A solid-state NMR study of structure and segmental dynamics of semicrystalline elastomer-toughened nanocomposites

Significant enhancement in toughness and elongation at a break in nanocomposites based on polyamide-6 (PA6) and silicate platelets (MMT, 5 wt %) was achieved by co-adding of a small amount of ethylene-methyl acrylate copolymer (EMA, 5 wt %). To investigate the observed changes in this triple component system at a molecular level, motional behavior of polymer segments was probed by recently developed 2D techniques of H-1-C-13 dipolar solid-state NMR spectroscopy. Owing to measurements performed separately in amorphous and crystalline domains, a large collection of high-quality site-specific dipolar profiles could be analyzed with respect to a distribution of motional amplitudes: (i) in both crystal modifications as well as in "constrained" domains of the amorphous phase of PA6, the amplitudes of rotational-diffusion motion of CH2 groups (10-33 degrees) increase from NH and CO groups toward the central part of the repeat units, (ii) comparing both crystal modifications, less-tight packing of polymer chains in gamma-form increases fluctuation angle about 5, and (iii) the presence of silicate platelets in amorphous PA6 matrix enhances motional amplitudes of polymer segments in nanocomposites. The determined order parameters were further used to estimate an upper bound for the change in the Gibbs free energy resulting from structural restrictions occurring in the investigated systems. As a result of the increased conformational entropy, the Gibbs free energy in nanocomposites is somewhat reduced (Delta G = 0.4-0.5 kcal/ mol). This fact, together with temperature-induced enhancements of the motional amplitudes, supports the explanation that fast and relatively low-amplitude motions occurring in the glassy state of polyamide-6 can be considered as mechanically active, and as a potential supplementary factor that may help to enable the larger-amplitude motions (jumps) associated with glass-transition temperature. Finally, the enhancement of ductility observed in the triple-component system was attributed to the formation of EMA copolymer domains that are surrounded by clay platelets. The size of these " core-shell" particles is sufficiently large (50-200 nm) for the internal dynamics of polymer chains not to be significantly hindered by rigid components. Polymer chains in these completely phase-separated domains exhibit large-amplitude motions and undergo the most prominent temperature-induced changes.