Enhanced pervaporation performance of PDMS membranes based on nano-sized Octa[(trimethoxysilyl)ethyl]-POSS as macro-crosslinker

A homogeneous distribution of inorganic fillers and their integration with polymer matrix is the primary challenge for advancement of mixed matrix membranes (MMMs) in pervaporation. Due to well-defined structure and multi-functional groups attached to the apex silicon atoms, polyhedral oligomeric silsesquioxane (POSS) provides a high possibility of good compatibility with diverse polymer matrix at the molecular level. In this work, soluble and reactive octa[(trimethoxysilyl)ethyl]-POSS(OPS) was synthesized, which was used as macro-crosslinker and reinforced porous fillers of polydimethylsiloxane (PDMS) synchronously to prepare a series of OPS/PDMS MMMs. The OPS/PDMS MMMs were characterized by FT-IR, XRD, SEM, TGA, DSC, PALS, vapor adsorption measurement and tensile test. It was found that OPS dispersed homogeneously in PDMS matrix and achieved good integration with PDMS based on SEM and DSC characterization. PALS results indicated that compared with pure PDMS membrane, OPS/PDMS MMMs with proper OPS loading showed much larger radii of free volume cavities (r(3), r(4)) and fractional free volumes (f(v3), f(v4) and f(v)). The numerous hydrophobic POSS cages and enlarged PDMS' free volume would effectively improve the diffusion selectivity towards ethanol. Vapor adsorption results showed that the chemical incorporation of moderate amount OPS into PDMS not only improved ethanol vapor adsorption amount, but also depressed water adsorption amount in PDMS, which would obviously improve PDMS' sorption selectivity to ethanol. As applied to the bio-ethanol recovery from aqueous solutions, the prepared OPS/PDMS MMMs exhibited a simultaneous increase in permeation flux and separation factor, breaking the permeation flux-separation factor trade-off limitation. As OPS loading was 7.5 weA, the separation factor of OPS/PDMS MMMs reached the maximum value of 16.4 (109% higher than that of pure PDMS membrane), with permeation flux of 253.1 g/m(2) h (32% higher than that of pure PDMS membrane). Experimental results showed that an increase in operation temperature resulted in an increase in permeation flux, but decrease in permeability, separation factor and selectivity. The pervaporation performance of OPS/PDMS MMMs kept stable during two days operation time. This work may provide useful insights of high-performance MMMs with a POSS embedded selective layer for biofuel separation.