HIGH-PERFORMANCE AROMATIC POLYIMIDE FIBERS .2. THERMAL-MECHANICAL ANDDYNAMIC PROPERTIES

A family of high-temperature, high-modulus aromatic polyimide fibres h as been dry-jet wet spun from either its gel state or isotropic soluti on, followed by high-temperature drawing. In this report, thermal and dynamic mechanical properties of one of the family members, a segmente d rigid-rod polyimide synthesized from 3,3',4,4'-biphenyltetracarboxyl ic dianhydride (BPDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphen yl (PFMB), are presented in detail. Mechanical properties of these BPD A-PFMB fibres can be improved remarkably by drawing due to drastic inc reases in overall orientation, crystal orientation and crystallinity. These three structural parameters, however, do not show parallel chang es with increasing draw ratio. It has been observed that the linear co efficient of thermal expansion (CTE) of BPDA-PFMB fibres after drawing generally show negative values in the solid state when low stresses a re applied during measurements. For as-spun fibres, the CTEs are const ant over a certain applied stress region, which is on the same order o f magnitude as CTEs of in-plane oriented BPDA-PFMB films along the fil m surface. This may be an indication that within this region the stres s 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 regi on, then level off when the applied stress becomes high. Dynamic mecha nical data indicate two relaxation processes in as-spun fibres above r oom temperature: an a relaxation corresponding to the glass transition and a beta relaxation which is a subglass transition. In the fibres w ith 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 ha nd, crystallinity dependent. The Arrhenius activation energy (about 16 0 kJ mol-1) of the beta relaxation in as-spun fibres is about 50 kJ mo l-1 lower than that of drawn fibres, indicating that the cooperativity of molecular motion in the fibre changes with orientation and crystal linity.