Miniemulsion Droplets Research Articles

Encapsulation of magnetic nickel nanoparticles via inverse miniemulsion polymerization

AbstractIn this work, the encapsulation of magnetic nickel nanoparticles in polyacrylamide particles was performed via inverse miniemulsion polymerization. The dispersion of nickel nanoparticles was characterized in polar solvents including water, ethanol, and dimethyl sulfoxide using different stabilizers. The best results were obtained when the nonionic stabilizer poly(ethylene glycol) octadecyl ether (Brij 76) was used to stabilize the nickel nanoparticles in dimethyl sulfoxide. In addition, the block copolymer poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) was used as a surfactant to create inverse miniemulsions while minimizing the coalescence of the miniemulsion droplets. Different types of salts such as zinc, nickel, and sodium nitrates were tested as lipophobes to retard Ostwald ripening. Transmission electron microscopy images of polyacrylamide/nickel particles synthesized with zinc and nickel salts as lipophobes indicate that nickel nanoparticles are embedded in the polyacrylamide matrix. Magnetization curves show that the saturation magnetization of polyacrylamide/nickel particles is only slightly below that of the pure nickel nanoparticles. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Characterization of Costabilizers in Retarding Ostwald Ripening of Monomer Miniemulsions

The Ostwald ripening behavior of ST or MMA miniemulsions stabilized by various types of costabilizers upon aging at 25°C was investigated. The general validity of the quadratic empirical model with two parameters K1 and K2 was verified and it showed great potential to describe the general features of the Ostwald ripening process involved in miniemulsions stabilized by a relatively wide range of volume fraction of costabilizer in the disperse phase. The greater the K1/RO,1 (or K2/RO,1) value, the more effective is the costabilizer in stabilizing miniemulsion droplets against the diffusional degradation process. RO,1 is the Ostwald ripening rate corresponding to the single component of ST (or MMA). Some useful correlations of the K1/RO,1 (or K2/RO,1) data with the water solubility (or solubility parameter) of the bulk costabilizer were established. Polymeric costabilizers showed quite different behavior in stabilizing miniemulsions compared to low molecular weight costabilizers. Incorporation of hydrophilic functional groups into the costabilizer structures greatly reduced their capabilities to enhance the storage stability of miniemulsion products.

Stabilization of Miniemulsion Droplets by Cerium Oxide Nanoparticles: A Step toward the Elaboration of Armored Composite Latexes

Stable methyl methacrylate (MMA) miniemulsions were successfully prepared using for the first time cerium oxide (CeO(2)) nanoparticles as solid stabilizers in the absence of any molecular surfactant. The interaction between MMA droplets and CeO(2) nanoparticles was induced by the use of methacrylic acid (MAA) as a comonomer. Both MAA and CeO(2) contents played a key role on the diameter and the stability of the droplets formed during the emulsification step. Cryo-transmission electron microscopy (TEM) images of the suspensions formed with 35 wt % of CeO(2) showed the presence of polydisperse 50-150 nm spherical droplets. More surprisingly, some nonspherical (likely discoidal) objects that could be the result of the sonication step were also observed. The subsequent polymerization of these Pickering miniemulsion droplets led to the formation of composite PMMA latex particles armored with CeO(2). In all cases, the conversion was limited to ca. 85%, concomitant with a loss of stability of the latex for CeO(2) contents lower than 35 wt %. This stability issues were likely related to the screening of the cationic charges present on CeO(2) nanoparticles upon polymerization. TEM images showed mostly spherical particles with a diameter ranging from 100 to 400 nm and homogeneously covered with CeO(2). Besides, for particles typically larger than 200 nm, a buckled morphology was observed supporting the presence of residual monomer at the end of the polymerization and consistent with the limited conversion. The versatility of these systems was further demonstrated using 35 wt % of CeO(2) and replacing MMA by n-butyl acrylate (BA) either alone or in combination with MMA. Stable monomer emulsions were always obtained, with the droplet size increasing with the hydrophobicity of the oil phase, pointing out the key influence of the wettability of the solid stabilizer. The polymerization of Pickering miniemulsion stabilized by CeO(2) nanoparticles proved to be an efficient strategy to form armored composite latex particles which may find applications in coating technology.

