The structure and deformation behaviour of poly(p-phenylene benzobisoxazole) fibres

Abstract

The relationship between structure and mechanical properties in as-spun and heat-treated high modulus poly (p-phenylene benzobisoxazole) (PBO) fibres has been examined using a combination of electron microscopy, mechanical testing, and Raman microscopy. The structure of the fibres has been determined by obtaining longitudinal sections, and electron diffraction has shown that skin regions are significantly more oriented than the fibre cores. Heat treatment of the fibres at elevated temperatures produces an improvement in the level of crystallinity especially in core regions. Heat treatment also produces an increase in fibre modulus but for fibres heat treated at 650° C there is a significant decrease in strength compared with ones heat treated at 600° C. Well-defined intense Raman spectra were obtained from individual fibres and three main bands at 1280, 1540 and 1615cm−1 have been identified. All three bands are sensitive to the level of applied strain with the 1280cm−1 being the most sensitive, shifting by-7.9cm−1% strain for PBO fibres heat treated at 600° C. The dependence of the sensitivity of the position of the 1615cm−1 band to strain upon fibre structure has been examined in detail. The rate of shift of band position with strain increases with fibre modulus. It is shown that these shifts in Raman bands are a direct reflection of molecular deformation within the fibres.