Pickering emulsion polymerization of graphene oxide-stabilized styrene

Polystyrene (PS) particles were prepared via Pickering emulsion polymerization using graphene oxide (GO) as the stabilizer. The results show that pH is an important factor in the stability of Pickering emulsions. The effects of two different phase initiators, the water phase initiator potassium persulfate and the oil phase initiator azobisisobutyronitrile, on the morphology of PS particles in Pickering emulsion polymerization had been investigated in detail. Wrinkled particles were prepared using the water phase initiator, and spherical particles were prepared using the oil phase initiator. In addition, hexadecane was used as the auxiliary stabilizer in the polymerization, which narrowed the diameter distribution of the PS spheres, and the hollow PS spheres were fabricated. The size of the GO particles also influenced the final morphology of the particles. Nano-sized polymer particles were grafted onto the surface of micro-sized GO. Small GO particles were suitable for Pickering emulsion polymerization to prepare the composite particles. The thermogravimetric analysis of the prepared particles confirmed that they were PS/GO composite particles, which could have a wide range of potential applications, such as in catalysts, sensors, environmental remediation, and energy storage.     

Synthesis of micron-sized polymeric particles in soap-free emulsion polymerization using oil-soluble initiators and electrolytes

Soap-free emulsion polymerization of styrene using oil-soluble initiators and electrolytes was investigated to synthesize micron-sized polystyrene particles. It was clear that an oil-soluble initiator, such as AIBN, worked like a water-soluble initiator in soap-free emulsion polymerization of styrene to prepare monodispersed particles with negative charges, probably because of the polarization of the electron-attractive functional groups decomposed from the initiators and the pi electron cloud of benzene in a styrene monomer. The addition of an electrolyte enabled secondary particles to effectively promote hetero-coagulation for particle growth by reduction of an electrical double layer and prevention of self-growth. Changing the concentration and type of electrolyte enabled us to control the size up to 12 μm in soap-free emulsion polymerization of styrene using AIBN. Conventionally, organic solvents and surfactants have been used to prepare micron-sized polymeric particles, but this method enabled the synthesis of micron-sized polymeric particles in water using electrolytes without surfactants.     

Inverse emulsion polymerization of acrylamide. I. Contribution to the study of some mechanistic aspects

The inverse emulsion polymerization of aqueous solution of acrylamide in toluene has been studied at 40°C using a blend of surfactants as emulsifying system and oil soluble azo initiators. The azo compound partition between the phases has been measured and the effects of their nature and concentration on the polymerization kinetics have been investigated. The influence of other parameters on the kinetics and particle size of the inverse latex have also been investigated: the nature and amount of the emulsifier system, the stirring rate, and the presence of oil-soluble inhibitor. The particle-size analysis using electron microscopy or dynamic light-scattering methods showed the presence of two populations of particles in the initial monomer emulsion and in the final inverse latex: one with very tiny particles (20 nm diam) and the other with larger particles (80–400 nm diam) which is highly polydispersed. The average size of these large particles undergoes a sharp decrease at a certain percent conversion depending upon the stirring rate. The evolution of the particle size distribution may result from a balance between coalescence and dispersion of the emulsion droplets under the effect of prevailing shear rate due to agitation. Concerning the initiation process, the very low solubility of the azo compound in the aqueous solution, together with the effect of the stirring rate and the presence of an oil-soluble inhibitor on the polymerization kinetics lead to the conclusion that most of the initiaton originates from the capture of radicals or oligomeric radicals produced in the oil phase or in the interfacial layer.     

Methyl methacrylate emulsion polymerization at low monomer concentration: Kinetic modeling of nucleation,particle size distribution,and rate of polymerization

The results of a mathematical model developed in the authors' previous work are discussed and compared against final number (N) and size distribution of particles (PSD) and the rate of polymerization (RP) experimental data of methyl methacrylate (MMA) emulsion polymerization above the critical micelle concentration (cmc) of the surfactant. On the basis of the model results, the hypothesis that the observed bimodal PSD can be ascribed to secondary nucleation as proposed in the literature is questionable. It is discussed that this PSD can also be caused by differences in the growing rate of different‐size particles as predicted for styrene emulsion polymerization. Because of the small particle size obtained at low initial monomer concentration, the high rate of free‐radical desorption reduces the accumulation of these species; therefore, the autoacceleration effect is less pronounced for the conditions under study compared with the usual behavior of the RP during MMA emulsion polymerization above cmc. Similarities and differences between model predictions and experimental data are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2547–2556, 2001     

