Particles In Emulsion Polymerization Research Articles

Monomer Transport by Collisions in (Mini) Emulsion Polymerization, a Personal Perspective

AbstractTransport of monomer from droplets to growing latex particles in emulsion polymerization in general is assumed to proceed via diffusion through the aqueous phase. Especially in miniemulsion polymerizations the direct transfer of very hydrophobic species from droplet to droplet is assumed to also proceed via collisions. Amongst the hydrophobic species where this is shown to play a role are monomers, initiators, inhibitors and (catalytic) chain transfer agents. It is well known that the reactor geometry and the stirring speed can have a profound effect on emulsion polymerizations. The 1972 paper of Nomura on the effect of stirring on emulsion polymerization is cited more than 100 times and until today keeps scientists intrigued. Diffusion limitations of monomer going from the droplet into the aqueous phase can occur for very hydrophobic monomers. The alternative route of transport via collisions is often not considered. In this perspective, paper will discuss the evidence for collision based transfer in miniemulsion polymerization and also consider whether collision based monomer transport can play a role in regular emulsion polymerizations.

Monomer Concentration in Polymer Particles in Emulsion Polymerization

AbstractThe rate of an emulsion polymerization depends on the monomer concentration at the locus of polymerization in the polymer particles. This note suggests a shortcut method of specifying monomer concentration for new monomers, or monomers for which thermodynamic swelling data is unavailable. While the monomer concentration in a polymer particle in moles per liter varies widely, when described as volume fraction monomer, the variance is much smaller. In fact, for ten monomers for which experimental values are available, the volume fraction monomer in a polymer particle in equilibrium with an aqueous phase saturated with monomer of that polymer is 0.67 ± 0.04. Hence, if no other value is available, one could assume a monomer volume fraction monomer of 0.67 with a good degree of accuracy.

A New Methodology for Measuring the Stability of Emulsion Polymer Particles

SummaryA new methodology for the rapid and robust evaluation of the electrostatic stabilization of latex particles has been developed. It is based on the interpretation of the backscattering profile obtained using a Turbiscan Lab®. The experimental approach employs Multiple Light Scattering (MLS) in agitated measurement cells so the colloidal stability study can be performed at moderate concentration of latexes particles. Coalescence of the latex particles at different volume fraction was provoked by the addition of an aliquot of a concentrated solution of monovalent electrolyte (NaCl). The backscattering spectra were treated in order to follow the variation of the stability as a function of salt concentration, and from this variation to identify the critical coagulation concentration (CCC) of the system under investigation, in a rapid and robust manner.

Comparative Study of Solution Properties of Amphiphilic 8-Shaped Cyclic-(Polystyrene-b-Poly(acrylic acid))2 and Its Linear Precursor

Amphiphilic 8-shaped cyclic-(polystyrene-b-poly(acrylic acid))2 with two rings and its linear precursor, i.e., 8-shaped cyclic- and linear-(PS–PAA)2, were successfully prepared by a combination of atom transfer radical polymerization (ATRP) and “click” chemistry. Using various methods, we characterized those intermediates and resultant copolymers and studied their association properties in solutions. As expected, the average aggregation number (⟨Nagg⟩) increases with the molar fraction of styrene for a given overall degree of polymerization. Our results reveal that the cyclization leads to a smaller ⟨Nagg⟩ but slightly larger and looser aggregates, presumably due to the topological constraint of the two rings (8-shaped). Using these amphiphilic chains as emulsifying agents, we found that 8-shaped cyclic-(PS–PAA)2 chains are less effective in stabilizing latex particles in emulsion polymerization because each cyclic chain occupies a smaller interfacial surface area than its linear counterpart. Further, using pyrene as a model hydrophobic molecule, we investigated their solubilization powers. Our results reveal that 8-shaped cyclic- and linear-(PS–PAA)2 chains have a similar ability in loading hydrophobic pyrene molecules, different from our original expectation, presumably because the hydrophobic PS block is too short and the hydrophilic PAA rings are too small. The current study provides a better understanding of the complicated topological constraint on the solution properties of 8-shaped cyclic amphiphilic copolymers.

