Poly(ethylene oxide)-block-poly(hexyl acrylate) Copolymers as Templates for Large Mesopore Sizes─A Detailed Porosity Analysis

Mesoporous materials with defined pore geometry act as important models for porous substances being applied in various fields of materials research due to their large surface area─from catalysis to coatings and from solar cells to batteries and capacitors. Thus, understanding structure-property relationships requires the capability of deliberately and precisely tuning the mesoporosity, i.e., pore diameter, connectivity, and wall thickness. However, especially for the interesting pore size range between 35 and 70 nm, only a few convenient block copolymer templates are available using micellar self-assembly. In this study, we synthesized poly(ethylene oxide)-block-poly(hexyl acrylate) copolymers (PEO-b-PHA) by a supplemental activator reducing agent atom transfer radical polymerization (SARA ATRP) and employed them as soft templates for the preparation of ordered mesoporous metal oxide powders with spherical mesopores of ca. 40 nm in diameter, as shown by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and small-angle X-ray scattering (SAXS). With the aid of argon physisorption, STEM-based tomography, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), we performed in-depth elucidation of pore shape and their mutual connection. In the case of mesoporous silica, 40 nm spherical mesopores are connected to 3-4 adjacent pores by 15 nm pore windows as well as 1-2 nm-sized micropores. These micropores seem to originate from single PEO chains penetrating the 17 nm thick pore wall. Compared to such mesoporous silica, mesoporous, crystalline zirconia possesses significantly higher pore accessibility. Furthermore, we prepared a set of PEO-b-PHA block copolymers with different block lengths, showing that mainly the PHA block length governs the mesopore size and thus enables mesopore size tuning. These results highlight that PEO-b-PHA is a promising template for the preparation of mesoporous metal oxides (in particular, crystalline ones) with tailored mesopore sizes, which enables systematic studies on property-porosity relationships.