Miniemulsion production

The variation of the functional monomer concentration is reflected in the PA-FllK spectra of the miniemulsion products. An increasing of the HEMA content in the polymer composite can be seen at the signiflcant carbonyl stretching band at 1600 cm which increased in the spectra from product 1 to product 3 with increasing of the HEMA content in the monomer composition. Nevertheless, next to the typical copolymer bands for poly(S-co-HEMA), signiflcant silica core bands are visible in all spectra. The monomer com-... 

The morphology of the S-co-HEMA polymer composite exhibits a typical miniemulsion product of spherical particles with a narrow particle size distribution (Figure 7). 

The SEM image of the miniemulsion product (Figure 11) shows spherical particles with a narrow particle size distribution and an average particle size of 150 nm. This confirms the results from the dynamic light scattering measurements. 

Figure 5.2. A schematic representation of the mechanism for the transport of monomer between a small monomer droplet and a large droplet. Monomer molecules tend to diffuse from the small monomer droplet to the large droplet due to the Ostwald ripening effect. This will cause a concentration gradient for costabilizer between these two monomer droplets. However, the very hydrophobic costabilizer in the small monomer droplet cannot be dissolved in water, diffuse across the continuous aqueous phase, and then enter the large droplet. Thus, monomer molecules in the large monomer droplet are forced to migrate back to the small droplet in order to relax the concentration gradient for costabilizer (temned the osmotic pressure effect), and a relatively stable miniemulsion product is obtained.

More recently, Priego-Capote et al. reported on the production of MIP nanoparticles with monoclonal behaviour by miniemulsion polymerisation [63]. In the synthetic method that they employed, they devised to use a polymerisable surfactant that was also able to act as a functional monomer by interacting with the template (Fig. 4). The crosslinker content was optimised at 81% mol/mol (higher or lower contents leading to unstable emulsions). In this way, the authors were able not only to produce rather small particles (80-120 nm in the dry state) but also to locate the imprinted sites on the outer particle surface. The resulting MIP nanobeads were very effective as pseudostationary phases in the analysis of (/ ,S)-propranolol by CEC. 

Research (Fontenot and Schork 1993a, b) indicates that miniemulsion polymerization can provide benefits over the current process technology of conventional emulsion polymerization. Among these are a process which is much more robust to contamination and operating errors, a more uniform copolymer composition when used for copolymerization, and a final product which is far more shear-stable than the product of conventional emulsion polymerization. 

Ethylene can be polymerized using an aqueous miniemulsion consisting of an organo-transition metal catalyst at ethylene pressures of 10-30 bar and temperatures of 45-80°C resulting in large particles of about 600 nm [129]. A maximal productivity of 2520 kg PE per g atom active metal was achieved, which represents about 60% of the productivity of the same catalyst when used in ethylene suspension polymerization in organic phase. 

Fig. 23a,b. Reaction products of molten iron salt miniemulsions ( inorganic polymerizations ) a Fe203 particles obtained from FeCl3 droplets b Fe304 particles obtained from FeCl2/FeCl3 droplets... 

The polymerization process of two monomers with different polarities was carried out in direct or inverse miniemulsions using the monomer systems AAm/MMA and acrylamide/styrene (AAm/Sty). The monomer, which is insoluble in the continuous phase, is miniemulsified in the continuous phase water or cyclohexane in order to form stable and small droplets with a low amount of surfactant. The monomer with the opposite hydrophilicity dissolves in the continuous phase (and not in the droplets). Starting from those two dispersion situations, the locus of initiation (in one of the two phases or at the interface) was found to have a great influence on the reaction products and on the quality of the obtained copolymers, which can act as hydrogels. 

As shown above, the miniemulsion is a very efficient system for production of copolymer particles from hydrophobic and hydrophilic monomers. In the case of direct (oil-in water) miniemulsion, if the hydrophilic monomer is used in smaller quantities, there is a possibility to form an amphiphilic copolymers close to the interface of the nanoparticles. This shell region of the polymeric particle can be considered as a hydrogel shell. The structure of the hydrogel shell mainly depends on the monomer(s) concentration, reactivity ratios of the monomers, their solubility in water, and the type of surfactant used. 

Over the past 25 years, miniemulsion polymerization has grown from being the subject of a single paper to being the focus of a great deal of academic and industrial research. During that time, some products have been commercialized based on this technology, and in the next few years a number of new... 

In their original discovery of miniemulsion polymerization, Ugelstad and co-workers [5] used either cetyl alcohol (CA water solubility estimated at 6x10 [43]) or hexadecane (HD water solubihty estimated at 1x10 [43]) to retard monomer diffusion from submicron monomer droplets. Both CA and HD, referred to here as costabilizers, are volatile organic components and are therefore not entirely desirable in the final product. Other researchers have used polymers, chain transfer agents, and comonomers as stabiUzers, as will be discussed later. 

