribution of surface energy to the elastic modulus. In addition, these data suggest that the confinement of the supermolecular structures within nano-objects formed during nanofiber processing is responsible for the observed modification in elastic modulus.