Rheology of organoclay nanocomposites: Effects of polymer matrix/organoclay compatibility and the gallery distance of organoclay

The linear dynamic viscoelasticity, and transient and intermittent shear flows of organoclay nanocomposites, which were prepared with poly(ethylene-ran-vinyl acetate) (EVA), poly(ethylene-ran-vinyl alcohol) having 53 mol % vinyl alcohol (EVOH-53), or poly(ethylene-ran-vinyl acetate-ran-vinyl alcohol) having 5 mol % vinyl alcohol (EVAOH-5) were investigated. For the study, two organoclays (Southern Clay Products) were employed: W Cloisite 30B (having the gallery distance of 1.85 nm) treated with a surfactant (MT2EtOH) containing tallow, quaternary ammonium chloride, and hydroxyl groups and (ii) Cloisite 15A (having the gallery distance of 3.15 nm) treated with a surfactant (2M2HT) containing hydrogenated tallow, quaternary ammonium chloride but no hydroxyl group. Thus, a total of six nanocomposites were prepared by melt blending using a Brabender mixer. Before taking rheological measurements, the nanocomposites were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The linear dynamic viscoelastic measurements indicate that the dynamic storage modulus of the (EVAOH-5)/Cloisite 30B and EVA/Cloisite 15A nanocomposites increased as the temperature increased from 120 to 180 degreesC, while the dynamic storage modulus of the (EVAOH-5)/Cloisite 15A and EVA/Cloisite 30B nanocomposites decreased with increasing temperature. The intermittent shear flow experiments indicate that structural reorganization occurred, during the rest period upon cessation of the initial transient, in the (EVAOH-5)/Cloisite 30B and EVA/Cloisite 15A nanocomposites, while there was very little evidence of such for the (EVAOH-5)/Cloisite 15A and EVA/Cloisite 30B nanocomposites. The unusual rheological observations made in this study are explained in terms of the compatibility between the polymer matrix and organoclay and the gallery distance of organoclay. In-situ Fourier transform infrared spectroscopy indicates that EVAOH-5 and Cloisite 30B, which both have hydroxyl groups, formed hydrogen bonds even at 180 degreesC, the highest experimental temperature employed, while little evidence was found indicating the formation of hydrogen bonds between EVAOH and Cloisite 15A.