Preparation of Various Nanomaterials via Controlled Gelation of a Hydrophilic Polymer Bearing Metal-Coordination Units with Metal Ions

We investigated the gelation of a hydrophilic polymer with metal-coordination units (HPMC) and metal ions (Pd-II or Au-III). Gelation proceeded by addition of an HPMC solution in N-methyl-2-pyrrolidone (NMP) to a metal ion aqueous solution. An increase in the composition ratio of the metal-coordination units from 10 mol% to 34 mol% (HPMC-34) increased the cross-linking rate with Au-III. Cross-linking immediately occurred after dropwise addition of an HPMC-34 solution to the Au-III solution, generating the separation between the phases of HPMC-34 and Au-III. The cross-linking of Au-III proceeded from the surface to the inside of the HPMC-34 droplets, affording spherical gels. In contrast, a decrease in the ratio of metal-coordination units from 10 mol% to 4 mol% (HPMC-4) decreased the Pd-II cross-linking rate. The cross-linking occurred gradually and the gels extended to the bottom of the vessel, forming fibrous gels. On the basis of the mechanism for the formation of gels with different morphologies, the gelation of HPMC-34 and Au-III provided nanosheets via gelation at the interface between the Au-III solution and the HPMC-34 solution. The gelation of HPMC-4 and Pd-II afforded nanofibers by a facile method, i.e., dropwise addition of the HPMC-4 solution to the Pd-II solution. These results demonstrated that changing the composition ratio of the metal-coordination units in HPMC can control the gelation behavior, resulting in different types of nanomaterials.