Importance of axial stretch on the determination of Young’s modulus of electrospun poly(ε-caprolactone) nanofibres by atomic force microscopy

Abstract

With increasing interest in the use of polymeric nanofibres for biomedical applications such as composite materials and tissue scaffolding, accurate determination of their mechanical properties is essential. Fibre orientation and the stiffness of individual fibres determine the overall elastic modulus of nanofibrous materials. However, accurate measurements of the elastic properties of single fibres are challenging at the nanoscale, and distinguishing between results arising from competing models can be difficult. We report here on investigations of the Young’s modulus of single poly(ε-caprolactone) (PCL) electrospun nanofibres by measuring the deflection of fibres due to a loading force applied by an atomic force microscope (AFM). Although such testing is often performed with the tacit assumption that bending resistance alone is responsible for the fibre response, we found that consistent results could only be obtained if the overall fibre stretch is taken into account. The Young’s modulus we measured for electrospun PCL fibres with diameters ranging from 100 to 400 nm was 0.48 ± 0.03 GPa, which is similar to the modulus of bulk PCL, with no apparent dependence on diameter. Our findings highlight the importance of the assumptions used in the analysis of bending data, as discounting the effects of axial stretch and pre-existing tension typically lead to an overestimate of the Young’s modulus.