Method to produce nanoporous polymeric membranes
A method for making a flat polymer foam having a core layer of nano-sized open, interconnected cells that includes saturating a solid-state polymer with a supercritical fluid, allowing the gas to desorb for at least 35 minutes, and then heating the gas-saturated solid polymer for at least 3 minutes while constraining the foam in the thickness dimension. Any skin layer formed on the exterior may be removed via polishing, thus creating a foam with an open structure from side to side. The foam can be used as a battery separator.
1. A method for making a flat, blister-free foam with an open, interconnected cellular structure, comprising:
saturating a polymer with a gas in a vessel at conditions wherein the gas is supercritical and producing a gas-saturated polymer;
desorbing gas from the gas-saturated polymer;
placing the gas-saturated polymer in a press;
in the press, heating the gas-saturated polymer at a temperature to allow cell growth while the press applies a clamping force to maintain flatness and produces a flat blister-free foam; and
removing a solid skin layer from the foam to expose an open, interconnected cellular structure.
2. The method of claim 1 , wherein the gas-saturated polymer is desorbed for about 35 minutes or greater.
3. The method of claim 1 , wherein the gas-saturated polymer is heated in the press for about 3 minutes or greater.
4. The method of claim 1 , wherein the polymer is a thermoplastic polymer.
5. The method of claim 1 , wherein the polymer is polyetherimide.
6. The method of claim 1 , wherein the clamping force is 1 ton or less.
7. The method of claim 1 , wherein the foam includes an open, interconnected nanocellular structure, wherein the cellular structure comprises cells having a diameter of 50 to 100 nm.
8. The method of claim 1 , further comprising polishing the foam to remove a skin layer.