Tuning Pore Dimensions of Mesoporous Inorganic Films by Homopolymer Swelling

The functionality and applications of mesoporous inorganic films are closely linked to their mesopore dimensions. For material architectures derived from block copolymer (BCP) micelle co-assembly, the pore size is typically manipulated by changing the molecular weight corresponding to the pore-forming block. However, bespoke BCP synthesis is often a costly and time-consuming process. An alternative method for pore size tuning involves the use of swelling agents, such as homopolymers (HPs), which selectively interact with the core-forming block to increase the micelle size in solution. In this work, poly(isobutylene)-block-poly(ethylene oxide) (PIB-b-PEO) micelles were swollen with poly(isobutylene) HP in solution and co-assembled with aluminosilicate sol with the aim of increasing the resulting pore dimensions. An analytical approach implementing spectroscopic ellipsometry (SE) and ellipsometric porosimetry (EP) alongside the more commonly used atomic force microscopy (AFM) and small angle x-ray scattering in transmission (SAXS) and grazing-incidence (GISAXS) modes enabled to study the material evolution from solution processing through to the manifestation of the mesoporous inorganic film after BCP removal. In-depth SE/EP analysis evidenced an increase of over 40% in mesopore diameter with HP swelling and a consistent scaling of the overall void volume and number of pores. Importantly, our analytical tool-box enabled us to study the effect of swelling on the connecting necks between adjacent pores, with observed increases as high as ≈35%, knowledge of which is crucial to sensing, electrochemical and other mass transfer-dependent applications.