Effect of Molecular Weights of Poly(acrylic acid) on Crystallization of Calcium Carbonate by the Delayed Addition Method

Abstract

The effect of poly(acrylic acid) (PAA) on the mineralization of CaCO3 was studied by varying the molecular weights (PAA1.2k, Mw=1200; PAA25k, Mw=25000; PAA250k, Mw=250000) as well as by changing the addition time of the sodium salts of PAA (PAA-Na) to an aqueous solution of calcium carbonate. The precipitation of CaCO3 was carried out by a double jet method. Stable vaterite crystals were successfully obtained by delaying the addition of all the sodium salts of PAA from 1 to 60 min, and the resulting particles were formed by a spherulitic growth mechanism. The vaterite particles modified with PAA25k-Na and PAA250k-Na showed higher stability than the ones stabilized by PAA1.2k-Na in an aqueous solution. However, in the initial presence of PAA-Na, amorphous calcium carbonate (ACC), vaterite or calcite were induced under various conditions. It is interesting to find that the ACC product induced by PAA1.2k-Na was more stable than that induced by PAA25k-Na in an aqueous solution as well as in a dry state. It was also found that the CaCO3 particles formed through the nano-aggregation mechanism might be induced by the strong inhibiting effects of PAA-Na, while the spherulitic growth mechanism might be due to the insufficient inhibiting efficiency for the crystallization. These results suggest that the selective interaction of PAA-Na with CaCO3 at different stages as well as the inhibiting strength varied with the chain lengths of PAA-Na could play an important role for the controlling of the crystal nucleation and growth during the crystallization process.