Flower-Like Colloidal Particles through Precipitation Polymerization of Redox-Responsive Liquid Crystals.

We report on the synthesis of monodisperse, flower-like, liquid crystalline (LC) polymer particles by precipitation polymerization of a LC mixture consisting of benzoic acid-functionalized acrylates and disulfide-functionalized diacrylates. Introduction of a minor amount of redox-responsive disulfide-functionalized diacrylates (≤10 wt %) induced the formation of flower-like shapes. The shape of the particles can be tuned from flower- to disk-like to spherical by elevating the polymerization temperature. The solvent environment also has a pronounced effect on the particle size. Time-resolved TEM reveals that the final particle morphology was formed in the early stages of the polymerization and that subsequent polymerization resulted in continued particle growth without affecting the morphology. Finally, the degradation of the particles under reducing conditions was much faster for flower-like particles than for spherical particles, likely a result of their higher surface-to-volume ratio.

On Particle Size Distributions in Catalytic Chain Transfer Emulsion Polymerization: Chain-Extension and the Use of Derived Macromonomers as Reactive Surfactants in Emulsion Polymerization.

Catalytic chain transfer emulsion polymerization (CCTP) and subsequent chain extension via reversible addition-fragmentation chain transfer (RAFT) were used to synthesize amphiphilic macromonomers (MM), in the form of polymer latexes. The macromonomers consisted of two blocks whose first was a random copolymer of methacrylic acid and methyl methacrylate, at 35:65 mol:mol, while the second block was n-butyl methacrylate P[(MAA-co-MMA)-block-PBMA]. The block copolymer colloids were disintegrated and micellized upon addition of ammonia. The resulting nanosized polymer dispersions were used as reactive surfactants in the emulsion polymerization of n-butyl methacrylate. For this, a dual stage slow-fast monomer feed profile was used. The final polymer latexes were in the sub-100 nm range for the particle diameter at 30% w/w total polymer content. The emphasis of the work is to discuss and find an explanation for the observed particle size distributions in the three consecutive emulsion polymerization steps. The particle size distribution of the ω-unsaturated macromonomer latex synthesized by CCTP emulsion polymerization was found to be much broader than expected. This discrepancy is attributed to an extended particle nucleation period. The chain extension step in the macromonomer latex preparation showed considerable secondary nucleation. The presence of water-soluble macromonomer species from the CCTP emulsion polymerization step assured that control of chain growth persisted. The use of the amphiphilic macromonomers as reactive surfactants in the form of a nanosized aggregate seed dispersion showed that the average particle diameter could be tuned and that the molecular weight distributions could be regulated, when monomer starved conditions were used in the emulsion polymerizations.

Introducing Porosity in Colloidal Biocoatings to Increase Bacterial Viability.

A biocoating confines nongrowing, metabolically active bacteria within a synthetic colloidal polymer (i.e., latex) film. Bacteria encapsulated inside biocoatings can perform useful functions, such as a biocatalyst in wastewater treatment. A biocoating needs to have a high permeability to allow a high rate of mass transfer for rehydration and the transport of both nutrients and metabolic products. It therefore requires an interconnected porous structure. Tuning the porosity architecture is a challenge. Here, we exploited rigid tubular nanoclays (halloysite) and nontoxic latex particles (with a relatively high glass transition temperature) as the colloidal "building blocks" to tailor the porosity inside biocoatings containing Escherichia coli bacteria as a model organism. Electron microscope images revealed inefficient packing of the rigid nanotubes and proved the existence of nanovoids along the halloysite/polymer interfaces. Single-cell observations using confocal laser scanning microscopy provided evidence for metabolic activity of the E. coli within the biocoatings through the expression of a yellow fluorescent protein. A custom-built apparatus was used to measure the permeability of a fluorescein sodium salt in the biocoatings. Whereas there was no measurable permeability in a coating made from only latex particles, the permeability coefficient of the composite biocoatings increased with increasing halloysite content up to a value of 1 × 10-4 m h-1. The effects of this increase in permeability was demonstrated through a specially developed resazurin reduction assay. Bacteria encapsulated in halloysite composite biocoatings had statistically significant higher metabolic activities in comparison to bacteria encapsulated in a nonoptimized coating made from latex particles alone.

