Nuclear magnetic resonance studies of resorcinol-formaldehyde aerogels |
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Authors: | Moudrakovski Igor L Ratcliffe Christopher I Ripmeester John A Wang Li-Qiong Exarhos Gregory J Baumann Theodore F Satcher Joe H |
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Institution: | Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada K1A 0R6. |
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Abstract: | In this article, we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, resorcinol]/catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized (129)Xe NMR in combination with solid-state (13)C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both coadsorption of chloroform and 2D EXSY (129)Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY (129)Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micropores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (V(g)/S) ratios for aerogels are measured for the first time without the use of geometric models. The V(g)/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in V(g)/S. |
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