Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Rheological, morphological, and interfacial properties of compatibilized PLA/PBAT blends

Abstract: The focus of this paper was to gain a true understanding of the impact of a multifunctional epoxide (Joncryl®;ADR-4368) on the interfacial properties of biopolymer blends based on poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT). The effect of Joncryl on the shear rheological, morphological, and interfacial properties of the blends was systematically investigated. For the deformed drop retraction experiments, different sandwich model systems (droplet/matrix), representing various scenarios of compatibilization, were prepared, aiming to probe the role of the epoxy-functionalized chains on the interface. The decrease of the interfacial tension in the modified/compatibilized PLA_PBAT and the formation of the PLA-Joncryl-PBAT copolymer were highlighted. A new relaxation peak relative to this copolymer was detected by the relaxation spectrum. Transient start-up shear and nonlinear stress relaxation experiments were carried out and confirmed the obtained results. In addition, the interface contribution was demonstrated using the Lee and Park model. The relaxation time increased with the amount of added Joncryl. Hence, the coexistence of chain extension/branching chains coupled to the PLA-Joncryl-PBAT copolymer formation had to be taken into account to explain the improved mechanical properties.

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

Abstract: A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.

Effect of Si doping on the dislocation structure of GaN grown on the A-face of sapphire

Abstract: Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.