Study of emulsion polymerization stabilized by amphiphilic polymer nanoparticles

Submicron-sized polystyrene (PS) microspheres with a relatively narrow particle size distribution can be easily produced through emulsion polymerization induced by γ-ray at room temperature using a new type of amphiphilic cross-linked poly(stearyl methacrylate-co-acrylamide-co-acrylic acid) particles as stabilizer. The properties of these amphiphilic particles were described, including morphology, size, ζ potential, and contact angles. The effect of the pH value and the content of amphiphilic particles on the formation and stability of emulsions were also investigated. Meanwhile, the obtained PS microspheres were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, through observing the morphology and size of emulsion droplets at different times under an optical microscope, we found it is interesting that Pickering emulsions formed initially disappeared gradually, which is different from the common Pickering emulsions stabilized by inorganic particles. Thus, the mechanism was further discussed.     

Monitoring the effects of homogeneity methanol/water/monomer on the mode of polymerization of styrene: dispersion polymerization versus emulsion polymerization

The polymerization of styrene (St) was carried out with varying amounts of methanol in aqueous medium. As methanol content decreased (to 50 %), the phase of polymerization mixture (methanol/water/monomer) changed to a heterogeneous state; the homogeneous state was obtained in samples that contain 75 and 100 % methanol. In order to verify the mechanism of polymerization in heterogeneous and homogeneous mixtures, the nucleus formation rate during polymerization, the stability equilibrium of the media and seeded particles, and the size of particles and their growth in polymerization were experimentally being monitored. With the homogeneous mixture in 75 wt% methanol, dumbbell, triangle, and peanut-like particles have been formed. On the other hand, the characteristics of the polymerization products were different from those typically obtained in the emulsion polymerization and in the sample with 100 wt% methanol dispersion polymerization. In the sample with 100 % methanol and in one with 50 % methanol, monodispersed spherical particles are formed in the final conversion. Thus, homogeneity in the aqueous methanol mixture can be a critical factor in determining the polymerization modes between dispersion and emulsion polymerization.     

Sterically stabilized emulsion polymerization of styrene: Pseudo‐semicontinuous approach

The sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (R_p) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the R_p versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003     

The control of structure in emulsion polymerization

Free-radical addition polymerization can be carried out using four different processes: mass or bulk, solution, suspension, and emulsion polymerization. Of these four processes, emulsion polymerization is unique because it is a heterogeneous process, in which the polymerization reactions can take place in three different sites: in the continuous aqueous phase, on the surface of growing particles, and within the growing particles. This unique feature of emulsion polymerization offers many possibilities for designing different polymers and latexes: e.g., high-molecular-weight polymers, uniform copolymers, copolymers of difficult-to-copolymerize monomers, functionalized (surface-modified) latexes, uniform size latex particles, grafted latexes, and structured latexes having core-shell, microdomain structures, interpenetrating polymer networks, etc. This paper will describe several aspects of the control of structure in emulsion polymerization.     

Conducting polystyrene/polyaniline blend through template-assisted emulsion polymerization

Electrically conducting polystyrene (PS)/polyaniline blends have been prepared through a one-step “anilinium-surfactant template”-assisted emulsion polymerization at room temperature. The self-assembled cylindrical An⁺PDPSA^? micelle formed inside the PS matrix can act as a structure directing template cum dopant. Morphological observation under scanning electron microscopic studies revealed that during the progress of polymerization, the initially formed nanostructured conducting polyaniline was changed into cubic/hexagonal/lamellar particles and finally transformed into a percolated structure inside the PS matrix. Blend was further characterized by UV-Vis spectroscopy, FTIR spectroscopy, X-ray diffraction, electrical conductivity, thermal stability, dielectric property, rheological property, and electromagnetic shielding efficiency. The key finding of this work is that the conductive blend prepared through micelle-guided polymerization exhibited superior electrical conductivity (9.6 S/m) with low percolation threshold concentration (5 wt%), excellent thermal stability, electromagnetic interference (EMI) SE of 1–10 dB which makes it a promising candidate for EMI shielding and antistatic discharge matrix for the encapsulation of microelectronic devices.     

Emulsion polymerization of styrene. I. Review of experimental data and digital simulation

Isothermal emulsion polymerization at 60°C of styrene in a batch reactor were studied by using sodium lauryl sulfate as surfactant and potassium persulfate as initiator source. The concentrations of surfactant and initiator were varied during the runs. The polymerization evolution was followed as samples were taken at regular intervals. These emulsion samples were analyzed for monomer conversion, rate of polymerization, as well as for the size and the size distribution of the particles. The molecular weight and molecular weight distribution were obtained by gel permeation chromatography. Our study showed that fresh nucleation takes place even at high conversion, causing a continuous shifting toward broadening of particle size distribution. Contrary to the theory of Smith and Ewart, which assumes a constant number of particles during interval II of the polymerization reaction, our digital simulation of the reaction presents better experimental results with a variable number of particles, and indicates that the Hui–Hamielec model for termination constant k_t as function of conversion is not applicable under our working conditions.     

