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A model for the viscosity of glass-forming molecular liquids is proposed in which a ''flow event'' requires a local volume increase. The activation energy for a flow event is identified with the work done in shoving aside the surrounding liquid; this work is proportional to the high-frequency shear modulus, which increases as the temperature decreases. The model is confirmed by experiments on a number of molecular liquids.Glass formation is a universal property of supercooled liquids. 1-9 For simple liquids rapid cooling is required to avoid crystallization. For most complex liquids supercooling causes no problems; in fact, many complex liquids are difficult to crystallize. The glass transition takes place when the viscosity of the supercooled liquid becomes so large that molecular motion is arrested. The laboratory glass transition is dynamic and not a phase transition, although many workers in the field believe it to be a manifestation of an underlying equilibrium second-order phase transition. For cooling rates of order Kelvin per minute, the glass transition takes place when the viscosity, , is around 10 13 poise ͑P͒. In the following, the glass transition temperature, T g , is defined as the temperature at which ϭ10 13 P.The linear shear mechanical properties of a liquid are determined by the shear modulus as function of frequency, G()ϭGЈ()ϩiGЉ(). At low frequencies G()ϭi. At high frequencies liquid becomes solidlike and G͑͒ approaches a limiting value, lim →ϱ G()ϭG ϱ . In terms of and G ϱ , the average shear relaxation time, , is given 3 by Maxwell's expression

Contrasts between beta relaxation in equilibrium viscous liquids and glasses are rationalized in terms of a double-well potential model with structure-dependent asymmetry, assuming structure is described by a single order parameter. The model is tested for tripropylene glycol where it accounts for the hysteresis of the dielectric beta loss peak frequency and magnitude during cooling and reheating through the glass transition.

Dielectric relaxation measurements on supercooled triphenyl phosphite show that time-temperature superposition (TTS) is obeyed for the primary relaxation process at low temperatures. Measurements on other molecular liquids close to the calorimetric glass transition indicate that TTS is linked to an omega(-1/2) high-frequency decay of the loss, while the loss peak width is nonuniversal.

Understanding the origin of the dramatic temperature and density dependence of the relaxation time of glass-forming liquids is a fundamental challenge in glass science. The recently established 'density-scaling' relation quantifies the relative importance of temperature and density for the relaxation time in terms of a material-dependent exponent. We show that this exponent for approximate single-parameter liquids can be calculated from thermoviscoelastic linear-response data at a single state point, for instance an ambient-pressure state point. This prediction is confirmed for the van der Waals liquid tetramethyl-tetraphenyl-trisiloxane. Consistent with this, a compilation of literature data for the Prigogine-Defay ratio shows that van der Waals liquids and polymers are approximate single-parameter systems, whereas associated and network-forming liquids are not.

It is shown that the shoving model expression for the average relaxation time of viscous liquids, according to which the activation energy is proportional to the instantaneous shear modulus, follows largely from a classical "landscape" estimation of barrier heights from curvature at energy minima. Although the activation energy in this reasoning involves both instantaneous bulk and shear moduli, the bulk modulus contributes less than 8% to the temperature dependence of the activation energy. This reflects the fact that the physics of the two models are closely related.

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Aging to the equilibrium liquid state of organic glasses is studied. The glasses were prepared by cooling the liquid to temperatures just below the glass transition. Aging following a temperature jump was studied by measuring the dielectric loss at a fixed frequency using a microregulator in which 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 aging times. Data for the following five glass-forming liquids are presented: dibutyl phthalate, diethyl phthalate, 2,3-epoxy propyl-phenyl-ether, 5-polyphenyl-ether, and triphenyl phosphite. The aging data were analyzed using the Tool-Narayanaswamy formalism. The following features are found for all five liquids: (1) The liquid has an "internal clock," a fact that is established by showing that aging is controlled by the same material time that controls the dielectric properties. (2) There are no so-called expansion gaps between the long-time limits of the relaxation rates following up and down jumps to the same temperature. (3) At long times, the structural relaxation appears to follow a simple exponential decay. (4) For small temperature steps, the rate of the long-time exponential structural relaxation is identical to that of the long-time decay of the dipole autocorrelation function.