Nano-porous polymer membranes are important for technologies in polymer membrane separations, rechargeable batteries, polymer electrolyte membrane fuel cells and film electrodes in flexible electronics/optoelectronics. The major two objectives of this thesis are to (1) develop dimensional stable nanoporous membranes that can be applied as thermal-fuse separators in safer batteries, which can also be facilitated with high power density; and to (2) prepare and investigate thin/ultra-thin membranes/graphene composite membranes and the mechanism of strengthening effects.

Battery safety has been of critical concerns and there are renewed interest in developing safer membranes for enhancing the inherent safety of lithium ion batteries. The preparation of a robust and safer self-composite ul...[ Read more ]

Nano-porous polymer membranes are important for technologies in polymer membrane separations, rechargeable batteries, polymer electrolyte membrane fuel cells and film electrodes in flexible electronics/optoelectronics. The major two objectives of this thesis are to (1) develop dimensional stable nanoporous membranes that can be applied as thermal-fuse separators in safer batteries, which can also be facilitated with high power density; and to (2) prepare and investigate thin/ultra-thin membranes/graphene composite membranes and the mechanism of strengthening effects.

Battery safety has been of critical concerns and there are renewed interest in developing safer membranes for enhancing the inherent safety of lithium ion batteries. The preparation of a robust and safer self-composite ultra-high molecular weight polyethylene (UHMWPE) membrane was reported. The self-composite membrane consists of ~200 nm nanopores homogeneously embedded inside interpenetrating nano-fibrilliar "shish kebab" networks. It performs the thermal fuse function by selectively melting its kebab crystals whilst the elongated shish fibrillary backbones remain intact. Simulated thermal fuse function tests show that the newly prepared separator displays a 300% increase in tensile strength (550 MPa), 300% increase in puncture resistance (1.5 N µm^-1) as well as an 18,000 times increase in impedance when lateral dimensions are kept constant. Cells prepared using the UHMWPE separators also exhibit a 10% higher energy density and better cyclability than those using commercial separators. Hence, our newly prepared ultra-thin and dimensionally stable membrane will enhance the safety protections for rechargeable batteries with low impedance for high-energy and power density.

There has been tremendous effort in exploring 2D graphene's ultimate reinforcement potential for polymer composites. Hitherto lack of freestanding polymer thin-films for the construction of ideal composite with full surface alignment of 2D graphene for direct mechanical property characterization has been a bottleneck for validating graphene's reinforcement limit. We synthesized a freestanding biaxial-oriented nanoporous ultrahigh molecular weight polyethylene (UHMWPE) 200 nm-thick membrane and aligned its top surface by spanning a chemical vapor deposition (CVD) monolayer graphene. The assembled graphene/UHMWPE (gPE) porous membrane exhibited unprecedented tensile strengths of 880 MPa, 70 times higher than stainless steel's specific strength! Its Young's moduli and tensile strengths are in accord with the upper bound predictions of the classical theory of mixtures corroborating graphene's maximum limit in mechanical property reinforcement. We discovered graphene's simultaneous toughening effect by a six-fold crack-propagation-rate retardation. Our findings may lead to further breakthroughs of 2D materials-reinforced polymer composites.