Radical polymerization of polar unsaturated monomers in direct microemulsion systems

Advances in Colloid and Interface Science 80 Ž1999. 85᎐149

Radical polymerization of polar unsaturated monomers in direct microemulsion systems Ignac ´ Capek Polymer Institute, Slo¨ ak Academy of Sciences, Dubra ´ ¨ ska´ cesta 9, 842 36 Bratisla¨ a, Slo¨ akia

Abstract Polymerization in orw microemulsions is a new polymerization technique which allows preparation of ultrafine latex particles within the size range 5 nm - particle radius - 50 mm. This article presents a review of the current literature in the field of radical polymerization of polar traditional monomers in direct microemulsion systems. Besides a short introduction into some kinetic aspects of polymerization in microemulsions, we mainly focus on the formation of orw microemulsions and the radical polymerization and copolymerization of alkyl Žmeth.acrylates and other polar monomers. We present kinetic data of radical polymerization of hydrophilic or polar monomers which are partly soluble in water and contribute to the surface activity of reactants. Effects of initiator, emulsifier and monomer concentration, and the type of additive are evaluated. The influence of several parameters, such as temperature, intensity of the incident light, the nature of emulsifier, monomer and initiator, etc., were also investigated. These results indicate that the nature of the emulsifier and the surface activity of monomer play a decisive role in the polymerization process. The surface active monomer dominates the formation of stable monomer microemulsion, improves the colloidal stability of final polymer microemulsion and decreases the critical amount of emulsifier in the microemulsion monomer droplets. It has been recognized that photopolymerization in the micellar systems is a very attractive way to prepare polymers displaying high molecular weights with high rates of reaction. Besides variation of kinetic, colloidal and molecular weight parameters with the reaction conditions, the type of components of reaction mixture or microemulsion, etc. are discussed. 䊚 1999 Elsevier Science B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 2. Formation of direct Žoilrwater. microemulsions . . . . . . . . . . . . . . . . . . . . . . . . 88 0001-8686r99r$ - see front matter 䊚 1999 Elsevier Science B.V. All rights reserved. P I I: S 0 0 0 1 - 8 6 8 6 Ž 9 8 . 0 0 0 5 9 - 1

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3. Kinetics of radical polymerization of conventional polar monomer in direct microemulsion system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.1 Microemulsion polymerizations of alkyl acrylates . . . . . . . . . . . . . . . . . . . . . 96 3.1.1 Homopolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.1.2 Copolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3.1.3 Reaction locus in the microemulsion system . . . . . . . . . . . . . . . . . . . . 107 3.2 Microemulsion polymerizations of alkyl methacrylates . . . . . . . . . . . . . . . . . . 109 3.2.1 Homopolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 3.2.2 Copolymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 3.3 Microemulsion polymerization of other unsaturated polar monomers . . . . . . . . 121 4. Photopolymerization in microemulsion systems . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.2 Polymerization of alkyl Žmeth.acrylates . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 4.3 Polymerization of other unsaturated monomers . . . . . . . . . . . . . . . . . . . . . . 140 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

1. Introduction Mixtures of oil, water and an emulsifier have attracted much attention in colloid science. Oil and water are essentially not miscible and coexist as a water and oil phase, each saturated with traces of the other component. Emulsifiers are fairly soluble in one or both solvents but form a true molecular solution of emulsifier molecule monomers at low concentrations only. At higher concentrations of emulsifier they aggregate into micelles. Three- or four-component mixtures containing water, oil, an emulsifier and coemulsifier form not only kinetically stable emulsions but also thermodynamically microemulsions. Microemulsions appear to be excellent media for facilitating chemical reactions. They solubilize a large number of very different compounds, they possess a large internal interface, and they form spontaneously. As a consequence a variety of chemical reactions have been studied including ester hydrolysis w1x, the formation of macrocyclic lactones w2x, nucleophilic displacement reactions w3x, the Wacker reaction w4x, photochemical reactions w5x, acid-base equilibria w6x, and a large number of enzyme-catalyzed reactions w7x. The studies on chemical reactions in microemulsion media deal mainly with the physical chemistry of the systems themselves. Reactions were studied either as probes for clarifying the physical properties of the microemulsions, or for investigating the influence of an organized reaction medium on the kinetics of the reactions. However, for performing a synthesis the concentrations of the reactants have to be much higher Žin the range of 1 mol dmy3 .. Increasing concentrations of additives cause increasing problems to control the phase behavior of the microemulsion. This is especially true if high concentrations of electrolytes, amphiphilic polymers, reactive polymers, etc. are added to a microemulsion stabilized by an ionic emulsifier. Microemulsions act as attractive media for polymerization reactions. Polymeriza-

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tion in microemulsions is a new polymerization technique which allows the preparation of ultrafine latex particles within the size range 10 nm - D - 100 nm and with narrow size distribution w8,9x. However, the formulation of polymerizable microemulsions is subject to severe constraints, due in large part to the high emulsifier level Ž; 10%. needed for achieving their thermodynamic stability. This fact, together with the requirement of high polymer contents in most applications, raises the problem of keeping specific emulsifier-coemulsifier, monomer-emulsifier and monomer-coemulsifier interactions, which are disrupted in the presence of a large amount of polymer tending to destabilize the polymer microemulsion or to produce large-sized polymer particles. While microemulsions can be used as potential media for polymerization in which large molecular-weight polymers with narrower molecular weight distribution ŽMWD. may be achieved, little has been done on the kinetics of the polymerization. This may be due in part to the difficulties in directly obtaining kinetic data using conventional analytical techniques, because of the relatively higher polymerization rate and the various complications associated with microemulsions. The microemulsion polymerization system consists of three phases: an aqueous phase Žcontaining initiator, emulsifier, coemulsifier and some amount of monomer., emulsified monomer droplets or the monomer swollen micelles and monomer swollen polymer particles. Water is a most important ingredient of the microemulsion polymerization system. It is inert and acts as the locus of initiation Žthe formation of primary and oligomeric radical., the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particles micelles to particles and the component of complex emulsifierrcoemulsifierrwater. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. The emulsifier provides sites for the particle nucleation and stabilizes growing of final polymer particles. Even though conventional emulsifiers Žanionic, cationic and non-ionic. are commonly used in microemulsion polymerization, other non-conventionals are also used which include surface active macromonomers, reactive emulsifiers, etc. Reactive emulsifiers, which are surface active emulsifiers with an unsaturated group, are chemically bound to the surface of polymer particles. This reduces the critical amount of emulsifier needed for stabilization of polymer microparticles. In addition to the mechanical actions and interfacial energy considerations which will act to reduce the degree of dispersion of an emulsion, there are other considerations which act to limit the stability of emulsions. One such factor is the phenomenon, commonly termed Oswald ripening, in which large drops are found to grow at the expense of smaller ones, results from differences in the chemical potential of molecules in small particles relative to those in large ones. Such differences arise from the fact that the pressure Žchem. potential. of material inside a drop is inversely proportional to the drop radius. The solubility of the dispersed phase may be so low that diffusion from small to large particles will be exceedingly slow. To reduce the rate of droplets growth due to Oswald ripening can be achieved by using Žco.emulsifiers which form a barrier to the passage of dispersed phase molecules into the continuous phase. For example, an important group of coemulsifiers are short-length alcohols. They have been widely researched

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and some interesting results on their behavior together with emulsifiers in water have been obtained Žsee later.. The most commonly used water-soluble initiator is potassium, ammonium or sodium salt of peroxodisulfate. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc. are also used in microemulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. The initiation of microemulsion polymerization is a two-step process. It starts in water by the primary free radicals derived from the water-soluble initiator. The second step occurs in the monomer-swollen micelles by entered oligomeric radicals. During the polymerization, in a conventional emulsion polymerization, monomer is located in the following four locations: Ž1. monomer droplets; Ž2. inactive monomer swollen micelles; Ž3. active micelles that become monomer-swollen polymer particles where polymerization occurs; and Ž4. solute monomer in an aqueous phase. Two characteristics of orw microemulsion polymerization are different from those of conventional emulsion polymerization: Ž1. no monomer droplets and no inactive micelles exist. Ž2. The system is optically transparent. If we use the oil-soluble initiator then most of the initiator exists in microemulsion droplets; thus the formation of primary radicals by decomposition of initiator occurs in the monomer droplets or monomer-swollen polymer particles. The most significant difference between emulsions Žopaque. and microemulsions Žtransparent . lies in the fact that stirring of a crude emulsion or increasing the emulsifier concentration usually improves the stability. This is not the case with microemulsions, which appear to be dependent for their formation on specific interactions among the constituent molecules. If these interactions are not realised, neither intensive stirring nor increasing the emulsifier concentration will produce a microemulsion. On the other hand, once the conditions are right, spontaneous formation occurs and little mechanical work is required. Basically, microemulsion is prepared by titration of a course emulsion by a coemulsifier Žsecond surface active substance . w10x. This article presents a review of the current literature in this field. There is a short introduction into some kinetic aspects of polymerization in emulsion and microemulsion systems and their differences. In this review we summarize and discuss the amphiphilic properties of anionic and non-ionic emulsifiers, the aggregation of amphiphilic molecules into micelles, the microemulsion formation due to the interaction or complex formation between emulsifier and coemulsifier, the effect of organized aggregation of emulsifier and coemulsifier on the polymerization process, and the kinetics of radical polymerization and copolymerization of polar unsaturated monomers in microemulsion systems. We mainly focus on the kinetic aspects of microemulsion polymerization of polar unsaturated monomers. 2. Formation of direct (oil r water) microemulsions An emulsifier is a molecule that possesses both polar and non-polar moieties, i.e. it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as

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monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration ŽCMC., they associate spontaneously to form aggregates ᎏ micelles. The formation of micelles is controlled by the chemical equilibrium between emulsifier monomers and larger micellar aggregates. At low concentrations the emulsifier dissolves as free monomers but as soon as the emulsifier concentration exceeds the CMC the monomer concentration remains roughly constant and the emulsifier aggregates into micelles. The formation of micelles is based on expressions of the free energy change Ža micellar aggregate containing i molecules of emulsifier.. The free energy change contains a contribution which accounts for the contacts remaining between emulsifier alkyl chains and water. This contribution is usually written as ␥ i a i , where ␥ i is the interfacial tension for the contact between alkyl chains and water and a i is the surface area per emulsifier at the surface of the hydrophobic core of an aggregate containing i emulsifiers. However, an interfacial tension, ␥ i , is a macroscopic quantity which usually refers to planar interfaces, whereas the micelle, core-water interface is a microscopic nature and highly curved. In aqueous solutions, at concentrations not too large with respect to the CMC, say in the range CMC to 10 CMC, ionic emulsifiers form spherical or close to spherical micelles w11x. Micelles are responsible for many of the processes such as: Ž1. enhancement of the solubility of organic compounds in water; Ž2. catalysis of many reactions; Ž3. alteration of reaction pathways, rates and equilibria; and Ž4. reaction loci for the production of polymer products, etc. The outer-core region of the micelle, commonly referred to as the palisade layer, may provide a medium of intermediate polarity that effects the energetics of transition state formation. The primary influence of micelles is to concentrate all reactants in or near the micelles. Micellar catalysis exploits the ability of surfactants that are virtually water-insoluble. When ionic surfactants are employed, polar or ionic reactants that are freely soluble in water, may also be concentrated near the micelles by electrostatic or dipole interactions w12x. The increase in reaction rates depends on a number of factors, such as the particle size and number, the location of the solubilized reactant in the micelle, etc. Non-polar compounds partition into the micelle core while more polar compounds are formed closer to the micelle-water interface. The extent of solubilization, ionic charge of micelle, and the shape of the micelle are also important factors. It was observed that the titration of a coarse emulsion by a coemulsifier leads in some cases to the formation of a transparent microemulsion. Transition from opaque emulsion to transparent solution is spontaneous and well defined. Zero or very low ␥ i obtained during the redistribution of the coemulsifier plays a major role in the spontaneous formation of microemulsions. The value of ␥ i may be temporarily lowered to zero while the coemulsifier diffused through the interface and redistributed itself between the water and oil phase. The transfer and redistribution at the interface initially lowers the ␥ i . As soon as the interface curls and droplets are formed, the ␥ i increases. During the titration of an emulsion with a coemulsifier, the system often undergoes viscosity changes before clearing. Upon addition of coemulsifier the viscosity of the fine-emulsion varies Žincreases and

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then decreases. and when the system cleared the viscosity of the microemulsion mostly increased again. The transparency of such a system is mostly ) 85%. During the addition of a coemulsifier to an emulsion, excess coemulsifier accumulates at the orw interface during the process, reducing the ␥ i . One of the important characteristics of organized emulsifier phases in waterremulsifierroil systems is their ability to swell Žor dissolve. a large amount of water and oil when the hydrophile-lipophile property of the emulsifier just balances w13x. All water and oil are swelled Ždissolved. above a certain emulsifier concentration and a single solution phase Žemulsifier phase. is obtained. The hydrophilelipophile balance ŽHLB. required to form an orw or wro microemulsion depends on the oil, as well as on the emulsifier structure. Surface activity of a solute is defined as the ability to reduce the surface tension at an interface without requiring concentrations so large that the distribution between solute and solvent is blurred. Thus, the emulsifier structure is an important factor for the formation of wro microemulsions. An emulsifier has a partly hydrophobic and partly lipophilic molecule. The organic tail has affinity for the organic phase and the polar head has affinity for the aqueous phase. A balance between these affinities dictates in which phase the emulsifier resides. Some emulsifiers need a coemulsifier in order to reside in the water phase and form an oil domain in microemulsions. Coemulsifiers, for example, such as long chain alcohols, participate in the interfacial region of the microemulsions, screen the repulsive forces between the charged emulsifier head groups and thus enhance the aggregation of the emulsifier molecules. Microemulsion formation involves: Ž1. a large increase in the interface Že.g. a droplet of radius 120 nm will disperse ; 1800 microdroplets of radius 10 nm ᎏ a 12-fold increase in the interfacial area.; and Ž2. the formation of a mixed emulsifierrcoemulsifier film Žcomplex. at the oilrwater interface, which is responsible for a very low ␥ i . Microemulsions Žmonomer swollen micellar solution, micellar emulsions, or spontaneous transparent emulsion. are dispersions of oil and water made with emulsifier and coemulsifier molecules. In many respects, they are small-scale versions of emulsions. They are homogeneous on a macroscopic scale but heterogeneous on a molecular scale. They consist of oil and water domains which are separated by emulsifier monolayers. Microemulsions are thermodynamically stable, transparent or translucent, and homogeneous systems with a particle size of about 5᎐50 nm. This may seem at first sight surprising, but it is due to the very low interfacial tensions between oil and water microdomains. Usual emulsifiers cannot lower the interfacial tensions between oil and water to such ultralow values. A coemulsifier, such as a short-chain alcohol, alkyl amine, etc. is frequently necessary. When an apolar solvent Žoil., water, a salt, and an emulsifier are blended together and allowed to equilibrate, two or more phases may appear. In many cases, almost all of the emulsifier resides in one of the phases together with various proportions of oil and water. The phase containing the bulk of the emulsifier is called a microemulsion phase. Winsor classified the microemulsion systems into three types: the orw in equilibrium with oil Žsuch as micelles., the wro in equilibrium with water Žsuch as reverse micelles., and bicontinuous Žo q w. in

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equilibrium with oil and water. These systems are also called Winsor microemulsion types I, II, and III, respectively. The Winsor type II ŽWII. system consists of three phases and is a transition stage between the two extreme systems I, and II. Formation of each of these systems depends on the conditions and composition of the phases and each type can be formed from another type by varying one or more of the parameterŽs. of the system Žsalt, coemulsifier, emulsifier structure and their concentrations .. Thus, depending upon the proportions of components and the influence of certain parameters Žionic strength, temperature, nature of the oil, emulsifier and coemulsifier, etc.., the phase diagram presents single or multiphase domains. In WI systems, orw microemulsion is in equilibrium with excess oil, while WII systems consist of wro microemulsion in equilibrium with excess water. WI and WII microemulsions are of globular form, while the WIII middle phase is thought to be bicontinuous with an interface showing a constant mean curvature. The volume fractions of oil and water were not that important and the microemulsion type and stability were determined primarily by the nature of the emulsifier. There are three corner-stones guiding practical microemulsion formulation which address this problem: Ž1. the Bancroft rule w14x, Ž2. Griffin HLB scale w15x and Ž3. Shinoda phase inversion temperature w16x. According to Bancroft, the phase in which the emulsifier is predominantly dissolved tends to be the continuous phase, water-soluble emulsifiers tend to stabilize orw emulsions, while oilsoluble surfactants stabilize wro emulsions. Griffin suggested an empirical HLB scale which characterizes the tendency of emulsifiers to form orw and wro microemulsions. Emulsifiers with low HLB values Ž; 4. tend to stabilize wro emulsions, while those with high HLB values Ž; 20. stabilize orw emulsions. The HLB approach does not take into account, however, the effects of temperature and the nature of the oil on emulsion stability. According to Shinoda and Friberg orw emulsions are stable in the Winsor I region at temperatures ; 20⬚C below the phase inversion, wro emulsions are stable above the phase inversion temperature ŽPIT. Žin the Winsor II region.. In the vicinity of the PIT point ŽWinsor III region., where oil, water and bicontinuous microemulsion phases coexist in a three-phase equilibrium, neither emulsion is stable. If the HLB shifts to hydrophilic, the amount of water swelled between emulsifier aggregates increases rapidly and that of oil decreases, and vice versa. The HLB investigations led to the existence of a wro microemulsion Ž3 - HLB - 7. followed by a phase inversion domain Ž7 - HLB - 9. and by an orw microemulsion Ž9 - HLB - 17.. Meanwhile, experimental evidence of the existence of bicontinuous Žzero and near-zero average mean curvature. structures was found, especially in cases where the microemulsions are in equilibrium with both excess oil and water w17x. Besides, the HLB required to form an orw or wro microemulsion depends not only on the emulsifier type, but also on the oil type. Surface activity of a solute is defined as the ability to reduce the surface tension at an interface without requiring concentrations so large that the distribution between solute and solvent is blurred. The formation of microemulsion type can also be understood from the emulsifier parameter ¨ra = l, where ¨ is the emulsifier hydrocarbon volume, a is the polar

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head area, and l is the length of the emulsifier tail. When the ratio ¨ra = l is larger than unity, the aggregate curvature will be toward the water. This corresponds to a situation where the oil is penetrating the emulsifier tails andror the electrostatic repulsion between the charged head group is low. When the ratio is less than unity we have a situation where the electrostatic repulsion is larger andror the oil is not penetrating the emulsifier tails: spherical micelles are formed for ¨ra = l - 1r3; cylindrical micelles for 1r3 - ¨ra = l - 1r2; bilayers, vesicles for 1r2 - ¨ra = l - 1, and inverted structures for ¨ra = l ) 1. Spontaneous microemulsion formation is a function of the selection of primary emulsifier and coemulsifier and the right procedure capable of favoring redistribution between phases. It is not only dependent on simple thermodynamic stability but also on the occurrence of kinetic conditions favorable to the dispersion of the dispersed phase into the orw system. The microemulsions are thermodynamically stable, even though their occasional path dependence properties may reflect activation energy barriers that they must overcome during their formation. The driving forces for these processes are small and may explain why the method of preparation sometimes affects their formation. Microemulsions are formed when the emulsifier and coemulsifier, in just the right ratio, produce a mixed adsorbed film that reduces the ␥ i below zero w18x. It was concluded that ␥ i must have a metastable negative value, giving a negative free energy variation responsible for spontaneous dispersion. The ␥ i , in the presence of a mixed film is given by

␥ i s ␥orw y ␲ i

Ž1.

