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Carboxylic acid comonomers are often added to latex formulations to improve colloidal stability and to help control the physical structure of composite (core-shell type) particles. We have performed a systematic study of the incorporation of methacrylic acid (MAA) within styrene/butyl acrylate copolymer seed latices, and determined the eventual effect on the morphology of the composite particles when using a second-stage monomer that is either polar (MMA) or nonpolar (BMA). These latices have been produced in batch and semibatch reactions in the pH range of 3-7. At low pH, the MAA groups are not ionized, but at the higher pH they may be nearly completely ionized. Here, we report that for batch reactions carried out within the above pH range, the latex particle morphologies of the PMMA second-stage systems change dramatically with increasing pH, while those for the PBMA system do not change at all. These results show that one cannot easily generalize the effect of acid comonomers on the morphology of composite latex particles, as this depends upon the choice of the copolymers in the latex and the process characteristics of the polymerization reaction. C ontrol of the morphology of latex particles has been a well-practiced art within industry for some time now, given its great importance in determining the physical properties of composite polymer systems. These structured latex particles find applications in modern coatings and adhesives, impact modifiers, medical diagnostics, etc. Among the significant number of variables used to produce structured latex particles, two of them stand out due to the fact that in small amounts they make large differences in the latex properties. These two variables are crosslinking and the incorporation of carboxylic acid comonomers. This article deals with the latter.Acid comonomers, particularly acrylic and methacrylic acids but also fumaric and itaconic acids, are very commonly added to latex recipes. While this is not common when preparing impact modifiers, it is nearly always done for latices that are intended for the coatings markets. Often the acids are added to improve the mechanical, freeze-thaw, and pigment-mixing stability of the latices. There have been many studies done to determine the location of the acid polymer in the latex particles and in the aqueous phase surrounding them. 1-4 Another area of study has been the partitioning of the acid monomers between the aqueous and particle phases during polymerization. Acrylic, itaconic, and fumaric acids are highly partitioned to the water phase even at pH levels below the pKas of these acids, while methacrylic acid (MAA) is more reasonably balanced between the phases. 5 The use of acrylic acid (AA) tends to produce significant amounts of water-soluble polymer, and that portion of the acid copolymer that is in the particles is usually located near the outer surface of the latex particles. With MAA, there is significantly less water-soluble polymer formed and the MAA copolymer in the particle is somewhat more evenly dispersed ...

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