Acid distribution, carboxylated latices

Garjria C, Vijayendran B.R (1983) Acid Distribution in Carboxylated Vinyl-Acryhc Latexes J. Appl. Pol3rm. Sci. 28 1667-1676. 

The polymerisation of styrene and acrylic acid by seeded batch emulsion polymerisation was investigated. The effects of acrylic acid content and pH on the polymerisation rate and the amount of carboxyhc acid groups in the final latex product was studied. Aqueous conductometric titration and nonaqueous potentiometric titration were used to determine the distribution of the functional groups over the aqueous phase, the latex particle surface and the interior of the latex particle. The carboxylic acid group distribution along with kinetic results provided information about the process of incorporation of acrylic acid into the latex product. In order to increase the surface incorporation efficiency a two-step process in which a shot of acrylic acid was performed in the last stage of the reaction of investigated. 23 refs. 

A study was made of the impact of incorporation of a small amount of carboxylic monomers (acrylic acid or methacrylic acid) into the latex particles in the limited flocculation process, often encountered in the semi-batch surfactant-free emulsion polymerisation of pure butyl acrylate. The possibility of producing carboxylated polybutyl acrylate latices with a smaller particle size was evaluated. The resultant latex was characterised to gain a better understanding of the effect of the surfactant-free technique on their physical properties, e.g. zeta potential, distribution of acrylic acid or methacrylic acid in the particles, and stability towards the added salt, compared with the conventional emulsion polymerisation system stabiUsed by surfactants. 35 refs. 

Until recently, there was only one report about the use of reactive costabilizers in miniemulsion polymerization [125]. In that study, dodecyl methacrylate (DMA) and stearyl methacrylate (SMA) were been used as cosurfactants with SDS and compared with cetyl alcohol (CA) and hexadecane (HD). It has been shown that DMA behaves like CA, whereas SMA displays a behavior similar to HD in terms of droplet size stability as well as in the particle size distribution of latexes. However, the distribution obtained using these reactive hydrophobes is in both cases somewhat narrower than for the model compounds. More recently, the same team published a study where in the polymerization of styrene in miniemulsions stabilized using DMA or SMA, small quantities of acrylic acid or methacrylic acid were added [126]. The authors were chiefly interested in the nucleation mechanism. Surprisingly, the addition of these hydrophilic monomers tends to favor nucleation within the droplets more than homogeneous nucleation, which is the dominating mechanism in the absence of these water-soluble monomers. The explanation lies in the fact that the styrene-carboxylic co-oligomers, because they are much more hydrophilic, are more reluctant to nucleate new particles. 

In order to compare the PMMA results with those obtained with the carboxylic acrylic latex, the concentration of surface carboxyls must be determined. Acid location analysis (5 was carried out for this purpose. Briefly, the latexes were titrated conductometrically with 0.1N NaOH followed by a titration of the aqueous phase from which the particles had been removed by centrifugation. The difference in the two titrations provided the distribution between surface and soluble acid. The deficit between the total acid thus determined and the concentration of acrylic acid used in the polymerization was termed "buried". Although some drift occurred in the conductance with time, an equilibration time of approximately 10 minutes per addition of sodium hydroxide was generally sufficient to yield stable readings. 

Many investigators have studied polymer surfaces for years [74,75] and have been successful in determining combinations of two or more valence states [76,77] by the mathematical process of deconvoluting the peak assignments [78]. It was only recently that latexes were examined by ESCA. Davies et al. [79] prepared a series of homopolymers of poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), and also poly[(methyl methacrylate)-co-(butyl methacrylate)] (PMMA-PBMA), by surfactant-free emulsion polymerization. It was found that the surface of the latex film was rich in PMMA, which may possibly be explained by the reactivity ratios for the MMA/BMA system (ri = 0.52 and rj = 2.11) [80], Recently, Arora et al. carried out angle-dependent ESCA studies on a series of films prepared from core-shell ionomeric latexes (with a polystyrene core and a styrene/n-butyl acrylate/ methacrylic acid copolymer shell) to determine the distribution of carboxyl groups in the films [81,82]. 

An important characteristic of a carhoxylated latex is the way in which the carboxylic-acid groups in the latex are distributed within individual polymer molecules and particles, and within the latex as a whole. In principle, acid groups can be present at the end of the reaction in one or more of five principal forms ... 

It has already been noted previously that high pH favours partitioning and polymerisation of the carboxylic acid in the aqueous phase. Several studies have been made of the distribution of carboxylic-acid groups amongst the various possible forms at the end of the polymerization reaction, and of the way in which that distribution is affected by the nature of the acid monomer (29-40 21,22). The general conclusion reached from acid-group location studies is that the less hydrophilic carboxylic-acid monomers, such as methaciylic acid, tend to become buried in the latex particles, whereas the more hydrophilic acids, such as acrylic and itaconic acids, tend to become incorporated at the surface of the latex particles or as water-soluble and surface-active polymers. An impressive illustration of the marked reluctance for methacrylic acid to polymerize in the aqueous phase is to be found in the observations of Nishida et al. (21) on the emulsion copolymerization of methyl methacrylate and methacrylic acid. 

In reaction systems in which carboxylation is effected by the use of the half-esters of unsaturated dicarboxylic acids and mono-hydric alcohols, acid-catalysed hydrolysis of the half-ester can be a factor which complicates the distribution of carboxylic-acid groups in the final latex (15) the shorter is the alkyl chain length of the alcohol from which the ester was prepared, the greater is the tendency to hydrolysis. The presence of interaction products of carboxylic acids and one or more of the main monomers can be a further complicating factor an example is the foimation of surface-active products by Biels-Alder reaction between dienes and unsaturated acids thus butadiene can react with maleic acid to form 1,2,3 6-tetrahydrophthalic acid (15) ... 

After indicating the reasons for current industrial interest in carhoxylated latices, a general review is given of the production of these latices by emulsion copolymerization of the main monomers with minor amounts of unsaturated carboxylic acids. In addition to discussing the types of carboxylating acid which can be used, consideration is also given to surfactants, initiators, modifiers, electrolytes and post-polymerization treatments. The factors which affect the distribution of carboxylic-acid groups in the final latex are discussed these include the hydrophilicity of the carboxylic-acid monomer, the pH of the reaction system, and procedural aspects such as the way in which the acid monomer is added to the reaction system. The effects of carboxylic-acid monomers upon the rate of polymerization and the mechanism of particle nucleation are also reviewed. 

Carboxyl and amino-functionalized latex particles were synthesized [67] by the miniemulsion polymerization of styrene and acrylic acid or 2-aminoethyl methacrylate hydrochloride, and the effect of hydrophilic comonomer and surfactant type (nonionic versus ionic) on the colloidal stability, particle size, and particle size distribution was analyzed. The reaction mechanisms of particle formation in the presence of nonionic and ionic surfactants were proposed. 

---