Monomer Droplets Research Articles

Preparation of polymeric/inorganic nanocomposite particles in miniemulsions: II. Narrowly size-distributed polymer/SiO2 nanocomposite particles

Narrowly size-distributed poly(styrene-co-methyl methacrylate) (poly(St-co-MMA))/SiO2 nanocomposite particles (NCPs) were efficiently prepared through miniemulsion polymerization by using a special use strategy of surfactants. A trace amount of sodium dodecyl sulfate (SDS) was post-added to monomer miniemulsions stabilized with polyoxyethylene (20) sorbitan monolaurate (Tween-20). The number fraction of the poly(St-co-MMA)/SiO2 NCPs reached 97% in the as-synthesized emulsion stabilized with 0.15g of Tween-20 and 2.5mg of the post-added SDS. The highly efficient formation of poly(St-co-MMA)/SiO2 NCPs was realized through controlled mergence between the monomer droplets and latex particles during polymerization. The influence of the amount of post-added SDS, surface hydrophobic modification degree of SiO2 nanoparticles, and SiO2 content on the particle properties of latex particles was systematically investigated. The amount of post-added SDS and surface hydrophobic modification degree of SiO2 nanoparticles are two determining factors for the efficiency of polymer/SiO2 NCP formation.

Efficient preparation and formation mechanism of polymer/SiO2 nanocomposite particles in miniemulsions

Poly(styrene-co-methyl methacrylate) (poly(St-co-MMA))/SiO2 nanocomposite particles (NCPs) were prepared through miniemulsion polymerization by using polyoxyethylene (20) sorbitan monolaurate (commercial name: Tween-20) as the sole surfactant. The number fraction of poly(St-co-MMA)/SiO2 NCPs in the as-synthesized emulsion could exceed 90% in some well-designed systems, indicative of high efficiency of poly(St-co-MMA)/SiO2 NCP formation. Each poly(St-co-MMA)/SiO2 NCP contained many SiO2 NPs, thus exhibiting a high composite efficiency between the poly(St-co-MMA) and SiO2 NPs. The mechanistic investigation indicated that the enhanced efficiency of the poly(St-co-MMA)/SiO2 NCP formation is due to the improved homogeneity in the distribution of SiO2 among the monomer droplets and the effectively suppressed nucleation of plain monomer droplets through controlled coalescence of the monomer droplets with the latex particles.

Online Monitoring of a Styrene Monomer and a Dimer in an Emulsion via Laser Ionization Time-of-Flight Mass Spectrometry.

Laser ionization time-of-flight mass spectrometry was applied to the online monitoring of a styrene monomer and dimer in an emulsion. During the measurement of a styrene monomer oil-in-water emulsion for this study, a styrene dimer, 1,3-diphenylpropane, was dropped into the emulsion. As a result, signal spikes from both analytes occurred simultaneously, which suggested that either the dimer had moved to the monomer droplets or that the monomer and dimer droplets had aggregated. We concluded that this method could be useful for the direct monitoring of monomers and oligomers in the early stages of emulsion polymerization.

Towards the production of high performance lauryl methacrylate based polymers through emulsion polymerization

Water-borne polymeric dispersions of superhydrophobic monomers can be catalogued as high performance materials, particularly towards applications involving excellent water-barrier properties. Their industrial production however is challenging due to their low water solubility. This work illustrates the polymerization of superhydrophobic monomers lauryl methacrylate and isobornyl acrylate in aqueous media, while avoiding the use of energy intensive procedures and keeping particle sizes and surfactant concentrations relatively low, by implementing a novel emulsion polymerization technique. The success is attributed to the presence of an emulsifier with very low CMC and its ability to stabilize nano-sized monomer droplets in water. The polymerization of superhydrophobic monomer is explained by the transport of monomer to the polymer particles through a collision-based monomer transport mechanism and by radical capture of the nano-sized monomer droplets. The likelihood of both mechanisms is assessed.

