Thickness Dependent Penetrant Gas Transport Properties and Separation Performance Within Ultrathin Polysulfone Membrane: Insights from Atomistic Molecular Simulation Serene Sow Mun Lock, Kok Keong Lau , Azmi Mohd Shariff, Yin Fong Yeong, Mohamad Azmi Bustam Department of Chemical Engineering, Research Center for CO 2 Capture, Universiti Teknologi PETRONAS, Perak Darul Ridzuan 32610, Malaysia Correspondence to: K. K. Lau (E-mail: laukokkeong@utp.edu.my) Received 29 May 2017; accepted 11 September 2017; published online 00 Month 2017 DOI: 10.1002/polb.24523 ABSTRACT: Simulation technique has been employed to eluci- date the effect of thickness upon confinement to gas transport properties in pure and binary mixtures within ultrathin polysul- fone membranes. It is found that the gas diffusivity, solubility, and permeability are improved with increment in membrane thickness, which can be rationalized through bigger free volume in thicker polymeric membranes attributed to diminishing chains relaxation. The effect is found to be exceptionally perceptible in thinner polymeric films beneath 400 A ˚ . Accuracy of the simula- tion methodology has been validated by demonstrating good accordance with actual gas permeability data. As for binary con- dition, the gas transport properties are demonstrated to be comparatively lower than its pure counterpart due to competitive sorption and barrier for diffusion in the presence of secondary gas molecules. In addition, quantitative re-evaluation of pub- lished correlations and establishment of new empirical models have been conducted to associate membrane thickness effect to gas transport characteristics. V C 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 00, 000–000 KEYWORDS: empirical models; membranes; molecular model- ing; molecular simulation; thin films; transport properties; ultrathin polymeric membrane INTRODUCTION Polymeric membranes play a pivotal role in gas separation applied in industrial application attributed to their various advantages, such as occupying a relatively smaller footprint, chemical free, cost-effective, high process flexibility, simplicity, and high energy efficiency. 1–7 Key to the extensive application of membrane system in industry is polymeric layer with superior performance, which forms the selective barrier to control relative rate of gas transport properties, has been fabri- cated in the magnitude of ultrathin size upon nanoscale dimen- sion. 3 It has long been recognized that permeance is directly correlated to the path length through which a certain gas mole- cule permeates. This circumstance contributes to reduction in membrane film thickness to achieve ultrathin membranes to be undoubtedly the most effective solution to accomplish higher permeation flux. 7–10 The selective layer is commonly made of glassy polymer, which governs the gas transport properties, such as diffusivity, solubility, permeability, and selectivity, in the thick- ness of 1000 Å or less. 4,11,12 Although departure of morphological changes and physical properties from bulk polymeric structure has been identified and reported in various published literature, 13 the systematic studies on thickness dependency of gas transport properties at the nanoscale dimension has been limited and often obscure. Some of the literatures have reported that gas transport characteristic should be virtually thickness inde- pendent 14,15 while some of the tributes have been devoted to concluding the opposite remarks. 16–22 For instance, Shish- atskii et al. reported increment in gas diffusion coefficients when film thickness of poly(vinyltrimethyl silane) (PVTMS) membrane is increased, while gas permeability coefficients, which are typically dependent upon the diffusion coeffi- cients, are in the contrary being found to be independent of system size. 14 The independent characteristic of gas perme- ability behavior has been further supported in published experimental data by Yoshida et al. for commercial Flemion V R membranes. 15 Nonetheless, a pool of supporting published literature data, such as those provided by Taylor et al. (water diffusion in organic material), 16 Korvezee and Mol (perme- ability of water vapor in high polymeric membrane), 17 Long & Thomson (diffusion and permeability coefficients of water vapor in polymers), 18 McCall et al. (diffusivity coefficient and Additional Supporting Information may be found in the online version of this article. V C 2017 Wiley Periodicals, Inc. JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 00, 000–000 1 JOURNAL OF POLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER