Toughness of Silicone Block Copolyimides

Block copolyimides were synthesized from 3,3^′,4,4^′-benzophenonetetracarboxylic dianhydride by reaction with 4,4^′-methylenedianiline and poly(dimethylsiloxane) diamines. Aromatic diamine-terminated poly(dimethylsiloxanes) were prepared by the base-catalyzed equilibration of octamethylcyclotetrasiloxane with l,3-bis(4-aminophenoxypropyl)tetramethyldisiloxane. The amount of poly(dimethylsiloxane) incorporated in the copolyimides was 5–15 wt %. The selected molecular weight of poly(dimethylsiloxane) diamine was 4500. The fracture toughness of copolyimides was found to increase by a factor of 3 with only 5 wt % poly(dimethylsiloxane)-modified copolyimide without sacrificing the mechanical strength or thermal stability. The glass transition temperatures of the copolyimides were very close to that of the homopolyimide over the range of poly(dimethylsiloxane) content from 5 to 15 wt %. The morphology and toughening mechanisms have been examined, as have the large-scale mechanical properties of copolyimide films.