Effect of Morphology on Rheological Properties of Ethylene-vinyl Acetate Copolymer (eva) (containing 40 Wt% Va Content) Nanocomposites

The combination of Ethylene-vinyl acetate copolymer (EVA) with nanoclay has wide commercial applications (such as packaging films and cables), which depend on the vinyl acetate contained in the main chain. As the vinyl acetate content increases, the copolymer presents less crystallinity and different mechanical behavior. EVA with 40 wt% vinyl acetate content (EVA40)/ organoclay nanocomposites were prepared via a melt intercalation method with different clay loadings (2.5, 5.0, 7.5, and 10.0 wt%). X-ray diffraction confirmed the formation of exfoliated nanocomposites in all cases with disappearance of the characteristic peak corresponding to the d-spacing of the pristine organoclay. Transmission electron microscopy studies also revealed an exfoliated morphology of the nanocomposites. Morphology and thermal property of the nanocomposites were further examined by means of scanning electron microscopy (SEM) and thermo gravimetric analysis (TGA), respectively. Rheological properties of the EVA40/organoclay nanocomposites were investigated using a rotational rheometer with parallel-plate geometry in both steady shear and dynamic modes, demonstrating remarkable differences with the clay contents in comparison to that of pure EVA40 copolymer. Introduction Polymer/clay nanocomposties are a new emerging class of organic-inorganic hybrid materials. With nanoscopic dispersion of a tiny amount of inorganic clay (montmorillonite, saponite, kaolinite, etc.) with platelet-like structures in a polymeric matrix [1, 2], they often exhibit remarkable improvement of material properties when compared with those for pure polymer or conventional polymer composites (microor macro-composites) with such fillers as glass fiber and carbon fiber. These improvements include enhanced physical and mechanical properties of high tensile moduli and strength [3, 4], increased dimensional stability [5, 6], decreased gas permeability and flammability [7], improved solvent and UV resistance [8], improved electrical property and increased biodegradability of biodegradable polymers [9]. The main reason for these enhanced properties in the nanocomposites is caused by the very high aspect ratios (10-1000) and/or large surface area of exfoliated clay particles and the strong interfacial interaction between the silicate platelets of 1nm layer thickness and the polymer matrix [1-9]. Furthermore, the clay is naturally abundant, cheap, and environmentally friendly. Among various polymeric materials for polymer/clay nanocomposites, ethylene vinyl acetate copolymer (EVA), a random copolymer consisting of ethylene and vinyl acetate (VA) as a repeating unit, has been recently adopted for its nanocomposite system due to its potential engineering application in the fields of packaging films and adhesives [10, 11]. The crystallinity of the EVA, controlled by the vinyl acetate contents, affects the properties of the obtained EVA. As the VA content increases, the copolymer presents less crystallinity, more polarity and different physical behaviors [12, 13]. Common grades have vinyl acetate contents ranging from 1%~50% by weight depending upon the application [14]. Recently, several groups have extensively studied EVA copolymer nanocomposites with various vinyl acetate (VA) contents [15-18]. In this study EVA/organoclay nanocomposite were prepared with rather high (40 wt%) VA content compared to those previously reported and characterized their physical and rheological properties [19]. Since the vinyl acetate group is polar, it is well known that increasing the VA content in the EVA increases the polarity of EVA [20] and thus affects characteristics of EVA40/clay nanocomposite. Both steady shear and dynamic rheological properties of exfoliated EVA40/clay nanocomposites with different clay loadings will be discussed in detail.