TECHNOLOGY FOCUS - POLYMERS - Preferred process: 100 parts by weight of substituent are used with 100 parts by weight polymer. A high-energy beam is used to bind the substituent to the polymer chain and/or an initiator placed between the substituent and the polymer to promote binding. The porous film may be formed from the liquid state onto a plaque or the film may be formed separatelyPreferred modifier: this is selected from monoallyl isocyanurate, diallyl isocyanurate, isocyanurate triallyl, triallyl cyanurate, ethylene glycol di-(meth)-acrylate, trimethylpropantri-(meth)-acrylate, diallyl phthalate, divinyl benzene, vinyl toluene, vinyl pyridine, triallyl phthalate, vinyl trichlorosilane, vinyl tris (p-methoxy ethoxy) silane, vinyl triethoxy silane, vinyl trimethoxy silane, approximatelyg-(meth)-acryloxy propyl)trimethoxy silane, approximatelyg-(methyl)-acryloxy propyl)triethoxy silane, approximatelyg-(methyl)-acryloxy propyl)methyl dimethoxy silane and silicone acryl. The modifier is a composition having a level of lowest unoccupied molecular orbital (LUMO) at least 0.3eV with the opening group of the polymer and a level of highest unoccupied molecular orbital at most -10.1 eV. The preferred substituent for bonding to the polymer carries a structure -SiOSi-Preferred polymer: polybenzoimidazole, polyimide, polyether imide, polyamide imide, polyphenylene sulfide, polyether sulfone, polysulfone, polyether ether ketone, polymethyl pentene, aramid, polyvinylidene fluoride, polyamide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate polyarylate, polyacetal and polyphenylene ether.Preferred battery: the negative and positive electrodes are stacked on top of each other separated by the film (F) EXAMPLE - 30 parts by weight of saturated polyester ( Vylon KS001 and 6 parts by weight of approximatelyg-methacryloxy propyl trimethoxysilane modifier were dissolved at 125 degrees C in70 parts by weight n-methyl pyrrolidone to produce polymer solution (S1). This solution was applied with a blade onto a negative electrode, which was immersed in water for 5 minutes and dries to produce a porous film on a negative electrode (S2). The electrode (S2) was plunged into a solution of dicumyl peroxide (1% solution in EtOH) and then dried. The product was then sealed in an atmosphere of Argon and heated for 2 hours at 150 degrees C to achieve the reaction scheme (I)