Effects of fabrication conditions on the microstructures and performances of smart gating membranes with in situ assembled nanogels as gates

Smart gating membranes with in situ assembled poly(N-isopropylacrylamide) (PNIPAM) nanogels as gates are successfully prepared via vapor-induced phase separation (VIPS) with different exposure time periods, temperatures and relative humidities of the water vapor. Effects of the fabrication conditions on microstructures as well as thermo-responsive and mechanical performances of the membranes are investigated. Both the membrane microstructure and the movement of blended PNIPAM nanogels in the membrane forming solution can be controlled by adjusting the fabrication conditions. With increasing the exposure time, the membrane microstructure undergoes a transition from typical liquid-induced phase separation (LIPS) structure (unsymmetrical finger-like porous structure) to typical VIPS structure (symmetric cellular-like porous structure). The critical time periods for the microstructure transition of membranes prepared with vapor temperature and relative humidity of 25°C/90%, 25°C/70% and15 °C/70% are about 1.5min, 2min and 10min respectively. The performances of membranes are heavily dependent on the microstructures. The membranes with unsymmetrical finger-like porous structures show large thermo-responsive factor (R39/20, the ratio of water flux at 39°C to that at 20°C), with the maximum R39/20 value being 43.2. All the membranes with symmetric cellular-like porous structures exhibit strong mechanical property and large water flux at 39°C, which is higher than the volume phase transition temperature (VPTT) of PNIPAM nanogels. The results provide valuable guidance for rational design and fabrication of smart gating membranes with desirable performances.