Computational insights on the role of film thickness on the physical properties of ultrathin polysulfone membranes

Abstract

Although it has been reported that physical properties of polymeric membranes inherit thickness dependent characteristics, typically when they are subjected to confinement at an ultrathin dimension (<1000 Å), deviations from their bulk counterpart are still not completely understood. An empirical investigation of physical properties for an ultrathin membrane at laboratory scale is difficult, time consuming, and costly which is attributed to challenges to fabricate defect-free films with ultrathin thickness and that requires special instruments at critical conditions. In our current work, a Soft Confining Methodology for Ultrathin Films was conducted to simulate ultrathin polysulfone polymeric membranes of varying thicknesses, l, to resemble their actual size in the thickness dimension. Subsequently, physical properties of the constructed ultrathin films, e.g., density and glass transition temperature, have been elucidated from an atomistic insight. Quantitative empirical models have been proposed to capture thickness-dependent physical properties upon ultrathin confinement. In addition, free volume and cavity distribution was also quantified in order to elucidate the evolution in membrane morphology and to satisfy a previous research gap of deficiency in system dimension dependent cavity sizes. On the whole, it was found that a thinner structure exhibits higher structural density and lower glass transition temperature, as well as lower free volume and cavity sizes. The findings from the present work are anticipated to propose an alternative from a molecular simulation aspect to circumvent complexities associated with experimental preparation and testing of ultrathin polymeric membranes, while providing direct elucidation and quantification of thickness-dependent physical properties in order to enhance understanding at a molecular perspective.