Dynamics of propylene glycol and its oligomers confined in clay
Department of Applied Physics, Chalmers University of Technology, SE-412 96, Göteborg, Sweden
2 Rutherford-Appleton Laboratory, OX11 0QX, Chilton, Didcot, UK
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The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with (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 , deduced from neutron scattering at a momentum transfer Q of about , is in perfect agreement with the dielectric -relaxation time, although neutron scattering does not only probe the main (-) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature is proportional to Q 2, indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that (or the dielectric -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.
© EDP Sciences, Società Italiana di Fisica, and Springer-Verlag, 2003