where ␥orw is the orw interfacial tension without the film present and ␲ i is the interfacial surface pressure of the film. The emulsifier and coemulsifier, when properly selected, form a mixed film at the oilrwater interface, resulting in an interfacial pressure exceeding the initial positive interfacial tension. At equilibrium ␥ i becomes zero. The high emulsifierrwater ratio ensures that the emulsifier is undissociated. The non-ionized amphiphatic molecules separate the emulsifier ion-pairs sufficiently to prevent the repulsion between them that would otherwise occur, and indeed convert the repulsion in the monolayers into an attraction by forming complexes with emulsifier. Osmotic Ž␲ . surface pressures can be obtained spontaneously by the monolayer penetration of an alkyl alcohol with ionic surface agents such as the salts of alkyl sulfate injected into the underlying solution. If the surface pressure is held constant, or below 60 dynesrcm but above 35 dynesrcm, immediate expansion of the interfacial area takes place as the molecules of ionic emulsifier penetrate the monolayer at the airrwater interface. The analogous penetration of the mixed film by oil molecules at the oilrwater interface increasing the surface area is the basis for the formation of microemulsions. A maximum packing and a minimum ␥ i were observed when the ratio SDSrcoemulsifier Žalcohol. in the adsorbed layer tends towards unity w19x. The

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penetration of coemulsifier favors an increase in density and a lowering of ␥ i . Then the layer becomes really condensed, and more coemulsifier Žbutanol. can penetrate the interface only if the emulsifier ŽSDS. molecules leave it. But the interfacial activity of butanol is lower than that of SDS, and the interfacial tension decreases. The dependence of the mean area or ␥ i versus the mole fraction Ž n but . of coemulsifier Ž1-butanol. is described by a curve with a district minimum at n but s 0.5. Rosano et al. w20x have considered the dynamic role of the coemulsifier by lowering the ␥ i during the titration of a coarse emulsion into a transparent dispersion. It has been pointed out that during the addition of a coemulsifier to an emulsion Žorw or wro., excess coemulsifier accumulates at the orw interface during transport, reducing the value of Ž) 0. ␥ i . The emulsifier retards the coemulsifier transport, a prolonged low ␥ i helps in the formation of a large increase in the interfacial area. Eventually, ␥ i regains a positive value responsible for the resolution of the system into microemulsion droplets. The interfacial tension may be temporarily lowered to zero while the alcohol diffused through the interface and redistributed itself between the aqueous and oil phases. Stability, in turn, is not only dependent on the value of ␥ i , but mainly on the structure of the interfacial film surrounding the individual droplet. Therefore, low ␥ i appears to be required for stability to occur at any degree. For a given oilremulsifier pair, coemulsifier steric requirements determine the volume of the dispersed phase that can be stabilized. Emulsifier, coemulsifier, oil and water are four interacting variables that determine the size of the dispersed phase droplet when microemulsions are formed. Eventually ␥ i increases while the orw interface curls and droplets are formed, allowing a barrier to form at the interface and preventing coalescence. Microemulsions are isotropic and low viscosity solutions. A direct microglobule consists of a spherical organic core surrounded by a monolayer shell of amphiphilic molecules whose polar heads are in contact with the continuous aqueous medium. For example, in the microemulsions ŽSDSralcoholrtoluenerwater., the emulsifier is located at the interfaces, whereas the alcohol molecules can be found in the bulk phases. At low concentration of the dispersed phase, it is composed of identical spherical isolated droplets. At higher concentration, the structure of the system depends on the interactions between droplets. If they are repulsive, the collisions are very short and no overlapping between interfaces of colliding droplets occurs. If the interactions are attractive, the duration of collisions increases, and transient of droplets are formed. Attractive interactions are not observed in the hard-sphere droplets. Interface overlapping occurs during collisions, allowing exchanges between touching droplets. These exchanges are achieved by hopping ions or molecules. When the connectivity is achieved, a steep increase of the component exchange rate or diffusion rate is observed, which has been analyzed as a percolation process. The role of water is of great importance for the understanding of different structures in microemulsions. The decrease in the water diffusion rate when the water volume fraction is decreased can be ascribed to two factors: Ž1. the obstruc-

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tion effect due to the emulsifierroil aggregates, which hinders the diffusion path of the water molecules and Ž2. the solvatation of the charged head groups and counterions. The number of ‘bound’ water molecules is quite high and may be the result of the strongly hydrated polar groups Žions.. It is assumed that the diffusion of the hydration water is much slower than that for free water; the free water and the hydration water are in fast exchange. The amount of hydrate water per surfactant is fundamentally characteristic of lyotropic aggregates. The disorder at the emulsifierrwater interface results in increased interactions between micelles. On the basis of the experimental observations, a molecular rearrangement of the emulsifierrcoemulsifierrwater interface is proposed to quantitatively explain the increased attractive interactions leading to the viscosity maximum w21x Žsee later.. The structure of four- or five-component microemulsion systems Žemulsifierrcoemulsifierralcoholroilrwater . is dramatically dependent on the structure of coemulsifier. Changing the coemulsifier from butanol to decanol at constant feed composition changes Dwater by two orders of magnitude, while Doil remains quite constant. With a long-chain alcohol as coemulsifier, the structure is of a distinct miniemulsion droplet type. Lower alkanols Že.g. methanol, ethanol, and to a great extent propanol. are miscible with water, whereas higher alkanols starting from butanol are moderately, poor, or very poorly soluble in water, and therefore can be used as oil substitutes. In fact, their hydroxyl groups have hydrophilicity and in conjunction with ionic and non-ionic amphiphiles lower alkanols can assist the formation of microemulsions and are, therefore, categorized as coemulsifiers. Butanol and pentanol are frequently used for this purpose w22x. From the foregoing discussion it appears that the transparent systems are prepared under the following considerations: Ž1. enough emulsifier has to be present to cover the interfacial area. Ž2. The primary emulsion be as finely dispersed as possible. Ž3. A large increase in the interfacial area by addition of coemulsifier Ž1᎐2 orders in magnitude.. Ž4. Formation of a mixed emulsifierrcoemulsifier film at the orw interface. Ž5. A low value of ␥ i is a necessary step in microemulsion formation. Once ␥ i is sufficiently low Ž- 10y3 dynercm., spontaneous dispersion occurred with little or no mechanical work required. Ž6. The role of coemulsifier is to reduce the rigidity of the interfacial film, allowing the transition from a well-organized phase towards an isotropic microemulsion. Ž7. The internal interfaces are determined to be flexible and highly disorganized. Ž8. The flexible interface is an absolute requirement for maintaining some microemulsion type systems. Ž9. No distinct separation into hydrophobic and hydrophilic domains is observed. Ž10. No formation of extended aggregates. Microemulsions usually behave like Newtonian fluids; their viscosity is comparable to that of water, even at high droplet concentration, probably because of reversible droplet coalescence. Indeed the microstructure evolves constantly due to constituent exchanges. This is an important feature that strongly effects the dynamic properties of microemulsions. Bulk viscosity of a polymer solution depends on the length, weight, size, configuration and structure of polymer. In the case of emulsifier micellar solutions, the linear and non-linear viscoelastic proper-

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ties of threadlike micelles have been predicted to resemble entanglement of ordinary polymers w23x. They explained the increasing viscosity in terms of entanglement of threadlike micelles, which evolve to a network structure. The spherical micelles with no particular structuring have less influence on viscosity but the threadlike micelles forming a random loop structure do have a greater influence. For example, the macroscopic viscosity shows a strong increase above ; 20% emulsifier Žnonylphenol ŽEO. 25 -OH. concentration, which should be associated with formation of micellar clusters or other supermicellar structures w24x. However, ␶c Žthe rotational correlation time. values of the spin probe ŽSTPO: tetramethyl stearoyl pyperidin-1-oxyl radical. do not at all follow this increase, remaining very low ␶c F 1.3 = 10y1 0 s. However, the spin probe in regular, viscous surroundings, such as glycerolrwater mixtures, monitors the macroscopic viscosity, attaining values as high as ; 10y8 s. Hence, it can be concluded that in this case, the macroscopic viscosity is due to long-range structures, while the spin probe senses the local, low viscosity of the external aqueous phase, moving relatively free in the network holes. The high values of macroscopic viscosities Ž; 100 cP. indicate the formation of large andror interconnected aggregates. Macroscopic viscosity is related to the microstructures of microemulsions, but viscosity data in general are not easily interpreted w25x. In combination with conductivity measurements, viscosity can give more information about the structure of microemulsions as it was reported in w26x. Conductivity is a measure of the mobility of the charge carriers in the aqueous domains, while viscosity mirrors the interactions between microstructures. Well below the percolation threshold the viscosity of a wro microemulsion is Newtonian up to high shear rates. At the percolation threshold the viscosity steeply increases due to clustering or shape changes of the droplets. Ordered structures such as bicontinuous structures show viscoelastic effects. Besides NMR self-diffusion coefficients w27x and viscosity parameters, electrical conductivities w28x are often utilized in studies of the structure of microemulsions. In oil-emulsifier mixtures the conductivity shows quite a remarkable change over many orders of magnitude upon the addition of water. The conductivity is very low in water-free oil-emulsifier mixtures, but as small amounts of water are added, the conductivity increases due to hydration of the emulsifier molecules w29x. If more water is added closed structures, e.g. water droplets surrounded by emulsifier monolayers, are formed and a maximum conductivity is reached. With the further addition of water the conductivity can increase drastically as a result of percolation of the water droplets. For bicontinuous structures the conductivity is relatively high as the charge carriers are transported through a continuous medium in the same way as in an aqueous electrolyte solution w30x. For micellar solutions, as well as for other microheterogeneous phases, oxygen represents a relatively unique case for which an efficient radical scavenger can penetrate inside a micelle without changing significantly the micellar size, shape, polarity, etc. The partition coefficient of oxygen between SDS micelles and water is 2.9.

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3. Kinetics of radical polymerization of conventional polar monomer in direct microemulsion system 3.1. Microemulsion polymerizations of alkyl acrylates 3.1.1. Homopolymerization The coemulsifier activity of alkyl acrylates in the microemulsion polymerization was investigated with short and medium-long alkyl acrylates, such as methyl ŽMA., ethyl ŽEA., butyl ŽBA., hexyl ŽHA. and 2-ethylhexyl ŽEHA. acrylate w31x. Microemulsion polymerizations of BA and EHA reached in a short time a conversion close to 100%. On the contrary, the MA and EA polymerizations proceeded very slowly at higher conversion and the decrease in the rate was more pronounced with MA. For example, the limiting conversions 90% ŽEA. and 60% ŽMA. were reached after 2 h. The polymerization rate-conversion curve is described by a curve with a maximum at a certain conversion. The position of the maximum rate varies with the monomer type as follows: 10% Ž MA. - 20% Ž EA. - 20᎐30% Ž BA,4 ᎐ 6 . - 30% Ž HA. - 40᎐50% Ž EHA.

Ž2.

The monomerrpolymer weight ratio Ž ␻mr p . or the monomer volume fraction in particle Ž Vf . was observed to vary with the monomer type as follows:

␻mr p :0.4 Ž EHA. - 1.7 Ž BA. - 2.9 Ž EA. - 4.5 Ž MA. Vf :0.55 Ž EHA. - 0.65 Ž BA. - 0.78 Ž EA. - 0.85 Ž MA.

Ž3. Ž4.

These data indicate that the critical conversion at which monomer microemulsion droplets disappear increases from MA to EHA. The values of ␻mr p and Vf thus, vary strongly with the monomer type. The monomer saturation parameters Žthe equilibrium monomer concentration, the monomerrpolymer weight ratio, . . . . are regulated by both the solubility of the polymer in its monomer and the interfacial tension between the monomerrpolymer particle surface and the aqueous phase. The thermodynamic quality of solvent Žmonomer. decreases from MA to EHA and so does the monomer swelling ability. Thus, the richest monomer core will appear in the MA system. On the contrary the EHA-swollen micelles or the EHArpolyEHA particles are supposed to have the monomer-saturated interface. The co-emulsifier activity of monomer which increases here with the alkyl chain length Žfrom methyl to hexyl. favors the formation of micelles with the monomersaturated shell w32x. The reaction order x Ž R p vs. wAPSx x . was found to vary as follows: 0.54 Ž EHA. ) 0.51 Ž MA. ) 0.46 Ž BA. ) 0.17 Ž EA.

Ž5.

The reaction order x for MA, BA and EHA systems indicates efficient particle nucleation andror mutual termination. In the case of MA the water phase polymerization dominates the overall polymerization. The very low value of x for

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97

EA is suggested to result from the gel effect which is known to depress the dependence of the rate of polymerization on the initiator Žor other additives. concentration w33x. Both the monomer-saturated micelle Žparticle. core ŽEA does not act as an efficient co-emulsifier. and the large k p favor the gel effect. The rate per particle, however, is independent of initiator concentration. This behavior results from the very large number of micelles and the relationship Nmic 4 Np . Thus, the polymer particles can only slightly compete with monomer-containing micelles in capturing radicals. The rate of polymerization was found to increase linearly with wmonomerx for MA, BA and EHA but exponentially increases for EA. The reaction orders y Ž R p vs. wmonomerx y . were estimated to be 1.3, 1.2 and 0.9, for BA, MA and EHA, respectively. This finding indicates that the concentration of monomer at the reaction loci increases with increasing total monomer concentration, i.e. the equilibrium monomer concentration approach typical for the emulsion polymerization is not applicable for the microemulsion polymerization system. Besides, the number of microemulsion droplets is supposed to increase with increasing alkyl acrylate concentration. On the contrary, in the polymerization of EA the reaction order 1 was kept only at very low monomer concentrations and abruptly increased with monomer concentration Ž y s 2.2 at wEAx ) 0.7 mol dmy3 .. Here the preferred location of monomer within the micelle core favors the miniemulsion or emulsion polymerization approach. The particle diameter of swollen polymer particles reached its maximum at very low conversion ŽFig. 1.. In the EHA and HA runs the particle size decreased with conversion in the whole conversion range. The decrease was more pronounced with EHA. In the other two systems ŽBA and EA. the dependence of the particles vs. conversion was described by a curve with a minimum at 20% for BA and 35% conversion for EA. The minimum was much more pronounced with EA. However, the microemulsion polymerization of MA led to the formation of macroemulsion. The abrupt increase in the size of the monomerrpolymer particles at the beginning of the polymerization Žcompared to the size of the original microemulsion droplets. was experimentally confirmed by the appearance of increased turbidity. The particle size, thus, varies strongly with conversion in the EA, HA and EHA runs. Light changes in the particle size with conversion were observed in the BA microemulsion. In the range of low conversion Ž10᎐20% conversion. the particle size changes with the monomer type as follows: 200 nm Ž EHA. ) 60 nm Ž HA. ) 50 nm Ž EA. ) 46 nm Ž BA.

Ž6.

Alkyl chain length of the co-emulsifier has a strong influence on the formation of microemulsions w34x. The alcohols with the short chain-length up to six carbons form percolating microdroplets while those with longer alkyl chain length above six carbons form non-percolating hard-sphere Žminiemulsion. droplets. The alkyl acrylate or its polymer is expected to Ž1. screen the repulsive forces between the charged surfactant head groups and microemulsion particles, and Ž2. change the internal structure of the interface Žclose-packing of emulsifier and co-emulsifier..

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Fig. 1. Variation of the average particle diameter Ž D . in the microemulsion polymerization of alkyl acrylates initiated by APS with the reaction time and the alkyl acrylate type. Recipe: 100 g water, 10 g monomer, 20 g SDS, 0.025 g NaHCO3 , 0.023 g APS, temp. 60⬚C. Monomer type: EA Ž䢇., BA ŽB., HA Ž^., EHA Ž`..

The coemulsifier behavior of alkyl methacrylate is confirmed by the increase of the amount of monomer that can be incorporated, for a given SDS concentration, when the molar ratio of alkyl acrylate per SDS is increased. The spherical BArSDS micelles with no particular structuring have no influence on viscosity. However, the formation of long-chained oligomers in the interface can lead to the formation of random loop-structured micelles which should have a greater influence on viscosity. Indeed, the dynamic viscosity Ž␩rcP. reached the maximum value after the nucleation of polymer particles: BA: Ž ␩rc P . r%con¨ .:2.5r0, 2.9r10, 2.8r19, 2.5r54, 2.0r98

Ž7.

EHA: Ž ␩rc P . %con¨ .:2.8r0, 3.1r14, 3.0r21, 2.7r47, 2.0r97

Ž8.

A co-emulsifier, butyl acrylate, participates in the interfacial region of the oil in water microemulsion, partly screening the repulsive forces between the charged surfactant head groups. A co-emulsifier, EHA, which has a longer alkyl chain, screens the repulsive forces more between the charged surfactant head groups, and thus enhances the aggregation of the surfactant molecules which leads to the formation of larger aggregates and stronger interactions Ža larger dynamic viscosity.. It was found that the rate of three-component microemulsion polymerization of BA ŽSDSrBArwater. initiated with APS is ; 2᎐3 times that with AIBN and 8᎐9 times that with DBP w35,36x:

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

R p ,APS ) R p ,AIBN G R p ,DBP

99

Ž9.

However, the rate of initiation in water with APS is ; 17 times that with DBPw . The estimated R i and observed R p disfavor the dominant role of the water-phase polymerization. Besides, the concentration of DBP in water is of the order 10y6 or 10y7 mol dmy3. Under such conditions ŽwAPSx s 10y6 mol dmy3 . the polymerization does not start. Thus, these small amounts of initiator in water are not sufficient to start the polymerization or to nucleate the micelles. Thus, the rate of initiation is a complex function of AIBN or DBP concentration in both phases. The rate of initiation varies with the reaction medium and initiator type as follows: R i ,APS 4 R i ,AIBN Ž in water .

and

R i ,AIBN G R i ,DBP 4 R i ,APS Ž in monomer. Ž 10 .

The overall rate of initiation is given by the sum of both events in water and monomer phase according to the partitioning of radicals between monomer and water. Thus, the initiating radicals may be formed in both phases, water and micelles Žparticles., respectively. The R p A winitiatorx x plot was used to evaluate the termination mechanism with the following values of x: 0.53 Ž AIBN. ) 0.46 Ž APS. ) 0.24 Ž DBP. .

Ž 11.

The reaction order x F 0.5 supports the instantaneous termination, i.e. the entry of oligomeric Žmobile. radical terminates the entangled growing radical. In the microemulsion systems the efficient capture of radicals by micelles delay termination which increases the reaction order above 0.4 typical for the classical emulsion polymerization w37,38x. However, the efficient chain transfer to monomerrthe exit of transferred radicals to water events influence the termination. An efficient water phase termination leads to instantaneous termination with x close to 0.5. On the contrary, the multiple entryrexit events of transferred radicals delay termination and increase the reaction order above 0.5. The correction of the reaction order on the particle number Žthe dependence of the rate per particle vs. winitiatorx x . led to the smaller reaction order x⬘: 0.29 Ž APS. - 0.32 Ž DBP. - 0.37 Ž AIBN. .

Ž 12.

The values of x⬘ close to 0.3 favor efficient termination known as the primary radical termination. This includes reactions of growing entangled radicals with entered Žmonomeric. mobile ones or agglomeration of unstable primary particles. The similar values of the reaction orders for all initiators result from the similar termination mechanism. This should result from the dominant role of the monomer transferrexit events because the difference in the initiation locus between APS and AIBN Žor DBP. would lead to the different values of x⬘. In favor of the chain transfer to monomer and participation of monomeric radicals in termination is the independence of the molecular weight on the initiator concentration and a slight

100

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

dependence of the N vs. winitiatorx z : z : 0.17 Ž APS. ) 0.06 Ž AIBN. ) y0.07 Ž DBP. .

Ž 13.

