The Structure and Properties of Aramid Fibres

Abstract

Aramid fibres are very tough and strong polymer materials. The modulus of elasticity of these fibres can range from 60 to 140 GPa, whereas the strength reaches a value of 4.5 GPa at 2.5 cm gauge length with an elongation at break of more than 4%. The origin for these properties is found in the intermolecular interactions resulting in a semirigid conformation of the poly(p-phenylene terephthalamide) chain. X-ray and electron diffraction studies have demonstrated that the structure is paracrystalline. On the basis of a single-phase structural model the elastic properties of this fibre have been fully explained.

Another outstanding property of the aramid fibres is the low rate for creep and stress relaxation. Presumably this is due to the semi-rigid nature of the chains, the high crystallinity of the structure and the hydrogen bonding between the chains. For a better understanding of the visco-elasticity of the fibre it is necessary to know the structural phenomena that happen during creep and stress relaxation. A useful tool is provided by the dynamic compliance measurement, because in well-oriented fibres it is linear related to the second moment of the chain orientation distribution. Experiments have shown that during creep and stress relaxation a progressive contraction of this distribution takes place, which is caused by shear deformation of the crystallites. These results have lead to a further development of the series model, which now incorporates viscoelasticity.