pyrrolidone dmso dimethylsulfoxide dmac n ndimethylacetamide dmf dimethylformamide py pyridine thf tetrahydrofuran the value of n in the polymer structure can be determined from the molecular weight and molecular weight distribution obtained by size exclusion chromatography gpc measurement to a stirred solution of 0 495 g 1 25 mmol diamine i in 8 ml anhydrous dmac was added gradually 0 4025 g 1 25 mmol of 3 3 39 4 4 39 benzophenonetetracarboxylic dianhydride btda the mixture was stirred at room temperature for 2 hours under argon atmosphere the inherent viscosity of the poly amic acid was 2 01 dl g 1 measured in dmac at a concentration of 0 5 g dl 1 at 30 u00b0 c the poly amic acid was cast on a clean glass plate which was then put into an oven to be baked at 80 u00b0 c to form a solid film through evaporation of the solvent and was further baked in the oven at 110 150 180 210 230 and 250 u00b0 c for 20 minutes at each temperature a polyimide thin film was thus obtained the inherent viscosity of the polyimide was 0 90 dl g 1 measured in concentrated h 2 so 4 98 at a concentration of 0 5 g dl 1 at 30 u00b0 c the elementary analysis values the mechanical strength thermal properties and molecular structure are listed as follows elementary analysis c 43 h 26 n 2 o 3 n calculated values c 75 60 h 3 81 n 4 10 analytical values c 74 14 h 3 90 n 4 02 ______________________________________mechanical properties tensile strength elongation at break initial modulus mpa gpa ______________________________________139 8 1 81______________________________________ glass transition temperature 256.