Method of forming an asymmetric membrane

A method of forming an asymmetric membrane comprising:

• providing a porous substrate having a first major surface and a second major surface;
• applying a polymerizable composition to the porous substrate providing a coated porous substrate, the polymerizable composition comprising
• i) at least one polymerizable species;
• and
• ii) at least one photoinitiator;
• and
• exposing the coated porous substrate to an ultraviolet radiation source having a peak emission wavelength less than 340 nm to polymerize the polymerizable species, wherein the photoinitiator residing at the first major surface is exposed to a greater peak irradiance of ultraviolet radiation than the photoinitiator residing further into the thickness of the porous substrate thus reducing the amount of polymerization within the pores of the substrate through the thickness of the substrate, thereby providing an asymmetric membrane, the asymmetric membrane having a polymerized material retained within the porous substrate, the polymerized material having a concentration greater at the first major surface than at the second major surface.