Mechanistic Aspects of Aqueous Heterogeneous Radical Polymerization of Styrene under Compressed CO2

Radical polymerization of styrene in aqueous emulsions pressurized by CO2 has been investigated with a view to development of low energy miniemulsion polymerization. The general requirement of high energy mixing to generate miniemulsions comprising sub‐micrometer‐size monomer droplets has long been an impediment to wide‐spread industrial application. It is demonstrated by use of online dynamic light scattering and visual observation that CO2 pressurization to the so‐called transparency pressure of an emulsion comprising styrene/hexadecane/Dowfax 8390 (anionic surfactant)/water leads to a decrease in droplet size as well as an increase in emulsion stability. However, mechanistic investigations of radical polymerization under CO2 pressure reveal that these polymerizations do not proceed as true miniemulsion polymerization systems (i.e., via exclusive monomer droplet nucleation), but are also characterized by a significant contribution of particle formation via secondary nucleation as in an ab initio emulsion polymerization. image

Harnessing the interaction between surfactant and hydrophilic catalyst to control eATRP in miniemulsion.

In contrast with previous accounts, it is reported that a single, strongly hydrophilic Cu complex can control an electrochemically mediated atom transfer radical polymerization (eATRP) in oil-in-water miniemulsion in the presence of anionic surfactants, such as sodium dodecyl sulfate (SDS). The anionic surfactant interacted strongly with cationic copper complexes, enabling controlled polymerization by a combination of "interfacial" and "ion-pair" catalysis, whereby ion pairs transport the catalyst to the monomer droplets. The ion-pair system was assembled in situ by mixing commercially available reagents (NaBr, SDS, and traditional hydrophilic copper complexes). Polymer purification was very facile because after reaction >99% of the hydrophilic copper complexes spontaneously left the hydrophobic polymer particles.

Inline Turbidity Measurements of Batch Emulsion Polymerization

The aim of the present study is to investigate applications of inline turbidity as a tool to monitor optical properties during batch emulsion polymerization and during emulsification with high sample rates. For this purpose, an InPro 8200 turbidity probe with a Trb 8300 transmitter is used in combination with an RC1e Mettler Toledo reaction calorimeter. Reproducible turbidity inline monitoring during emulsification and polymerization is achieved. Turbidity measurements allow the observation of droplet formation as well as the determination of the time needed to reach a certain emulsion state. Therefore, turbidity can be used to optimize the duration of emulsification. Higher stirring speed and longer duration lead to smaller monomer droplets and to faster reaction. Furthermore, inline turbidity can be used to detect the start of the reaction and to identify emulsion stability problems. image

Innovative one-step synthesis of hollow polymer particles by microsuspension polymerization of styrene and methyl acrylate with Mg(OH)2 as dispersant

Several methods consisting of two or multi-step processes have been so far proposed for the preparation of sub- and micrometer-sized hollow polymer particles. In this study, we proposed an innovative one-step synthesis of the hollow polymer particles by applying microsuspension copolymerization of styrene and methyl acrylate with Mg(OH)2 as dispersant. In this method, Mg(OH)2 acted not only as dispersant, which covered densely at the surface of the monomer droplets, but also caused hydrolysis reaction of MA unit within styrene-methyl acrylate copolymer particles during the polymerization due to giving alkaline pH in the aqueous medium. It is also important that methyl acrylates are predominantly polymerized over styrene at the initial stage of the microsuspension copolymerization.

Preparation of Polymer Composite Particles by Phase Separation Followed by Suspension Polymerization

The novel method for preparing the polymer composite particles has been developed. It was tried to prepare polymer composite particles composed of polystyrene and carbon black with the phase separation method followed by suspension polymerization. In order to prepare the polymer composite particles with the more uniform diameter, the styrene monomer droplets containing carbon black were formed with phase separation emulsification in which ethyl alcohol and water were used as the good solvent and the poor solvent for styrene monomer, respectively. In the experiment, the surfactant species and their concentrations, the pouring velocity of water and the weight ratio of carbon black to styrene monomer were mainly changed. Water was poured at the given pouring velocity into ethyl alcohol in which styrene monomer and an initiator were dissolved and carbon black was dispersed beforehand. The spherical polymer composite particles containing carbon black were prepared with Tween 20 and Tween 80 of nonionic surfactants and the irregular polymer composite particles were prepared with PVA, SDS and Kotamine. The diameters of polymer composite particles increased with the pouring velocity of water and with the weight ratio of carbon black to styrene monomer.

