Control of Ion Transport in Sulfonated Mesoporous Polymer Membranes.

We investigated proton conductivity and the permeability of monovalent cations across sulfonated mesoporous membranes (SMMs) prepared with well-defined pore sizes and adjustable sulfonic acid content. Mesoporous membranes with three-dimensionally continuous pore structure were produced by the polymerization-induced microphase separation (PIMS) process involving the reversible addition-fragmentation chain transfer (RAFT) copolymerization of styrene and divinylbenzene in the presence of a polylactide (PLA) macrochain transfer agent and subsequent PLA etching. This allowed us to control pore size by varying PLA molar mass. Postsulfonation of the mesoporous membranes yielded SMMs whose pore structure was retained. The sulfonic acid content was adjusted by reaction time. While proton conductivity increased with increasing ion exchange capacity (IEC) without noticeable dependence on the pore size, ion permeability was strongly influenced by the pore size and IEC values. Decreasing pore size and increasing IEC resulted in a decrease in ion permeability, suggesting that ions traverse across the membrane via the vehicular mechanism, through the mesoporous spaces filled with water. We further observed that the permeability of the vanadium oxide ion was dramatically suppressed by reducing the pore size below 4 nm, which was consistent with preliminary vanadium redox flow battery data. Our approach suggests a route to developing permselective membranes by decoupling proton conductivity and ion permeability, which could be useful for designing separator materials for next-generation battery systems.

Syndiotactic polystyrene nanofibrils in silica nanotube reactors: understanding of synthesis with ultrahigh molecular weight.

Nanofibrils of ultrahigh molecular weight syndiotactic polystyrene (sPS) have been synthesized in a silica nanotube reactor (SNTR) using a metallocene catalyst in conjunction with methylaluminoxane cocatalyst. Very thin sPS nanofibrils (<10 nm) grown at the catalytic sites on the pore walls aggregate to form intertwined, rope-like nanofibrils with 30-50 nm diameters, which further intertwine into even larger 200 nm diameter polymer nanofibrils. The extrusion of nanofibrils synthesized inside the SNTR was directly observed by scanning electron microscopy, and the individual SNTR containing a single polymer nanofibril was separated and observed by transmission electron microscopy. The sPS synthesized in the SNTR has ultrahigh molecular weight (Mw = 928,000 g/mol) with a large fraction of 2 000,000-5,000,000 g/mol molecular weight polymers.