In Situ Observation of Ca²⁺ Diffusion-Induced Superstructure Formation of a Rigid Polyanion

Diffusion of multivalent metallic ions into aqueous solution of rigid, negatively charged macromolecules of high concentration is an effective approach to prepare macroscopically anisotropic hydrogels. However, the mechanism for superstructure formation is still not clear. By observing the mixing process of a small drop of CaCl₂ solution with solution of a rigid polyanion, poly­(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), under the polarizing optical microscope, the diffusion profile of Ca²⁺ and detailed anisotropic gelation process of PBDT are revealed. Diffusion of Ca²⁺ into the surrounding PBDT solution immediately induces the formation of physical liquid crystalline (LC) gel with concentric alignment of PBDT. The thickness d of this region increases with diffusion time t, obeying the diffusion law d ∼ t^1/2. A thin ring of constant width (∼100 μm) with radial alignment of PBDT appears at the diffusion/reaction front, ahead of the concentric alignment region. When two drops of CaCl₂ fluxes meet, their outside thin rings interact with each other and the PBDT in this contacting region orients ±45° to the midline of the two drops. From these observations, we rationally contend that the internal stress induced by the contraction of gel phase is responsible for the ion diffusion-induced PBDT orientations. This structure formation mechanism gives insight into other diffusion-directed anisotropic gelation systems.