Preparation of polymeric/inorganic nanocomposite particles in miniemulsions: I. Particle formation mechanism in systems stabilized with sodium dodecyl sulfate

Polymeric/inorganic nanocomposite particles (NCPs) were tentatively prepared through miniemulsion polymerization using sodium dodecyl sulfate (SDS) as the sole surfactant. The representative inorganic nanoparticles (NPs), SiO2 NPs, were pre-dispersed in monomer solutions. Particle morphology and size of the latex particles in the emulsions prepared from miniemulsions were investigated in terms of the SiO2 and SDS content. The results indicate that the number fraction of polymer/SiO2 NCPs (fNCP) in the latex particles was below 4%, and that plain polymer particles dominated in all of the emulsions regardless of the SiO2 and SDS content when the sonication power was 95W. The centrifugation experiments showed that there were three types of particles in the emulsions, namely, a dominant amount of plain polymer particles, a small amount of polymer/SiO2 NCPs, and a very small amount of sub-micrometer polymer/SiO2 aggregates. These particles evolved from small plain monomer droplets, medium monomer droplets containing SiO2 NPs, and micrometer monomer droplets containing many SiO2 NPs, respectively. Formation mechanisms of these three types of particles were proposed. On the basis of the mechanism, we used higher sonication power to improve the homogeneity of the number distribution of SiO2 NPs among monomer droplets. The fNCP value markedly increased to ∼39% upon increase of the sonication power to 190W. However, the formation efficiency of polymer/SiO2 NCPs could not be improved with further increase of the sonication power.

Successive growth and applications of polymeric particles with controllable size and shapes

Nonspherical particles resembling sea pineapples were synthesized by successive growth technique during soapless emulsion polymerization for various applications. First, highly cross-linked seed particle dispersion was synthesized by emulsifier-free emulsion polymerization with acrylic acid as co-monomer for the formation of surface carboxylic groups. Then, a successive growth scheme was applied to the seeds by swelling the particles with monomer droplets, followed by polymerization. The sea pineapple-shaped particles could be produced by adjusting the amount of monomer during the swelling step of the third growth. As a demonstrative application, the seed or sea pineapple-shaped particles could be used as templates for the synthesis of porous inorganic particles by spray drying technique. The resulting porous particles could be adopted as photocatalyst for the decomposition of organic molecules such as methylene blue. As another application, the dye molecules could be adsorbed onto the second grown particles to produce dye-doped nanospheres. Finally, the sea pineapple-shaped particles could be self-organized into supra-aggregates using toluene emulsions as confining geometries. Collectively, successively grown particles were found to be efficient building blocks to prepare the unusually packed structures or functionalized into colored products.

Features of Emulsion Polymerization – How Does the Monomer Move from the Droplets into the Latex Particles?

SummaryA series of rather unorthodox swelling experiments with the aid of a recently developed method proves the important influence of monomer droplets for polymer swelling under heterogeneous conditions. This assumption can be used consistently to explain former results of styrene emulsion polymerization in dependence on the stirring rate speed.

Electrochemical Atom Transfer Radical Polymerization in Miniemulsion with a Dual Catalytic System.

An electrochemical approach was used to control atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) in miniemulsion. Electropolymerization required a dual catalytic system, composed of an aqueous phase catalyst and an organic phase catalyst. This allowed shuttling the electrochemical stimulus from the working electrode (WE) to the continuous aqueous phase and to the dispersed monomer droplets. As aqueous phase catalysts, the hydrophilic Cu complexes with the ligands N,N-bis( 2-pyridylmethyl)-2-hydroxyethylamine (BPMEA), 2,2'-bipyridine (bpy), and tris(2-pyridylmethyl)amine (TPMA) were tested. As organic phase catalysts, the hydrophobic complexes with the ligands bis(2-pyridylmethyl)-octadecylamine (BPMODA) and bis[2-(4-methoxy-3,5-dimethyl)-pyridylmethyl]octadecylamine (BPMODA*) were evaluated. Highest rates and best control of BA electropolymerization were obtained with the water-soluble Cu/BPMEA used in combination with the oil-soluble Cu/BPMODA*. The polymerization rate could be further enhanced by changing the potential applied at the WE. Differently from traditional ATRP systems, reactivity of the dual catalytic system did not depend on the redox potential of the catalysts but instead depended on the hydrophobicity and partition coefficient of the aqueous phase catalyst.

