Scientific publications

Double and triple network (TN) elastomers can be made by infusing monomers into a single network (SN) polymer, causing it to swell, and then polymerizing and cross-linking the monomers. The result is a double network (DN) elastomer in which one network is stretched and the other is in hydrostatic compression. TN systems are made by repeating the process starting with the DN material. The multinetwork (MN) elastomers exhibit a Mullins effect in which softening occurs upon a first cycle of loading, with the elastomer stiffness recovered above the previous maximum strain. The Mullins effect is attributed to rupture of the stretched network, eliminating the constraint on the compressed network, thereby motivating straining at the lower stiffness of the remaining material. A model for this process is developed, based on the previous work of Horgan et al. (2004, “A Theory of Stress Softening of Elastomers Based on Finite Chain Extensibility,” Proc. R. Soc. A, 460(2046), pp. 1737–1754). In the proposed model, a composite stiffness for the MN system is developed and a damage process introduced to degrade the contribution of the stretched network. The damage model is designed to account for the progressive elimination of chains that are most highly loaded in the stretched network, so that the undamaged stiffness is restored when the strain rises above levels previously experienced. The proposed model reproduces the behavior of the Mullins effect in the MN system.

The theoretical basis is developed for a strategy for enhancing the toughness of elastomers that have been damaged by the creation of a surface crack. The approach involves soaking the damaged elastomer for a limited time in a bath of the elastomer monomers, an ultra-violet initiator and cross-linkers to allow them all to diffuse into the specimen. The elastomer is then withdrawn from the bath at a stage where the diffusing species have only penetrated to a limited degree into the component, so that they are concentrated near its surface, including adjacent to the crack. Very soon thereafter the specimen is subjected to ultra-violet light, thereby causing the diffused species to polymerize and cross-link. As the diffusing monomers will have caused the elastomer to have swollen, the polymerization process will lock this volume expansion in place near the elastomer surface, and will have done so adjacent to the crack. Since a local expansion of the material around the crack and adjacent to its surfaces will reduce the energy release rate when the crack is forced to propagate, the swelling will have effectively toughened the elastomer against the growth of the pre-existing surface crack.

A viscoelastic constitutive law is developed for a hydrogel to encompass deviatoric deformations where the viscosity of the solvent contributes to the deviatoric stress of the system. The law is obtained in Kelvin–Voigt form by adding together the stress in the polymer network, the osmotic and solvent pressures and the viscous stress induced in the solvent. Transient behavior of the hydrogel is presented in terms of an example where transport of the solvent through the gel and viscosity due to deviatoric deformation interact to enable response that would otherwise be precluded, showing the significance of including the deviatoric viscosity. In addition, a simple relaxation problem is solved for a swollen hydrogel where volume of the hydrogel is preserved, namely small strain shear as induced in a rheometer.

In this paper we systematically investigated effect of separator morphology on the performance of Li-S batteries. We tested two kinds of commercially available non-woven fibrous separators made of polypropylene in conjunction with trilayer PP/PE/PP porous separator. Among the non-woven separators, the fibers of the separator of first kind are rough and more acidic than the fibers of the separator of second art, which are smooth. It was found that batteries with smooth separator demonstrate higher Coulombic efficiency, higher charging/discharging capacity and better cycling stability. We attribute better performance of batteries with smooth fibers of separators to lower acidity than that of rough fibers. We believe that negatively charged polysulfides are unable to adsorb on more acidic rough fibers and diffuse readily to lithium electrode. In contrast, polysulfides are able to adsorb on the surface of less acidic smooth fibers that reduces their polysulfides shuttle and rapid decay of battery capacity. Strategic deployment of non-woven Freudenberg and Celgard separator in combination leads to high active mass utilization, superior wettability, reduced short circuit tendency caused by dendritic growth and slower capacity decay.

A stretchable gel-polymer electrolyte (GPE) based on polyepichlorohydrin terpolymer as a host polymer, a mixture of 1,3-dioxolane and 1,2-dimethoxyethane as plasticizers and lithium bis(trifluoromethane)sulfonimide (LiTFSI) as salt was developed and studied in detail. GPEs were prepared in a two-step process. The cross-linked polyepichlorohydrin network was developed using ethylene thiourea (ETU) and magnesium oxide (MgO) as cross-linkers. An immersion method was used for distribution of LiTFSI-salt in the network film. The effects of electrolyte uptake on mechanical and thermal properties of GPEs were studied in detail. Dynamic mechanical analysis (DMA) shows a GPE with a storage modulus higher than 0.6 MPa at ambient temperature (298 K). Differential scanning calorimetry (DSC) analysis was carried out on the GPE to demonstrate the change in glass transition temperature caused by the addition of two plasticizers. Impedance spectroscopy was used to study the ionic conductivity in the GPE. The highest ionic conductivity was found to be 2.4 × 10−4 S/cm at 298 K. The lithium-sulfur cells in conjunction with the elastomeric gel-polymer electrolyte showed characteristic cyclic voltammograms and charge-discharge plateaus indicating a two stage reduction process and a one stage oxidation process. The battery demonstrated initial specific capacity of 700–800 mA h/gsulfur which faded slowly with cycle number. Our present work could pave a new route to fabricate flexible rechargeable batteries with a cross-linked soft polymer electrolyte.

Wegen der vorteilhaften Eigenschaften im Trockenlauf sind thermoplastische Kunststoffe bereits in vielen tribologischen Anwendungen vertreten, hohe Temperaturen führen den Werkstoff dabei jedoch schnell an seine Grenzen. In dieser Studie wird der Wärmeentwicklung im tribologischen Kontakt durch einen energieabsorbierenden Füllstoff entgegengewirkt. Mechanische, thermische und tribolo-gische (Pin-on-Disk) Prüfungen zeigen deutlich, dass durch Nutzung energieverzehrender Effekte das Einsatzfeld kunststoffbasierter Tribocompounds stark erweitert werden kann.

A ball-milling approach was developed to investigate the constituents of isolated nacre tablets of the gastropod Haliotis glabra in aqueous suspension without additional chemical additives. The obtained particle mixtures were characterized using X-ray crystallography as well as scanning and transmission electron microscopy. Aragonite nanoparticles retained their crystal structure even after 14 h of ball milling. The long-term stability of the particle mixtures varied as a function of the ball-milling duration. An increased milling time led to rod-like stable assemblies of aragonite nanoparticles. Selected area electron diffraction investigations revealed that the longitudinal axes in about one-third of these nanoparticle rods were oriented along the crystallographic c-axis of aragonite, indicating oriented attachment of the aragonite nanoparticles. These in vitro observations support the idea that a two-stage process, separated into crystallization of nanoparticles and oriented assembly of nanocrystals, could also occur in vivo.

ABSTRACT Polycarbonate (PC) nanocomposites containing different nano-SiO2 contents were prepared using a twin-screw extruder followed by injection molding. The mechanical behavior and environmental stress cracking (ESC) resistance of the nanocomposites were investigated. Compared to neat PC, the incorporation of nanofiller leads to a slight improvement in Young's modulus, tensile strength, and notched impact strength properties. Interestingly, the nanofiller enhanced the environmental stress cracking resistance of PC in different fluids (isopropanol, methanol, aqueous urea solution, and deionized water) vastly. The correlation of the mode I critical stress intensity factor with the Hansen solubility parameter shows a very good agreement for different fluids and thus allows the prediction of the stress cracking behavior as a function of the filler content for different stress-cracking agents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45451.