Influence of initiators on the growth of poly(ethyl 2-cyanoacrylate) nanofibers Pratik J. Mankidy a , Ramakrishnan Rajagopalan b , Henry C. Foley a, b, c, * a Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, United States b Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, United States c Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, United States article info Article history: Received 19 October 2007 Received in revised form 4 March 2008 Accepted 5 March 2008 Available online 14 March 2008 Keywords: Polymer nanofibers Poly(ethyl 2-cyanoacrylate) Synthesis abstract The type of anionic initiator used to polymerize ethyl 2-cyanoacrylate was found to influence the morphology of the polymer formed via vapor phase polymerization. Depending upon the type of initi- ator, polymerization of ethyl 2-cyanoacrylate resulted in either the formation of neat polymer nanofibers (w200 nm in diameter) or thin films. Based on the classification of anions using Hard Soft Acid Base principles, we found that harder anions favored polymer film formation while softer ones favored polymer nanofibers. Infrared (IR) spectroscopy, scanning electron microscopy (SEM) and gel permeation chromatography (GPC) were used to characterize the structure, morphology and molecular weight of the synthesized polymers, respectively. Finally, a mechanism of formation of different polymer morphologies is proposed. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The importance of polymer nanofibers is apparent from the vast volume of literature in recent years focusing on synthesis methods, properties and applications of nanofibers. Nanofibrous polymer structures offer a high surface area to volume ratio which can be exploited for a variety of applications as summarized by recent reviews [1,2]. These applications include making use of the me- chanical properties of nanofibers for reinforced composites [3], tissue scaffolding material [4], controlled drug delivery applications [5] or as filtration media [6]. Currently there are two major approaches for the fabrication of polymer nanofibers, template-based methods or electrospinning. Template-based methods [7] involve either extrusion of a polymer melt through nano-sized pores of a template, for example through an anodized aluminum oxide membrane [8] or extrusion poly- merization of polymer nanofibers via catalyst immobilized on the walls of a nanotemplate, such as mesoporous silica [9]. This method, however, makes removal of the fibers from the template complex. Electrospinning [1,2] involves the use of a high voltage source to generate electrically charged polymer jets, which are collected on a substrate as a mat of nanofibers. This technique necessitates that the polymer be either melt or solvent processable and be able to withstand the high voltage electric fields. A lesser used approach for polymer nanofibers formation is template-less formation of nanofibers during polymerization. Polyaniline nano- fibers have been synthesized this way either extrinsically by adding ‘structure-directing’ agents during polymerization [10], or in- trinsically by interfacial polymerization schemes [11]. Poly- acetylene fibrils ranging in diameters from 3–20 nm have also been reported for certain conditions of Ziegler–Natta polymerized acetylene [12]. This synthesis method of polymer nanofibers presents an interesting and truly ‘bottom-up’ route that is an alternative to template-based processes or electrospinning for the mass production of nanofibers. We recently demonstrated a novel technique for facile growth of poly(ethyl 2-cyanoacrylate) [PECA] nanofibers without a template by vapor phase polymerization of the ethyl 2-cyanoacrylate [ECA] monomer [13]. ECA commonly known as Superglue Ò undergoes anionic polymerization initiated by a variety of covalent or ionic bases, including water [14]. Scheme 1 illustrates this polymeriza- tion via initiation by a nucleophile (Nu  ) and propagation by sub- sequent addition of ECA monomer units to the carbanion end of the growing chain. In our previous studies, it was observed that vapor phase po- lymerization of ECA carried out under high relative humidity conditions resulted in different morphologies of the polymer depending upon the type of initiator used. For certain initiators, such as NaCl, a mass of nanofibers were obtained, while for other initiators, for example NaOH, a textured polymer film was obtained. In a relevant study, Doiphode et al. [15] grew similar nanofibers of polycyanoacrylate on electrospun fibers by first exposing the electrospun fibers to water vapor and then to cyanoacrylate vapor. The water vapor condenses as tiny droplets on the electrospun fibers thereby rendering tiny islands of initiator that start the * Corresponding author. Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, United States. E-mail address: hfoley@ist.psu.edu (H.C. Foley). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Polymer 49 (2008) 2235–2242 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.03.018 Polymer 49 (2008) 2235–2242