Abstract
A copolyimide has been synthesized from 3,3',4,4′-tetracarboxybiphenyl dianhydride (BPDA), 3,3′-dimethyl-4,4′-diaminobiphenyl (OTOL) and p-phenylene diamine (PPD) in a mole ratio of 100/70/30 through a one-step polymerization at elevated temperature. It has been spun as fibres by dry-jet wet spinning. The tensile modulus of the fibres reaches 120 GPa, with a tensile strength of 3.2 GPa. Such high tensile properties are the result of the high draw ratio (up to x8) while annealing at 400°C. The homopolyimide (BPDA-OTOL) is found to adopt a triclinic crystal lattice, based on wide angle X-ray and electron diffraction experiments. In the copolyimide, it's crystal lattice is disturbed by the introduction of the PPD comonomer. As the draw ratio of the copolymer fibre is increased, the crystallinity, overall orientation and the apparent axial crystallite sizes also increased, as evidenced by wide angle X-ray diffraction, optical birefringence and density measurements. These structural developments critically affect the tensile properties of the fibres. The glass transition temperature of the copolyimide is 308°C, based on thermal mechanical measurements. Dimensional stability is retained up to about 300°C. At around 100–250°C, a subglass transition is observed in dynamic mechanical measurements, and is assigned as a β relaxation. The activation energy of this relaxation is 140 hJ/mol, based on its frequency dependence. The α relaxation corresponds to the glass transition. As the crystallinity increases with drawing, this relaxation is suppressed due to the restriction of molecular segmental motion.
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