Microfibrillar composites based on polyamide/polyethylene blends. 1. Structure investigations in oriented and isotropic polyamide 6

The present paper discloses the structural changes caused by heating of polyamide 6 (PA6) samples with different thermal and mechanical histories in the 30-240 °C range. Wide and small-angle X-ray scattering (WAXS and SAXS) of synchrotron radiation, as well as solid-state nuclear magnetic resonance spectroscopy (NMR) measurements are performed. The NMR spectra show that in both isotropic and oriented samples there is a co-existence of α and γ-PA6 crystalline forms. Deconvolution of the WAXS patterns is performed to follow the temperature dependence of the unit cell parameters of the α and γ-forms and also of the equatorial (ECI) and total crystallinity indexes (CI), evaluating the contributions of the two crystalline phases. Estimates for the long spacing and for the average thicknesses of the crystalline (lc) and amorphous (la) phases within the lamellae are calculated as a function of the heat treatment employing analysis of the linear correlation function calculated from the SAXS patterns. The X-ray results allowed the conclusion that upon heat treatment up to 160-200 °C, intensive transitions between the PA6 crystalline forms take place, whereby the content of the initial major crystalline phase decreases and that of the initial minor one increases reaching almost 1:1. Close to 200 °C a general trend toward increasing the content of the α-form is registered. The influence of annealing and quenching after melting on the PA6 crystalline structure is also studied.