Monomer Droplets Research Articles

The influence of cavitation radicals on the suspension polymerization of n‐butyl methacrylate during ultrasonic emulsification

AbstractThe influence of cavitation‐generated free radicals on the suspension polymerization of n‐butyl methacrylate under ultrasonic irradiation was investigated experimentally and by kinetic modeling. A kinetic model of radical polymerization was proposed, considering the interaction of free radicals generated in cavitation bubbles with both monomer molecules and free radicals formed within the monomer droplets. It was found that the primary contribution to the polymerization acceleration come from cavitation radicals generated inside the monomer droplets, whereas the impact of cavitation radicals formed in the dispersion medium decreases as the fraction of monomer in the mixture decreases. Increasing the concentration of cavitation radicals was shown to lead to a reduction in the average chain length and the dispersity of terminated chains. It was demonstrated that the computational results are in good agreement with experimental data.Highlights Kinetic model for suspension polymerization in ultrasonic field is proposed. Impact of cavitation radicals on suspension polymerization kinetics is studied. Chain length and dispersity decrease with increasing amount of cavitation radicals.

Hydrogel-Embedded Polydimethylsiloxane Contact Lens for Ocular Drug Delivery.

Topical administration is the commonly preferred method of administering ophthalmic formulations, with the majority of available medications in the form of eye drops or ointments. However, the topical application of ophthalmological medications has less bioavailability and a short residence time because of the physiological and anatomical constraints of the eye, making efficient ophthalmic drug delivery a challenging task. Microfluidic contact lenses have the advantage of delivering drugs into the eye in a controlled and on-demand manner. Here, we showcase the use of hydrogel-embedded microcavities on PDMS-based contact lenses for ocular drug delivery applications. The fabrication technique adopted here is the spontaneous formation of the spherical cavity by hydrogel monomer droplet, followed by the simultaneous thermal curing of hydrogel and PDMS, creating a spherical cavity as small as 150 μm. The spherical cavity is embedded with pH-responsive hydrogel for on-demand drug delivery. The drug loaded in the hydrogel matrix is released into the ocular environment by diffusion. The spherical cavity with a narrow opening restricts the diffusion to a minimum under normal ocular pH conditions(pH > 6). When the ocular pH reduces (pH < 6), the pH-responsive hydrogel inside the spherical cavity deswell and accelerates the drug release.

Controlling Morphology of Polymer Particles Synthesized from Condensed Monomer Droplets

Controlling Morphology of Polymer Particles Synthesized from Condensed Monomer Droplets

Polymer blend particles with dual-phase morphology prepared using a novel one-pot method via a phase inversion emulsion procedure

We develop a new method for preparing polymer blend particles with dual-phase morphology based on a phase inversion emulsion process. Using this method, the polystyrene (PS)/ethylene–vinyl acetate (EVA) copolymer particles were prepared and analyzed. The preparation procedure included a emulsification of liquid styrene monomer with dissolved EVA and a following free radical polymerization to convert the monomer droplets into solid blend particles.The obtained PS/EVA particles had a dual-phase morphology. With increasing EVA content, the structure of EVA dispersed in PS matrix switched from spherical, stripe to nearly co-continuous morphology and the particles become significantly tougher and the maximum impact strength reached 9.13 KJ/m2. This method provides the opportunity to prepare a number of new kinds of polymer blend particles with tailored performance even though the composed polymers are condensation polymers or unreachable at atmospheric pressure.

Batch-Operated Condensed Droplet Polymerization to Understand the Effect of Temperature on the Size Distribution of Polymer Nanodomes

