Nanocomposites from styrene-butadiene rubber (SBR) and multiwall carbon nanotubes (MWCNT) part 1: Morphology and rheology S.K. Peddini a , C.P. Bosnyak b , N.M. Henderson b , C.J. Ellison a , D.R. Paul a, * a Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA b Molecular Rebar Design, LLC, 13477 Fitzhugh Rd, Austin, TX 78736, USA article info Article history: Received 16 July 2013 Received in revised form 30 October 2013 Accepted 1 November 2013 Available online 13 November 2013 Keywords: MWCNT-SBR masterbatch Rheology Dilution abstract Because of the exceptionally high modulus and aspect ratios of multiwall carbon nanotubes (MWCNT), there has been much interest in using them as reinforcing agents for polymer composites. However, the commercial implementation of such nanocomposites has generally met with very limited success owing to poor dispersion of the MWCNT in the polymer matrix. A strategy that overcomes many of these difficulties is described here with a view towards incorporating MWCNT with carbon black or silica for improved elastomer performance in such applications as tires. Key issues are control of the MWCNT surface functionality for proper individual tube dispersion, their aspect ratio for a balance of mechanical performance versus melt processability and an appropriate masterbatch concentration for ease of further formulation by rubber goods manufacturers. Styrene-butadiene rubber (SBR), commonly used as a tread stock for tires, is employed here as the matrix for creation of a masterbatch with oxidized MWCNT (12.3 e15 wt.%). Masterbatch rheology is necessary to understand how to achieve good dispersion and conformation of the MWCNT in the final product. Rheological characterization of the masterbatch nanocomposites and their dilutions over shear rate ranges relevant for processing will be described. Scanning transmission electron microscopy (STEM) investigations have revealed that this process pro- duces good dispersion of the MWCNT’s in the SBR matrix. The distribution of diameters, contour lengths, and end-to-end distances of the MWCNT in these formulations has also been determined. Effective tube aspect ratios for the nanocomposites with various MWCNT loadings were estimated by analysis of the rheological data for uncured specimens and the dynamic mechanical properties of cured composites using the GutheGoldeSmallwood theory. These materials do not show a high level of electrical con- ductivity as might be expected from a percolation concept, signifying excellent tube dispersion and formation of a bound rubber layer on the discrete MWCNT. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Since the documented discovery of carbon nanotubes (CNTs) by Iijima [1] in 1991, polymereCNT composite materials have been the subject of much academic and industrial research using different polymers including elastomers to improve mechanical and elec- trical properties [2e11]. CNTs can be classified by the number of walls in the tube, singlewall, doublewall and multiwall (MW), each wall can be further classified into chiral or non-chiral forms. Carbon nanotubes are generally manufactured commercially via a chemical vapor phase deposition process using iron, cobalt or nickel cata- lysts. The nanotubes usually exist as agglomerated balls or bundles, due to van der Waals attraction forces between them [12], as taken directly from the reactor and can contain significant amounts (5e 25 wt.%) of residual catalysts and char. Based on the method of synthesis, CNTs exhibit very high aspect ratios, i.e., length to diameter ratio, up to 1000 [13], and also have a very high tensile modulus. As a result, carbon nanotubes are predicted to have sig- nificant utility as a reinforcing agent in polymer composites. However, commercial utilization of carbon nanotubes in these ap- plications has been hampered by the general inability to reliably separate and disperse individualized carbon nanotubes in the polymer matrix. To reach the full potential of performance enhancement by carbon nanotubes in polymer composites, the aspect ratio should be substantially greater than 10, but not so high as to cause problems, such as high viscosity, that preclude pro- cessing. The maximum aspect ratio for a given tube length is reached when each tube is fully separated from others. A bundle of carbon nanotubes, for example, has an effective aspect ratio in composites of the order of the bundle length divided by its * Corresponding author. Tel.: þ1 512 471 5392; fax: þ1 512 471 0542. E-mail address: drp@che.utexas.edu (D.R. Paul). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.11.003 Polymer 55 (2014) 258e270