Micro‐Raman study of the fatigue fracture and tensile behaviour of polyamide (PA 66) fibres

Abstract

The micro/nano structural evolution of two different types of ultra-high-performance polyamide (PA) 66 fibres, before and after tensile fatigue loading and failure, was studied using Raman spectroscopy. The nanostructure of these PA fibres consists of amorphous and crystalline domains. Two probes were considered: (i) low-wavenumber collective modes at ∼100 and ∼70 cm −1 as representative of the crystalline and amorphous domains, respectively; and (ii) N-H stretching mode at ∼3300 cm −1 , as representative of the inter-chain behaviour. The analysis of the wavenumber distribution revealed that the skin/core heterogeneity is a maximum for the amorphous domains. The residual stress in the skin was evaluated at about 200 and 400 MPa from calibrations plots for crystalline and amorphous domains, for the two types of fibres. This stress disappears after thermal treatment around T g . In situ analysis at different strain levels shows that the amorphous regions accommodate the stress up to ∼350 MPa, from which point elastic behaviour is observed. The analysis of a series of fibres failed in fatigue showed that amorphous domains are highly stressed during the failure and remnant compression can be measured. The extent of the stressed region extends to typically 100-400 µm from the fracture. Examination of the N-H wavenumber shift shows that the fracture squeezes the polymer chains together. Fatigue failure is explained by the loss of compliance in the amorphous region.

Raman and IR micro-analysis of high performance polymer fibres tested in traction and compression

“…6). A core-skin effect is obvious [1,3,4] from the wavenumber shift across the fibre diameter. These variations indicate a weak tension of the crystalline chains and a strong compression of the amorphous bonds in the fibre core.…”

“…The tensile behaviour of the crystalline and amorphous macromolecular chains can be investigated by follow-up of the collective mode T' chains [3,4]. This shows an initially viscoplastic behaviour until 450-500 MPa (~10-12 % of lengthening), slightly varying according to the fibre producer.…”

“…The mechanisms correlating texture and nano-/micro-properties remain to be clarified. We will try here to present the broad possibilities that IR and Raman microscopes offer in the study of the texture and the (nano)mechanical behaviour of fibres [3,4] by closely analysing the spectra with the techniques of deuteration developed for the study of compounds with hydrogen bonding (protonic conductors in particular [5]). We will take PA66 polyamide fibres The analysis in (load or strain-) controlled traction and hydrostatic compression (in a diamond anvil cell [6]) clearly highlights different thresholds of tension/compression for "crystalline" and "amorphous" bonds, in agreement with the macroscopic mechanical analyses.…”

“…We will focus our discussion on the differentiation of bonds belonging to a crystalline "area" from those pertaining to "amorphous areas" by using the technique of isotopic dilution H/D [5]. The conclusions will be applied to the 4 discussion of recently published results on measurements under controlled stress [3,4,6]. Silk was studied either in the gel state, directly in the Bombyx mori glands, or in the threaded state.…”

Nanomechanics of single keratin fibres: A Raman study of the α‐helix →β‐sheet transition and the effect of water

“…11 Attention was paid to the reproducibility of the data treatment with respect to the Raman mode assignments found in the literature. In this regard, the following assumption was made: Lorentzian shape for narrow peaks and a Gaussian form for broad peaks.…”