Size-controlled polymer nanodomes (PNDs) benefit a broad cross-section of existing and emerging technologies. Condensed droplet polymerization (CDP) is a vacuum-based synthesis technology that produces PNDs from monomer precursors in a single step. However, the effect of synthesis and processing conditions on the PND size distribution remains elusive. Towards size distribution control, we report the effect of substrate temperature, on which monomer droplets condense, on the size distribution of PNDs. We take a reductionist approach and operate the CDP under batch mode to match the conditions commonly used in condensation research. Notably, despite the rich knowledge base in dropwise condensation, the behavior of nonpolar liquids like a common monomer, i.e., 2-hydroxyethyl methacrylate (HEMA), is not well understood. We bridge that gap by demonstrating that dropwise condensation of HEMA follows a two-stage growth process. Early-stage growth is dominated by drop nucleation and growth, giving rise to relatively uniform sizes with a lognormal distribution, whereas late-stage growth is dominated by the combined effect of drop coalescence and renucleation, leading to a bimodal size distribution. This new framework for understanding the PND size distribution enables an unprecedented population of PNDs. Their controlled size distribution has the potential to enable programmable properties for emergent materials.

Encapsulation of Cadmium-Free InP/ZnSe/ZnS Quantum Dots in Poly(LMA-co-EGDMA) Microparticles via Co-flow Droplet Microfluidics.

Quantum dots (QDs) are semiconductor nanocrystals that are used in optoelectronic applications. Most modern QDs are based on toxic metals, for example Cd, and do not comply with the European Restriction of Hazardous Substances regulation of the European Union. Latest promising developments focus on safer QD alternatives based on elements from the III-V group. However, the InP-based QDs lack an overall photostability under environmental influences. One design path of achieving stability is through encapsulation in cross-linked polymer matrices with the possibility to covalently link the matrix to surface ligands of modified core-shell QDs. The work focuses on the formation of polymer microbeads suitable for InP-based QD encapsulation, allowing for an individual protection of QDs and an improved processibility via this particle-based approach. For this, a microfluidic based method in the co-flow regime is used that consists of an oil-in-water droplet system in a glass capillary environment. The generated monomer droplets are polymerized in-flow into poly(LMA-co-EGDMA) microparticles with embedded InP/ZnSe/ZnS QDs using a UV initiation. They demonstrate how a successful polymer microparticle formation via droplet microfluidics produces optimized matrix structures leading to a distinct photostability improvement of InP-based QDs compared to nonprotected QDs.

Free-radical polymerization in a droplet with initiation at the interface

The free-radical polymerization inside monomer droplets surrounded by an immiscible liquid of the initiator solution is studied before the gel effect by means of the generalized method of moments taking into account diffusion of radicals and polymer chains. The influence of the droplet radius and initiator concentration in the surrounding phase on the polymerization kinetics and molecular mass characteristics of the synthesized polymer is analyzed. It is found that the polymerization kinetics matches the bulk polymerization when the droplet is small enough (kinetic regime). However, increasing the droplet radius leads to a diffusion-kinetic regime, in which the polymerization rate becomes significantly lower. As a result, the average length and the dispersity of the produced polymer chains depend significantly on the monomer droplet size. The dependence of the effective polymerization rate coefficient on the droplet radius and the initiator concentration is discussed.

Preparation and Functionalization of Cellulose Nanofibers/Polymer Composite Particles Using Various Kinds of Core Polymers

In the present study, we investigated the development of dry powders comprising polymer particles covered with cellulose nanofibers （CNFs）. CNFs with uniform widths can prepared from wood cellulose by TEMPO （2,2,6,6-tetramethylpiperidine-1-oxyl）-mediated oxidation, which introduces carboxy groups onto the surface of the cellulose. Divinylbenzene （DVB） monomer droplets are stabilized by CNFs in an aqueous emulsion system, and it is possible to obtain poly-DVB （pDVB） microparticles that are densely covered with CNFs by polymerizing the DVB in such a system. Herein, we developed an approach to fabricating CNF-shelled composite microparticles with cores comprising various types of polymers, and investigated the properties of their carriers.We found that it was easier to stabilize the monomer droplets in an emulsion if the CNFs were modified with quaternary ammonium （QA） cations. This strategy enabled us to produce CNF-shelled composite microparticles with various types of core polymers. Moreover, the mechanical properties of these composite particles were superior to those of polymer microparticles without a CNF shell.The electronic interaction with the surface carboxy groups of the CNF shell endowed the composite particles with the pH-sensitive ability to adsorb/desorb various cationic molecules, such as antibacterial agents, metal ions, polymers, and fluorescent or redox dyes. Interestingly, when fluorescent dyes were adsorbed onto the composite particles, the emission wavelength was shorter than in the solid state, which suggests that interaction with the carboxy groups dissociated on the surfaces of the particles prevented the aggregation of the fluorescent dye.The functionalized cellulose nanofibers/polymer composite particles produced by the process have desirable mechanical and carrier properties, and are therefore potentially very useful for industrial applications and in the security field.

