Preparation and controlled self-assembly of Janus magnetic nanoparticles

Janus magnetic nanoparticles (~20 nm) were prepared by grafting either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) to the exposed surfaces of negatively charged poly(acrylic acid) (PAA)-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. Controlled aggregation of the Janus nanoparticles was observed at low pH values, with the formation of stable clusters of approximately 2-4 times the initial size of the particles. Cluster formation was reversed, and individually dispersed nanoparticles recovered, by restoring the pH to high values. At intermediate pH values, PSSNa Janus nanoparticles showed moderate clustering, while PDMAEMA Janus nanoparticles aggregated uncontrollably due to dipolar interactions. The size of the stable clusters could be controlled by increasing the molecular weight of the grafted polymer, or by decreasing the magnetic nanoparticle surface availability for grafting, both of which yielded larger cluster sizes. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension resulted in a further increase in the final cluster size. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.     

Amphiphilic silica nanoparticles at the decane-water interface: insights from atomistic simulations

The properties of 3 nm-diameter silica nanoparticles with different surface chemistry were systematically investigated at the decane-water interface using molecular dynamics simulations. Our results show that the decane-water interfacial tension is not much influenced by the presence of the nanoparticles. The three-phase contact angle increases with nanoparticle surface hydrophobicity. Contact angles observed for the nanoparticles at 300 and at 350 K differ very little. The contact angle of the nanoparticle with randomly dispersed hydrophobic groups is smaller than that observed in Janus nanoparticles of equal overall surface chemistry composition. The energy necessary to desorb Janus nanoparticles from the interface is usually higher than that required to desorb the corresponding homogeneous nanoparticles. Desorption from the interface into the aqueous phase is preferred over that into the organic phase for all except one of the nanoparticles considered. Structural and dynamic properties including nanoparticle rotational relaxation, solvent density profiles, and solvent residence autocorrelation functions near the nanoparticles are also presented. The data are useful for designing Pickering emulsions.     

Preparation of monodisperse SiO2 nanoparticles by membrane emulsification using ideally ordered anodic porous alumina

Monodisperse SiO2 particles of nanometer dimensions were fabricated by membrane emulsification using ideally ordered anodic porous alumina. For the preparation of monodisperse emulsion droplets, the dispersed phase was pressed through a porous alumina membrane into the continuous phase. After solidification treatment of the emulsion droplets, prepared spherical SiO2 nanoparticles with uniform sizes were obtained. From scanning electron microscope observation of the obtained particles, it was confirmed that the size distribution of SiO2 nanoparticles is relatively narrow.     

Microcapsule synthesis via RAFT photopolymerization in vegetable Oil as a green solvent

Polymeric capsules with an aqueous core have great potential for a wide range of applications, for example food/biomedical applications. However, synthesis of such capsules often involves the use of toxic organic solvents. Herein, an organic solvent‐free approach is developed for the synthesis of polymeric microcapsules with an aqueous core. The method is based on RAFT polymerization of divinyl monomer around the periphery of inverse emulsion water droplets acting as templates, with an amphiphilic macroRAFT species fulfilling the dual roles of RAFT agent and colloidal stabilizer. Vegetable oils, which are non‐toxic and renewable, are used as the continuous phase of these inverse emulsions, which are prepared using membrane emulsification to control the emulsion droplet size and size distribution. Relatively monodisperse emulsions with tunable droplet size in the range of approximately 10–30 µm are prepared, followed by the RAFT polymerization step to generate polymeric microcapsules having similar size as the initial droplets. This approach will be beneficial for various applications where toxic solvents need be minimized or removed completely to avoid adverse effects. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 831–839     

One-step microwave preparation of well-defined and functionalized polymeric nanoparticles

Well-defined colloidal polymeric nanoparticles are important in advanced biomedical and optical technologies. We report a facile microwave methodology to prepare narrowly dispersed cross-linked polymeric nanoparticles at high solids content through a surfactant-free emulsion polymerization process. The nanoparticle size was controlled by using cross-linkers with enhanced reactivity through a one-step microwaving process, significantly simplifying the nanoparticle synthetic process. The successful size control was realized by confining the cross-linking to intraparticle cross-linking rather than interparticle cross-linking. We also discovered that the superheating/dielectric heating effect associated with microwave irradiation could be utilized to effectively reduce the nanoparticle size.     

