Silica core-polystyrene shell nanoparticle synthesis and assembly in three dimensions

Macroscale superlubricity induced by film-forming polymer brush-grafted colloidal additives

HYPOTHESIS

Modifying surfaces with concentrated polymer brushes (CPBs) is an effective way to reduce friction of tribo-pairs lubricated with liquids. We investigate the hypothesis that colloids grafted with CPBs (hybrid colloids) can deposit onto tribo-substrates by varying the solvent quality with respect to the polymer, in order to obtain ultra-low coefficients of friction (CoFs), so-called superlubricity.

EXPERIMENTS

Hybrid colloids are synthesized and characterized, and a dynamic light scattering compares their swellings in aqueous solutions of glycerol or polyethylene glycol. A mini-traction machine with viscoelastic tribo-pairs is used for lubrication experiments. Adsorption of colloids and film structures are tested using a quartz crystal microbalance and an atomic force microscope.

FINDINGS

The solvent controls whether hybrid colloids spontaneously adsorb to the substrate under quiescent conditions or require contact forces to enable (tribo-)deposition. In both cases, the friction in the boundary-mixed lubrication regimes is lower upon increasing the degree of swelling of CPBs and upon increasing coverage of deposited colloids. The greatest lubrication enhancement and surface coverage occur for the spontaneously adsorbed colloids, with ultra-low CoFs of order 10^-3 over a large range of speeds. The results demonstrate the potential for hybrid colloids to be used as solvent dispersible "friction modifier additives".

Self-Assembly of Metal-Organic Framework (MOF) Nanoparticle Monolayers and Free-Standing Multilayers

We report the first self-assembled porous monolayer and free-standing multilayer films composed of metal-organic framework (MOF) nanoparticles. Self-assembled MOF monolayers (SAMMs) were assembled at a liquid-air interface to produce films that are 87 wt % (89 vol %) MOF. Monolayer self-assembly was aided by growing a layer of poly(methyl methacrylate) (PMMA) on the particle surface using a histamine anchor. SAMMs could be stacked to obtain MOF multilayers, including alternating MOF/polymer heterostructures. SAMMs were coated on silicon microparticles, and a MOF film constructed of only five stacked layers could be manipulated as a free-standing, opalescent film. These monolayers are a significant advancement for obtaining highly functional porous membranes and coatings.

Nanoparticle synthesis via bubbling vapor precursors in bulk liquids

Conventional methods for preparing polymer nanoparticles and organic-inorganic composite nanoparticles use solution based top-down processes with surfactants and mechanical stirring. Examples of such processes include emulsion polymerization of monomers to produce polymer nanoparticles and sol-gel reactions involving hydrolysis of inorganic precursors to produce inorganic materials (such as silica and titanium nanoparticles). Here, we show that vaporized precursors of various compounds can be used as reactants to produce polymer, inorganic, and composite nanoparticles. The bubbling action of precursor vapor in a reactant vessel provides a constant supply of precursor species while aiding their rapid mixing in the bulk solution liquid. The vaporization and bubbling processes require only small amounts of energy to prepare nanoparticles or core-shell nanoparticles without forming unwanted side products. Compared to other available techniques, this approach enables precise control of nanoparticle size and shell thickness as functions of vapor supply time and temperature without surfactants. Our approach can potentially be applied to fabricate functional nanomaterials using organic and inorganic precursors for medical, electrical, optical, magnetic and/or catalytic applications.

USAXS analysis of concentration-dependent self-assembling of polymer-brush-modified nanoparticles in ionic liquid: [I] concentrated-brush regime

Using ultra-small angle X-ray scattering (USAXS), we analyzed the higher-order structures of nanoparticles with a concentrated brush of an ionic liquid (IL)-type polymer (concentrated-polymer-brush-modified silica particle; PSiP) in an IL and the structure of the swollen shell layer of PSiP. Homogeneous mixtures of PSiP and IL were successfully prepared by the solvent-casting method involving the slow evaporation of a volatile solvent, which enabled a systematic study over an exceptionally wide range of compositions. Different diffraction patterns as a function of PSiP concentration were observed in the USAXS images of the mixtures. At suitably low PSiP concentrations, the USAXS intensity profile was analyzed using the Percus-Yevick model by matching the contrast between the shell layer and IL, and the swollen structure of the shell and "effective diameter" of the PSiP were evaluated. This result confirms that under sufficiently low pressures below and near the liquid/crystal-threshold concentration, the studied PSiP can be well described using the "hard sphere" model in colloidal science. Above the threshold concentration, the PSiP forms higher-order structures. The analysis of diffraction patterns revealed structural changes from disorder to random hexagonal-closed-packing and then face-centered-cubic as the PSiP concentration increased. These results are discussed in terms of thermodynamically stable "hard" and/or "semi-soft" colloidal crystals, wherein the swollen layer of the concentrated polymer brush and its structure play an important role.

Agglomeration-Free Preparation of Modified Silica Nanoparticles for Emulsion Polymerization-A Well Scalable Process

To prepare modified silica nanospheres for emulsion polymerization, a new agglomeration-free change of dispersion media has been developed. Nanosized silica spheres were synthesized by the Stöber method and directly modified with a silane coupling agent. To prepare these particles for subsequent polymerization, the dispersion medium was changed in a two-step process from ethanol to water without agglomeration of the particles. The emulsion polymerization leads to hemispherical single-core-structured silica-polystyrene composite particles. The thickness of the polymer shell can be altered by varying the amount of styrene. The developed change of dispersion media provides nonagglomerated modified silica particles for the encapsulation with polystyrene and enables the synthesis of narrowly distributed single-core composite particles. The developed process is a promising approach for the preparation of nanoparticles for subsequent polymerization and can be scaled-up for industrial applications.

Significantly improving mechanical, thermal and dielectric properties of cyanate ester resin through building a new crosslinked network with unique polysiloxane@polyimide core–shell microsphere

Tough, rigid and thermally resistant resins with outstanding dielectric properties were developed based on polysiloxane@polyimide core–shell microspheres and cyanate ester.