Emulsion Polymerization Systems Research Articles

Preparation and Properties Analysis of Polyaniline/Nano-SrFe<sub>12</sub>O<sub>19</sub> Composites with Different Morphologies

Polyaniline/nano-SrFe12O19 composites were prepared by in situ polymerization method for the first time. Rod-like and flower-like morphologies composites were synthesized in the phase of an emulsion polymerization system. It was found that the morphology of obtained PANI/nano-SrFe12O19 composites depended on the content of SrFe12O19 of the reaction system. A possible mechanism for the formation of the different morphologic composites had been proposed. The magnetic properties of the PANI/nano- SrFe12O19 composites were inspected by VSM. The possible mechanism for magnetic variation had been proposed in the paper.

Role of electrolytes in the preparation of nanoparticles via the emulsion polymerization of vinyl pivalate

By controlling both the kind of ion and the ionic strength of electrolytes in an emulsion polymerization system of vinyl pivalate containing about 1% sodium lauryl sulfate as a surfactant, nanoparticles of polyvinylpivalate having a diameter of about 25 nm were successfully prepared. The use of high concentrations of lithium chloride and lithium sulfate (∼1.0 mol L −1) prevented the nanoparticles from aggregating and produced nanoparticles sizes of 25–50 nm. Ammonium acetate and sodium acetate, on the other hand, accelerated the aggregate of the nanoparticles. These phenomena were examined in detail and found to be similar to the Hofmeister phenomena and the combination rule proposed by Craig et al.

A new method to determine monomer concentration in the polymer particles of emulsion polymerization systems by dynamic light scattering

AbstractEmulsifier‐free emulsion polymerization of styrene was performed in the presence of small amount of methacrylic and itaconic acids as carboxylic acid monomers and potassium persulfate as an initiator at 70°C to prepare monodisperse polymer particles. Diameter of monomer swollen polymer particles (dpswol) was measured by dynamic light scattering for samples taken from the reaction mixture during the Intervals II and III of the emulsion polymerization. Graphically treatment of dpswol versus conversion data allowed us for the first time to directly determine the critical monomer conversion (xc), from which constant monomer concentration in the polymer particles (CMP) during the Interval II was then calculated. xc and CMP were obtained to be 0.379 and 5.68, respectively. CMP value is in good agreement with that obtained by centrifugation method and those reported in the literature for the similar system. Attempts were also made to evaluate the average number of growing chain per particle ( $\overline n$) during the Interval II of emulsion polymerization of styrene. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Emulsifier-Free, Organotellurium-Mediated Living Radical Emulsion Polymerization of Butyl Acrylate

Organotellurium-mediated living radical polymerization (TERP) of n-butyl acrylate (BA) in emulsifier-free emulsion polymerization system was carried out for the first time, using the in situ nuclea...

Preparation and characterization of polypyrrole/nano-SrFe 12O 19 composites by in situ polymerization method

Polypyrrole (PPY)/nano-SrFe 12O 19 composites were synthesized by in situ polymerization method. The samples were characterized by TEM, SEM, XRD and IR technology. Spherical, conglobulation-like and arborization-like polypyrrole/SrFe 12O 19 composites were synthesized in an emulsion polymerization system for the first time. It was found that the morphology of PPY/nano-SrFe 12O 19 composites depended on the SrFe 12O 19 content of the reaction system. A possible mechanism for the formation of the different morphologic composites had been proposed. It was found that the saturation magnetization ( M S) and remanent magnetization ( M r) for the composites decreased with the decrease of the nano-SrFe 12O 19 content. The coercivity force ( H c) for the composites increased with the increase of PPY content when the PPY content was at a low value, and then decreased with the increase of PPY content. The possible mechanism for the phenomenon had been proposed. The conductivity was measured, and the variation mechanism of the composites conductivity was analyzed in the paper.

