Dynamics of propylene glycol and its oligomers confined in clay

Abstract.

The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with $M_{{\rm W}} = 4000$ (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time $\langle\tau\rangle$, deduced from neutron scattering at a momentum transfer Q of about $1 \; {\rm {\AA}^{-1}}$, is in perfect agreement with the dielectric $\alpha$-relaxation time, although neutron scattering does not only probe the main ($\alpha$-) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature $1/\langle\tau\rangle$ is proportional to Q ², indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that $\langle\tau\rangle$ (or the dielectric $\alpha$-relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation.