High-performance aromatic polyimide fibers .6. Structure and morphology changes in compressed BPDA-DMB fibers

A high-performance aromatic polyimide fiber has been spun from a high molecular weight polyimide synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-dimethyl-4,4'-diaminobiphenyl (DMB). The fiber exhibits not only excellent tensile properties and high temperature resistance but also a high compressive strength of 655 MPa. Morphological observations of BPDA-DMB fibers indicate that the fiber shows a skin-core structure and microfibrillar textures. A banded texture can also be found with a spacing of about 2 mu m, which may be introduced by the liquid crystalline behavior that appears during processing. Compressed BPDA-DMB fibers form kink and microkink bands over different size scales. The detailed formation mechanism of these banded textures is discussed. The structure parameter changes during compression - including crystal unit cell parameters, apparent crystallinity, crystal and overall orientation, and apparent crystallite sizes - are monitored. It is found that after restretching, the crystal and overall orientation is almost fully recovered while the apparent crystallinity is not recoverable. The structural changes during compression are critically associated with the loss of macroscopic mechanical property in the fibers.