Latex polymerization conditions

For some apphcations, eg, foam mbber, high soHds (>60%) latices are requited. In the direct process, the polymerization conditions are adjusted to favor the production of relatively large average particle-size latices by lowering the initial emulsifier and electrolyte concentration and the water level ia the recipe, and by controlling the initiation step to produce fewer particles. Emulsifier and electrolyte are added ia increments as the polymerization progresses to control latex stabiUty. A latex of wt% soHds is obtained and concentrated by evaporation to 60—65 wt % soHds. 

We have found that in the system of presulfate initiator, the PVAc latexes are not dissolved transparently in the methanol-water mixture [8], and in the system of HPO initiator, the extraction of the polymer from the PVAc latex films with acetone greatly depends on the polymerization condition [9]. These results suggest that if a polymerization method can be found in which the grafting polymerization of VAc onto PVA is controlled to the minimum, a large portion of PVAc in the latex film will have a chance of extraction with solvents. In this Chapter, the preparations of the unique porous films from the PVAc latexes containing PVA as a protective colloid by an extraction of the PVAc particles with acetone and the characteristic properties of the porous films are summarized. 

To prepare (true) latices, the monomers are emulsified in water with stirring and addition of emulsifiers, which help stabilize the monomer droplets. The molecular weight of the polymer molecules in the resultant latex can be controlled by the concentration and decomposition rate of added polymerization initiators. The residual monomer content can be reduced by optimizing the polymerization conditions and can also later be eliminated by steam distillation. 

Polymerization conditions with temperatures of 60 °C were mild compared with conventional ATRP in solution, where typically temperatures around 100 °C are applied. Despite these mild conditions, high polymerization rates and high conversion within only 3 h were achieved. Moreover, it could be demonstrated that the catalyst, being covalently bound to the surfactant, can easily be removed by filtering off the latex and subsequent washing with methanol. By this procedure the copper content of the resulting PMMA was reduced from the theoretical value of 0.73% to 0.06% and the polymer appeared colorless while the washing fraction was colored [65]. 

To determine the effect of different polymerization conditions on the polymer endgroups produced, polymerizations were carried out using the standard bicarbonate buffer as well as other variations. Table V (13,16) shows that the use of the persulfate-bicarbonate combination with and without emulsifier gave latexes of final pH 7-8 with only sulfate groups. The addition of 10 5 silver ion gave a latex of pH 8.5, but with weak-acid groups, presumably because of oxidation of the sulfate groups. 

The morphology of two-stage (styrene//styrene-butadiene) and (styrene-butadiene//styrene) latex particles was found to vary from a core-shell structure to a complete phase separation with various two-phase structures in between, depending on polymerization sequence, polymerization conditions, polymer compatibility, molecular weights, polymer phase ratio, etc. 

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

A quantitative method has been developed to separate free and graft copolymers in an ABS sample. The ABS powder is dispersed in MEK and then introduced into the cells of a preparative ultracentrifuge. After the reproducibility of the procedure was ascertained, the method was used to determine the grafting parameters of samples polymerized under specific conditions. This analytical technique is well suited to demonstrate how the grafting efficiency or grafting density is influenced by various polymerization conditions such as mercaptan content, monomer flow rate, emulsifier content, or polybutadiene content. The effects of other variables such as temperature, the initiator system, and characteristics of the polybutadiene latex can also be demonstrated. 

The procedure by which an emulsion polymerization is carried out has a profound effect upon the resulting latex and polymer properties. Indeed, latexes and polymers with quite different performance characteristics can be produced from the same reaction formulation by appropriate control of the type of polymerization process and conditions used. In this chapter, the two types of batchwise process are described with particular emphasis upon the control that can be exercised over polymerization conditions and product properties. An enormous volume of material on this subject exists in the scientific and patent literature, and cannot be fully documented here. Instead, general principles are presented and illustrated using specific examples which serve as good starting points for researching relevant literature on the topics covered. 

In principle, the characterization of polymers prepared in emulsion could be achieved by using approaches and techniques. similar to those available for polymers in general. Special attention for products of emulsion polymerization, however, seems justified since emulsion polymers usually show a number of features, characteristic of their origin. Therefore, in this chapter, latex polymer characterization will be considered in relation to the pertaining emulsion polymerization conditions. 

Two general features of emulsion polymerization reaction systems for the production of carboxylated rubber latexes are of special interest The first b that the polymerization usually takes place under acidic conditions (c. pH 3-4) and not under the alkaline conditions which are usual for the production of non-functionalized synthetic rubber latexes. Polymerization is carried out under acidic conditions in order to encourage the carboxylic acid monomer to become copolymerized into the molecxiles of rubber being prcxluced. If the reaction is carried oui under alkaline conditions, then the carboxylic acid monomer is present mainly as a carboxylate salt which partitions strongly in favour of the aqueous phase If it polymerizes at all under these conditions, polymerization occurs mainly ir the aqueous phase of the reaction system, and the polymer molecxiles in whicl it becomes incorporated are far more hydrophilic than are the majority of the polymer molecxiles, which are produced in the latex particles. The requiremen... 

