A method of making a microporous membrane, said membrane comprising a single integral layer comprising a first microporous surface; a second microporous surface; and a porous bulk between the first microporous surface and the second microporous surface, wherein the bulk comprises at least a first region and at least a second region; wherein (i) the first region comprising a first set of pores having outer rims, prepared by removing introduced dissolvable silica nanoparticles, the first set of pores having a first controlled pore size, and a second set of pores connecting the outer rims of the first set of pores, the second set of pores having a second controlled pore size in the range of 0.2 to 0.4 of the first controllable pore size, and a polymer matrix supporting the first set of pores; (ii) the second region comprising a third set of pores having outer rims, prepared by removing introduced dissolvable silica nanoparticles, the third set of pores having a third controlled pore size, and a fourth set of pores connecting the outer rims of the third set of pores, the fourth set of pores having a fourth controlled pore size in the range of 0.2 to 0.4 of the third controlled pore size, and a polymer matrix supporting the third set of pores; and (iii) wherein the third controlled pore size is at least 20% greater than, or at least 20% less than, the first controlled pore size; said method comprising: (a) casting a first polymer solution containing dissolvable silica nanoparticles having a diameter from 50 to 1000 nm and a density of 1.96 g/cm 3 or less onto a substrate or onto a substrate pretreated with a preconditioning or releasing agent selected from polyvinyl alcohol (PVOH), polyvinylpyrrolidone (PVP), poly(acrylic acid), and poly(methacrylic acid); (b) casting a second dissolvable silica nanoparticle-containing polymer solution onto the cast solution of (a); (c) carrying out phase inversion and obtaining a membrane; (d) dissolving the nanoparticles and obtaining a nanoparticle-depleted membrane; and (e) washing the nanoparticle-depleted membrane. (i) the first region comprising a first set of pores having outer rims, prepared by removing introduced dissolvable silica nanoparticles, the first set of pores having a first controlled pore size, and a second set of pores connecting the outer rims of the first set of pores, the second set of pores having a second controlled pore size in the range of 0.2 to 0.4 of the first controllable pore size, and a polymer matrix supporting the first set of pores; (ii) the second region comprising a third set of pores having outer rims, prepared by removing introduced dissolvable silica nanoparticles, the third set of pores having a third controlled pore size, and a fourth set of pores connecting the outer rims of the third set of pores, the fourth set of pores having a fourth controlled pore size in the range of 0.2 to 0.4 of the third controlled pore size, and a polymer matrix supporting the third set of pores; and (iii) wherein the third controlled pore size is at least 20% greater than, or at least 20% less than, the first controlled pore size; (a) casting a first polymer solution containing dissolvable silica nanoparticles having a diameter from 50 to 1000 nm and a density of 1.96 g/cm 3 or less onto a substrate or onto a substrate pretreated with a preconditioning or releasing agent selected from polyvinyl alcohol (PVOH), polyvinylpyrrolidone (PVP), poly(acrylic acid), and poly(methacrylic acid); (b) casting a second dissolvable silica nanoparticle-containing polymer solution onto the cast solution of (a); (c) carrying out phase inversion and obtaining a membrane; (d) dissolving the nanoparticles and obtaining a nanoparticle-depleted membrane; and (e) washing the nanoparticle-depleted membrane. <EOS>