Biomimetic Silver-Containing Colloids of Poly(2-methacryloyloxyethyl phosphorylcholine) and Their Film-Formation Properties

The synthesis of stable dispersions of hybrid colloids comprising copolymers of biocompatible 2-hydroxyethyl methacrylate (HEMA) and zwitterionic, biomimetic 2-methacryloyloxyethyl phosphorylcholine (MPC) incorporating antibacterial AgBF(4) by inverse miniemulsion is described. The prepared hybrid colloids were designed to provide both antibacterial and antifouling properties for the formation of interesting, multifunctional films. The obtained particles had sizes in the range of 130-160 nm with two different weight ratios of MPC to HEMA (1:10 and 2:5) and AgBF(4) contents between 0% and 15%. The silver salt takes on the role of the lipophobe in stabilizing the miniemulsion droplets against Ostwald ripening and is reduced after polymerization to Ag nanoparticles by gaseous hydrazine. Subsequently, the hybrid particles are transformed into smooth and stable films with thicknesses between 145 and 225 nm by simple drop casting and solvent annealing. The dispersions and films were thoroughly characterized by DLS, TEM, SEM, EDX, TGA, UV-vis spectroscopy, ICP-OES, XRD, AFM, and contact angle measurements. After immersion into water, the films did not show detectable leakage of silver, so they could be employed as dual-functional antifouling and antibacterial coatings.

Preparation and Characterization of Anisotropic Submicron Particles From Semicrystalline Polymers

AbstractThe synthesis of colloidally stable submicron particles of syndiotactic polystyrene (sPS) and isotactic polystyrene (iPS) is reported. Model particles based on poly‐L‐lactic acid (PLLA), atactic polystyrene (aPS), sPS, and iPS are prepared by the evaporation of a solvent present in miniemulsion droplets. The degree of crystallinity of the particles is found to decrease with their size, as shown by DSC and WAXS measurements. Remarkably, nonspherical particles can be formed in the dispersed state with sPS and iPS, whereas PLLA and aPS particles always display spherical morphologies.

CO2 responsive reversible aggregation of nanoparticles and formation of nanocapsules with an aqueous core

We report the use of commercially available long-chain tertiary amines as smart switchable surfactants or hydrophobic oils in miniemulsions. Miniemulsion droplets were stabilized by employing the charged tertiary amine as surfactants to prevent coalescence. PMMA and PS nanoparticles were obtained by polymerizing the monomer droplets. Furthermore, the dispersions could be aggregated by bubbling argon (Ar) at 60 °C or by increasing their pH. In both cases, the nanoparticles could be re-dispersed by charging the dispersions with CO2 at room temperature. The same tertiary amines in their neutral form were used as soft liquid templates in miniemulsion droplets. Core–shell nanoparticles, with the hydrophobic tertiary amine as the core and polyvinylformal (PVF) as the shell, were prepared by the emulsion-solvent evaporation technique. By charging the nanocapsules with CO2, the hydrophobic core was transformed to the water soluble ammonium bicarbonate salt. Thus, aqueous dispersions of polymer nanocapsules with a hydrophilic core could be prepared without the conventional transfer of capsules from oil to aqueous dispersions. This method provides a direct pathway to synthesize polymer nanocapsules mimicking natural compartmentalized systems such as liposomes and cells, with aqueous media present inside and outside the compartments.