Aqueous RAFT polymerization-induced self-assembly (PISA): amphiphilic macroRAFT self-assembly vs. monomer droplet nucleation (miniemulsion polymerization)

The effect of monomer droplet size on the particle formation mechanism in aqueous heterogeneous polymerization using a hydrophilic macroRAFT agent has been investigated.

Osmotic Pressure as Driving Force for Reducing the Size of Nanoparticles in Emulsions.

We describe here a method to decrease particle size of nanoparticles synthesized by miniemulsion polymerization. Small nanoparticles or nanocapsules were obtained by generating an osmotic pressure to induce the diffusion of monomer molecules from the dispersed phase of a miniemulsion before polymerization to an upper oil layer. The size reduction is dependent on the difference in concentration of monomer in the dispersed phase and in the upper oil layer and on the solubility of the monomer in water. By labeling the emulsion droplets with a copolymer of stearyl methacrylate and a polymerizable dye, we demonstrated that the migration of the monomer to the upper hexadecane layer relied on molecular diffusion rather than diffusion of monomer droplets to the oil layer. Moreover, surface tension measurements confirmed that the emulsions were still in the miniemulsion regime and not in the microemulsion regime. The particle size can be tuned by controlling the duration during which the miniemulsion stayed in contact with the hexadecane layer, the interfacial area between the miniemulsion and the hexadecane layer and by the concentration of surfactant. Our method was applied to reduce the size of polystyrene and poly(methyl methacrylate) nanoparticles, nanocapsules of a copolymer of styrene and methyl methacrylic acid, and silica nanocapsules. This work demonstrated that a successful reduction of nanoparticle size in the miniemulsion process can be achieved without using excess amounts of surfactant. The method relies on building osmotic pressure in oil droplets dispersed in water which acts as semipermeable membrane.

Carbon Quantum Dot-Catalyzed, Highly Efficient Miniemulsion Atom Transfer Radical Polymerization Induced by Visible Light.

Owing to the benefits of using natural or artificial light sources as a stimulus, photoinduced reversible-deactivation radical polymerization (photoRDRP) techniques have been recognized to be a powerful "green" platform for the preparation of well-defined polymers. However, the development of highly efficient visible light-induced photoRDRP processes in aqueous dispersed media remains a challenge due to light scattering and refraction by monomer droplets or colloidal particles. In this work, an efficient green photocatalyst, carbon quantum dots (CQDs), was introduced to visible light-mediated miniemulsion atom transfer radical polymerization (ATRP), leading to highly efficient polymerizations with reaction rates (>80% monomer conversion within 1 h) much higher than in previous studies. This heterogeneous photocatalytic system is presumed to involve three catalytic cycles in (i) the aqueous phase, (ii) the oil-water interface, and (iii) the monomer droplets. The effect of different polymerization parameters on the polymerization reaction was investigated, including the amounts of surfactant and CQDs, CuBr2 dosage, and solid content. Excellent temporal control of the polymerization was illustrated by "ON/OFF" polymerizations, and natural sunlight was also used as an energy source. This novel CQDs-catalyzed miniemulsion photoATRP process may be easily extended to other aqueous dispersion RDRP systems. As an extension of our previous work (J. Am. Chem. Soc. 2022, 144 (22), 9817-9826) we also developed a "one-pot" method for the rapid preparation of heterogeneous hydrogels.