For laboratory investigations of miniemulsions, a variety of high-shear devices have been used, although sonication has been the most popular. Soni-cation, however, may not be very practical for the large-scale production of commercial miniemulsion polymers. An effective alternative to sonication is also driven by the need to design an efficient miniemulsion polymerization process. A continuous process places greater demand on the shear device in terms of energy consumption and dissipation. 

The vast majority of miniemulsion polymerizations reported in the literature have been stabilized with anionic surfactants, probably because of the widespread application of anionic surfactants in macroemulsion polymerization, and due to their compatibility with neutral or anionic (acid) monomers and anionic initiators. However, Landfester and coworkers [70, 71] have used the cationic surfactants cetyltrimethyl ammonium bromide (CTAB) and cetyltri-methyl ammonium tartrate for the production of styrene miniemulsions. They report that these surfactants produce similar particle sizes to anionic surfactants used at the same levels. Bradley and Grieser [72] report the use of dodecyltrimethyl ammonium chloride for the miniemulsion polymerization of MMA and BA. 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

In the case of nanoencapsulations of solids, or the incorporation of high molecular weight, highly water-insoluble additives (such as polymers, oligomers, alkyds) into polymer particles, macroemulsion polymerization will not work, since the high molecular weight material will remain in the monomer droplet as the monomer is transported out. At the end of the reaction, the additive will remain in the depleted monomer droplets, rather than in the polymer particles. Clearly, these products can only be made via miniemulsion polymerization. 

It should be apparent by now that miniemulsion polymerization systems have some properties that ought to be exploitable when making polymer colloidal products with unique or improved properties. This section will discuss some of these documented and potential applications. 

Results from the polymer-costabilized miniemulsion polymerizations are shown in Table 2. Droplet sizes were found to vary between 115.1 and 121.0 nm. These are in accord with measurements made by Fontenot [140] for MMA miniemulsions stabilized with hexadecane. The sizes of the particles in the final products were close to the sizes of the droplets, ranging from 102.6 to 108.1 nm, with polydispersities ranging from 1.011 to 1.027. The ratio of the number of particles to the number of droplets (N /N ) was found to be between 0.95 and 1.08. Therefore, the majority of the droplets were nucleated to form polymer particles. Droplet nucleation led to polymerization rates comparable to those for the corresponding macroemulsions. For equal concentrations of initiator, 0.01 Maq, the rates are 0.199 and 0.233 gmol/min L q for the mini- and the macroemulsion polymerizations, respectively. 

An enhanced robustness can benefit a process in a number of ways. Since the polymer-stabilized miniemulsions are less susceptible to disturbances, their polymerization is less hkely to be affected by operator error, fluctuations in feed stream concentrations and residual contaminants in the reaction vessel. Many monomers contain species that can act as inhibitors or retarders as a result of monomer production, storage, or processing. These contaminants also cause batch-to-batch variability in particle number in macroemulsions. Therefore, miniemulsion polymerization may be an alternative to seeded polymerization as a way of maintaining robust control of particle number. 

However, this does not preclude mini emulsion copolymerization in a CSTR for extremely water-insoluble comonomers. In spite of the fact that the copolymer composition in the continuous miniemulsion is less than that predicted using the homogeneous copolymerization reactivity ratios, the miniemulsion copolymer might be more uniform than the macroemulsion copolymer, where the possibility of significant droplet nucleation could lead to two separate homopolymers or, at the very best, copolymers of various composition. Therefore, it is very important to use CSTR data to scale up a continuous miniemulsion copolymerization product to take into account the different particle growth kinetics for batch and continuous reactors. 

While all of these results are far from providing commercial products, they highlight the possibilities for alternative polymerization chemistries in miniemulsions. 

Willert and Landfester [321] have polymerized amphiphilic copolymers from miniemulsion systems. The hydrophobic monomer was miniemulsified, while the hydrophilic monomer resides in the continuous phase. Polymerization was found to take place in the droplet phase, at the interface, or in the continuous phase the quality of the product depended strongly on the primary... 

Amphiphilic lipopeptides with a hydrophobic paraffinic chain containing from 12 to 18 carbon atoms and a hydrophilic peptidic chain exhibit lyotropic meso-phases and good emulsifying properties. The X-ray diffraction study of the mesophases and of dry lipopeptides showed the existence of three types of mesomorphic structures lamellar, cylindrical hexagonal and body-centred cubic. Two types of polymorphism were also identified one as a function of the length of the peptidic chain and the other as a function of the water content of the mesophases. The emulsifying properties of the lipopeptides in numerous pairs of immiscible liquids such as water/ hydrocarbons and water/base products of the cosmetic industry showed that small amounts of lipopeptides easily give three types of emulsions simple emulsions, miniemulsions and microemulsions. 

Production of polymer nanosuspensions by miniemulsion or suspension polymerisation. 

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