Poly(p-phenylene terephthalamide) fibers were treated in the range of 250~400℃ under free-end in air.Their mechanical properties and structure parameters were characterized by mechanical testing

Fourier transfer infrared spectroscopy

wide angle X-ray diffraction

sonic velocimetry and viscocimetry.It was found that the tenacity of aged samples decreased rapidly at the early stage of ageing due to the disorientation of molecular chains.Then

thermal degradation made the tenacity to keep on decreasing with time

which was consistent with the second-order kinetics equation with apparent activation energy of 32.4 kJ/mol.With the increase of ageing temperature

an obvious decrease in tenacity was found.However

the tenacity decreasing rate slowed down above 350℃

which could be related to the occurrence of slight cross-linking

the increasing of crystallinity and the perfection of the crystallites.The modulus of aged sample was lower than that of untreated sample

since the disorientation of molecular chains in amorphous region was dominant below 350℃.The modulus of aged sample increased above 350℃ because of more perfect crystallization structure

which was indicated by the smaller distortion of second kind lattice

gII

and larger value of a

b axis due to the combination of adjacent crystallites.