Colloidal particles at fluid interfaces: behaviour of isolated particles.

The adsorption of colloidal particles to fluid interfaces is a phenomenon that is of interest to multiple disciplines across the physical and biological sciences. In this review we provide an entry level discussion of our current understanding on the physical principles involved and experimental observations of the adsorption of a single isolated particle to a liquid-liquid interface. We explore the effects that a variation of the morphology and surface chemistry of a particle can have on its ability to adhere to a liquid interface, from a thermodynamic as well as a kinetic perspective, and the impact of adsorption behaviour on potential applications. Finally, we discuss recent developments in the measurement of the interfacial behaviour of nanoparticles and highlight open questions for future research.

Structure and behaviour of vesicles in the presence of colloidal particles.

This review highlights recent studies that investigate the structural changes and behaviour of synthetic vesicles when they are exposed to colloidal particles. We will show examples to demonstrate the power of combining particles and vesicles in generating exciting supracolloidal structures. These suprastructures have a wide range of often responsive behaviours that take advantage of both the mechanical and morphological support provided by the vesicles and the associated particles with preset functionality. This review includes applications spanning a variety of disciplines, including chemistry, biology, physics and medicine.

Roughening up polymer microspheres and their diffusion in a liquid.

A simple, versatile approach for the roughening of polymer microparticles surfaces via a deformation technique in the presence of an inorganic matrix is presented here. The process consists of straightforward steps: (1) preparation of a bicomposite colloidal sol, that is polymer particles and inorganic particles, dispersed in a liquid, (2) drying of the mixture onto a suitable hard substrate, (3) heating the dried film above the glass transition temperature of the polymer, and (4) re-dispersion and chemical etching of the inorganic medium. The primary driver is capillary imbibition of the polymer melt into the inorganic colloidal template. In addition, 2D particle tracking experiments of dispersed rough particles in water were performed to probe the diffusional behaviour of the roughened objects in comparison with their smooth precursors. We show that, despite large scale roughness (up to 10% asperity size with respect to particle diameter), Stokes law is obeyed and the particle motion can be modelled simply with the Stokes-Einstein-Sutherland relation.

Mechanistic Insight into the Synthesis of Silica-Based "Matchstick" Colloids.

We report an insight into the synthesis of silica-based "matchstick"-shaped colloidal particles, which are of interest in the area of self-propulsion on small length scales. The generation of aqueous emulsion droplets dispersed in an n-pentanol-rich continuous phase and their use as reaction centers allows for the fabrication of siliceous microparticles that exhibit anisotropy in both particle morphology, that is, a "matchstick" shape, and chemistry, that is, a transition-metal oxide-enriched head. We provide a series of kinetic studies to gain a mechanistic understanding and unravel the particle formation and growth processes. Additionally, we demonstrate the ability to select the aspect ratio of the "matchstick" particle in a straightforward manner.

Synthesis of "hard-soft" janus particles by seeded dispersion polymerization.

The majority of studies on Janus particles focus on those that show amphiphilicity, with distinct hydrophilic and hydrophobic domains. Here, we demonstrate the synthesis of a different class of Janus particles: "hard-soft" biphasic dumbbell- or peanut-shaped particles with distinct lobes of "soft" poly(n-butyl acrylate) and "hard" poly(styrene). The particles are made by seeded dispersion polymerization of butyl acrylate in the presence of poly(styrene) seed particles. Surface nucleation by capture of the oligoradicals onto the surface of the seed particles thereby forming a distinct new polymer phase is found to be the formation mechanism of these particles. The total available poly(styrene) seed surface area plays a significant role in the size and number of poly(butyl acrylate) lobes grown off a single particle. At particularly low values of the surface area, we observe the formation of multilobe particles. We further demonstrate that our synthesis method is versatile and can be extended to the submicrometer domains by using seed particles of 200 nm in diameter.

