Effect of solvents on electro-spinnability of polystyrene solutions and morphological appearance of resulting electrospun polystyrene fibers

Abstract

The effects of solvents and their properties on electro-spinnability of the as-prepared polystyrene (PS) solutions and the morphological appearance of the as-spun PS fibers were investigated qualitatively by means of a scanning electron microscope (SEM). The eighteen solvents used were benzene, t-butylacetate, carbontetrachloride, chlorobenzene, chloroform, cyclohexane, decahydronaphthalene (decalin), 1,2-dichloroethane, dimethylformamide (DMF), 1,4-dioxane, ethylacetate, ethylbenzene, hexane, methylethylketone (MEK), nitrobenzene, tetrahydrofuran (THF), 1,2,3,4-tetrahydronaphthalene (tetralin), and toluene. The PS solutions in 1,2-dichloroethane, DMF, ethylacetate, MEK, and THF could produce fibers with high enough productivity, while the PS solutions in benzene, cyclohexane, decalin, ethylbenzene, nitrobenzene, and tetralin were not spinnable. Qualitative observation of the results obtained suggested that the important factors determining the electro-spinnability of the as-prepared PS solutions are high enough values of both the dipole moment of the solvent and the conductivity of both the solvent and the resulting solutions, high enough boiling point of the solvent, not-so-high values of both the viscosity and the surface tension of the resulting solutions.

Introduction

Electrospinning is an interesting process for producing non-woven fibers with the average diameters in the range of micro to nanometers. In this process, a continuous filament is drawn from a polymer solution or a melt through a spinneret by high electrostatic forces to deposit on a grounded-metal collective screen [1]. The appearance of the collected fibers depends on many factors: (a) viscoelastic force which has been found to depend on solution concentration [2], [3], [4], [5], molecular weight average of the polymer [3], and viscosity of the solution [5], [6], [7], (b) surface tension which has been found to depend on solution concentration, molecular weight average of the polymer, and surface tension of the solvent [6], (c) gravitational force which is dependent on solution density, and (d) electrostatic force which has been found to depend on the applied electrostatic field (i.e. an applied electrostatic potential divided by a collection distance) and the conductivity of the solution [5], [6].

In the electrospinning process of polymer solution, solvent is one of the main contributor for solution properties, e.g. conductivity. Water, a good solvent for poly(ethylene oxide) (PEO), has been successfully used in the electrospinning process of PEO [6], [8], [9]. When ethanol was added to water to attain mixed solvent systems, the diameters of the electrospun PEO fibers became bigger and the beaded fibers earlier observed disappeared [6]. The reason was claimed to the reduction of charge density carried by a charged jet and the increase in viscosity and evaporation rate of the mixed solvent, respectively. Dimethylformamide (DMF) was successfully used as a solvent for the electrospinning of polyacrylonitrile [10] and polyurethaneurea copolymer [4]; whereas, for biodegradable poly(p-dioxanone-co-l-lactide)-block-poly(ethylene glycol) copolymer, a mixed solvent of dichloromethane and DMF had to be employed [11]. Solvents with high vapor pressures, e.g. carbondisulfide (CS₂), have been discussed as the cause of the observed nanoporous morphology of electrospun polystyrene (PS) fibers [12]. Trichloromethane or chloroform (CHCl₃) was found to be the best solvent for producing highly textured poly(methyl methacrylate) (PMMA) fibers [12].

In a recent preliminary report [13], six solvents (i.e. acetic acid, acetonitrile, m-cresol, toluene, THF, and DMF) with different properties (e.g. density, boiling point, solubility parameter, dipole moment, and dielectric constant) were used to prepare electrospun polystyrene (PS) fibers. Fiber diameters were found to decrease with increasing density and boiling point of the solvents. A large difference between the solubility parameters of PS and a solvent was responsible for the bead-on-string morphology observed. Productivity of the fibers (the numbers of fiber webs per unit area per unit time) was found to increase with increasing dielectric constant and dipole moment of the solvents. Among the solvents investigated, DMF was the best solvent that provided PS fibers with highest productivity and optimal morphological characteristics. The beadless, well-aligned PS fibers with a diameter of ≈0.7 μm were produced from 10% (w/v) PS solution in DMF at an applied electrostatic field of 15 kV/10 cm, a N₂ flow rate of 101 ml/min, and a rotational speed of the collector of 1500 rpm.

In the present contribution, solutions of PS in eighteen different solvents were prepared in three different concentrations (i.e. 10%, 20%, and 30% (w/v), respectively). These solutions were tested for some basic properties, i.e. viscosity, surface tension, and conductivity. The effects of solvents and their properties on electro-spinnability of the as-prepared PS solutions and morphological appearance of the obtained PS fibers were qualitatively observed by means of a scanning electron microscope (SEM).