HIGH-PERFORMANCE AROMATIC POLYIMIDE FIBERS .2. THERMAL-MECHANICAL AND DYNAMIC PROPERTIES

A family of high-temperature, high-modulus aromatic polyimide fibres has been dry-jet wet spun from either its gel state or isotropic solution, followed by high-temperature drawing. In this report, thermal and dynamic mechanical properties of one of the family members, a segmented rigid-rod polyimide synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB), are presented in detail. Mechanical properties of these BPDA-PFMB fibres can be improved remarkably by drawing due to drastic increases in overall orientation, crystal orientation and crystallinity. These three structural parameters, however, do not show parallel changes with increasing draw ratio. It has been observed that the linear coefficient of thermal expansion (CTE) of BPDA-PFMB fibres after drawing generally show negative values in the solid state when low stresses are applied during measurements. For as-spun fibres, the CTEs are constant over a certain applied stress region, which is on the same order of magnitude as CTEs of in-plane oriented BPDA-PFMB films along the film surface. This may be an indication that within this region the stress applied is at the same level as the internal stress frozen into the fibres during spinning and drawing. Glass transition temperatures (T(g)) of as-spun fibres show a linear decrease at low applied stress region, then level off when the applied stress becomes high. Dynamic mechanical data indicate two relaxation processes in as-spun fibres above room temperature: an a relaxation corresponding to the glass transition and a beta relaxation which is a subglass transition. In the fibres with a draw ratio of above three times, the alpha relaxation is totally suppressed. This reveals a rigid fraction (above T(g)) dependence of this relaxation in the fibres. The beta relaxation is, on the other hand, crystallinity dependent. The Arrhenius activation energy (about 160 kJ mol-1) of the beta relaxation in as-spun fibres is about 50 kJ mol-1 lower than that of drawn fibres, indicating that the cooperativity of molecular motion in the fibre changes with orientation and crystallinity.