Nanocomposites from styrene-butadiene rubber (SBR) and multiwall carbon nanotubes (MWCNT) part 1: Morphology and rheology

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 -15 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 produces 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 Guth-Gold-Smallwood theory. These materials do not show a high level of electrical conductivity as might be expected from a percolation concept, signifying excellent tube dispersion and formation of a bound rubber layer on the discrete MWCNT. (C) 2013 Elsevier Ltd. All rights reserved.