The rate vs. conversion data plot is described by a curve with a maximum at a certain conversion ŽFig. 2.. The maximum rate of polymerization was observed at 30% with APS and 20% with DBP, respectively. According to the micellar model w37,38x the decrease in the rate is attributed to the depletion of the free emulsified monomer droplets. In the emulsion polymerization of butyl acrylate the monomer droplets disappear at ; 40᎐50% conversion, and the monomerrpolymer Ž60r40. weight ratio Ž ␻mr p . in the particle is ; 1.7 Žthe equilibrium monomer concentration, wBAxeq s 4.4 mol dmy3 . w39x. The monomerrpolymer weight ratio for the BA microemulsion polymerization at 40᎐50% conversion was observed to be in the range 0.7᎐1.0. This indicates that the microemulsion polymerization of BA proceeds under the monomer-starved conditions already at medium conversions. Besides, the increase of number of large polymer particles with conversion Žat medium and high conversions. pronounces more the decrease of monomer concentration in polymer particles. Simulation of microemulsion polymerization of styrene led to the same conclusion, i.e. the concentration of monomer in particles at medium conversions is lower than the equilibrium monomer concentration in the monomer-swollen latex particles w40,41x. The decrease of the rate of polymerization beyond 20% or 30% conversion is caused by the depletion of free monomer Žemulsified droplets or monomer-swollen micelles. phase or the decrease of monomer in the polymer particles and micelles. These findings indicate that the equilibrium monomer concentration concept typical for the classical emulsion polymerization is not valid for the microemulsion polymerization. The values of the overall Žapparent. activation energy Eo,a Žat ; 20% conversion. are found to differ from those for the emulsion polymerization: 100 kJ moly1 Ž APS, Micr.. ) 85 kJ moly1 Ž DBP, Micr.. ) 51 kJ moly1 Ž APS, Emuls.. ) 32 kJ moly1 Ž DBP, Emuls..

Ž 14.

The Eo,a value for the microemulsion polymerization initiated by APS agrees very well with that for the solution polymerization but differs from that for the emulsion polymerization. Thus, the percolation is supposed to depress the compartmentalization of reaction loci due to which the microemulsion polymerization is governed by pseudo-bulk reaction conditions. Besides, at low and medium conversion the Eo,a value is nearly constant. The strong decrease of the Eo,a value is observed above 60 or 70% conversion. This is discussed in terms of the increased deactivation of entered radicals within the micelles containing a small fraction of free monomer. The unreacted monomer Ža coemulsifier. is mostly complexed within the monomer droplet interface due to which it shows low activity in growth events. The new particles are found to be generated throughout the microemulsion

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

101

Fig. 2. Variation of the rate of polymerization in the microemulsion polymerization of butyl acrylate with conversion and APS concentration. Recipe: 100 g water, 10 g BA, 20 g SDS, 0.025 g NaHCO3 , temp. 60⬚C. wAPSx = 10 5 rŽmol dmy3 .: 1000 Ž`., 500 ŽI., 100 Ž䉫., 60 Žq., 5 Ž)..

polymerization of BA w42x. This behavior results from much higher concentration of emulsifier required in microemulsion polymerization andror much higher concentration of monomer-swollen micelles. Besides the small fraction of monomer-swollen micelles is nucleated or used for the stabilization of polymer particles. The monomer-containing micelles, thus, strongly compete with the polymer particles in capturing radicals and formation of polymer in primary particles Žwith D ; 5 nm ᎏ nucleated monomer-swollen micelles.. The reaction order x from R p A wSDSx x was found to be y0.96 and y0.66 for APS and DBP, respectively. The negative reaction order x strongly deviates from the micellar model case 2 Ž x s 0.6 w37,38x.. In monomer microemulsions, the hydrocarbon tails of DSS make up an appreciable fraction of the droplet volume so that a true core solubilized monomer is suppressed. It was found that more than 60% of the coemulsifier Žhere BA. reside in the interface w43x. An emulsifierrcoemulsifier Žmonomer. interface volume Ža mixed film., thus, contains the largest fraction of monomer present in the system. The volume fraction, ␾ h t , of the hydrocarbon tails of SDS in the monomer-swollen micellar phase was estimated using 2.11 = 10y4 m3rmol and 1.12 = 10y4 m3rmol as the molar volumes of a C 12 hydrocarbon chain and BA, respectively w44,45x to vary with the wSDSx Žwith wBAx s 0.78 mol dmy3 . as 0.5 Ž 0.42 mol dmy3 . - 0.52 Ž 0.45. - 0.56 Ž 0.52. - 0.57 Ž 0.55. - 0.64 Ž 0.69. .

Ž 15 .

The value in the brackets denotes the wSDSx. These results indicate that the volume fraction of monomer parallels the rate of polymerization. Thus, the

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decrease in R p may be ascribed to the decrease of monomer concentration in the vicinity of the reaction loci. The concentration of monomer in droplet cores is equal to the bulk monomer system whereas in the interface area is much lower due to the presence of other constituent molecules Že.g. the hydrocarbon tails of emulsifier which dilute monomer.. If the reaction loci are located in the droplet core the rate of polymerization is high and comparable with the emulsion system. In other cases when the interface zone is the reaction medium the rate of polymerization is lower due to the lower concentration of monomer Ža dilution approach.. The strong deviation from the micellar model case 2 Ž y s 1. shows the relationship R p A N y. The experimental data show that the rate of polymerization decreases with increasing particle concentration, i.e. y is ; y0.6 for APS and y0.53 for DBP, respectively. The dependence of the rate per particle Ž R p p . vs. wSDSx z , z is y1.4 for APS and y1.0 for DBP, respectively. The rate per particle is larger for the APS system and decreases in both systems with increasing wSDSx, i.e. the decrease in the R p p results from the decrease of both the monomer concentration and the radical concentration in particles, as follows: the number of radicals per particle, Q, were estimated from the experimental data of R p , N and wBAxeq , and the literature value of k p Ž1360 dm3 moly1 sy1 . w46x for different wSDSx using the Smith-Ewart rate equation w37x. The Q value is close to 0.5 in the systems with the smallest wSDSx and abruptly decreases with increasing wSDSx up to 0.1. The low value of Q much below 0.5 can be attributed to the high desorption rate of monomeric radicals generated by the transfer reaction to monomer and emulsifier inside the particles or micelles w47x. The exit of transferred radicals from particles is very effective in the microparticles due the fact that the radical desorption is inversely proportional to the particle size. The particles with diameter below 20 nm, however, were not detected by LS measurements. This indicates that the primary particles are very unstable and coagulate between themselves or with large Žstable. particles. The abrupt increase in the size of the polymer particles after the start of polymerization is attributed to strong collision of primary particles with monomer-swollen micelles w48x. The transparent microemulsions disfavor the idea that the increase in particle size is caused by the nucleation of large monomer droplets. However, the accumulation of polymer chains in micelles at the beginning of polymerization destabilizes the microemulsion which leads to the strong agglomeration of primary particles and micelles. This is the case with microemulsions, which appear to be dependent for their formation on specific interactions between emulsifier, co-emulsifier, water and monomer w43x. A change in the structure of microemulsion systems orrand the nature of oil-phase may turn the microemulsion to mini- or macroemulsion. The fate of desorbed monomeric or emulsifier radicals in the aqueous phase with APS was evaluated by considering the following kinetic parameters w40,41x: taq ' 0.02 s; t prop s 0.07 s; and t ter s 10 s

Ž 16 .

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103

By comparing the values of taq , t prop and t ter , it was concluded that the most probable process of primary radical or a desorbed radical is an entry or re-entry into the micelles. The microemulsion polymerization of BA proceeds quantitatively in the runs with wBAx ) 0.78 mol dmy3 , whereas with wBAx - 0.4 mol dmy3 the limiting conversion appears w49x. Variation in the final conversion with the weight ratios monomerrwater and monomerremulsifier, respectively, is expressed as Žwith APS.: 100% conv. Ž 0.1, 0.5. ) 92% conv. Ž 0.064, 0.32. ) 84% conv. Ž 0.026, 0.13.

Ž 17.

The first value in brackets denotes the weight ratio monomerrwater and the latter the weight ratio monomerremulsifier. The final conversion, thus, increases with the weight ratios monomerrwater and monomerremulsifier, respectively. The increase in the weight ratio monomerrwater from 0.026 to 0.1 may slightly influence the water-phase polymerization Žthe formation of oligomers and their nature . but not the growth events in particles. The small weight ratios monomerremulsifier orrand monomerrinitiator seem to disfavor the growth events. This can be explained by the dead-end polymerization Žthe monomer is complexed in the interface area. w50x, the chain transfer to emulsifier Žformation of less reactive radicals. w51x and the close-packing of BA Žcoemulsifier. with SDS in the micelle interface. The reaction order x from R p A wBAx x was found to be 1.8 and 1.2 for BA and BArBPR Žbutyl propionate ŽBPR. ᎏ non-polymerizing monomer. with DBP, respectively. The values of the reaction order x ) 1 is supposed to result from the increase of both the monomer concentration and the number of particles. Thus, the fraction of effective monomer concentration Žminus the incorporated in the middle interface . increases with increasing total monomer concentration. In favor is the relationship R p p A wMx z where exponent z varies in the range from 2᎐3, respectively. The BA molecules have three solubilization sites within microemulsions. These sites are the hydrocarbon Žmonomer. micelle core, the droplet Žmicelle. surface and the continuous phase. At low BA contents, the hydrocarbon tails of SDS make up an appreciable fraction of the droplet volume so that a true core solubilized monomer is unlikely. With increasing wBAx, the droplets contain more monomer and the fraction of the droplet volume occupied by the tails is proportionally less so that a core of solubilized monomer could be postulated. The size of polymer particles was found to increase with the monomer concentration. For example, with increasing wBAx from 0.3 to 1.0 mol dmy3 increased D from ; 25 to 60 nm, respectively. This finding indicates that the particle growth Žcaused by aggregation of sticky BArPBA particles and droplets andror by polymerization of absorbing monomer. is proportional to the wBAx. The formation of larger particles, however, is more pronounced in the BA runs. The presence of BPR, thus, somehow depresses the particle growth. The molecular weights were found to increase with increasing wBAx or with the ratio wBAxrwSDSx. For example, with increasing wBAx from 0.4 to 0.8 mol dmy3 increased M v = 10 5 from 4 to 1.4, respectively. The reaction order z Ž M v A wBAx z . is found to be 0.9 and 0.7 for APS and DBP, respectively.

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3.1.2. Copolymerization The copolymer microlatexes prepared by the microemulsion copolymerization of BA and acrylonitrile ŽAN. were stable and bluish, but less translucent than the monomer microemulsions due to the larger size of the polymer particles Žthe particle diameter is ; 40 nm. and the higher refractive index of the ANrBA copolymer w52x. Thus, the monomer microemulsions become increasingly turbid as the polymerization proceeds. No phase separation is observed during the copolymerization with the molar fraction n BA G 0.2. The ANrBA copolymer microemulsion latexes were less translucent than the BA polymer latexes probably due to the larger particles. In all APS runs the final conversion close to 100% was reached. On the contrary, the limiting conversion ; 90% was reached in the systems with DBP Žthe half life of DBP is ; 70 h at 60⬚C. w53x. The limiting conversion was ascribed to the glass transition temperature approach or the phase separation due to which the initiator molecules become immobilized in polymer matrix. Under such conditions the cage effect and the dead-end polymerization are operative. The maximum rate with APS or DBP is located at 15% or 20% conversion, respectively. In the microemulsion polymerization of BA R p,max is at 30᎐40% conversion. Thus, the addition of AN shifts the rate maximum to the lower conversion. Thus, the disruption of a barrier to radical entry into monomer droplets is favored by the formation of a larger number of oligomeric radicals Ža water phase polymerization of AN.. The increased radical entry rate shifts the gel effect to lower conversion. The polymerization initiated with APS is much faster than with DBP. The increase in the rate with conversion is attributed to the increase in the particle concentration and the gel effect. The decrease of the rate of polymerization or the rate per particle beyond ; 20% is due to the strong decrease of the monomer concentration in the particles. The maximum rates of copolymerization of BA and AN or the rate per particle as a function of initiator type and concentration obey the following equations: R p ,max A w APSx

0.48

, R p ,max A w DBPx

0.65

, R p p A w APSx

0.05

and

R p p A w DBPx

0.11

.

Ž 18 . The high water-solubility of AN, the water-phase polymerization and radical entryrexit events are assumed to be responsible for the deviation from the value 0.4 typical for the classical emulsion polymerization of hydrophobic monomers w37,38x. The exponent 0.65 indicates that the unimolecular termination may be operative. This behavior may be discussed also in terms of high radical entry rate into micelles and delayed termination of exited radicals. The slight dependence of the rate per particle on winitiatorx is assumed to result from the very low radical entry efficiency into particles. The increasing amount of BA in the comonomer mixture of AN to the BA increases the rate of polymerization and the decrease is more pronounced in the runs with DBP:

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

R p ,max A Ž n BA .

1.0

for APS and R p ,max A Ž n BA .

1.15

for DBP

105

Ž 19.

where n BA is the mole fraction of BA in the comonomer mixture. The values of k p Ž k p,BA s 1360 dm3 moly1 sy1 , k p,AN s 5000 dm3 moly1 sy1, w46,51,53x. for BA and AN, however, should favor the opposite trend. For example, the oligomer radicals formed in water which enter the micelles andror precipitate from water Žthe homogeneous nucleation and the formation of occluded radical.. The importance of the water-phase polymerization came from Guo’s approach w40,41x suggesting that the desorbed monomeric ŽAN. radicals take part in the water-phase propagation, i.e. taq s 0.02 s, t prop s 0.0002 s, and t ter s 10 s. The t prop s 0.07 s for the BA microemulsion polymerization was estimated. The addition of AN strongly decreased the t prop to 0.0002 s. This finding indicates that the water-phase polymerization strongly influences the polymerization process and the nucleation proceeds by the homogeneous mechanism. The particle size is found to increase with increasing conversion. However, the dependence of N versus conversion in the microemulsion ANrBA copolymerization is described by a curve with a maximum at 70% conversion for APS and at 50% for DBP, respectively. The decrease of the copolymer particle concentration with increasing conversion in the high conversion range is caused by the agglomeration of polymer particles as a result of adsorption of PAN oligomers on the particle surface due to which the surface particle morphology changes. The polymerization of AN under the same reaction conditions does not lead to the formation of stable polymer latexes Žyields coagulum.. The nature of the particle surface will clearly determine the adsorption characteristics of SDS. It has been shown that emulsifier adsorption at a particlerwater interface decreases with increasing polarity of polymer w54x. The formation of coagulum and large-sized polymer particles were observed in the emulsion copolymerization of AN and BA Žthe particles mostly grow by agglomeration. w55x. This observation supports the view that the SDS molecules will well adsorb on the polyŽbutyl acrylate. BA particle surface but poorly on the polyacrylonitrile particle surface. At very high conversion the comonomer-free mixture is rich in AN w51,55x. Under such conditions the polymer chains or particle surfaces are rich in AN units. The reduced adsorption of SDS by the particles disfavor the colloidal stability of the polymer latex. This observation supports the view that the SDS molecules will well adsorb on the PBA-rich particle surface appearing at low conversion but poor on the polyacrylonitrile-rich particle surface appearing at high conversion. The dependence of Mn versus conversion Žfor both APS and DBP. is described by a curve with a maximum at ; 20% conversion. The Mn values for APS were twice as large as for DBP. The independence of Mn on the initiator concentration ŽAPS and DBP. supports the idea that termination is regulated by the chain-transferrexit events. The strong increase of Mn with increasing BA fraction in the comonomer feed Mn A Ž n BA .

0.93

for APS and Mn A Ž n BA .

1.1

for DBP

Ž 20.

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106

results from the increased contribution of growth events in polymer particles, increased number of micelles or particles and decreased chain transferrexit events. The dependence of the dynamic viscosity Ž␩ . vs. conversion is described by a curve with a maximum at ; 40% conversion. The abrupt increase in viscosity Žat ; 40% conversion. is probably caused by the interaction between the sticky monomer-swollen polymer particles themselves or with monomer-swollen micelles Žthe percolation.. A surface roughness Ž‘hairiness’ . of the monomer-swollen polymer particles is the main reason for the low particle mobilities Žor high viscosity. w56x. The surface active properties of BA and the effect of a chain-transfer agent on the polymerization process were investigated in the microemulsion polymerization and copolymerization of BA, AN, and styrene ŽSt. w57x. The addition of isopropanol ŽIPA. was found to decrease the rate of polymerization and the decrease was more pronounced in the copolymerization: y0 .2

R p ,max,BA A w IPA x 0

and

y0.65

R p ,max,BArAN A w IPA x 0

Ž 21.

This finding was attributed to the increased exit of radicals andror the transfer of reaction loci from the polymer particles into the aqueous phase. The presence of hydrophilic AN is supposed to pump IPA from water into polymer particles due to which the chain-transfer events are more pronounced in polymer particles. Besides, the exit of IPA-transferred radicals favors the water-phase termination events or decreases the average degree of polymerization of oligomeric radicals Žsuppresses the homogeneous nucleation .. Variations of the rate of polymerization with the monomer feed composition, and the IPA concentration are as follows: y1 .1

R p ,max,B A w ANx 0

y7 .5

R p ,max,D A w ANx 0

w BAx 0.24 , 0 w Stx 01.5

y3.7

R p ,max,C A w ANx 0

w BAx 0.8 0 , Ž 22.

where system A: the weight ratio ANrBA s 1.0, system B: the weight ratio ANrBA s 3.0, system C: the weight ratio ANrBA s 3.0, and the system D: the weight ratio ANrSt s 3.0. In all runs the rate of polymerization decreases with increasing AN fraction and the decrease is more pronounced in the system with the higher total monomer concentration. Besides, the negative reaction order is much larger in the copolymerization systems with St. These findings may be attributed to several contributions: Ž1. the different partition of monomers between the reaction phases; Ž2. the different chain transfer and exit events; and Ž3. the different copolymerization parameters. Besides, BA itself acts as a coemulsifier w36x and therefore BA favors the formation of a larger number of particles. The St monomer is situated mainly in the core of micelles or particles where it takes part in growth events. The AN monomer is situated in water and in the core of micelles and particles and favors polymerization in both phases.

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The number of particles was found to vary in a complex way with increasing wANx, wBAx, wStx and wIPAx. The exponents a, b, c, and d from the relationships N A wIx i for IŽ i .: wANx a, wBAx b , wStx c and wIPAx d are estimated to be: the BA system: b s 1.4, d s 0.56; system B: a s y0.41, b s 9.3, d s y3.7; system C: a s y3.7, b s 0.8; system D: a s y7.5, c s 1.5

Ž 23 .

IPA is found to increase the number of particles in BA runs while the opposite trend is found in the ANrBA runs. The copolymerization of ANrSt microemulsion does not lead to the formation of the stable polymer Žmicro.particles. This behavior was attributed to the surface inactivity of both AN and styrene. 3.1.3. Reaction locus in the microemulsion system The location of reaction loci in the microemulsion polymerization was investigated by the polymerization in the presence of radical scavenger ŽRS. and following the decay rate of stable radicals Ž4-stearoyloxy-2,2,6,6-tetramethyl piperidine-1-oxyl ŽSTPO, an oil-soluble., Ž4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl ŽHTPO, a water-soluble., and 1,1-diphenyl-2-picrylhydrazyl ŽDPPH, soluble in both phases.. in the presence of water and oil-soluble initiators w42,58x. The measure of the radical formation is the product k d . f which was followed as a function of a waterand oil-soluble stable radical and it was estimated by using the following equation ln  1 y Žw R ⭈st x o y w R ⭈st x o . r2 w I x o 4 s yk d ft s yXt

Ž 24 .

where subscripts o and t denote the initial concentration and the concentration of stable radicals at time t and f is the initiator efficiency. It was observed that the DPPH molecules Žsoluble in both water and monomer. are consumed effectively by both APS and DBP radical fragments in the BA microemulsion systems. From the decay data of DPPH using Eq. Ž24. the following k d . f ; 0.03 sy1 and 0.05 sy1 for APS and DBP, respectively, were obtained. These results show that the decay is similar in both systems because DPPH partitions between water and oil phases. The APS is assumed to be consumed in water and DBP in the monomer swollen particles. The equilibrium is kept by diffusion of DPPH from oil phase into a water phase in the case with APS. On the contrary, the consumption of DPPH in monomer phase by DBP radicals means that the water phase serves as a reservoir of DPPH. This is the reason why similar decay curves were obtained. HTPO is found to decay differently in BA and BPR microemulsions initiated by APS. The decay of HTPO is considerably faster in BPR than in BA ŽTable 1.. This difference was attributed to the organization of the system which was more pronounced with BA. In favor was the decrease of X with increasing SDS in the system. An alternative explanation may be based on the fact that the competition of monomer and HTPO radicals with initiator radicals takes place in runs with BA. As this is not the case with BPR, the decay of HTPO in the presence of BPR will

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Table 1 Interaction parameters of HTPO and STPO with different initiators RSrmonomera

Initiatorb

106 . Xc Žsy1 .

