Weakly Ionically Bound Thermosensitive Hyperbranched Polymers

Ultrathin Films of MXene Nanosheets Decorated by Ionic Branched Nanoparticles with Enhanced Energy Storage Stability

Two-dimensional (2D) materials such as MXenes have shown great potential for energy storage applications due to their high surface area and high conductivity. However, their practical implementation is limited by their tendency to restack, similar to other 2D materials, leading to a decreased long-term performance. Here, we present a novel approach to addressing this issue by combining MXene (Ti3C2Tx) nanosheets with branched ionic nanoparticles from polyhedral oligomeric silsesquioxanes (POSS) using an amphiphilicity-driven assembly for the formation of composite monolayers of nanoparticle-decorated MXene nanosheets at the air-water interface. The amphiphilic hybrid MXene/POSS monolayers allow for the fabrication of organized multilayered films with ionic nanoparticles supporting the nanoscale gap between MXene nanosheets. For these composite multilayers, we observed a 400% enhancement in specific capacitance compared to pure drop-cast MXene films. Furthermore, dramatically enhanced electrochemical cycling stability for ultrathin-film electrodes (<400 nm in thickness) with a 91% capacitance retention over 10,000 cycles has been achieved. Our results suggest that this insertion of 0D ionic nanoparticles with complementary interactions in between 2D MXene nanosheets could be extended to other hybrid 0D-2D nanomaterials, providing a promising pathway for the development of hybrid electrode architectures with enhanced ionic transport for long-term energy cycling and storage, capacitive deionization, and ionic filtration.

Reconfiguration of Langmuir Monolayers of Thermo-Responsive Branched Ionic Polymers with LCST Transition

Thermo-responsive ionic polymers have the ability to form adaptive and switchable morphologies, which may offer enhanced control in energy storage and catalytic applications. Current thermo-responsive polymers are composed of covalently attached thermo-responsive moieties, restricting their mobility and global dynamic response. Here, we report the synthesis and assembly at the water-air interface of symmetric and asymmetric amphiphilic thermo-responsive branched polymers with weakly ionically bound arms of amine-terminated poly(N-isopropylacrylamide) (PNIPAM) macro-cations. As we observed, symmetric branched polymers formed multimolecular nanosized micellar assemblies, whereas corresponding asymmetric polymers formed large, interconnected worm-like aggregates. Dramatic changes in localized and large-scale chemical composition confirmed the reversible adsorption and desorption of the mobile PNIPAM macro-cations below and above the low critical solution temperature (LCST) and their non-uniform redistribution within polymer monolayer. Increasing the temperature above LCST led to the formation of large interconnected micellar aggregates because of the micelle-centered aggregation of the hydrophobized PNIPAM macro-cationic terminal chains in the aqueous subphase. Overall, this work provides insights into the dynamic nature of the chemical composition of branched ionic polymers with weakly ionically bound thermo-responsive terminal chains and its effect on both morphology and local/surface chemistry of monolayers at LCST transition.

All-Atom Simulation Method for Zeeman Alignment and Dipolar Assembly of Magnetic Nanoparticles

Magnetic nanoparticles (MNPs) can organize into novel structures in solutions with excellent order and unique geometries. However, studies of the self-assembly of smaller MNPs are challenging due to a complicated interplay between external magnetic fields and van der Waals, electrostatic, dipolar, steric, and hydrodynamic interactions. Here, we present a novel all-atom molecular dynamics simulation method to enable detailed studies of the dynamics, self-assembly, structure, and properties of MNPs as a function of core sizes and shapes, ligand chemistry, solvent properties, and external field. We demonstrate the use and effectiveness of the model by simulating the self-assembly of oleic acid ligand-functionalized magnetite (Fe₃O₄) nanoparticles, with spherical and cubic shapes, into rings, lines, chains, and clusters under a uniform external magnetic field. We found that the long-range electrostatic interactions can favor the formation of a chain over a ring, the ligands promote MNP cluster growth, and the solvent can reduce the rotational diffusion of the MNPs. The algorithm has been parallelized to take advantage of multiple processors of a modern computer and can be used as a plugin for the popular simulation software LAMMPS to study the behavior of small MNPs and gain insights into the physics and chemistry of different magnetic assembly processes with atomistic details.