Impact of Entanglement Density on Solution Electrospinning: A Phenomenological Model for Fiber Diameter

The rheological properties of poly(N-isopropylacrylamide, PNIPAM) in dimethylformamide solvent were investigated and correlated with solution electrospinnability. The jet diameter was measured by using the light scattering technique during the electrospinning in the straight jet region prior to the jet whipping. The diameter of the straight jet end is independent of the solution concentration (or viscosity within the range of 15-2000 rnPa.s). Thus, the final fiber diameter d(f) observed on the grounded collector is dominantly controlled by the jet-whipping process. According to the present PNIPAM solution and other different polymer solutions, d(f) is correlated with the solution concentration phi. A master curve is constructed by using the following equation: d(f)/d(f,e) = (phi/phi(e))(2.5), where d(f,e) is the diameter of the fibers electrospun from the solutions with an entanglement concentration of phi(e), above which the specific viscosity starts to increase with phi according to phi(3.7) or phi(4.7), depending upon the given polymer/solvent pair. The derived exponent of 2.5 is in good agreement with the theoretical exponent value of 2.3 provided that d(f) is proportional to the entanglement density v(phi) similar to phi(2.3) (entangled strands per unit volume of the solution). Our results imply that the plateau modulus (elasticity) of the entangled polymer solution rather than its viscosity plays a major role in determining the final fiber diameter. The entangled polymer solutions behave like elastic swollen gels during electrospinning because of the high deformation rates. We propose that the deformation-induced structure formation in the jet eventually results in the fiber with the concentration-dependent diameter.