106 .kd fd Žsy1 .

HTPOrBA

APS DBP AIBN

0.85e 3.46f 1.35

2.89 7 9.6

STPOrBA

APS DBP AIBN

5.57 1.3 1.09

2.89 7 9.6

HTPOrBPR

APS DBP AIBN

3.68 3.55 1.082

2.89 7 9.6

STPOrBPR

APS DBP AIBN

2.26 0.71 1.08

2.89 7 9.6

a

RS Žinhibitor.rmonomer. Type of initiator. c The value of X determined from experimental data according to Eq. Ž24.. d The theoretical values of k d and f w52x for homogeneous systems at 60⬚C. e ESR signal is split during the reaction. f ESR signal is split during the reaction, X was determined from the data before the splitting occurred. b

be faster. Another specific feature of ESR measurements of HTPOrAPSrBA systems is the splitting of the triplet ESR spectrum after some reaction time or consumption of certain fraction of HTPO. This was attributed to the diffusion of stable radicals or their products into the interface. Besides, the entry of oligomeric radicals into micelles or particles may initiate growth events and reactions with HTPO. This includes the chain-transfer events, immobilization of HTPO molecules in polymer matrix, etc. This behavior was not observed with BPR. Since the polymerization of BA starts only after the elapse of the inhibition period when 90% of the initial concentration of HTPO was consumed, HTPO acts as the ‘ideal’ inhibitor of the polymerization. The rest of the inhibitor is deactivated by some reactions andror by its immobilization in the polymer matrix. The reaction of HTPO and STPO Žwith BA or BPR. with free radicals derived from AIBN takes place with equal rate, approximately. At the end of the inhibition period, the polymerization starts in the system containing 8% of the initial HTPO amount. The orientation of the NO group of HTPO and STPO in the micelle is assumed to govern the reaction of RS. The NO group in STPO is directed towards the water phase and the hydrophobic alkyl chains are directed to the micelle core. This may bring about a more difficult interaction with AIBN radicals when compared with HTPO where the OH group is oriented towards the water phase and the NO group towards the micelle core. The decay of HTPO in microemulsions containing BA or BPR initiated by DBP or AIBN is considerably faster than that of STPO. The values of X for STPOrBA or STPOrSTPOrBPR are lower than those for HTPO runs. This was discussed in

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the following terms: Ž1. STPO molecules are supposed to be located in the micelle interface as a coemulsifier, Ž2. HTPO radicals are mainly dissolved in water and a small fraction of HTPO molecules are located in the interface in such a way that their OH groups project to the water phase while the NO groups project to the core of the micelle. Free radicals of DBP Žformed mainly in the oil phase., therefore, will easily react with HTPO. Besides, after some reaction time, the splitting of the ESR signal occurs in the BA microemulsion polymerization. Similar to the APS initiation, the reaction by-products or immobilization of HTPO radicals may take place. Polymerization, in both systems, is faster already at the reduction of RS to 20 wt.% of the original value. However, quite different results were obtained with the surface-active STPO inhibitor. Here the oil-soluble STPO molecules react slightly with the radicals derived from the oil soluble initiator DBP. This should result from the different location of DBP and STPO radical in the systems, the low initiation efficiency andror the high germinate recombination of primary radicals. In the case of the oil-soluble initiator two main mechanisms w51x for the production of radicals are suggested; Ž1. in the monomer-swollen polymer particles formed radicals desorb to the continuous phase and Ž2. in the continuous phase formed radicals are generated from the fraction of the oil-soluble initiator dissolved in water and initiate the growth of the polymer chains ŽDBP is insoluble or sparingly soluble in water.. Besides, the kinetics of suspension polymerization in small-size particles is suppressed. The polymerization initiated by DBP or AIBN, however, was found to proceed in the presence of STPO but the polymerization is very slow, the rate is by one order in magnitude smaller than that without STPO. The polymerization behavior agrees very well with the low k d f value obtained for this system ŽTable 1.. The low radical concentration in water and interface means that the consumption of STPO by both primary radicals derived from DBP or AIBN and entering oligomeric ŽPBA. radicals is not efficient. The lower reactivity of all initiators in the presence of BPR favors the participation of entered oligomeric radicals on deactivation of STPO stable radicals within the micelle interface. The retardation effect of STPO may be attributed to the close-packing Ža complex formation. of emulsifier with co-emulsifier ŽSTPO. which decreases the concentration of free STPO molecules in the system. Due to the formation of an interfacial complex of the emulsifierrcoemulsifier at the surface of the monomer droplets the concentration of free STPO is decreased. This complex is known to make a barrier to radical entry into microemulsion droplets w59x. However, as soon as a certain amount of polymer was present in the monomer droplets the disruption of the packing of the emulsifierrcoemulsifier surface takes place leading to release of components forming the complex. The release of STPO molecules during the particle nucleation and polymerization reduces the growth events or the rate of polymerization. 3.2. Microemulsion polymerization of alkyl methacrylates 3.2.1. Homopolymerization The effect of coemulsifier Ž n-pentanol. on the microemulsion polymerization of

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MMA initiated by KPS or AIBN was investigated by Feng and Ng w60x. The four-component system was composed of MMA, SDS, pentanol, and water. The maximum Žinitial. rate of polymerization initiated by KPS Ž11.3% conversionrmin. is larger than that Ž8.1% conversionrmin. initiated by AIBN. Besides, the induction time is much longer with AIBN compared with KPS. This was explained by the fact that the water-insoluble AIBN must migrate into the oil phase through the water-oil interface to initiate the polymerization, while KPS can be dissociated readily in the continuous water phase. It was noted that at about 20 min the conversion of the AIBN-initiated polymerization exceeded the KPS initiated one. In the range of MMA concentration studied Žfrom 25% to 33%., the initial polymerization rates and final conversion are found to be dependent on the monomer concentration. When monomer concentration is further increased, the microemulsion shifts to the wro region. The reaction rate in wro microemulsion was found to decrease rapidly after initiation. Moreover, the initial rate was significantly lower than in the orw region. It is observed that in the MMA system the polymerization is initiated much faster than in the styrene system. Since the solubility of MMA in water Ž1.56 wt.% at 20⬚C. is much higher than that of styrene Ž0.031 wt.% at 25⬚C. w51x, it is suggested that MMA polymerization occurs in the continuous phase of the orw microemulsion before the initiator migrates across the emulsifier boundary into the oil phase. On the other hand, the much-less water-soluble styrene results in a lower initial rate. The molecular weights of PMMA values were found to be relatively low which contributed to the presence of coemulsifier, i.e. the chain transfer to coemulsifier and the decrease of monomer concentration at the reaction loci disfavor the growth events. The broad-molecular weight distribution Ž MwrMn varied from 2 to 4. can be taken as evidence that the chain transfer events contribute to the termination mechanism. The polymer from the AIBN-initiated system had slightly higher molecular weight compared to that of the KPS-initiated system. The estimated overall activation energy was found to be very low, ; 26.8 kJ moly1 . This seems to be somehow related to the low molecular weight polymers formed. Jayakrishnan and Shaw first demonstrated that the polymerization of MMA can be performed in direct microemulsions with single emulsifier systems Žwithout coemulsifier, three component microemulsions. w61x. They used oil soluble initiators and a mixture of Aerosol MA 80 and Pluronic L-31 for microemulsifying MMA, but also used CTAB as an emulsifier. It was found that the rate of polymerization increased with increasing initiator and emulsifier concentrations. However, the polymerization rates in the microemulsions were lower than those in the classical emulsion systems. The microemulsion system was not transparent and stable during the polymerization process. Unstable and non-spherical latex particles were observed using TEM. In addition they concluded that the adverse effect of emulsifier would result in a decreased reaction rate. The TEM observations of their microemulsion systems showed either no latex particles or extremely small, non-spherical particles. The same cationic microemulsion system Žthree-component microemulsion. was applied by Ferrick et al. w62x. Microemulsion initiated by AIBN produced stable polymer microemulsions.

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Ternary systems Žwithout coemulsifier. containing water, MMA, and cetyltrimethylammonium bromide ŽCTAB. could be continuously changed from turbid emulsions to transparent microemulsions by merely increasing the emulsifier concentration w63x. The viscosities of both series A Ž; 5 wt.% MMA. and B Ž; 9 wt.% MMA. increased very slowly from about 1 to 3 cPs at 30⬚C as the CTAB concentration increased from 1 to 8 wt.%. But the viscosity increased quite rapidly on further increasing the CTAB concentration Ž8 cPs for 10 and 13 wt.% of CTAB for A and B series, respectively.. The polymerization of such microemulsions led to formation of stable microlatexes. The strong dependence of R p on KPS concentration Ž0.82. for the emulsion polymerization ŽCTAB - 5 wt.%. was taken as evidence of the homogeneous nucleation of MMA in the aqueous phase as well as the initiation in emulsified monomer droplets. This is in contrast with the low power dependency Ž0.33. of R p on wKPSx for the microemulsion polymerization ŽA, 7 wt.% CTAB. which contained no excess oil droplets. The low dependency of R p on wCTABx 0.31 was observed for emulsion polymerizations in contrast with wCTABx 0.58 for microemulsion polymerizations. The low CTAB concentration dependency Ž0.31. was attributed to the homogeneous nucleation process. In other words, anion-radicals produced by KPS might react with some MMA molecules in the aqueous phase to form surface-active compounds of anion radicals. On the contrary, in the microemulsion polymerization the micellar mechanism is operative with high radical entry efficiency. Microemulsions of MMA produce stable microlatexes of particles ranging from 16 to 32 nm in R h . The particle sizes for series B Ž R h s 25᎐32 nm. were generally larger than those of series A Ž R h s 16᎐25 nm. due to higher MMA concentrations in the former systems. In series A, Np increased from about 9 = 10 14 rml of an emulsion containing 1 wt.% CTAB to about 7 = 10 15 rml for a 9 wt.% CTAB microemulsion. At a given CTAB concentration, Np was much smaller for series B than that of series A. High molecular weights of PMMA Ž5᎐7 = 10 6 . were obtained. They did not seem to be significantly affected by the wCTABx or winitiatorx. Each latex particle containing one to two polymer chains was obtained from microemulsion polymerization Žseries A. while it was about four from emulsion polymerization at the same MMA concentration. The overall activation energies Ž Eo . were about the same for emulsion Ž81 kJ moly1 . and microemulsion Ž85 kJ moly1 . as obtained from the Arrhenius plot. The cationic emulsifiers ŽCTAB and DDAB, or their mixtures. were found to be efficient for the preparation of stable polyMMA microlatexes w64x. The polymerization of the ternary ŽDTABrMMArwater or DDABrMMArwater. microemulsion was initiated by both oil- and water-soluble initiators. A mixture of DTAB and DDAB Ždidodecyldimethylammonium bromide. emulsifiers was found to be very efficient for preparation of microlatexes. They claim that the initiation of polymerization proceeds in the aqueous phase independent of the type of initiator. A pronounced induction or nucleation period was taken as a support for this statement. The cationic emulsifier DTAB Ždodecyltrimethylammonium bromide. was used

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

112

in the three-component microemulsion polymerization of butyl methacrylate ŽBMA, a monomer with low solubility in water. w65x. The polymerization was followed as a function of monomer and emulsifier in parent microemulsions, type and concentration of initiator, and temperature. Transparent one-phase microemulsions of BMA, water, and DTAB were found at 25 and 60⬚C at the water-rich corner of the phase diagram Ž6 wt.% BMA, DTABrwater ratio s 15r85, . . . .. The extent of the microemulsion region is larger than that for the DTABrwaterrstyrene system w66x. This was ascribed to the more amphiphilic character of BMA which may behave as coemulsifier. The decrease in microemulsion conductivity upon increasing BMA content suggests micellar growth. Transparent microemulsions become increasingly turbid as the reaction proceeds because of particle growth and the increase in the refractive index difference between the particles and the suspending medium. Final latexes range from bluish-transparent to opaque, depending on BMA and emulsifier contents, and contain spherical particles with diameters in the range from 20 to 30 nm. Two reaction rate intervals were observed: one in which the reaction rate increases monotonically and which ends at low conversions Ž- 0.2᎐0.3., followed by another in which the polymerization rate decreases steadily. The rate maximum was attributed to the combined effects of the increase in the number of particles and the decrease in the average number of free radicals per particle and of monomer concentration per particle. Reaction rates and final conversions increase with increasing emulsifier concentration: R p A w DTABx

2 .7

Ž 25.

The strong increase in R p may be attributed to the increase in micelle or particle concentration. As a result, the probability of capture of free radicals by microemulsion droplets increases and so does the reaction rate. Increasing initiator concentration yields faster polymerization rates and higher conversions, regardless of the type of initiator employed: R p A w V y 50 x R p A w KPSx

1.0

0.47

Ž 6 wt.% of BMA, DTAB r water s 15r85.

Ž 6 wt.% of BMA, DTAB r water s 15r85.

Ž 26. Ž 27.

The increasing polymerization rate with initiator concentration is a consequence of the increasing flux of free radicals, which increases the probability of radical capture by droplets or by monomer in the aqueous phase to induce formation of oligomers to produce active particles. However, polymerizations are faster and conversions are higher with V-50 than with KPS. This was explained by one ᎏ or a combination ᎏ of the following phenomena: Ž1. differences in values of the decomposition rate constant of V-50 and KPS, Ž2. electrostatic interactions between microemulsion droplets and the charged radicals, and Ž3. chain-transfer reactions to counterions. Polymerization of BMA is very rapid and faster than those reported

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

113

for styrene and MMA. This was attributed to the difference in k p and coemulsifier efficiency of monomers. As expected, reaction rates increase with increasing temperature for both initiators because the initiator decomposition rate increases with temperature. Reaction rate follows an Arrhenius behavior with temperature with activation energies of 131.4 kJ moly1 Ž31.4 kcalrmol. for V-50 and 148 kJ moly1 Ž35.4 kcalrmol. for KPS. These values are similar to those reported for the polymerization of MMA w67x and styrene in DTAB in microemulsions. They are, however, much larger than those reported for the microemulsion polymerization of MMA Žsee above.. The MWD Ž MwrMn . was reported to vary from 1.1 to 1.7 as a function of temperature, the initiator type and concentration, and BMA content. The MWD was smaller and Mw Ž; 4 = 10 6 . larger in runs with V-50. Plots of the instantaneous number MWD versus molecular weight are linear and parallel at all conversions with the following equation: P Ž M . s exp Ž yk t r , M Mrk p Mo .

Ž 28 .

where M and Mo is the molecular weight of polymer and monomer, respectively. From the slope of these plots, a value of k t r ,M rk p of 6.5 = 10y5 was estimated. Also, both the particle size and weight-average molecular weight remain constant throughout the reaction and the number of polymer chains per particle is unusually low Ž2᎐3.. The particle size increases with BMA concentration as D A w BMAx

0.12

Ž 29 .

The particle size is independent of initiator type but it decreases with initiator concentration to an exponent y0.08 for V-50 and y0.05 for KPS. Molecular weight decreases with initiator concentration Žwith an exponent of y0.15 for V-50 and y0.17 for KPS. and temperature. Particle size decreases as reaction temperature increases, with both KPS and V-50. It was found that D A Ty0 .79 for V - 50 and D A Ty0.94 for KPS

Ž 30.

A kinetic analysis of the microemulsion polymerization of hexyl methacrylate ŽHMA. in mixed dodecyltrimethylammoniumrdidodecyldimethylammonium bromide cationic microemulsion was performed and used to test mechanistic assumptions by Morgan et al. w68x. The time dependence of growing radicals in particles RU is given by U

U

U

U2

d R rdt s ␳ o q k t r C o Ž 1 y f . R y k c Raq y k t Raq

Ž 31.

where CM s C o Ž1 y f ., CM is the monomer concentration in the growing particles, C o is an initial monomer concentration in the particles at the point at which sufficient polymer has formed to absorb all available monomer, ␳ o is the rate of production of primary free radicals from initiator decomposition, k t r ,M is the rate

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

114

constant for radical transfer to monomer, k c is the pseudo-first-order rate coefficient for capture of aqueous free radicals by micelles or particles, and k t is the second-order rate constant for bimolecular termination in the aqueous phase. The equation for the radical population balance within the particles is U

U

U

U

d R rdt s k c Raq Ž Nmic y 2 R rNmic . y k t r C o Ž 1 y f . R

Ž 32 .

The authors suggested three approaches with different simplification of the reaction scheme. In the first limit Žcase 1. it was assumed that Ž1. there is no bimolecular termination, either in the aqueous phase or in the particles and that Ž2. capture of aqueous free radicals is fast; i.e. k c is large and hence Naq is negligible. Consequently all radicals generated in the aqueous phase, either those obtained from initiator decomposition or those that exit from particles, are passed immediately to a droplet or dead particle and began propagating. These assumptions amount to the statement that entry into growing particles is negligible compared to entry into droplets or dead particles, so that bimolecular termination in the particles can be neglected. The re-entry is the most likely fate of exited free radicals. The considered next Žcase 2. approach of bimolecular termination in the particles by entry of a second radical, while ignoring aqueous termination and retaining the assumption of fast capture Žlarge k c .. In the case 3, the aqueous phase termination is non-negligible, while termination in particles is ignored. In the case of a monomer such as HMA with very low aqueous solubility, this is unlikely to be a major contributor to the kinetics. There are two limits for k c ᎏ the diffusional and the propagation limit. The dependence of the maximum rate of polymerization on conversion was described by a curve with a maximum at ; 17% conversion. The dramatic decrease in the rate beyond 17% conversion was discussed in terms of strong decrease in the radical entry rate andror reduced initiation rate. The maximum rate of polymerization was found to increase linearly with increasing initiator wV-50x. The linearity of these plots  R p,max vs. wIx 0.5 and t R p,max Žthe time of R p,max . vs. wIxy0 .54 suggests a constant initiator efficiency, a situation which should only arise in the limit of 100% efficiency. Similar data were found in the microemulsion initiated by KPS. Thus, it appears that the reaction mechanism is identical for both cationic and anionic initiators. In order to determine whether termination of exited free radicals in the aqueous phase plays an important role, the case 3 rate equations were solved numerically Žfor runs with V-50.. It was observed that the reaction rate is an order of magnitude smaller than the experimental value, and the shape of the rate profile is inconsistent with the experimental results. The disagreement was taken as evidence that the aqueous phase does not play a role in this reaction, as might be expected for a monomer of low water solubility, and also that the reaction kinetics are not controlled by propagational entry. Thus, the case 1 is valid for the microemulsion polymerization of HMA. Thus, the microemulsion appears to behave as an effective radical scavenger, rapidly capturing and aggregating the initiating radicals. Very small polymer microparticles with particle diameter 18᎐30 nm were pre-

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115

pared by the microemulsion polymerization of MMA by Larpent and Tadros w69x. A mixture of non-ionic emulsifiers Žnonylphenol-modified polyŽethylene oxide.s. such as Sybperonic NP, Lutensol AP, etc. was used. Their procedure to obtain the monomeric microemulsions is based on the classical Schulman approach for microemulsification ᎏ the titration approach. Polymeric microparticles were prepared only in the case of a redox initiation at room temperature. In other thermal initiations, the pure microemulsion phase was left, and emulsion-based particles were obtained. For the MMA orw microemulsions w70x, the solubilization of polar MMA in the interface was found to be significant. This was attributed to the favorable interaction of MMA with water. The aggregation number of SDS decreases with increasing coemulsifier concentration. Similar results were found for systems consisting of pentanolrSDSrwater and butanolrSDSrbrine. The progressive decrease in surface charge of the SDS micelles upon addition of alcohol was reported w71x. The interaction of alcohol with the head groups of SDS opened up the surface region and gave a decrease in surface charge density. The incorporation of MMA in the interface was expected to influence the polymerization. The effect of polarity or surface-activity of alkyl methacrylates Žmethyl ŽMMA., ethyl ŽEMA., butyl ŽBMA. and 2-ethylhexyl ŽEHMA. initiated by APS. on the three-component microemulsion polymerization was investigated by Capek et al. w72x. The dependence of the rate versus conversion is described by a curve with a maximum at low conversion. The position of the maximum rate for EMA, BMA and EHMA is located in the conversion range 20᎐40%. The rate of polymerization was found to rise with increasing alkyl chain length of monomer. For example, the relative rate Ž R p,alkylrR p, MMA . of polymerization Žin the monomer concentration range from 0.34 mol dmy3 to 0.39 mol dmy3 . at 20% conversion increases in the following order: 1.0 Ž MMA. - 1.6 Ž EMA. - 3.4 Ž BMA. - 5.3 Ž EHMA.