Facile synthesis of cauliflower-like hydrophobically modified polyacrylamide nanospheres by aerosol-photopolymerization

Amphiphilic copolymers consist of hydrophilic and hydrophobic monomer units have attracted great technological attention recently, owing to their unique properties and their ability to stabilize various interfaces in aqueous systems. This paper presents a novel and facile approach to produce spherical polyacrylamide, polystyrene and hydrophobically modified polyacrylamide (HM-PAM), as one of the most important type of amphiphilic copolymers, using a continuous aerosol-photopolymerization for the first time. To this end, the monomer droplets were generated by an atomizer, then photopolymerization was initiated ‘‘in flight’’ by ultraviolet (UV) irradiation of the aerosol monomer droplets containing photoinitiator within the average aerosol residence time of 30s in the photoreactor. To control the aqueous solubility, size and morphology of the resulting HM-PAM copolymers, different styrene/acrylamide monomer concentrations were introduced to the system. The textural properties of the obtained materials were characterized by TEM, SEM and N2 adsorption-desorption isotherms. The results revealed that the copolymers and polyacrylamide have cauliflower-like morphologies, while polystyrene has a nearly smooth morphology. Moreover, chemical compositions of the obtained materials were investigated by several characterization techniques such as 1H NMR, FT-IR, TGA and elemental analysis. 1H NMR and elemental analysis of resulting copolymers microstructures displayed successful incorporation of styrene in copolymer with a good agreement compared to initial ratios. The production of amphiphilic copolymer of acrylamide and styrene using the facile aerosol-photopolymerization method might open new avenues to produce various amphiphilic copolymers with a variety of hydrophobic and hydrophilic monomers with desirable properties.

On the miniemulsion polymerization of very hydrophobic monomers initiated by a completely water‐insoluble initiator: thermodynamics, kinetics, and mechanism

ABSTRACTSuccessful miniemulsion polymerizations of very hydrophobic monomers, such as lauryl methacrylate and 4‐tert‐butyl styrene, initiated by very hydrophobic (i.e., completely water‐insoluble) lauroyl peroxide, are reported. Conversion‐time histories, as well as final latex properties, for example, the particle size distribution, are different from similar miniemulsion polymerizations in the presence of water‐soluble initiators. The observed differences can be attributed to the average number of radicals inside a miniemulsion particle; the system obeys Smith‐Ewart case I rather than Case II kinetics. Albeit the pairwise generation of radicals in the monomer droplets, substantial polymerization rates are observed. Water, present in the droplet interfacial layer, is supposed to act as chain transfer agent. The product of a chain transfer event is a hydroxyl radical, exit of this hydroxyl radical allows for the presence of single radicals in particles. The proposed mechanisms allow for agreement between initial droplet and final particle size distributions in miniemulsion polymerization initiated by lauroyl peroxide. © 2016 The Authors Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2731–2745

Continuous flow reactor for miniemulsion chain photopolymerization: Understanding plugging issue

Plugging is probably one of the most challenging issues facing further continuous polymerization process development. Starting with the photopolymerization of n-butyl acrylate miniemulsion in a continuous photoreactor composed by fluoropolymer coiled tubing, we show that three parameters have a critical role on the occurrence of plugging: solids content (⩾30wt%), surfactant concentration (⩽1wt%) and tubing diameter (⩽1mm). In contrast, monomer droplet stability, size and flow rate have a minimal impact. The use of nanodroplets, as individual reactors able to confine the solid products to these droplets, is in no way an efficient strategy to prevent channel clogging. Polymer adsorption occurs locally on macroscopic nucleation sites, where polymer build-up leads gradually to plugging. Based on interfacial tension measurements, we show adhesional wetting as the main trigger of plugging. In this process driven by the high cohesive energy of water, the monomer droplets not originally in contact with the reactor wall makes contact with that surface by displacing water, adhere to it, and polymerize.

Preparation of Poly(ionic liquid) Hollow Particles with Switchable Permeability.

