Sorption and permeation of gases in hyper-cross-linked hybrid poly(POSS-imide) networks: An in silico study

Networked hybrid materials containing various types of organic imides covalently bonded with polyhedral oligomeric silsesquioxanes (POSS) have recently been developed. One of their possible applications is for gas-separation under extreme conditions of pressure and/or temperature. Experimental results indicate that their permeation properties depend on the specific organic dianhydride precursor used in the synthesis. In this work, atomistic models of 6FDA-based and PMDA-based poly(POSS-imide) crosslinked networks have been used to investigate the sorption, desorption, relaxation and permeation properties of such materials at the molecular level and their dependence on the nature of the imide linker. Results have been compared to available experimental data. A highly-efficient parallelised version of the excluded volume map sampling test particle insertion (EVMS-TPI) method was used to obtain the infinite dilution solubilities of a number of small penetrants (N ₂ , O ₂ , CH ₄ and CO ₂ ) in these model networks. Analyses of the Boltzmann-weighted probability densities for the distribution of the insertion energies showed that very reliable estimates of solubilities could be obtained. Extensive molecular dynamics simulations were then performed to obtain the complete model sorption isotherms for CO ₂ and CH ₄ at 35 °C up to pressures of over 60 bar using an iterative technique which matches the chemical potential of the penetrant in the poly(POSS-imide) phase to that in the corresponding gas phase. The sorption-induced changes in volume, the significant space available to penetrants, the mean insertion energies and their distributions were characterized. Comparisons were not only made between both types of poly(POSS-imide) to characterize the effect of the linker, but also with (hypothetical) uncrosslinked mixtures of the POSS and dianhydride molecules in order to elucidate the effects of crosslinking. Given the known conditioning properties of CO ₂ , desorption isotherms were also obtained for this penetrant. These showed significant hysteresis. Prolonged exposure to high concentrations of CO ₂ was also found to lead to slow relaxations that have significant effects on the solubility. Permeabilities were obtained for methane and carbon dioxide at 100 °C, 200 °C and 300 °C at a pressure of ~ 2 bar. Differences in permeabilities were largely diffusion dependent.