Superparamagnetic Polymer Emulsion Particles from a Soap-Free Seeded Emulsion Polymerization and their Application for Lipase Immobilization

Using emulsion copolymer of styrene (St), glycidyl methacrylate (GMA) and 2-hydroxyethyl methacrylate (HEMA) as seed latexes, the superparamagnetic polymer emulsion particles were prepared by seeded emulsion copolymerization of butyl methacrylate (BMA), vinyl acetate (VAc) and ethylene glycol dimethacrylate in the presence of the seed latexes and superparamagnetic Fe3O4/SiOx nanoparticles (or Fe3O4-APTS nanoparticles) through a two-step process, without addition of any emulsifier. The magnetic emulsion particles named P(St-GMA-HEMA)/P(BMA-VAc) were characterized by transmission electron microscope and vibrating sample magnetometry. The results showed that the magnetic emulsion particles held a structure with a thinner shell (around 100nm) and a bigger cavity (around 200nm), and possessed a certain level of magnetic response. The resulting magnetic emulsion particles were employed in the immobilization of lipase by two strategies to immobilized lipase onto the resulting magnetic composites directly (S-1) or using glutaraldehyde as a coupling agent (S-2), thus, experimental data showed that the thermal stability and reusability of immobilized lipase based on S-2 were higher than that of S-1.

Synthesis and Properties of Polyacrylate Emulsion Modified by Pentaerythritol Tetraacrylate

A crosslinking core-shell structured polyacrylate emulsion polymer was synthesized by semi-batch seeded emulsion polymerization with methyl methacrylate and butyl acrylate as main monomers, and pentaerythritol tetraacrylate as a cross-linking agent. Chemical structure and micro morphology structure of the obtained polymer emulsion particles were characterized by Fourier transform infrared spectra (FTIR) and transmission electron microscope (TEM), respectively. Influences of cross-linking agent content on water resistance, thermal property, and mechanical property of the emulsion films were investigated. Compared to the film of the unmodified polyacrylate emulsion, water absorption capacity of modified polyacrylate emulsion film with 1.0% PET4A declined to 4.7%, tensile strength at break increased from 1.35MPa to 3.03MPa, and elongation at break increased to 171.7%; 5%,10% and 50% weight loss temperature of the modified emulsion increased 17°C,32°C and 45°C, respectively.

Comparison of High Resolution Schemes for Solving Population Balances

The physical properties of particulate products such as crystals and polymers are strongly dependent on their size distribution which is usually modeled by a population balance equation (PBE). The PBE is an integro-partial differential equation and due to its hyperbolic nature, steep fronts are encountered in the solution profile. When solved numerically, the discretization of the PBE has to be performed with care to avoid numerical diffusion, and dispersion and appropriate numerical methods have to be chosen for that purpose. In this work, an extensive investigation of the performance of several high resolution discretization methods when applied to the PBE is performed with the objective to determine the most promising among them in terms of accuracy and computation time. The comparison is performed on standard benchmark problems and on a practical PBE that arises from modeling the growth of a seed of particles in emulsion polymerization.

Incorporation of nonionic emulsifier inside methacrylic polymer particles in emulsion polymerization

The incorporations of polyoxyethylene lauryl ether (Emulgen 109P) and polyoxyethylene nonylphenyl ether (Emulgen 911) nonionic emulsifiers inside poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(iso-butyl methacrylate) (Pi-BMA) particles prepared by emulsifier-present emulsion polymerizations were examined. To measure the amounts of the incorporated nonionic emulsifiers, optimum compositions of 2-propanol aqueous solutions to remove the nonionic emulsifier from the particle surfaces without removal from the insides were determined. The amount of the incorporation measured by gel permeation chromatography was increased in the order of PMMA > PEMA > Pi-BMA, which accorded with the order of miscibility between each polymer and the emulsifier.

Incorporation of Nonionic Emulsifiers Inside Particles in Emulsion Polymerization: Mechanism and Methods of Suppression

Emulsion polymerizations of styrene were carried out using two kinds of polyoxyethylene lauryl ether nonionic emulsifiers having different hydrophilic-lipophilic balances (HLB): Emulgen 109P (HLB 13.6); and Emulgen 150 (HLB 18.3). In both cases, incorporation of emulsifier inside polystyrene (PS) particles was clearly observed, as previously reported for the emulsion polymerization of styrene and methacrylic acid using polyoxyethylene nonyl phenyl ether (Emulgen 911, HLB 13.7) nonionic emulsifier. The generality of the incorporation phenomenon of nonionic emulsifier inside polymer particles in emulsion polymerization was clarified. In the case of Emulgen 109P, which is more hydrophobic than Emulgen 150, about 30% of the total amount was incorporated inside the PS particles, higher than for Emulgen 150 (15%). The difference seemed to be ascribed to the difference in the affinities between the nonionic emulsifiers and styrene, which cause the incorporation of emulsifier. On the basis of this idea, suppression of the incorporation was achieved by decreasing the polymerization temperature and the monomer-feed rate. This strongly supports the proposed incorporation mechanism.

