Morphologies of oriented alginate gels crosslinked with various divalent metal ions

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Abstract
Optically anisotropic cylinder-shaped alginate gels crosslinked with various divalent metal ions such as Ca, Cu, Zn, and Pb were prepared by the flow-gelation method in which semidilute sodium alginate solution was extruded through a tube into metal nitrate salt solutions. Effects of metal ions on the morphological and structural features of the gels were examined mainly by polarized optical microscopy. The chain orientation was characterized by a planar orientation where the mean chain orientation is primarily parallel to the gel surface and normal to the total flow direction, due to diametric expansion and longitudinal shrinkage on gelation. The degree of chain extension and orientation was shown to increase in the order Pb > Cu > Ca > Zn. Scanning electron microscopy of the gels dried rapidly under vacuum showed that all the metal alginate gels had cylindrically developed sheet strucures. Molar ratios of metal ions bound to alginate chains were estimated, using plasma spectroscopy, to be in the range 0.42–0.45.