Ž 33 .

A similar trend was observed with the relative rate per particle Ž R p p,rel . as a function of the monomer type Žin the monomer concentration range from 0.34 mol dmy3 to 0.39 mol dmy3 .: R p p ,rel : 1.0 Ž MMA. - 1.3 Ž EMA. - 6.0 Ž BMA. - 27.7 Ž EHMA.

Ž 34.

The rate of polymerization, the rate per particle or the relative rate per particle increase with increasing alkyl ester chain length of methacrylate monomer. This increase is attributed to the increase in the micelle number, the value of k p and to the decrease of the desorption rate of radicals. For example, the fast EHMA polymerization results from the coemulsifier activity of monomer Žhigh concentration of microdroplets. and decreased exit of monomer transferred radicals. The hydrophobic EHMA radicals are assumed to be located mostly in the polymer particle phase. Indeed, the average number of radical per particle n increased from MMA to EHMA as follows

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0.003 Ž MMA. - 0.005 Ž EMA. - 0.03 Ž BMA. - 0.17 Ž EHMA.

Ž 35.

On the contrary, the very small size of polymer particles and the hydrophilic nature of MMA and EMA monomeric radicals Žthe water solubility of MMA and EMA is ; 1.0᎐1.5 g per 100 g water w49x. support the radical exit events. The relationship R p,max vs. wAPSx was used to discuss the termination mechanism. The reaction order x on wAPSx was found to vary as follows: 0.43 Ž EHMA. - 0.45 Ž MMA. - 0.5 Ž BMA. - 0.52 Ž EMA.

Ž 36.

An increase in x Žabove 0.4. may result from the high nucleation activity of microemulsion droplets. In the microemulsion polymerization, however, the probability of the bimolecular termination reactions between the large macroradicals and small Žoligomer radicals derived from the initiator, monomeric, . . . . ones is very small, because only a very small fraction of radicals may enter the large polymer particles Ž Nmic 4 Np .. The rapid desorption of monomer transferred radicals and their bimolecular termination in the continuous phase could also explain the reaction order 0.5. Besides, the nucleation of micelles in district domain forming a bunch of primary particles and their agglomeration could also lead to instantaneous termination and the reaction order 0.5. The reaction order x s 1.7᎐2 Žfrom the plot R p vs. wmonomerx x . is supposed to be caused by inhomogeneous distribution of monomer in the micelles andror the increase in the number of polymer particles. The condensed phase Žclose packing of emulsifierrcoemulsifier . can be saturated with a certain amount of monomer. In the range of large monomer concentration monomer concentrates in the micelle core. The polymerization of surface-active monomer Žcoemulsifier. proceeds in the interface zone where the concentration of monomer is lower due to the dilution of monomer by the alkyl chains of emulsifier. On the contrary, the polymerization in the monomer micelle core proceeds under the bulk conditions due to which the rate is high Žwith the gel effect.. Besides, the increase of N with wmonomerx z favors the growth events: z r monomer: 0.24 Ž MMA. - 0.83 Ž EMA. - 1.0 Ž BMA. - 1.65 Ž EHMA.

Ž 37.

The value of z can be taken as a relative measure of the coemulsifier efficiency of monomer which increases from MMA to EHMA. A similar trend is found in the dependence N vs. winitiatorx y : y r monomer: 0.2 Ž MMA, EMA. - 0.3 Ž BMA. - 0.55 Ž EHMA.

Ž 38.

The increase in the reaction order y may be discussed in terms of increasing both the colloidal stability of primary particles and radical entry rate into microdroplets. The viscosity-average molecular weight Ž M v = 10y6 . was found to vary with the monomer type as follows: 4.8 Ž MMA. - 5.7 Ž EMA. - 6.0 Ž BMA. - 6.3 Ž EHMA.

Ž 39 .

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The molecular weight parallels the k p , the barrier to a radical entry and the number of micelles. The higher the number of micelles Žlower radical entry rate per particle. or the stronger barrier to radical entry the higher the lifetime of growing radicals. Texter et al. w73,74x have investigated the polymerization of tetrahydrofurfuryl methacrylate ŽTHFMA. in three-component microemulsions made with AOT, THFMA and water initiated by KPS. Polymerization of these microemulsions is very fast, after an initial induction period. Conversions greater than 90% are achieved within 15᎐20 min. The initially transparent microemulsions become bluish to opaque as polymerization proceeds, indicating the growth of particles. For example, the particle diameter increased from ; 13 to 20 nm by increasing conversion from 5 to 100% conversion. QUELS and TEM measurements confirmed that the particles are spherical and monodisperse and that they range in diameters from 15 to 50 nm. The polymerization rate shows the two characteristic intervals: an increasing rate interval followed by a decreasing rate interval. The maximum rate appears at ; 35᎐40% conversion. The maximum rate was observed to strongly increase with THFMA concentration, i.e. the reaction order on wTHFMAx is much above 1. This is not the case with styrene. This behavior is supposed to result from the solubilization of THFMA in the interface which increases number of reaction loci and parallels the rate of polymerization. The MWD is found to be relatively narrow throughout the entire polymerization process and slightly decreased Žfrom 1.4 to 1.1. with increasing conversion Žfrom 7 to 98%.. The Mw was observed to increase monotonically from 4.2 = 10 6 to 15.1 = 10 6 by increasing conversion from 7 to 98% conversion. The increase in the molecular weight and particle size provide conclusive evidence of particle and polymer chain growth during polymerization. Further, the apparently small particle sizes and large polymer molecular weights suggest that the polymer chains are collapsed and that only one, or at most several, macromolecules make up each particle. These findings support the hypothesis that in microemulsion polymerization, latex particles, after their formation, recruit monomers from uninitiated monomer droplets. The similarities and differences in the polymerization of MMA and iso-BMA ŽiBMA. in a ternary component system ŽMMArCTABrwater or iBMAr CTABrwater. have been investigated from a turbid emulsion to a transparent microemulsion only by increasing the emulsifier concentration w75x. The effect of the monomer concentration of a relatively polar MMA and a less polar iBMA on the kinetics was examined. The clearrturbid boundaries were established from titrations. As was illustrated, the polymerization rate-conversion curves show three distinct regions for the emulsion polymerization of MMA at relatively low CTAB concentrations. But the rate plateau region Žinterval II. disappeared in microemulsion polymerization of MMA at relatively high CTAB concentrations. In spite of the difference in the solubilities of these two monomers Ž1.5 and 0.0023 vrv% for MMA and iBMA w51x. in the aqueous phase, the rate dependencies on the emulsifier concentration of both monomers were found to be about 0.3

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for the emulsion polymerization and about 0.6 for the microemulsion polymerization with monomer concentrations higher than 5 wt.%. However, at a low monomer concentration Ž3 wt.%., different negative rate dependencies of y0.93 and y1.2 were obtained for microemulsion polymerization of MMA and iBMA, respectively. The results are discussed in terms of different particle nucleation mechanisms. The emulsion polymerization system is suggested to involve micellar, homogeneous, and fine monomer-droplet nucleations, but the principal loci of particle formation for the microemulsion system are the microemulsion droplets. The decreasing rate of microemulsion polymerization with increasing emulsifier concentration at very low monomer concentration was attributed to the monomer dilution effect due to the increasing number of micelles. When the entered oligomeric radicals begin growing in these monomer-deficient microemulsion droplets, the limiting amount of monomer in the uninitiated microemulsion droplets is siphoned to the growing particles. Thus, polymerization proceeds under monomer-starved conditions. But these monomer-starved microemulsion globules can still capture radicals without leading to a successful polymerization. Particle diameter for latexes was observed to decrease drastically from about 158 nm to 43 nm as the CTAB concentration increased from 1 to about 6 wt.%. It then decreased slowly in the microemulsion regions. For example, the dependency of Np A wCTABx 2.4 was obtained for emulsion and microemulsion of iBMA. The pronounced difference between the iBMA and MMA microemulsion polymerizations is that the former showed a strong positive dependence of Np on the CTAB concentration while the latter exhibited a negative dependence. Thus, the exponential dependencies of Np on CTAB concentration are much higher than the theoretical value of 0.6 w37,38x. The high dependence was attributed to radical desorption and continuous nucleation. Thus, owing to the easy escape of radicals from smaller sized iBMA particles, their Np dependency is much higher than that of the MMA run. The negative dependency of Np for the MMA microemulsion run is assumed to be due to the decrease of Np arising from particle coalescence. This is because MMA also functions as a coemulsifier, but iBMA does not. A negative relationship between log Mw and log wCTABx was found for both iBMA and MMA Ž; 3 wt.%. throughout the emulsion and microemulsion regions, i.e. Mw A wCTABxy0 .45. However, there was no significant effect of CTAB concentration on Mw at a higher concentration Ž; 5%. of both iBMA and MMA. The pronounced difference between the iBMA and MMA in the molecular weight parameters is that the former gave polymers with much broader MWD. Besides, the MWD decreased Že.g. from 19 to 7. with increasing wCTABx Žfrom 1 to 9 wt.%. for iBMA while it was constant for MMA Ž; 5᎐7.. 3.2.2. Copolymerization Hydroxyalkyl Žmeth.acrylates comonomers such as 2-hydroxypropyl methacrylate ŽHPMA., 2-hydroxyethyl acrylate ŽHEA., and 2-hydroxybutyl acrylate ŽHEA. have proved to act as coemulsifiers in orw microemulsion copolymerization with styrene or with a mixture of monomers w76x. The coemulsifier behavior of HEA, HBA and HPMA is confirmed by the increase of the amount of styrene that can be

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incorporated, for a given SDS concentration, when the molar ratio hydroxyester per SDS is increased. The greatest solubilization enhancement is observed with HPMA in agreement with its highest effectiveness. Furthermore, measurements of the surface tension of aqueous solutions of SDS show a decrease of the CMC when hydroxyesters are added. The decrease of the CMC value demonstrates that comicelization occurs: HEA and HBA, partly located at the interface, decrease the ionic repulsion between SDS anionic groups. The polymerization of styrenerhydroxyester microemulsions allows the synthesis of well-defined highly functionalized nanoparticles, in the 15᎐25 nm diameter range. Associations of SDS with HEA Žlow molar ratio., HBA, or HPMA lead to monodisperse suspensions of nanoparticles of ; 20 nm diameter. On the other hand, associations of CTAB or DBS Žsodium dodecylbenzene sulfonate. with HEA at high ratio hydroxyesterremulsifier give rise to broader size distributions. With a poorly water-soluble emulsifier like CTAB, the progressive precipitation of the emulsifier during the polymerization, owing to the disappearance of the hydroxyester, gives rise to polydisperse suspensions of larger particles. This approach is suitable for styrene microemulsions and also for mixtures of styrene with other monomers such as methacrylic acid ŽMAA. or vinylbenzyl chloride ŽVBC. giving rise to polyfunctionalized nanoparticles. The mechanism of microemulsion copolymerization of styrene depends on the location of the radical initiator: when the initiating system is located in the continuous aqueous phase, a simultaneous random copolymerization takes place ᎏ the polymerization rate and the final conversion are enhanced in the presence of a surface active comonomer. On the contrary, when an oil-soluble initiator is used and introduced within the monomer dispersed phase, a two-step copolymerization is observed: styrene polymerizes first Žup to 60᎐70%. and then HPMA. These results demonstrate that the relative location of the reactive species and the so-called compartmentalization effect play a major role. The mechanism obviously depends on the microenvironment and the local monomer concentrations at the region where the free radicals are produced and where the initiation takes place. For example, when the free radicals are produced within the microemulsion droplets, the local concentration of styrene at the reaction site is much higher than that of HPMA, so that the polymerization proceeds first with styrene until concentration of a surface active monomer at the reaction locus reaches a significant value. When free radicals are produced in the aqueous phase, initiation occurs in the coemulsifier-rich interfacial region so that HPMA acts as a phase transfer agent for radicals. The rate of polymerization and the shape of conversion curves were reported to depend on the monomer and initiator concentrations. The maximum rate is the initial one. The highest rate with 100% conversion within 10 min is obtained when the molar ratio R ŽH 2 O 2rascorbic acid. is lowered to 1 or when the initiator concentration is increased to 1.5 mol of H 2 O 2rmol of styrene. For high molar ratios R and lower concentrations of initiator, the yield of production of radicals is lower: the reaction rate is lowered and rapidly reaches a plateau Žthe limiting conversion, 60% and

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larger.. Accordingly, for a given R, the reaction rate and the final conversion increase when the amount of styrene in the microemulsion is increased. However, the mechanism of photoinitiation polymerization by UV light in the presence of DMPA Žoil-soluble photoinitiator, 2,2-dimethoxy-2-phenylacetophenone. is much more complex as it is presented. It depends, for example, on the reactivity of ketyl radicals, the presence of hydrogen donors, reactivity and hydrophobicity of transferred radicals Žhydrogen donor radicals., etc. The polymerization rate and the final conversion of both monomers depend on the molar ratio styrenerHPMA: when the concentration of styrene is higher than those of HPMA Žmolar ratio 1.5., the polymerization of styrene is slightly faster and reaches a higher conversion. While the opposite is observed for a molar ratio of styrenerHPMA lower than 1. The results clearly demonstrate that the microemulsion of styrene Žusing pentanol as coemulsifier. is greatly enhanced in the presence of the surface active comonomer and that the initial rate is much higher even for a lower monomer concentration. Moreover, the concentration of radical initiator required to obtain 100% conversion is much lower in the presence of HPMA. These results were interpreted by assuming that the initiation step occurs at the interface, where local HPMA concentration is expected to be high, or in the aqueous phase, where HPMA is slightly soluble. The methacryloyl-terminated polyŽethylene oxide. ŽPEO-MA. macromonomers have recently received more attention due to their amphiphilic nature. They can readily be organized to micelles w77x, and their rate of emulsion polymerization is much higher compared to that in solution w78,79x. The polymerizable MA groups of the PEO-MA macromonomers are concentrated in the monomer-swollen micelles in emulsion system to facilitate the Žco.polymerization with comonomer to produce stable and monodisperse latex particles. The long PEO chains of PEO-MA macromonomers anchored on the surface of latex particles provide a permanent steric stabilization. Liu et al. w80x prepared monodisperse polystyrene microlatexes by copolymerization of styrene and PEO-MA ŽEO.40 ŽCH 2 .11 MA4 macromonomer. This macromonomer forms micelles in water. Its CMC is 6.6 = 10y5 mol dmy3 and polymerizes very quickly in water. It is readily copolymerized with other monomers Žstyrene, etc.. in emulsion w81x. The monomer reactivity ratio of styrene Ž r St . obtained is 0.74, and the relative reactivity of the macromonomer towards polystyrene radical is 1.35. This ternary system of styrene, ŽEO.40 ŽCH 2 .11 MA macromonomer, and water can form turbid emulsions. The latex particles produced by this Žmicro.emulsion polymerization are much smaller than those produced by the dispersion polymerization of styrene in the presence of PEO-MA macromonomer due to the different polymerization mechanisms. The locus of the copolymerization is believed to occur near the surface of styrene-swollen polymer particles where the MA groups are concentrated. The latexes became stabilized when only 6 wt.% styrene and 10.5 wt.% PEO-MA were copolymerized. It was shown that only about 22% of the latex particle surface occupied by the copolymerized PEO-MA was sufficient to stabilize the latex particles. Toward the completion of the copolymerization, about 80% of the particle surface was enriched by the copolymerized PEO-MA.

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Nearly monodisperse latex particles with diameter ranging from about 30 to 90 nm were obtained by lowering the feed weight-ratio of PEO-MA to styrene from 1r2 to 1r8. When the weight ratio of the monomers ŽPEO-MArstyrene. was increased from about 0.1 to 0.5, the particle size distribution Ž DwrDn . increased slightly from 1.03 to 1.16. The particle size increases rapidly in a very early stage of polymerization, and it remains almost unchanged thereafter. The KPS concentration in the range 0.5᎐3.0 mM did not effect the particle size or the particle distribution of the latexes. By plotting the rate of polymerization at an early stage of polymerization against the KPS concentration, the relationship R p vs. wIx 0.69 is obtained. This was attributed to higher radical capture efficiency of the macromonomer composed micelles. The observed inverse effect of KPS concentration on Mw is expected because of the increasing rate termination. 3.3. Microemulsion polymerization of other unsaturated polar monomers Microemulsions of vinyl acetate ŽVAc. in three or four-component systems were kinetically studied by Donescu et al. w82᎐84x. The organic phase consisted of 30r70 monomerralcohol solutions. The aqueous solutions of nonyl phenol ethoxylated with 25 mol of ethylene oxide ŽNPŽEO. 25 -emulsifier. was used. The increase of emulsifier entails the widening of the homogeneity range. Ethanol, n-propanol, and n-butanol were used as a coemulsifier. Copolymerization was performed with di-2-ethyl-hexyl maleate. Polymerizations were initiated by a water-soluble APS or oil-soluble AIBN. It was observed that the increase of alcohol concentration decreases the dissociation of APS and the rate of initiator decomposition. The decrease of conductivity and the decomposition rate in the presence of the emulsifier is assigned to the PEO chains of the emulsifier that become strong complexing agents for the ions of the initiator w85x. The complexing causes the initiator molecule mobility to decrease, which reduces its possibility to interact with the alcohol. The dependence of the rate of polymerization Žwith APS. vs. the Žwrw q o. ratio Ž R . is described by a curve with a minimum at ; R s 65 Žo s VAcrEthanol s 30r70, w s 33 wt.% NPŽEO. 25 .. On the contrary, the rate of copolymerization Žwith AIBN. decreased linearly with increasing the ratio R. This complex behavior was discussed in terms of the complex formation between APS and emulsifier. Variations in the continuous phase with the concentration of reactants and composition of reaction systems was reported to influence both the rate of radical formation and the rate of polymerization. In the systems with APS Žwith higher content of water. the complex formation APSremulsifier vary the rate of initiation and the rate of polymerization as well. In the copolymerization runs Žwater content was lower than 30%. the rate of copolymerization decreased with decreasing amount of monomer in the reaction systems. The polymerization of VAc in three component orw microemulsions stabilized with cationic emulsifier, CTAB, was reported by Lopez et al. w86x. Initiation is achieved by thermal decomposition of a water soluble initiator ᎏ V-50. Mi-

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croemulsions are transparent and exhibit low viscosities at emulsifier concentrations up to 20 wt.%. For higher CTAB concentrations, microemulsions are transparent but highly viscous. Microemulsion polymerizations were carried out under the former conditions. At constant weight ratio emulsifierrwater, the conductivity increased with increasing VAc concentration. This behavior was attributed to the partial hydrolysis of VAc. The dependence of the rate of polymerization on conversion was described by a curve with a maximum at ; 20% conversion. The increase was ascribed to the increase of the particle concentration and the decrease above 20% conversion to the decreased concentration of monomer at the reaction loci. The dependence R p ,max A w V - 50x

0 .7

Ž 40 .

indicates strong increase of the rate with initiator concentration. This is attributed to efficient homogeneous nucleation. Similar reaction orders 0.7 and 0.64 were also found for the emulsion polymerization of VAc w87,88x. Particle size was reported to slightly increase with conversion but final particle size appears to be independent of initiator concentration Ž30᎐36 nm.. Molecular weight Ž; 4 = 10 5 . slightly increases with conversion. However, Mw is independent of initiator concentration. The MWD was observed to decrease from 2.6 to 1.8 by increasing wV-50x from 0.1 to 0.5 wt.%. Besides, the microemulsion polymerization of VAc does not yield highly branched polymers at high conversion as was found in the emulsion polymerization. In the emulsion polymerization of VAc, a Mw grows much faster leading to highly branched polymer structures and to large polydispersity ratios ŽMWD ) 20.. Information about the chain-stopping mechanism was obtained by examining the MWD using Eq. Ž28.. The slope of this plots gives a value k t r ,M rk p of 3.3 = 10y4 , which is similar to the value reported in the literature Ž1.5 = 10y4 . for VAc at 60⬚C w53x. Thus, the chain transfer to monomer appears to be the dominant mechanism of termination. The absence of highly branched polymers at high conversion indicates that the reaction locus in the microemulsion polymerization is the monomerrpolymer particle interface where the concentration of PVAc is low.

