Controlling microstructural evolution in pechini gels through the interplay between precursor complexation, step-growth polymerization, and template confinement

The mechanisms driving microstructure formation in template-confined Pechini-type gel systems involving solid solutions of cerium oxide with alkaline earth metals are investigated. Three-dimensionally ordered macroporous microspheres and more extended bicontinuous networks with hierarchical porosity are synthesized directly from a Pechini sol-gel precursor within a colloidal crystal template. The type of morphology generated is related to the mechanisms of phase separation in the precursor, namely, nucleation and growth vs spinodal decomposition. These mechanisms are, in turn, determined by the citric acid concentration in the initial precursor solution and by electrostatic interactions of the precursor with the polymeric template. Microspheres generated by the nucleation-and-growth pathway can be produced between 1-3 μm in size, with polydispersities below 15%. They retain the ordered porous network left by removal of the template. The number of nucleation sites (i.e., oligomers and longer chains of complexed metal) is dependent on the reactant imbalance between metal-citrate complexes and ethylene glycol, as predicted by step-growth polymerization statistics. This method expands existing phase-separation techniques currently exploited in metal alkoxide systems to the production of microstructure in ceramic oxides.