Controlled crystallization of hierarchical and porous calcium carbonate crystals using polypeptide type block copolymer as crystal growth modifier in a mixed solution

Abstract

Various kinds of nearly spherical calcium carbonate (CaCO₃) crystals with hierarchical and porous structures can be prepared using poly(ethylene glycol)-b-poly(aspartic acid) (PEG-b-pAsp) as a crystal growth modifier in a mixed solvent composed of N,N-dimethylformamide (DMF) and cyclohexanol. The results reveal that the porosity or specific surface area of these CaCO₃ crystals can be tuned by altering the volume ratio (R) of DMF/cyclohexanol in solution, and the pore size of the obtained spherical particles can be ranged from several tens to hundreds of nanometres. Additionally, most of the obtained calcium carbonate samples can be assigned to vaterite or a mixture of calcite and vaterite, which are well crystalline and are influenced by the R value. Interestingly, unique hierarchical and porous microspheres can be prepared at polymer concentrations of ∼ 0.5 g L⁻¹ and an R value of ∼ 1.0, respectively. It has been proposed that the formation of the specific CaCO₃ crystals with hierarchical and porous structures could be ascribed to the collodial aggregation transition and self-assembly of calcium carbonate precursor in a desirable mixed solvent. This specific synthesis strategy in a mixed solvent again emphasizes that it is possible to synthesize other inorganic/organic hybrid materials with exquisite morphology and specific textures.