Miniemulsion Polymerization Technology

Vikas Mittal

• Publisher: Wiley-Scrivener
• Year: 2011
• Print ISBN: 9780470625965
• eBook ISBN: 9780470923160
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Explains miniemulsion technology and techniques and why they have many distinct advantages over the conventional emulsion polymerization technology

Miniemulsion Polymerization Technology comprises 10 papers by many of the world's experts on the subject. It summarizes the recent advances in miniemulsion polymerization technology including the advances on the selection of surfactants and co-surfactants, the expansion of miniemulsion technology in various polymers and co-polymer systems, and the use of miniemulsion polymerization for the synthesis of advanced polymer particle morphologies.

There have been a large number of texts on emulsion and other forms of polymerization methods, but miniemulsion polymerization, though it provides unique routes for polymer particle synthesis, has been neglected.

This edited volume:

• Details the use of miniemulsion polymerization in encapsulation, core shell functional particles, nitroxide mediated polymerization, atom transfer radical polymerization or radical addition fragmentation chain transfer polymerization, to generate advanced polymer nanoparticles or organic-inorganic composite particles
• Examines the wide spectrum of commercial possibilities of miniemulsion polymerization
• Provides both introductory material as well as deep insights into the synthesis of polymer particles

1

Miniemulsion Polymerization: An Overview*

V. Mittal

BASF SE, Polymer Research, 67056 Ludwigshafen, Germany

Abstract

Miniemulsion polymerization exhibits distinct advantages over the conventional emulsion polymerization. The polymerization of monomer droplets is directly achieved when the diffusion of the monomer from the droplets to polymer particles is not required. This helps to polymerize water insoluble monomers. It also allows the presence of various system components like initiators, costabilizers, etc. directly at the site of the polymerization thus allowing better control. Conventionally, volatile hydrophobes or costabilizers like cetyl alcohol or hexadecane have been used. A number of advances have been reported in recent years on the use of costabilizers which are more compatible to the polymerization system. Use of polymers as costabilizers was reported to be very effective even though the polymer forms a poor costabilizer. Similarly, the comonomers and initiator have also been used as costabilizers. The use of chain transfer agents as costabilizers also opens the opportunities for molecular weight control in the polymer particles. These advances ensure that the particles are free from any low molecular weight impurity or volatile components. The living polymerization techniques like nitroxide mediated polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer are also well suited for miniemulsion processes in order to generate specific morphologies in polymer particles and to control the molecular weight and its distribution in the particles.

Keywords: diffusion, monomer droplet, costabilizer, surfactant, initiator, micelles, chain transfer, comonomer, miniemulsion, conversion, colloidal stability, controlled living polymerization.

1.1 Introduction to Polymerization Techniques

Free radical polymerization can be carried out by using a number of different techniques. The simplest of these techniques is the bulk polymerization. In this technique, monomer is in the liquid form and the generated polymer is in solid form. Though the reaction mixture is free from any unwanted impurities or contamination leading to clean polymer, however, the viscosity of the system increases significantly due to the generation of polymer chains during the course of polymerization and thus the mixing of the system becomes extremely difficult leading to very broad molecular weight distributions in the polymer chains. Additionally, the polymer chains do no diffuse freely in the highly viscous medium leading to the accumulation of radicals at particular sites causing the polymerization rate to increase exponentially. Solution polymerization is an alternative method in which a solvent is used in which the monomer and polymer are soluble. The use of solvents eliminates the problems of higher viscosity and heat dissipation associated with bulk polymerization and allows one to stir the reaction medium easily. However, the choice of solvents must be proper; otherwise extensive chain transfer to solvent can take place resulting in only low molecular weight polymer chains. Precipitation polymerization is another form of polymerization in which the polymer is not soluble in the monomer or the reaction medium and precipitates out soon from the solution. Thus, precipitation polymerization starts as homogenous polymerization, but is soon turns into a heterogeneous polymerization. Dispersion polymerization is also similar to precipitation polymerization that the polymer formed is not soluble in monomer or organic solvent. After the formation of polymer particles, these particles are stabilized by added particle stabilizer and the polymerization proceeds in the polymer particles by the absorption of monomer into the polymer particles. Suspension or bead polymerization is a method in which monomer droplets are directly polymerized to generate high molecular weight polymers. In this method, water insoluble monomer is suspended in water with the aid of suspension stabilizers. The initiator used is also water insoluble or monomer soluble. The size of monomer droplets can be controlled depending on the ratio of monomer to the dispersion medium, i.e., water, the speed of agitation to generate droplets as well as by the amount of stabilizing agents.

During the polymerization, the monomer droplets polymerize independently and each droplet can be visualized as a bulk polymerization happening inside the droplet. Inverse suspension polymerization is also possible in which a water soluble monomer is used and its droplets are generated in an organic solvent. Initiators used are also water soluble and the monomer droplets are stabilized similarly by using suspension stabilizers. Emulsion polymerization is the one of the most versatile technique to generate small particles. With this technique, water insoluble monomers are polymerized by suspending them in water in the form of emulsion droplets stabilized by surfactants. The initiators used are water soluble in contrast to suspension polymerization where water insoluble initiators are used, the most common being potassium persulphate (KPS). Polymerization of extremely low water soluble monomers is very difficult with conventional emulsion polymerization. The low solubility of the monomer would not allow its diffusion to the polymer particles through the aqueous medium. Miniemulsion polymerization has been developed for such purposes, in which the monomer droplets generated by using high shear in the presence of an ionic surfactant and a co-surfactant or hydrophobe like hexadecane, are directly polymerized. The droplets and hence resulting polymer particles are generally in the size range of 50–500 nm. There are other forms of polymerization techniques like microemulsion, melt polycondensation and solution polycondensation etc. Figure 1.1 lists the large number of polymerization techniques used to synthesize a variety of polymers [1].

