Analysis of HNBR-montmorillonite nanocomposites - Morphology, orientation and macroscopic properties

Morphologies and dynamic mechanical properties of hydrogenated acrylonitrile butadiene rubber (HNBR) filled with organophilic layered silicates (OLS) were investigated. The results from transmission electron microscopy (TEM), small angle X-ray scattering (SAXS, especially X-ray diffraction on larger structures) and dynamic mechanical analysis (DMA) favor the notion of local filler-filler network-like clusters consisting of exfoliated silicate layers. Especially, processing under high shear forces leads to an anisotropic structure of the network. Due to the orientation of the clusters the aspect ratio (at small deformations) increases. This is in accord with the large reinforcing effect observed for these materials. For example, the stress at 50% elongation from tensile test can be increased at 33% with 5 phr OLS compared to the unfilled reference. We estimate the activation energy of the immobilized polymer layer around the filler structures and show that the values depend on the spatial direction (the values are between 3.4 kJ/mol and 4 kJ/mol). As a result, the DMA-analysis could be used as a supporting method to evaluate orientation effects of the filler structures. Furthermore, from SAXS experiments, measured in the three directions in space, and from TEM analysis one could conclude that small amounts of intercalated structures are still present. These structures are orientated, too. The results make evident that the orientation of intercalated structures should be taken into account, especially if SAXS is used to estimate the degree of exfoliation.