The Significance of Percolation on the Dynamics of Polymer Chains Bound to Carbon Black
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The Significance of Percolation on the Dynamics of Polymer Chains Bound to Carbon Black Alan I. Nakatani, Robert Ivkov,# Peter Papanek,#,‡ Catheryn L. Jackson, Henry Yang,* Leszek Nikiel,* and Michel Gerspacher* National Institute of Standards and Technology, Polymers Division, Gaithersburg, MD 20899 # - NIST Center for Neutron Research ‡ - Department of Materials Science, University of Pennsylvania * - Sid Richardson Carbon Company, Fort Worth, TX 76106 INTRODUCTION A critical need in the fundamental understanding of reinforcement in filled polymers is the characterization of the polymer-filler interface and the dynamics of the polymer in this interfacial regime. In carbon black filled polymers, one of the central themes in the mechanism of reinforcement is that of “bound” polymer. Understanding the dynamics of this bound polymer may be key to arriving at an understanding of reinforcement mechanisms in filled polymers. The interactions between polymers and filler surfaces are also key in the development of more advanced nanocomposite materials. We have previously utilized inelastic neutron scattering methods to examine the variation of bound polymer dynamics as a function of carbon black type for a single, initial carbon black concentration.1 An apparent change in the distribution in backbone motions was observed in the bound polymer compared with the pure polymer. In this study, we extend our prior work to examine the bound polymer dynamics as a function of the type of carbon black and the initial concentration of carbon black. The results suggest that two types of dynamic behavior are observed as a function of the initial carbon black concentration. This critical cutoff concentration may be related to the percolation threshold of the carbon black and suggests that quantifying the amount of bound polymer is insufficient for understanding the relationship between mechanical behavior and bound polymer content. EXPERIMENTAL We have compared the dynamics of bound polymer using inelastic neutron scattering measurements performed on a Fermi chopper time-of-flight (TOF) spectrometer. The data is presented in a “density of states” formalism, which gives the distribution of the vibrational energies, g(ε), in the sample over the accessible energy range of the instrument. The vibrational energy range covered by the neutron method can reach very low energies (i.e. frequencies) which are lower than the range accessed by infrared or Raman techniques, hence, longer wavelength, lower energy motions can be probed. We examine samples containing only bound polymer by performing a Soxhlet extraction on all samples to remove all unbound polymer and compare all results to the pure, uncompounded polymer. Materials Three different types of carbon black were compounded into a synthetic, commercial polyisoprene (Natsyn 2200, Goodyear Tire and Rubber Co.).2 Each carbon black is identified by KK4.2.1
the specific grade (N299, G299, or N762). The G299 carbon black is the graphitized version of the N299 carbon black and was prepared by heati
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