Thermo-processable polyimides with high Tg and high thermo-oxidative stability as derived from 2,3,3',4'-biphenyltetracarboxylic dianhydride

Aromatic copolyimides were prepared using 2,3,3',4'-biphenyltetracarboxylic dianhydride (asymmetric BPDA or a-BPDA) and several aromatic diamines with flexible ether linkages to obtain melt-processable polyimides (PIs) with balanced properties, i.e. high T-g, high thermo-oxidative stability, low melt viscosity, and sufficient toughness. The use of a-BPDA for the present purpose is based on our previous results which showed that a-BPDA-based PIs always show both higher Tg and much higher thermoplasticity than the corresponding symmetric BPDA-derived PIs. On the basis of the structure-property relationship, a copolyimide system was focused upon as a promising candidate. The molecular weight of the selected copolyimide was controlled using an end-capping reagent to balance between T-g, the melt viscosity, and toughness. The effect of the polymerization route (one-pot and two-step) on the properties was also discussed. An a-BPDA-based copolyimide accomplished a high T-g at 249degreesC, a low melt viscosity of 8200 Poise at 400degreesC, and high thermo-oxidative stability concurrently with considerably high film toughness (elongation at break = 66%).