On inverse miniemulsion polymerization of conventional water-soluble monomers

https://doi.org/10.1016/j.cis.2010.02.006Get rights and content

Abstract

Inverse monomer miniemulsions can be generated by sonification of the polar monomer, water, stabilizer and costabilizer in organic solvents as the unpolar continuous phase. The inverse miniemulsion obtains its stability by using a combination of effective surfactant and osmotic pressure agent, so called lypophobe, which is practically insoluble in the continuous phase and prevents the minidroplets from Ostwald ripening. Inverse miniemulsions are typically sterically stabilized with a nonionic surfactant blend so as to provide a relatively condensed interface. The monomer droplet nucleation proceeds under an uncomplete coverage of the monomer and polymer particles with surfactant. Inverse monomer miniemulsions can be easily polymerized to latexes by using water and oil-soluble initiators. The rate of inverse miniemulsion polymerization of water-soluble monomers increased with increasing both initiator and emulsifier concentrations. The inverse polymerization is very fast and the high conversion is reached during a few minutes. The dependence of the polymerization rate vs. conversion can be described by a curve with the two rate intervals. The abrupt increase in the polymerization rate can be attributed to the increased number of reaction loci and the gel effect. The partitioning of unsaturated monomers between the aqueous and continuous phases favours the contribution of homogeneous nucleation. The desorption of monomeric radicals from the small polymer particles favours the polymerization in the continuous phase. The miniemulsion polymerization and copolymerization is ideal process for the preparation of composite nanoparticles with different structures. This procedure can be used to develop novel thermally responsive polymer microspheres, for example, based on N-isopropylacrylamide monomer. The composite magnetic nanoparticles are prepared by polymerization of both water-soluble and oil-soluble monomers in the presence of water- and oil-soluble iron oxide nanoparticles. The inverse miniemulsion copolymerization of acrylic acid and sodium acrylate in the presence of inorganic nanoparticles and substances produces poly(acrylic acid-co-sodium acrylate)/inorganic phase composite nanoparticles. The presence of hydrophobic monomer in the miniemulsion system favours the formation of hollow nanoparticles. The composite latex particles owned better thermal stability and higher colloidal stability than pure latex particles.

Introduction

Formation of a stable emulsion and miniemulsion from two immiscible liquids in the micrometer or submicrometer range can normally only be obtained by the addition of surface active compounds. Emulsions and miniemulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surfactants. To date, surfactant molecules are usually added to an inverse emulsion polymerization recipe to provide the system with colloidal stability. An essential problem in the preparation of monomer-containing inverse miniemulsion is its stability before and during the polymerization of the monomer. What distinguishes the inverse emulsion polymerization systems from the direct ones is that the emulsifier is less distinct aggregate at the particle surface and the monomer is dissolved in the dispersed aqueous phase. These dispersions are thermodynamically unstable, and water separation and shelf-life have major concerns.

Most of the miniemulsions are kinetically stable, but under an ideal condition thermodynamically stable miniemulsions can also be prepared. The fact that the droplet size increases continuously with the aging time is a manifestation of the thermodynamic instability. Incorporation of lypophobe into the recipe can effectively retard the diffusional degradation of monomer saturated aqueous droplets and maintain the very small droplet size generated by homogenization. This small droplet size (or large total droplet surface area) means that most of the emulsifier species are adsorbed onto the droplet surface. Therefore, little free emulsifier is available for micelle formation in the subsequent miniemulsion polymerization. That is, the nucleation and polymerization in these tiny monomer droplets may control the reaction kinetics.

Some substances (lypophobe) can provide the monomer droplets with the osmotic pressure effect. Besides, the lypophobe can also initiate the formation of a complex close-packed structure on the droplet surface and the formation of monomers with higher dissociation degree or higher hydrophilic nature. Diffusional degradation of the monomer droplets can be retarded or even prevented by addition of lypophobe. This is because lypophobic additives lower the Gibbs free energy of the miniemulsion droplets, thereby decreasing the driving force for diffusion of the monomer into the continuous phase. The term “miniemulsion polymerization” is also commonly used for dispersed systems in which particle nucleation predominantly occurs in the submicron minidroplets stabilized against Ostwald ripening [1], [2].

