Enhancing Mechanical Strength of Electrospun Nanofibers by Thermal Crosslinking and Coaxial Electrospinning

Abstract

Electrospinning of nonwoven nanofibrous mats has received significant attention in recent years due to the high versatility and porosity of electrospun mats. Specifically, considerable interest has developed in using electrospun nanofiber mats as breathable dressing layers, separator layers in lithium-ion batteries, etc. For example, the high porosity and high pore interconnectivity of nanofiber mats allows them to exhibit superior electrochemical characteristics and high overall battery performance. However, electrospun mats generally suffer from poor mechanical strength, creating the risk of a short circuit if a rip or tear were to appear. Many methods exist to improve the mechanical strength of electrospun nanofiber mats. Composite structures, such as multilayer or coaxial mats, can be used to improve the average mechanical strength of the fibers, while post-treatments can be used to improve the inter-fiber bonding to increase mechanical strength. However, many of these techniques impact the complexity and scalability of electrospinning or impact physical properties such as porosity. Alternatively, thermal crosslinking of fibers by heat treatment has emerged as a simple, scalable method of significantly improving mechanical strength, but typically results in considerable shrinkage. In this work, coaxial electrospinning is combined with thermal treatment to produce a novel method of improving the mechanical strength of nanofiber mats, without incurring significant dimensional shrinkage. Coaxial PAN/PVDF-HFP mats showed no significant shrinkage when tested at temperatures up to 240 ºC for 20 minutes, compared to the homogenous PVDF-HFP mats, which displayed a shrinkage of 94% when treated at 190 ºC for 20 minutes. When treated at 178 ºC for up to 30 minutes, the coaxial fibers consistently showed changes in thickness of less than 10% and no significant change in area. More importantly, the reductions in thickness and volume experienced by the coaxial mats were much more uniform across the varied treatment times when compared to those of the homogenous PVDF-HFP samples. The as-spun coaxial fibers showed a decrease in porosity compared to homogenous PVDF-HFP (95% to 79%) but remained much more porous than the commercial PP separator (41%). In addition, no significant change in average porosity in the coaxial samples occurred following treatment at 178 ºC for 20 minutes. Coaxial samples heat treated at 178 ºC for 5 minutes demonstrated a mechanical strength of 7.72 MPa, a 22% increase when compared to the as-spun coaxial fibers, and a 54% increase compared to the as-spun homogenous PVDF-HFP. Elongation at break decreased from 17.8% to 5.3% following the 5-minute heat treatment, showing a significant reduction compared to the elongation at break for as-spun PVDF-HFP (79.7%), and PVDF-HFP treated at 178 ºC for 5 minutes (40.6%). Therefore, the proposed technique of combining heat treatment with coaxial morphologies demonstrates significant potential for improving mechanical strength without dimensional shrinkage.