the process of producing a reverse osmosis membrane by the steps of preparing a liquid mixture of n - vinyl pyrrolidone , maleic anhydride and maleimide , wetting a porous membrane with the mixture and irradiating the wetted membrane with laser energy with energy sufficient to polymerize the mixture to form a terpolymer of n - vinyl pyrrolidone , maleic anhydride and maleimide on the membrane .