Chemical vapor deposition of transparent superhydrophobic anti-Icing coatings with tailored polymer nanoarray architecture

Transparent superhydrophobic coatings are highly desirable in outdoor optoelectronic devices for their anti-wetting, self-cleaning, and anti-icing abilities. Superhydrophobicity requires surface microstructure that accommodates trapped air in between the liquid–solid interface, which would, however, compromise coating transparency. This trade-off can be balanced by creating tailored microtopography on low surface energy coating, which is usually achieved by rather complex multi-step procedures. Herein, we demonstrate a facile one-step method that creates transparent superhydrophobic coatings with tailored nanocone array structures via initiated chemical vapor deposition (iCVD). This novel iCVD process employs condensed nanosize monomer droplets as nucleation centers and subsequently grows vertically aligned polymer nanocones. The height and density of the nanocones can be well controlled by adjusting the deposition conditions. The resultant polymer nanocone array coatings (NC) exhibit better water repellency and anti-icing performance as well as higher light transmittance than iCVD rough coatings (RC) with irregular surface microstructure. The icing delay time of coatings with optimized nanocone array structure reached a remarkable 540 s, which is 38.5 and 2.2 times that of pristine and RC-coated glass. Further tests reveal good self-cleaning, excellent durability and stability of the NC coating, indicating potential for outdoor optoelectronic device protection.

Gamma irradiation-induced pH-responsive poly(methyl methacrylate- acrylic acid-divinyl benzene) hybrid polymer particles for dye treatment

ABSTRACT The environmentally friendly method is used to fabricate spherical poly(methyl methacrylate-methyl acrylate-divinylbenzene) (P(MMA-MA-DVB)) hybrid particles by gamma irradiation-induced suspension polymerization without an initiator. The MMA: MA: DVB of 60: 20: 20 was used to encapsulate 10 wt% oleic coated-magnetic nanoparticles using ultrasonication for monomer droplet preparation. A stable suspension with approximately 90% monomer conversion was produced using 5 kGy gamma-ray to initiate the polymerization. A large quantity of submicrometer-sized free polymer particles as a by-product was formed in an aqueous medium where the radicals were generated by gamma irradiation of water. However, the number of free polymer particles decreased with the decrease in monomer droplet size. The obtained particles were hydrolyzed to form P(MMA-AA-DVB) particles presenting a negative surface charge (> −30 mV) in an alkaline condition. It was found that the prepared particles presented dye adsorption efficiency as the methylene blue (MB) concentration reduced with adsorption time. The MB adsorption efficiency also increased with pH due to the presence of PAA as a pH-responsive material. Moreover, the hybrid polymer particles were easily separated by the application of an external magnet. Therefore, the fabricated pH-responsive hybrid polymer particles would be served as an alternative dye adsorbent for wastewater treatment.

Four-Level Structural Hierarchy: Microfluidically Supported Synthesis of Polymer Particle Architectures Incorporating Fluorescence-Labeled Components and Metal Nanoparticles.

Hierarchical assemblies of functional polymer particles are promising due to their surface as well as physicochemical properties. However, hierarchical composites are complex and challenging to form due to the many steps necessary for integrating different components into one system. Highly structured four-level composite particles were formed in a four-step process. First of all, gold (Au) nanoparticles, poly(methyl methacrylate) (PMMA) nanoparticles, and poly(tripropylene glycol diacrylate) (poly-TPGDA) microparticles were individually synthesized. By applying microfluidic techniques, polymer nano- and microparticles were formed with tunable size and surface properties. Afterwards, the negatively charged gold nanoparticles and PMMA particles functionalized with a positively charged surface were mixed to form Au/PMMA assemblies. The Au/PMMA composites were mixed and incubated with poly-TPGDA microparticles to form ternary Au/PMMA/poly-TPGDA assemblies. For the formation of composite-containing microparticles, Au/PMMA/poly-TPGDA composites were dispersed in an aqueous acrylamide-methylenebisacrylamide solution. Monomer droplets were formed in a co-flow microfluidic device and photopolymerized by UV light. In this way, hierarchically structured four-level composites consisting of four different size ranges─0.025/0.8/30/1000 μm─were obtained. By functionalizing polymer nano- and microparticles with different fluorescent dyes, it was possible to visualize the same composite particle under two different excitation modes (λex = 395-440 and λex = 510-560 nm). The Au/PMMA/poly-TPGDA composite-embedded polyacrylamide microparticles can be potentially used as a model for the creation of composite particles for sensing, catalysis, multilabeling, and biomedical applications.

