Evaluating Nano-Scale Shifts: Quantifying Mesopore Shrinkage in Porous Polymers via NMR Cryoporometry

Nanoporous materials synthesized in a wet-chemical process often show different pore sizes in their as-prepared state in the reaction solution and their dried state. Depending on the materials, these changes can be reversible or irreversible. Determination of the exact pore sizes of such materials in their as-prepared state is challenging as conventional characterization methods for pore size determination only work on dry materials. Cryoporometry is applicable directly to wet materials though. Thus, it offers a unique opportunity for analyzing unaffected pores of materials prior to drying. With this method the "true" size distributions of pores in wet materials are accessible. Shrinkage of gels upon ambient drying is a known phenomenon. However, exact numbers are not accessible even though being essential for process design. Here, nuclear magnetic resonance (NMR) cryoporometry is used for determining pore sizes of sol-gel derived gels before and after drying, allowing for quantitative assessment of drying-induced pore shrinkage, an immensely valuable information barely accessible with other methods. The magnitude of pore shrinkage in the gels depends strongly on synthesis parameters. Higher monomer concentrations can mitigate drying-induced pore shrinkage. The quantitative information obtained with cryoporometry will allow for optimization and efficient design of porous materials for various applications.