Effects of Laplace Pressure on Propagation and Termination in Aqueous Heterogeneous Free Radical Polymerization

Summary: The influence of the Laplace pressure in polymer emulsion particles during aqueous heterogeneous free radical polymerization on the polymerization kinetics has been investigated. Calculations were carried out based on experimentally reported pressure dependences of propagation and termination rate coefficients. The results suggest that in most cases the effects are not likely to be significant, although under conditions of very small particles (diameter <20 nm) and high interfacial tensions effects of the order of a few percent on propagation (increase in rate) and termination (decrease in rate) were predicted. (kp/kt0.5)/(kp/kt0.5)0 as a function of particle radius as a result of the Laplace pressure.

Adsorption of block copolymers at latex surface and their utilization in emulsion polymerization

Amphiphilic block copolymers have been investigated for their utilization in emulsion polymerization of butyl methacrylate. Special attention has been paid to the adsorption mechanism of the block copolymers from systematic measurements of equilibrium adsorption isotherms. A series of well-defined water-soluble amphiphilic block copolymers, composed of poly(butyl methacrylate) and poly(sodium methacrylate) blocks, were synthesized by anionic polymerization of butyl methacrylate and tert-butyl methacrylate followed by the thermal deprotection of the tert-butyl ester groups and final hydrolysis. The number density of emulsion polymer particles N P varied as [copolymer] α, α lying between 0.44 and 0.73 according to the hydrophilic content of the copolymers. In contrast with SDS taken as a reference emulsifier, the adsorption of the copolymers was very strong and this provided quite an efficient stabilization of the polymer particles during emulsion polymerization, even at low concentrations (<10 −4 mol L −1) and low coverages (<10% of the interfacial area).

Monodispersed and nanosized dendrimer/polystyrene latex particles

Emulsion polymerization of styrene was carried out using dendrimer DAB-dendr-(NH 2) 64 as seed. The size and size distribution of the emulsion particles were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS), and the effects of emulsion polymerization conditions on the preparation of emulsion particle were investigated. It has been found that the nanosized dendrimer/polystyrene polymer emulsion particles obtained were in the range of 26∼64 nm in diameter, and were monodisperse; the size and size distribution of emulsion particles were influenced by the contents of dendrimer DAB-dendr-(NH 2) 64, emulsifier and initiator, as well as the pH value.

On‐line monitoring of the evolution of number of particles in emulsion polymerization by conductivity measurements. II. Model validation

AbstractA model to describe the evolution of conductivity measurements in emulsion polymerizations was presented in the first part of this work [Santos et al., J Appl Polym Sci, 2002]. The model was built on the assumption that surfactant is partitioned among the three principle phases of the polymerizing latex. Model parameters were estimated by fitting the model responses to experimental conductivity data obtained in different runs performed with similar solids content, reactor temperature, and initiator concentration. A soft‐sensor strategy was then proposed for monitoring of the number of polymer particles (Np) by combining the conductivity model with the available conversion, temperature, and conductivity signals. The main objective of the current work is to validate the conductivity model in a broader range of operation conditions, to follow the evolution of Np (nucleation/coagulation) on‐line under different reaction conditions. A series of batch and semibatch polymerization runs were carried out aiming to evaluate the model performance when different solids content, surfactant concentration, and reactor temperatures are used. Additionally, simulation studies were performed to assess the model sensitivity to the fluctuations of the conversion and temperature signals. Results showed that the model is able to perform accurate predictions of Np even when disturbances of ±2°C in temperature and ±0.03 in monomer conversion take place during the polymerization process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 941–952, 2004

Online monitoring of the evolution of the number of particles in emulsion polymerization by conductivity measurements. I. Model formulation

AbstractA conductivity meter is an inexpensive instrument that can easily be installed in polymerization reactors. This instrument can be used to monitor ionic species without time‐consuming calibrations. A probe is inserted into the media, providing in situ measurements of conductivity in real time. For emulsion polymerization reactions, the conductivity meter can respond to changes in the ionic surfactant concentration, allowing the determination of surfactant dynamics in the media. The surfactant concentration can then be related to the changes in the surface area of the polymer particle phase, which can be linked to nucleation or coagulation phenomena. In this study, a conductivity meter was coupled to a calorimetric reactor to provide in situ and online measurements of conductivity during the emulsion polymerization of styrene, with sodium dodecyl sulfate as an anionic surfactant and with potassium persulfate as a free‐radical initiator. A semiempirical model was built to describe the conductivity signal as a function of the latex composition and the reactor temperature. The model was inverted and combined with the available conductivity signal, conversion, and temperature measurements and was able to accurately predict the number of polymer particles in the latex and the surfactant concentrations in the many phases, without online measurements of the particle size. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1213–1226, 2003

Behavior of free radical transfer between aqueous phase and latex particles in emulsion polymerization

The free radical transfer between aqueous phase and latex particles was investigated in the seeded emulsion polymerization using potassium persulfate (KPS) as an initiator. The effects of seed particle size, initiator concentration and polymerization temperature on the radical entry into and/or exit from the particles were emphatically examined. The results suggest that the aqueous-phase radical entry into the particles should be a competitive process, i.e. the competition between radical diffusion from the bulk aqueous phase to the interface of particles and its reactions in the aqueous phase determines whether it may be adsorbed into the particles or not. This implies that the reactions and properties of radicals in the aqueous phase play an essential role in particle nucleation in the emulsion polymerization.