Hierarchical self-assembly of 'hard-soft' Janus particles into colloidal molecules and larger supracolloidal structures.

Here we report the self-assembly of 'hard-soft' micron-sized Janus particles into clusters in aqueous media. The assembly process is induced by the desorption of a polymeric stabiliser from the particles, that is polyvinylpyrrolidone (PVP). Upon contact through collision and coalescence of the soft polymeric lobes, the newly formed clusters adopt a minimized surface area to volume ratio, thereby forming distinct microscopic supracolloidal analogues of simple molecular valance shell electron pair repulsion (VSEPR) space-fill structures. To explain this behaviour, the colloidal stability of our particle suspensions were studied with and without an adsorbed steric surfactant. Simulations of expected cluster morphology, compared with those from cryo-SEM analysis support the mechanism of assembly driven by surface area minimization in the case of soft-soft interactions. Altering the soft lobe size with respect to the hard lobe indicates a moderate effect on number of primary particles per cluster. Additionally, higher order structures of clusters containing a number of primary particles exceeding what is possible for a 'solid' core cluster are observed. As such, we also investigated the formation of suprastructures using a high number of 'hard-soft' Janus particles and verified their effective Pickering stabilization of air bubbles.

Surfactant-free miniemulsion polymerization of n-BA/S stabilized by NaMMT: films with improved water resistance.

The use of sodium montmorillonite clay as a stabilizer in the surfactant-free emulsion polymerization of n-butyl acrylate/styrene (n-BA/S) was assessed. It was shown that the use of the clay alone did not yield the desired armored latex particles. A functional comonomer, that is, a phosphate ester of poly(ethylene glycol) monomethacrylate, was used to improve the interaction between the polymer and clay, thus allowing for the clay platelets to adhere to the surface of the polymer particles. The morphology of the films obtained for these two different scenarios was similar and resembled a honeycomb structure. However, their water-resistance properties differed drastically. The water absorption and water vapor permeation rate were much lower in the hybrid n-BA/S/clay films in the presence of the functional monomer than in the films obtained without the functional monomer.

Multicompartmental Janus microbeads from branched polymers by single-emulsion droplet microfluidics.

We describe a versatile and facile route for the preparation of Janus microbeads using single emulsion droplet-based microfluidics, in which water droplets that contain a mixture of branched poly(N-isopropylacrylamide)-co-(poly(ethylene glycol)diacrylate)-co-(methacrylic acid) and colloidal particles form the basis of our approach. The colloidal particles, poly(methyl methacrylate) microspheres or titanium dioxide particles, and iron oxide nanoparticles are spatially positioned within the water droplets through gravity and an externally applied magnetic force, respectively. Evaporation of water leads to gel formation of the branched copolymer matrix as a result of physical cross-linking through hydrogen bond interactions, fixing the spatial position of the colloidal particles. The thermo- and pH-responsive nature of the branched poly(N-isopropylacrylamide) (PNIPAm)-based copolymer allows for the disintegration of the polymer network of the Janus microbeads and a triggered release of the colloidal content at temperatures below the lower critical solution temperature (LCST) and at increased pH values.

Influence of Janus particle shape on their interfacial behavior at liquid-liquid interfaces.

We investigate the self-assembly behavior of Janus particles with different geometries at a liquid-liquid interface. The Janus particles we focus on are characterized by a phase separation along their major axis into two hemicylinders of different wettability. We present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders, and Janus discs. Using the pendant drop technique, we monitor the assembly kinetics following changes in the interfacial tension of nanoparticle adsorption. According to the evolution of the interfacial tension and simulation data, we will specify the characteristics of early to late stages of the Janus particle adsorption and discuss the effect of Janus particle shape and geometry. The adsorption is characterized by three adsorption stages which are based on the different assembly kinetics and different adsorption mechanisms depending on the particle shape.