Inverse emulsion and miniemulsion polymerizations are in wide commercial use because of their many advantages, however, the process itself is not without drawbacks. The inverse micellar aggregates are summarized in Scheme 1. The miniemulsion polymerization is usually much faster than the solution or bulk polymerization, the resulting polymer is in a form of latex and it has much higher average molecular weight and broader molecular weight distribution. The miniemulsion polymerization can easily be carried up to a relatively high conversion of monomer to polymer; hence problems with residual monomer are minimized. Only a small fraction, if any of the unsaturated monomer is present in the micelles or dissolved in the continuous phase. Most monomer molecules dwell in the monomer-saturated aqueous droplets.

The first general mechanism for any type of inverse emulsion polymerization was proposed in 1987 by Hunkeler et al. [3]. This was subsequently expanded and developed into a kinetic model for homopolymers of acrylamide and copolymers of acrylamide and quaternary ammonium cationic monomers [3]. From the viewpoints of the classical micellar and homogeneous nucleation models [4], [5], [6], [7], [8], [9], [10], [11], the polymerization resulting exclusively from monomer droplet nucleation would have no Region II, in which a constant monomer concentration in the reaction loci is observed. Furthermore, there is no or only a minor dependence of the polymerization kinetics on the transport of monomer across the aqueous phase, which may be the rate-limiting step in some emulsion polymerization systems. Miniemulsion polymerization refers to the reaction in the disperse system with a rather complex particle formation mechanism. The mechanism primarily involves monomer droplet nucleation and some contribution of micellar and homogeneous nucleations. The miniemulsion polymerization technique involves dispersion of a large number of homogenized monomer droplets in oil with the aid of emulsifier, coemulsifier, and cosolvents. These monomer saturated aqueous minidroplets may remain stable during polymerization by using a hydrophobic emulsifier in combination with a lypophobe to depress agglomeration and diffusion of monomer from small droplets to large ones.

The minidroplets have a very large total droplet surface area and, therefore, may compete effectively with the monomer-swollen micelles (if they are present) for the oligomeric radicals generated in water. Furthermore, the particle nuclei formed via homogeneous nucleation can be captured by these minidroplets. After capture of the oligomeric radicals from the continuous phase, the polymerizing minidroplets are then transformed into the monomer-swollen polymer particles (latex particles). These nucleated droplets or highly monomer-swollen polymer particles might have a larger number of radicals per particle n̅ than the monomer-swollen polymer nanoparticles generated by the micellar mechanism. This is the reason why the polymerization kinetics can be governed by pseudo-bulk kinetics rather than by the Smith–Ewart zero-one kinetics. In miniemulsion polymerization, radicals enter the droplets and initiate the polymerization of monomer inside these droplets. The polymerization system under such a condition, especially with large minidroplets, might be regulated by the solution kinetics with participation of the gel effect [12]. In the early stage of polymerization, the nucleated minidroplets contain a large fraction of monomer and also may have a larger n̅ due to the faster radical entry rate. Furthermore, the gel effect of water soluble acrylic monomers is operative slightly above 20% conversion. As a result, the miniemulsion polymerization rate is very fast.