Atom transfer radical emulsion polymerization (emulsion ATRP) of styrene with water-soluble initiator

This study presents styrene emulsion polymerization initiated in aqueous media through an atom transfer radical polymerization (ATRP) mechanism. The water-soluble initiator employed in this process has been synthesized by our team by reacting diethanolamine with α-bromoisobutyryl bromide. The complexation of CuBr was realized by using a bicomponent complexation system comprised of 2,2′-bipyridine and N,N,N′,N′,N″-pentamethyldiethylenetriamine. The initiator ratio influence on the obtained emulsion was studied. The obtained latexes and polymer particles have been characterized by dynamic light scattering, scanning electron microscopy, and gel permeation chromatography.     

Chemical properties of water-soluble,nonionic azo compounds as initiators for emulsion polymerization

Four kinds of water-soluble, nonionic azo compounds were studied in terms of their decomposition rate and initiator efficiency in radical polymerization, and then used for emulsion polymerization. They had relatively low initiator efficiency from 0.09 to 0.46. It was attributed to the susceptibility to a cage effect, depending on their molecular size and hydrophobicity. Four azo compounds initiated emulsion polymerization but nonionic latex particles were not obtained unexpectedly. Methanol-containing medium results in the formation of a bimodal particle size distribution as well as a bimodal molecular weight distribution. © 1996 John Wiley & Sons, Inc.     

Particle formation under monomer‐starved conditions in the semibatch emulsion polymerization of styrene. I. Experimental

Particle formation and particle growth compete in the course of an emulsion polymerization reaction. Any variation in the rate of particle growth, therefore, will result in an opposite effect on the rate of particle formation. The particle formation in a semibatch emulsion polymerization of styrene under monomer‐starved conditions was studied. The semibatch emulsion polymerization reactions were started by the monomer being fed at a low rate to a reaction vessel containing deionized water, an emulsifier, and an initiator. The number of polymer particles increased with a decreasing monomer feed rate. A much larger number of particles (within 1–2 orders of magnitude) than that generally expected from a conventional batch emulsion polymerization was obtained. The results showed a higher dependence of the number of polymer particles on the emulsifier and initiator concentrations compared with that for a batch emulsion polymerization. The size distribution of the particles was characterized by a positive skewness due to the declining rate of the growth of particles during the nucleation stage. A routine for monomer partitioning among the polymer phase, the aqueous phase, and micelles was developed. The results showed that particle formation most likely occurred under monomer‐starved conditions. A small average radical number was obtained because of the formation of a large number of polymer particles, so the kinetics of the system could be explained by a zero–one system. The particle size distribution of the latexes broadened with time as a result of stochastic broadening associated with zero–one systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3940–3952, 2001     

Synthesis and characterization of polychloromethylstyrene nanoparticles of narrow size distribution by emulsion and miniemulsion polymerization processes

Polychloromethylstyrene nanoparticles of sizes from 12.0 ± 2.3 to 229.6 ± 65 nm were prepared by the emulsion and miniemulsion polymerization of chloromethylstyrene in an aqueous continuous phase in the presence of potassium persulfate as initiator, sodium octylbenzenesulfonate as surfactant, and hexadecane as costabilizer for the miniemulsion polymerization process only. The influence of various polymerization parameters (e.g., concentration of the monomer, initiator, the crosslinker monomer, and the surfactant) on the properties of the particles (e.g., size, size distribution, and yield) has been elucidated. The polychloromethylstyrene nanoparticles formed via the emulsion polymerization mechanism possess smaller diameter and size distribution than those formed under similar conditions via the miniemulsion polymerization mechanism. Other differences between these two polymerization mechanisms have also been elucidated. For future study, we wish to use these nanoparticles for the covalent immobilization of bioactive reagents such as proteins to the surface of these nanoparticles for various biomedical applications.     

