Glassy dynamics of polymers confined to nanoporous glasses revealed by relaxational and scattering experiments
Federal Institute of Materials Research and Testing, Unter den Eichen 87, D-12205, Berlin, Germany
2 Department of Physics, University Rostock, Universitätsplatz 3, D-18051, Rostock, Germany
3 Institute Max von Laue - Paul Langevin, 6 rue Jules Horowitz, B.P. 156, F-38042, Grenoble Cedex 9, France
4 Institute for Solid State Research, D-52425, Jülich, Germany
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The glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system revealed by dielectric spectroscopy, temperature-modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature-modulated DSC agree quantitatively indicating that both experiments sense the glass transition. For PPG the segmental dynamics is determined by a counterbalance of adsorption and confinement effect. The former results form an interaction of the confined macromolecules with the internal surfaces. A confinement effect originates from an inherent length scale on which the underlying molecular motions take place. The increment of the specific-heat capacity at the glass transition vanishes at a finite length scale of 1.8 nm. Both results support the conception that a characteristic length scale is relevant for glassy dynamics. For PDMS only a confinement effect is observed which is much stronger than that for PPG. Down to a pore size of 7.5 nm, the temperature dependence of the relaxation times follows the Vogel-Fulcher-Tammann dependence. At a pore size of 5 nm this changes to an Arrhenius-like behaviour with a low activation energy. At the same pore size vanishes for PDMS. Quasielastic neutron scattering experiments reveal that also the diffusive character of the relevant molecular motions --found to be characteristic above the glass transition-- seems to disappear at this length scale. These results gives further strong support that the glass transition has to be characterised by an inherent length scale of the relevant molecular motions.
© EDP Sciences, Società Italiana di Fisica, and Springer-Verlag, 2003