Nanostructure of xanthan networks

Abstract

Investigations of structure formation in linear and cross-linked polymer systems including one based on xanthan, an exopolysaccharide, attract theoretical and practical interest because of their widespread applications. Freeze-fracture transmission electron microscopy is used to study the structural topology formed in aqueous xanthan solutions and xanthan hydrogels. The data enable us to visualize for the first time an intact structure of xanthan formed in dilute and semidilute solutions at concentrations ranging from 0.002 to 0.5 wt %. In addition to single macromolecules, the dilute xanthan solutions are shown to contain microgels. When the concentration grows to values close to an overlap concentration (C*), a weak gel structure appears, being composed by a continuous 3D network with relatively large meshes. Macromolecular aggregation is observed in the network skeleton when switching to semidilute entangled solutions (>C**); it is accompanied by a compaction of network structure. The cross-linking of polymer chains by polyvalent Cr3+ ions gives a network skeleton composed of aggregated macromolecules at lower concentrations (around C*). Macromolecular aggregation in the skeleton of a network polymer structure occurring both in the absence and in presence of Cr3+ cations is indicative of a microphase separation into polymer-poor and polymer-rich regions.