An investigation into the relationship between processing, structure and properties for high-modulus PBO fibres. Part 1. Raman band shifts and broadening in tension and compression

Abstract

This study is concerned with an investigation into the relationship between morphology and mechanical properties for poly(p-phenylene benzobisoxazole) (PBO) fibres performed using Raman spectroscopy. The three fibres studied have different modulus values (in the range 180–360 GPa), as a result of different processing conditions leading to different molecular orientation and the presence of density fluctuations along the fibre axis. It is found that the 360 GPa modulus fibre has a strain-induced Raman band shift value of −12 cm⁻¹/% which is the highest reported value for aromatic polymer fibres such as PBO and p-aramids. The strain-induced Raman band broadening value of this fibre is the lowest among the fibres studied. Compressive strengths for the PBO fibres were also measured and found to be similar (0.29–0.35 GPa). It was also shown that the Raman band broadening becomes less in compression, which is different from the behaviour of carbon fibres and a characteristic feature of PBO fibres. The relationship between the band broadening and fibre morphology is discussed in detail.

Introduction

The PBO (poly-p-phenylene benzobisoxazole) high-modulus, high-strength super fibre has been commercialised by Toyobo Co. Ltd. under the trade name of Zylon^®. It is based upon the molecule:

The Young's modulus of PBO is higher than that of any other commercial fibre, but there still exists a gap between the values of actual and theoretical moduli. We have recently succeeded in improving PBO fibres further in laboratory experiments by adopting a non-aqueous coagulation system and a heat-treatment method [1]. It is important to implement a slow coagulation process during fibre production because the pre-structure of the fibre induced before heat-treatment determines both the final fibre structure and its mechanical properties. The modulus of the improved PBO fibre thus obtained can be as high as 360 GPa. The fibres also show an absence of the four-point small-angle X-ray scattering (SAXS) pattern that is characteristic of the morphology of heat-treated PBO fibre obtained using the normal (aqueous) coagulation process [1]. The apparent crystal size measured is almost the same as for the HM fibre and the molecular orientation is slightly higher for the improved fibre. This implies that factors other than just molecular orientation are concerned with improving moduli. It is thus the objective of this study to use Raman spectroscopy, which is a powerful technique to detect molecular deformation in fibres under stress [2], [3], [4], [5], to elucidate the mechanism of modulus improvement for this new experimental PBO fibre. This first paper is concerned with the relationship between Raman band peak shift, peak broadening, and fibre morphology for PBO fibres processed under different conditions.