Sorption-Deformation Interplay in Hierarchical Porous Polymeric Structures Composed of a Slit Pore in an Amorphous Matrix
Résumé
Wood, as a natural adsorbent and significant building material, features complex biopolymer composition and hierarchical porous structure, endowing it with distinguished sorption properties accompanied by sorption-induced deformation. In former molecular simulation studies, sorption, deformation and coupled 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 mesoscopic slit-pore between two slabs of amorphous and microporous cellulose, aims at modeling sorption and sorption-induced deformation in hierarchical cellulosic porous structures. Specifically, the atomic system is numerically modeled by a hybrid workflow combining molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The results clarify the multiple sorption/deformation mechanisms in the porous materials with different slit-pore sizes, including water filling in micropores, surface covering at slit-air interface and subsequent capillary condensation in mesopores. In particular, before capillary condensation, the sorption behavior of AC matrix of the hybrid system is almost the same as for bulk AC, in which sorption and deformation enhance each other through sorption-induced swelling and additional sorption in the newly-created voids. Upon capillary condensation, however, interaction between the micropores and mesopore emerges. Water molecules in mesopore exert a negative hydrostatic pressure or tensile stress perpendicular to the slab surface on the matrices, resulting in an increase in porosity and water content, a decrease in distance between the center of mass (COM) of the slabs and thus a thinning of the slit-pore. Notably, the micropore space in slabs induced by either sorption stress in matrices or capillary forces in slit-pores is saturated with water once it has been created. As described by Bangham's Law, the surface area of the rough slit-pore slab increases proportionally to surface energy variation during surface covering. For the system comprised by compliant polymers like AC, however, the surface area enlargement is not resulting in an in-plane swelling as expected, but in an in-plane shrinkage along with an increase in local roughness or irregularity (an accordion effect).
Domaines
Physique [physics]Origine | Fichiers produits par l'(les) auteur(s) |
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