Micro-Raman study of the compressive behaviour of advanced PA66 polyamide fibres in a diamond-anvil cell

The micro/nanostructural evolution of ultra-high performance PA66 polyamide fibres has been studied by Raman spectroscopy in a diamond-anvil cell, up to 10 GPa. The nanostructures of these PA fibres consists of amorphous and crystalline (αI-type) regions (oriented + crystalline index ∼ 60%). Information on the “crystalline” part of the fibre is obtained from three probes: (i) a low wave number collective mode at ∼100 cm−1 representative of the “friction/shear” of crystalline chains; (ii) NH stretching mode at ∼3300 cm−1, and (iii) Amide I (CO) stretching mode at ∼1650 cm−1; the last two modes are representative of the inter-chain and intra-chain behaviour, respectively. A reversible transition between two regimes was evidenced at ∼3.7 GPa and assigned to the onset of the “contact” between neighbouring polyamide chains, i.e. the onset of possible fatigue. In the first regime (I), the “friction” wave number increased linearly (7.5 cm−1 GPa−1) whilst the distance between neighbouring (crystalline) chains decreased from ∼0.293 to ∼0.285 nm; in the second regime (II), the “friction” wave number still increased but at a much slower rate (<4.5 cm−1 GPa−1) and the CO stretching mode of the amide group started to increase (∼1.6 cm−1 GPa−1), indicating that the chains were squeezed together in all directions. The NH stretching mode continued to decrease, slowly (dN⋯O down to 0.289 or 0.2865 nm according to the data fitting procedure). Different behaviours are expected for amorphous and crystalline “regions”.