Sorption-Deformation Interplay in Hierarchical Porous Polymeric Structures Composed of a Slit Pore in an Amorphous Matrix

LJ Hua and C Zhang and A Shomali and B Coasne and D Derome and J Carmeliet, LANGMUIR, 39, 11345-11356 (2023).

DOI: 10.1021/acs.langmuir.3c01103

Prevailingabsorbents like wood-derived porous scaffolds or polymericaerogels are normally featured with hierarchical porous structures.In former molecular simulation studies, sorption, deformation, andcoupled sorption-deformation have been studied for single-scale materials,but scarcely for materials where micropores (<2 nm) and mesopores(2-50 nm) coexist. The present work, dealing with a mesoscopicslit pore between two slabs of microporous amorphous cellulose (AC),aims at modeling sorption-deformation interplay in hierarchical porouscellulosic structures inspired by polymeric modern adsorbents. Specifically,the atomic system is modeled by a hybrid workflow combining moleculardynamics (MD) and grand canonical Monte Carlo (GCMC) simulations.The results clarify the multiple sorption/deformation mechanisms inporous materials with different slit-pore sizes, including water fillingin micropores, surface covering at the solid-air interface,and subsequent capillary condensation in mesopores. In particular,before the onset of capillary condensation, the sorption behaviorof the AC matrix in the hybrid system is almost the same as that ofbulk AC, in which sorption and deformation enhance each other throughsorption-induced swelling and additional sorption in the newly createdvoids. Upon capillary condensation, the interaction between the microporesand the mesopore emerges. Water molecules in the mesopore exert anegative hydrostatic pressure perpendicular to the slab surface onthe matrices, resulting in an increase in porosity and water content,a decrease in distance between the centers of mass (COMs) of the slabs,and thus a thinning of the slit pore. As described by Bangham'sLaw, the surface area of the rough slit-pore slab increases proportionallyto the surface energy variation during surface covering. For a systemcomposed of a compliant polymer like AC, however, the surface areaenlargement does not result in an in-plane swelling as expected butinstead in an in- plane shrinkage along with an increase in local roughnessor irregularity (an accordion effect).

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