SYNTHESIS OF CALCIUM CARBONATE-CHITOSAN COMPOSITES VIA BIOMIMETIC PROCESSING

The crystal growth of calcium carbonate on a chitosan substrate was achieved using a supersaturated calcium carbonate solution, at different concentrations of polyacrylic acid (PAA) as an additive. Several techniques have been employed to characterize the systems. The pH of the solution as the one of indices was used to monitor the crystallization. In the absence of polyacrylic acid, the pH of the solution changed from 6.00 to 8.50 during the crystallization; meanwhile, sporadic nucleation and crystallization was observed via optical microscopy. By introducing polyacrylic acid to the systems, positively charged protonated nitrogen and negatively charged carboxylate ions were produced by reaction between the amino group in chitosan and the carboxyl group in polyacrylic acid, which were detected by ATR-IR and XPS techniques. These charges induced calcium carbonate nucleation of calcite and vaterite crystals on the chitosan-film surface: The average size of the vaterite phase was about 15 nm, determined by XRD. The pH of the solution changed from 5.80 to 9.25 during the crystallization; moreover, the crystals showed spherical morphology, which consisted of a large number of small particles with a diameter of about 0.2 mu m. (C) 1995 John Wiley and Sons, Inc.