Characterization of aqueous micellar solutions of amphiphilic block copolymers of poly(acrylic acid) and polystyrene prepared via ATRP. Toward the control of the number of particles in emulsion polymerization

A series of diblock, triblock and star-block copolymers composed of polystyrene and poly(acrylic acid) were synthesized by ATRP. The structure of the copolymers, the size of the blocks and the composition were varied, keeping however a short polystyrene block and a poly(acrylic acid) content larger than 60 mol% to allow solubility in alkaline water. Their micellization was studied by static and dynamic light scattering and the influence of their structural characteristics on the aggregation number, N agg, was examined at low salt concentration and alkaline pH. It was shown that micelles were in thermodynamic equilibrium and that N agg followed the power law N agg∼ N A −0.9 N S 2 (with N A, the total number of acrylic acid units in the copolymer and N S, the total number of styrene units), that is characteristic of amphiphile micelles formed from strongly segregated block copolymers. Moreover, N agg was independent of salt concentration in the investigated range. The same copolymers were previously used as stabilizers in emulsion polymerization [Macromolecules 34 (2001) 4439]. The final latex particle concentration, N p, was compared with N m, the initial micelle concentration. This enabled us to conclude that among the block copolymers studied, those with high acid content behaved like low molar mass surfactants. In contrast, those with low acid content formed stable micelles that could be directly nucleated to create latex particles, allowing a good control over N p.

MATHEMATICAL MODELING OF PARTICLE MORPHOLOGY DEVELOPMENT INDUCED BY RADICAL CONCENTRATION GRADIENTS IN SEEDED STYRENE HOMOPOLYMERIZATION

The development of concentration gradients in the seeded emulsion homopolymerization of styrene has been studied using a mathematical model. Under ideal conditions, emulsion polymer particles are expected to be relatively uniform across their radius. When this is true, kinetic models can be reliably used to predict the reaction rates, molecular weight, and composition (for copolymerizations). However for large particles, large radial gradients in the polymer radical concentration can develop during polymerization. While gradients in the monomer concentration are not observed, the radical concentration gradients result in non-uniform reaction rates across the particle radius, rendering conventional models unreliable. This study examines the effect of seed particle diameter and various kinetic parameters on the development of particle morphology.

Variation of the morphology of a carboxylated polymer film by alkali treatment

The variation of the morphology of a carboxylated polymer film cast from tetrahydrofuran solution, in which styrene/methacrylic acid copolymer particles produced by emulsion copolymerization were dissolved, before and after alkali treatment at temperatures higher than the glass-transition temperature was observed with a scanning electron microscope. The treated film had a porous structure. This result provides important evidence for the formation mechanism of the multihollow structure in submicron-sized, carboxylated polymer emulsion particles by the stepwise alkali/acid method and the alkali/cooling method which the authors proposed.

Swelling behavior of emulsion polymer particles: comparison between fully cross-linked and partially cross-linked poly(acrylate) latexes

Measurement of swelling is used to estimate the extent of cross-linking of latex particles prepared by emulsion polymerization. In this paper we report swelling data as a function of solvent activity for two latexes: (1) a poly(butylacrylate) (PBA) latex, and (2) a poly(isodecylacrylate) latex. In the latter case comparison is made between the swelling behavior of fully cross-linked, i.e., networks without free polymer chains, and partially cross-linked latexes. It is demonstrated that the equilibrium swelling properties are only slightly affected by the presence of uncross-linked chains. Swelling gives reliable results for the cross-link density only if the latex does not contain free chains.

An overview of the preparation and use of emulsion polymer particles for the toughening of plastics

AbstractThe rationale for rubber‐toughening of brittle plastics is presented together with a discussion of the advantages of using pre‐formed toughening particles prepared by emulsion polymerisation. Rubber‐toughened acrylic and epoxy resin materials are used to illustrate the preparation and use of pre‐formed emulsion polymer particles for toughening. For both types of material, tensile properties are controlled by the volume fraction of rubbery polymer, essentially independent of the size and morphology of the toughening particles. However, under the additional constraint ahead of propagating cracks, toughening particle size and morphology are shown to be important, and to have different effects in the two types of material due to inherent differences in the mechanism of yielding of the matrix polymer.