Biosourced Polymeric Emulsifiers for Miniemulsion Copolymerization of Myrcene and Styrene: Toward Biobased Waterborne Latex as Pickering Emulsion Stabilizer.

Biobased waterborne latexes were synthesized by miniemulsion radical copolymerization of a biosourced β-myrcene (My) terpenic monomer and styrene (S). Biobased amphiphilic copolymers were designed to act as stabilizers of the initial monomer droplets and the polymer colloids dispersed in the water phase. Two types of hydrophilic polymer backbones were hydrophobically modified by terpene molecules to synthesize two series of amphiphilic copolymers with various degrees of substitution. The first series consists of poly(acrylic acid) modified with tetrahydrogeraniol moieties (PAA-g-THG) and the second series is based on the polysaccharide carboxymethylpullulan amino-functionalized with dihydromyrcenol moieties (CMP-g-(NH-DHM)). The produced waterborne latexes with diameters between 160 and 300 nm and were composed of polymers with varying glass transition temperatures (Tg,PMy = -60 °C, Tg,P(My-co-S) = -14 °C, Tg,PS = 105 °C) depending on the molar fraction of biobased β-myrcene (fMy,0 = 0, 0.43, or 1). The latexes successfully stabilized dodecane-in-water and water-in-dodecane emulsions for months at all compositions. The waterborne latexes composed of low Tg poly(β-myrcene) caused interesting different behavior during drying of the emulsions compared to polystyrene latexes.

Monomer Transport in Emulsion Polymerization III Terpolymerization and Starved‐Feed Polymerization

AbstractA simple mathematical model of the transport of monomer from the monomer droplets to the polymerizing polymer particles during batch emulsion homopolymerization has been developed and published previously. A Damkohler number for monomer transport in emulsion polymerization is proposed as the ratio of the maximum rate of polymerization divided by the maximum rate of monomer transport out of the monomer droplets. This Damkohler number is calculated from literature values for a number of common monomers. Following standard practice for the use of such a Damkohler number in other reaction‐with‐transport systems, monomers with a Damkohler number above 0.1 are considered to be transport limited. In a third paper the analysis is extended to batch binary copolymerization. Here the analysis is extended to the more industrially relevant cases of batch terpolymerization, starved‐feed copolymerization, and the combination of the two (starved‐feed terpolymerization).

Ouzo polymerization: A bottom-up green synthesis of polymer nanoparticles by free-radical polymerization of monomers spontaneously nucleated by the Ouzo effect; Application to molecular imprinting

HypothesisTop-down fabrication of polymer nanoparticles from preformed polymers by spontaneous colloid formation under influence of the Ouzo effect is a widely applied concept whereas bottom-up free-radical polymerization of monomers nucleated under the Ouzo regime have found limited application after a seminal report almost half a century ago. We hypothesized that the approach would be of value today as a versatile method for green synthesis of polymer nanoparticles, including molecularly imprinted ones. ExperimentsTernary mixtures of pentaerythritol triacrylate, ethanol, and water were prepared by either a one-pot-one-step batch-wise procedure or a continuous-flow process using a 3D-printed micro-fluidic mixer. The mixtures were subjected to free-radical polymerization. Cortisol was added as a template to generate a molecular memory through molecular imprinting. Characterization of the fabricated nanoparticles was carried out by dynamic light scattering, electron microscopy, and binding studies. FindingsCompositions in the Ouzo region of the ternary phase diagram provided spontaneous and instantaneous formation of nucleated monomer droplets. Free-radical polymerization, promoted by heat or UV light, transformed the droplets into polymer nanoparticles in a green and sustainable route. Addition of cortisol created polymer nanoparticles with high affinity for cortisol in aqueous media. Competition studies showed some cross-reaction with other steroids, comparable to that found with antibodies, and complete discrimination of structurally unrelated drug molecules.

