Postprocesses in tubular electrospun nanofibers A. Arinstein 1,2 and E. Zussman 2 1 Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel 2 Department of Mechanical Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel Received 6 August 2007; published 9 November 2007 The postprocesses that occur in coelectrospun polymer nanofibers are investigated. The high rate of solvent evaporation during the electrospinning of fibers results in such a rapid formation of the shell of the tubular nanofibers, that the polymer molecules composing the fibers are in a nonequilibrium state. This stretched state of macromolecules is assumed to be stabilized in a solid matrix, and can account for the anomalous properties of the nanofibers. During this processing stage a considerable amount of solvent remains inside the tubular nanofibers. The evaporation of the solvent continues for several minutes, and is accompanied by further evolution of the nanofibers in both their microstates and macrostates. In this paper, we examine possible macrostate modifications of the nanofibers, such as radial buckling. The theoretical model which describes the kinetics of the solvent evaporation is found to be in good agreement with experimental observations. Thus, the physical parameters of the system in question can be estimated, and the conditions of fiber shell instability that produce buckling of the tubular nanofibers can also be predicted. DOI: 10.1103/PhysRevE.76.056303 PACS numbers: 47.61.k, 81.05.Lg, 81.16.c I. INTRODUCTION Electrospinning is a commonly used process for fabricat- ing polymer-based nanofibers 1–4. In this process, a poly- mer solution is extruded from a spinneret, and in the pres- ence of a sufficiently strong electric field, a jet is formed at the tip. This jet then undergoes extreme elongation thereby stretching the polymer molecules within it. As this is occur- ring, the rapid solvent evaporation fixes the polymer matrix in this stretched, yet nonequilibrium state. This process al- lows for the fabrication in a single stage and in less than 10 ms, of nanofibers ranging from 100 to 1000 nm in diam- eter. The morphology and mechanical properties of the col- lected fibers are commonly studied by focusing on the pa- rameters of the electrospinning process 5–9. Many studies have examined polymer rheology 10, while others have focused on controlling the electrostatic field, or determining the effect of various setup configurations 11. However, the effect of the evaporation rate on the physical features of the electrospun fibers has not been studied in detail. The very rapid evaporation process is a challenging prob- lem for experimental investigation. However, theoretical analysis and computer simulations can clarify some aspects of the problem in question. For example, the role that the high evaporation rate plays in the fabrication of polymeric electrospun nanofibers has already been discussed by Koom- bhongse et al. 12 and treated quantitatively by Guenthner et al. 13. In particular, it was demonstrated that when the evaporation is very fast, the polymer density at the fiber/ vapor interface increases sharply, thus creating a polymer density gradient that acts as a barrier, or skin, that resists further solvent evaporation. These results are in good agreement with our presumption that despite the rapid evaporation that has occurred, the collected electrospun nanofibers still contain a significant amount of solvent. The presence of the solvent, which now evaporates even slower due to the barrier that has formed, apparently results in “relaxation” of the fabricated nanofi- bers. This relaxation causes certain postprocesses to take place within the system. Therefore, the effect of this residual solvent is crucial to understanding the mechanisms that af- fect the nanofibers’ mechanical properties, and provides the motivation for the presented work. The structure of this paper is as follows: First, we de- scribe the experimental system which was designed to allow us to observe the evaporation kinetics within the tubular electrospun nanofibers containing the solvent. Next, we pro- pose a physical model of the processes observed in the ex- periments. Based on this model and the experimental obser- vations, we are able to elucidate the effect of some of the system parameters. Finally, we discuss one of the possible phenomena which occurs as a result of solvent evaporation, i.e., the radial buckling of nanofibers, which is occasionally observed in our experimental system. II. EXPERIMENTAL SYSTEM As mentioned above, the investigation of the very rapid solvent evaporation occurring in the electrospinning process is a complex experimental problem. Therefore, we focus on a special case of the system discussed by Guenthner et al. 13. Their theoretical analysis and simulation have shown that the polymer molecules in the electrospun fibers are inhomoge- neously distributed in the cross section of the nanofiber, forming a compact surface or “shell.” Also, a spongy core is formed as a result of the entrapped solvent which cannot quickly escape, due to the low penetrability of the formed shell. Clearly, a tubular nanofiber filled with solvent can be considered as a special case of the Guenther et al. system. In our experimental system, we used tubular nanofibers, thereby enabling us to observe the decrease in the amount of solvent inside the nanofibers due to evaporation. In this manner, we are able to track the kinetics of solvent evaporation through the shell. The method of fabricating such tubular nanofibers is pre- sented in detail in Ref. 14. Briefly, two polymer solutions PHYSICAL REVIEW E 76, 056303 2007 1539-3755/2007/765/0563037 ©2007 The American Physical Society 056303-1