Structural Changes from the Pure Components to Nylon 6−Montmorillonite Nanocomposites Observed by Solid-State NMR

Abstract

Multinuclear solid-state NMR was used to characterize the molecular structures in nylon 6-montmorillonite nanocomposites in comparison with the two pure components. Both the polymer and the clay were studied. 27 Al two-dimensional multiple quantum magic angle spinning (MQMAS) measurements reveal the existence of an additional four-coordinated aluminum site in the nanocomposites compared to in the pure clay. This site is most probably induced by interactions of the polymer chains with the silicate surface. A highly mobile component is observed only in the 1 H NMR spectra of all nanocomposite samples, whose relative content increases with increasing clay content and which is generated during the extrusion process. High-resolution 1 H and 13 C NMR spectra of these mobile molecules are recorded and assigned to be a tertiary amine that is formed during the melt extrusion process as a result of the loss of one methyl group of the organic modifier dimethyl di(hydrogenated tallow) ammonium ion. As a result of the presence of paramagnetic Fe 3+ inside the clay, the proton spin-lattice relaxation time of the polymer is strongly influenced. This effect can be used to get information on the degree of clay exfoliation. It was found that at higher clay contents on average just two platelets are stuck together. With this, the distance between clay platelets is calculated and correlated with the analysis of TEM (transmission electron microscopy) measurements. 15 N cross-polarization magic angle spinning (CPMAS) spectra show an increase of the fraction of the γ-crystalline phase at the cost of the α-crystalline phase upon increasing clay content.