Glass transition and relaxation dynamics of propylene glycol-water solutions confined in clay
Journal article, 2014
The molecular dynamics of aqueous solutions of propylene glycol (PG) and propylene glycol methylether (PGME) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy and differential scanning calorimetry. As typical for liquids in confined geometries the intensity of the cooperative alpha-relaxation becomes considerably more suppressed than the more local beta-like relaxation processes. In fact, at high water contents the calorimetric glass transition and related structural alpha-relaxation cannot even be observed, due to the confinement. Thus, the intensity of the viscosity related alpha-relaxation is dramatically reduced, but its time scale as well as the related glass transition temperature T-g are for both systems only weakly influenced by the confinement. In the case of the PGME-water solutions it is an important finding since in the corresponding bulk system a pronounced non-monotonic concentration dependence of the glass transition related dynamics has been observed due to the growth of hydrogen bonded relaxing entities of water bridging between PGME molecules [J. Sjostrom, J. Mattsson, R. Bergman, and J. Swenson, Phys. Chem. B 115, 10013 (2011)]. The present results suggest that the same type of structural entities are formed in the quasi-two-dimensional space between the clay platelets. It is also observed that the main water relaxation cannot be distinguished from the beta-relaxation of PG or PGME in the concentration range up to intermediate water contents. This suggests that these two processes are coupled and that the water molecules affect the time scale of the beta-relaxation. However, this is most likely true also for the corresponding bulk solutions, which exhibit similar time scales of this combined relaxation process below T-g. Finally, it is found that at higher water contents the water relaxation does not merge with, or follow, the alpha-relaxation above T-g, but instead crosses the alpha-relaxation, indicating that the two relaxation processes are independent of each other. This can only occur if the two processes do not occur in the same parts of the confined solutions. Most likely the hydration shell of the interlayer Na+ ions is causing this water relaxation, which does not participate in the alpha-relaxation at any temperature.