The properties of the latex product may be affected by the particle nucleation mechanism as well. The preferable formation of polymer inside the minidroplets would depress the interaction of radicals with emulsifier or chain transfer of radicals to emulsifier, which generally leads to incorporation of emulsifier units into the polymer matrix and changes in the physical properties of the latex product. The chemically incorporated emulsifier on the particle surface increases the colloidal stability of latex particles. The miniemulsion polymerization, if other nucleation mechanisms are eliminated, might produce a latex product with a more uniform particle size distribution (PSD). The shrinkage of monomer/polymer particles with the progress of polymerization promotes the release of the originally adsorbed emulsifier due to which generation of a crop of particle nuclei in the continuous phase may appear and the PSD may become broader. In order to suppress this occurrence the minidroplet surface should be only partially saturated with adsorbed emulsifier. Under such conditions there is no release of surfactant molecules from unsaturated particle surface. On the contrary, any surface active substances formed during the polymerization as byproduts are adsorbed by such polymer particles.

Polymerization of the monomer droplets of such miniemulsions turned out to be very promising and extends the possibilities of classical emulsion polymerization especially for the preparation of nontraditional composite nanoparticles [13], [14]. Due to the fact that the polymerization time is usually shorter than the growth of the droplets by collisions, the polymerization in carefully prepared miniemulsions results in latex particles which have about the same size as the initial droplets. In the case of appropriately formulated miniemulsions where polymerization is initiated in each droplet and the solubility of the monomer in the continuous phase is low, the ideal, limiting case of a 1:1 copy of the droplets to the particles can be obtained [15]. For the formulation of miniemulsions, a wide variation of ionic and nonionic surfactants and amphiphilic block copolymers could be used in order to get stable polymer dispersions in different size ranges.

Miniemulsion polymerization is a process in which a liquid monomer (or a solution of the monomer) is dispersed into a continuous phase using: (1) a high energy input process (ultrasounds, high shear rate…), (2) a surfactant and (3) a compound that is solubilised in the droplets and exhibits extra-low solubility in the continuous phase. Under these conditions, the initial droplet size could be in the range 100–500 nm. In the presence of an initiator solubilised into the reaction system, the submicronic size has important consequences on the polymerization mechanism as compared to ab inito emulsion polymerization [16]. Indeed, droplet nucleation becomes significant and competes with other possible nucleation processes. Moreover, polymerization taking place in the droplets does not involve monomer diffusion contrary to polymer particles prepared by emulsion polymerization.

The hydrophilic surfactant prevents oil droplets from coalescence either by electrostatic repulsion or steric stabilization [17]. The costabilizer used in oil/water miniemulsion, a hydrophobe, creates osmotic pressure in oil droplets to prevent molecular diffusion from small to large droplets (Scheme 2) [18]. As miniemulsion polymerization is carried out in a nonaqueous solvent, such a polymerization is called water-in-oil (W/O) or inverse (mini)emulsion polymerization. The hydrophobic surfactant prevents water droplets from coalescence by steric stabilization. The surfactants used in the inverse miniemulsion polymerization are mostly hydrophobic nonionic surfactants and partly amphiphilic block copolymer [19], [20]. Hydrophobic nonionic surfactants like Span 80, Tween 85 or anionic surfactants such as AOT (sodium bis-2-ethylhexylsulfosuccinate) are among the most commonly used in inverse mini- and emulsion polymerization (Scheme 2) [21], [22]. As the osmotic pressure agent or antioswaldt ripening agent (lypophobe) is usually used a highly hydrophilic salt or low-molecular weight electrolyte, such as NaCl, NaOH, Na2SO4 and MgSO4. Anions of unsaturated monomer acids can function as a costabilizer and they can provide the osmotic pressure. The role of a lypophobe in the inverse miniemulsion polymerization is to suppress molecular diffusion or so-called Ostwald ripening effects by introducing an osmotic pressure to maintain the droplet stability during polymerization.

The miniemulsion offers several advantages to more conventional bulk (or solution) processes, including higher polymer concentrations in the ultimate product. During the following conversions (e.g., polymerization or polyaddition), it was shown that each droplet can be considered as an independent nanoreactor, and ideally, the droplets convert in a one-to-one copy process under preservation of particle number and the amount and relative composition of material in each droplet. For polymerization in inverse emulsions and miniemulsions, a large variety of hydrophilic monomer can be used, such as hydroxyethyl methacrylate, (meth)acrylamide, or (meth)acrylic acid [22]. Because these systems possess different solubilities in the continuous phase and influences the interfacial tensions in different ways, the each polymerization mechanism should be treated and discussed in different approach.

