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Experimental data are reported on moisture diffusion and the elastoplastic response of an intercalated nanocomposite with vinyl ester resin matrix and montmorillonite clay filler at room temperature. Observations in diffusion tests show that water transport in the neat resin is Fickian, whereas it becomes anomalous (non-Fickian) with the growth of the clay content. This transition is attributed to immobilization of penetrant molecules on the surfaces of hydrophilic clay layers. Observations in uniaxial tensile tests demonstrate that the response of vinyl ester resin is strongly elastoplastic, whereas an increase in the clay content results in a severe decrease of plastic strains observed as a noticeable reduction of curvatures of the stress-strain diagrams. This is explained by slowing down of molecular mobility in the host matrix driven by confinement of chains in galleries between platelets. Constitutive equations are developed for the anomalous moisture diffusion through and the elastoplastic behavior of a nanocomposite. Adjustable parameters in these relations are found by fitting the experimental data. Fair agreement is demonstrated between the observations and the results of numerical simulation. A striking similarity is revealed between changes in diffusivity, ultimate water uptake and the rate of plastic flow with an increase in the clay content.

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a b s t r a c tObservations are reported on a polymer composite (polyamide-6 reinforced with short glass fibers) in tensile relaxation tests with various strains, tensile creep tests with various stresses, and cyclic tests with a stress-controlled program (ratcheting with a fixed maximum stress and various minimum stresses). Constitutive equations are developed in cyclic viscoelastoplasticity of polymer composites. Adjustable parameters in the stress-strain relations are found by fitting observations in relaxation tests and cyclic tests (16 cycles of loading-unloading). It is demonstrated that the model correctly predicts experimental data in creep tests and dependencies of maximum and minimum strains per cycle on number of cycles up to fatigue fracture of specimens. The influence of strain rate and minimum stress on number of cycles to failure is studied numerically.

a b s t r a c tObservations are reported on high-density polyethylene in uniaxial tensile tests with constant strain rates and relaxation tests at various temperatures ranging from 25 to 90°C. A constitutive model is derived for the nonlinear viscoelastic and viscoplastic behavior of semi-crystalline polymers at three-dimensional deformations. Adjustable parameters in the stress-strain relations are found by fitting the experimental data. It is demonstrated that (i) the model correctly approximates the observations and (ii) material parameters are independent of strain rate and change consistently with temperature.

The study deals with the Payne effect (a substantial decrease in the storage modulus of a particle-reinforced elastomer with an increase in the amplitude of mechanical oscillations). The influence of temperature, concentration of filler and amplitude and frequency of strains is analyzed on the mechanical response of filled rubbery polymers. Constitutive equations are derived using the concept of two interpenetrating networks: one comprises semiflexible polymeric chains connected to temporary junctions, whereas the other is formed by aggregated filler clusters. Adjustable parameters are found by fitting experimental data for natural rubber, bromobutyl rubber and styrene-butadiene rubber reinforced by carbon black and polymeric particles. The critical concentration of particles is determined that characterizes transition from an ensemble of disjoint clusters to the network of filler. The volume fraction of filler corresponding to this transition is found to be close to theoretical predictions based on the percolation theory, as well as to experimental data for isolator-conductor transition.

A series of nanocomposites prepared by melt-blending of Cloisite organoclays with ethyleneco-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.