Single fibre deformation studies of poly(p-phenylene benzobisoxazole) fibres part I Determination of Crystal Modulus

Abstract

This paper constitutes the first part of a study to assess the influence of processing conditions on the final properties of poly(p-phenylene benzobisoxazole) PBO fibres. Three different samples were selected: as-spun (AS), high-modulus (HM), and ultra-high modulus (HM+) fibres. Synchrotron radiation was used to obtain single-fibre diffraction patterns. It is the first time this approach is taken to estimate the effects of deformation on the crystal properties of PBO fibres. The crystal modulus of the different types of fibre was calculated from the variation with stress of the c-spacing determined from the shift of the (005) and (006) reflections. The HM fibre was found to have the highest crystal modulus of the three fibres, with AS and HM+ PBO being lower. In comparison with tensile data, none of the fibres were found to have a Young's modulus near to the crystal modulus value, although the HM+ fibre was closest due to its production route. These results could be compared to previous diffraction experiments, where the crystal modulus of PBO fibres were determined using fibre-bundles, assuming homogeneous stress in the bundle. Also, Raman spectroscopy experiments were carried out to examine the differences in Raman bandshift rates in response to both stress and strain. The Raman results showed both the HM and HM+ fibres to have stress-induced bandshifts of approximately −4 cm−1/GPa. The AS fibre value was significantly lower, this being attributed to the non-uniformity of the fibre cross-section. The strain-induced Raman bandshifts were found to be dependent on the tensile modulus of the fibre.