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An active droplet system, programmed to repeatedly move autonomously at a specific velocity in a well‐defined direction, is demonstrated. Coulombic energy is stored in oversaturated interfacial assemblies of charged nanoparticle‐surfactants by an applied DC electric field and can be released on demand. Spontaneous emulsification is suppressed by an increase in the stiffness of the oversaturated assemblies. Rapidly removing the field releases the stored energy in an explosive event that propels the droplet, where thousands of charged microdroplets are ballistically ejected from the surface of the parent droplet. The ejection is made directional by a symmetry breaking of the interfacial assembly, and the combined interaction force of the microdroplet plume on one side of the droplet propels the droplet distances tens of times its size, making the droplet active. The propulsion is autonomous, repeatable, and agnostic to the chemical composition of the nanoparticles. The symmetry‐breaking in the nanoparticle assembly controls the microdroplet velocity and direction of propulsion. This mechanism of droplet propulsion will advance soft micro‐robotics, establishes a new type of active matter, and introduces new vehicles for compartmentalized delivery. 

Visualizing the network of a solvent‐swollen polymer gel remains problematic. To address this challenge, open transmission electron microscopy (TEM) was applied to thin gel films permeated by a nonvolatile ionic liquid. The targeted physical gels were prepared by cooling concentrated solutions of poly(ethylene glycol) in 1‐ethyl‐3‐methyl imidazolium ethyl sulfate [EMIM][EtSO₄]. During the cooling, gelation occurred by a frustrated crystallization of the dissolved polymer, leading to a percolated, solvent‐permeated semicrystalline network in which nanoscale polymer crystals acted as crosslinks. Crystalline features ranging from ~5 to ~200 nm were observed, with the visible network strands dominantly consisting of long curvilinear crystallites of ~15–20 nm diameter. Nascent spherulites irregularly decorated the network, creating a complex structural hierarchy that complicated analyses. Lacking diffraction contrast, TEM did not visualize the many disordered, fully solvated PEG chains present in the voids between crystals. Recognizing that a network's three dimensionality is ambiguous when assessed through two‐dimensional microscopy projections, a small gel region was studied by TEM tomography, revealing a nearly isotropic three‐dimensional arrangement of the curvilinear crystallites, which displayed remarkably uniform cylindrical cross sections.

 

Research involving polymer zwitterions typically involves the preparation of ammonium-based structures and their study as coatings or gels that impart hydrophilicity and/or antifouling properties to substrates and materials. More recent synthetic advances have produced a significant expansion in polymer zwitterion chemistry, especially with respect to the composition of the cationic moieties that open new possibilities to examine polymer zwitterions as amphiphiles, functional surfactants, and components of complex emulsions. This article describes the synthesis of new zwitterionic sulfonium sulfonate monomers and their use as starting materials in controlled free radical polymerization to yield the corresponding polymers. These novel polymer zwitterions bear sulfonium sulfonate groups, that possess an inverted dipole directionality relative to prior examples that yields different and unexpected physical and chemical properties. For example, the polymer zwitterions described here are soluble in a wide range of nonaqueous solvents and possess significantly greater stability against nucleophiles relative to their dipole-inverted counterparts. Additionally, the amphiphilic character of these sulfonium sulfonate polymers makes them amenable to use as surfactants for stabilizing oil-in-water emulsions, a feature that is not possible using conventional ultrahydrophilic polymer zwitterions. 

By combining the unique characteristics of molecular bottlebrushes (MBBs) and the properties of stimuli‐responsive polymers, we show that MBBs with randomly grafted poly(n‐butyl acrylate) and pH‐responsive poly(2‐(N,N‐diethylamino)ethyl methacrylate) (PDEAEMA) side chains are efficient and robust pH‐responsive emulsifiers. Water‐in‐toluene emulsions were formed at pH 4.0 and disrupted by increasing the pH to 10.0. The emulsion generation and disruption was reversible over the ten cycles investigated, and the bottlebrushes remained intact. The exceptional emulsion stability stemmed from the high interfacial binding energy of MBBs, imparted by their large molecular size and Janus architecture at the interface, as evidenced by the interfacial jamming and wrinkling of the assemblies upon reducing the interfacial area. At pH 10.0, PDEAEMA became water‐insoluble, and the MBBs desorbed from the interface, causing de‐emulsification. Consequently, we have shown that the judicious design of MBBs can generate properties of particle emulsifiers from their large size, while the responsiveness of the MBBs enables more potential applications.