Performing Encapsulation of dsDNA and a Polymerase Chain Reaction (PCR) inside Nanocontainers Using the Inverse Miniemulsion Process

We report the encapsulation of dsDNA molecules with a defined number of base pairs (476 bp and 790 bp) and their subsequent amplification by polymerase chain reaction (PCR) inside nanosized polymeric capsules/droplets. In the first set of experiments, the dsDNA template and PCR reagents were encapsulated in crosslinked potato starch using the inverse (water-in-oil) miniemulsion technique. After redispersion of the capsules in a water-surfactant mixture, PCR was performed inside the crosslinked starch nanocapsules. In the second set of experiments, the PCR was performed inside the aqueous nanodroplets before capsule formation, and then each miniemulsion droplet was covered with a polybutylcyanoacrylate (PBCA) shell which was formed through anionic polymerization directly at the droplet interface. The PCR efficiency was quantitatively evaluated by fluorescence spectroscopy, using a DNA-specific dye called SYBR® Green which intercalates between the base pairs of the dsDNA.

Towards copper-free nanocapsules obtained by orthogonal interfacial “click” polymerization in miniemulsion

A facile method to produce nanocapsules by copper-free interfacial "click"-polymerization as orthogonal reaction for the encapsulation of functional molecules is successfully performed using stable miniemulsion droplets. Difunctional azides and alkynes have been used for polymerization around the miniemulsion droplets, leading to the formation of nanocapsules. The results were compared with copper-catalyzed systems.

Heck Reactions in Aqueous Miniemulsions

Carrying out organic reactions in water-based nanoreactors represents a ‘green’ method for the preparation of organic compounds. This process eliminates the need for solvents, thus reducing the effect of high volumes of solvent on the environment. In this work, we demonstrate a successful Heck cross-coupling reaction, one of the most used approaches to form C–C bonds using a palladium catalyst, in a miniemulsion. The miniemulsion droplet sizes were small (25 to 42 nm), and the reactions resulted in high conversions of three different products with high trans stereoisomers.

Design and characterization of functionalized silica nanocontainers for self-healing materials

Silica nanocapsules functionalized with thiol or amine groups with sizes between 100 and 400 nm could be successfully synthesized using the interface of direct miniemulsion droplets for the hydrolysis and condensation of silicon alkoxides. We show that healing agents such as monomers and catalysts can be successfully encapsulated in the silica shell. The catalyst was encapsulated as a solution to allow a better mobility when released for a self-healing reaction. The functional groups were quantified by 29Si MAS-NMR, XPS, and chemical titration. Therefore a precise picture for the gradient of concentration of functional groups inside the shell could be given. The encapsulated reagents were found to be still active and the self-healing reaction could be successfully monitored by solid-state NMR spectroscopy and thermogravimetry.

Radical Photopolymerization in Miniemulsions. Fundamental Investigations and Technical Development

A photochemical means to convert acrylate monomer emulsions into polymer nanolatexes has been explored using radical miniemulsion photopolymerization. Our aim is to offer a complete overview through a stepwise mechanistic investigation, addressing first the key aspect of the electronic excitation of the photoinitiator in monomer miniemulsions and ending with the implementation of a photochemical reactor. In a first step, the photon flux absorbed by different photochemical initiators was investigated as a function of the miniemulsion droplet size. A series of miniemulsion polymerizations were also performed and the effects of the primary experimental parameters on the reactions kinetics, molar masses, and colloidal properties were assessed. As expected, droplet size, incident photon flux, and the nature of the photochemical initiator were found to be the primary reaction parameters. Finally, miniemulsion polymerizations were performed in an annular photochemical reactor in batch and semibatch processes.

Experimental Validation of Intermediate Termination in RAFT Polymerization with Dithiobenzoate via Comparison of Miniemulsion and Bulk Polymerization Rates

AbstractTo understand if either of two controversial models for the retardation by RAFT agents is applicable, styrene polymerization using dithiobenzoate as the RAFT agent is carried out in both bulk and miniemulsion systems with the same rates of radical generation and the same RAFT agent concentrations. Miniemulsion polymerization with average diameters of the miniemulsion droplets of ≈107 nm is by far faster than in bulk, and the obtained rate of polymerization agrees well with the calculated results assuming a bimolecular termination between propagating radical and intermediate radical, generated by the addition reaction of propagating radical to the RAFT agent, which shows that the intermediate termination is the major reason for rate retardation by the RAFT agent.magnified image

