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°C 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 T g for Oligo-1.5, resulting in a viscosity as low as 200 Poise at 300°C. Whereas, a melt viscosity for Oligo-10 was 20 000 Poise at 365°C. The glass transition temperatures of these cured oligomers were 341°C and 308°C, 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°C. 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 such as alkyl and methylene groups are very effective in the molecular design of heat resistant, addition-type polyimides. The excellent properties exhibited in a-BPDA based polyimides demonstrate a promising potential for future aerospace applications.