Solid-State NMR Investigation of Paramagnetic Nylon-6 Clay Nanocomposites. 1. Crystallinity, Morphology, and the Direct Influence of Fe3+ on Nuclear Spins

Several exfoliated nylon-6/clay nanocomposites (NnC's) were investigated and compared with pure nylon-6 using solid-state NMR, both proton and 13 C. NnC's had nominally 5 mass % clay and were generated both by blending and by in situ polymerization (IsP). Most of the studied NnC's contained layered, naturally occurring montmorillonite clays having nonstoichiometric amounts of nonexchangeable Mg 2+ and Fe 3+ ions that substitute into octahedral Al 3+ sites along the midplane of the 1-nm-thick clay layers. The Fe 3+ ions impart a useful paramagnetism to the clay. Each Mg 2+ ion leaves an embedded negative charge that must be neutralized with some cation at the surface of the clay. All clays were initially treated with a cationic so-called organic modifier (OM), often a substituted ammonium ion, which increases the clay layer spacing, attaching ionically to the surface of the clay layers. Clay is found to promote growth of the γ-crystalline phase of nylon-6 for both blended and IsP NnC's; α-crystallites are characteristic of the pure nylon-6. Stability of the γ-phase to annealing at 214 °C was investigated. Conversion of γ- to α-crystallinity during annealing was minimal, except for an injection-molded IsP NnC, which had been exposed to a temperature of 295 °C during molding. This high processing temperature produced an irreversible change. An attempt was made to understand, at least qualitatively, the nature of the spectral density of magnetic fluctuations associated with the paramagnetic Fe 3+ sites in the clay. For this purpose, we looked directly at the influence of Fe 3+ on the 13 C and proton observables in organically modified clays (OMC). We agree with other investigators that the spectral density of paramagnetic fluctuations at the surface of the clay is determined mainly by spin-exchange interactions between Fe 3+ sites; thus, the spectral density can be altered by changing the Fe 3+ concentration. Moreover, we find that the spectral density is very wide, having strong contributions all the way from mid-kHz fluctuations to MHz fluctuations near the proton Larmor frequencies. Significant variations in the α/γ ratio were also observed in the injection-molded disk, which reflect either a processing-induced heterogeneity in clay dispersion or a significant variation in cooling history from region to region. Proton spin diffusion and multiple-pulse methods were utilized to compare morphologies for a diamagnetic NnC and nylon-6 with the same thermal histories. Long spacing, crystallinity, and the mobility of the noncrystalline nylon-6 segments are very similar for NnC's and nylon-6.