Previously, it was reported that a remarkable tendency of fibrillation of poly(p-phenyleneterephthalamide) (PPT) fiber on its deformation was related to the pillar-shaped microfibrillar structure and that this tendency may lead to poorer fatigue resistance compared with other synthetic fibers when it was used as a reinforcement for rubber.
We found that these problems could be solved by copolymerization of PPT to the extent that the spinning dope of the copolymers showed optical anisotropy.
The fine structure of such copolymer fibers was studied by the wide and small angle X-ray scattering and the birefringence measurement.
The following results were obtained.
1) Copolymer fiber have the extended chain structure similar to PPT fiber, and the tenacity of copolymer fiber is same as that of PPT fiber.
2) Diaminocomonomer unit (NN'-bisaminobenzoilethylene diamine or piperazine) of copolymer fiber is forced to be included in crystals of PPT, and a crystal of copolymer fiber becomes more paracrystalline than that of PPT fiber. Therefore the cross section of copolymer fiber is more uniform than that of PPT fiber and the pillar-shaped microfibrillar structure found in PPT fiber disappears in copolymer fiber.
3) Such modification of fine structure leads to the improvement of mechanical propertiess of PPT fiber, such as increased elongation-at-break, knot tenacity and knot elongation.