Polyamide flat sheet membranes with microporous surface structure for nanoparticle retention

The present disclosure provides a flat-sheet polyamide membrane comprising a first major surface and a second major surface and a separation layer and a porous substructure in the cross section of the membrane between the first major and the second major surface, wherein the average pore size diameter in the separation layer is smaller than the average pore size diameters on the first and second major surfaces, wherein the average pore size diameter on the first major surface is larger than the average pore size diameter on the second major surface, wherein the separation layer is closer to the second major surface than to the first major surface. The present disclosure further provides a method for producing such membranes and a use of the membranes for nanofiltration or ultrafiltration purposes.