Thin-film formation of calcium carbonate crystals: Effects of functional groups of matrix polymers

We report on the preparation of the thin films of calcium carbonate (CaCO3) crystals on solid matrixes, i.e., cellulose, chitosan, and chitin in the presence of an acidic macromolecule. The crystal growth from CaCO3 solution containing poly(acrylic acid) as a soluble acidic polymer results in the formation of CaCO3 thin-film crystals about 0.8 mum in thickness on these solid matrixes. The crystallite size is estimated to be about 30 nm by X-ray measurements and SEM observation. The derivatives of these polysaccharides, whose functional groups of the OH and NH groups are protected, have been prepared and employed as insoluble solid matrixes. No precipitation is observed for the crystallization on the polysaccharide derivatives possessing no proton-donating group even in the presence of poly(acrylic acid). Rhombohedral calcite crystals are obtained from the solution in the absence of the acidic polymer. The infrared spectra for the solid matrixes soaked in the solution show that the acidic polymer is adsorbed on the solid matrixes. The thin-film crystals are obtained only when the acidic polymer exists both in the solution and on the solid matrixes. The balance of the crystallization by the acidic polymer adsorbed on the surface and the inhibition by the acidic polymer in the solution should be important for the thin-film formation because the films are obtained in certain ranges of the concentration of the acidic polymer. The thin films grown on cellulose and chitin consist of only calcite, while those grown on chitosan mainly consist of vaterite. The effects of the concentration and the molecular weight of poly(acrylic acid) on the polymorphs of the thin-film crystals have been examined. The polymorphs of the thin films developed on chitosan are dependent on the concentrations of poly(acrylic acid) in the solution. Poly(acrylic acid) with a higher molecular weight leads to the deposition of thin-film crystals consisting of less stable polymorphs on these polymer matrixes.