Anisotropic effects and master curves for rubbers with sp2 carbon allotropes towards light weight materials

This work presents the preparation of lightweight rubber materials with nanosized sp2 carbon allotropes and discusses the anisotropic nonlinear mechanical behavior of composites based on these nanofillers. Composites were prepared with either poly(styrene-co-butadiene) or poly(1,4-cis-isoprene) as the polymer matrix and either carbon black (CB) or carbon nanotubes (CNT) or hybrid CB/CNT as the filler systems. The initial modulus of the composite (G’min) was determined through dynamic mechanical shear tests and was correlated with the specific interfacial area (i.a.), calculated through the product of filler surface area, density and volume fraction. Common correlation was established, the equation of the common interpolating curve was derived and was used to design composites with the same modulus and lower density, substituting part of CB with lower amount of the carbon allotrope with larger surface area, CNT. Anisotropic nonlinear mechanical behavior was found for nanocomposites based on CNT and poly(1,4-cis-isoprene), prepared by melt blending, calendering and compression molding. An orthotropic and transversally isotropic response was observed: dynamic-mechanical moduli were very similar inside the sheet plane and very different from those in the orthogonal direction. Hence, energy dissipation is not isotropic in CNT filled rubber composites. Such mechanical behavior was correlated with the material structure: alternate areas containing large or low CNT amount and preferential orientation of CNT were observed. In spite of this anisotropic behavior, the validity of the above mentioned mastercurve was confirmed.