Dual control of external surface and internal pore structure of small ordered mesoporous silica particles directed by mixed polyion complex micelles

Abstract

A versatile approach has been developed to prepare small mesoporous silica particles with simultaneous control of the internal ordered pore structure and the external particle surface. Mixed polyion complex (PIC) micelles are used as silica structure-directing agents: they result from the complexation of a polybase with two polyacid double-hydrophilic block copolymers (DHBC) having either a poly(ethylene oxide) (PEO) based-block or a polyacrylamide (PAM) block. The ionizable block in both DHBC is poly(acrylic acid), which complexes oligochitosan to form the core of the electrostatic complex. By varying the architecture of the PEO-based block (linear or comb-shaped) and the synthesis parameters, it is possible to modulate the pore structure from 3D cage-like to 2D-hexagonal and lamellar mesostructures. Replacing a fraction of the PEO-based polymers with DHBC having a polyacrylamide block that has no affinity for silica is shown to affect silica-micelle interactions and material growth. While the PEO chains interact with silica to form the hybrid interface, the PAM chains act as capping agents and control the external surface of the particles. Increasing the relative amount of PAM-based DHBC leads to the formation of small discrete mesoporous silica particles that are reduced in size to 200 nm. The particle size reduction and particle surface stabilization by PAM chains can be explained by considering not only the existence of a mixed corona of PAM and PEO in PIC micelles but also the differentiated solubility of these two neutral blocks induced by silica condensation. Thus, the present strategy allows independent decrease of the particle size and tuning of its pore structure.