4. Photopolymerization in microemulsion systems 4.1. Introduction Micellar aggregates formed by emulsifiers containing both hydrophobic cores and hydrophilic surfaces have been used as model reactors for photophysical and photochemical reactions w89,90x. The capacity of micelles to incorporate a wide range of solutes is one of their most significant properties. In particular, the ability of micelles to vary the rate of bimolecular Žinitiation, propagation, etc.. processes is due to their capacity to concentrate or isolate the reaction components Žmonomer, initiator, additive, etc...

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Photopolymerization in the micellar systems is a very attractive way to prepare polymers displaying high molecular weights with high rates of reaction as it was exemplified by experiments in direct w91᎐93x micelles, microemulsions w94,95x, emulsions w96x or vesicles w97x. The influence of several parameters, such as temperature, intensity of the incident light, the nature of emulsifier, monomer and initiator, etc. was investigated. These results indicate that the nature of the photoinitiator plays a decisive role in the polymerization process. In this context, some attempts were made, using time-resolved laser spectroscopy, to determine the reaction locus. Besides, the effect of photoinitiator type Žhydrophilic or hydrophobic. with respect to the micro-heterogeneous structure of monomer-swollen micelles on the polymerization behavior was studied. One particular advantage of a photopolymerization is that the initiating source can be removed from the system within a few seconds. Moreover, as for chemical initiation, the lifetime of the initiating free radicals produced by UV light in the reaction medium is usually of the order of microseconds. A second advantage of the method is that it permits experiments over a wider range of temperatures that is usually impossible with any chemical initiator. A full understanding of the elementary processes in the micellar systems implies a knowledge of the location of the reactants with respect to their structure and the emulsifier aggregates. The large gradient of polarity and viscosity prevailing at the interface between the micellar and the bulk phase, are supposed to influence the reactions. The low polarity and high viscosity of the micellar region influences the photophysical processes in opposite directions, for example, low polarity decreases the fluorescence intensity while high viscosity enhances it. A small increase in fluorescence intensity of a probe Žphotoinitiator. in SDS micelles indicates that the viscosity effect of micelle was larger than the polarity effect. The longer and flexible aliphatic chain of CTAC may make the movement of probe molecules towards the micelle core easier, so that the probe molecules in CTAC micelles are in less polar and less viscous environment than in SDS micelles. The biphasic decay curve of a probe in micellar solution indicates that a probe resides in two different environments w98x. Fluorescence probes have become widely used for the study of the structure of organized Žmicellar. media w99x. Peak position, fluorescence quantum yield, fluorescence polarization, excimer formation, andror the ratio between the intensity of different vibronic bands of the fluorescence probe are sensitive to the changes in local polarity andror viscosity. Pyrene and its derivatives are the most popular fluorescence probes for sensing local polarity in direct micellar systems or microemulsions. Particularly, the ratio of intensities of the first Ž; 372 nm. and third Ž; 383 nm. vibronic bands of pyrene, I1rI3 , is a very sensitive parameter w100x. The pyrene I1rI3 ratio decreases, varying from 1.87 in water to ; 0.6 in pure aliphatic hydrocarbon. Intermediate values are obtained in micellar systems. It is assumed that pyrene is located in the interface and, probably, is displaced towards the non-polar interior when a coemulsifier is added w99x. When less than 4% Žwrw. pentanol was added to a 5% Žwrw. SDS aqueous solution, the pyrene I1rI3 ratio decreased from 1 to 0.83. When more pentanol was added Žfrom 4% to 10%., no

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further changes in polarity were sensed by the probe, indicating some kind of saturation of the pentanol in the micellar interface and probably the formation of swollen micelles, with an alcohol-containing core. For example, a relatively low value Ž I1rI3 s 0.87. was found for the system ŽME 1 . containing 0.2 M SDS and 0.6 M pentanol Ž Vfr,St s 0.02, volume fraction of styrene., smaller than that obtained in pure pentanol Ž0.98. or pure SDS micelles Ž1.1. indicating a deeper solubilization in the micellar core of the probe pyrene in the presence of pentanol. Similar results were obtained for the system 0.5 M SDS q 1.0 M pentanol Ž Vfr,St s 0.04. ŽME 2 .. In the first case small and spherical micelles are formed while in the second one large, disk-like micelles are formed. Introduction of styrene into the solution results in a progressive increase in I1rI3 . In view of the relatively high value of I1rI3 in pure styrene Ž1.07. the results indicate solubilization of styrene in the micellar interior thus influencing the pyrene spectral behavior. Similar results were also found with the same emulsifiercoemulsifier systems but toluene or butylbenzene as the solubilized oils w101᎐103x. Another factor that must be considered is that micellar solutions, and microemulsions differ in the compactness of the globular surface film Žinterface ., surface charge density, and extent of water penetration. For example, emulsifiers with smaller polar groups, such as SDS compared to CTAB, are less penetrated by water. Addition of a small amount of an alcohol Žcoemulsifier. causes also an exclusion of water and a tightening of the interface film. Changes in the relative amounts of the components also lead to changes in the size of the microdroplets which induce a different average distance between pyrene molecules and the interface w104x. Another quantity measured is the ratio I E rI3 of the excimer-band maximum and the monomer third-peak intensity which depends, of course, on the rate constant for excimer formation k E but mainly on the number of probe molecules occupying each micelle w103x. Excimer formation decreases when solvent viscosity increases, which is reflected by the excimer-to-monomer peak intensity ratio. The increase of the effective microviscosity at the pyrene solubilization site decreases the value I E rI3 . For example, in the styrene saturated polymer particles these measurements led to the conclusion that two distinct solubilization sites should be available for pyrene after polymerization: on the particle surface and in the interior of the particles. In that case only surface-active solubilized probes would have a high enough probability of excimer formation, since diffusion controlled excimer formation in the polymer interior is not feasible w105x. Ferrick et al. w106x have reported the photoprobing studies with pyrene on the nature of the hydrophobic polymerized core of the microemulsion polystyrene particles and on the rigidity of this region. When pyrene is excited with UV light, it produces a steady state emission spectrum which is very structured. A more polar environment produces a larger I1rI3 ratio. Each system Žmicelles and polystyrene particles. started with 2 = 10y2 mol dmy3 of CTAB and 1 = 10y4 mol dmy3 of pyrene Žthe I1rI3 ratio is ; 1.19., and spectra were obtained for a water dilution series of each system to 1 = 10y4 mol dmy3 of CTAB Žthe I1rI3 ratio is ; 1.66.. Dilution of the CTAB only solution resulted in a change in the pyrene I1rI3 ratio

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due to the transition from emulsifier micelles to free molecules. Once the solution was diluted below the CMC of CTAB Ž0.9 = 10y3 mol dmy3 ., there were only free molecules, the pyrene’s I1rI3 ratio in this system was similar to the I1rI3 ratio Žclose to 1.66 value. of pyrene in water. Dilution of the large Ž27.3 nm radius. particles did not display this drop in the I1rI3 ratio Ž; 1.11.; the pyrene remained in a less polar environment such as the interior of a polymer particle. This demonstrated that the particles do stay intact upon dilution. The I1rI3 ratios which were obtained for the dilution series of the small Ž5.5 nm radius. particles were in between those of the CTAB only case and the large particles. This result can be interpreted in one of two ways. Either the smaller latex particles are less stable upon dilution than the large ones or more of the pyrene is closer to the more polar surface of the particles because of their small size. The CTAB only system showed an increasing effect of quenching with increasing quencher ŽCTAB. concentration. This behavior is typical of micelle systems. The latex particle systems, on the other hand, displayed a levelling off of the effect of the quencher. In order for the pyrene to be quenched, it andror the quencher must move in the particle or micelle. Reactant movement is not hindered in the CTAB only system, but movement is restricted in the latex particle systems. In the particle systems, only pyrene in the CTAB surface layer of the particle is quenched initially, the pyrene in the interior of the particles exhibiting no quenching by the CPC Žcetylpyridinium chloride.. More pyrene fluorescence quenching was observed in the smaller particle system than the larger, and this points to the fact that smaller particles are less rigid. The smaller the particles the larger the surface particle zone. Thus, in the smaller particles the larger portion of pyrene is located in the particle shell region. Basically, the model of photopolymerization used to describe a micellar polymerization does not differ from the one which is currently reported in bulk or solution photopolymerization w107x. ŽA. Light excitation of photoinitiators Žmostly carbonyl type. leads to transient states of which the triplet is generally considered to be the photo active species. From this excited triplet two different photochemical pathways are possible: Ž1. cleavage to radicals )

A y B Ž initiator . ª Ž A y B . ª excited state

½

⭈ ⭈ A...B

radical pair

5

ª sol

⭈ Aq B⭈

single radicals

initiation ¤ monomer ¢ Ž2. chemical reaction in the excited state producing radicals; the excited state of photoinitiator undergoes one-electron photoreduction in the presence of an electron-rich compound Ži.e. alkylamines.. This reaction generates two species: a reduced form of the carbonyl and an amine-derived radical: ) C s 0 Ž qh ␯ . ª

w) C s 0x ) excited Žtriplet. state

yN s Ž alkyl amine . x

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126

 ) C s 0 ⭈⭈⭈ yN s4

)

CTC ⭈

⭈

⭈

⭈

q

) y OH q y H s  ) y O q y N s 4 ¢ pair of neutral radicals

pair of radical ions

In this process of initiation, the active radical is mostly the amine-derived radical w108x. ŽB. However, several specific differences have to be considered in the quantitative approach of the initiation efficiency: Ž1. there is a tremendous difference between the lifetimes of a macrocage formed by the aggregate and that surrounding a two-center intermediate in homogeneous solution, Ž2. the separation of the pair of free radicals is only possible if one of them crosses the interface micellerbulk phase, Ž3. a strong interaction coupled with irreversible chemical reaction takes place between the emulsifier and the initiator and Ž4. the compartmentalization of initiating radicals in the micelles. The rate of initiation R i can be explained as the product of several factors, each of them characterizing the efficiency of an elementary step of the overall process: R i s KIabs ␾ ist ␾ctc ␾rm ␾rent f trer Nmic

Ž 41.

where K is the constant, Iabs is the intensity of absorbed light, ␾ s are the yield of intersystem crossing Ž ␾ ist ., formation of the charge transfer complex Ž ␾ctc ., formation of primary or monomeric radical Ž ␾rm ., f trer is the efficiency of the chain transfer events, exit of radicals and re-entry of desorbed radicals, and Nmic is the number of micelles. 4.2. Polymerization of alkyl (meth)acrylates The relative efficiencies of water- and oil-soluble photoinitiators ᎏ benq Ž zophenone ŽBP. derivatives  BP-R, where R s H ŽBP., CH 2 SOy BPy. and 3 Na q y q CH 2 NŽCH 3 . 3 Cl ŽBP .4 ralkylamines Žtriethylamine, TEA, methyldiethanolamine, MDEA, dimethyldodecylamine, DMDA. in the photopolymerization of MMA in the micellar systems were investigated by Fouassier et al. w93,107x. The rate of polymerization was found to increase in the following order: BPq4 BPyG BP. At first sight, BPq proves to be a very efficient photoinitiator, the quantum yield of polymerization ␾m s R prIabs which represents the number of monomer molecules polymerized per absorbed photon, is estimated to be 260. This result is comparable with that for very efficient photoinitiator systems. The higher reactivity of BPq is observed in SDS and cationic emulsifiers ŽCTAC and DTAC ᎏ dodecyltrimethylammonium chloride.. The experimental results led to the conclusion that Ž1. a water-solubilizing group is not sufficient to improve the efficiency of a given photoinitiator in micellar polymerization, and Ž2. the most important factor is not the sense of the interaction between the charges carried by the substituent and by the emulsifier since BPq is observed to be more efficient both in SDS and CTAC micelles. In SDS

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micelles, chloro-substituted BP, which is located in a polar waterlike environment Žlike BPq. exhibits a photoinitiating efficiency comparable to that of BP. The binding constant of MMA to SDS micelle is 1000 My1 , which means that about 87% of MMA is incorporated in the micellar pseudophase ŽwSDSx s 0.5 M.. In CTAC, the conclusion is the same with about 80% incorporated. This means that interaction between SDS and MMA favors the location of MMA in the micelle interface and that the rest of the monomer is located in the aqueous phase. The charge transfer complex ŽCTC s BPramine. undergoes proton transfer to yield a triplet radical pair between a ketyl radical ŽK . and an amine derived radical ŽA .. The pair converts into a single radical pair and the separation of this pair leads to free radicals: 䢇

䢇

CTC ª 3 w K A x ª 1 w K A x ª K q A 䢇

䢇

䢇

䢇

䢇

䢇

Ž 42 .

In general, the exit of the ketyl radical must be as high as possible, in order to favor the initiation of polymerization. It was reported that the exit of a radical Žfrom the corresponding radical pair. depends on the nature of solute Že.g. BP, BPy, . . . ., emulsifier ŽSDS, CTAC . . . ., or counter-radical R w109x. The conclusion was the mobility of the partners is a predominant factor for the escape of the radicals from the radical pair cage. The use of a dodecyl-BP Žwhose exit of the K radical is rather hindered. leads to a low rate of polymerization. However, the superiority of the system BPqrDMDA compared with that of BPyrDMDA is undoubtedly not related to the exit ratio of the corresponding K radicals. The efficiency of the proton transfer reaction depends presumably on the nature of the two partners Žketone and amine. and could explain increase of R p for BPq and the decrease for BPy when going from MDEA to DMDA. An important chemical factor governing the reaction efficiency seems to be the reactivity of the ketyl radicals which act as terminating agents of the growing polymer chains. Their concentration depends on acid base equilibrium between the exited species and their efficiency in the termination reactions is strongly related to their chemical nature. In direct micelles formed with anionic and cationic emulsifiers, BPq appears as the most efficient photoinitiator: the derived ketyl radicals are weakly reactive, the electron transfer process with amine is the most efficient, and the monomer quenching is the less pronounced. The dependence of R p on amine concentration is described by a curve with a plateau at high amine concentration. BPq, BPy, and BP led to comparable DP values Ž; 1500᎐2000., although the corresponding R p of BPq exceeds those of BPy and BP by a factor of about 10. Upon increasing pH or ionic strength of the solution, DP has a tendency to decrease. The polydispersity of the samples remains lower than 2 in every case, although a shoulder appears in the low molecular region when the initiator is BPy. Thus, in the case of BP and BPy, the termination reactions seem to occur with a higher rate than in the presence of BPq since one could expect that the more efficient the initiation process the lower the DP. A complete survey of the initiation events in the microemulsion polymerization of MMA using typical oil- and water-soluble thioxanthane  TX-R, where R s Cl 䢇

䢇

䢇

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

128

ŽCTX., OCH 2 COOH ŽATX., and C 12 H 25 ŽDTX.4 derivatives and alkyl amines was given by Fouassier et al. w110x. The low water solubility of TXs which goes along with high binding constant to DSS micelles, causes the probe to strongly interact with the aggregates. The ATX is suggested to interact weakly with the micelles, the CTX is in the polar region of the micelles, the long alkyl chain DTX is considered as lying in a slightly less polar environment of the micelle. The addition of an amine ŽAH: MDEA, DMDA. leads to a shortening of the triplet-state lifetime and the generation of a long-lived transient. Its absorption increases when the triplet lifetime decreases, according to the usual process that consists of an electron transfer from the lone pair of the nitrogen to the carbonyl groups Žyielding a charge transfer complex, CTC., followed by a proton transfer Že.g. from the ␣ position of the nitrogen.: T1Ž ) C s O . q y N y CH 2 y ª ) C ⭈y Oy ⭈⭈⭈ y⭈q N y CH 2 y

½

<

<

5

x ⭈q

y

⭈

⭈.

⭈

N y CH y Ž A q ) C y OH <

No significant interaction between the ketyl-type radical and the monomer was observed, which suggests that the amine derivative species A is the only initiating radical. The rate of polymerization was found to increase in the following order: 䢇

ATX F CTX < DTX

Ž 43 .

In homogeneous systems, DTX and CTX exhibit very similar efficiencies, whereas the rates are very different in SDS micellar systems. In these runs ␾ ist approaches unity which suggests that the electron-transfer reaction is not the predominant factor effecting the efficiency of the initiating system. The polymerization behavior was discussed in terms of the proton transfer efficiency, the reactivity of the amine-derived radical towards the monomer, the acid-base equilibrium of the ketyl radical, the presence of ketyl radical and ketyl radical anion, the localization effects of both the TX and amine, the electronic effects of the substituents, and the micropolarity effects on the rate constants. Each of these parameters is not independent of the others, and no simple and direct correlation has been demonstrated so far between one of them and the efficiency of the system. DTX is supposed to act as a coemulsifier which favors both the colloidal stability of microparticles and formation of larger number of microdroplets. The saccharide compounds Žglucose, cellobiose, maltose, fructose, sucrose, xylose, etc.. were found to initiate the polymerization of MMA in the aqueous micellar SDS system w111x. Photopolymerization even without saccharide proceeds. However, the rate of polymerization increases by the addition of these compounds: None - glucose - maltose - fructose Ž ␭ G 254 nm.

Ž 44.

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129

At a lower wavelength of irradiation Ž ␭ G 254 nm. MMA itself is directly activated and polymerization occurs. At ␭ s 365 nm no polymerization of MMA was observed in the absence of saccharides. The values of R p can be compared with data of the emulsion polymerization of MMA photoinitiated by benzoin derivatives Ž R p ; 150 = 10 5 mol dmy3 sy1 , absorbed light ; 0.3 = 10 16 photon cmy3 sy1 at ␭ s 365 nm.. The rate of polymerization increases with increasing the incident light intensity Iinc and the concentration ŽSac. of the saccharide: 0.6 R p A Iinc

and

R p A w Sacx

0.45

Ž 45.

which deviates from the classical emulsion polymerization. The larger values of reaction orders may result from the increased nucleation rate andror the high radical capture efficiency of monomer droplets. Microemulsion polymerization of butyl acrylate ŽBA. in the presence of sodium 12-butinoyloxy-9-octadecenate ŽSBOA. emulsifier initiated by ␥-rays was kinetically investigated by Xu et al. w112x. The overall maximum rate of polymerization Žat 20᎐30% conversion. increased with increasing monomer concentration at constant emulsifier concentration and dose rate: R p ,max A w BAx

0.93

Ž 46 .