Styrene-based polymerised high internal phase emulsions using monomers in the internal phase as co-surfactants for improved liquid chromatography.

Poly(styrene-co-divinylbenzene)-based monoliths were prepared from the polymerisation of water-in-monomer high internal phase emulsions, where the water-soluble monomers acrylamide (AAm) or poly(ethylene glycol) diacrylate (PEGDA) (M w 258) were also included in the 90 vol% internal phase. Both AAm and PEGDA were found to act as co-surfactants, resulting in the obtainment of monoliths with greater homogeneity in some cases. As a result these materials demonstrated significantly improved chromatographic performance for the separation of a standard mixture of proteins using reversed-phase liquid chromatography, in comparison to monoliths prepared with no internal phase monomer. In particular, the columns grafted with PEGDA were capable of separating a more complex mixture consisting of seven components. The inclusion of monomers in the internal phase also allowed for the functionalisation of the monolith's surface where the degree of polymerisation that occurred in the internal phase, which was governed by the monomer content in the internal phase and initiation location, determined whether polymeric chains or a hydrogel were grafted to the surface. A monolith grafted with AAm was also found to be capable of retaining polar analytes as a result of the increase in surface hydrophilicity.

Polymer vesicles with a colloidal armor of nanoparticles.

The fabrication of polymer vesicles with a colloidal armor made from a variety of nanoparticles is demonstrated. In addition, it is shown that the armored supracolloidal structure can be postmodified through film-formation of soft polymer latex particles on the surface of the polymersome, hereby effectively wrapping the polymersome in a plastic bag, as well as through formation of a hydrogel by disintegrating an assembled polymer latex made from poly(ethyl acrylate-co-methacrylic acid) upon increasing the pH. Furthermore, ordering and packing patterns are briefly addressed with the aid of Monte Carlo simulations, including patterns observed when polymersomes are exposed to a binary mixture of colloids of different size.

Textured Microcapsules through Crystallization.

This work demonstrates the fabrication of surface-textured microcapsules formed from emulsion droplets, which are stabilized by an interlocking mesh of needle-like crystals. Crystals of the small-organic-compound decane-1,10-bis(cyclohexyl carbamate) are formed within the geometric confinement of the droplets, through precipitation from a binary-solvent-dispersed phase. This binary mixture consists of a volatile solvent and nonvolatile carrier oil. Crystallization is facilitated upon supersaturation due to evaporation of the volatile solvent. Microcapsule diameter can be easily tuned using microfluidics. This approach also proves to be scalable when using conventional mixers, yielding spikey microcapsules with diameters in the range of 10-50 µm. It is highlighted that the capsule shape can be molded and arrested by jamming using recrystallization in geometric confinement. Moreover, it is shown that these textured microcapsules show a promising enhanced deposition onto a range of fabric fibers.

Unraveling mechanistic events in solids-stabilized emulsion polymerization by monitoring the concentration of nanoparticles in the water phase.

The fate of nanoparticles used as stabilizers in solids-stabilized, or Pickering, emulsion polymerization for the formation of armored hybrid polymer latexes was studied. We showed that disk centrifugation can be used as a powerful quantitative tool to analyze and determine the concentration of nanoparticles in the water phase throughout solids-stabilized emulsion polymerizations. We performed a series of emulsion polymerizations using vinyl acetate, vinyl pivalate, methyl methacrylate, or butyl acrylate in presence of silica nanoparticles (Ludox TM-40, ca. 25 nm in diameter). The developed method to quantify the number of silica nanoparticles in the water phase proved to be an invaluable tool for determining key features of the polymerization process. The obtained concentration profiles versus monomer conversion explained the existence of limited coalescence of armored particles in the later stages of the solids-stabilized emulsion polymerization process of vinyl acetate, leading to nonspherical structures. Moreover, we demonstrated that the correlation of the measured number of silica nanoparticles present in the water phase with the average particle sizes of the latex particles provided excellent predictions for the coverage of the armored layer of nanoparticles on the surfaces of the polymer particles, corresponding to observed packing patterns.

