Swelling carboxylated latexes

The expansion characteristics of carboxylic latex particles have been measured using three independent techniques sedimentation, which uses the change in particle density due to swelling to determine the change in particle size viscometry, which measures volume changes and photon correlation spectroscopy, which measures the diffusion coefficient of the particles. The sedimentation technique offers precise measurements at low shear but requires relatively... 

If the swelling behavior of carboxylated latexes could be characterized only by the increase in the volume fraction, i.e., in particle size, Eq. 1 would still hold but this is not the case in carboxylated latexes. The latex particles are swelled to a great extent, and the polymer segments are dissolved into the aqueous phase and interact between the particles by the hydrogen bonds and chain entanglements. Thus, Eq. 1 is not expected to hold at all and the interparticle interactions are expected to be dominant in the viscosity development of the latex. 

From the above Results and Discussion, one could speculate the following scheme given in Figure 14, of alkali-swelling and/ or dissolving behaviors of the MMA-MAA carboxylated latexes. 

Nishida, S., "Preparation, Characterization and Alkali-Swelling Behavior of Carboxylated Latexes", Ph.D. Dissertation, Lehigh University, 1980. 

While most studies of particle swelling have dealt with emulsion polymers containing high acid levels, we have restricted our attention to latexes containing relatively low incorporated acid (ca.2-3%) to avoid substantial solubilization of the particles. Questions under current consideration are related to the location of incorporated carboxyl groups within the latex particles, the morphology of expanded particles, and the detailed mechanism of expansion. Information of this type is valuable not only from a fundamental standpoint but is essential in the investigation of very practical problems such as the viscosity stability of latex formulations. 

The results are generally consistent with a broader treatment of the techniques for measuring particle swelling of carboxylic emulsion polymer latexes reported elsewhere in this Monograph (9). The broader study, which was carried out independently but concurrently, has shown that the magnitude and pH of maximum expansion depends on dilution and ionic strength. Studies of the concentration dependence in the dilute regime and more concentrated systems are underway. 

This microscopic difference in the copolymer composition could influence the particle morphology, especially the distribution of the carboxyl groups within the latex particle, which in turn could be expected to influence the alkali-swelling behavior. 

Figure 14. Schematic explanation of alkali-swelling and/or dissolving behaviors of carboxylated MMA-MAA copolymer latexes...

The extraordinary increases in viscosity exhibited by the semi-continuous latexes with MAA contents of 15, 20 and 25% were attributed to the balance of repulsive forces due to dissociated carboxyl groups with attractive forces due to hydrogen bonds, which results in structure formation extending within the entire system. On the other hand the moderate increase in viscosity of the batch latex was attributed to limited swelling, and moderate interaction between the particles and restricted to the surface regions of the particles. 

Also it should be noted that this computerized HDC analysis for particle-size measurements oi these latexes was within 2% oi that measured by TEM. Since the particle-size determinations by the two different methods were in close agreement it was accepted that the HDC eluant composition was minimizing the particle-swelling phenomenon of the carboxylated S/B particles. 

By combining thermodynamically-based monomer partitioning relationships for saturation [170] and partial swelling [172] with mass balance equations, Noel et al. [174] proposed a model for saturation and a model for partial swelling that could predict the mole fraction of a specific monomer i in the polymer particles. They showed that the batch emulsion copolymerization behavior predicted by the models presented in this article agreed adequately with experimental results for MA-VAc and MA-Inden (Ind) systems. Karlsson et al. [176] studied the monomer swelling kinetics at 80 °C in Interval III of the seeded emulsion polymerization of isoprene with carboxylated PSt latex particles as the seeds. The authors measured the variation of the isoprene sorption rate into the seed polymer particles with the volume fraction of polymer in the latex particles, and discussed the sorption process of isoprene into the seed polymer particles in Interval III in detail from a thermodynamic point of view. 

The water-solubilfeed polymers lie between tiie latexes and polymer solutions i. e., between tiie colloidal and molecular sizes. These water-solubilized polymers are often prepared by the neutralization of carboxyl-containing latex particles with base, to give swelling and partial disintegration of the particles. 

The thickening mechanisms of linear carboxyl-containing emulsion polymers have been studied in considerable detail. The polymer molecules of AST emulsions are initially in a coiled configuration within individual latex particles of submicrometer size, and the viscosity of the diluted latex emulsion is similar to that of water prior to neutralization. On the addition of base, the carboxyl groups are ionized, and hydrophilic polymer is formed within the particles. Depending on various factors, which will be elaborated on later in this chapter under the section entitled Factors Affecting the Swelling Dissolution Behavior of Conventional ASTs , the particles may only swell or dissolve completely, or the surface polymer may dissolve and leave swollen cores. 

In an examination of the shear behaviour of latexes in which the particles were carboxylated terpolymers of styrene, butyl acrylate and ethyl acrylate Husband and Adams [94] found that shear coagulation was sensitive to pH. Their results are illustrated in Figure 3.29. Both the latexes used swelled with increase of pH... 

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