SYNTHESIS AND CHARACTERIZATION OF POLYIMIDES FROM 3,7-DIAMINOPHENOTHIAZINIUM CHLORIDE (THIONINE)

A new series of thermally stable, organic solvent soluble polyimides has been synthesized by reacting the tricyclic heterocyclic molecule 3,7-diaminophenothiazinium chloride, thionine, with four different dianhydrides. With the exception of the polyimide obtained from pyromellitic dianhydride and thionine, the polyimides were synthesized by a single-stage polymerization reaction at an elevated temperature. For these polymers, the conditions necessary for the polymerization to proceed did not permit the isolation of the intermediate poly(amic acid). To assess the reactivity of the primary amine groups of the thionine molecule, model compounds were prepared by reacting thionine with phthalic anhydride. This reaction was studied in different solvents and under various experimental conditions so that the intermediate diamic acid could be isolated. Subsequent treatment of the bis(phthalic diamic acid) at elevated temperatures yielded the bis(phthalic diimide). The polyimides obtained from thionine and the dianhydrides 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) were soluble in many of the common organic solvents, whereas the polyimides obtained from the dianhydrides pyromellitic dianhydride (PMDA) and naphthalenetetracarboxylic dianhydride (NTCDA) were soluble in fewer organic solvents but dissolved completely in strong acids without decomposition. The differences in solubility of these polyimides are attributed to the differences in the dianhydride flexibility and polarity. Differential thermal analyses and thermogravimetric studies established that these polyimides were thermally stable up to 500-degrees-C in air. The polyimides prepared with pyromellitic dianhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, or 1,4,5,8-naphthalenetetracarboxylic dianhydride exhibited a 10% weight loss in the region of 507-degrees-C in air. The polyimide synthesized with 3,3',4,4'-benzophenonetetracarboxylic dianhydride exhibited a 10% weight loss at 539-degrees-C in air. All four polyimides exhibited improved thermal stability ranging from 10 to 80-degrees-C in a nitrogen atmosphere. Characterization of the polyimides by X-ray diffraction showed that the polyimide obtained from thionine and PMDA had a greater degree of crystallinity than the other polyimides. Computer simulations of the geometry of each polyimide indicated a preference by that polyimide for its own unique helical structure conformation.