Macromolecular Nanotechnology Surface Plasticization of Poly(ether ether ketone) T. Iqbal ⇑ , B.J. Briscoe, P.F. Luckham Chemical Engineering Department, Imperial College, South Kensington, London SW7 2AZ, UK article info Article history: Received 15 March 2011 Received in revised form 22 September 2011 Accepted 26 September 2011 Available online 5 October 2011 Keywords: PEEK Nanoindentation Plasticization Solubility parameter abstract This paper describes a study of the surface plasticization and antiplasticization of an amor- phous and a semicrystalline poly(ether ether ketone) (PEEK) in solvent environments using nanohardness method. A range of solvents (octane, chloroform, tetrachloroethane, acetone, dichlorobenzene, polyethyleneglycol (PEG), methanol and water) based on the Hilder- brand’s Solubility Parameter were selected as solvent environments. The results of the nan- oindentation hardness experiments performed on the virgin and the solvent immersed polymeric surfaces are reported. The surface plasticization or antiplasticization is reported on the basis of the softening or the hardening of the near surface layers (61 lm) after immersion of the polymeric surfaces in the solvent environments. Surface plasticization of the amorphous PEEK has been observed in organic solvents. The chlorine containing solvents have severely degraded the hardness of the amorphous polymer. A surface hardening of the amorphous PEEK has been observed after immersion in water. Semicrystalline PEEK was seen to exhibit a considerable inert behaviour to common organic solvents but chlorinated organic solvents and water have caused a decrease in the surface mechanical properties. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Most often applications of polymers involve the contact of the polymeric surface with potentially aggressive envi- ronments, such as solvents, lubricating oils, detergents and cleaning solutions. The polymeric surface often be- comes plasticised, or softened, if exposed to such chemical environments for a long period of time. The plasticization of the polymer produces softening of the surface which deteriorates the surface mechanical properties and ulti- mately the optical characteristics of the polymer. Dissolu- tion of a solid polymeric surface in a solvent is invariably a slow process. The dissolution process can be divided into two stages. A swollen gel is produced in the first step, where the solvent molecules diffuse into the polymer structure. This process takes place in the case when the polymer–polymer molecular interactions are comparable to the affinity of the solvent to polymer molecule. If the solvent–polymer interactions are stronger, a gel like solu- tion of the polymer will be formed in the solvent. The sol- vent penetration through the polymeric surface reduces the intermolecular forces causing a partial relaxation of the polymer chains with a strain relaxation. The rate of dif- fusion of the solvent determines in part the depth to which surface plasticization will take place. The rate of relaxation of the swelling strain is also a crucial factor [1]. Since the Fick’s law relationship do not consider the time dependent structural changes taking place at the polymer–solvent interface front, the solvent diffusion process cannot be described only based on the concentration gradient. Alfred et al. [2] have categorized the diffusion behaviour based on the comparison between the rate of penetrant mobility and the polymer segment relaxation; (1) Case 1 (Fickian) diffusion, the penetrant mobility is much less than the polymeric segmental relaxation rate. 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.09.022 ⇑ Corresponding author at. Chemical Engineering Department, KSK Campus, University of Engineering & Technology, Lahore, Pakistan. Tel.: +92 (0) 3008430776; fax: +44 020 7594 5638. E-mail addresses: t.iqbal07@imperial.ac.uk, tanveer@uet.edu.pk (T. Iqbal). European Polymer Journal 47 (2011) 2244–2258 Contents lists available at SciVerse ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj MACROMOLECULAR NANOTECHNOLOGY