HIGH-PERFORMANCE AROMATIC POLYIMIDE FIBERS .3. A POLYIMIDE SYNTHESIZED FROM 3,3',4,4'-BIPHENYLTETRACARBOXYLIC DIANHYDRIDE AND 2,2'-DIMETHYL-4,4'-DIAMINOBIPHENYL

A new high molecular weight aromatic polyimide has been synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-dimethyl-4,4'-diaminobiphenyl (DMB) in p-chlorophenol at elevated temperature. BPDA-DMB fibers have been spun by a dry-jet wet spinning method. The fibers were elongated and annealed at elevated temperatures above 400-degrees-C to achieve excellent mechanical properties. In seven times drawn fibers, the BPDA-DMB molecule packs into a triclinic unit cell with dimensions of a = 2,10(2) nm, b = 1,523(8) nm, c = 4,12(7) nm, alpha = 61,2(6)-degrees, beta = 50,7(7)-degrees, and gamma = 78,9(6)-degrees with the number of chain repeating units per unit cell (Z) is sixteen. After annealing at elevated temperatures, the fibers produce a small modification of the unit cell [a 2,048(6) nm, b = 1,529(5) nm, c = 4,00(2) nm, a = 62,1(3)-degrees, beta = 52,2(3)-degrees and gamma = 79,6(3)-degrees]. By increasing the draw ratio, both the crystallinity and crystal orientation increase. The BPDA-DMB fibers possess a decomposition temperature of 530-degrees-C in nitrogen and 500-degrees-C in air at a 5% weight loss when the heating rate is 10-degrees-C/min. After extensive drawing, BPDA-DMB fibers exhibit a tensile strength of 3,3 GPa and a tensile modulus of over 130 GPa. Dynamic mechanical behavior of the fibers show both alpha (glass transition) and beta (sub-glass transition) relaxations above room temperature. The nature of the sub-glass transition behavior is described as a noncooperative motion attributed to the diamine portion of the molecule. The activation energy for this relaxation in as-spun fibers is 109 kJ/mol and increases to 144 kJ/mol by increasing the draw ratio. This beta relaxation is found to be crystallinity dependent. The alpha transition is also suppressed by crystallinity which increases with draw ratio.