Surface confinement induced amorphization of polyethylene oxide in high-performance porous polyethylene films

Abstract

We report a new nanoconfinement phenomenon whereby surface confinement leads to the amorphization of polyethylene oxide (PEO) in porous ultrahigh molecular weight polyethylene membrane (UHMWPE). The surface confinement was achieved through a monolayer attachment of PEO onto the UHMWPE nanofibril surfaces‒pore bounding edges‒in the membrane via amphiphilic interactions of polypropylene oxide (PPO) blocks in the triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO). Full amorphization of PEO is corroborated through the lack of phase transitions under differential scanning calorimetry (DSC), and liquid-like vibrational absorption responses under Fourier transform infrared spectroscopy (FTIR). Monolayer confinement of PEO in the membrane is quantified using a combination of Brunauer-Emmett-Teller (BET), time-of-flight secondary ion mass spectrometry (ToF-SIMS) molecular depth profiling and scanning electron microscopy (SEM). The total amount of PEO confined in the porous membrane is 7.7 wt% agreeing with that expected from the total BET surface area of the UHMWPE membrane; ToF-SIMS depth profiling using argon cluster ion reveals the homogeneous PEO distribution along the membrane depth; and surface topology analysis by SEM shows no obvious topological changes after PEO confinement.