This article reports the synthesis of a new nanoporous UHMWPE membrane separator that exhibits a three-fold increase in tensile strength upon a thermally-induced pore closure. High-performance biaxially oriented nano-porous ultra-high molecular weight polyethylene (UHMWPE) membranes with tensile strengths greater than 100 MPa and a volumetric porosity over 60% were prepared. These membranes exhibit two melting endotherms at 130 and 150 °C, respectively. Annealing of the nanoporous membranes under lateral dimensional constraints 145 °C for 7 seconds leads to the formation of surface-pore-free toughened films with a 350% increase tensile strength. This drastic increase in tensile strength is due to the melting induced recrystallization of kebab crystals and densification of membrane in the thickness of the film. The rapid pore-closing functionality accompanied by the accompanied toughness enhancement makes these membranes ideal for use as explosion-proof membrane separators.