An investigation into the relationship between processing, structure, and properties for high-modulus PBO fibers. II. Hysteresis of stress-induced Raman band shifts and peak broadening, and skin-core structure

Abstract

This paper is the second of a series of studies to elucidate the relationship between the structure and mechanical properties of poly(p-phenylene benzobisoxazole) (PBO) fibers processed in different ways. Deformation hysteresis effects with stress and strain have been followed using Raman Spectroscopy and are discussed in terms of modulus improvement and fiber morphology. Selected-area electron diffraction has been employed upon longitudinal ultra-thin section of the fibers to detect the differences in skin-core structure. It is found that the stress–strain curve and Raman band shifts with stress and strain in PBO as-spun (AS) fibers show noticeable mechanical hysteresis. This indicates that plastic structural changes occur upon loading in tension, which may give the fibers a more homogeneous structure. No significant mechanical hysteresis is found for the commercial AS and heat-treated (HM), and new (HM+) PBO fibers. The HM+ fiber is found to show a more enhanced skin-core structural difference than the AS and HM fibers even though it has the highest Young's modulus.

Keywords:

• Raman spectroscopy
• Poly(p-phenylene benzobisoxazole) fibers
• Mechanical properties
• Fiber morphology

Acknowledgments

The authors would like to express their gratitude to the EPSRC for financial support and to Toyobo Co. Ltd for allowing publication. The authors would also like to extend their thanks to Mr. John Hutton (Electron microscope unit of the Medical School, University of Manchester) and Mr. Peter Kenway (Manchester Materials Science Centre) for their introduction of ultra-microtomy and TEM analysis.