Precipitation of metal oxide nanoparticles using a miniemulsion technique

A method to precipitate nanoparticles using a miniemulsion technique is described, in which miniemulsion droplets between 100 and 1000 nm in size serve as nanoreactors enabling both the control of particle formation and particle growth. The application of miniemulsion droplets to synthesise nanoparticles comprises three advantages: first, the size of the precipitated particles is limited by the reactant concentration within the emulsion droplet; second, particle agglomeration is prevented as nanoparticle collision outside the nanoreactor is avoided; and third, easy technical scale up can be realized by increasing emulsion volume and thus the number of nanoreactors, while local conditions within the reactors are not changed. The miniemulsion technique is an easy scalable process which allows defined synthesis of particles by precipitation reactions. The miniemulsion technique involves first the preparation of a stable water-in-oil miniemulsion by high pressure homogenisation. Whereas a water soluble reactant is provided within the aqueous droplets, another oil- as well as water-soluble reactant can be introduced to the emulsion after homogenisation. The precipitation reaction is induced by the diffusion of the second reactant into the emulsion droplet. Together with this contribution, a method is described and discussed which uses a high pressure homogenisation process to produce stable water-in-oil miniemulsions serving as a reaction medium to precipitate metal oxides.

DNA Amplification via Polymerase Chain Reaction Inside Miniemulsion Droplets with Subsequent Poly(n‐butylcyanoacrylate) Shell Formation and Delivery of Polymeric Capsules into Mammalian Cells

There is a growing interest in the development of stable nanocapsules that could deliver the bioactive compounds within the living organism, and to release them without causing any toxic effects. Here the miniemulsion droplets were first used as "nanoreactors" for the amplification of single-molecule dsDNA template (476 and 790 base pairs) through PCR. Afterwards, each droplet was surrounded with a biodegradable PBCA shell by interfacial anionic polymerization, enabling therefore to deliver the PCR products into the cells. The size of the initial miniemulsion droplets and the final polymeric capsules was in the range of 250 and 320 nm, mainly depending on the type of the continuous phase and presence of dsDNA template molecules. The formation of PCR products was resolved with gel electrophoresis and detected with fluorescence spectroscopy in the presence of DNA specific dye (SYBRGreen). TEM studies were performed to prove the formation of the polymeric shell. The shell thickness was measured to be within 5-15 nm and the average molecular weight of the formed PBCA polymer was around 75000 g · mol(-1) . For the cell uptake experiments, the obtained nanocapsules were transferred from the organic phase into aqueous medium containing a water-soluble surfactant. The effect of the surfactant type (anionic, cationic or non-ionic) on the HeLa cell viability and nanocapsule uptake behavior was studied by CLSM and FACS. Confocal analysis demonstrated that nanocapsules stabilized with cationic (CTMA-Cl) and non-ionic (Lutensol AT50) surfactants show almost the same uptake, whereas capsules redispersed in anionic (SDS) surfactant possess a 30% higher uptake. The release of the encapsulated material within the cell was studied on the example of Cy5-labeled oligonucleotides showing the colocalization with mitochondria of MSCs cells.

Mesoporous CeO2 nanoparticles synthesized by an inverse miniemulsion technique and their catalyticproperties in methane oxidation

Cerium(IV) oxide nanoparticles were synthesized using an inverse miniemulsiontechnique with cerium nitrate hexahydrate as precursor. The resultingnanocrystallites are as small as 5 nm with a specific surface area of158 m2 g − 1 aftercalcination at 400 °C. With the addition of cetyltrimethylammonium bromide (CTAB) or (poly(ethylene oxide)-b-poly(propyleneoxide)-b-poly(ethylene oxide)) triblock copolymers (PEO–PPO–PEO) as template inthe miniemulsion droplets, the specific surface area can be increased up to255 m2 g − 1. The miniemulsions were characterized by dynamic light scattering (DLS) and theobtained oxides were examined by x-ray diffraction (XRD), nitrogen sorption (BET andBJH), and transmission electron microscopy (TEM). The catalytic activity of the resultingceria was investigated for the temperature-programmed oxidation (TPO) of methane.

