Poly(methyl phenyl siloxane) in Random Nanoporous Glasses: Comparison of coated and uncoated confining surfaces
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0899-N09-05.1
Poly(methyl phenyl siloxane) in Random Nanoporous Glasses: Comparison of coated and uncoated confining surfaces Andreas Schönhals1,*, Harald Goering1, Christoph Schick2 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
ABSTRACT The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS) and temperature modulated DSC (TMDSC). As confining hosts random nanoporous glasses with nominal pore sizes of 2.5 nm – 20 nm have been used. Epically it is focused on the influence of a surface treatment on the dynamical behavior. 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. There is no influence of the surface treatment on the glassy dynamics for this pore size. 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 where the apparent activation energy depends on pore size. For silanized pores a higher value (102 kJ / mol) is found than for natives pores (73 kJ / mol). For a pore size of 2.5 nm the activation energy is independent of the surface treatment. The value of ca. 40 kJ / mol points to a real localized process. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale which can be regarded as minimal length scale for glass transition to appear. The actual value depends on surface treatment. From this difference the thickness of an immobilized boundary layer of about 1 nm is estimated for uncoated pores. INTRODUCTION In nanotechnology and for basic understanding the investigation of structure and dynamics of matter confined to restricting geometries of a nanometre scale is of extreme importance [1-3]. From the theoretical side one reason for such studies is to investigate the influence of finite size effects on the properties of molecules. One important point in this field is to unravel the glass transition (α-relaxation) which is an immediate problem of soft matter physics [4]. It is not known today if liquids freeze into a glassy state instead of solidifying as a crystal because in the neighbourhood of a given molecule others have to re-arrange cooperatively to reach the minimum of a complex energy landscape. By enclosing molecules into nanoporous cages one limits the size of such an environment or length scale ξ and therefore one may hope that the dynamics and consequently the freezing is changed. The acceleration of the α-relaxation of salol [3] confined to nanoporous glasses compared to the bulk (confinement effect) and the transition from a Vogel / Fulcher / Tammann (VFT-) temperature dependence of the α-relaxation time to an Arrhenius-like behavior reported
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