Structure and dynamic mechanical properties of melt intercalated polyamide 6 - Montmorillonite nanocomposites

Polymer-layered silicate nanocomposites (PLSN), based on polyamide 6 (PA6) and montmorillonite (MMT) modified with an octadecylammonium salt, were produced via melt compounding in 6 co-rotating twin-screw extruder. Wide angle X-ray diffraction (WAXD) and TEM revealed a PLSN containing 3.3% by weight (wt.-%) of MMT to exhibit a mixed exfoliated/intercalated morphology, consisting mainly of individual silicate lamellae together with some intercalated stacks, resulting in a mean value of 1.8 lamellae per particle. In contrast, a PLSN containing a higher level of 7.2 wt.-% MMT exhibited a more ordered intercalated structure, consisting mainly of a distribution of lamellae stacks with a mean value of 3.8 lamellae per particle. The dispersion of MMT in the PLSN generated very large, polymer-filler interfacial areas, resulting in significant increase in the volume of constrained PA6 chain segments. Consequently, significant changes in the ratio of alpha/gamma crystallites and in the thermal behaviour of the matrix PA6 were observed during WAXD, DSC and dynamic-mechanical thermal analysis (DMTA) studies of the PLSN. In particular, damping data from DMTA showed relaxations between T-g and T-m resulting from amorphous polymer chain segments constrained at the polymer-filler interface, indicating the formation of a continuous phase of constrained polymer. In contrast, a PA6 microcomposite formed using unmodified MMT generated much lower polymer-filler interfacial area, with most of the MMT residing within large, poorly wetted aggregates. Consequently, changes to the thermal behaviour of the matrix PA6 were much less significant than those induced in the PLSN.