Shear and extensional rheology of EVA/layered silicate-nanocomposites

Nanocomposites of ethylene-vinyl acetate copolymer (EVA) with 28 wt.% vinyl acetate content (EVA28) was prepared by melt intercalation with different loadings of organically-modified bentonite clay. The microstructure and morphology of the nanocomposites were examined by X-ray diffraction (XRD) and electron microscopy. XRD and transmission electron microscopy (TEM) indicated that EVA28 nanocomposites had predominantly exfoliated morphologies. The dynamic and steady shear rheological properties of the nanocomposites showed remarkable differences in comparison to that of pure EVA28 copolymer. Linear viscoelastic parameters were enhanced at all frequencies investigated and indicated the presence of a percolated network structure. Steady shear measurements revealed that the elasticity of EVA28 nanocomposites were dependent on the silicate loading at high shear stresses. Uniaxial extensional viscosities were found to increase with silicate loading and in general exhibited strain hardening behavior. Beyond a critical strain, nanocomposite extensional viscosities were almost identical with that of the unfilled EVA28, suggesting that at high strains, silicates have little effect on the extensional viscosities. An attempt has been made to propose a possible mechanism that describes the influence of clay layers on the uniaxial extensional flow field. Images obtained from transmission electron microscopy (TEM) and an environmental scanning electron microscopy (ESEM) were used to support the proposed mechanism.