A film comprising a copolyester wherein said copolyester comprises repeating units derived from an aliphatic glycol, an aromatic dicarboxylic acid, and a comonomer (M) selected from the group consisting of N,N′-bis-(2-hydroxyalkyl)-bicyclo-[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylic diimide (BODI) and 2-hydroxyalkyl-2-[p-(2-hydroxyethoxycarbonyl)phenyl]-1,3-dioxo-2H-isoindole-5-carboxylate (DOIC), wherein the number of carbon atoms is the 2-hydroxyalkyl group is 2, 3 or 4, and wherein comonomer (M) constitutes a proportion of the glycol fraction of the copolyester.