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 an d compared with pure nylon-6 using solid-state NMR, both proton and C-13. N nC'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 Mg2+ and Fe3+ ions that substitute into octahedral Al3 sites along the midplane of the 1-nm-thick clay layers. The Fe3+ ions impa rt a useful paramagnetism to the clay. Each Mg2+ ion leaves an embedded neg ative charge that must be neutralized with some cation at the surface of th e clay. All clays were initially treated with a cationic so-called organic modifier (OM), often a substituted ammonium ion, which increases the clay l ayer spacing, attaching ionically to the surface of the clay layers. Clay i s found to promote growth of the gamma -crystalline phase of nylon-6 for bo th blended and IsP NnC's; alpha -crystallites are characteristic of the pur e nylon-6. Stability of the gamma -phase to annealing at 214 degreesC was i nvestigated. Conversion of gamma- to alpha -crystallinity during annealing was minimal, except for an injection-molded IsP NnC, which had been exposed to a temperature of 295 degreesC during molding. This high processing temp erature produced an irreversible change. An attempt was made to understand, at least qualitatively, the nature of the spectral density of magnetic flu ctuations associated with the paramagnetic Fe3+ sites in the clay. For this purpose, we looked directly at the influence of Fe3+ on the C-13 and proto n observables in organically modified clays (OMC). We agree with other inve stigators that the spectral density of paramagnetic fluctuations at the sur face of the clay is determined mainly by spin-exchange interactions between Fe3+ sites; thus, the spectral density can be altered by changing the Fe3 concentration. Moreover, we find that the spectral density is very wide, h aving strong contributions all the way from mid-kHz fluctuations to MHz flu ctuations near the proton Larmor frequencies. Significant variations in the alpha/gamma ratio were also observed in the injection-molded disk, which r eflect either a processing-induced heterogeneity in clay dispersion or a si gnificant variation in cooling history from region to region. Proton spin d iffusion and multiple-pulse methods were utilized to compare morphologies f or a diamagnetic NnC and nylon-6 with the same thermal histories. Long spac ing, crystallinity, and the mobility of the noncrystalline nylon-6 segments are very similar for NnC's and nylon-6.