Confinement effects on glass forming liquids probed by dynamic mechanical analysis

Abstract

Many molecular glass forming liquids show a shift of the glass transition Tg to lower temperatures when the liquid is confined into mesoporous host matrices. Two contrary explanations for this effect are given in literature: First, confinement induced acceleration of the dynamics of the molecules leads to an effective downshift of Tg increasing with decreasing pore size. Secondly, due to thermal mismatch between the liquid and the surrounding host matrix, negative pressure develops inside the pores with decreasing temperature, which also shifts Tg to lower temperatures. Here we present novel dynamic mechanical analysis measurements of the glass forming liquid salol in Vycor and Gelsil with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic susceptibility data can be consistently described with the assumption of two relaxation processes inside the pores: A surface induced slowed down relaxation due to interaction with rough pore interfaces and a second relaxation within the core of the pores. This core relaxation time is reduced with decreasing pore size d, leading to a downshift of Tg ∝ 1/d in perfect agreement with recent DSC measurements. Thermal expansion measurements of empty and salol filled mesoporous samples revealed that the contribution of negative pressure to the downshift of Tg is small (< 30%) and the main effect is due to the suppression of dynamically correlated regions of size ξ when the pore size d approaches ξ.

Dynamic mechanical analysis of confined glass-forming liquids

“…As already mentioned in the Section 1, due to the huge inner surface of the host matrices, surface interactions between the molecules and the pore surface can drastically influence the results [12]. To get rid of these surface effects, we have chemically covered the pore surfaces with trimethylsilyl OSi(CH 3 ) 3 groups, which replace the OH À groups of the natural SiO 2 surface, thereby creating a hydrophobic inner surface ( Figure 2).…”

“…Since most of the DMA measurements of salol in confinement have already been published in a sequence of recent papers [11][12][13]; here we will present only a summary of the main results.…”

“…and Y 00 (T, !) for all temperatures and measured frequencies ( Figure 6 of [12]). The two peaks in Y 00 (T ) as well as the two shoulders in Y 0 (T ) could be identified with the relaxation of the molecules in the pore centre (low temperature side) and near the pore walls (high temperature side), respectively.…”

“…As usual, with increasing measurement frequency, the curves shift to higher temperatures. Inspecting the data in more detail (Figure 6), revealed a so-called 'double S-shaped behaviour' of Y 0 (T ) and a 'double peak structure' or a 'shoulder' in Y 00 (T ) [12]. The behaviour of glass-forming liquids in confinement has been recently studied by the group of Binder [22,23], where it was shown, that the relaxation time of molecules increases strongly when approaching rough pore walls.…”

“…Very recently, we have studied [11][12][13][14] the glass-freezing behaviour of various molecular glass-forming liquids (salol, ortho-terphenyl (OTP), toluene) in mesoporous geometries of Vycor and Gelsil at different pore sizes (2-10 nm) using a dynamic mechanical analyzer in the frequency range 0.1-100 Hz.…”