JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2007; 38: 100–109 Published online 22 September 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jrs.1631 Micro-Raman and IR study of the compressive behaviour of poly(paraphenylene benzobisoxazole) (PBO) fibres in a diamond-anvil cell Philippe Colomban, * Achraf Aidi-Mounsi and Marie-H ´ el ` ene Limage Laboratoire Dynamique, Interactions et R ´ eactivit ´ e UMR 7075 CNRS & UPMC, 2 rue Henry-Dunant, 94320 Thiais, France Received 22 February 2006; Accepted 19 July 2006 The micro/nano structural evolution of PBO (poly(paraphenylenebenzobisoxazole), commercial name Zylon) fibres has been studied by Raman and IR spectroscopy in a diamond-anvil cell, up to ∼15 GPa. Different modes were considered in the 900–1700 cm -1 (Raman) and 800–3300 cm -1 (IR) spectral range. The material behaviour appears more similar to that of a densely packed inorganic structure than to the behaviour previously observed for polyamide fibres, and is related to the compact arrangement of the rigid heterocyclic rings. Wavenumber shift starts with increase in pressure and a transition between the two regimes was revealed at ∼2–2.5 GPa, and it was assigned to the loss of order between neighbouring PBO chains. A good correlation is observed between the macroscopic mechanical properties and the nanomechanics at the chemical bond scale. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: vibrational spectroscopy; fibre; PBO; compression; diamond-anvil cell INTRODUCTION The effect of high pressures on inorganic, organic and even biological compounds have been investigated, 1–4 but the effect of pressure on the micro/nanostructure of polymeric materials remains largely unexplored. Compressive proper- ties of fibres are less understood than their tensile properties and indirect methods have been developed to characterize them at the macroscopic scale (elastic loop, beam bending, single fibre composites, tensile recoil 5 ). Spectroscopic analy- ses of the specimen as a function of pressure have great potential for the examination of decreasing inter- and intra- segment/chain distances, and provide an experimental tool that is selectively sensitive to changes in bonds and local chemical environments. Diamond-anvil cells (DACs) have proven to be useful tools for studying the response of differ- ent materials to applied pressure, 6 using both IR and Raman microscopes. With an appropriate medium, DACs provide a perfect hydrostatic pressure environment for studying changes in the compound. The present study examines the behaviour of poly(p-phenylenebenzobisoxazole) (PBO, commercial name Zylon, Toyobo Co. Ltd.) fibres under hydrostatic pressure up to ¾15 GPa. Built with rigid aro- matic heterocyclic rings (Fig. 1(a)), the PBO fibre shows L Correspondence to: Philippe Colomban, Laboratoire Dynamique, Interactions et R´ eactivit´ e UMR 7075 CNRS & UPMC, 2 rue Henry-Dunant, 94320 Thiais, France. E-mail: philippe.colomban@glvt-cnrs.fr excellent tensile and mechanical strength and high heat resistance: 7–12 the fibre modulus reaches ¾350 GPa and has a tensile strength 5.8 GPa, much higher than that of other polymers and inorganic fibres. Figure 1(b) compares the ten- sile stress–strain curves: the pure elastic behaviour of PBO fibre is evident. Note that the ultimate stress of a polymer fibre is directly related to the density, i.e. to the number of chemical bonds in the unit volume. IR and Raman analyses were reported and peak assign- ment to vibrational modes was made on the basis of normal- mode calculations. 13 – 15 Crystal size and morphology have been discussed. 8–10,13,15 According to its high modulus and its processing route, the PBO fibre exhibits an oriented molecu- lar structure, but small-angle X-ray scattering measurements show that the fibre includes periodical density fluctuations along the fibre axis (Fig. 2) together with elongated voids. 13 PBO chains form elongated rigid rods, but the interactions between the neighbouring chains are expected to be weak because only the Van der Waals bonds and electrostatic forces connect the molecular chains. In the present study, a comparison will be made with the results from the same labo- ratory on the compressive behaviour of polyamide-66 (PA66) fibres, a semi-crystalline polymer (poly(hexamethylene adi- pamide)), with chains that are hydrogen-bonded through N ÐÐÐ O bridges 16 A high orientation of the molecular chains along the fibre axis is desirable in order to achieve high mechan- ical properties. In earlier papers we have pointed out Copyright  2006 John Wiley & Sons, Ltd.