Sorption-Induced Deformation of Nanoporous Materials

Abstract

Sorption-induced deformation is ubiquitous in nanoporous media, but underlying mechanisms are not yet fully understood, and a reliable modeling of this phenomenon is absent. Moreover hysteresis in sorption and swelling isotherms is observed but its origin not yet fully understood and not modeled. In this thesis the sorption-induced deformation of nanoporous media is studied systematically with different approaches. Three different nanoporous materials are considered: microporous polymers, microporous polymer-based composites and mesoporous materials. With the help of molecular simulations, the coupling mechanisms between sorption and deformation are revealed and the sorption and strain isotherms, as well as their hysteresis, are quantitatively modeled. With the knowledge gained at molecular level, a macroscopic description of sorption-induced deformation is given with the help of a dependent domain model. Molecular simulations demonstrate that microporous polymers swell upon water sorption as water molecules have a tendency to create more space between the flexible polymer chains for accommodating their presence. Sorption hysteresis is found to be related to deformation: polymers swell to form water–polymer hydrogen bonds upon adsorption but these bonds do not break upon desorption at the same chemical potential, which leads to sorption hysteresis. This hysteresis also manifests itself in other physical properties such as heat of sorption and bulk modulus. The influence of temperature and stress state on the coupled behavior is also examined. It is found that, when relating observable variables to the correct independent variables, hysteresis disappears as such explaining the actual origin of hysteresis. As a statement, hysteresis does not exist when looking at it from the correct driving potential. With the knowledge acquired on the bulk microporous polymer, the sorption-induced deformation of a microporous polymer-based composite, with cellulose nanocrystal (CN) as reinforcement and amorphous cellulose (AC) as matrix, is studied. Two competitive mechanisms are found regarding the coupling between sorption and deformation. The first mechanism is the reinforcing effect through CN-AC mechanical interaction, which constrains the sorption-induced swelling of the matrix and results in a reduction of sorption amount and of hysteresis in both sorption and deformation. The second mechanism is the CN-water interaction, enhancing water sorption in the matrix at the CN-matrix interface, increasing the sorption-induced swelling of the matrix and increasing the resulting hysteresis in sorption and deformation. Sorption-induced deformation in mesoporous materials is studied at single pore level with two atomistic models, a slit pore and a cylindrical pore. Two driving mechanisms are revealed for both slit and cylindrical pore models. At high relative vapor pressure, pore deformation is governed by Laplace pressure as the pore gets filled with liquid due to capillary condensation. At low pressure, when liquid films are formed on the pore surfaces and the pore remains mainly filled by vapor phase, the strain is driven by the attractive solid-fluid forces and the in-plane pressure within the film. Because of the interplay of these deformation mechanisms, the strain changes from shrinkage to expansion upon increase of pressure. The thesis ends with a Dependent Domain Model (DDM), developed to describe the coupled behavior at the macroscale of microporous polymers. The DDM is based on poromechanics taking into account the mechanical behavior of the solid and the influence of different pore sizes. The proposed dependent domain model captures the governing mechanism of the coupled behavior and provides a deeper understanding in sorption-induced deformation. Sorption-induced deformation is simulated and addressed systematically in this thesis regarding different materials, different coupling mechanisms and different scales. The simulation results agree with experiments and the proposed mechanism can explain the experimental results well. The outcome of this research provides a theoretical framework for modeling sorption-induced deformation of a great variety of nanoporous materials. Though the emphasis is laid at molecular scale, an upscaling approach is provided to connect the information at different scales.