Polymer-Assisted 3D Printing of Inductor Cores

Poly(glycerol monomethacrylate) (PGMA) prepared by reversible addition–fragmentation chain transfer polymerization was investigated as an additive for high-loading iron oxide nanoparticle (IOP) 3D printable inks. The effect of adjusting the molar mass and loading of PGMA on the rheology of IOP suspensions was investigated, and an optimized ink formulation containing 70% w/w IOPs and 0.25% w/w PGMA98 at pH 10 was developed. This ink was successfully 3D printed onto various substrates and into several structures, including rectangles, high aspect ratio cylinders, letters, spiral- and comb-shaped structures, and thin- and thick-walled toroids. The effect of sintering on the mechanical properties of printed artifacts was investigated via four-point flexural and compressive testing, with higher sintering temperatures resulting in improved mechanical properties due to changes in the particle size and microstructure. The printed toroids were fabricated into inductors, and their electrical performance was assessed via impedance spectroscopy at a working frequency range of 0.001–13 MHz. There was a clear trade-off between electrical properties and sintering temperature due to a phase change between γ-Fe2O3 and α-Fe2O3 upon heating. Nevertheless, the optimized devices had a Q factor of ∼40 at 10 MHz, representing a superior performance compared to that of other inductors with iron oxide cores previously reported. Thus, this report represents a significant step toward the development of low-cost, fully aqueous, high loading, and 3D printable ceramic inks for high-performance inductors and functional devices

Synthesis and Characterization of All-Acrylic Tetrablock Copolymer Nanoparticles: Waterborne Thermoplastic Elastomers via One-Pot RAFT Aqueous Emulsion Polymerization

Reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization is used to prepare well-defined ABCB tetrablock copolymer nanoparticles via sequential monomer addition at 30 °C. The A block comprises water-soluble poly(2-(N-acryloyloxy)ethyl pyrrolidone) (PNAEP), while the B and C blocks comprise poly(t-butyl acrylate) (PtBA) and poly(n-butyl acrylate) (PnBA), respectively. High conversions are achieved at each stage, and the final sterically stabilized spherical nanoparticles can be obtained at 20% w/w solids at pH 3 and at up to 40% w/w solids at pH 7. A relatively long PnBA block is targeted to ensure that the final tetrablock copolymer nanoparticles form highly transparent films on drying such aqueous dispersions at ambient temperature. The kinetics of polymerization and particle growth are studied using 1H nuclear magnetic resonance spectroscopy, dynamic light scattering, and transmission electron microscopy, while gel permeation chromatography analysis confirmed a high blocking efficiency for each stage of the polymerization. Differential scanning calorimetry and small-angle X-ray scattering studies confirm microphase separation between the hard PtBA and soft PnBA blocks, and preliminary mechanical property measurements indicate that such tetrablock copolymer films exhibit promising thermoplastic elastomeric behavior. Finally, it is emphasized that targeting an overall degree of polymerization of more than 1000 for such tetrablock copolymers mitigates the cost, color, and malodor conferred by the RAFT agent

Preparation of Double Emulsions using Hybrid Polymer/Silica Particles: New Pickering Emulsifiers with Adjustable Surface Wettability

A facile route for the preparation of water-in-oil-in-water (w/o/w) double emulsions is described for three model oils, namely, <i>n</i>-dodecane, isopropyl myristate, and isononyl isononanoate, using fumed silica particles coated with poly­(ethylene imine) (PEI). The surface wettability of such hybrid PEI/silica particles can be systematically adjusted by (i) increasing the adsorbed amount of PEI and (ii) addition of 1-undecanal to the oil phase prior to homogenization. In the absence of this long-chain aldehyde, PEI/silica hybrid particles (PEI/silica mass ratio = 0.50) produce o/w Pickering emulsions in all cases. In the presence of 1-undecanal, this reagent reacts with the primary and secondary amine groups on the PEI chains via Schiff base chemistry, which can render the PEI/silica hybrid particles sufficiently hydrophobic to stabilize w/o Pickering emulsions at 20 °C. Gas chromatography, ¹H NMR and X-ray photoelectron spectroscopy provide compelling experimental evidence for this in situ surface reaction, while a significant increase in the water contact angle indicates markedly greater hydrophobic character for the PEI/silica hybrid particles. However, when PEI/silica hybrid particles are prepared using a relatively low adsorbed amount of PEI (PEI/silica mass ratio = 0.075) only o/w Pickering emulsions are obtained, since the extent of surface modification achieved using this Schiff base chemistry is insufficient. Fluorescence microscopy and laser diffraction studies confirm that highly stable w/o/w double emulsions can be achieved for all three model oils. This is achieved by first homogenizing the relatively <i>hydrophobic</i> PEI/silica hybrid particles (PEI/silica mass ratio = 0.50) with an oil containing 3% 1-undecanal to form an initial w/o emulsion, followed by further homogenization using an aqueous dispersion of relatively <i>hydrophilic</i> PEI/silica particles (PEI/silica mass ratio = 0.075). Dye release from the internal aqueous cores into the aqueous continuous phase was monitored by visible absorption spectroscopy. These studies indicate immediate loss of 12–18% dye during the high speed homogenization that is required for double emulsion formation, but no further dye release is observed at 20 °C for at least 15 days thereafter

