Influence of metal ions on the alkali-swelling behavior of carboxylated acrylic polymer latexes

The alkalization of carboxylated acrylic polymer latexes by sodium hydroxid e gives rise to swelling of the particles. For a poly(n-butyl acrylate) lat ex copolymerized with 15 wt % methacrylic acid (MAA) and 7 wt % acrylonitri le the particle volume increases by a factor of 30. The alkali-swelling doe s not depend on the type of monovalent cation used in the base (LiOH, NaOH, KOH, NH4OH). In contrast, when bivalent cation bases such as Ca(OH)(2) are employed no latex swelling is observed during neutralization because of io nic crosslinking of the copolymer chains. Crosslinking also takes place whe n the bivalent cations (Ca2+ Zn2+ Mg2+) are added as chlorides to dispersio ns with latexes previously swollen by sodium hydroxide. In these experiment s the original size of the latexes is reached again at a molar ratio MAA: b ivalent metal ion of 2:1, i.e. at charge compensation of the carboxyl group s. The shrinking behavior is almost independent of the type of bivalent met al ion used. On the other hand, it is more pronounced when trivalent cation s such as Fe3+ are added. In general, the experiments demonstrate that the alkali swelling of acrylic latexes is dominated by electrostatic forces.