The physics above and below the yield stress is unified by a simple model for viscoplasticity that accounts for the nonlinear rheology of multiple yield stress fluids. The model has a rate-dependent ...relaxation time, allows for plastic deformation below the yield stress, and indicates that rapid elastic deformation aids yielding. A range of commonly observed rheological behaviors are predicted, including the smooth overshoot in the loss modulus and the recently discovered contributions from recoverable and unrecoverable strains in amplitude sweeps.
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An alternative indentation method to study materials that present a time-dependent behaviour (e. g. viscoelasticity) is the dynamic stiffness measurement technique, which provides fast and accurate ...values of the elastic properties through the calculation of the storage modulus (related to the elastic recovery of the material), and the loss modulus (related to the damping), using indentation cycles with oscillating load or displacement. We have made dynamic measurements on four amorphous materials (CNx, amorphous carbon, fused silica and polypropylene). Such materials have a relatively low density compared to crystalline materials of similar composition and atoms are able to move into internal free space during deformation. The results suggest that all the amorphous materials have a similar time-dependant behaviour associated with the restrictions to movement of atoms (or chains) by neighbouring atoms in the amorphous structure.
Cross-linker control of vitrimer flow El-Zaatari, Bassil M; Ishibashi, Jacob S. A; Kalow, Julia A
Polymer chemistry,
09/2020, Volume:
11, Issue:
33
Journal Article
Peer reviewed
Open access
Vitrimers are a class of covalent adaptable networks (CANs) that undergo topology reconfiguration
via
associative exchange reactions, enabling reprocessing at elevated temperatures. Here, we show ...that cross-linker reactivity represents an additional design parameter to tune stress relaxation rates in vitrimers. Guided by calculated activation barriers, we prepared a series of cross-linkers with varying reactivity for the conjugate addition-elimination of thiols in a PDMS vitrimer. Surprisingly, despite a wide range of stress relaxation rates, we observe that the flow activation energy of the bulk material is independent of the cross-linker structure. Superposition of storage and loss moduli from frequency sweeps can be performed for different cross-linkers, indicating the same exchange mechanism. We show that we can mix different cross-linkers in a single material in order to further modulate the stress relaxation behavior.
The rate of stress relaxation in a vitrimer can be modulated by changing solely the structure of the cross-linker electrophile.
The mechanical properties of extracellular matrices can control the function of cells. Studies of cellular responses to biomimetic soft materials have been largely restricted to hydrogels and ...elastomers that have stiffness values independent of time and extent of deformation, so the substrate stiffness can be unambiguously related to its effect on cells. Real tissues, however, often have loss moduli that are 10 to 20% of their elastic moduli and behave as viscoelastic solids. The response of cells to a time-dependent viscous loss is largely uncharacterized because appropriate viscoelastic materials are lacking for quantitative studies. Here we report the synthesis of soft viscoelastic solids in which the elastic and viscous moduli can be independently tuned to produce gels with viscoelastic properties that closely resemble those of soft tissues. Systematic alteration of the hydrogel viscosity demonstrates the time dependence of cellular mechanosensing and the influence of viscous dissipation on cell phenotype.
Materials that exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write three-dimensional printing inks and filled rubbers. Their key design ...feature is the ability to transition behaviorally from solid to fluid under sufficient load or deformation. Despite its widespread applications, little is known about the dynamics of yielding in real processes, as the nonequilibrium nature of the transition impedes understanding. We demonstrate an iteratively punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solid-like and fluid-like contributions to a yield stress material’s behavior to be separated in a time-resolved manner. Using this protocol, we investigate the overshoot in loss modulus seen in materials that yield. We show that this phenomenon is caused by the transition from primarily solid-like, viscoelastic dissipation in the linear regime to primarily fluid-like, plastic flow at larger amplitudes. We compare and contrast this with a viscoelastic liquidwith no yielding behavior, where the contribution to energy dissipation from viscous flow dominates over the entire range of amplitudes tested.
