Quasielastic neutron scattering of propylene glycol and its 7-mer confined in clay

Abstract

The dynamics of propylene glycol (PG) and its 7-mer confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. Experiments were carried out in the temperature range 300–420 K and in two different scattering geometries in order to make the elastic Q-vector parallel and perpendicular to the clay platelets for a scattering angle of 90° (Q≈1.33 Å−1). The results indicate that at least three dynamical processes are present in the experimental time window of approximately 3–80 ps, although only two processes could be resolved at a given temperature. The fast process is independent of the chain length as well as the confinement, and in accordance with previous studies of the bulk liquids it is assigned to a rapid local motion of hydrogens in the polymeric backbone. The slow process has mainly a rotational character at T=300 K and it is assigned to the methyl group rotation. At higher temperatures this rotational motion is hidden behind a Q-dependent jump diffusion process. The calculated diffusion constant is roughly the same for our confined PG and 7-PG, and also similar to the corresponding bulk liquids (except for PG at high temperatures where the bulk liquid shows a considerably faster diffusion). However, the diffusion seems to increase slightly faster for confined 7-PG with increasing temperature. This is an interesting finding since a clear opposite trend was observed for the bulk liquids. The methyl group rotation at low temperatures (≈300 K) is significantly faster in the confined liquids. Both the methyl group rotation and the jump diffusion process seem to be present also in the direction perpendicular to the clay platelets for the 7-mers, but not for the monomers where only the fast local motion of hydrogens is observed in that direction. Furthermore, the pure translational diffusion (with a quasielastic broadening proportional to Q2) observed for the bulk liquids was never obtained for our confined liquids. The different results for the confined and bulk liquids are discussed in terms of confinement and free volume effects and surface interactions.