Surfactants (emulsifiers of various chemical nature) are usually applied as stabilizers of disperse systems, they are rather stable, poorly destmcted under the influence of natural factors, and contaminate the environment. The principal possibility to synthesize emulsifier-free latexes was shown. In the absence of emulsifier (but in emulsion polymerization conditions) with the usage of persrrlfate-lype irritia-tors (e g., ammonirrm persulfate), the particles of acrylate latexes can be stabilized with ionized endgroups of macromolecules. The ion radicals appearing in... 

Besides two-component LIPNs, three-component LIPNs have also been studied through three-stage emulsion polymerization processes (Zhang et al. 1991, 1994 Isao et al. 1992). These authors synthesized poly(n-butyl acrylate) cross-linked with ethylene glycol dimethacrylate as the seed latex. Styrene and divinylbenzene were added at the second stage. The third stage was linear poly(methyl methacrylate). Starved polymerization conditions resulted in more regular-shaped latex particles than batch addition of monomer. 

This chapter covers the preparation, characterization, and biomedical application of thermally sensitive particles. Thermosensitive hydrogel is prepared by precipitation polymerization of A-alkylacrylamide or A-alkylmethacrylamide as a principal water-soluble monomer, a water-soluble cross-linker (for instance, A-methylenebisacrylamide), and an initiator (such as Azobis-amidinopropane derivatives, potassium persulfate, or basically any charged initiator). The core-shell latexes are produced by a combination of emulsion and precipitation polymerization, such as the preparation of polystyrene core and poly(A-isopropylacrylamide) shell or encapsulation of coUoidal seed using alkylacrylamide derivatives. During the elaboration of such stimuli-responsive particles, various aspects should be considered (1) a water-soluble cross-linker is needed, (2) the polymerization temperature should be higher than the LCST of the corresponding linear polymer, and (3) the production of water-soluble polymer (which can be controlled by monitoring the polymerization conditions). The polymerization mechanism has been clearly discussed and well illustrated, but the nucleation step remains questionable and requires further work. 

Preparation of a vinyl chloride copolymer powder by copolymerizing a PVC latex under suspension polymerization conditions with a comonomer [249]. 

A latex can be used to form conductive blends of thermoplastic rubber with TT-conjugated polymers such as polyaniline, polypyrrole, and poly(3-methoxythiophene) made from Fe(ni), Fe(ii), Cu(ii)-H202 in-situ aqueous oxidation systems. The composite films were then obtained by hot pressing. Suitable polymerization conditions involved using HCl, aniline... 

If the seed latex particles can barely be swollen by the second-stage monomer and a water-soluble initiator is used, then the subsequent seeded emulsion polymerization will be localized near the particle surface layer. Thus, the postformed polymer tends to form a surface layer around the seed latex particle. An example of this kind of morphological structure of latex particles is the seeded emulsion polymerization of methyl methacrylate in the presence of a polyvinylidene chloride seed latex. On the other hand, free radical polymerization can take place inside the seed latex particles. In this manner, various morphological structures of latex particles such as the perfect core/shell, inverted core/shell, dumbbell-shaped, and occluded structures can be achieved, depending on various physical parameters and polymerization conditions. 

Polymer latex nanoparticles can be prepared in many materials such as polystyrene and acrylate with controllable size, through radical-initiated polymerization in heterogeneous media (Figure 14.2). The sizes of latex nanoparticles are very dependent on the polymerization conditions. To yield nanosized particles, the polymerization is usually carried out in miaoemulsions [34], For some applications, two or more monomers are used. For example, for polystyrene nanoparticles, divinylbenzene (DVB) is used as a cross-linker to improve the structural performance [35] and methacrylic acid (MAA) or methacrylate (MMA) is used as a co-monomer to provide the nanoparticles with desirable surface chemistry [36,37], Furthermore, some fluorochromes or magnetic materials are incorporated into polymer nanoparticles, to render the particles multifunctional [38,39],... 

Polymeric binder can be added to the network either as an aqueous latex dispersion or as a solution that should be dried prior to lamination in this process. In either case, the polymer should form a film and join adjacent fibers together and thus improve the stress transfer characteristics of the fibrous network. Provided that the proper film forming conditions are available, the property profile of the bonded network is determined to a significant degree by the properties of the polymeric binder at the temperature of use [20,22]. For example, if a softer type of product is desired, a binder with a relatively low glass transition temperature Tg) is often chosen. 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

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