Crystal structure and mechanical properties of ODPA-DMB polyimide fibers

Abstract

A new polyimide has been synthesized from 4,4′-oxydiphthalic anhydride (ODPA) and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB) via a one-step polycondensation reaction. This polyimide can be dissolved in polar organic solvents and spun into fiber form using a dry-jet wet-spinning method. The crystal unit cell of highly drawn and annealed ODPA-DMB fibers has been determined to be triclinic with a = 1.05 nm, b = 0.871 nm, c = 2.14 nm, α = 45.6°, β = 53.7° and γ = 61.4°. It has been found that crystallinity and orientation are critically dependent upon draw ratio. Furthermore, the overall orientation and crystal orientation developed in the ODPA-DMB fibers exhibit different behavior. In the high draw ratio region (a draw ratio of above 3), the degree of crystal orientation levels off to reach a constant while the overall orientation develops continuously. Tensile properties of the fibers in this region also show significant improvement. This indicates that the enhancement of the tensile properties must be associated with molecular orientation in the non-crystalline region of the fibers. The pre-existing structure developed during the fiber spinning also drastically affects subsequent drawing and annealing processes and ultimate tensile properties due to the competition between molecular orientation and crystallization.