HIGH-PERFORMANCE AROMATIC POLYIMIDE FIBERS .5. COMPRESSIVE PROPERTIESOF BPDA-DMB FIBER

A new polyimide has been synthesized from 3,3',4,4'-biphenyltetracarbo xylic dianhydride (BPDA) a nd 2,2'-dimethyl-4,4'-diaminobiphenyl (DMB) . A high-strength, high-modulus, high-temperature fibre has been devel oped from this polyimide via a dry-jet wet spinning method. The tensil e strength of BPDA-DMB fibres is 3.3 GPa and the tensile modulus is ar ound 130 GPa. The compressive strength of the fibres has been investig ated through a tensile recoil test (TRT), while the fibre morphology a fter compression has been studied via polarized light microscopy (PLM) and scanning electron microscopy (SEM). From the TRT measurements, we have observed that the compressive strength of this fibre is 665 (+/- 5) MPa, which is higher than those of other aromatic polymer fibres. T he effect of fibre diameter on the compressive strength of BPDA-DMB fi bres is not substantial. The critical compressive strain for this fibr e at which the kink bands start appearing under the observation of PLM is at 0.51-0.54%. Subglass relaxation processes have been observed an d the measure of an apparent relaxation strength may serve as one of t he factors which significantly affect the compressive strength of the fibres. Tensile tests of pre-compressed fibres reveal a continuous los s in tensile strength (up to 30%) with increasing the compressive stra in (up to 2.6%). PLM and SEM observations show that during the compres sion BPDA-DMB fibres form regularly-spaced kink bands at +/-60 degrees (+/-2 degrees) with respect to the fibre axis. The kink band density initially increases with the compressive strain, and reaches a maximum at around 1.1%. Further increase of the compressive strain decreases this density due to the merge of the neighbouring bands. The size of k ink bands also correspondingly increases within this compressive strai n region. The morphological observation via SEM implies the existence of a skin-core structure and microfibrillar texture which are common f eatures in polymer fibres.