ABSTRACT
The microscopic interfacial structures for a series of metal-organic framework/polymer composites consisting of the Zr-based UiO-66 coupled with different polymers are systematically explored by applying a computational methodology that integrates density functional theory calculations and force field-based molecular dynamics simulations. These predictions are correlated with experimental findings to unravel the structure-compatibility relationship of the MOF/polymer pairs. The relative contributions of the intermolecular MOF/polymer interactions and the flexibility/rigidity of the polymer with respect to the microscopic structure of the interface are rationalized, and their impact on the compatibility of the two components in the resulting composite is discussed. The most compatible pairs among those investigated involve more flexible polymers, i.e. polyvinylidene fluoride (PVDF) and polyethylene glycol (PEG). These polymers exhibit an enhanced contact surface, due to a better adaptation of their configuration to the MOF surface. In these cases, the irregularities at the MOF surface are filled by the polymer, and even some penetration of the terminal groups of the polymer into the pores of the MOF can be observed. As a result, the affinity between the MOF and the polymer is very high; however, the pores of the MOF may be sterically blocked due to the strong MOF/polymer interactions, as evidenced by UiO-66/PEG composites. In contrast, composites involving polymers that exhibit higher rigidity, such as the polymer of intrinsic microporosity-1 (PIM-1) or polystyrene (PS), present interfacial microvoids that contribute to a decrease in the contact surface between the two components, thus reducing the MOF/polymer affinity.
ABSTRACT
Density functional theory calculations have been performed in order to extract the charge distribution in the aluminium-containing MIL-53 structure, to allow further computational studies of adsorption in these materials. Both cluster and periodic methods have been used and the charges calculated for each atom constituting the organic and inorganic part of the material, were discussed. Preliminary grand canonical Monte Carlo simulations, based on a consistent set of potential parameters and this newly derived charge distribution, predicted for enthalpies of adsorption for CO(2) at low coverage in the "large" and "narrow" pore versions of MIL-53 (Al) to be significantly different. These calculated enthalpies reproduced the two distinct ranges of values observed by microcalorimetry on either side of 6 bars quite well. This agreement between experiment and simulation validated our previous assumption, suggesting a structural switching of the hybrid material during the adsorption process. The microscopic mode of interaction between the hybrid porous framework and the CO(2) adsorption was then carefully analysed in both of the MIL-53 (Al) structures.
