Fe ³⁺ -Coordination-Directed Assembly of Robust Fe-Incorporated Polymethylsilsesquioxane Xerogels with Branched Networks

Polymethylsilsesquioxane (PMSQ) xerogels represent a compelling alternative to conventional silica aerogels for ambient pressure drying, owing to their pendant methyl groups that confer low skeletal density (∼1.40 g/cm³), mechanical flexibility, and intrinsic hydrophobicity. However, their practical application has been limited by insufficient mechanical strength. Here, we address this limitation by developing Fe-incorporated PMSQ (Fe-PMSQ) xerogels via a synergistic acid-base catalytic route using FeCl₃ and NH₃·H₂O. The total base molarity (<i>c</i>_base) governs the sol pH, which in turn regulates Fe³⁺ hydrolysis speciation and the condensation kinetics of methyltriethoxysilane. Within an optimal pH range of 7.0-7.5, Fe(OH)₃ dominates the speciation and coordinates with cetyltrimethylammonium bromide (CTAB) micelles via Fe³⁺-directed assembly. This process templates a branched architecture that diverges from the conventional spherical or coral-like PMSQ networks. The resulting structure features Fe³⁺ nanoclusters acting as densely cross-linked nodes that form multiple Si-O-Fe linkages and restrict Si-OH condensation into rigid branches, while CTAB promotes local Fe³⁺ enrichment at network junctions. Gelation completes within 2 h with only about 10% linear shrinkage, surpassing the performance of both pristine PMSQ and Al-incorporated PMSQ benchmarks. Deviations from this optimal pH range hinder gelation: at pH ∼6.5, gelation is either delayed beyond 60 h or leads to macroscopic stratification, whereas above 7.5, rapid sol-precipitate phase separation occurs. The hierarchically branched structure endows the Fe-PMSQ xerogels with exceptional mechanical robustness, enabling them to withstand up to 80% compression strain without fracture and exhibit exponential strain stiffening between 80% and 90% strain. Moreover, the materials endure 400 compression cycles at 60% strain without failure and show stress recovery after rest periods of over 4 days. These properties collectively ensure structural integrity under demanding loading conditions, positioning Fe-PMSQ xerogels as viable candidates for industrial applications.