Macroporous polymer thin film prepared from temporarily stabilized water-in-oil emulsion

We report a simple and convenient method for fabricating ordered porous structure in a polymeric thin film. A temporarily stabilized water-in-oil emulsion, where aqueous droplets were dispersed in the medium of polymer-organic solvent solution, was utilized for the preparation of porous structure. The water-in-oil emulsion was simply prepared by sonicating the mixture of water and polymer-organic solvent solution without any colloid stabilizer. The growth of aqueous droplets was profoundly retarded by dissolving a small amount of sucrose, selectively soluble in the dispersed phase. The prepared emulsion was recovered onto a substrate through dip-coating and subsequently air-dried to get a well-ordered porous polymer film. The polymer content in the polymer solution phase and the compositional ratio of the aqueous phase to the polymer solution phase was optimized to fabricate well-ordered structures.     

Effect of Physical Properties and Emulsification Conditions on the Microsphere Size Prepared Using a Solvent Extraction Process

Microspheres were prepared using a hydrocarbon-perfluorocarbon solvent extraction process. The effect of the physical properties and the emulsification conditions on the mean microsphere size was investigated. The viscosity of the dispersed and the continuous phase greatly affected the microsphere size. Smaller microspheres were produced at the same mixing intensity when the viscosity of the dispersed phase decreased. Increased continuous phase viscosity reduced the coalescenceof the droplets and hence smaller microspheres were produced. The mean microsphere size first decreased as the volume ratio of the dispersed phase to the continuous phase increased but upon further increase the mean microsphere size increased. The effect of the volume ratio on the microsphere size was linked to the surfactant concentration. The stability of the studied hydrocarbon-in-fluorocarbon emulsion is poor. One reason for the poor stability is the high density difference between the phases. The emulsion droplets were solidified by siphoning part of the emulsion in the fresh continuous phase, which extracted the solvent from the dispersed phase. The effect of emulsion transfer time between the emulsification and solidification steps on the particle size was studied but no significant effect was observedduring the controlled time interval.     

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and M?ssbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents. Copyright 1999 Academic Press.     

Synthesis of nonspherical superparamagnetic particles: in situ coprecipitation of magnetic nanoparticles in microgels prepared by stop-flow lithography

We present the synthesis of nonspherical magnetic microparticles with multiple functionalities, shapes, and chemistries. Particle synthesis was performed in two steps: polymeric microparticles functionalized homogenously with carboxyl groups were generated using stop-flow lithography, and then in situ coprecipitation was used to grow magnetic nanoparticles at these carboxyl sites. With successive growth of magnetic nanoparticles, we obtained polymeric particles with saturation magnetizations of up to 42 emu/g microparticle. The growth in the magnetic nanoparticle mean size and polydispersity was determined from the magnetization curves obtained following each growth cycle; nanoparticle sizes were limited by the physical constraint of the effective mesh within the hosting gel microparticle. Particles with spatially segregated domains of varying magnetic properties (e.g., Janus particles, particles with step changes in magnetite concentration, etc.) can be synthesized readily using this approach.     

Controllable preparation of nanoparticle-coated chitosan microspheres in a co-axial microfluidic device

In this work, we describe a novel and simple microfluidic method for fabricating nanoparticle-coated chitosan microspheres. Uniform droplets of aqueous chitosan solution were dispersed into an oil phase containing partially hydrophilic nanoparticles via a co-axial microfluidic device. Recirculating flow in the continuous phase in the area between drops enhanced mixing and allowed the nanoparticles to coat the surface of the droplets as they passed through the channel. The chitosan droplets were then crosslinked with glutaraldehyde and nanoparticle-coated microspheres were obtained. SEM characterization shows that the microspheres are monodispersed with uniform nanoparticle distribution on the surface. The dispersity, size and composition of the microspheres could all easily be controlled by changing the microfluidic flow parameters and three different types of nanoparticles were successfully used to synthesize hybrid microspheres to demonstrate the method's versatility.     

