Molecular design of heat resistant polyimides having excellent processability and high glass transition temperature

The relationship between the imide structures-and morphology are discussed in order to develop heat resistant polyimides having excellent processability and toughness. Addition-type imide oligomers consisting of asymmetric 2,3,3 ' ,4 ' -biphenyltetracarboxylic dianhydride (a-BPDA) and 3,4 ' -oxydianiline (3,4 ' -ODA) and/or 4,4 ' -oxydianiline (4,4 ' -ODA) with 4-phenylethynyl phthalic anhydride (PEPA)were synthesized and characterized. The imide oligomers derived from 3,4 ' -ODA; 4,4 ' -ODA (50:50) comonomer having molecular weights of 5240 g mol(-1) (Oligo-10) and 1340 g mol(-1) (Oligo-1.5) showed good solubility in aprotic solvents such as DMAc and NMP, and were successfully cured at 370 degreesC for one hour. The thermal curing process, and thermal and rheological properties of the imide oligomers were investigated by FT-IR, differential scanning calorimetry, thermogravimetric analysis, and dynamic rheometry. It was observed that the melt flow dramatically decreased above the Tg for Oligo-1.5, resulting in a viscosity as low as 200 Poise at 300 degreesC. Whereas, a melt viscosity for Oligo-10 was 20 000 Poise at 365 degreesC. The glass transition temperatures of these cured oligomers were 341 degreesC and 308 degreesC, respectively. In addition to the excellent melt property, the cured oligomers exhibited good thermo-oxidative stability. Furthermore, the cured imide oligomer consisting of a-BPDA and 4,4 ' -ODA with PEPA (Oligo-4.5) exhibited over 13% flexural elongation and a T-g of 343 degreesC. Their T-300 carbon fibre composites were also well consolidated demonstrating excellent processability and properties. It is concluded that amorphous, aromatic imide structures without any weak linkages.