Relationship between aged and vapor-deposited organic glasses: Secondary relaxations in methyl-m-toluate

Abstract

In situ interdigitated electrode broadband dielectric spectroscopy was used to characterize the excess wing relaxations in vapor-deposited and aged glasses of methyl-m-toluate (MMT, Tg = 170 K). MMT displays typical excess wing relaxations in dielectric spectra of its supercooled liquid and glasses. Physical vapor deposition produced glasses with degrees of suppression of the excess wing relaxation that varied systematically with deposition conditions, up to a maximum suppression of more than a factor of 3. The glass deposited at a relatively high temperature, 0.96 Tg (163 K), showed the same amount of suppression as that of a liquid-cooled glass aged to equilibrium at this temperature. The suppression of the excess wing relaxation was strongly correlated with the kinetic stability of the vapor-deposited glasses. Comparisons with aged MMT glasses allowed an estimate of the structural relaxation time of the vapor-deposited glasses. The dependence of the estimated structural relaxation times upon the substrate temperature was found to be stronger than Arrhenius but weaker than Vogel-Fulcher-Tammann dependence predicted from extrapolation of relaxation times in the supercooled liquid. Additionally, this work provides the first example of the separation of primary and secondary relaxations using physical vapor deposition.

How to "measure" a structural relaxation time that is too long to be measured?

“…In practice, Kasting et al observe that χ ∼ − log t w during the aging, and they extrapolate (in some cases by more than 6 orders of magnitude) this logarithmic behaviour to the independently measured equilibrium value. The extrapolated relaxation times deviate significantly from a fit performed for T > T g using the VFT law [59].…”

“…To our knowledge, this method has not yet been used to study glasses with large relaxation times such as vapor deposited glasses, although the dynamics at timescales corresponding the β relaxation has been analysed [58,59]. Importantly, there are many indications that the structural relaxation times in ultrastable glasses may be more than 8 orders of magnitude larger than in ordinary glasses, but the timescale of the β process itself is changed by a much smaller factor of at most one order of magnitude [60,61].…”

“…Ediger and coworkers [58,59] have followed the evolution of the dynamics in the frequency range corresponding to the β process in several molecular glass-formers to infer the relaxation time of ultrastable glasses by comparing two distinct sets of experiments, based on dielectric measurements, χ (ω): (i) perform an isothermal aging experiment after a rapid quench from high temperature to a final temperature T , and measure how the amplitude of the β process evolves with the waiting time t w spent since the quench; (ii) directly measure the amplitude of the β process for the equilibrium ultrastable glass at the same temperature T . By construction, the data in the first set of measurements must (slowly) converge to the result of the second set in the limit of large t w .…”

“…To this end, we follow the spirit of Ref. [59] and compare two independent sets of numerical measurements. First, we measure the dynamics of equilibrium samples at various pressures, Z, over a large time window.…”

Dielectric properties of vapor-deposited propylbenzenes

“…As a result, such deposition conditions lead to a distinct separation of the primary and secondary processes, where the amplitude of the  process is still resolved and the -process onset is shifted upward along the temperature scale. 38 The delayed onset of This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”