Biopolymer microparticle and nanoparticle formation within a microfluidic device

This paper reports a novel microfluidic method for the production of cross-linked alginate microparticles and nanoparticles. We describe a continuous process relying on both thermodynamic and hydrodynamic factors to form microdroplets. A rapid cross-linking reaction thereafter allows solidification of the polymer droplets either within the microfluidic device or "off-chip" to form alginate micro- and nanoparticles. Monodisperse droplets are generated by extruding an aqueous alginate solution using an axisymmetric flow-focusing design. As they flow downstream in the channel, due to water and the continuous phase being partially miscible, the water diffuses very slowly out of the polymeric droplets into the transport fluid, which causes the shrinkage of the drops and the condensation of the polymer phase. The resulting size of the solid particles depends on the polymer concentration and the ensuing balance between the kinetics of the cross-linking reaction and the volume loss due to solvent diffusion. This work details both a single-step microfluidic technique for the formation of alginate microparticles of sizes ranging from 1 to 50 microm via near-equilibrium solvent diffusion within a microfluidic device and thereafter a two-step method, which was shown to generate biopolymer nanoparticles of sizes ranging from 10 to 300 nm. These novel methodologies are extremely flexible and can be extended to the preparation of micro- and nanoparticles from a wide range of single or mixed synthetic and biologically derived polymers.     

Oil core-polymer shell microcapsules prepared by internal phase separation from emulsion droplets. I. Characterization and release rates for microcapsules with polystyrene shells

Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield.     

NMR studies into colloidal stability and magnetic order in fatty acid stabilised aqueous magnetic fluids

We report the physico-chemical characterisation of fatty acid stabilised aqueous magnetic fluids, which are ideal systems for studying the influence of nanoparticle aggregation on the emergent magnetic resonance properties of the suspensions. Stable colloids of superparamagnetic magnetite, Fe(3)O(4), nanoparticle clusters in the 80 to 100 nm size range were produced by in situ nanoparticle growth and stabilisation, and by suspending pre-formed nanoparticles. NMR relaxation analysis shows that the magnetic resonance properties of the two types of suspension differ substantially and provides new insights into how the relaxation mechanisms are determined by the organisation of the nanoparticles within the clusters.     

Production of porous suspension polymer beads with a narrow size distribution using a cross-flow membrane and a continuous tubular reactor

The work described focuses on a two-stage process for the production of large porous suspension polymer beads of defined particle size and narrow size distribution. Emulsification has been performed using purpose built cross-flow membrane equipment, which allows controlled production of large emulsion droplets with a much narrower size distribution. The work described compares the production of large emulsion droplets of monomer prepared both by agitation and using a cross-flow membrane. The effects of variations in the pore size of the membrane and flow-rates on the size of the emulsion droplets produced are also investigated. The second stage of the process is polymerisation of performed monomer emulsion droplets using a continuous tubular reactor. Samples polymerised using such a method show a narrower size distribution than similar systems polymerised as a batch.     

PBD/PDMS共混物分散相的聚并捕获行为

借助显微-剪切装置在线研究了低速剪切场下SiO2纳米粒子含量、分散相聚丁二烯(PBD)浓度和剪切速率对PBD/聚二甲基硅氧烷(PDMS)不相容体系中聚并捕获行为的影响.结果表明,聚并捕获所形成的液滴尺寸与形状规整度由粒子含量、分散相浓度和剪切速率等因素共同决定.在较低的SiO2纳米粒子含量或较高的分散相浓度下,PBD液滴在低剪切场下发生聚并捕获,形成尺寸较大、形状不规则的液滴.增加SiO2纳米粒子含量或减小分散相浓度,能够减小分散相的尺寸并提高分散相的规整度.增加剪切速率能有效地减小分散相的尺寸并提高分散相的规整度.     

Cross-linked polyvinylpyrrolidone nanoparticles: a potential carrier for hydrophilic drugs

Injectable hydrogel polymeric nanoparticles of polyvinylpyrrolidone cross-linked with N,N'-methylene bis-acrylamide and encapsulating water-soluble macromolecules such as FITC-dextran (FITC-Dx) have been prepared in the aqueous cores of reverse micellar droplets. These particles are 100 nm and below in diameter with a narrow size distribution. When dispersed in aqueous buffer these particles appear to be transparent and give an optically clear solution. Lyophilized powder of these nanoparticles is redispersable in aqueous buffer without any change in the size and morphology of the particles. The efficiency of FITC-Dx entrapment by these nanoparticles is high (>70%) and depends on the amount of cross-linking agent present in the polymeric material. The release of the entrapped molecules from these nanoparticles depends on the degree of cross-linking of the polymer, particle size, pH of the medium, and extent of loading, as well as temperature.     

