The effect of molecular weight and fiber diameter on the mechanical properties of single, electrospun PCL nanofibers

Biomaterials based on electrospun nanofibers are increasingly used in biomedical applications. Polycaprolactone (PCL) is a biocompatible, biodegradable, FDA-approved, synthetic polymer suitable for such applications. In this study, we used a nanomanipulation technique based on a combined atomic force/optical microscope to investigate key mechanical properties of electrospun PCL nanofibers as a function of PCL molecular weight (150 kDa, 114 kDa, and 74 kDa) and fiber diameter (20–250 nm). Mechanical properties extracted from different types of stress-strain curves included maximum stress and strain, elasticity, Young’s modulus, strain-dependent energy loss/storage, and stress relaxation time. We found that PCL molecular weight had a marginal effect on mechanical properties. In contrast, fiber diameter had a strong effect on fiber modulus. Specifically, extensibility, elastic limit, and stress relaxation time ranged from 123% to 133%, 16–44%, and 21–14 s, respectively. Energy loss/cycle was 35% for low strains (strain, 10–20%) and 65% for high strains (strain > 80%). The Young’s modulus decreased from 3000 MPa to 500 MPa as the diameter increased from 40 nm to 100 nm; above 100 nm, the diameter-dependence waned, and the fiber modulus approached the PCL bulk modulus of about 300 MPa. The diameter-dependence of the modulus opens the intriguing possibility of producing electrospun PCL nanofibers with very different stiffnesses simply by adjusting the fiber diameter.