Mapping the Influence of Solubility and Dielectric Constant on Electrospinning Polycaprolactone Solutions

Abstract

Solvent–polymer interactions critically influence not only the viscoelasticity and the critical minimum solution concentration required for electrospinning but also the diameter, crystallinity, tensile strength, aspect ratio, and morphology of the electrospun fibers. Hence, a good understanding of the solvents and nonsolvents available and electrospinnable for a polymer of interest is important. The electrospinnability–solubility map uniquely presents the solubility and the electrospinnability of all solvents for a polymer in a single figure. Poly(ε-caprolactone) (PCL), an important polymer in biomedical applications, has been electrospun in a few conventional solvent systems, but a comprehensive mapping of its solvents for electrospinning has not been performed. Based on 49 common solvents of diverse solubility parameters and functional groups, the spinnability–solubility graph for electrospinning PCL solutions was mapped for the first time to enable a comprehensive understanding of the processability of all solvent choices for electrospinning PCL solutions. Furthermore, to date, many studies have demonstrated the importance of the dielectric constant (relative permittivity) of solvents in solution electrospinning, but few have systematically investigated its influence for a broad range of solvent systems. Based on the comprehensive PCL solvent map, this work studies the influence of dielectric constant of solvent systems on the electrospinning of PCL solutions. PCL ( M n = 80 000 g/mol) fiber diameters <100 nm were achieved when the dielectric constant of a solvent system was ∼19 and above, below which fibers or relics of diameters from submicrometer to millimeter range were produced. A detailed investigation was carried out on solvent systems with a calculated range of dielectric constants by mixing acetic acid and formic acid—two solvents with significantly different dielectric constants but the same functional group and comparable in other physical properties influential in electrospinning. With increasing dielectric constant, the required applied voltage to achieve stable jetting increased, the frequency of bead-on-string morphology decreased, and the interfiber spacing increased without affecting the total mass of fibers spun per unit time. In addition, the dissolution and electrospinnability of poor or nonsolvents of PCL were tested at temperatures 10 °C or higher than the ambient temperature. A unique and novel morphology of electrospun fibers within electrosprayed relics was generated for the first time when electrospinning PCL in 2-ethoxyethyl acetate, a nonsolvent at an ambient temperature of 20–22 °C and a partial solvent of PCL at an elevated temperature of above 30 °C.