Polymer Entanglement Loss in Extensional Flow: Evidence from Electrospun Short Nanofibers Israel Greenfeld, Eyal Zussman Department of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel Correspondence to: I. Greenfeld (E - mail: green_is@netvision.net.il) Received 24 May 2013; revised 16 June 2013; accepted 17 June 2013; published online 17 July 2013 DOI: 10.1002/polb.23345 ABSTRACT: High strain rate extensional flow of a semidilute poly- mer solution can result in fragmentation caused by polymer entanglement loss, evidenced by appearance of short nanofibers during electrospinning. The typically desired outcome of electro- spinning is long continuous fibers or beads, but, under certain material and process conditions, short nanofibers can be obtained, a morphology that has scarcely been studied. Here we study the conditions that lead to the creation of short nanofibers, and find a distinct parametric space in which they are likely to appear, requiring a combination of low entanglement of the polymer chains and high strain rate of the electrospinning jet. Measurements of the length and diameter of short nanofibers, electrospun from PMMA dissolved in a blend of CHCl 3 and DMF, confirm the theoretical prediction that the fragmentation of the jet into short fibers is brought about by elastic stretching and loss of entanglement of the polymer network. The ability to tune nanofiber length, diameter and nanostructure, by modifying var- iables such as the molar mass, concentration, solvent quality, electric field intensity, and flow rate, can be exploited for improving their mechanical and thermodynamic properties, leading to novel applications in engineering and life sciences. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1377–1391 KEYWORDS: beads; disentanglement; electrospinning; fibers; fracture; networks; nanotechnology; nanofibers; inorganic poly- mers; jet; PMMA; short nanofibers; viscoelastic properties INTRODUCTION The high strain rate of an extensional flow, such as the flow of an electrospinning semidilute polymer solution jet, can result in fragmentation as a consequence of loss of the polymer network entanglement, leading to appearance of short nanofibers varying from a few tens of nanometers to a few microns in diameter. Short nanofibers are a unique morphology of electrospun polymeric nanofib- ers, in addition to the continuous nanofibers and nanobeads morphologies. Typically, long continuous nanofibers or beads are the preferred outcome in electrospinning applications, while short nanofibers are an undesirable defect. However, electrospun short polymeric nanofibers are expected to find important applications in engineering and life sciences. 1 Although short nanofibers can be produced by methods such as solution precipitation 2 or ultrasonication of continuous fibers, 3 their creation by electrospinning without any post- processing offers unique structures and enhanced mechani- cal properties. Discontinuous beaded fibers and elongated beads are known to appear in low-concentration solutions, when the concen- tration is sufficiently high to avoid excessive dilution and creation of beads from droplets. 4–6 By gradually increasing the concentration, continuous fibers begin to appear. Bead- free short fibers with aspect ratios of 10–200 were obtained by varying the polymer–solvent system and molecular weight. 1 Polymer entanglement was identified as the key fac- tor affecting the transition from the bead morphology, through that of elongated beads or short fibers, to that of continuous fibers. 1,4,6 Short nanofibers have been scarcely explored, and further study is needed to measure the dominant factors that influ- ence their creation and properties. Furthermore, the nanoscale mechanism that leads to short nanofiber formation requires clarification, particularly with regards to how disentanglement initiates the fragmentation of the electrospinning jet. Short nanofibers can be created in a semi–dilute entangled solution, depending on the polymer architecture (linear or branched), molar mass, solution concentration and solvent quality. Generally, at given electric field intensities and flow rates, lower concentrations and poorer solvents decrease chain entanglement, and consequently increase the likelihood that short nanofibers will appear during electrospinning (Fig. 1). Also, short nanofibers are frequently accompanied by beads-on-string and free beads [Fig. 1(b,d)], a phenomena associated with low viscosity and high surface tension. V C 2013 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2013, 51, 1377–1391 1377 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER