Calcium carbonate crystal growth on a chitosan substrate was achieved using a supersaturated calcium carbonate solution. Heterogeneous nucleation of the calcium carbonate crystals was significantly influenced by the chitosan surface profile. Poly(acrylic acid) (MW = 2000, PAA2K) was chosen to modify chitosan surfaces in aqueous solution. The adsorption of PAA2K occurred on the chitosan film surface. The thicknesses of the adsorbed chitosan films were determined by ellipsometry. X-ray photoelectron spectroscopy (XPS) was used to determine the surface structure of the modified chitosan films. Calcium carbonate crystal sizes and morphology were observed under a polarized optical microscope and a scanning electron microscope (SEM). PAA2K promoted the heterogeneous nucleation of calcium carbonate crystals on the chitosan film and at the same time suppressed homogeneous nucleation in solution.

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SYNOPSISThe 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 pm. I NTRO DUCT10 NCeramic-polymer composites produced by natural organisms are known to have properties far beyond those that can be achieved in present technological materials.' The mollusk shell, for example, starting with relatively weak structural ingredients, calcium carbonate and chitin, is one such ceramic-polymer composite. In nacre, the overall composite is more than 95 volume percent calcium carbonate, with the remainder a n organic matrix of protein and chitin. The inorganic phase consists of highly oriented aragonite platelets forming the bricks and the organic matrix forming the mortar in between.' Even though nacre consists of a high amount of the inorganic component, it has excellent fracture toughness and high strength. Bone, another ceramic-polymer * To whom correspondence should be addressed. Instead of trying to duplicate the entire biomineralization process, scientists are dividing the process into stages that can be understood and adapted to practical processing scheme^.^ Hydroxyapatite crystallization from a n aqueous solution on decalcified collagen has been reported by Weiner and cow o r k e r~.~ Sikes and Wheeler have tested the effect of a variety of acid-containing peptides on calcium carbonate crystallization by using the kinetic app r~a c h .~ Compressed Langmuir monolayers have also been used for controlling the crystallization of calcium carbonate? Kinetic studies of hydroxyapatite crystallization on various substrates have been 687

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