Rubbery Polymer-Inorganic Nanocomposite Membranes: Free Volume Characteristics On Separation Property

The rational design of polymer-inorganic nanocomposite membranes relies heavily on the precise insight and elaborate control of the interface. Presently, the direct exploration of the hierarchical structure of nanocomposite membranes still remains elusive. In this study, we propose a facile and generic method to quantitatively probe the interfacial structure by complementary positron annihilation lifetime spectroscopy (PALS) and molecular dynamics simulation (MDS) techniques. As proof-of-principle, we choose anisotropic inorganic nanotube embedded rubbery polymer membrane as model, which imparts the interface between soft polymer and rigid inorganic. It is deduced that the segmental chain mobility in the vicinity of the polymer-inorganic interface is substantially restrained, which creates numerous nano-sized voids for molecular transport, and dramatically enhances the fractional free volume (FFV) of the membranes. The PALS results reveal that incorporation of titanate nanotubes (TNTs) narrows the free volume pore radius distribution of the membranes. Quite interestingly, we find that the rubbery membranes can also exhibit simultaneously increased permeability and permeaselectivity, and tentatively elucidate this unusual phenomenon by relating the separation properties to the free volume characteristics of the membranes.