Counterintuitive Gas Transport through Polymeric Nanocomposite Membrane: Insights from Molecular Dynamics Simulations

Abstract

Completely contrary to classical composite theory, the gas permeability of certain rigid polymers was frequently found to increase upon addition of nonporous, nanoscale inorganic particles. Until now, the underlying mechanism remains elusive. In this study, polycaprolactone–TiO2 nanocomposite model was computationally constructed to clarify this issue. The molecular dynamics simulation results indicated that such counterintuitive behaviors arose from an extra region with depleted matrix phase and, hence, higher free volume at the polymer–filler interface. Owing to its inaccessibility and delicacy, this interfacial region was indiscernible by experimental means. However, it could be qualitatively visualized and quantitatively measured by simulated density field and density profile, respectively. By reproducing the thermodynamic property of polycaprolactone, we also conducted, for the first time, a comparative simulation of how polymer chains that shared the same primary structure but differed in rigidity behaved differently when packing around the highly curved nanoparticle surface. Such discrepancy was further found to correlate well with opposite trends in gas self-diffusivity in the resultant polycaprolactone–TiO2 nanocomposite model. Based on these results, the molecular mechanism leading to the formation of the interphase whose properties differed significantly from the bulk polymer was proposed. Coupled with previous data experimentally obtained, the present study offered a generic framework for understanding the molecular basis of interfacial architecture in polymeric nanocomposites, which was crucial in designing membrane devices with tailored permeability for specific applications, covering from breathable leather coating, through ultrahigh barrier blood sacs in ventricular assist devices, up to advanced gas separation membrane not subject to the empirical permeability/selectivity trade-off.