Mechanistic Insights into Diblock Copolymer Nanoparticle–Crystal Interactions Revealed via <i>in Situ</i> Atomic Force Microscopy

Recently, it has become clear that a range of nanoparticles can be occluded within single crystals to form nanocomposites. Calcite is a much-studied model, but even in this case we have yet to fully understand the details of the nanoscale interactions at the organic–inorganic interface that lead to occlusion. Here, a series of diblock copolymer nanoparticles with well-defined surface chemistries were visualized interacting with a growing calcite surface using <i>in situ</i> atomic force microscopy. These nanoparticles comprise a poly­(benzyl methacrylate) (PBzMA) core-forming block and a non-ionic poly­(glycerol monomethacrylate) (Ph-PGMA), a carboxylic acid-tipped poly­(glycerol monomethacrylate) (HOOC-PGMA), or an anionic poly­(methacrylic acid) (PMAA) stabilizer block. Our results reveal three modes of interaction between the nanoparticles and the calcite surface: (i) attachment followed by detachment, (ii) sticking to and “hovering” over the surface, allowing steps to pass beneath the immobilized nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. By analyzing the relative contributions of these three types of interactions as a function of nanoparticle surface chemistry, we show that ∼85% of PMAA₈₅-PBzMA₁₀₀ nanoparticles either “hover” or become incorporated, compared to ∼50% of the HOOC-PGMA₇₁-PBzMA₁₀₀ nanoparticles. To explain this difference, we propose a two-state binding mechanism for the anionic PMAA₈₅-PBzMA₁₀₀ nanoparticles. The “hovering” nanoparticles possess highly extended polyelectrolytic stabilizer chains and such chains must adopt a more “collapsed” conformation prior to successful nanoparticle occlusion. This study provides a conceptual framework for understanding how sterically stabilized nanoparticles interact with growing crystals, and suggests design principles for improving occlusion efficiencies

Mechanistic Insights into Diblock Copolymer Nanoparticle–Crystal Interactions Revealed via <i>in Situ</i> Atomic Force Microscopy

Recently, it has become clear that a range of nanoparticles can be occluded within single crystals to form nanocomposites. Calcite is a much-studied model, but even in this case we have yet to fully understand the details of the nanoscale interactions at the organic–inorganic interface that lead to occlusion. Here, a series of diblock copolymer nanoparticles with well-defined surface chemistries were visualized interacting with a growing calcite surface using <i>in situ</i> atomic force microscopy. These nanoparticles comprise a poly­(benzyl methacrylate) (PBzMA) core-forming block and a non-ionic poly­(glycerol monomethacrylate) (Ph-PGMA), a carboxylic acid-tipped poly­(glycerol monomethacrylate) (HOOC-PGMA), or an anionic poly­(methacrylic acid) (PMAA) stabilizer block. Our results reveal three modes of interaction between the nanoparticles and the calcite surface: (i) attachment followed by detachment, (ii) sticking to and “hovering” over the surface, allowing steps to pass beneath the immobilized nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. By analyzing the relative contributions of these three types of interactions as a function of nanoparticle surface chemistry, we show that ∼85% of PMAA₈₅-PBzMA₁₀₀ nanoparticles either “hover” or become incorporated, compared to ∼50% of the HOOC-PGMA₇₁-PBzMA₁₀₀ nanoparticles. To explain this difference, we propose a two-state binding mechanism for the anionic PMAA₈₅-PBzMA₁₀₀ nanoparticles. The “hovering” nanoparticles possess highly extended polyelectrolytic stabilizer chains and such chains must adopt a more “collapsed” conformation prior to successful nanoparticle occlusion. This study provides a conceptual framework for understanding how sterically stabilized nanoparticles interact with growing crystals, and suggests design principles for improving occlusion efficiencies

