Cavity Energetic Sizing Algorithm Applied in Polymeric Membranes for Gas Separation

Abstract

Polymeric membranes demonstrate the fastest emerging field in membrane gas separation since it has huge reproducibility for large scale production and low cost fabrication. To date, many research works have emerged to propose different polymeric materials for membrane synthesis and fabrication in order to enhance gas separation performance. It has been suggested that the underlying factor that distinguishes the difference in separation efficiency among varying polymer is the amount of free volume formed throughout the polymeric matrix. Therefore, in current work, through adaptation of atomistic models by a combination of molecular dynamics and Monte Carlo or commonly known as the Cavity Energetic Sizing Algorithm (CESA), the cavity size distributions of several commonly adapted polymeric membranes for gas separation have been determined. The accuracy of the simulated molecular structure has been verified by comparing the simulated and measured experimental density. It is found from the study that that the CESA algorithm is capable of capturing the free volume distribution within the polymeric matrix, with the higher free volume polymeric membrane inheriting higher average cavity size [PTMSP (10.30A) >PPO (4.86A) >Matrimid® 5218 (4.01A) >PSF (3.25A)]. In addition, the higher free volume polymers, which exhibit shift towards the larger cavity sizes, also reveal higher gas permeability for gas molecules. The sieving capability of the polymer is also demonstrated to be correlated with the kinetic diameter of the gas penetrants as compared to the cavity size. In future work, the methodology is proposed to be employed as a preliminary step to predict polymeric membrane morphology in order to evaluate the feasibility of any polymers before being applied in gas separation.