55 research outputs found
Communication: Direct tests of single-parameter aging
This paper presents accurate data for the physical aging of organic glasses
just below the glass transition probed by monitoring the following quantities
after temperature up and down jumps: the shear-mechanical resonance frequency
(around 360 kHz), the dielectric loss at 1 Hz, the real part of the dielectric
constant at 10 kHz, and the loss-peak frequency of the dielectric beta process
(around 10 kHz). The setup used allows for keeping temperature constant within
100 micro Kelvin and for thermal equilibration within a few seconds after a
temperature jump. The data conform to a new simplified version of the classical
Tool-Narayanaswamy aging formalism, which makes it possible to calculate one
relaxation curve directly from another without any fitting to analytical
functions
Rheological model for the alpha relaxation of glass-forming liquids and its comparison to data for DC704 and DC705
Dynamic shear-modulus data are presented for the two silicone oils DC704 and
DC705 for frequencies between 1 mHz and 10 kHz at temperatures covering more
than five decades of relaxation-time variation. The data are fitted to the
alpha part of a phenomenological model previously shown to describe well the
dynamic shear modulus of squalane, which has a large beta process [Hecksher
\textit{et al.}, J. Chem. Phys. \textbf{146}, 154504 (2017)]; that model is
characterized by additivity of the alpha and beta shear compliance and by a
high-frequency decay of the alpha process in proportion to in
which is the angular frequency. The fits of the alpha part of this
model to the DC704 and DC705 data are compared to fits by a Havriliak-Negami
type model, the Barlow-Erginsav-Lamb model, and a Cole-Davidson type model. At
all temperatures the best fit is obtained by the alpha part of the squalane
model. This strengthens the conjecture that so-called -relaxation,
leading to high-frequency decays proportional to , is a general
characteristic of the alpha relaxation of supercooled liquids [Dyre, Phys. Rev.
E {\bf 74}, 021502 (2006); Nielsen \textit{et al.}, J. Chem. Phys.
\textbf{130}, 154508 (2009); Pabst \textit{et al.}, J. Phys. Chem. Lett.
\textbf{12}, 3685 (2021)]
Physical ageing studied by a device allowing for rapid thermal equilibration
Ageing of organic glasses to the equilibrium liquid state is studied by
measuring the dielectric loss utilizing a microregulator where temperature is
controlled by means of a Peltier element. Compared to conventional equipment
the new device adds almost two orders of magnitude to the span of observable
ageing times. Data for five organic glass-forming liquids are presented. The
existence of an "inner clock" is confirmed by a model-free test showing that
the ageing of structure is controlled by the same material time that controls
the dielectric properties. At long times relaxation is not stretched, but
simple exponential, and there is no "expansion gap" between the limits of the
relaxation rates following up and down jumps to the same temperature
Frequency Dependent Specific Heat from Thermal Effusion in Spherical Geometry
We present a novel method of measuring the frequency dependent specific heat
at the glass transition applied to 5-polyphenyl-4-ether. The method employs
thermal waves effusing radially out from the surface of a spherical thermistor
that acts as both a heat generator and thermometer. It is a merit of the method
compared to planar effusion methods that the influence of the mechanical
boundary conditions are analytically known. This implies that it is the
longitudinal rather than the isobaric specific heat that is measured. As
another merit the thermal conductivity and specific heat can be found
independently. The method has highest sensitivity at a frequency where the
thermal diffusion length is comparable to the radius of the heat generator.
This limits in practise the frequency range to 2-3 decades. An account of the
3omega-technique used including higher order terms in the temperature
dependency of the thermistor and in the power generated is furthermore given.Comment: 17 pages, 15 figures, Substantially revised versio
Shear and dielectric responses of propylene carbonate, tripropylene glycol, and a mixture of two secondary amides
Propylene carbonate and a mixture of two secondary amides, N-methylformamide and Nethylacetamide, are investigated by means of broadband dielectric and mechanical shear spectroscopy. The similarities between the rheological and the dielectric responses of these liquids and of the previously investigated tripropylene glycol are discussed within a simple approach that employs an electrical circuit for describing the frequency-dependent behavior of viscous materials. The circuit is equivalent to the Gemant-DiMarzio-Bishop model, but allows for a negative capacitive element. The circuit can be used to calculate the dielectric from the mechanical response and vice versa. Using a single parameter for a given system, good agreement between model calculations and experimental data is achieved for the entire relaxation spectra, including secondary relaxations and the Debye-like dielectric peak in the secondary amides. In addition, the predictions of the shoving model are confirmed for the investigated liquids
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