Self-assembly of amphiphilic peanut-shaped nanoparticles.

We use computer simulation to investigate the self-assembly of Janus-like amphiphilic peanut-shaped nanoparticles, finding phases of clusters, bilayers, and micelles in accord with ideas of packing familiar from the study of molecular surfactants. However, packing arguments do not explain the hierarchical self-assembly dynamics that we observe, nor the coexistence of bilayers and faceted polyhedra. This coexistence suggests that experimental realizations of our model can achieve multipotent assembly of either of two competing ordered structures.

Packing patterns of silica nanoparticles on surfaces of armored polystyrene latex particles.

Fascinating packing patterns of identical spherical and discotic objects on curved surfaces occur readily in nature and science. Examples include C(60) fullerenes, (1, 2) 13-atom cuboctahedral metal clusters, (3) and S-layer proteins on outer cell membranes. (4) Numerous situations with surface-arranged objects of variable size also exist, such as the lenses on insect eyes, biomineralized shells on coccolithophorids, (5) and solid-stabilized emulsion droplets (6) and bubbles. (7) The influence of size variations on these packing patterns, however, is studied sparsely. Here we investigate the packing of nanosized silica particles on the surface of polystyrene latex particles fabricated by Pickering miniemulsion polymerization of submicrometer-sized armored monomer droplets. We are able to rationalize the experimental morphology and the nearest-neighbor distribution with the help of Monte Carlo simulations. We show that broadening of the nanoparticle size distribution has pronounced effects on the self-assembled equilibrium packing structures, with original 12-point dislocations or grain-boundary scars gradually fading out.

Synthesis of Janus and Patchy Particles Using Nanogels as Stabilizers in Emulsion Polymerization.

Polymer nanogels are used as colloidal stabilizers in emulsion polymerization. The nanogels were made by the covalent cross-linking of block copolymer micelles, the macromolecular building blocks of which were synthesized using a combination of catalytic chain transfer emulsion polymerization and reversible addition-fragmentation chain transfer (RAFT) of methacrylate monomers. The use of the nanogels in an emulsion polymerization led to anisotropic Janus and patchy colloids, where a latex particle was decorated by a number of patches on its surface. Control of the particle size and patch density was achieved by tailoring of the reaction conditions, such as varying the amount of nanogels, pH, and salt concentration. Overall, the emulsion polymerization process in the presence of nanogels as stabilizers is shown to be a versatile and easily scalable route toward the fabrication of Janus and patchy polymer colloids.

Effect of shearing stress on the radial heterogeneity and chromatographic performance of styrene-based polymerised high internal phase emulsions prepared in capillary format.

Poly(styrene-co-divinylbenzene) monoliths were prepared from the polymerisation of water-in-monomer high internal phase emulsions consisting of a 90 vol% internal phase and stabilised by the non-ionic surfactant Span 80®. The materials were prepared in capillary housings of various internal diameters ranging from 150 µm to 540 µm by simply passing the emulsion through the capillaries. When low shear (300 rpm) was used for emulsification, the droplet and resulting void size distributions were observed to shift towards lower values when the emulsions were forced through capillaries of internal diameter less than 540 µm and all columns exhibited significant radial heterogeneity. When high shear was employed (14 000 rpm) the resulting emulsions preserved their structure when forced through these capillaries and possessed narrower void size distributions with no obvious radial heterogeneity observed upon curing. This resulted in significantly improved chromatographic performance for the separation of a standard mixture of proteins when compared to the materials prepared under low shear.