Molecular Weight Distribution (Soluble and Insoluble Fraction) in Emulsion Polymerization of Acrylate Monomers by Monte Carlo Simulations

A Monte Carlo simulation model for the semibatch emulsion polymerization of acrylate monomers was developed. The model accounts for the complex kinetics of acrylate monomers (presence of chain-end and midchain radicals), the compartmentalization of emulsion polymerization systems and the development of the entire molecular weight distribution (MWD) as well as the branching density. It was found that the MWD produced in the semibatch process was bimodal (a sharp and extremely high Mw peak and a broad and lower Mw mode), the midchain radicals were predominant during the polymerization, and most of the branches were short and produced by the backbiting mechanism. Interestingly, the bimodality of the MWD could be avoided under certain experimental conditions (those who led to a decrease of particles with more than two radicals), but this did not prevent the formation of polymer chains of molecular weights above 107 g/mol (which are typically insoluble). The model also shows the importance of the presence of midchain radicals in the microstructure of the polymer formed. Thus, the fraction of termination by combination and disproportionation of the midchain radicals had a significant influence on the location of the high molecular weight peak. The predictions of the Monte Carlo model were in good agreement with experimental data and a previously developed deterministic mean-field theory model for the seeded semibatch emulsion polymerization of n-butyl acrylate.

Particle Size Distributions in Electrosterically Stabilized Emulsion Polymerization Systems: Testing the “Mid-Chain-Radical” Hypothesis

It has been found [Thickett, S. C.; Gaborieau, M.; Gilbert, R. G. Macromolecules 2007, 40, 4710-4720] that rate and characterization data on the seeded growth of latex particles electrosterically stabilized by a hairy layer of anchored poly(acrylic acid) (pAA) can be qualitatively and quantitatively explained by mechanistic events involving mid-chain radicals (MCRs). A considerable range of data was found to be consistent with the supposition that these MCRs are formed by abstraction on the pAA during polymerization, and are slow to propagate but quick to terminate, and thus are a cause of radical loss; they may also undergo P-scission leading to secondary particle formation. Results are reported here for four additional types of experiments on the evolution of the particle size distribution (PSD) in these systems; all these experiments have the potential to refute the mechanistic hypothesis. The experiments comprise (i) evolution of the PSD of small particles, (ii) evolution of the PSD in a bimodal system (competitive growth), (iii) growth rates of separate experiments of small and large particles with the same total amount of pAA, and (iv) the amount of secondary nucleation (formation of small particles) with both pAA and also with poly(ethylene oxide) steric stabilizers. In each case, the data are qualitatively and quantitatively consistent with the predictions of the mid-chain-radical mechanisms and thus support the general applicability of this effect.

Developing a hybrid emulsion polymerization system to synthesize Fe3O4/polystyrene latexes with narrow size distribution and high magnetite content

AbstractA hybrid emulsion polymerization was formulated for synthesizing Fe3O4/polystyrene composite latex. This system, containing binary droplets that are magnetic (Mag)‐droplets with a diameter of 100–200 nm and styrene (St)‐droplets with a diameter of 3–4 μm, was obtained by mixing Mag‐miniemulsion and St‐macroemulsion. With extremely low surfactants concentration (≪critical micelle concentration, CMC), the nucleated loci are selectively controlled in the Mag‐droplets, as the result of smaller droplet size and larger surface ratio. Both water‐soluble potassium persulfate (KPS) and oil‐soluble 2,2′‐azobis(2‐isobutyronitrile) was adopted to initiate the polymerization. In the presence of KPS, magnetic polystyrene latices with particles size of 60–200 nm, narrow size distribution, and high magnetite content (86 wt % measured by TGA) were attained successfully. The synthesized magnetic Fe3O4/polystyrene latices assembled into well‐ordered hexagonal structure in the surface of a carbon supported copper grid. The influence of various parameters on various aspects of the as‐synthesized Fe3O4/polystyrene was investigated in detail: type of initiator on composite morphology, feed ratio of Mag‐miniemulsion and St‐macroemulsion on magnetite content, and hydrophobic agent or amount of surfactant on size and size distribution. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5285–5295, 2007

Particle Formation in ab Initio RAFT Mediated Emulsion Polymerization Systems

Amphiphilic, RAFT-capped, (acrylic acid)x(styrene)y diblock copolymers (x = 10, y ) 10, 5, 0) were synthesized and used as stabilizers in emulsion polymerization. Above the critical micelle concentration (cmc) of the diblocks and under appropriate reaction conditions micelles of the more hydrophobic diblocks were sufficiently nonlabile to be nucleated and act as seed particles for latex particle formation. The key parameters which allow control over the system are diblock hydrophobicity and initiator concentration. A homogeneous nucleation mechanism is most likely to operate below the cmc of the diblocks.

