Relationships between structure and rheology in model nanocomposites of ethylene-vinyl-based copolymers and organoclays

A series of nanocomposites prepared by melt-blending of Cloisite organoclays with ethylene-co-vinyl acetate (EVA) and ethylene-co-methyl acrylate (EMA) copolymers were investigated by using small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and rheological techniques. SAXS and TEM results confirmed mixed clay intercalation and exfoliation in all tested nanocomposites. The melting temperature, T _m, and crystalline structure (orthorhombic) in EMA and EVA were not significantly affected by the presence of organoclays, indicating that the clay particles were predominantly confined to the amorphous phase. Rheological properties above T _m were very similar in EVA and EMA nanocomposites. Both systems exhibited pseudo-solid rheological behavior in small-strain oscillatory shear experiments, yet they could yield and flow under a steady shear, which is characteristic of physical gelation. The pseudo-solid rheological behavior in EVA and EMA nanocomposites becomes more pronounced at higher contents of organoclay and at higher temperatures. SAXS results indicated that the silicate gallery spacings (d), intercalated by EVA and EMA chains, decreased with increasing temperature. This can be attributed to the reduced compatibility between organoclay and polymer (i.e., a LCST-type phase behavior). The unusual rheological properties of the nanocomposites at high temperatures were probably due to the formation of a 3D network of clay tactoids. Novel analytic models were proposed to describe rheological data from meltlike to gellike behaviors in EVA- and EMA-organoclay nanocomposites.