| abstract |
An aromatic condensation polyimide film that is approximately 90% transparent at 500 nm, useful for thermal protective coatings and the like and the process for preparing same are disclosed. An essential feature of the invention for achieving maximum optical transparency films requires utilizing recrystallized and/or sublimated specific aromatic diamines and dianhydride monomers and introducing bulky electron-withdrawing groups and separator groups into the polymer molecular structure. The incorporation of bulky electron-withdrawing groups in the diamine portion of the polymer structure serves to reduce the formation of inter-chain and intra-chain charge transfer complexes which normally cause large absorptions in the UV-visible range. Incorporation of separator atoms into either the diamine or dianhydride monomers serves to reduce the amount of conjugation and inter-and intra-chain electronic interactions and thereby lessen charge transfer complex formation. |