Carboxylated isoprene/styrene copolymer latexes were prepared, and the concentration of surface carboxyl groups and the freeze–thaw stability of these latexes were measured. It is clarified that the introduction method of the carboxylic monomer governs the distribution of carboxyl groups in latex particles. Introduction of the carboxylic monomer prior to the initiation of polymerization localizes only 12–13% of total carboxyl groups on the surface of particles, whereas introduction of the carboxylic monomer at the last stage of polymerization localizes more than 60% of total carboxyl groups on the surface of particles, though the percentage is strongly dependent on the conversion when the monomers are shot. These latexes show a remarkable increase in the freeze–thaw stability with the increase in the degree of neutralization of surface carboxyl groups in a fairly narrow range. There exists a linear relationship between the surface area occupied by a surface carboxyl group and the degree of neutralization of surface carboxyl groups at which the remarkable increase in the freeze–thaw stability is observed. The result suggests that the freeze–thaw stability is substantially determined by the density of carboxylate ion on the surface of particles.
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Sakota et al. (1976) studied this question.