Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses
Federal Institute of Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany
2 University of Rostock, Department of Physics, Universitätsplatz 3, 18051 Rostock, Germany
3 Institut Laue-Langevin, 6, rue Jules Horowitz, BP. 156, 38042 Grenoble, France
4 Institute for Solid State Research, Research Centre Jülich, 52425 Jülich, Germany
The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.
© EDP Sciences, Springer-Verlag, 2007