The emulsion polymerization of vinyl acetate. Factors controlling particle surface area and rate of polymerization

An emulsion polymerization system with uniform continuous addition of vinyl acetate monomer, Pluronic F68 surfactant, and persulfate initiator has been examined with variation of the surfactant concentration over a tenfold range. The particle surface area per unit weight of emulsion was found to vary directly as the surfactant/monomer ratio, as also did the emulsion viscosity. At constant polymer/emulsion weight the number of particles per unit emulsion weight varied directly as the cube of the surfactant concentration. It is shown that these relationships apply also to other monomers, such as styrene and methacrylate esters. The solubility of vinyl acetate in a range of Pluronic F68 aqueous solutions was determined, and it was shown that the rate of polymerization is dependent on the solubility of the monomer in the surfactant solution. It is concluded that when a water-soluble initiator is used, polymerization proceeds in the aqueous phase. The principal factors controlling the rate of polymerization in the emulsion polymerization of vinyl acetate are, consequently, the initiating system and the concentration of monomer in the aqueous phase. Solubilization characteristics indicate that the surfactant concentration will have a much greater effect on the less water-soluble monomers, such as styrene, than on the more soluble ones, such as vinyl acetate.     

Preparation of poly(vinyl chloride) latexes by polymerization of stabilized monomer droplets

Poly(vinyl chloride) latexes have been prepared by polymerization in micron and submicron sized monomer droplets. Monomer emulsions with excellent long time stability were obtained by diffusional swelling of vinyl chloride monomer into preformed, stable polydisperse pre-emulsions of water-insoluble oils or monodisperse, oligomer styrene seed particles. It was found that the size and size distribution of the final latex particles were determined by those of the parent monomer emulsions. Except for the secondary particles formed during polymerization, the size and size distributions of the latex particles were found to be com-parable to those of the monomer emulsions employed, indicating a complete nucleation of the parent emulsion droplets. The extent of secondary particle formation was found to be very dependent upon the emulsifier concentration as well as on the type and amount of initiator used. © 1995 John Wiley & Sons, Inc.     

A theoretical study of the emulsion polymerization of styrene: The effect of the initial dispersion of the system on the polymerization rate and size distribution of polymer-monomer particles

A mathematical model of emulsion polymerization that takes into account the possibility of formation of polymer-monomer particles from monomer microdroplets is proposed. It is shown that the theoretically calculated polymerization rate, the concentration of polymer-monomer particles, and their size distribution are governed by the initial state of the original emulsion system (monodisperse or bidisperse system of microdroplets).     

Reversible addition–fragmentation chain transfer polymerization of methyl methacrylate in emulsion

Theoretical simulations showed that for controlled/living radical polymerization in an emulsion system, some of the earliest born particles could be superswollen to a size close to 1 μm. We hypothesized that the superswelling of these particles would lead to colloidal instability. Under the guidance of the simulation results, reversible addition–fragmentation chain transfer (RAFT) emulsion polymerization of methyl methacrylate (MMA) was carried out. Experimental results showed that increasing the initiation rate, surfactant level, and targeted molecular weight could improve the colloidal stability of the RAFT polymerization of MMA in an emulsion. The experimental results were in full accord with the theoretical predictions. The poor control of the molecular weight and polydispersity index was found to have a close relationship with the colloidal instability. For the first time, we demonstrated that RAFT polymerization could successfully be implemented with little coagulum, good control of the molecular weight, and a low polydispersity index with the same process used for traditional emulsion polymerization but with higher surfactant levels and initiation rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:2837–2847, 2006     

Effect of the presence of polymer in miniemulsion droplets on the kinetics of polymerization

Solution of polystyrene in styrene were dispersed in an aqueous gel phase comprising sodium lauryl sulfate, cetyl alcohol, and water using an emulsification process known to produce monomer droplet sizes inthe submicron size range (referred to as miniemulsion droplets). The shelf-life stabilities of these miniemulsions were studied to determine their relative droplet sizes, and the emulsions were concommitantly polymerized in an isothermal batch reaction calorimeter. The polymerization kinetics and final particle sizes produced were compared with miniemulsion and conventional emulsion polymerizations prepared using equivalent recipes without the addition of polystyrene. The results indicate that polymerization of miniemulsions prepared from polymer solutions produce significantly different kinetics than both miniemulsion and conventional emulsion polymerizations. In general, a small amount of polymer greatly increases the rate of polymerization and the final number of particles produced in the polymerization to the extent where even conventional polymerizations carried out above the critical micelle concentration of the surfactant polymerize more slowly. The results are explained by considering the system to be comprised of small, stable pre-formed monomer-swollen polymer particles which are able to efficiently capture aqueous phase radicals. This enables the system to produce a large final number of particles, similar to the initial number of pre-formed polymer particles, as opposed to miniemulsions and micelles in which only a relatively small fraction of the initial number of species (droplets or micelles) become polymer particles. © 1994 John Wiley & Sons, Inc.