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.
Broadband
dielectric spectroscopy (BDS) and differential scanning
calorimetry (DSC) are combined to study the effect of changes in the
surface chemistry on the segmental dynamics of glass-forming polymer,
poly(methylphenylsiloxane) (PMPS), confined in anodized aluminum oxide
(AAO) nanopores. Measurements were carried for native and silanized
nanopores of the same pore sizes. Nanopore surfaces are modified with
the use of two silanizing agents, chlorotrimethylsilane (ClTMS) and
(3-aminopropyl)trimethoxysilane (APTMOS), of much different properties.
The results of the dielectric studies have demonstrated that for the
studied polymer located in 55 nm pores, changes in the surface chemistry
and thermal treatment allows the confinement effect seen in temperature
evolution of the segmental relaxation time, τ
α
(T) to be removed. The bulk-like evolution of the segmental relaxation
time can also be restored upon long-time annealing. Interestingly,
the time scale of such equilibration process was found to be independent
of the surface conditions. The calorimetric measurements reveal the
presence of two glass-transition events in DSC thermograms of all
considered systems, implying that the changes in the interfacial interactions
introduced by silanization are not strong enough to inhibit the formation
of the interfacial layer. Although DSC traces confirmed the two-glass-transition
scenario, there is no clear evidence that vitrification of the interfacial
layer affects τ
α
(T) for nanopore-confined polymer.
Rigid molecular glass-formers with no internal degrees of freedom nonetheless have a single secondary β-relaxation. For a rigid and planar molecule, 1-methylindole (1MID), although a secondary relaxation is resolved at ambient pressure, its properties do not conform to the rules established for rigid molecules reported in early studies. By applying pressure to the dielectric spectra of 1MID, we find the single secondary relaxation splits into two. The slower one is pressure sensitive showing connections to the α-relaxation as observed in other rigid molecules, while the faster one is almost pressure insensitive and dominate the loss at ambient pressure. The two secondary relaxations, identified to associate with the out-of-plane and in-plane rotations of the rigid and planar 1MID, are resolved and observed for the first time by increasing density via elevating pressure.
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