PURPOSE: To obtain a composite membrane having a desired surface characteristic, strength and chemicals resistance by contacting a washed porous membrane with a soln. of a monomer of a second polymer, cross-linking agent and polymerization initiator to polymerize the monomer, and cross-linking the second polymer with surfaces of a first polymer.
CONSTITUTION: A porous membrane substrate is formed having a mean pore size of 0.001-10 μ formed from a first polymer. And surfaces of the pore in the porous membrane are wetted by washing the porous substrate. Then the porous membrane is contacted with a soln. of a monomer of the second polymer, a cross-linking agent for the monomer and a polymerization initiator for the monomer. By polymerizing the monomer by free radical polymerization, without blocking the pores of the porous membrane, the second polymer is cross-linked spreading over surfaces of the first polymer. Thus the composite porous membrane formed by applying a permanent coating onto all of the porous membrane contg. inner micropore walls, on the porous membrane substrate is obtained.
