Solid-State NMR Investigation of Paramagnetic Nylon-6 Clay Nanocomposites. 2. Measurement of Clay Dispersion, Crystal Stratification, and Stability of Organic Modifiers

Abstract

In this second paper of a two-part series dealing mainly with NMR characterization of nylon-6/clay nanocomposites (NnC's) having nominally 5 mass % clay, measurements with application to processing are featured. The paramagnetism of the montmorillonite clays, discussed in the first paper, allowed us to use the corresponding spin-diffusion-moderated reduction in longitudinal proton relaxation time, T1H, for two purposes, namely, to rank the quality of clay dispersion in NnC families with the same formulation and to investigate morphological stratification of the nylon-6 α- and γ-crystallites with respect to the clay surface. In a group of three NnC's with the same formulation but different melt-blending conditions, variations in T1H correlated well with previously published TEM assessments of the quality of the clay dispersion. Also, in a set of samples from an injection-molded, in situ polymerized NnC disk where strong variations in α/γ ratios were observed, it was found that these differences did not arise from processing-induced inhomogeneities in clay concentration; rather, variations in cooling histories throughout the disk was the more probable cause. In these latter samples, well-defined stratification of the γ-phase (versus the α-phase) crystallites nearer the clay surface did not occur until after annealing at 214 °C. We also examined the dependence of NnC T1H's on the static field of the measurement. It is clear that the magnitude of the paramagnetic contribution to T1H is a function of field and of Fe3+ concentration in the clay. Trends support the notion that spin-exchange interactions between the electrons on different Fe3+ ions largely define the spectral density of magnetic fluctuations near the clay surface. Some attention was, therefore, given to optimizing Fe3+ concentrations for the best NnC characterization. Finally, we investigated the chemical stability of a particular organic modifier (OM), which is used to pretreat the clay prior to melt blending. The OM, dimethyl, dehydrogenated-tallow ammonium ion, was followed in the process of blending this modified clay with nylon-6 at 240 °C. It was found that when such a clay surface was exposed to the nylon-6 during blending, most of the OM on that surface decomposed, releasing a free amine with one methyl and two tallow substituents. However, subsequent melting at 240 °C produced no further decomposition. The implication is that the combination of temperature and shear stress in blending causes decomposition, not just temperature alone. The susceptibility to chemical decomposition varied strongly with the OM. Ironically, extensive decomposition of the OM did not result in poor mixing; in fact, as judged by T1H, the NnC with the best dispersion of clay also had the most extensively degraded OM. The implications of this degradation for the physical properties have not been explored in detail.