Tailoring Pore Size, Structure, and Morphology of Hierarchical Mesoporous Silica Using Diblock and Pentablock Copolymer Templates

Mesoporous materials of tailored pore size, structure, and morphology are of interests for a wide range of applications. It is important to develop synthetic methods that will allow for easy processing and facile structure modification. Here, we present the preparation of hierarchically structured bimodal mesoporous silicas using water-soluble poly(lactic acid-co-glycolic acid)-b-poly(ethylene oxide) (PLGA-b-PEO) diblock copolymer and poly(lactic acid-co-glycolic acid)-b-poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(lactic acid-co-glycolic acid) (PLGA-b-PEO-b-PPO-b-PEO-b-PLGA) pentablock copolymers as templates. The block copolymers were synthesized through a step-growth polymerization method using a commercial Pluronic F68 macroinitiator. Mesoporous silica samples were obtained by sol-gel chemistry in acidic aqueous solutions. Hexagonally (p6mm) ordered mesoporous silica particles were obtained in the presence of a PLGA-PEO diblock copolymer and exhibited bimodal pore size distributions in the range of 2-9 nm. Core-shell type mesoporous silica particles were obtained in the presence of the PLGA-PEO-PPO-PEO-PLGA pentablock copolymer and exhibited a large pore diameter up to 20 nm with distinct bimodal pore size distributions. The pore size increased when using a longer pentablock copolymer template in strong acid. The physicochemical properties were investigated using small-angle X-ray scattering (SAXS), nitrogen adsorption-desorption, transmission electron microscope (TEM), solid-state ²⁹Si nuclear magnetic resonance (NMR), and scanning electron microscope (SEM), respectively.