Taking Advantage of Supramolecular Structure in Melt and Solution Electrospinning
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Taking Advantage of Supramolecular Structure in Melt and Solution Electrospinning Matthew T. Hunley1, Matthew G. McKee2, Pankaj Gupta3, Garth L. Wilkes3, and Timothy E. Long4 1 Virginia Tech, Blacksburg, VA, 24061 2 Procter and Gamble, Cincinnati, OH, 45202 3 Department of Chemical Engineering, Virginia Tech, Blacksburg, VA, 24061 4 Chemistry, Virginia Tech, 2110 Hahn Hall (0344), Blacksburg, WA, 24061 Electrospinning has recently emerged as a promising polymer processing technique.1-6 Using a large electric potential, micro- and nanoscale fibers can be drawn from solution or melt. In this process, the high potential, typically on the order of 5-30 kV, deforms a droplet of polymer solution into a Taylor cone. When the potential forces overcome the surface tension of the Taylor cone, the polymer accelerates towards a grounded or oppositely charged collector. At low viscosities, the accelerating polymer solution or melt will break up into droplets during flight. At sufficiently high viscosities, the accelerating polymer will form a stable jet between the droplet and the target.3-5,7 During the flight of the jet, solvent evaporates rapidly, resulting in a constantly decreasing jet diameter and a subsequent increase in surface charge density. Once the charge density reaches a critical value, a bending instability occurs in the jet, resulting in chaotic whipping and significant stretching and drawing.8 Electrospinning has been shown to produce fibers and fibrous mats with repeatable and predictable diameters on the order of nanometers and micrometers. These fibrous assemblies have been shown applicable in drug-delivery,9-11 filtration,12,13 separations, and tissue engineering.11,14 This presentation concentrates on our groupís recent advances in electrospinning, including semi-empirical relationships for the electrospinning behavior of neutral, non-associating polymers and polyelectrolytes; electrospinning behavior of low molar mass amphiphiles, including mixtures of phospholipids; and melt electrospinning of star-shaped polylactides with thermally-reversible associating groups. Recent work in our laboratories, as well as that of Shenoy, Wnek, et al., has shown that the formation of uniform electrospun fibers relies on the presence of entanglements within solution.3,15 In solution, at very low polymer concentrations (the dilute regime), separate polymer chains rarely interact with each other, remaining isolated in solution. At higher concentration, polymer chains interact with each other, but remain predominantly independent (semi-dilute unentangled regime). At the critical concentration for entanglements, Ce, the polymer chains become crowded and overlap with each other (the semi-dilute entangled regime). It was shown by our group and by Wnek, et al., that fiber formation in electrospinning first occurs above Ce.3,4,7,15 At these concentrations, fibers and droplets form, leading to beaded fibers. Uniform fibers with no droplets form at concentrations of two times Ce, where slow viscoelastic relaxations domi
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