Inverse-emulsion polymerization is a widely applied technology for the preparation of high molecular weight water-soluble macromolecules [23], owing to the fact that high concentrations of monomers can be contained within the aqueous droplets, while maintaining a monomer/polymer latex. These polymers, which are generally based on acrylamide copolymerized with anionic or cationic monomers, are applied as surfactants, compatibilizers, flocculants, as retention aids in papermaking, in the treatment of potable water and mining wastes, as rheology modifiers in oil recovery and cosmetics, and for aqueous solid–liquid separations in general [24]. When considering applications in waste-water treatment, the overall performance of the flocculant is a function of the polymer chemistry, structure, as well as concentration. Furthermore, a polymer's efficiency is related to the ability to invert the water-in-oil system completely, and rapidly, into an excess of water, or brine, depending on the application. Therefore, the complete liberation of the polymer, encapsulated in the water droplets, is essential.

We have given a brief summary of the most recent research development in the field of inverse miniemulsion polymerization, including the kinetics and mechanism of this process. Even this brief account has revealed the versatility of inverse miniemulsion polymerization for making high-molecular weight polymers and copolymers and composite nanomaterials and nanoconjugates with an excellent properties.

Section snippets

General

Miniemulsions are understood as liquid/liquid emulsions consisting of small, long-time stable, and narrowly size distributed droplets with diameters between 50 and 500 nm. The process of miniemulsion can be best described by applying extreme shear forces to a system consisting of a continuous phase, a dispersed phase, a surfactant, and an osmotic pressure agent. For example, in the reported inverse miniemulsions [19], a liquid hydrophilic monomer was emulsified in a hydrophobic phase such as

Kinetic studies

There is no true Region II for the miniemulsion polymerization system. The occasionally observed constant polymerization rate in the conversion range of 20–50% may result from the equilibrium between two opposing effects, that is, the growing population of latex particles and the decreased monomer concentration at the reaction loci. Besides, the gel effect (the Smith–Ewart case 3 or pseudo-bulk polymerization system) may counterbalance the continuously decreased monomer concentration in the

Traditional inverse latexes

Inverse emulsion (including miniemulsion and microemulsion) was usually applied to the polymerization of hydrophilic monomers, such as acrylamide (AAm), acrylic acid (AA), methacrylic acid (MA), N-isopropylacrylmide (NIPAM) and others. Due to the fact that the polymerization time is usually shorter than the growth of the droplets by collisions, the polymerization in carefully prepared miniemulsions results in latex particles which have about the same size as the initial droplets. In the case of

General

Composite nanoparticles are widely used in numerous technological and medical applications; e.g., as ceramic precursors, filler materials, pigments, recording materials, in electronics, catalysis, medical diagnostics, and many others. Several techniques have been developed for the generation of such particles, some based on physical principles, some based on chemical principles. With mechanical grinding or so-called colloidal milling, particles smaller than 1 μm are not accessible, the grain

Conclusion

The inverse miniemulsion polymerization technique involves dispersion of a large number of homogenized monomer droplets in oil with the aid of emulsifier, coemulsifier, lypophobe, different additives and (co)solvents. These monomer saturated aqueous minidroplets may remain stable during polymerization by using a hydrophobic emulsifier in combination with a lypophobe to depress agglomeration and diffusion of monomer from small droplets to large ones. Miniemulsion polymerization using an

Acknowledgements

This research is supported by the VEGA project No. 2/0037/10 and 2/0160/10, SAV-FM-EHP-2008-01-01 project, the APVV projects No. 0362-07 and 0030-07 and ASFEU project 26240220011 OPVaV-2008/4.2/01-SORO.

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