Extended Mechanistic Description of Particle Growth in Electrosterically Stabilized Emulsion Polymerization Systems

Polymeric (steric and electrosteric) stabilizers, based on water-soluble polymers such as poly(acrylic acid) and poly(ethylene oxide), are widely used to make surface coatings by emulsion polymerization and form a hairy layer which provides colloidal stability to the latex particles. It is shown by a combination of NMR and rate studies that the growth of these polymer colloids is dominated by a hitherto unsuspected combination of mechanisms: an abstraction reaction in the hairy layer which results in a radical on the water-soluble polymer which is slow to propagate but quick to terminate (Macromolecules 2006, 39, 6495-6504) and can also undergo beta-scission reactions. Termination with radicals which would otherwise lead to particle growth leads to radical loss, i.e., a slower polymerization rate, while beta-scission leads to radicals which can enter the water phase and cause secondary particle formation. Both of these effects are undesirable from a manufacturing point of view, and the newly discovered mechanism could be used to mitigate these undesirable features.

Solution of Population Balance Equations for Emulsion Polymerization: Zero−One and Zero−One−Two Systems

A numerical method was developed to solve the sets of population balance equations (PBEs) that govern the evolution of the particle size distribution (PSD) in emulsion polymerization processes and, more specifically, in zero−one and zero−one−two systems. The technique is based on the finite-volume method and uses high-resolution schemes and a generalized fixed pivot technique to discretize the growth and aggregation terms, respectively. Implicit−explicit methods are used to integrate the semidiscrete equations in time. The performance of the method was evaluated by comparing the computed distributions and/or their moments to analytical solutions determined for selected cases. In all four cases analyzed, good qualitative and quantitative agreement was found between numerical and analytical results. The present algorithm made possible to simulate, for the first time, the evolution of PSD for an actual emulsion polymerization system using the zero−one−two model.

Mechanism of Radical Entry in Electrosterically Stabilized Emulsion Polymerization Systems

The kinetics and mechanism for the process of entering latex particles from the aqueous phase (radical entry) in electrosterically stabilized emulsion polymerization particles were studied. Polystyrene particles stabilized with differing lengths of poly(acrylic acid) chains bound to the surface were synthesized using RAFT-controlled radical polymerization techniques to ensure that the hydrophilic block was of low polydispersity; these latexes had been studied previously to examine the exit rate coefficient (k) in such systems via gamma-relaxation experiments. Particles stabilized with poly(acrylic acid) chains were shown to have a significantly lower average number of per particle (n) (and hence reaction rate) than that of equivalent systems stabilized with a conventional surfactant when used in seeded chemically initiated dilatometry experiments with styrene. Assuming that these emulsion systems obey second-order loss kinetics (i.e., an exited radical will re-enter another particle to either propagate or terminate, a phenomenon well accepted for styrene emulsion systems), the calculated entry rate coefficient (F) was significantly lower than that predicted by the control by aqueous phase growth mechanism for radical entry. Excellent agreement with the accepted entry model was found to occur when radical loss was assumed to be a first-order process with respect to (n) over bar (i.e., an exited radical is terminated in the aqueous phase) or if significant amounts of aqueous phase termination is introduced into the evolution equations of initiator-derived oligomers. These results may be explained by rapid transfer of radical activity to a poly(acrylic acid) chain on the surface, forming midchain radicals that increase the likelihood of termination events in the poly(acrylic acid) surface layer.

Core-shell poly(styrene/sulfonatedn-hydroxy ethyl aniline) latex particles prepared by chemical oxidative polymerization in emulsion polymerization

