The effect of increasing
pressure and two-dimensional (2D) confinement
on the dynamics of glass-forming polymer poly(methylphenylsiloxane)
(PMPS) was investigated with the use of dielectric spectroscopy. We
demonstrate that the glass-forming polymer confined to nanoporous
alumina might obey the density scaling relation similar to that in
the bulk and that the same value of the scaling exponent is used to
superimpose the α-relaxation time measured under different thermodynamic
conditions. Our comprehensive analysis of the relaxation processes
detected in the dielectric loss spectra of PMPS allows us to identify
the Johari–Goldstein β-relaxation which for a bulk polymer
shows up as a well-resolved peak while under 2D nanoconfinement only
as an excess wing. In contrast to previous studies, we provide dielectric
evidence of an additional α′-relaxation, slower than
the segmental (α-) dynamics, which is related to the chain dynamics
of PMPS.
In
the presence of nanoscale confinement, the role of surface effects
come to the fore in determining the glass-transition dynamics of the
molecular liquids and polymers. Therefore, by playing with the surface
chemistry it is possible to understand better the mechanism which
governs the behavior of glass-forming systems at the nanoscale. In
this work, we have combined dielectric and calorimetric data to study
surface and confinement effects for highly polar glass-forming liquid
S-Methoxy-PC constrained within anodic aluminum oxide membranes of
different pore sizes. The inner surface of the pores was modified
either by silanization or atomic layer deposition (ALD) coatings.
For the tested substance in native nanopores, we have observed two
glass transition events in the calorimetric response accompanied by
a characteristic deviation of the temperature dependence of the α-relaxation
time from the Vogel–Fulcher–Tamman law. We found that
depending on the hydrophobicity of the ALD layer the glass transition
temperature of the interfacial layer tends to decrease, the α-relaxation
peak broadens, and the molecular mobility slows down compared to the
native pores. These changes are more visible with increasing hydrophobicity
of the surface. Silanization was also found to eliminate at least
partially the effects caused by nanopore confinement. However, in
this study the most pronounced effects were observed only for pores
with large diameters.
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