Electron Spin Resonance on polyurethanes e Vapor grown carbon nanofiber composites Mircea Chipara a, * , Richard A. Vaia b , Ali Nasar c a The University of Texas e Pan American, Edinburg, TX 78541, USA b Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA c CNC Coatings, Rochdale, Lancashire OL11 1HY, United Kingdom article info Article history: Received 20 October 2013 Received in revised form 20 February 2014 Accepted 24 February 2014 Keywords: Polyurethanes-vapor grown carbon nanofibers Electron Spin Resonance Free radicals Electrical conductivity Percolation Polarons abstract Electron Spin Resonance investigations on polyurethane e vapor grown carbon nanofiber composites are reported. The research focuses on the free radicals generated during the processing step and on the ESR spectrum of the conducting electrons delocalized over the carbon nanofiber. The deactivation of free radicals due to their interaction with the electrons residing on carbon nanofiber was analyzed and related to the enhanced stability of these composites. The dependence of the resonance line parameters on the concentration of the nanofiller is studied in detail. Experimental data indicates that the electrical conductivity is due to uncoupled electrons delocalized over the conducting domains of the nanofiller and suggests that the electrical conductivity is dominated by polarons. The percolation of the electrical conductivity is also reflected in the ESR data. The interactions between uncoupled electronic spins, as revealed by the line width dependence on the concentration of carbon nanofibers are discussed in detail. Published by Elsevier Ltd. 1. Introduction Polyurethanes are extremely interesting materials due to their shape memory capabilities [1]. The addition of carbon nanotubes or nanofibers enhances their mechanical properties (and in particular increases of the Young modulus [2,3,4], hardness [5], elongation at break [2,4], and stress at break [4]), improves the thermal con- ductivity [6,7], augments the thermal stability of the polymeric matrix [4,6,8], and adds electrically conducting capabilities [7,9,10]. High concentration of carbon nanotubes (typically above 10% phr; where phr stands for parts per hundred parts of matrix, which frequently is a resin) may result in the decrease of the elongation at break. Typically, the tensile strength is enhanced as the loading is increased [2,5,8]. However, nanofillers may destroy or lower the memory shape capabilities of the polymeric matrix. Electron Spin Resonance (ESR) spectroscopy is a powerful technique in the study of uncoupled electronic spins (such as free radicals and conducting electrons residing on carbon nanotubes or nanofibers) [11]. The ESR spectrum reflects the resonant absorption of energy from an ensemble of uncoupled electronic spins placed in a magnetic field of intensity H. The energy is absorbed from the magnetic component of an electromagnetic field of frequency v. For most commercially available spectrometers, the frequency lies in the microwave range (X band or 9  10 9 Hz). The consequence of this resonant absorption is the flip of the electronic spin. The resonant condition equals the splitting of the energy level (Zeman levels) in the external magnetic field to the energy of the electro- magnetic radiation; hv ¼ gm B B where h is the Planck’s constant, m B is the Bohr’s magneton for the electron, B is the intensity of the external magnetic field, and g is the so called g factor, Lande factor, or gyromagnetic factor. For a free uncoupled electronic spin, without relativistic corrections g ¼ 2.00. The study of conducting electrons by ESR is frequently referred as Conduction Electron Spin Resonance [11,12]. Typically, the ESR spectrometer records both the spectra of free radicals and of con- duction electrons, if both are present in the system under investi- gation. ESR spectroscopy has just recently started to be utilized in the study of carbon-based nanostructures, as cleaner materials became available [13]. Early carbon nanostructures contained an important fraction of magnetic residues originating from catalyst’s * Corresponding author. The University of Texas e Pan American, Department of Physics and Geology,1201 W. University Drive, Edinburg, TX 78541, USA. Tel.: þ1 956 605 5123; fax: þ1 956 665 2423. E-mail addresses: mchipara@utpa.edu, chipara@yahoo.com (M. Chipara). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2014.02.018 0042-207X/Published by Elsevier Ltd. Vacuum xxx (2014) 1e5 Please cite this article in press as: Chipara M, et al., Electron Spin Resonance on polyurethanes e Vapor grown carbon nanofiber composites, Vacuum (2014), http://dx.doi.org/10.1016/j.vacuum.2014.02.018