Impact of a tert-butyl alcohol-cyclohexane system used in unidirectional freeze-casting of SiOC on compressive strength and mass transport

Highlights: • Tert-butyl alcohol-cyclohexane phase-diagram reveals an eutectic point at around −24 °C. • Honeycomb-like pore structure templated by tert-butyl alcohol-cyclohexane mixture. • Pore sizes range from 11-57 µm depending on the solvents and freezing condition. • Compressive strengths reach 5-39 MPa depending on the solvents. • Isothermal wicking rate can be adjusted from fast to slow depending on pore structure. Macroporous SiOC monoliths were prepared by solution-based freeze-casting of a polymethyl siloxane using cyclohexane (CH) and tert-butyl alcohol (TBA) as a novel template media. Samples with TBA amounts of 100, 90, 80 and 0 wt% were stable during preparation at −20 °C. Using TBA or CH creates prismatic or dendritic pore structures, respectively, while a mixture of these solvents generates honeycomb-like pore structures. A constant freezing rate produces homogeneous pore window sizes while freezing with velocity gradients produces inhomogeneous pore window sizes. Variations in the amount of TBA led to pore sizes between 11 and 57 µm and consequently to water permeabilities of 4.4 × 10^-13 to 1.4 × 10^{- 11}m². The dendritic pore structure has the highest compressive strength (39 MPa) due to its smallest pore sizes (16–20 µm) and secondary dendrites. In wicking experiments, these structural properties and the lowest permeability resulted in the slowest wicking rate in contrast to prismatic pore structures with the biggest pore sizes (31–57 µm) and highest permeability. Honeycomb-like pore structures allow medium wicking rates with the pore size being the main influencing factor. The adjustment of the solvent allows tailoring the mass transport and mechanical properties as key elements in capillary transport applications.