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8 results for: All records
Author ORCID ID is 0000000342703866
Full Text and Citations
  1. We analyze conductivity of polymerized ionic liquids with focus on fundamental limitations hindering faster charge transport in polymer electrolytes. We emphasize that to achieve the required ionic conductivity ~10 -3 S/cm in dry polymer electrolytes, a decoupling of ion transport from segmental dynamics is required. We demonstrate that two competing mechanisms control decoupling of ion transport: electrostatic interactions that dominates for small ions such Li, and elastic force that dominates for large ions. Our experimental results indeed confirm significant contribution of the elastic force to the energy barrier controlling transport of large ions. We also emphasize importance of ion-ion correlationsmore » that strongly affect charge transport (conductivity) even at the same ion diffusivity. Our analysis suggests that these correlations suppress ion conductivity in polymer electrolytes by about ten times. At the end, we formulate some ideas on design of polymer electrolytes with high ion conductivity.« less
  2. In this paper, we present detailed studies of ionic conductivity in several polymerized ionic liquids (PolyILs) with different size of mobile ions. Presented analysis revealed that charge diffusion in PolyILs is about 10 times slower than ion diffusion, suggesting strong ion–ion correlations that reduce ionic conductivity. The activation energy for the ion diffusion shows a nonmonotonous dependence on the mobile ion size, indicating a competition between Coulombic and elastic forces controlling ion transport in PolyILs. The former dominates mobility of small ions (e.g., Li), while the latter controls mobility of large ions (e.g., TFSI). We propose a simple qualitative modelmore » describing the activation energy for the ion diffusion. In conclusion, it suggests that an increase in dielectric constant of PolyILs should lead to a significant enhancement of conductivity of small ions (e.g., Li and Na).« less
  3. The addition of nanoparticles to a polymer matrix is a well-known process to improve the mechanical properties of polymers. Many studies of mechanical reinforcement in polymer nanocomposites (PNCs) focus on rubbery matrices; however, much less effort concentrates on the factors controlling the mechanical performance of the technologically important glassy PNCs. This paper presents a study of the effect of the polymer molecular weight (MW) on the overall mechanical properties of glassy PNCs with attractive interaction by using Brillouin light scattering. We found that the mechanical moduli (bulk and shear) have a nonmonotonic dependence on MW that cannot be predicted bymore » simple rule of mixtures. The moduli increase with increasing MW up to 100 kg/mol followed by a drop at higher MW. We demonstrate that the change in the mechanical properties of PNCs can be associated with the properties of the interfacial polymer layer. The latter depend on the interfacial chain packing and stretching, as well as polymer bridging, which vary differently with the MW of the polymer. These competing contributions lead to the observed nonmonotonic variations of the glassy PNC moduli with MW. Our work provides a simple, cost-effective, and efficient way to control the mechanical properties of glassy PNCs by tuning the polymer chain length. Our finding can be beneficial for the rational design of PNCs with desired mechanical performance.« less
  4. The self-diffusion of a single large particle in a fluid is usually described by the classic Stokes–Einstein (SE) hydrodynamic relation. However, there are many fluids where the SE prediction for nanoparticles diffusion fails. These systems include diffusion of nanoparticles in porous media, in entangled and unentangled polymer melts and solutions, and protein diffusion in biological environments. A fundamental understanding of the microscopic parameters that govern nanoparticle diffusion is relevant to a wide range of applications. Here in this work, we present experimental measurements of the tracer diffusion coefficient of small and large nanoparticles that experience strong attractions with unentangled andmore » entangled polymer melt matrices. For the small nanoparticle system, a crossover from suppressed to enhanced diffusion is observed with increasing polymer molecular weight. We interpret these observations based on our theoretical and simulation insights of the preceding article (paper 1) as a result of a crossover from an effective hydrodynamic core–shell to a nonhydrodynamic vehicle mechanism of transport, with the latter strongly dependent on polymer–nanoparticle desorption time. In conclusion, a general zeroth-order qualitative picture for small sticky nanoparticle diffusion in polymer melts is proposed.« less
  5. We theoretically study the diffusion of a single attractive nanoparticle (NP) in unentangled and entangled polymer melts based on combining microscopic “core–shell” and “vehicle” mechanisms in a dynamic bond percolation theory framework. A physical picture is constructed which addresses the role of chain length (N), degree of entanglement, nanoparticle size, and NP–polymer attraction strength. The nanoparticle diffusion constant is predicted to initially decrease with N due to the dominance of the core–shell mechanism, then to cross over to the vehicle diffusion regime with a weaker N dependence, and eventually plateau at large enough N. This behavior corresponds to decoupling ofmore » NP diffusivity from the macroscopic melt viscosity, which is reminiscent of repulsive NPs in entangled melts, but here it occurs for a distinct physical reason. Specifically, it reflects a crossover to a transport mechanism whereby nanoparticles adsorb on polymer chains and diffuse using them as “vehicles” over a characteristic desorption time scale. Repetition of random desorption events then leads to Fickian long time NP diffusion. Complementary simulations for a range of chain lengths and low to moderate NP–polymer attraction strengths are also performed. They allow testing of the proposed diffusion mechanisms and qualitatively support the theoretically predicted dynamic crossover behavior. In conclusion, when the desorption time is smaller than or comparable to the onset of entangled polymer dynamics, the NP diffusivity becomes almost chain length independent.« less
  6. The modification of polymer dynamics in the presence of strongly interacting nanoparticles has been shown to significantly change themacroscopic properties above the glass transition temperature of polymer nanocomposites (PNCs). However, much less attention has been paid to changes in the dynamics of glassy PNCs. Analysis of neutron and light scattering data presented herein reveals a surprising enhancement of local dynamics, e.g., fast picosecond and secondary relaxations, in glassy PNCs accompanied with a strengthening of mechanical modulus. Here we ascribe this counter-intuitive behavior to the complex interplay between chain packing and stretching within the interfacial layer formed at the polymer-nanoparticle interface.
  7. Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer backbone and free counterions, are considered as an ideal electrolytes for various electrochemical devices, including fuel cells, supercapacitors and batteries. Despite large structural diversity of these systems, all of them reveal a universal but poorly understood feature - a charge transport faster than the segmental dynamics. Here, to address this issue, we have studied three novel polymer electrolyte membrane for fuel cells as well as four single-ion conductors including highly conductive siloxane-based polyIL. Our ambient and high pressure studies revealed fundamental differences in the conducting propertiesmore » of the examined systems. Finally, we demonstrate that the proposed methodology is a powerful tool to identify the charge transport mechanism in polyILs in general and thereby contribute to unraveling the microscopic nature of the decoupling phenomenon in these materials.« less
  8. Conductivity in polymer electrolytes has been generally discussed with the assumption that the segmental motions control charge transport. However, much less attention has been paid to the mechanism of ion conductivity where the motions of ions are less dependent (decoupled) on segmental dynamics. We present that this phenomenon is observed in ionic materials as they approach their glass transition temperature and becomes essential for design and development of highly conducting solid polymer electrolytes. In this paper, we study the effect of chain rigidity on the decoupling of ion transport from segmental motion in three polymerized ionic liquids (polyILs) containing themore » same cation–anion pair but differing in flexibility of the polymer backbones and side groups. Analysis of dielectric and rheology data reveals that decoupling is strong in vinyl-based rigid polymers while almost negligible in novel siloxane-based flexible polyILs. To explain this behavior, we investigated ion and chain dynamics at ambient and elevated pressure. Our results suggest that decoupling has a direct relationship to the frustration in chain packing and free volume. Finally, these conclusions are also supported by coarse-grained molecular dynamics simulations.« less

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