Modelling ZIF interfaces in polymer composites Jasmine Lightfoot, Sharifah Alkandari 1 , Bernardo Castro Dominguez University of Bath, United Kingdom
Although polymer films are widely used in gas barrier and separation technologies, pure polymeric materials are limited due to a trade-off existing between gas selectivity and permeability. One option to enhance separation performance is through the addition of filler particles, which can absorb penetrating molecules or increase the effective diffusivity of certain gases. In our research group, we have demonstrated the success of cellulose acetate/ ZIF-67 composite films, which exhibit improved gas permeability and selectivity for CO 2 /N 2 , CO 2 /CH 4 , and O 2 /N 2 . In particular, we have shown experimentally that the performance of composite films can be further boosted through alternative manufacturing processes. Molecular simulations were performed to explain the observed improvements in barrier performance between ZIF-containing cellulose acetate films prepared through electrospraying (asymmetric membrane), compared to traditional mixing (mixed matrix membrane). Bulk and slab systems of ZIF-67 were generated and validated, before being used as the basis for a cellulose acetate/ ZIF composite model. In this study, the morphology of polymer chains at the interface was compared with those in a neat cellulose acetate bulk system. It was demonstrated that chains were preferentially elongated parallel to the surface, and exhibited lower mobility and higher density in proximity to the ZIF. Conversely to polymer slabs, which show little to no penetration of gas molecules over the course of a molecular simulation, carbon dioxide molecules readily entered and were retained within the ZIF inorganic matrix. We propose that in mixed matrix membranes, where particles are sparsely dispersed, the surrounding rigid, highly dense shell of cellulose acetate blocks oncoming gases, which are instead redirected to surrounding amorphous polymer. In asymmetric membranes, the alignment of particles normal to the gas flux force penetrant gases through the performance enhancing ZIF. Imperfections in the electrospun layer, which appear as gaps between adjacent ZIF particles, are instead plugged by interacting, dense cellulose acetate factions. As particles are closely packed, the option of bypassing the zeolitic framework via amorphous polymer is not possible in asymmetric membranes, resulting in enhanced separation performance.
IP11
© The Author(s), 2023
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