Comprehensive Analysis of Key Parameters Determining Formation and Structural Properties of Sol-Gel-Derived Nanoporous Polymers.

This study presents a comprehensive investigation on the relationship between structure, synthesis parameters, and porous properties of sol-gel-derived polymer gels. The formation of the porous gels is monitored with in situ small-angle X-ray scattering, in situ nuclear magnetic resonance spectroscopy (NMR), and NMR cryoporometry. The transition of the reaction solution to a solid gel is governed by the consumption of the phenolic monomer. Primary particle growth and nanopore formation proceed during this short time period and are completed when all resorcinol is consumed. The kinetics of these processes are temperature-dependent and they are completed within 12 min at 120 °C and within 60 min at 80 °C. Extending the reaction time further results in enhanced cross-linking of the polymer, as observed by solid-state 13C NMR spectroscopy. Extended reaction time, i.e., higher degree of polymer cross-linking, enhances pore stability and reduces gel shrinkage during drying, resulting in xerogels with larger pore volume, larger external surface area, and larger average pore sizes. This work rationalizes molecular-scale transformation of polymers with macroscopic properties, thus providing a rational tool for tuning aerogel/xerogel performance through synthesis design.