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

Abstract

Several exfoliated nylon-6/clay nanocomposites (NnC's) were investigated and compared with pure nylon-6 using solid-state NMR, both proton and 13C. 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 Mg2+ and Fe3+ ions that substitute into octahedral Al3+ sites along the midplane of the 1-nm-thick clay layers. The Fe3+ ions impart a useful paramagnetism to the clay. Each Mg2+ 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 Fe3+ sites in the clay. For this purpose, we looked directly at the influence of Fe3+ on the 13C 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 Fe3+ sites; thus, the spectral density can be altered by changing the Fe3+ 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.

Nylon 66/clay nanocomposite structure development in a twin screw extruder

“…The second melting temperature (T mII ) detected is at around 2528C for all the nanocomposites studied, which is Form II or c-type peak. Previous studies on PA6/clay nanocomposites [32][33][34][35] have shown that the presence of clay promotes the growth of c crystal phase of PA6. It appears that the introduction of clay into PA66 also increases the amount of c form crystal and disturbs the perfect a nylon crystals during the heating cycle of DSC measurements.…”

Development and characterization of hybrid materials based on biodegradable PLA matrix, microcrystalline cellulose and organophilic silica

“…Generally speaking, the decrease in the relaxation parameter is a consequence of an increase in the molecular mobility of chains due to the increase in their freedom [8,[17][18][19] . Changes in the domain curves such as baseline mean that the samples have an amorphous or non-ordered organization, involving a mixture of more than one chain organization [8,19] .…”

“…The broad absorption band between 400 cm -1 and 3000 cm -1 corresponds to the vibrations of different basic types at the surface (Si-OH) [17,25] . The C-H stretch bands of methyl groups derived from dimethyl dichloro silane inserted into silica were observed in the region between 2850-2990 cm -1 .…”

Crossed characterisation of polymer-layered silicate (PLS) nanocomposite morphology: TEM, X-ray diffraction, rheology and solid-state nuclear magnetic resonance measurements

“…The NMR methodology to assess the nanodispersion degree has been developed at the National Institute of Standards and Technology by VanderHart et al 24,25 . It is based on proton In a recent work 27 , the dispersed microstructure of the clays in the nanocomposites was characterized by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy (HR-TEM).…”