No abstract

Shear and extensional rheological measurements were conducted in conjunction with laser light scattering (LLS) on ethylene-vinyl acetate copolymer (EVA) nanocomposites. The materials were prepared by melt-mixing EVA and commercially acquired layered silicates. Wide Angle X-Ray Scattering (WAXS) was used to ascertain the degree of layer swelling. This could be attributed to the intercalation of polymer chains into the interlayer of the silicates. The nanocomposites prepared were determined to be predominantly intercalated in nature. In shear rheological tests, the nanocomposites exhibited an increase in viscoelastic properties compared to the pure EVA. The extent of this property enhancement was not as pronounced as had been reported in many instances with respect to other polymer nanocomposites. This could be attributed to the absence of a network structure normally observed in an exfoliated system. The extensional rheological tests showed an increase in extensional flow properties. This was confirmed by the LLS, which indicated that the filled systems had higher deformability than the unfilled one.

Ethylene-vinyl acetate copolymer (EVA)/montmorillonite (MMT) clay nanocomposites with varying degree of intercalation and exfoliation have been prepared using direct melt blending techniques with various degrees of polarity (9, 18, and 28 wt% vinyl acetate [VA]) and two different types of clay modification. Morphological characterization using wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) have indicated/confirmed the presence of intercalation and/or a combination of intercalation and exfoliation existing in the nanocomposites. The effects of these (simple intercalation or mixed intercalation/exfoliation) states and the effect of changing matrix polarity (by changing VA wt% content) on the nanocomposite mechanical behavior were studied. There is sufficient evidence from the mechanical studies that 1) the presence of nanoclay can simultaneously improve modulus and strength of the nanocomposites, and 2) the mechanical properties are a combined function of the clay concentration and the nanocomposite morphology (due to the VA wt% and presence of clay). It is shown here that interrelation between the VA wt% content and the clay exfoliation affects the mechanical properties in a way that has a positive and increasing slope with increasing loading of clay. It is shown that a clear understanding of the nanocomposite mechanical properties can be obtained from its morphological analysis. POLYM. ENG. SCI., 45:889 -897, 2005.

The structure and deformation characteristics of ethylene-vinyl acetate (EVA)-layered silicate nanocomposites were studied using steady shear and extensional rheology. EVAs with VA contents of 18% and 28% by weight and a commercially modified montmorillonite clay were melt compounded in a twin-screw extruder. Nanocomposites of 2.5%, 5% and 7.5% by weight were produced. WAXS measurements have revealed that EVA chains had intercalated into the silicate layers and expanded the interlayer distance. TEM showed that the morphologies of the nanocomposites were of mixed intercalated/exfoliated. The nanocomposites exhibited an increase in steady shear viscosities compared to the unfilled EVA polymers. At high loading there was a possibility of yield stress owing to network structures. Elasticity of the nanocomposites was compared using first normal stress differenceshear stress plots and the modified Cole-Cole plots, with both methods showing contradictory results. Melt strengths of both EVAs were enhanced with the addition of silicate fillers; however, this was at the expense of the materials' extensibility. Addition of silicate fillers also brought with it flow instabilities like draw resonance and ductile failure, both of which lead to limitations in polymer processing.Compounding of EVA pellets and C30B was carried out using a Brabender twin-screw extruder. The pellets and C30B were initially premixed before introducing into the twin-screw extruder. The compounding temperature was set at 100°C with an extrusion speed of 70 rpm. EVA 18 and EVA28 nanocomposites with clay loadings of 2.5 wt%, 5 wt% and 7.5 wt% were 308 Brought to you by | Purdue University Libraries

Linear viscoelastic behaviors of ethylene-vinyl acetate (EVA)-layered silicate nanocomposites were investigated. EVA with vinyl acetate (VA) content of 18 and 28% by weight and commercially modified montmorillonite clay (Cloisite® 30B) were melt blended in a twin-screw extruder. Nanocomposites of 2.5, 5 and 7.5% by weight were produced. Wide angle X-ray scattering was used to ascertain the degree of layer swelling that could be attributed to the intercalation of polymer chains into the interlayer of the silicates. Transmission electron microscopy was used to analyze the dispersion and extent of exfoliation of the layered silicates in the polymer matrix. All nanocomposites were found to have mixed intercalated/exfoliated morphologies. Both storage and loss moduli and complex viscosity showed improvement at all frequencies tested with increase in silicate loading. Terminal zone behavior was also shown to disappear gradually with silicate content. Increase in silicate loading had caused the divergence of viscosity profile from low-frequency Newtonian plateau to non-Newtonian slope corresponding to a possible finite yield stress. The gradual disappearances of terminal zone and Newtonian homopolymer-like characteristics with silicate loading were attributed to the formation of lattice spanning three-dimensional network structures.

No abstract

Ethylene-vinyl acetate copolymers with vinyl acetate (VA) contents of 9 wt% (EVA9) and 18 wt% (EVA18), and commercially modified montmorillonite clay were melt blended in a twin-screw extruder to produce nanocomposites with various silicate loadings (2.5, 5, and 7.5 wt%). EVA9 was melt compounded with Cloisite 1 15A (C15A) and EVA18 was melt compounded with Cloisite 1 30B (C30B). Wide-angle X-ray scattering and transmission electron microscopy have indicated that EVA9 nanocomposites were predominantly intercalated in morphology while EVA18 nanocomposites possessed mixed intercalated/exfoliated structures. Light scattering was conducted in conjunction with meltdrawing experiments to analyze structural evolution of the drawn filaments following its exit from the die of a single-screw extruder. The scattering patterns were processed using Guinier's approximation. The parameter used to characterize deformation was the radius of gyration of optical inhomogeneities in the direction of extension and orthogonal to it. The ratio of these radii of gyration gives the deformation ratio. It was found that increasing silicate content decreased the deformation ratio for all the filled EVA18 nanocomposites. EVA9 with 2.5 and 5 wt% fillers showed an increase in deformation compared with unfilled EVA9, but the 7.5 wt%filled material showed a decrease. These observations were attributed to the relatively strong polymer-filler interactions. POLYM. ENG. SCI.,

No abstract