A method of forming an asymmetric membrane comprising: providing a porous substrate having a first major surface; interstitial pores; and a second major surface; applying a polymerizable composition to the first major surface, the interstitial pores, and the second major surface of the porous substrate providing a coated porous substrate, the polymerizable composition comprising i) at least one polymerizable species having a free-radically polymerizable group selected from a (meth)acryloyl group, an acryloyl group and a vinyl group; and ii) at least one free-radical photoinitiator; and exposing the coated porous substrate to an ultraviolet radiation source having a peak emission wavelength in the range of 200 nm to 300 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 concentration of the polymerized material at the first major surface and within the openings or pores nearest the first major surface being greater than at the second major surface, and wherein the polymerizable composition does not contain a photoblocker; and wherein the free-radical photoinitiator can selectively absorb radiation from the ultraviolet radiation source. providing a porous substrate having a first major surface; interstitial pores; and a second major surface; applying a polymerizable composition to the first major surface, the interstitial pores, and the second major surface of the porous substrate providing a coated porous substrate, the polymerizable composition comprising i) at least one polymerizable species having a free-radically polymerizable group selected from a (meth)acryloyl group, an acryloyl group and a vinyl group; and ii) at least one free-radical photoinitiator; and i) at least one polymerizable species having a free-radically polymerizable group selected from a (meth)acryloyl group, an acryloyl group and a vinyl group; and ii) at least one free-radical photoinitiator; and exposing the coated porous substrate to an ultraviolet radiation source having a peak emission wavelength in the range of 200 nm to 300 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 concentration of the polymerized material at the first major surface and within the openings or pores nearest the first major surface being greater than at the second major surface, and wherein the polymerizable composition does not contain a photoblocker; and wherein the free-radical photoinitiator can selectively absorb radiation from the ultraviolet radiation source. <EOS>