Figure 1.1 Various polymerization techniques to generate a wide spectrum of polymers.

Reproduced with permission from Nova Science Publishers [1].

1.2 Emulsion and Miniemulsion Polymerization

The common mode of particle synthesis in emulsion polymerization is achieved by micellar nucleation method, though there is also the presence of homogenous nucleation especially in water soluble monomers. The surfactants like sodium dodecyl sulphate are added which at a concentration higher than the critical micelle concentration in the aqueous phase form micelles. These micelles owing to their hydrophobic nature inside the inner space are an ideal site for the radical entry as well as propagation of polymerization. The structure of surfactant is generally amphiphilic, with one part hydrophobic and the other part hydrophilic. These molecules thus arrange themselves in a way that their hydrophilic parts are in interface with water. Every surfactant has a different critical micelle concentration value and it should be considered carefully while using different kinds of surfactants. The micelles generally have a size of 10 nm and generally 100–200 surfactant molecules form a micelle [2,3]. It is generally known that the surface tension of the solution decreases with the addition of surfactant at critical micelle concentration. However, it is not only the surface tension that is affected by the surfactant, rather a host of other properties of the solution are affected at critical micelle concentration.

Once the monomer is added to the system, a small amount of monomer enters the micelles and some gets dissolved in the aqueous phase owing to the partial solubility in water. However, the majority of the monomer is generally present in the form of monomer droplets. These droplets are stabilized by the adsorption of surfactant molecules on the surface. The number of micelles is much larger than the number of droplets and the droplet size may fall in the range of tens of micrometers [2,4]. When the polymerization is initiated by the addition of the initiator and after achieving the required polymerization temperature, the radicals are generated in the aqueous phase. The generated radicals have two possibilities to propagate further: to enter either the micelles or the monomer droplets. However, the experimental studies report that it is very rare that the radicals enter the monomer droplets. This is because of very large number of micelles present in the system as well as the architecture of the micelles provides ideal conditions for the monomer polymerization. When the radicals enter the micelles and start polymerizing the monomer contained in these micelles, the polymer particles form. These growing polymer particles are then supplied by the monomer molecules from the monomer droplets by diffusion through the aqueous phase. The termination of the radicals is quite slow as at a particular time during polymerization, there is rarely more than one radical per particle.

The conventional emulsion polymerization is thus divided into three intervals as shown in Figure 1.2. On addition to the aqueous phase, the monomer enters the micelles as well as forms the monomer droplets apart from dissolution in water to some extent based on the solubility of the monomer as shown in Figure 1.2a [2]. The first interval, also termed as particles formation phase, is then initiated. The radicals re generated in the aqueous phase due to the decomposition of initiator. These radicals enter the micelles and initiate monomer polymerization leading to the generation of polymer particles. The number of particles keeps on increasing in this interval which also results in the continuous enhancement in the polymerization rate. The system, as shown in Figure 1.2b, thus consists of polymer particles, monomer droplets, and the inactive micelles. The particles keep on increasing in size, thus requiring more and more surfactant to stabilize the increasing surface area. This leads to the adsorption of the dissolved surfactant in the aqueous phase on the surface of the particles and the surfactant concentration thus falls much below the critical micelle concentration. This results into the destabilization of the remaining micelles and they provide their surfactant to stabilize the growing particles. The number of particles generated from total micelles in the beginning is generally in the range of 0.1%. At the end of first interval, all of the micelles either are polymer particles or are destabilized to lose the surfactant. In the second interval, the particles keep on growing in size and no new particles are nucleated thus leading to the constant rate of polymerization. As the particles grow in size during the course of polymerization, they deplete the monomer content present in them. This depletion is continuously replenished by the absorption of more monomer from the water phase, which has been dissolved in it. The water phase in return absorbs more monomer from the monomer droplets resulting in the reduction of the size of the monomer droplets as shown in Figure 1.2c. After a certain conversion of the monomer is achieved, the monomer droplets also disappear which forms the transition period between the second and third interval. As shown in Figure 1.2d, the particles in this interval keep on polymerizing the monomer enriching them. Thus, concentration of the monomer in the particles decreases, and subsequently the polymerization rate also decreases in this interval. The number of particles thus also remains the same as the second interval and after the monomer has been completely depleted, the polymerization rate climbs down to zero.

Figure 1.2 ...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright
• Preface
• 1: Miniemulsion Polymerization: An Overview
• 2: Multi-Functional Stabilizers in Miniemulsion Polymerization
• 3: Structured Copolymer Particles by Miniemulsion Polymerization*
• 4: Encapsulation of Inorganic Nanoparticles by Miniemulsion Polymerization
• 5: Polymeric Nanocapsules by Interfacial Miniemulsion Polymerization
• 6: Miniemulsion Polymerization of Vegetable Oil Macromonomers
• 7: Controlled/Living Radical Polymerization in Aqueous Miniemulsion
• 8: Inverse Miniemulsion Polymerization of Unsaturated Monomers
• 9: Double Miniemulsion Preparation for Hybrid Latexes
• 10: Surfactant Effect in Miniemulsion Polymerization for Biodegradable Latexes
• Index