Mechanism of nanocapsules formation by the emulsion-diffusion process

A detailed investigation into the mechanisms of nanocapsule formation by means of the two stages "emulsion-diffusion" process is reported. Such widely used process is still poorly understood. An emulsion of oil, polymer and ethyl acetate is fabricated as a first step; dilution with pure water allows ethyl acetate to diffuse out from the droplets, leaving a suspension of nanocapsules at the end. It has been shown that the size of nanocapsules was related to the chemical composition of the organic phase and the size of primary emulsion through a simple geometrical relationship. As a consequence, most of the properties of the nanocapsules were decided at the emulsification step. The influence of several formulation and processing parameters of the primary emulsion was studied accordingly. The thin polymer membrane of nanocapsules was observed by means of cryo-fracture electron microscopy. Finally two experiments were designed for a mechanistic investigation of the diffusion step. A step-by-step diffusion of the organic solvent takes place by successive partition equilibria of ethyl acetate between the droplets and aqueous phase. A time-resolved experiment shows the fast diffusion (less than 20 ms) related to the small droplet size of the emulsion.     

Preparation of hemispherical polymer particles via phase separation induced by microsuspension polymerization

Micrometer-sized, hemispherical polystyrene (PS) particles were successfully prepared by microsuspension polymerization of homogeneous styrene/hexadecane (HD) droplets dispersed in polyoxyethylene nonylphenyl ether (Emulgen 931) aqueous solution, followed by rapid removal of HD from formed PS/HD particles with a “Janus” structure. It was important for the formation of the morphology of Janus particles in thermodynamically stable state to carry out the polymerization slowly. The formation of by-product small PS particles by emulsion polymerization was suppressed by the additions of CuCl₂ as a water-soluble inhibitor and NaCl to decrease the solubility of styrene in the aqueous phase.     

A Comparison of Membrane Emulsification Obtained Using SPG (Shirasu Porous Glass) and PTFE [Poly(Tetrafluoroethylene)] Membranes

SPG (Shirasu porous glass) membrane emulsification used to prepare uniform polymeric microspheres is briefly reviewed, and the performance of a hydrophilically treated PTFE [poly(tetrafluoroethylerie)] membrane is described and compared with that of the SPG membrane. A mixture of styrene. divinyl benzene and hexadecane (HD) was extruded through the membranes and dispersed in an aqueous phase containing polyvinylalcohol (PVA) and sodium lauryl sulfate (SLS) as mixed stabilizers. A hvdrophilically treated PTFE membrane was used with a stainless steel mesh support so that the membrane would not expand to affect the pore size during the emulsification. The nominal pore size of the PTFE membrane was replaced with the calculated one using a theoretical expression derived from the force balance between the external pressure and the interfacial tension between oil and water phases. The emulsion droplets prepared with the PTFE membrane revealed a broader size distribution than those obtained with the SPG membrane, and the rate of emulsificaton was nearly same for both membranes. Droplet size control was readily possible. The performance was significantly affected by the adsorption behavior of the stabilizers on the membrane surfaces. The contact angle profile of oil droplets on the PTFE membrane implied that the hydrophilically treated PTFE membrane is still hydrophobic compared to the SPG membrane. This tendency was reflected by the dependence of the average droplet diameter (and coefficient of variation, CV) on the concentration and composition of mixed stabilizers.     

Preparation of calcium alginate microgel beads in an electrodispersion reactor using an internal source of calcium carbonate nanoparticles

An electrodispersion reactor has been used to prepare calcium alginate (Ca-alginate) microgel beads in this study. In the electrodispersion reactor, pulsed electric fields are utilized to atomize aqueous mixtures of sodium alginate and CaCO3 nanoparticles (dispersed phase) from a nozzle into an immiscible, insulating second liquid (continuous phase) containing a soluble organic acid. This technique combines the features of the electrohydrodynamic force driven emulsion processes and externally triggered gelations in microreactors (the droplets) ultimately to yield soft gel beads. The average particle size of the Ca-alginate gels generated by this method changed from 412 +/- 90 to 10 +/- 3 microm as the applied peak voltage was increased. A diagram depicting structural information for the Ca-alginate was constructed as a function of the concentrations of sodium alginate and CaCO3 nanoparticles. From this diagram, a critical concentration of sodium alginate required for sol-gel transformation was observed. The characteristic highly porous structure of Ca-alginate particles made by this technique appears suitable for microencapsulation applications. Finally, time scale analysis was performed for the electrodispersion processes that include reactions in the microreactor droplets to provide guidelines for the future employment of this technique. This electrodispersion reactor can be used potentially in the formation of many reaction-based microencapsulation systems.