Mixed matrix membranes with molecular-interaction-driven tunable free volumes for efficient bio-fuel recovery

Mixed matrix membranes (MMMs), consisting of inorganic fillers dispersed in a polymer matrix, are regarded as one of the most promising futuristic membranes. This work reports the utilization of molecular interactions to finely control the conformation and topology of polymer chains to fabricate high-performance polyhedral oligomeric silsesquioxanes (POSS)/polydimethylsiloxane (PDMS) MMMs. The influence of the incorporation of POSS on the polymer structure was systematically studied by molecular dynamics simulations combined with DSC, XRD and IR measurements. The surface and interfacial morphologies of the MMMs were observed through SEM, TEM and AFM characterizations. In particular, positron annihilation spectroscopy was employed to analyze the evolution of free volumes in the MMMs. Results indicated that facilely incorporating POSS into PDMS by molecular interactions could manipulate favorable interfacial morphology and tunable free volumes in MMMs. In the PDMS MMMs, the small free volumes were reduced and the large free volumes increased; these changes were beneficial for the preferential permeation of large-sized molecules through the polymeric membrane. As applied to the bio-butanol recovery from aqueous solutions, the prepared POSS/PDMS MMMs exhibited a simultaneous increase in permeability and selectivity, breaking the permeability-selectivity trade-off limitation, moreover transcending the upper bound of the state-of-the-art organophilic pervaporation membranes. Therefore, our work demonstrates that the proposed approach based on rationally creating molecular interactions can be expected to have broad applicability in fabricating high-quality MMMs for molecular separations.