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Abstract  

Through structural relaxation, the configuration of a viscous liquid changes to allow the Gibbs free energy to be minimum in response to temperature variations. In this review, the practical importance of relaxation in silicate melts is first illustrated by configurational heat capacity and entropy and their connection with viscosity via Adam-Gibbs theory. Relaxation effects on thermal expansion and compressibility are then examined, and the similarity of the kinetics of structural, enthalpy and volume relaxation is pointed out. Turning to microscopic mechanisms, we finally stress the importance of Si-O bond exchange and its decoupling with the motion of network-modifying elements near the glass transition.

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Abstract  

The thermoelastic behaviour of an unfractionated polystyrene was studied in the temperature interval 353–453 K in the regimes of isobaric cooling and of isothermal quasi-adiabatic loading, respectively. The main experimental results can be summarized as follows. 1. In the temperature interval far above the glass transition temperature Tg, both the temperature and volume relaxations of the polystyrene melt after sudden pressure jumps were completely reversible and proved to be simple exponential functions of the time. Therefore, by a straightforward application of Eqs (1) and (2) to the relevant thermoelastic data obtained in a single experimental run one can arrive at the reasonable values of the specific volume, specific heat capacity, thermal diffusivity and heat conductivity of the polymer in the equilibrium melt state. 2. In the temperature interval close to Tg, both the temperature and volume relations of the supercooled polystyrene melt in compression/expansion cycles became markedly asymmetric and non-exponential. The low values of the exponent β in the fractional-exponent Eq. (5) for the volume relaxation suggest a broad spectrum of relaxation times indicating the high degree of coupling between different mechanisms of the molecular motions involved.

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Abstract  

Structural relaxation for simple and more complex thermal histories is described by a phenomenological model based on a non-exponential relaxation function, the reduced-time concept and the nonlinear structural contribution to the relaxation time. The history, development of experimental techniques and data analysis is described. It is shown that the volume and enthalpy relaxation response can conveniently be compared on the basis of a fictive relaxation rate, R f. A simple equation relating R f and the parameters of the phenomenological model is given. The calculated data for moderate departures from equilibrium are in good agreement with our experiments and data previously reported in the literature.

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Abstract  

The structural relaxation of Ge38S62 glass has been studied by length dilatometry and calorimetry. The Tool-Narayanaswamy-Moynihan model was applied on obtained data of structural relaxation and parameters of this model were determined: Δh*= 4832 kJ mol-1, ln(A/s)= -811, β= 0.70.1 and x=0.60.1. Both dilatometric and calorimetric relaxation data were compared on the basis of the fictive relaxation rate. It was found that the relaxation rates are very similar and well correspond to the prediction of phenomenological model.

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Abstract  

the volume and enthalpy relaxation in a-PMMA subjected to temperature jumps in t g region has been analysed. The measured H and V data were compared with respect to aging time and proportionality between them as a slope of (∂H/∂V)T dependencies has been found. According to previous works the slope was identified as an apparent bulk modulus, K a. This method is applied to aging following temperature up-jumps after consolidation periods of varying lengths. the main finding is a marked increase of K a with consolidation time, approaching a limiting value in an asymptotic fashion.

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Summary Volume and enthalpy relaxation in polycarbonate subjected to double temperature jumps in the T g region has been analysed. It concerns both initial T down-jump from equilibrium above T g to consolidation temperature below T g and fina1 T up-jump to relaxation temperature, also below T g. The measured H and V data after T up-jump were compared with respect to aging time calculating (dH/dV) ratio denoted as aging bulk modulus, K a. According this new methodology H and V relaxation response after T up-jump demonstrates differences in relaxation responses.

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Summary Relaxation behavior of GeySe100-y (y=8 and 10) glasses related to the viscosity behavior was studied by dilatometry. The method of two consecutive temperature jumps was applied to study the volume relaxation. The relaxation response can be described by Tool-Narayanaswamy-Moynihan model and the parameters of this model ?h*, ß, x, A were determined using curve fitting method and characteristic times method. Viscosities of studied materials in the range of 108-1013 Pas were measured by penetration method. The calculated values of activation energies of viscous flow E? are close to the values of effective activation energies of relaxation ?h* for studied chalcogenide materials.

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It is evidenced that due to the kinetic character of the glass transition as a ‘freeze in’ process, PVT measurements extended over the glass transition range depend not only on the thermal history but also on the pressure acting during the formation of the polymeric glasses. As a consequence metastable glasses are formed which show during heating of the glassy polymer through the glass transition range ‘volume relaxation zones’, characterized by a retarded increase or even decrease of the volume. The width of the ‘relaxation zone’ increases with increasing pressure and depends additional on the mode of operation used during the PVT measurements. In the same time a pressure induced shift of the glass temperature to higher temperatures is observed, the shift being the greater the stiffer the polymer, i.e. the higher the glass temperature of the polymer at atmospheric pressure. Due to the metastable character of polymeric glasses the evaluation of universal equations of states is thus not ingenious for polymeric glasses, because the deduced EOS will be valid only for that given glass characterized by a well defined thermal and pressure history. Additionally the EOS is influenced by the unknown time dependent aging and relaxation processes within polymeric glasses.

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Isothermal and isobaric PVT-measurements of anionic polystyrenes

‘Relaxation zones’ within the glass transition range

Journal of Thermal Analysis and Calorimetry
Authors: A. Eckstein and H. A. Schneider

PVT-measurements of anionic polystyrenes on heating have shown that, depending on the mode of operation, specific ‘relaxation zones’ within the glass transition range are observed. Isothermal PVT curves exhibit always ‘relaxation zones’ independent whether the pressure is increased or decreased during the scan. The shift of the relaxation zones to higher temperatures is, however, higher for isothermal scans with increasing pressure. These ‘relaxation zones’ are explained by pressure-dependent changes of the state of the polymeric sample isothermally scanned within the glass transition range. At lower pressures the polymer is actually in the molten state, whereas at higher pressures it may be in the metastable glassy state and the actual state depends on the rate of pressure change. In isobaric PVT curves ‘relaxation zones” in heating scans are exhibited only if the pressure applied during glass formation differs from the pressure applied during the heating scan. The observed pressure-dependent shift of the glass temperatures to higher temperatures was higher for the studied polystyrenes of different molecular weight that had a higher glass temperature at normal pressure. But the specific molecular weight influence on the width of the ‘relaxation zone’ could not be ascertained. An attempt to calculate characteristic volume relaxation times failed because of insufficient precision of the measurements.

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linear dependence of density on degree of cure and temperature. In addition, the coefficient of linear thermal expansion should also change with proceeding polymerisation. For simplicity, the phenomen of volume relaxation recovery (see [ 34 , 35 ] has

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