Poly(ionic liquid) (PIL) particles with a single-hollow structure are prepared by suspension polymerization from monomer droplets consisting of the hydrophobic ionic liquid monomer [2-(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide, ethylene glycol dimethacrylate, and n-butyl acetate containing dissolved poly(n-butyl methacrylate). The obtained PIL hollow particles' shells can be changed from hydrophobic to hydrophilic by anion exchange using a LiBr/ethanol solution. In the case of hydrophilic PIL hollow particles, the water-soluble fluorescent materials can penetrate into the hollow structure, whereas in the case of hydrophobic PIL hollow particles, penetration of the fluorescent materials is restricted. In addition, the encapsulated water-soluble materials can be preserved into the hollow part by changing the shell property of the PIL particle encapsulated with the water-soluble materials from hydrophilic to hydrophobic.

Synthesis of polymeric nanoparticles containing reduced graphene oxide nanosheets stabilized by poly(ionic liquid) using miniemulsion polymerization.

Polymeric nanoparticles containing reduced graphene oxide (rGO) nanosheets have been prepared by aqueous miniemulsion radical polymerization of methyl methacrylate (MMA) utilizing poly(ionic liquid) (PIL) as stabilizer to effectively disperse the rGO nanosheets in the monomer phase. The PIL that gave the best results in terms of rGO dispersibility was a block copolymer of the ionic liquid monomer 1-(2-methacryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)amide ([Mbim][TFSA]) and MMA, the concept being that the MMA units impart solubility in the MMA monomer droplets whereas the IL units act as adsorption sites for rGO. The rGO dispersibility in vinyl monomer was demonstrated to be superior using the above PIL block copolymer compared to the corresponding statistical copolymer or PIL homopolymer. Overall, the approach developed demonstrates how PILs can be employed to conveniently switch (turn ON/OFF) the dispersibility of PIL/rGO via anion exchange reactions, which can be an efficient strategy for synthesis of polymer/rGO nanocomposite materials.

Piezoelectrically-driven production of sub 10 micrometer smart microgels

It is highly desirable to prepare crosslinked microgels whose diameters are comparable to that of a single mammalian cell (i.e., ∼10 μm) with desired monomers for potential biomedical applications. In this study, we have prepared a large number of stimuli-responsive poly(N-isopropylacrylamide) microgels through photo-polymerization in oil of aqueous monomer droplets ejected from a microdroplet generator. The crosslinked microgels as small as 8 μm in diameter could be prepared with a uniform shape and narrow size distribution, and their size could be controlled by adjusting the applied voltages. The size of microgels with LCST behavior was shown to be reduced to ∼5.7 μm with increases in the temperature. Furthermore, the functional monomers, such as fluorescent moieties and photothermal graphene oxide, could be incorporated within the polymer matrices, which introduces the possibility for practical applications such as on-demand drug release systems, tissue engineering, and soft actuators of these microgels.

Mechanistic investigation of nonisothermal suspension polymerization of vinyl chloride

Vinyl chloride suspension polymerization using different temperature trajectories was carried out in a pilot scale batch reactor. Detailed understanding of the conversion at which the primary particles become motionless (Xm) and the key effects of Xm on morphology development of PVC grains were provided. Motionless conversion is estimated for poly(vinyl chloride) (PVC) grains prepared with different temperature trajectories by cold plasticizer absorption measurements. The porosity of PVC grains (prepared isothermally and nonisothermally) shows a maximum at a certain conversion that is considered motionless conversion. With increasing monomer conversion, the cold plasticizer uptake decreases dramatically with conversions greater than motionless conversion until the monomer phase is completely exhausted (Xf) and continues to slightly decrease after Xf. The decrease in cold plasticizer absorption is more pronounced for PVC grains produced nonisothermally by lower initial temperature. The results obtained by scanning electron microscopy and Brabender® plastography showed that the changes in internal structure and fusion behavior of PVC grains after Xm would be much lower when early aggregates of primary particles are formed. Scanning electron microscopy photographs indicate that applying the variable temperature with negative slope accelerates networking between the primary particles inside the polymerizing monomer droplets. The Brabender® plastograph measurements indicate a lower time and temperature of fusion and a higher degree of gelation for nonisothermally produced resin in which the temperature trajectory follows a greater negative slope. J. VINYL ADDIT. TECHNOL., 24:84–92, 2018. © 2015 Society of Plastics Engineers