Norimasa Fukazawa

Turbine-like structured silica transcribed simply by pre-structured crystallites of linear poly(ethyleneimine) bounded with metal ions

The formation of hierarchical aggregates from linear polyethyleneimine (LPEI) was performed in aqueous media containing a series of divalent metal ions (MnII, FeII, CoII, NiII, CuII, ZnII) and their organization processes are investigated by transcribing the structure into hard silica frames which can be appropriately characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements. Coexisting metal salts interact with LPEIvia coordination bonds between metal ions and amine groups in ethyleneimine (EI) units to form water-soluble complexes. Without suitable complexation, LPEI crystallizes without restriction and forms non-well controlled hierarchical microstructures even in the presence of metal ions. The molar ratios of EI units and metal ions strongly affect the morphologies of crystalline precipitates. Low values of the ratios lead to intricate flower-like structures composed of leaf-like fibers assembled in a random fashion. On the other hand, an increase of the ratios makes the structures well-shaped and flattened for constructing 2-D turbine-like morphology. These structures consist of numerous sheet structures grown in a radial fashion. XRD and XRF measurements reveal that these resulting solids are mainly composed of hydrated LPEI and imply that coordinating metal ions are located in the peripherals of LPEI microstructures. Finally, temporal morphological evolution experiments indicate that a metal ion which can coordinate with EI units serves as a regulator in the crystallization of LPEI and abundant crystal growth can proceed.