Composite membranes with improved performance and/or durability and methods of use

An asymmetric composite membrane for selectively pervaporating a high octane compound from a feed mixture comprising the high octane compound and gasoline, the asymmetric composite membrane comprising:

• a porous substrate comprising opposite first and second major surfaces, and a plurality of pores;
• and
• a pore-filling polymer disposed in at least some of the pores so as to form a layer having a thickness within the porous substrate, with the amount of the pore-filling polymer at, or adjacent to, the first major surface being greater than the amount of the pore-filling polymer at, or adjacent to, the second major surface;
• wherein the pore-filling polymer is more permeable to the high octane compound than gasoline;
• wherein the composite membrane further comprises at least one of:
• (a) a liquid ionic compound mixed with the pore-filling polymer;
• or
• (b) an amorphous fluorochemical film disposed on the composite membrane.