Solid-state NMR investigation of paramagnetic nylon-6 clay nanocomposites.2. Measurement of clay dispersion, crystal stratification, and stability of organic modifiers

In this second paper of a two-part series dealing mainly with NMR character ization of nylon-6/clay nanocomposites (NnC's) having nominally 5 mass % cl ay, measurements with application to processing are featured. The paramagne tism of the montmorillonite clays, discussed in the first paper, allowed us to use the corresponding spin-diffusion-moderated reduction in longitudina l proton relaxation time, T-1(H), for two purposes, namely, to rank the qua lity of clay dispersion in NnC families with the same formulation and to in vestigate morphological stratification of the nylon-6 alpha- and gamma -cry stallites with respect to the clay surface. In a group of three NnC's with the same formulation but different melt-blending conditions, variations in T-1(H) correlated well with previously published TEM assessments of the qua lity of the clay dispersion. Also, in a set of samples from an injection-mo lded, in situ polymerized NnC disk where strong variations in alpha/gamma r atios were observed, it was found that these differences did not arise from processing-induced inhomogeneities in clay concentration; rather, variatio ns in cooling histories throughout the disk was the more probable cause. In these latter samples, well-defined stratification of the gamma -phase (ver sus the alpha -phase) crystallites nearer the clay surface did not occur un til after annealing at 214 degreesC. We also examined the dependence of NnC T-1(H) on the static field of the measurement. It is clear that the magnit ude of the paramagnetic contribution to T-1(H) is a function of field and o f Fe3+ concentration in the clay. Trends support the notion that spin-excha nge interactions between the electrons on different Fe3+ ions largely defin e 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 p rior to melt blending. The OM, dimethyl, dehydrogenated-tallow ammonium ion , was followed in the process of blending this modified clay with nylon-6 a t 240 degreesC. It was found that when such a clay surface was exposed to t he nylon-6 during blending, most of the OM on that surface decomposed, rele asing a free amine with one methyl and two tallow substituents. However, su bsequent melting at 240 degreesC produced no further decomposition. The imp lication is that the combination of temperature and shear stress in blendin g causes decomposition, not just temperature alone. The susceptibility to c hemical decomposition varied strongly with the OM. Ironically, extensive de composition of the OM did not result in poor mixing; in fact, as judged by T-1(H), the NnC with the best dispersion of clay also had the most extensiv ely degraded OM. The implications of this degradation for the physical prop erties have not been explored in detail.