The plateau of polymerization rate Žinterval II., which was scarcely observed at a low monomer concentration ŽwBAx F 1.4 mol dmy3 ., emerged at high monomer contents. For example the system with wBAx s 2.5 mol dmy3 the interval II was located in the conversion range 20᎐50%. The reaction order 0.93 on wBAx favors the non-equilibrium Žmicroemulsion. polymerization kinetics. However, the very fast polymerization and the appearance of interval II supports the contribution of emulsion polymerization. The rate of polymerization and the molecular weight were found to strongly decrease with increasing emulsifier concentration: R p ,max A w SBOAx

y1 .07

,

Mn A w SBOAx

y1.66

Ž 47.

These findings were discussed in terms of two factors; Ž1. the emulsifier layer around the particles retards the entry of radicals into the polymer particles. The higher the emulsifier concentration the lower penetration of initiating radicals into polymer particles. Ž2. The dilution approach. The monomer is diluted by hydrocarbon chains of emulsifier. Besides, the molecular weights Ž Mn ; 3 = 10 5 . were observed to increase with increasing wBAx and the increase was more pronounced at higher monomer concentration: Mn A w BAx

0.65

Ž 48 .

R p,max and Mn were found to be proportional to the 1.27th and 0.28th powers of the dose rate ŽDR.. It was not expected that the molecular weight increased with

130

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

increasing dose rate. This was attributed to two competitive factors with opposite factors: Ž1. R p increases and Mn decreases with increasing dose rate. Ž2. The emulsifier layer around a particle becomes thinner with increasing particle number Žor the dose rate. which causes the increase of both R p and Mn . As expected, the rate of polymerization increased and the molecular weight of PBA decreased with increasing temperature. Arrhenius plots of log R p and log Mn vs. 1rT were used to estimate the overall apparent activation energies 14 kJ moly1 and y7.8 kJ moly1 , respectively. Gratzel et al. w113x have performed the photoinduced polymerizations of cetyltrimethylammonium peroxodisulfate ŽCTAPS. containing monomeric microemulsions. Monomers employed comprise MMA, styrene ŽSt., divinylbenzene ŽDVB., acrylamide ŽAAm., and acrolein ŽAcrol.. Experiments for this study were aimed at producing translucent orw microemulsion systems which are capable of being polymerized, employing the monomers and initiators used in the polymerization as integral components of the microemulsions themselves. Irradiation by visible light in the presence of a sensitizer, or by UV light without a dye, of degassed solutions, formed polymeric species of various types. Highly efficient polymerization was found to occur under visible light by using RuŽbpy. 32q. Polymer microemulsions with a high percentage of water Ž65᎐80% by weight. using emulsifier CTAC Ž; 5 wt.%., 1-pentanol Ž; 10 wt.%., hexadecane Ž; 2.5 wt.%., CTACrCTAPS s 30 by weight and monomer Ž; 1.7 wt.%. were prepared. StrDVB monomer-solubilized microemulsions exhibited no polymerization when irradiated with monochromatic visible light Ž ␭ s 435 nm. in the presence of sensitizer. For the system AcrolrDVB, although the polymerization started by monochromatic light the inhibition period was twice as long as when irradiated by polychromatic light Ž20 min by monochromatic radiation; 10 min by broad spectrum light.. The AcrolrMMA system demonstrated no preference in its polymerization whether initiation be via monochromatic or polychromatic visible light. Apparently, the rate of production of radicals has to exceed a threshold value in order for the polymerization reaction to proceed, the number of initiating loci necessary for propagation changing with each system. In all three systems the most rapid and complete conversion occurs in the presence of RuŽbpy. 32q, CTAPS, and light ) 435 nm. Photopolymerization also takes place using eosin as sensitizer ŽS 2 O 82y as initiator. and h␥ ) 500 nm. Ultraviolet irradiations again with S 2 O 82y as initiator but with no lead likewise to polymerization but generally with less conversion. However, in the case of AcrolrDVB, UV irradiation leads to a slightly shorter inhibition period with a rate and conversion almost identical with that of the polymerization performed with eosin as sensitizer. In all samples essentially no polymerization was witnessed at room temperature without initiator andror sensitizer Žvisible irradiation. andror light. The thermal polymerization Ž60⬚C. gave an initial particle diameter distribution similar to that produced in the photopolymerization. Under the most favorable conditions, i.e. RuŽbpy. 32q as sensitizer, CTAPS as initiator, and h␥ ) 435 nm, the StrDVB system had an inhibition period of 20 min and a rate of polymerization corresponding to a slope of 11.62 mVrmin, the

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

131

AcrolrMMA system an inhibition period of 0 min and a slope of 25 mVrmin, and the AcrolrDVB system an inhibition period of 13 min and a slope of 37.6 mVrmin. QUELS measurements at various angles were performed on the three systems StrDVB, AcrolrMMA, and AcrolrDVB as a function of irradiation. In all three systems, the distribution of polymer particle diameters prior to irradiation falls between 1᎐100 nm. In the StrDVB system, after the initial irradiation period of 25 min, the distribution of small particle sizes is displaced by a factor of 10 relative to the pre-irradiation diameters, and there is an increase in the number of particles with diameter in the several hundred to 1000 nm range. After 55 min of irradiation the maximum in the diameter distribution is situated at ; 2000 nm. As was shown by the distribution curves at the end of the polymerization the range of particle diameters corresponds to that observed in EM measurements. The fixed geometry of S 2 O 82y allows rapid photoelectron transfer when an appropriate sensitizer ŽRuŽbpy. 32q or eosin. is excited by visible light. This photoinduced charge transfer is followed by facile initiation unencumbered by necessary diffusion through the aqueous continuous phase. The photolysis of the BA microemulsion was observed to initiate the polymerization w114᎐117x. The photopolymerization of BA was found to occur according to a radical mechanism, i.e. in the presence of oxygen or a radical scavenger the polymerization did not start. It was observed that a direct photolysis of the BA monomer Žin bulk or solution. or SDSrBA in solution did not lead to the formation of polymer. The polymerization, however, occurred in the microemulsion systems. Thus, the initiating radicals were generated by the photolysis of the monomer-swollen micelles. The impurities within the emulsifier or BA Žtraces of stabilizer. were found to decrease the rate of polymerization and not initiate the growth events. The conversion curves of the microemulsion polymerization of BA photoinitiated by UV light took on a shape more similar to that for ‘dead-end’ polymerization. The gel effect typical for emulsion systems seems to be operative at very low conversion. The conversion curve of the photopolymerization of BA differed from the thermally initiated polymerization of BA in both the shape and the final conversion ŽFig. 3.. The rate of photopolymerization increased abruptly with conversion and reached a distinct maximum at conversion below 10%. The increase was less pronounced in the thermally initiated polymerization where the maximum rate was observed at ; 20᎐25% conversion w35,36x and the rate varied slightly with conversion from 10% to 30%. The more significant difference was found in the values of the final conversions. The polymerization with APS Žafter ; 3 h. reached the total conversion, with AIBN ; 90% and with illumination only 80%. Table 2 shows that the final conversion Žthe limiting conversion. is much below 100% conversion. The polymerization in the high conversion range, thus, was found to be a function of the mole ratio wBAxrwSDSx. In the systems with the mole ratio wBAxrwSDSx ) 2.25, the largest polymer particles were formed Ž D ; 70 nm, the reaction system is translucent . and the polymerization was fast. The rate versus conversion dependence showed two distinct non-stationary regions.

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

132

Fig. 3. Variation of monomer conversion in the microemulsion polymerization of BA Žthermally. initiated by APS Ž^. and AIBN Ž䢇., and photoinitiated by a UV light Ž'. with the BA and SDS concentration. Recipe: 100 g water, 10 g BA, 20 g SDS and 0.025 g NaHCO3 . wAIBNx s 1 = 10y4 mol dmy3 Ž60⬚C., wAPSx s 5 = 10y5 mol dmy3 Ž60⬚C. and Iinc s 1.56 = 10y5 einstein dmy3 sy1 , 23⬚C.

The limiting Žfinal. conversion was found to increase with the mole ratio wBAxrwSDSx. This indicates that the certain fraction of ‘immobilized’ monomer ŽTable 2, column wBAx free . does not take part in initiation andror growth events in polymer particles. Thus, the rate of polymerization should follow the equation x

R p A w BAx effective

Ž 49.

Table 2 Variation of the kinetic and molecular weight parameters with the BA concentration a wBAx Žmol dm y3 .

Conv. Ž%. b1

b2

c1

0.78 0.468 0.312 0.156 0.078

10 10 10 8 8

10 10 10 10 7

77 52 46 40 30

wBAxfree Žmol dmy3 .

w␩ x

c2

1

2

1

2

1

2

77 66 63 48 40

0.18 0.19 0.17 0.09 0.06

0.18 0.16 0.12 0.08 0.05

4.89 3.5 2.82 2.0 ᎏ

4.89 5.26 5.2 2.8 1.9

2.85 1.83 1.37 0.87 ᎏ

2.85 3.17 3.18 1.45 0.81

Io s 1.56 = 10y5 einstein dmy3 sy1 . Žb. Conversion at R p,max . Žc. Final conversion at 3 h. Ž1. wSDSx s 0.693 mol dmy3 . Ž2. wBAxrwSDSx s 1.12. a

Mv .106

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

133

where wBAxeffective denotes the concentration of monomer charged minus the concentrations of monomer in the micelles and polymer particles Žburied within micelles and polymer particles.. Consequently, the amount of monomer available for the formation of initiating radicals should be very low at high conversions. The reaction order x from R p,max A wBAx x was found to be 1.2 at 10% or 1.5 at 20% conversion, respectively. The reaction order x may be discussed in terms of several contributions; Ž1. the monomer itself takes part in initiation: ␾ isc 3

h␥ 1

Ž BA. o ª Ž BA.U ª Ž BA.U

SDS Žmicelles .

ª

initiation

Ž 50.

where o denotes the ground state, 1 the singlet and 3 the triplet state, so that the quantum yield of the initiation ␾ i

␾ i A w BA) x or w SDS r BA) x

Ž 51 .

Consequently x

R p A w BAx , where x ) 1

Ž 52 .

Ž2. The effective monomer concentration increases with increasing the mole ratio wBAxrwSDSx and Ž3. the increase of emulsifier monomer droplet concentration. The gel effect is not ruled out. The quantum yield of ␾m of the polymerization lies in the range 10᎐100 for the BA concentrations varied from 0.08 to 0.78 mol dmy3 . The initiation efficiency, thus, increased with increasing wBAx Žin the microemulsion range.. The reaction order a from R A Iinc was found to be 0.6᎐0.7. The reaction order a ) 0.5 indicates that the first-order radical loss process is operative. In support is the linear dependence of ln R p p against time Žfirst-order kinetics. and non-linear dependence of 1rR p p against time Žsecond-order kinetics.. This also results from the high rate of particle nucleation. The average size of monomer swollen particles decreased Žfrom 56 nm to 30 nm. with conversion Žfrom 16 to 80%.. The abrupt increase in the size of the monomer-swollen particles Ž; 60 nm in diameter. at the beginning of polymerization compared to the size of monomeric microemulsion droplets Ž; 5 nm. was discussed in terms of the transport of monomer into polymer particles due to the fast polymerization andror agglomeration of unstable polyBA primary particles. The exitrre-entry events of transferred monomeric radical lead to the appearance of the cluster of unstable primary particles which collapse and form stable microparticles. The correction of the particle size on the monomer led to conclusion that the particle size was constant during the polymerization Ž; 30 nm in diameter, Fig. 4.. The number of particles increased with conversion during the polymerization. This finding resulted from the very large concentration of micelles Ž Nmic 4 Np . due to which the oligomer radicals are predominantly captured by micelles.

134

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

Fig. 4. Variation of the particle diameter Ž D, `. and the particle number Ž N, q. in the microemulsion polymerization of BA photoinitiated by the UV light with conversion. Recipe: 100 g water, 10 g BA, 20 g SDS, 0.025 g NaHCO3 , Iinc s 1.56 = 10y5 einstein dmy3 sy1 , 23⬚C.

In both runs the particle size and number increased with the monomer and SDS concentrations. The reaction order m or n Ž N A wBAx m or N A wBAx q wSDSx4 n . is 0.2 or 1.05, respectively. In the former case the mole ratio wBAxrwSDSx increased while in the latter case it was constant. The number of particles increased much more strongly in the latter case. This finding can be attributed to the increase of the radical concentration when the SDSrBA associate acts as an initiator. This behavior strongly differed from that for the thermally initiated microemulsion polymerization of BA where the number of particles decreased with both the BA and SDS concentration Ž N A wBAxy0 .4 and N A wSDSxy1 w42,49x. The viscosity-average molecular weight increased with increasing the BA concentration ŽTable 2, constant wSDSx.. This may be attributed to the increase of the monomer concentration at the reaction locus. The viscosity of the polymer microemulsion ŽcPars. varied with conversion as follows. On the contrary, the M v is nearly independent of wBAx q wSDSx4 at the constant ratio wBAxrwSDSx s 1.12. This is due to the same concentration of monomer at the reaction loci. The role of transferred and exit monomeric radicals in the polymerization process was investigated in the microemulsion polymerization of butyl acrylate initiated by UV light under steady illumination or illumination for varying initial periods w116x. These data are shown in Fig. 5 which shows two distinct non-stationary regions. The results show that the rate of polymerization is a function of both conversion and illumination. The limiting conversion in the polymerization proceeding under steady illumination appears at ; 80%. In the polymerization with the irradiation for varying initial periods the final conversion was much below 80% conversion. In the latter case the limiting conversion was found to increase

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

135

with the time of irradiation. The cessation of irradiation leads to the abrupt decrease in both the rate of polymerization and the final conversion. However, the number of particles increased after the cessation of illumination Žthe post-polymerization interval.. For example, the number of particles increased from ; 0.3 = 10 18 dmy3 Žafter the cessation of illumination at 16% conversion. to 0.6 = 10 18 dmy3 Ž; 50% final conversion. Žsee Fig. 5, the run Ž^... In the polymerization under steady illumination the number of particles at 50% conversion was observed to be 1.0 = 10 18 dmy3 . These results indicate that the nucleation of particles during the post-polymerization is most important. The initiating radicals formed by the decomposition of exited intermediates disappear shortly after the cessation of illumination which means that the nucleation events are performed by the desorbed radicals Žthe re-entry.. The reaction order, a, from R p A Ioa was found to be 0.7, which deviates from 0.5 found for the photopolymerization of acrylates in solution w118x. This was attributed to the contribution of immobilized macroradicals and efficient particle nucleation. By simulation of wR x values using the following equation 䢇

2 tr Žw R x o y w R x t . s 1r w R x o ,2 k 2 q tr w R x o ,2 䢇

䢇

䢇

䢇

Ž 53.

where k 1 is the unimolecular rate constant, k 2 is the bimolecular rate constant, wR x s wR x o,1 q wR x o,2 , o denotes the initial stage, t is the reaction time, wR x o is the total radical concentration, wR x o,1 is the concentration of stable Žentangled. radicals and wR x o,2 is the concentration of fast decaying radicals, the values of 䢇

䢇

䢇

䢇

䢇

䢇

Fig. 5. Variation of the rate of polymerization in the microemulsion polymerization of BA photoinitiated by a UV light with the irradiation time and conversion. Recipe: 100 g water, 20 g SDS, 10 g BA, 0.025 g NaHCO3 . Iinc s 8.92 = 10y6 einstein dmy3 sy1. Steady irradiation Ž'., an initial period of irradiation: Ž^. 10 min, Ž䢇. 5 min.

136

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

different parameters were estimated. For example, in the run 1 the light was turned off at 6% conversion and in the run 2 the light was turned off at 16% conversion, respectively. The final conversion of the run 1 was 24% and the run 2 48%. The run 1 contains only the fast decaying radicals, wR x o,2 . With increasing conversion decreases the fraction of mobile radicals and increases the fraction of more stable ones. Indeed, in the run 2 the fraction of entangled radicals ŽwR x o,1 . appears. At the same time, the rate constant k 2 decreased by one third. Both the decrease of monomer concentration in particles and the increase in the molecular weight hinder the movement of chain segments required for the second-order free radical decay. The very low value of Q ; 0.1᎐0.2 is attributed to the high desorption rate of monomeric radicals generated by the transfer reaction to monomer inside the particles and the low rate entry of radicals to particle w119x. The re-entry rate constant into micelles is ; 2᎐3 orders of magnitude larger than the rate constant for bimolecular termination in the aqueous phase w40,41x. Thus, the radicals are captured by micelles Ž Nmic 4 Np .. Under steady illumination, the number of polybutylacrylate chains inside each particle was estimated to be ; 12. The capture of additional radicals by the polymer particles is negligible and therefore the agglomeration of primary particles Žnucleated micelles. formed during the exitrre-entry events is suggested to be a dominant factor in regulating of the particle size and the number of polymer chains per particle. The molecular weight of polyBA in the macroemulsion polymerization parallels the rate of polymerization, i.e. the dependence is described by a curve with the maximum at 45% conversion ŽFig. 6.. The decrease of M v beyond 50% conversion was attributed to the decrease of monomer concentration in particles. The polyBA molecular weights are one order in magnitude larger in the photoruns than those in the emulsion polymerization and several times those in the thermally initiated microemulsion polymerization Žat the same conditions.. This difference may be discussed in terms of different contribution of the water-phase termination, the radical formation in micelles, the entry rate of radicals to particles and compartmentalization. The largest polymer molecules, however, were formed in the postpolymerization runs. For example, the M v increased from 4 = 10 6 Žthe cessation of irradiation at 4% conversion. to 7.7 = 10 6 Ž24% final conversion.. In the run under steady illumination the M v at 24% conversion was somewhat lower ᎏ 5 = 10 6 . From this behavior it appears that the bimolecular termination Žespecially the water-phase. is more depressed in the photopolymerization. The addition of AIBN Ž5 = 10y3 mol dmy3 . decreased the M v at 24% conversion to 1 = 10 6 . Thus, the radicals Žmobile. in water as well in micelles derived from AIBN seem to decrease the growth events. The appearance of a strong maximum of the relative viscosity at ; 20% conversion ŽFig. 6. was discussed in terms of several factors; Ž1. van der Waals attractions, Ž2. electrical double layer repulsion, Ž3. hydration and Ž4. steric attractions or repulsions. These interactions are supposed to vary with the particle size, the particle size distribution, the number of particles and the chemical 䢇

䢇

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

137

Fig. 6. Variation of the viscosity-average molecular weight Ž'. and the relative viscosity Ž^. of the microemulsion latex in the microemulsion polymerization of BA photoinitiated by a UV light with conversion. See other conditions in the legend for Fig. 5 Žsteady irradiation ᎏ the run Ž'...

structure of the interface of the polymer particles. The appearance of this maximum results from the strong change in the macroviscosity of the system Žstrong interparticle interactions, . . . .. Jansson et al. w120x reported that attractive hydrophobic interactions occurring between emulsion droplets are enhanced with the adsorption of the hydrophobic additives. Thus, the conversion of monomer to polymer favors the interparticle interactions Ždecreases the stability of Žnucleated. micelles.. During the course of polymerization, formed polymer products may, however, compete for a place at the interface between the aqueous and oil domains causing changes in both the structures. Interactions between micelles and particles vary with the chemical structure or internal organization of polymer, monomer and emulsifier in micelles or particles w121x. The accumulation of polymer chains in the micelles favors the interparticle interactions Žthe clustering process.. This is due to the fact that the polymer molecules vary the organization of the globules in solution. Collisions of such globules are assumed to lead to aggregation by bridging with polymer molecules Žthe flexible surface emulsifierrmonomer film.. The highly rigid Ždeveloped at higher conversion. interfacial emulsifierrpolymer surface film does not favor interparticle agglomeration. Generally, some additions Žcoemulsifiers. can influence the polymerization and colloidal parameters of microemulsion polymerization. Capek et al. w116,117x have used the hydrophobic and hydrophilic additives such as aromatic hydrocarbons, AN, BP, etc. in order to regulate the photoinitiated polymerization of BA microemulsions. In the homogeneous polymerization of acrylonitrile naphthalene

