Author's personal copy Nanocomposites of poly(ether ether ketone) with carbon nanofibers: Effects of dispersion and thermo-oxidative degradation on development of linear viscoelasticity and crystallinity Shriraj H. Modi a, b , Kimberly B. Dikovics a, b,1 , Halil Gevgilili a , Gaurav Mago c , Stephen F. Bartolucci d , Frank T. Fisher c , Dilhan M. Kalyon a, b, * a Highly Filled Materials Institute, Stevens Institute of Technology, Hoboken, NJ 07030, USA b Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA c Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA d U.S. Army Benét Laboratories, Armaments Research Development and Engineering Center, Watervliet, NY 12189-4000, USA article info Article history: Received 9 June 2010 Received in revised form 20 August 2010 Accepted 27 August 2010 Available online 6 September 2010 Keywords: PEEK Nanocomposites Carbon nanofibers abstract Poly(ether ether ketone), PEEK, is a widely used engineering plastic that is especially suitable for high temperature applications. Compounding of PEEK with carbon nanofibers, CNF, has the potential of enhancing its mechanical and thermal properties further, even at relatively low CNF concentrations. However, such enhancements can be compromised by myriad factors, some of which are elucidated in this study. Considering that the dispersion of the CNF into any high molecular weight polymer is a challenge, two different processing methods, i.e., melt and solution processing were used to prepare PEEK nanocomposites with low aspect ratio carbon nanofibers. The linear viscoelastic material functions of PEEK nanocomposites in the solid and molten states were characterized as indirect indicators of the dispersion state of the nanofibers and suggested that the dispersion of nanofibers into PEEK becomes difficult at increasing CNF concentrations for both solution and melt processing methods. Furthermore, the time-dependence of the linear viscoelastic material functions of the PEEK/CNF nanocomposites at 360e400  C indicated that PEEK undergoes thermo-oxidative cross-linking under typical melt processing conditions, thus preventing better dispersion by progressive increases of the mixing time and specific energy input during melt processing. The crystallization behavior of PEEK is also affected by the presence of CNF and degree of cross-linking, with the rate of crystallization decreasing with increasing degree of cross-linking and upon the incorporation of CNFs both for the solution and melt processed PEEK nanocomposites. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Poly(ether ether ketone), or PEEK, is an engineering plastic that is widely used especially in aerospace, automotive and health care applications [1,2] because of its superior mechanical and chemical properties [3]. PEEK has a glass transition temperature of 143  C and a melting temperature of approximately 343  C [4]. Such high transition temperatures permit its utilization at relatively high temperatures, but also require processing at correspondingly high temperatures, i.e., typically >360  C, at which the degradation of PEEK becomes an issue. The thermal degradation behavior of PEEK has been investigated under a variety of conditions [4e9]. These studies have generally kept PEEK at different temperatures for differing durations of time and then analyzed the resulting structure and properties. Day et al. [7] and Jonas and Legras [8] have indicated that the processing of PEEK at high temperatures gives rise to a progressive branching reaction leading to cross-linking. Patel et al. [9] have recently reviewed the * Corresponding author. Highly Filled Materials Institute, Stevens Institute of Technology, Hoboken, NJ 07030, USA. Tel.: þ1 2012168225. E-mail address: dilhan.kalyon@stevens.edu (D.M. Kalyon). 1 Present address: Chemical and Biological Engineering Department, Princeton University, NJ 08544, USA. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.08.060 Polymer 51 (2010) 5236e5244