Preparation and application of cross-linked PVA microspheres with narrow particle size distribution by suspension polymerization using uniform porous tube

PVA microspheres have high strength and plenty of hydroxyl groups, which has obvious advantages in a hydrophilic environment. However, the preparation of uniform larger size PVA microspheres with a diameter bigger than 100 μm is still extremely tough. Membrane emulsification is an excellent method to obtain a monodisperse microsphere, but the diameter is generally several microns to tens of microns. Inspired by the membrane emulsification method, in order to get over 100 μm microspheres, on titanium tube uniform micropores were produced by laser drilling. Uniform monomer droplets with vinyl acetate and cross-link agent 1,4-butanediol divinyl ether (BDVE) were all the time obtained by a simple device applying the tube as a membrane. After polymerization, cross-linked PVAc microspheres underwent alcoholysis. Cross-linked PVA microspheres ultimately were obtained. PVA microspheres' sizes varied from 120 μm to 800 μm with a maximum coefficient of variation of less than 13% were prepared by changing different pore size tubes. Cross-linked PVA microspheres have a good spheroidal shape, solvent resistance and good strength. Through the amination reaction, some hydroxyl groups were connected with amino groups, which was more conducive to the loading of drug molecules such as indomethacin.

Effect of monomer hydrophilicity on ARGET–ATRP kinetics in aqueous mini‐emulsion polymerization

AbstractActivators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP)‐based aqueous miniemulsion polymerization where the polymerization takes place in the stabilized monomer droplets is described. In this work, we compared styrene, n‐butyl methacrylate (nBMA) and tert‐butyl methacrylate (tBMA) and investigated the influence of their hydrophobicity on dispersity, molecular weight and particle stability based their partition coefficients (logP) (2.67, 2.23, and 1.86, respectively). Tetrabutylammonium bromide (TBAB) was used as a phase transfer agent for the controlled delivery of Cu2+‐Br/tris(2‐pyridylmethyl)amine (TPMA), a hydrophilic catalyst, into monomer droplets of varying hydrophobicity. The resulting dispersity and particle stability of each polymer is a function of its logP value, with the most hydrophobic monomer (styrene) displaying the narrowest dispersity and most control (Đ &lt; 1.3), and the most hydrophilic polymer poly(tert‐butyl methacrylate) (PtBMA) having reduced emulsion stability, determined by the observation of aggregate formation. Selected polymerization parameters, including effects of total ascorbic acid feed concentration and the monomer concentration and their effects on dispersity are reported. The controlled polymerizations of hydrophilic monomers using ARGET‐ATRP in miniemulsion conditions and understanding the effect of monomer hydrophilicity on the emulsion stability will broaden the use of ARGET‐ATRP in emulsion polymerization for the synthesis of polymer‐grafted nanoparticles with hydrophilic corona.

Shape-Controlled Nanoparticles from a Low-Energy Nanoemulsion.

Nanoemulsion technology enables the production of uniform nanoparticles for a wide range of applications. However, existing nanoemulsion strategies are limited to the production of spherical nanoparticles. Here, we describe a low-energy nanoemulsion method to produce nanoparticles with various morphologies. By selecting a macro-RAFT agent (poly(di(ethylene glycol) ethyl ether methacrylate-co-N-(2-hydroxypropyl) methacrylamide) (P(DEGMA-co-HPMA))) that dramatically lowers the interfacial tension between monomer droplets and water, we can easily produce nanoemulsions at room temperature by manual shaking for a few seconds. With the addition of a common ionic surfactant (SDS), these nanoscale droplets are robustly stabilized at both the formation and elevated temperatures. Upon polymerization, we produce well-defined block copolymers forming nanoparticles with a wide range of controlled morphologies, including spheres, worm balls, worms, and vesicles. Our nanoemulsion polymerization is robust and well-controlled even without stirring or external deoxygenation. This method significantly expands the toolbox and availability of nanoemulsions and their tailor-made polymeric nanomaterials.