Visible Mie Scattering from Hollow Silica Particles with Particulate Shells

A series of colloidal nanocomposite dispersions are synthesized by alcoholic dispersion polymerization of styrene in the presence of an ultrafine silica sol. The original core/shell polystyrene/silica nanocomposite particles have mean diameters ranging from 321 to 471 nm, as determined by dynamic light scattering. Upon calcination of the polystyrene cores, some shrinkage occurs but intact hollow silica shells are observed by transmission electron microscopy. On visual inspection, these silica residues display remarkable colors that vary depending on the particle diameter. When examined in transmittance mode (i.e., with an illuminated background) the silica powders appear yellow to red in color, but when viewed in reflectance (i.e., with a dark background) relatively intense blue/green colors are observed. The latter phenomenon has been analyzed by visible reflectance spectroscopy and the reflectance maximum depends on the dimensions of the silica shell, which are in turn dictated by the initial nanocomposite particle diameter. Small-angle X-ray scattering is used to determine the packing density of the silica nanoparticles, both in the original polystyrene/silica nanocomposite particles and in the calcined silica shells. Combined with geometrical considerations, this allows the equivalent <i>uniform</i> silica shell thickness to be calculated for a <i>particulate</i> silica shell and this parameter is then related to the theoretical predictions made by Retsch et al. for hollow particles comprising uniform silica shells (see Retsch, M.; Schmelzeisen, M.; Butt, H. J.; Thomas, E. L. <i>Nano Lett.</i>, <b>2011</b>, <i>11</i>, 1389)

Correcting for a Density Distribution: Particle Size Analysis of Core–Shell Nanocomposite Particles Using Disk Centrifuge Photosedimentometry

Many types of colloidal particles possess a core–shell morphology. In this Article, we show that, if the core and shell densities differ, this morphology leads to an inherent density distribution for particles of finite polydispersity. If the shell is denser than the core, this density distribution implies an artificial narrowing of the particle size distribution as determined by disk centrifuge photosedimentometry (DCP). In the specific case of polystyrene/silica nanocomposite particles, which consist of a polystyrene core coated with a monolayer shell of silica nanoparticles, we demonstrate that the particle density distribution can be determined by analytical ultracentrifugation and introduce a mathematical method to account for this density distribution by reanalyzing the raw DCP data. Using the mean silica packing density calculated from small-angle X-ray scattering, the real particle density can be calculated for each data point. The corrected DCP particle size distribution is both broader and more consistent with particle size distributions reported for the same polystyrene/silica nanocomposite sample using other sizing techniques, such as electron microscopy, laser light diffraction, and dynamic light scattering. Artifactual narrowing of the size distribution is also likely to occur for many other polymer/inorganic nanocomposite particles comprising a low-density core of variable dimensions coated with a high-density shell of constant thickness, or for core–shell latexes where the shell is continuous rather than particulate in nature

Mechanistic Insights into Diblock Copolymer Nanoparticle–Crystal Interactions Revealed via <i>in Situ</i> Atomic Force Microscopy

Recently, it has become clear that a range of nanoparticles can be occluded within single crystals to form nanocomposites. Calcite is a much-studied model, but even in this case we have yet to fully understand the details of the nanoscale interactions at the organic–inorganic interface that lead to occlusion. Here, a series of diblock copolymer nanoparticles with well-defined surface chemistries were visualized interacting with a growing calcite surface using <i>in situ</i> atomic force microscopy. These nanoparticles comprise a poly­(benzyl methacrylate) (PBzMA) core-forming block and a non-ionic poly­(glycerol monomethacrylate) (Ph-PGMA), a carboxylic acid-tipped poly­(glycerol monomethacrylate) (HOOC-PGMA), or an anionic poly­(methacrylic acid) (PMAA) stabilizer block. Our results reveal three modes of interaction between the nanoparticles and the calcite surface: (i) attachment followed by detachment, (ii) sticking to and “hovering” over the surface, allowing steps to pass beneath the immobilized nanoparticle, and (iii) incorporation of the nanoparticle by the growing crystals. By analyzing the relative contributions of these three types of interactions as a function of nanoparticle surface chemistry, we show that ∼85% of PMAA₈₅-PBzMA₁₀₀ nanoparticles either “hover” or become incorporated, compared to ∼50% of the HOOC-PGMA₇₁-PBzMA₁₀₀ nanoparticles. To explain this difference, we propose a two-state binding mechanism for the anionic PMAA₈₅-PBzMA₁₀₀ nanoparticles. The “hovering” nanoparticles possess highly extended polyelectrolytic stabilizer chains and such chains must adopt a more “collapsed” conformation prior to successful nanoparticle occlusion. This study provides a conceptual framework for understanding how sterically stabilized nanoparticles interact with growing crystals, and suggests design principles for improving occlusion efficiencies