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•DMA is one of the most powerful tools to study behaviour of polymer composites.•DMA study will help utilisation of natural fibre composites in construction field.•Natural fibre composites can be ...used for replacing steel, wood and concrete.
Dynamic mechanical analysis (DMA) is a versatile technique that complements the information provided by the more traditional thermal analysis techniques such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermal mechanical analysis (TMA). The dynamic parameters such as storage modulus (E′), loss modulus (E″), and damping factor (Tan δ) are temperature dependent and provide information about interfacial bonding between the reinforced fibre and polymer matrix of composite material. The dynamic parameters were ominously influenced by the increase in fibre length and loading but not in a geometric progression. Dynamic loading conditions are frequently stumble in civil infrastructure systems due to sound, winds, earthquakes, ocean waves and live loads. Vibration damping parameters shows prime importance for structural applications in order to enhance the reliability, performance, buildings comfort and in the alleviation of bridges hazards. DMA also predicts the effects of time and temperature on polymer sealants viscoelastic performance under different environments. Present review article designed to be a comprehensive source of reported literature involving dynamic mechanical properties of natural fibre reinforced polymer composites, hybrid and nano composites and its applications. This review article will provides a perfect data to explore its industrial application primarily as cheaper construction and building materials for doing further research in this topic.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Animals exploit the deformability of soft structures to move efficiently in complex natural environments. These soft structures are inherently compliant and enable large strains in components not ...typically found in robotics. Such capabilities have inspired robotic engineers to incorporate soft technologies into their designs. One goal in soft robotics is to endow robots with new, bioinspired features that permit morphologically adaptive interactions with unpredictable environments. Here, we review three key elements of bioinspired soft robots from a mechanics vantage point, namely, materials selection, actuation, and design. Soft materials are necessary for safe interaction and overall actuation of bio-inspired robots. The intrinsic properties of materials in soft robots allow for an “embodied intelligence” that can potentially reduce the mechanical and algorithmic complexity in ways not possible with rigid-bodied robots Finally, soft robotics can be combined with tissue engineering and synthetic biology to create bio-hybrid systems with unique sensing, dynamic response, and mobility. Bioinspired soft robots have the ability to also expedite the evolution of co-robots that can safely interact with humans.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A new class of bio‐based fully degradable block polyesters are pressure‐sensitive adhesives. Bio‐derived monomers are efficiently polymerized to make block polyesters with controlled compositions. ...They show moderate to high peel adhesions (4–13 N cm−1) and controllable storage and loss moduli, and they are removed by adhesive failure. Their properties compare favorably with commercial adhesives or bio‐based polyester formulations but without the need for tackifier or additives.
Well‐defined bio‐based triblock polyesters were prepared using a mixture of renewable monomers and a one‐pot synthetic method with a single catalyst. These materials are pressure‐sensitive adhesives and are easily degraded under acidic conditions.
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This work reports an experimental and theoretical analysis of the rheological properties of mineral oil-based SiO 2 nanofluid for their potential applications in transformer insulation. The flow ...electrification mechanism on the nanofluids with different surfactants such as cetyl trimethyl ammonium bromide (CTAB), oleic acid, and Span 80 is studied using a spinning disk technique. The results show a higher streaming current for the nanofluids with CTAB as a surfactant compared to oleic acid and Span 80. The rheological behavior of nanofluids is explored with the double gap concentric cylinder geometry. The variation of shear stress with shear rate follows a power law relationship along with a yield stress observed for all the nanofluids. A transition is seen from storage modulus to dominant loss modulus for the nanofluids during the frequency sweep analysis, whereas no transition is observed in the case of mineral oil. In addition, regression analysis using artificial neural network (ANN) algorithms are performed on the experimentally measured viscosity of the nanofluids in order to estimate theoretical parameters and provide insights into the streaming current formation. The desirable rheological characteristics of nanofluids are identified for achieving enhanced insulation performance in transformers.
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