The Linear And Nonlinear Viscoelastic Characterization And Analysis For New Polymeric Materials

Particle-filled elastomers often exhibit a reduction in peak stress after the initial extension under cyclic loading, a phenomenon known as the Mullins effect. In reported literature, long rest periods and elevated temperatures are often necessary conditions to even partially restore stiffness. The stress softening of the polymer nanoparticle composite discussed in this paper appeared to be completely reversible in a comparatively short time span (less than 17 hours) at room temperature. The Mullins effect recovery phenomenon was investigated by introducing rest periods of various durations following loading and unloading cycles. A mechanism of stress recovery was proposed based on the reduction of entanglements and weakening particle-matrix interactions. New entanglements and particle-chain interactions were considered major contributors to the recovery of the stiffness.The effect of silicate-based nanoparticles on the mechanical response was studied as function of rate and temperature using the time–temperature superposition principle. An apparent yielding was observed in the filled polymer while the pure PDMS network exhibited more typical elastomeric behavior. The time-temperature superposition principle was applied to capture the shear rate and temperature dependence of the stress response at of the filled PDMS at different strains and at the yield point. A power-law relationship was found to adequately describe the resulting master curves for shear stress. Using a triangular shear displacement profile, the effect of temperature on the recovery from a particularly pronounced Mullins effect was investigated as a function of rest time. Given adequate rest time (between 10 and 102 min), recovery was observed for the entire temperature range.A framework for linear viscoelastic analysis of sealants is presented for analyzing stresses resulting from thermally-driven deformations. The analysis method is used to estimate the stress states resulting from assumed diurnal temperature profiles for two representative Dow Corning silicone glazing sealants: a conventional elastomer, and a crosslinked hot melt adhesive containing a high volume fraction of a silicate-based nanoparticle filler. The latter exhibits considerably more rate and temperature dependence than conventional silicones. Estimates of the yielding envelope for a representative thermal cycle profile are provided, based on experimental results reported elsewhere for the time and rate dependent yielding of the hot melt. Since mechanical stresses resulting from the hygrothermal cycles are believed to contribute to the loss of mechanical durability that are sometimes experienced in operating PEM fuel cells, it is important to characterize the mechanical behavior of PEMs over a wide range of hygrothermal conditions. In this study, the linear and nonlinear viscoelastic properties of PEMs equilibrated with both humidified air and liquid water are characterized using a custom-built multi-station stress relaxation fixture. Significant nonlinearity is observed in the membrane, but becomes less pronounced at longer times. Cyclic tests with various strain levels were carried out on the membranes at 70oC in immersed conditions. The nonlinearity exhibited by the PEM under the larger strain levels was represented quite accurately with a Schapery unaxial hereditary single integral model. For this initial effort, material nonlinear parameters were chosen to simulate the stress output from larger strain levels. Complex loading profiles at various rates were used to validate the model and good agreement was achieved between experimental results and numerical predictions.