The kinetic behavior of emulsion polymerizations of styrene in the presence of sulfonated N-hydroxy ethyl aniline (SHEA) was investigated with two initiators: 2,2′-azobisisobutyronitrile (AIBN) and potassium persulfate (KPS). SHEA was synthesized using a stepwise polyurethane reaction method from 3-hydroxy-1-propane sulfonic acid sodium salt, isophorone diisocyanate (IPDI), andN-(2-hydroxyethyl) aniline. Stable core-shell poly(styrene/sulfonated N-hydroxy ethyl aniline, St/SHEA) latex particles were successfully prepared by using an appropriate amount of AIBN, in which SHEA plays the role of ‘surfmer’, i.e., acting as both a surfactant in the emulsion polymerization and a monomer in the chemical oxidative polymerization. The kinetic behavior was dissimilar to that of typical emulsion polymerization systems. A long inhibition period and low rate of polymerization were observed due to radical loss by the oxidative polymerization of SHEA. It was concluded, due to the low water-solubility of AIBN and retardation reaction by SHEA, that the initial loci of polymerization were monomer droplets. However, growing polymer particles as polymerization loci became predominant as polymerization proceeded. It was suggested that AIBN was more effective than KPS in the preparation of the core-shell type poly(St/SHEA) latex particles. With KPS, no substantial polymerization was observed in any of the samples.

Development of an ab Initio Emulsion Atom Transfer Radical Polymerization: From Microemulsion to Emulsion

Atom transfer radical polymerization (ATRP) has been successfully extended to an ab initio emulsion system using a "two-step" procedure, in which the final emulsion polymerization system was formed by adding monomer to an ongoing microemulsion ATRP. The newly developed AGET (activators generated by electron transfer) initiation technique was employed in the first stage of this ab initio ATRP. It allows using oxidatively stable Cu(II) species that is reduced in situ by ascorbic acid. The surfactant concentration in the final emulsion system was efficiently decreased to approximately 2 wt % (approximately 10 wt % vs monomer) by decreasing the catalyst concentration and changing the ratio of the monomer added at the microemulsion stage to the monomer added during the second stage. This two-step procedure avoids the necessity of transporting catalysts through the aqueous media during polymerization, resulting in a controlled emulsion polymerization, as evidenced by a linear first-order kinetic plot and formation of a polymer with a relatively narrow molecular weight distribution (Mw/Mn = 1.2-1.4). The polymerization typically reached 70-90% monomer conversion in 5-6 h. The resulting polymer had high chain-end functionality and was successfully chain extended to form in situ block copolymers by adding the second monomer to an ongoing emulsion polymerization. The stable latex from the ab initio emulsion ATRP had a particle size approximately 120 +/- 10 nm.

The dissociation rate coefficient of persulfate in emulsion polymerization systems

The effects of various components in an emulsion polymerization system on the dissociation rate coefficient of persulfate at 50°C are examined using iodometry. Styrene monomer is found to enhance greatly the dissociation, while there is either no effect or possibly a slightly reduced rate of dissociation with methyl methacrylate monomer. The saturated analogues of these monomers (ethylbenzene and methyl isobutyrate) also enhance the dissociation, although not as much as styrene. Thus, such analogues should not be used as model compounds for determining the effect of a monomer on decomposition rate. The presence of metal parts in the reactor (e.g. as part of the agitation system) also could enhance the dissociation. The accelerations are consistent with literature mechanisms involving transfer reactions of aqueous-phase species. These results have significant implication for the interpretation and prediction of entry efficiencies and particle formation rates in emulsion polymerization systems.

Synthesis and characterization of nanosilica/polyacrylate composite latex

AbstractThe nanosilica/polyacrylate organic–inorganic composite latex was synthesized by in‐situ emulsion polymerization of methyl methacrylate (MMA) and butyl acrylate (BA) in the presence of silica nanoparticles, which were modified by silane coupling agent. The surface properties and dispersibility of silica nanoparticles modification, chemical structure, Zeta potential, diameter distribution of the composite latex prepared, surface roughness, and thermal stability of the hybrid film formed by the composite latex were investigated by fourier transform infrared spectrometer (FTIR), transmission electron microscopy (TEM), Zeta meter, ZetaPlus apparatus (dynamic light scattering method), atomic force microscopy (AFM), and thermogravimetric analysis (TGA), respectively. After modification with silane coupling agent, silane was grafted onto the surface of silica nanoparticles to form the organic layers, which was able to efficiently prevent the silica nanoparticles from aggregating to individually homogeneous disperse in the in‐situ emulsion polymerization system and improve the compatibility of silica nanoparticles with the acrylate monomers. The nanosilica/polyacrylate organic–inorganic composite latex prepared had the properties of silica nanoparticles and pure polyacrylate latex but was not simply a combination. Strong chemical bonding tethered the silica and acrylate chains to form the core/shell structural composite latex. Consequently, the hybrid film formed by nanosilica/polyacrylate composite latex exhibited a smooth surface and better thermal properties than the pure polyacrylate film. POLYM. COMPOS. 27:282–288, 2006. © 2006 Society of Plastics Engineers

