Electrospinning of epoxy fibers: Effect of curing conditions on solution rheological behavior

The electrospinning of thermoplastic polymers is widely used in applications such as filters and coatings, but has only recently been applied to thermosetting polymers because of their chemical structure and reactivity. Epoxy is a thermosetting polymer which, when combined with a curing agent, chemically reacts to form a crosslinked matrix. In the present study, we demonstrate that to electrospin epoxy and obtain continuous micro and nanofibers, one must precisely control the curing reaction. Epoxy was mixed with triamine curing agent and, to enable electrospinnability, was dissolved in a mixture of tetrahydrofuran and dimethylformamide solvents. We identified a narrow working window wherein a proper solution for electrospinning is close to the gel point, right before the transition from liquid to solid gel state. The solution was characterized by means of (i) Fourier-transform infrared spectroscopy to monitor the extent of reaction, (ii) steady shear viscosity to detect the divergence near the gel point, and (iii) oscillatory loss and storage shear moduli to identify the liquid-to-gel transition. Based on these measurements, it was possible to monitor the chemical transformations that the epoxy solution underwent with time, such as chemical interconnections and gelation, and thus define the working window for electrospinning.