total pore volume before the copolymerization in situ of the microporous support sheet is occupied by carboxylic copolymer the proportion by weight of divinyl benzene if used may be between about 2 5 and 25 parts to 100 parts of unsaturated carboxylic acid it is also advisable to use only small amounts of polymerization initiator for instance less than about 5 parts by weight of benzoyl peroxide to 100 parts by weight of comonomers and little or no copolymerization accelerator such as dimethylaniline less than 2 parts this impregnation can be effected for instance by immersion of the porous sheet in a tank containing the liquid mixture and filtration under a vacuum of about 10 to 100 mm of mercury the sheet possibly on a support and in particular on a cathode is then introduced into an enclosure in which the temperature or actinic rays e g ultraviolet rays permit the action of the polymerization initiators the sheet may be immersed in a liquid for instance water it is important that the temperature is not too high that is generally less than about 150 u00b0 c and does not cause substantial modification of the structure of the microporous sheet due to.