Rate-Controlling Events for Radical Exit in Electrosterically Stabilized Emulsion Polymerization Systems

The mechanism controlling radical loss by exit (desorption) in electrosterically stabilized emulsion polymerization particles was obtained from kinetic studies. Using RAFT-controlled radical polymerization techniques, polystyrene particles stabilized with differing lengths of poly(acrylic acid) chains bound to the surface were synthesized, with the hydrophilic block of low polydispersity, and of various degrees of polymerization. After removal of the RAFT agent, these latexes were used in seeded emulsion polymerization experiments with styrene, with the radical loss kinetics studied through the use of gamma-radiolysis dilatometry. The size of the particles is such that they follow zero-one kinetics, so the sole rate-determining step for radical loss is by exit. The rate coefficient for exit (k) of these latexes was obtained directly from the non-steady-state relaxation period. A significant decrease in k occurs (by a factor of 10) relative to ionically stabilized latexes of corresponding size, even for particles with very small hydrophilic layers. The value of k was smaller for latexes with greater length of the hydrophilic block, consistent with the hypothesis that exit in electrosterically stabilized systems is bound by a restricted diffusion through the hydrophilic polymeric layer on the surface. Modification of the Smoluchowski treatment for diffusion-control led rate coefficients to allow for diffusion through two different regions provides an expression for the rate coefficient of radical desorption out of a particle in these systems; semiquantitative agreement with experiment was obtained.

Preparation of polystyrene/poly(vinyl acetate) nanocomposites with a core–shell structure via emulsifier-free emulsion polymerization

Polystyrene/poly(vinyl acetate) latex nanoparticles with a core–shell morphology in an emulsifier-free emulsion polymerization system were prepared with purified styrene and vinyl acetate (VAc) as monomers and 2,2′-azo bis(2-amino propane) dihydrochloride (ABA,2HCl) as the initiator and emulsifier. The optimized conditions of polymerization of VAc, on top of the already-formed polystyrene as a core polymer, with a core–shell morphology were obtained using various parameters such as volume ratio of the first and second stages, type of process, and reaction time. The morphologic structure of the nanoparticles was studied by scanning electron microscopy and transmission electron microscopy. The latex nanoparticles and polymers were characterized by differential scanning calorimetry. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2409–2414, 2006

Preparation of crosslinked composite nanoparticles

AbstractCrosslinked composite nanoparticles were prepared by adding a trifunctional monomer (trimethyol propane trimethacrylate) or a difunctional monomer (divinyl benzene) as a crosslinker into the emulsion polymerization system of styrene in the presence of inorganic nanosilica. A coupling agent, 3‐methacryloxypropyltrimethoxysilane (MPS), was added along with the monomer, crosslinker, and silica to improve the interfacial interaction between silica and polymer and thus to obtain high binding efficiency. The role of MPS was examined. The effects of crosslinkers on the kinetics of emulsion polymerization, monomer conversion, and yield were investigated. The morphology of the composite particles was observed by TEM. The particle size and size distribution of composite latex particles were measured by the dynamic light scattering method. The binding efficiency and swelling ratio were determined by reluxing the sample in xylene using a Soxhlet extraction apparatus. FTIR spectra and TGA verified the participation of crosslinker and silica. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1538–1544, 2005

Particle formation mechanism and kinetic model of ultrasonically initiated emulsion polymerization

A general kinetic model of particle formation in an ultrasonically initiated emulsion polymerization system is presented. This model takes into account homogeneous, micelle entry, and monomer droplet nucleation mechanism. The effects of the ultrasound in producing free radical, degrading free radical and influencing the fashion of the nucleation are also considered. Moreover, chain transfer to monomer and termination in the aqueous phase, capture of oligomer radicals by particles, and coagulation of particles are also considered. An analytical solution is obtained for the initial particle stage consideration. This model predicts that, if the desorption of radical from particles can be neglected, the concentration of the total radical in the aqueous phase is directly proportional to the cavitation concentration. Model predictions are in good agreement with experimental data obtained from the literature.