Emulsion latex characterization

Transmission Spectroscopy An Analysis Tool for Emulsion Latex Characterization... 

A considerable amount of work has been published during the past 20 years on a wide variety of emulsion polymerization and latex problems. A list of 11, mostly recent, general reference books is included at the end of this chapter. Areas in which significant advances have been reported include reaction mechanisms and kinetics, latex characterization and analysis, copolymerization and particle morphology control, reactor mathematical modeling, control of adsorbed and bound surface groups, particle size control reactor parameters. Readers who are interested in a more in-depth study of emulsion polymerization will find extensive literature sources. 

Complex colloids can be characterized advantageously by a combination of Fl-FFF with different analytical or other FFF techniques, yielding supplemental information. Examples reported in the literature are combinations of Fl-FFF and S-FFF for size (Fl-FFF) and density (S-FFF) as well as the thickness and density of the shell of core shell latexes [402],El-FFF for the charge and composition of emulsions [403],Th-FFF for the characterization of the size and composition of core shell latexes [404] and, finally, with SEC for the particle size distribution and stoichiometry of gelatin complexes with poly(styrene sulfonate) and poly(2-acrylamido-2-methylpropane sulfonate) [405]. 

Copolymers 59 [181] and terpolymers 60 [182] were synthesized by micellar copolymerization and characterized with respect to their molecular and solution properties. The subject of further investigations was the interaction with low molecular weight surfactants [181,183]. Another interesting use was made of hydrophobized sulfopropylammonio monomers as surface-active monomers (or surfmers ) [184]. Their use in emulsifler-free emulsion polymerizations [185] reduced the water uptake and improved the mechanical stability of the resulting filmed latexes. 

The technique of emulsion polymerization is characterized by the formation of the polymer in the form of a latex. The particle size distribution (PSD) of the latex and the molecular weight distribution (MWD) of the contained polymer are two important measurable parameters of the latex. Not only do they influence the end-use behavior of the product, but they also reflect the growth history of the emulsion polymerization process. In what follows, we review the theories that have been developed to describe the PSD and MWD-of emulsion polymers. 

Semicontinuous emulsion polymerizations are characterized by the continued addition of monomer to the reaction vessel. This permits the production of latexes with high weight percentage solids while allowing the initial burst of nucleation to be achieved in substantially aqueous surroundings. The theory for semicontinuous systems is substantially that set forth for Interval III of batch polymerizations, except that the materials balance equations [Eq. (17)] must be modified to include the flow of new material into the reactor. The effect of the monomer input is twofold first, the mass of material present in the system is increased and seccmd, the concentration of other reagents may be reduced. 

Emulsion polymenzaticm without the use of an emulsifier may be achieved even with a monomer with v ter solubility as low as thet of styrene provided one uses an initiator such as potassium persulfate which introduces ionic end groups into the polymer that can stabilize the polymer latex particles produced electrostatically. Emulsifier free emulsion polymerization is advantageous when the object is to obtain a well-characterized model colloid for use in experiments on colloidal stability, etc. Then it is usually desirable that the surfaces of the colloidal particles be clean. When an emulsifier is used in the iH eparation, its removal (e.g., by dialysis) is generally so incomplete that it is simpler to avoid its use in the first place. However, emulsifier-free latexes are necessarily dilute and consequently of little interest for commercial applications. 

The sedimentation FFF is shown schematically in Fig. 2a. The separation channel is situated inside a centrifuge rotor and the centrifugal forces are applied radially [8]. The method can be used for the analysis and characterization of various latexes, inorganic particles, emulsions, biological cells, and so forth. The retention parameter A depends on the effective mass of the particles ... 

Luna-Xavier, J.L., Guyot, A., and Bourgeat-Lami, E., Synthesis and characterization of silica/polyjmethyl methacrylate) nanocomposite latex particles through emulsion polymerization using a cationic azo initiator, J. Colloid Inteif. Sci., 250. 82, 2002. 

Those emulsion polymerizations for which initially polymer latex particles are not present, but in which particles are formed by some of the mechanisms described above, are known as ab initio. Seeded emulsion polymerizations are those in which at the beginning of the process there are preformed (and usually characterized) polymer latex particles this kind of polymerizations are commonly used in industry to avoid the variability of the process associated with the nucleation stage. 

Characterization of polymer latexes can be performed by techniques available for polymers in general and by other techniques specific for emulsions. 

Since the 1950s, batch emulsion polymerization has been used to prepare standard latexes which have specific particle sizes, narrow particle size distributions and well-defined particle surfaces, though most of the definitive research activity in this area occurred during the 1960s and 1970s [e.g. consult 35-37], The latexes can either be prepared as required or purchased from ranges which are commercially available. In either case, they invariably are polystyrene latexes which have been cleaned after preparation (by processes such as exhaustive dialysis and ion exchange [38]) and fully characterized. 

References to the characterization of emulsion polymers with IR or Raman spectroscopy are not numerous, and IR is used only in very specific cases. Only very few cases of the determination of copolymer composition with IR have been reported. An example where IR is utilized concerns the analysis of poly(methyl acrylate(MA)-co-styrene (S)) copolymers in chloroform at a concentration of 10% w/v [51]. Hergeth and Lange [52] used IR and Raman spectroscopy to study the stracture of core-shell latex particles of poly(vinyl acetate)(PVAc)-polystyrene (PS), and also obtained information about die interfacial layer between the two polymer phases. 

All latex samples prepared by emulsion polymerization are characterized by a broad distribution of molar masses, and in the case of copolymer latexes, a distribution of copolymer composition. Since the diffusion coefficient for a polymer depends upon both the chain length and the chemical structure, the polymers in any one film sample will be characterized by a rather broad distribution of Dcm values. Experiments to detormine in such systems actually yield a value averaged over the distribution, Dts. As will be seen below, since different components of the system contribute to the measured signal at different times, and the fastest diffusing species dominate the diffusion at early times, experi-mental values of Detr decrease with the ext t of interdiffiision. For such sanqiles, one is normally less int sted in the absoluie values of than in how extonal... 

Figure 210 2 Schematic representation of the early stages of emulsion polymerization illustrating three scales of observation macroscopic, microscopic and submicroscopic. (Reprinted with permission from E. D. Sudol, E. S. Daniels and M. S. El-Aasser, in Polymer Latexes Preparation, Characterization, and Applications, E. S. Daniels, E. D. Sudol and M. S. El-Aasser, (eds), ACS Symp. Sen, Vol. 492, 1992, p 1 Copyright 1992 American Chemical Society)...

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