NMR relaxation studies of slow motions of HDA hydrocarbons chains inside lamellar structures

Abstract

Measurements of H-1 and C-13 Nuclear Magnetic Resonance (NMR) for the nanocomposite materials formed by the intercalation of hexadecylamine (HAD) in metal oxides (TiO2, V2O5, CrO3 and MoO3), are reported. The H-1 NMR relaxation results reflects the complexity of the chain segments motions in these systems. The proton (and the carbon) mobility depends on the position in the HDA hydrocarbon chain to the restriction imposed by the anchoring of the head group. The nonexponential behavior of the proton magnetization is clearly a result of the anisotropic motion of the HDA chains. For most of the protons, the rate of motion is not fast enough to fulfill the condition for the nuclear spin-lattice relaxation (NSR) maximum in the laboratory frame but they fulfill the condition for the NSR in the rotating frame. The T-1 rho(-1) maximum at around 280K reflects the relative lower mobility of the protons of the HDA chain inside the low-dimensional space of the metal oxide. The description of the segmental motion of long chains require the use of an asymmetric distribution of correlation times to describe relaxation in flexible polymer chains.