Process for treating a liquid-wet polycarbonate membrane to improve its gas separation properties

A process for treating a liquid-wet polycarbonate membrane comprising the steps of:

• A. contacting at least one side of a polycarbonate membrane with water at a temperature which does not deleteriously affect the integrity of the membrane under conditions such the water removes substantially all of the residual solvent and non-solvent from the membrane;
• B. contacting at least one side of the membrane with air, an inert gas, or vacuum at a temperature under conditions such that substantially all of the water is removed from the membrane without deleteriously affecting the integrity of the membrane;
• and
• C. simultaneously or consecutively heat treating the membrane under conditions such that the membrane subsequently exhibits a decrease in gas flux of less than about 2 percent over 30 days under an operating temperature of less than about 25.degree. C. and an operating pressure of less than about 200 psig with an increase in gas selectivity of at least about 5 percent compared with a membrane which is not heat treated.