Poly(methyl phenyl siloxane) in Random Nanoporous Glasses: Comparison of coated and uncoated confining surfaces

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.