 

By combining the unique characteristics of molecular bottlebrushes (MBBs) and the properties of stimuli‐responsive polymers, we show that MBBs with randomly grafted poly(n‐butyl acrylate) and pH‐responsive poly(2‐(N,N‐diethylamino)ethyl methacrylate) (PDEAEMA) side chains are efficient and robust pH‐responsive emulsifiers. Water‐in‐toluene emulsions were formed at pH 4.0 and disrupted by increasing the pH to 10.0. The emulsion generation and disruption was reversible over the ten cycles investigated, and the bottlebrushes remained intact. The exceptional emulsion stability stemmed from the high interfacial binding energy of MBBs, imparted by their large molecular size and Janus architecture at the interface, as evidenced by the interfacial jamming and wrinkling of the assemblies upon reducing the interfacial area. At pH 10.0, PDEAEMA became water‐insoluble, and the MBBs desorbed from the interface, causing de‐emulsification. Consequently, we have shown that the judicious design of MBBs can generate properties of particle emulsifiers from their large size, while the responsiveness of the MBBs enables more potential applications.

 

As a class of semiconductor nanocrystals that exhibit high photoluminescence quantum yield (PLQY) at tunable wavelengths, perovskite nanocrystals (PNCs) are attractive candidates for optoelectronic and light‐emitting devices. However, attempts to optimize PNC integration into such applications suffer from PNC instability and loss of PL over time. Here, we describe the impact of organic and polymeric N‐oxides when used in conjunction with PNCs, whereby a significant increase in PNC quantum yield is observed in solution, and stable PL emission is obtained in polymeric nanocomposites. Specifically, when using aliphatic N‐oxides in ligand exchange with CsPbBr₃PNCs in solution, a substantial boost in PNC brightness is observed (~40% or more PLQY increase), followed by an alteration of the perovskite chemistry. When N‐oxide substituents are positioned pendent to a poly(n‐butyl methacrylate) backbone, the optically clear flexible nanocomposite films obtained have bright PL emission and maintain optical clarity for months. X‐ray diffraction is useful for characterizing the PNC crystalline structure following exposure to aliphatic N‐oxides, while electron microscopy (EM) and small‐angle X‐ray scattering (SAXS) measurements of the PNC‐polymer nanocomposites show this polymeric N‐oxide platform to cleanly disperse PNCs in flexible polymer films.

 

It has been shown both theoretically and experimentally that amphiphilic Janus particles are the most effective solid surfactants to stabilize interfaces. In most cases, the Janus particles investigated have uniform morphologies with Janus boundaries dividing the particle into halves. However, there are many examples of Janus particles where the hydrophilic and hydrophobic domains are not equally distributed. The effects of this uneven domain distribution on the mechanism and kinetics of Janus particle assembly, and final equilibrium state are not well‐understood. Dynamic pendant drop tensiometry offers a means to probe both the equilibrium assembly and the kinetics and mechanism of assembly. Here, the interfacial kinetics and assembly of spherical anisotropic Janus particles are investigated using dynamic pendant drop tensiometry. Systematic studies quantifying the time‐dependent interfacial behavior as a function of Janus particle morphology, chemical composition, particle concentration, and NaOH and HCl concentration are performed. These studies shed light on the assembly mechanism of more complex Janus particle morphologies and highlight their effectiveness as interface stabilizers.

 

The in‐plane packing of gold (Au), polystyrene (PS), and silica (SiO₂) spherical nanoparticle (NP) mixtures at a water–oil interface is investigated in situ by UV–vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine‐functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λ_max) and integrated intensities as the assemblies saturate for different concentrations of non‐plasmonic (PS/SiO₂) NPs. As the PS/SiO₂content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λ_maxchanges within the first few minutes of adsorption due to weak attractive inter‐NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non‐plasmonic NP content. Grazing incidence small angle X‐ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non‐plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase‐separated NP films.