Synthesis and Properties of New Heat-resistant Pyridine-based Polyimides Containing Preformed Amide Units

Anew diamine monomer containing pyridine and amide units was prepared via twostep reactions. Nucleophilic chloro-displacement reaction of 2,6-diaminopyridinewith 4-nitrobenzoyl chloride in the presence of propylene oxide afforded 2,6-bis(4-nitrobenzamido)pyridine and subsequent reduction of the nitro groups led to 2,6-bis(4-aminobenzamido)pyridine. The diamine was reacted with aromatic dianhydridesincluding pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, andhexafluoroisopropylidene diphthalic anhydride to form polyimides via two-step polycondensationmethod, and formation of poly(amic acid) followed by chemicalimidization. All the prepared materials and polyimides were fully characterized bycommon spectroscopic methods and elemental analysis. Physical and thermal propertiesof the polymers including solubility, inherent viscosity, thermal behaviour andthermal stability were studied and structure-property relations in these polyimides wereinvestigated. The inherent viscosity of the polyimides in NMP at a concentration of 0.5g.dL^-1 at 30°C was between 0.47-0.51 dL/g. The polymers showed high thermalstability while their solubilities were improved. The glass transition temperatures (Tg) ofthe polymers were about 244-283°C. The 10% weight loss of polymers as a maincriterion for their thermal stability was about 420-453°C. Char yield of the polymers at600°C was in the range of 49-71%. High thermal stability resulted from incorporation ofimide backbone, phenylation of chain, symmetry of structures to avoid the weaklinkages. The polymers revealed good solubility in dipolar aprotic solvents such as Nmethyl-2-pyrrolidone, N,N-dimethylacetamide, dimethylformamide, dimethylsulphoxideand also m-cresol. Improved solubility was attributed to the presence of pyridine polarunit and amide structures in the polyimide backbones.