High performance aromatic polyimide fibers, 3. A polyimide synthesized from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and 2,2′‐dimethyl‐4,4′‐diaminobiphenyl

Abstract

AbstractA 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°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, α = 61,2(6)°, β = 50,7(7)°, and γ = 78,9(6)° 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, α = 62,1(3)°, β = 52,2(3)° and γ = 79,6(3)°]. By increasing the draw ratio, both the crystallinity and crystal orientation increase. The BPDA‐DMB fibers possess a decomposition temperature of 530°C in nitrogen and 500°C in air at a 5% weight loss when the heating rate is 10°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 α (glass transition) and β (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 β relaxation is found to be crystallinity dependent. The α transition is also suppressed by crystallinity which increases with draw ratio.