138

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

ŽNph., anthracene ŽAnt. or BP acted as a photosensitizer where the rate of polymerization was found to depend on the square root of hydrocarbon concentration w118x. In the BA microemulsion, however, the addition of Nph or Ant decreased the rate of polymerization and the decrease was more pronounced with Ant. Table 3 shows that neither Nph Žor Ant. nor BP Žbenzophenone. act as a sensitizer even though the triplet activation energy varied from 68.6 kcal dmy3 for BP, 60.9 kcal dmy3 for Nph and to 42 kcal moly3 for Ant. Thus the energy transfer from each additive to BArSDS aggregates is not operative. The observed decrease in the BA polymerization rate, thus, may be interpreted as a consequence of competitive reactions between BA or SDSrBA Žformation of exciplex and initiating radicals. on the one hand and energy transfer from BA or SDSrBA to the ground state of additive molecules on the other, as well as a result of reactions between BA or SDSrBA and additives. The radicals Žfrom BA or SDSrBA. can react with Nph ŽAnt. in a reaction sequence as explanation of the inhibitory effect of aromatic hydrocarbons on thermally initiated polymerization of vinyl monomers. BP was found to very strongly decrease the rate of polymerization as well as the molecular weight of polyBA formed. Similar behavior of BP was reported in the photopolymerization of AN initiated by Nph where the results are discussed in terms of reaction of propagating radicals with ketyl radicals w118x. In the SDSrBA microemulsions BP in its triplet state decays according to several relaxation processes: Ž1. deactivation through a non-radiative process Ž k ., Ž2. generation of a radical pair through hydrogen abstraction from the emulsifier Ž k r ., and quenching by monomer Ž k q .. The decreased molecular weights in the photoruns thus may be attributed to the reactions of the triplet state BP  diphenylhydroxymethyl Žketyl. radicals4 with growing radicals. The polymerization does not depend on wNphx and is much slower with Ant and BP Ž ␭ s 365 nm.. Here Nph does not absorb light while Ant and BP do so. The decrease in molecular weight was observed with Ant and BP. These results indicate that the excited states Žradical intermediates. of Ant or BP take part in termination. Owing to the compartmentalization of Ant, bimolecular reactions which take place in toluene are avoided Žor depressed. in micelles. The exit of hydrophobic Ant radicals is hindered which should increase the cage effect or deactivation events and decrease the initiation events. The trends of R p indicate that the number of particles should be lower in the systems with additives. The experimental results show that the reverse is true. Thus, the rate of polymerization is inversely proportional to the number of particles. No definite statement is available yet to account for this effect. It is supposed that the short stoppage of particle growth caused by the additive decreases the flux of monomer from micelles to particles which may favor the nucleation of additional monomer-swollen micelles. The post-polymerization conversion interval Žor the initiation efficiency of additives. was found to increase in the following order: AN - Ant - without additive - BP - BOL - Nph

Ž 54.

Additive

0 Nph Ant BP AN

Rp,max Ž.104 . Žmol dmy3 sy1 .

Np Ž10y18 .rdm3

D Žnm.

Conv. Ž%.

Mn Ž10y6 .

b

e

b

c

d

d

f

g

d

f

g

d

6.5 5.4 1.9 1.3

1.0 1.2 3.4 4.9

10.0 11.6 23.6 12.7

80 75 75 70

85 80 80 75

46 46 35 34

56 60 40 42 44

51 51 36 36 41

1.75 1.6 3.6 3.5

0.4 0.36 1.26 1.0 0.9

1 1.1 2.6 3.0 1.6

6.0 4.9 4.4 2.0

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149

Table 3 Variation of the kinetic, colloidal and molecular weight parameters in the microemulsion polymerization of BA initiated by UV light with different additivesa

a wBAx s 0.6 mol dmy3 , wSDSx s 0.54 mol dmy3 , ␭ s 313 nm, wNphx s 3.86 = 10y3 mol dmy3 , wAntx s 3.87 = 10y3 mol dmy3 , wBPx s 3.74 = 10y3 mol dmy3 , Iinc s 1.1 = 10y5 einstein dmy3 sy1 . Žb. Conversion at the R p,max . Žc. Conversion at 4 h. Žd. Final conversion. Že. Ratios of the rates without and with additive. Žf. At 25% conversion Žlight was turned off.. Žg. Final conversion Žthe post-polymerization interval..

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140

Thus, AN favors the exit of radicals and their deactivation in water, Ant Žits triplet biradical form. deactivates the radicals within the polymer particles, butanol ŽBOL. increases the number of micelles Žas a coemulsifier., and no definite statement is available for Nph. The rate of polymerization is found to increase with pH and reach a maximum at pH s 7, and then decrease. The lower reactivity observed in the acidic media may be due to the protonation of emulsifier Žthe acid form., which may depress charge transfer events in the excited complex. In strongly alkaline media, the decrease is much more pronounced. Here, the negative surface charge of particle is supposed to suppress the percolation Žexchange of reaction components including monomer and radicals between negatively charged polymer particles. which disfavors initiation and growth events. Micelles and particles are very sensitive to small changes in the ionic strength of the aqueous solution andror pH. In SDS solution, the surface tension Ž␥ . decreases and hydrodynamic particle radius, R h , increases with Hq concentration. With increasing NaOH ŽpH ) 9., R h decreases. The increased negative charge favors the compression of the diffuse double layer of micelles Žparticles.. Indeed, the particle size of microlatex decreased from 48 to 39 nm in the pH range from 7 to 12. Small particles were formed at very low pH. This indicates that other factors influence polymerization. The addition of butanol was found to decrease the rate of polymerization but increased the particle number. The incorporation of butanol as a coemulsifier into microparticles is expected to change the attraction between SDS tails and BA. Under such conditions, the formation of radicals Žfrom SDSrBA exciplex. is suppressed as well as the growth events. The close packing of SDS and butanol generates the barrier for radical entry into monomer droplets. The dilution of BA in the interface andror in the bulk disfavors the growth events, i.e. the molecular weight decreased with increasing butanol concentration. 4.3. Polymerization of other unsaturated monomers Turro et al. w122x reported photoinitiated microemulsion polymerization of styrene with dibenzyl ketone ŽDBK. as an initiator and UV light with wavelength 313 nm as an initiating source. The system remains transparent during the whole polymerization process. The shape of the conversion-time curves was found to be similar to that for the precipitation polymerization. The initial rate was found to be a maximum one and the limiting conversion appeared at the conversion range 60᎐80%. The rate of polymerization was found to increase with increasing incident light intensity Ž Iinc . and initiator concentration ŽDBK.. These results can be summarized in the following relation; R p A w DBKx

0.2

0.2 . Iinc

Ž 55 .

The principle locus for initiation of polymerization is suggested to be in monomer droplets. The same dependency of polymerization rate on oil-soluble initiator

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141

concentration as in the miniemulsion polymerization suggests that there is a similarity in initiation mechanism between these two types of polymerizations. A higher initiation rate, from higher initiator concentration or higher light intensity, converted more droplets into particles, and thus gave a higher initial polymerization rate and also a faster coagulation rate of the primary particles. The decrease in particle number due to coagulation events for the rapid decay of polymerization rate after it reached the maximum in a short time. The decrease in monomer concentration and slight decrease in the transparency of the system also contributes to the decrease in polymerization rate with time. Results showed that all of the latexes have average diameter larger than the diameter of the original monomer microemulsion droplets Ž; 10᎐30 nm.. The particle sizes of the produced latexes are in the range of 30᎐60 nm, and the polydispersity indexes are in the range 1.05᎐1.08. A possible mechanism for the particle growth in microemulsion polymerization is a combination of coagulation and monomer diffusion. As soon as polymers were formed in droplets, microemulsion became less stable. The droplet-converted particles gradually coagulated to larger sizes so that interparticle electrostatic repulsion could better stabilize them. The latex polymerized for 120 min had a smaller apparent average Ž47 nm. than the latex polymerized 30 min Ž56 nm.. This was attributed to the fact that the former had a higher conversion; thus the particles had less tendency of ‘flattering’ on drying. However, the present research in the microemulsion polymerization indicates that the continuous generation of polymer particles may lead to the decrease of the average particle size. The molecular weights of the produced polymers were reported to be in the order of 10 5, and their polydispersity index Ž MwrMn . to be in the range 1.6᎐2.2. With increasing initiator and light intensity decreased the degree of polymerization ŽDP. as DP A w DBKx

y0 .4

y0 .2 . Iins

Ž 56.

As the reaction time increased from 10 to 120 min, both Mn and Mw of the polymer increased ; 50% and 30%, respectively. The absolute value of exponent is lower than that of the conventional emulsion polymerization initiated by watersoluble initiator Žy0.6.. Since DBK radicals also form diphenylethane as the cage combination product, the concentration of initial radicals ŽDBK radical fragments. should be proportional to wDBKx with a power much less than 1. For a water soluble initiator in conventional polymerization, however, all initiators contribute to producing radicals for polymerization. This may be the reason that the exponent value for wDBKx in microemulsion polymerizations is smaller than that for watersoluble initiator in conventional emulsions. For these reasons also the degree of polymerization decreases slightly with the increase in light intensity with the y0 .2 relation DP A Iins . The accurate and precise knowledge of growth events Žpropagation rate constant k p . is of paramount importance for a detailed understanding of the kinetics and

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mechanisms of, for example, emulsion or microemulsion polymerizations. Several methods of obtaining k p have been reviewed and one of the more reliable methods is pulsed-laser polymerization ŽPLP. w123x. This method comprises the generation of radicals through a photoinitiator, activated by a laser pulse. When termination of radicals in-between laser pulses is not complete, the pulse interval determines the growth time of a definite part of the chain. Because their termination predominantly occurs just after the radical concentration has been sharply increased through a successive laser pulse, this termination is mainly one by very short chain radicals. Superimposed onto a distribution of polymer terminated in-between laser pulses, the MWD resulting from PLP may therefore contain additional peaks resulting from chains that have grown an integer multiple of the time between pulses. The location of these peaks may in principle be measured by means of GPC and may be used to determine the propagation rate constant k p for free-radical polymerization in bulk. According to Olaj et al., the molecular weight Mi p at the low-molecular-weight side inflection point of the additional peaks is a good measure of k p , and, again according to Olaj et al., is largely unaffected by the termination constant or the mode of termination. Eq. Ž57. may be formulated as Mi p ,i s i. Mm .k p . w M x ␶

Ž 57.

where i. Mm is the molecular weight of a monomeric unit and i s 1,2,3 . . . , w M x is the effective concentration of monomer and ␶ is the time interval between laser pulses. PLP was applied in the four-component ŽstyrenerSDSrpentanolrwater. microemulsion polymerization of styrene by Holdroft et al. w124x. A high concentration of free radicals is produced by the initial high intensity laser pulse. These radicals initiate polymerization of the latex particles. The pulse which immediately follows generates an equally large number of radicals which enter nucleated particles, causing rapid termination of the growing polymer, and initiation of the particles which do not already contain a polymer radical. Growing polymer chains not terminated by this pulse are captured by subsequent pulses with increasing probability. The latter gives rise to polymer with multimodal molecular weight distributions. The intensity and repetition rate of laser pulses determines the distributions of molecular weights in a unique manner. Information on the kinetics of microemulsion polymerization were derived using the data plotted in Eq. Ž57.. The product k p w M x was determined to be 313 " 7. Since k p for styrene at 25⬚C is 80 moly1 dm3 sy1 w125x, the local concentration of monomer in the microemulsion particle can be calculated to be 3.91 " 0.08 mol dmy3 . This value is slightly lower than the value based on the relative concentration of styrene and benzene, calculated to be 4.37 mol dmy3 . However, the latter does not take into account the differences in partition coefficient between styrene and benzene, nor does it take into account dilution of monomer by coemulsifier Ž n-pentanol. and dodecyl chains of emulsifier w42x.

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Termination was assumed to be prematurely brought about by long-lived, mobile radicals generated by the laser pulse. The probability of these mobile radicals terminating a polymer radical located in a polymer particle will depend on their concentration. High concentrations of radicals generated by high intensity laser pulses provide the greatest probability of terminating polymer chains prematurely. The rate of radical production in the thermal polymerization of styrene microemulsions is much lower than that obtained during a laser pulse. As a result, thermally polymerized microemulsions are often characterized by high molecular weight products. Molecular weight distributions are usually large Ž) 4., indicating complicated steady state kinetics. Both of these observations are consistent with the findings using laser initiation. The low molecular weights observed with high intensity pulses indicates that termination of the polymer chains occurs prior to the subsequent laser pulse. Chain transfer cannot account for the termination mechanism since molecular weights should be able to exceed 1 = 10 6 before chain transfer manifests itself. Furthermore, thermal initiation and steady-state photoinitiation of styrene microemulsions yield polymers of high molecular weight, i.e. ) 10 6 . A mean value of k p s 339 dm3 moly1 sy1 has been derived from PLP experiments in StrAOTrwaterrNaCl runs. A very clear decrease in k p Ž260 and 265 dm3 moly1 sy1 . with the droplet diameter can be observed in two experiments with the smallest droplets Ž5.7 nm, 0.95 g of styrene.. Although less obvious, the same trend appears to be present in the experiments with larger droplet diameters Ž18.2 and 30.8 nm, 3 and 4.7 g of styrene, k p s 340 and 351 dm3 moly1 sy1 .. Since k p is usually assumed to be the same in the monomer phase of emulsion polymerization systems as in the extended bulk monomer phase Ž8.356 mol dmy3 at 60⬚C., the first conceivable explanation of this result might be that the monomer concentration w M x within the droplets is lower than the monomer concentration in bulk. A mean value of k p s 354 dm3 moly1 sy1 results from the bulk experiments. Besides, Guo et al. w126x reported variation of monomer concentration of styrene in micelles and partitioning of styrene monomer between the aqueous phase, the micelle shell and the polymer particles. At lower monomer concentration, the mixed micelles were present Žmonomer is solubilized in the micelle shell.. In this case the monomer concentration was strongly diluted. The dilution approach was also observed in the microemulsion polymerization of butyl acrylate in which butyl acrylate Žacts as a coemulsifier. was located mainly in the micelle shell w36x. Thus, the decrease of k p in the smaller micelles Žthe monomer core is reduced or not present. may be discussed in terms of polymerization within the micelle core where the monomer is lower. In the monomer saturated conditions Žthe micelles contain monomer core. the k p value is close to the bulk system. On the contrary when the polymerization proceeds under the monomer unsaturated conditions Žthe formation monomer core in micelles is depressed and initiation of polymerization may start in the micelle shell. the k p value is lower due the lower c m . Thus the value of k p can give some information about the partitioning of monomer between micelle shell and core under different reaction conditions.

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Acknowledgements This research is supported by the Slovak Grand Agency ŽVEGA. through grant number 2r5005r98. The author is also indebted to the Alexander von Humboldt Stiftung for financial support.

Nomenclature a ai APS AIBN AAm Acrol Ant AN AH BA BPR BMA BOL BP CTAB CTAC CMC CPC CTC CTAPS Cm Co DVB Df D DBP DTAC DTAB DDAB DP DMDA DBK DBS DMPA EA EMA EHA EHMA Eo,a EO EM f ftrer

the polar head area of emulsifier the surface area per emulsifier ammonium peroxodisulfate 2,2⬘-azobisizobutyronitrile acrylamide acrolein anthracene acrylonitrile amine butyl acrylate butyl propionate butyl methacrylate butanol benzophenone cetyltrimethylammonium bromide cetyltrimethylammonium chloride critical micelle concentration cetylpyridinium chloride charge transfer complex cetyltrimethylammonium peroxodisulfate monomer concentration in particles initial monomer concentration in particles divinylbenzene diffusion coefficient particle diameter dibenzoyl peroxide dodecyltrimethylammonium chloride dodecyltrimethylammonium bromide didodecyldimethylammonium bromide degree of polymerization dimethyldodecylamine dibenzyl ketone sodium dodecylbenzene sulfonate 2,2-dimethoxy-2-phenylacetophenone ethyl acrylate ethyl methacrylate 2-ethylhexyl acrylate 2-ethylhexyl methacrylate overall activation energy ethylene oxide electron microscopy the initiator efficiency the efficiency of the chain transfer events

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149 HA HPMA HEA HBA HLB HTPO I Iab s Iinc I1 rI3

IPA K k1 k2 kt r ,M kd kp kt kc KPS l LS M Mm Mpol Meq MWD MA MAA MMA MDEA Mv Mn Mw Nmic Np Nd Nph nBA orw PBA PAN PVAc PEO PEO-MA PIT PLP

hexyl acrylate 2-hydroxypropyl methacrylate 2-hydroxyethyl acrylate 2-hydroxybutyl acrylate hydrophilic-lipophilic balance 4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl stable radical initiator intensity of absorbed light incident light intensity the intensities of the first and the third vibronic peak in the probe fluorescence spectrum isopropanol constant unimolecular rate constant bimolecular rate constant rate constant for chain transfer to monomer the initiator decomposition constant the propagation rate constant the termination rate constant the pseudo first-order rate coefficient for capture of radicals by micelles potassium peroxodisulfate length of the emulsifier light scattering monomer molecular weight of monomer molecular weight of polymer the equilibrium monomer concentration Ž Mw rMn . molecular weight distribution methyl acrylate methacrylic acid methyl methacrylate methyldiethanolamine viscosity-average molecular weight number-average molecular weight weight-average molecular weight number of micelles number of polymer particles number of monomer droplets naphthalene mole fraction of BA oilrwater polyBA polyAN polyVAc polyŽethylene oxide. methacryloyl-terminated PEO phase inversion temperature pulsed-laser polymerization

145

146

PSD Rp Rp p Rp,max Ri Rs RS RU wR䢇 xo wR䢇 xo,1 wR䢇 xo,2 Rh r SDS STPO St Sac SBOA t taq

tprop tter T1 T TEA TEM TX VAc VBC Vfr,St V-50 ¨

Vf WI,II,III Q QUELS ␥i ␥orw ␲i ␶c ␻mr p ␩ ␾ist ␾CT C ␾rm ␾ht

␶ w␩ x ␳o

I. Capek r Ad¨ . Colloid Interface Sci. 80 (1999) 85᎐149 Ž Dw rDn . particle size distribution rate of polymerization rate per particle maximum rate of polymerization rate of initiation stable radical radical scavenger concentration of propagating radicals in particles total radical concentration the concentration of stable radicals the concentration of fast decaying radicals hydrodynamic radius particle radius sodium dodecyl sulfate tetramethyl stearoyl piperidine-1-oxyl stable radical styrene saccharide sodium 12-butinoyloxy-9-octadecene the reaction time the average residence time of radical in the aqueous phase before re-entry into a particle the average time to add one monomer unit in the aqueous phase the average time for termination reaction with another radical in the aqueous phase triplet state temperature triethylamine transmission electron microscopy thioxanthene vinyl acetate 3-vinylbenzyl chloride volume fraction of styrene 2,2⬘-azobisŽ2-amidinopropane. dichloride emulsifier hydrocarbon volume monomer volume ratio Winsor microemulsion type the average number of radicals per particle dynamic light scattering the interfacial tension interfacial tension oilrwater interfacial surface pressure rotational correlation time monomerrpolymer ratio dynamic viscosity the yield of intersystem crossing the yield of CTC formation the yield of primary radical formation volume fraction of the hydrophobic tails of emulsifier in the monomer-swollen micelles the time interval between laser pulses the intrinsic viscosity the rate of primary radical production

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