Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

In this work, the molecular dynamics of four organic compounds confined in silica pores of nominal diameter 6 and 20 nm are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with re ference to the bulk substances. By using organic compounds forming soft pla stic crystals on freezing as adsorbates, damage to the pore structures can be avoided. NMR lineshapes, spin-lattice relaxation times (T-1), spin-spin relaxation times (T-2) and diffusivities are reported as a function of temp erature. Since the porous grains are much greater than the distance travell ed by the molecules during the experiment, intracrystalline NMR parameters were obtained. However, the short T-2(similar to 1 ms) encountered in both the bulk and confined samples prohibited measurements of T, and the diffusi vity in the low-temperature ordered phases. The confinement in the pores gi ves rise to substantial changes in the phase behavior and molecular dynamic s. Thus, the H-1 lineshape observations of the confined samples clearly rev eal a narrow-line component superimposed on a broad resonance at temperatur es well below the transition point of the bulk material. In the freezing re gion, the narrow-line component is attributed to the surface layer and the undercooled liquid in the smaller pores that remains unfrozen. In the two-c omponent, low-temperature region, the narrow component corresponds to the s urface layer, while the broad component originates from the crystalline pha se at the center of the pores.