Microscopic Origins of Caging and Equilibration of Self-Suspended Hairy Nanoparticles

Evidence for Enhanced Tracer Diffusion in Densely Packed Interfacial Assemblies of Hairy Nanoparticles

Nearly monodisperse nanoparticle (NP) spheres attached to a nonvolatile ionic liquid surface were tracked by in situ scanning electron microscopy to obtain the tracer diffusion coefficient Dtr as a function of the areal fraction ϕ. The in situ technique resolved both tracer (gold) and background (silica) particles for ∼1-2 min, highlighting their mechanisms of diffusion, which were strongly dependent on ϕ. Structure and dynamics at low and moderate ϕ paralleled those reported for larger colloidal spheres, showing an increase in order and a decrease in Dtr by over 4 orders of magnitude. However, ligand interactions were more important near jamming, leading to different caging and jamming dynamics for smaller NPs. The normalized Dtr at ultrahigh ϕ depended on particle diameter and ligand molecular weight. Increasing the PEG molecular weight by a factor of 4 increased Dtr by 2 orders of magnitude at ultrahigh ϕ, indicating stronger ligand lubrication for smaller particles.

Enthalpic Interactions and Solution Behaviors of Solvent-Free Polymer Brushes

We performed molecular dynamics simulations to characterize the role of enthalpic interaction in impacting the static and dynamic properties of solvent-free polymer brushes. The intrinsic enthalpic interaction in the simulation was introduced using different attraction strengths between distinct species. Two model systems were considered: one consisting of binary brushes of two different polymer types and the other containing a mixture of homopolymer brushes and free molecules. In the first system, we observed that, when two originally incompatible polymers were grafted to opposing surfaces, the miscibility between them was significantly enhanced. A less favorable intrinsic enthalpic interaction in the brushes resulted in a more stretched chain configuration, a lower degree of inter-brush penetration, and faster segmental relaxation. In the second system, we characterized the solvent capacity of the homopolymer brushes from variations in the energy components of the system as a function of the number of free molecules. We determined that molecular absorption was driven by the release of the entropic frustration for the grafted chains in conjunction with the chemical affinity between the solutes and polymers. The solute distribution function within the inter-wall space showed that solute-polymer mixing in the middle of the gap occurred preferentially when the enthalpic interaction was more favorable. When this was not the case, absorption was predominantly localized near the grafting surface. From the mean square displacement of the solute, we found that the brush profiles restrained the molecular diffusion perpendicular to the grafting wall; the weaker the attraction from the brush, the higher the solute mobility.

Structure and Evolution of Quasi-Solid-State Hybrid Electrolytes Formed Inside Electrochemical Cells

Solid-state electrolytes (SSEs) formed inside an electrochemical cell by polymerization of a liquid precursor provide a promising strategy for overcoming problems with electrolyte wetting in solid-state batteries. Hybrid solid-state polymer electrolytes (HSPEs) created by in situ polymerization of a conventional liquid precursor containing electrochemically inert nanostructures are of particular interest because they offer a mechanism for selectively reinforcing or adding new functionalities to the electrolyte-removing the need for high degrees of polymerization. The synthesis, structure, chemical kinetics, ion-transport properties and electrochemical characteristics of HSPEs created by Al(OTf)₃ -initiated polymerization of 1,3-dioxolane (DOL) containing hairy, nano-sized SiO₂  particles are reported. Small-angle X-ray scattering reveals the particles are well-dispersed in liquid DOL. Strong interaction between poly(ethylene glycol) molecules tethered to the SiO₂  particles and poly(DOL) lead to co-crystallization-anchoring the nanoparticles in their host It also enables polymerization-depolymerization processes in DOL to be studied and controlled. The utility of the in-situ-formed HSPE, is demonstrated first in Li|HSPE|Cu half cells, which manifest Coulombic efficiencies (CE) values approaching 99%. HSPEs are also demonstrated in solid-state lithium-sulfur-polyacrylonitrile (SPAN) composite full-cell batteries. The in-situ-formed Li|HSPE|SPAN cells show good cycling stability and thus provide a promising path toward all-solid-state batteries.

Universal Polymeric-to-Colloidal Transition in Melts of Hairy Nanoparticles

Two different classes of hairy self-suspended nanoparticles in the melt state, polymer-grafted nanoparticles (GNPs) and star polymers, are shown to display universal dynamic behavior across a broad range of parameter space. Linear viscoelastic measurements on well-characterized silica-poly(methyl acrylate) GNPs with a fixed core radius (R_core) and grafting density (or number of arms f) but varying arm degree of polymerization (N_arm) show two distinctly different regimes of response. The colloidal Regime I with a small N_arm (large core volume fraction) is characterized by predominant low-frequency solidlike colloidal plateau and ultraslow relaxation, while the polymeric Regime II with a large N_arm (small core volume fractions) has a response dominated by the starlike relaxation of partially interpenetrated arms. The transition between the two regimes is marked by a crossover where both polymeric and colloidal modes are discerned albeit without a distinct colloidal plateau. Similarly, polybutadiene multiarm stars also exhibit the colloidal response of Regime I at very large f and small N_arm. The star arm retraction model and a simple scaling model of nanoparticle escape from the cage of neighbors by overcoming a hopping potential barrier due to their elastic deformation quantitatively describe the linear response of the polymeric and colloidal regimes, respectively, in all these cases. The dynamic behavior of hairy nanoparticles of different chemistry and molecular characteristics, investigated here and reported in the literature, can be mapped onto a universal dynamic diagram of f/[R_core³/ν₀)^1/4] as a function of (N_armν₀f)/(R_core³), where ν₀ is the monomeric volume. In this diagram, the two regimes are separated by a line where the hopping potential ΔU_hop is equal to the thermal energy, k_BT. ΔU_hop can be expressed as a function of the overcrowding parameter x (i.e., the ratio of f to the maximum number of unperturbed chains with N_arm that can fill the volume occupied by the polymeric corona); hence, this crossing is shown to occur when x = 1. For x > 1, we have colloidal Regime I with an overcrowded volume, stretched arms, and ΔU_hop > k_BT, while polymeric Regime II is linked to x < 1. This single-material parameter x can provide the needed design principle to tailor the dynamics of this class of soft materials across a wide range of applications from membranes for gas separation to energy storage.