Poly(methyl phenyl siloxane) in Random Nanoporous Glasses: Molecular Dynamics and Structure
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Poly(methyl phenyl siloxane) in Random Nanoporous Glasses: Molecular Dynamics and Structure Andreas. Schönhals1,*, Harald. Goering1, Christoph Schick2, Bernhard. Frick3 and Reiner Zorn4 1
Federal Institute of Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin,
Germany 2
University of Rostock, Department of Physics, 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
ABSTRACT 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). 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. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel / Fulcher / Tammann like behavior to an Arrhenius one. At the same pore size ∆cp vanishes. These results give strong evidence that the glass transition has to be characterized by an inherent length scale of the relevant molecular motions. Quasielastic neutron scattering experiments reveal a strong change even in the microscopic dynamic. INTRODUCTION Recently there is an increasing interest to study the properties of molecular systems in restricting space of a nanometer scale [1-4] like thin films or confined to zeolites or nanoporous glasses. One reason for that is the potential of such systems for applications in modern chemistry, medicine or nanotechnology. From the theoretical side one main reason for such studies is to investigate the influence of finite size effects on the properties of matter. One important point in this field is to unravel the glass transition (α-relaxation) which is an immediate problem of soft matter physics [5]. The most pronounced effect observed during cooling of a glass-forming system is the dramatic increase of the relaxation times τ or the viscosity over more than 14 orders of magnitude by decreasing temperature by a factor of 2 close to the glass transition temperature Tg. According to theoretical approaches this can be understood assuming a cooperative behavior of the molecular motions characterized by a length scale ξ which increases with decreasing temperature (see for instance [6,7]). Close to Tg ξ is expected to be in the order of a few nanometers [7,8]. Furthermore the existence of dynamical heterogeneities in glass-forming materials gives also a new length scale in the nanometer range [8,9]. By confining molecules to host systems with dimensions of a few molecules the relevance of an inherent length scale beeing responsible for glassy dynamics can be indirectly proven. The
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acceleration of the α-relaxation of salol [10] confined to nanoporous glasses compared to the bulk (confinement effect) and the transition from
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