Phase behavior of binary mixtures of block copolymers and a non-solvent in miniemulsion droplets as single and double nanoconfinement

Nanoparticles consisting of different molecular weight poly(styrene-block-methyl methacrylate) (P(S-b-MMA)) copolymers and nanocapsules consisting of the same copolymers, but additionally with hexadecane as liquid core material were prepared by the miniemulsion process. The dependence of the morphology of block copolymer assemblies on the nanoconfinement was investigated. We introduced two nanoconfinement parameters, that are the nanoparticle diameter D and the shell thickness d; D was controlled by varying the concentration of surfactant in the miniemulsion, while d was controlled by the ratio hexadecane/copolymer. As the diameter D of the high molecular weight (Mw ∼ 203,700 g mol−1) P(S-b-MMA) nanoparticles increased, first Janus-particles (at D 1800 nm) were obtained. Nanocapsules with 0 < d < D also showed an onion-like structure. In both cases the outmost layer was PMMA as identified by XPS and the lamellar thickness was in agreement with theoretical considerations. Nanoparticles and nanocapsules prepared with a low-molecular weight (Mw ∼ 19,500 g mol−1) P(S-b-MMA) displayed patchy structures. This is the first time that the morphology of block copolymers was studied under double nanoconfinement in colloids.

Dual lanthanide role in the designed synthesis of hollow metal coordination (Prussian Blue analogue) nanocages with large internal cavity and mesoporous cage

Prussian Blue (PB) analogue metal coordination nanocages comprised of mesoporous walls (ca. 3.5 nm pore width) encapsulating a cavity approaching ca. 100 nm in diameter (surfactant free) are presented as an advance in rational metal coordination polymer nanostructure design. The synthesis employs lanthanide ions (Gd(3+) or Er(3+)) which function initially as peripheral coordination crosslinkers of metallo-surfactant templated miniemulsion droplets, and, subsequently, as promoters in the removal of the organic component of those surfactants via metal-assisted ester hydrolysis. The success of this synthetic strategy relies entirely on the periphery coordination event occurring prior to the ester hydrolysis surfactant removal step. Crucially, this one-pot sequential synthesis was achieved using a newly developed metallo-surfactant designed to have a reduced ester hydrolysis rate. Syntheses of this innovative metallo-surfactant, intermediary PB analogue coordination polymer organo-nanoshells and the subsequent conversion to hollow metal coordination nanocages are fully characterised using a wide variety of techniques, including TEM, SEM, EFTEM, EDX, TGA, WAXD, NMR, N(2) adsorption, etc., and represent the first designed synthesis of hollow metal coordination nanocages containing a large nanoscale cavity (wall of hollow nanosphere is mesoporous; hence nanocage).

Polyelectrolyte nanocages via crystallized miniemulsion droplets

Polyelectrolyte nanocages were synthesized by interfacial cross-linking of monolayers of vinyl-functionalized surfactant molecules adsorbed by crystallized miniemulsion droplets. The monolayer-thick shell of these nanocages was confirmed by AFM analysis.

Monte Carlo simulations of a single polystyrene chain in spherical confinement

We report on Monte Carlo simulations of a single coarse-grained polystyrene chain in spherical confinement. To this end we employ a variant of the freely rotating chain model, the parameters of which are chosen to mimic polystyrene in good solvent conditions. Entanglements are analyzed as a function of molecular weight and capsid radius to provide an educated guess about the structure of a single polystyrene chain in a miniemulsion droplet. We also show that significant knotting occurs first when the radius of the confining sphere falls below the chainʼs radius of gyration.

Miniemulsion polymerization as a versatile tool for the synthesis of functionalized polymers

The miniemulsion technique is a particular case in the family of heterophase polymerizations, which allows the formation of functionalized polymers by polymerization or modification of polymers in stable nanodroplets. We present here an overview of the different polymer syntheses within the miniemulsion droplets as reported in the literature, and of the current trends in the field.