Anisotropic Deformation in a Polymer Slab Subjected to Fluid Adsorption
Résumé
Nanoporous adsorbents can mechanically swell or shrink once upon the accumulation of guest fluid molecules at their internal surfaces or in their cavities. Existing theories in this field attribute such sorption-induced swelling to a tensile force, while shrinkage is always associated to a contractive force. In this study, however, we propose that the sorption-induced deformation of a porous architecture is not solely dictated by the stress conditions, but can also be largely influenced by its mechanical anisotropy. In more detail, the sorption-induced deformation of a polymeric slab is investigated using a hybrid molecular dynamics (MD) and Monte Carlo (MC) algorithm. When subjected to water loading, the slab is found to swell along its normal direction and display an overall positive volumetric strain. Moreover, the surface roughness is enhanced as a response to the surface energy decrease induced by the water covering on the slab external surface. Unexpectedly, the inplane deformation of the slab material seems to be highly constrained so that it is far below its normal counterpart. This anisotropy is enhanced when decreasing the slab thickness. With the thickness of around 1.35 nm, an in-plane shrinkage is observed throughout all the hygroscopic range. A theoretical analysis based on a poromechanical model suggests that the anisotropic mechanical properties, which is common for a slab material 1 , is the essence of the constrained in-plane swelling or even shrinkage under the isotropic sorption-induced tensile forces. This study, unveiling overlooked mechanisms of the sorption-induced shrinkage in mechanically anisotropic materials, provides new insights in this field.
Sorption-induced deformation, referring to the mechanical response of a porous solid matrix to the accumulation of guest molecules at its internal or external surfaces or within its cavities 2 , is ubiquitous in natural materials. Such phenomena have also proven to be the driving force of several intriguing self-modulation processes, e.g., water-actuated movement of plant issue 3,4 . In the past decades, thanks to the development of material sciences, similar phenomena have been intensively reported with the emergence of nanoporous adsorbents. Meanwhile, the variations in materials properties concomitant with deformation, along with their application potential in various fields, e.g. sensing 2,5 , actuation 2,6 , natural gas recovery 2,7 and geological CO2 sequestration 2,8 , have arouse wide attention. Recent endeavors in this community are to suppress the side effects of the sorptioninduced deformation and to promote its positive impacts in practical applications. To achieve this, a deep understanding of the mechanisms at play is a priority. Sorption-induced deformation can be swelling, shrinkage or both. The pore topology of the sorbent plays a dominate role in the deformation characteristics. Here, we conform to the IUPAC pore classification, namely, micropores (width below 2 nm), mesopores (width within 2-50 nm) and macropores (width beyond 50 nm) 9 . Early-stage studies on microporous materials, such as charcoal 10-12 , reported a monotonous expansion upon the adsorbate loading. More recent studies on activated charcoal with graphite structure 13 and zeolite granules 14 indicated the possible existence of initial contraction. On the other hand, mesoporous materials, like Vycor glass 15 and silicon honeycomb 16 , possess two different regions of expansion separated by a region where contraction takes place 2 . Coal seams 8 and natural woods 17-19 with a wider pore-size distribution, ranging from micropores to mesopores or even macropores, are also studied, exhibiting a hybrid deformation pattern with both contraction and dilation. Numerous theoretical frameworks have been developed to interpret sorption-induced deformation. The first seminal work in this field was reported in 1928 by Bangham 11,12 , in which a linear correlation between the variations in surface area and the surface energy was postulated. Despite its success in predicting the monotonic expansion of charcoal samples, this theory neglected the difference between the surface stress and surface energy. The Shuttleworth equation 20 and Eriksson equation 21 were then proposed to fill this gap by putting forward a rational explanation to the shrinkage upon surface covering. Without considering the inter-wall interactions in a narrow space, the above-mentioned theories mainly aim at describing the adsorption on a surface rather than in nano-pores. Recently, thermodynamic theories incorporating the concepts of surface energy, solvation/ disjoining pressure and/or Laplace pressure were developed for nanoporous adsorbents. Among them, the models 22 based on the Derjaguin-Broekhoff-de-Boer (DBdB) 23 theory have proven to be quantitatively accurate in describing adsorption in mesoporous materials with the pore sizes larger than ~ 8 nm. Most of the theories, however, assume that the deformation is too small to impose a reciprocal influence on the sorption. This assumption is not relevant, i.e. applicable, for adsorbents with intricate porous structures (micropores) or highly-compliant scaffold. The poromechanical approach 24 , on the other hand, provides a solution for both microporous and mesoporous materials, since it takes into account the coupling between sorption and deformation. Generally, all the pre-existing theories on sorption-induced deformation attributes the swelling of the adsorbent to a sorption-induced tensile force (and, reciprocally, shrinkage is attributed to a contractive force). Moreover, anisotropic deformation is normally traced back to the anisotropy in sorption-induced forces 25 . However, in the present work, we report that the anisotropic deformation
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