Manipulating the microstructure and rheology in polymer-organoclay composites

A series of nanocomposites prepared by melt-blending of cloisite-based organoclays with poly(ethylene-vinylacetate) (EVA) and neutralized poly (ethylene-methacrylic acid) (EMA) copolymers were investigated via DSC, small-angle X-ray scattering (SAXS), and rheological techniques. SAXS results indicated partial clay exfoliation in all samples. In both EMA and EVA systems, the nominal melting temperature T-m and bulk crystallinity are not significantly affected by the presence of organoclays, suggesting that clay particles are predominantly confined in the amorphous phase. In rheological measurements (above T-m), the EVA-clay system demonstrated a solid-like rheological behavior under the small-strain oscillatory shear, yet it was able to yield and flow under a steady shear, which is the characteristic of physical crosslinking. In contrast, the EMA-clay system exhibited a melt-like rheological behavior, where the influence of organoclay on the thermorheological. behavior of the EMA composite was quite minimal. We propose that the carbonyl groups of vinylacetate in EVA interact with the clay surface, resulting in a strong physically crosslinking like interaction in the melt. On the other hand, the interaction between EMA and clay is weak because of repulsion between carboxyl anions and negatively charged clay surface.