An equation for the description of volume and temperature dependences of the dynamics of supercooled liquids and polymer melts

A recently proposed expression to describe the temperature and volume dependences of the structural (or alpha) relaxation time is discussed. This equation satisfies the scaling law for the relaxation times, tau = f(TV^g), where T is temperature, V the specific volume, and g a material-dependent constant. The expression for the function f is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover, providing a description of the dynamics with a minimal number of parameters. The results herein can be reconciled with previously found correlations of the isochoric fragility with both the isobaric fragility at atmospheric pressure and the scaling exponent g.Comment: to be published in the special edition of J. Non-Crystalline Solids honoring K.L. Nga

Determination of the Thermodynamic Scaling Exponent from Static, Ambient-Pressure Quantities

An equation is derived that expresses the thermodynamic scaling exponent, g, which superposes relaxation times and other measures of molecular mobility determined over a range of temperatures and densities, in terms of static, physical quantities. The latter are available in the literature or can be measured at ambient pressure. We show for 13 materials, both molecular liquids and polymers, that the calculated g are equivalent to the scaling exponents obtained directly by superpositioning. The assumptions of the analysis are that the glass transition is isochronal and that the first Ehrenfest relation is valid; the first assumption is true by definition, while the second has been corroborated for many glass-forming materials at ambient pressure. However, we find that the Ehrenfest relation breaks down at elevated pressure, although this limitation is of no consequence herein, since the appeal of the new equation is its applicability to ambient pressure data.Comment: 9 pages, 3 figures, 1 tabl

Volume Effects on the Glass Transition Dynamics

The role of jamming (steric constraints) and its relationship to the available volume is addressed by examining the effect that certain modifications of a glass-former have on the ratio of its isochoric and isobaric activation enthalpies. This ratio reflects the relative contribution of volume (density) and temperature (thermal energy) to the temperature-dependence of the relaxation times of liquids and polymers. We find that an increase in the available volume confers a stronger volume-dependence to the relaxation dynamics, a result at odds with free volume interpretations of the glass transition.Comment: 9 pages 5 figure

Effect of entropy on the dynamics of supercooled liquids: New results from high pressure data

We show that for arbitrary thermodynamic conditions, master curves of the entropy are obtained by expressing S(T,V) as a function of TV^g_G, where T is temperature, V specific volume, and g_G the thermodynamic Gruneisen parameter. A similar scaling is known for structural relaxation times,tau = f(TV^g); however, we find g_G < g. We show herein that this inequality reflects contributions to S(T,V) from processes, such as vibrations and secondary relaxations, that do not directly influence the supercooled dynamics. An approximate method is proposed to remove these contributions, S_0, yielding the relationship tau = f(S-S_0).Comment: 10 pages 7 figure

Density Scaling and Dynamic Correlations in Viscous Liquids

We use a recently proposed method [Berthier L.; Biroli G.; Bouchaud J.P.; Cipelletti L.; El Masri D.; L'Hote D.; Ladieu F.; Pierno M. Science 2005, 310, 1797.] to obtain an approximation to the 4-point dynamic correlation function from derivatives of the linear dielectric response function. For four liquids over a range of pressures, we find that the number of dynamically correlated molecules, Nc, depends only on the magnitude of the relaxation time, independently of temperature and pressure. This result is consistent with the invariance of the shape of the relaxation dispersion at constant relaxation time and the density scaling property of the relaxation times, and implies that Nc also conforms to the same scaling behavior. For propylene carbonate and salol Nc becomes constant with approach to the Arrhenius regime, consistent with the value of unity expected for intermolecularly non-cooperative relaxation.Comment: revisio

What can we learn by squeezing a liquid

Relaxation times for different temperatures, T, and specific volumes, V, collapse to a master curve versus TV^g, with g a material constant. The isochoric fragility, m_V, is also a material constant, inversely correlated with g. From these we obtain a 3-parameter function, which fits accurately relaxation times of several glass-formers over the supercooled regime, without any divergence below Tg. Although the 3 parameters depend on the material, only g significant varies; thus, by normalizing material-specific quantities related to g, a universal power law for the dynamics is obtained.Comment: 12 pages, 4 figure

Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids

Viscosities and their temperature, T, and volume, V, dependences are reported for 7 molecular liquids and polymers. In combination with literature viscosity data for 5 other liquids, we show that the superpositioning of relaxation times for various glass-forming materials when expressed as a function of TV^g, where the exponent g is a material constant, can be extended to the viscosity. The latter is usually measured to higher temperatures than the corresponding relaxation times, demonstrating the validity of the thermodynamic scaling throughout the supercooled and higher T regimes. The value of g for a given liquid principally reflects the magnitude of the intermolecular forces (e.g., steepness of the repulsive potential); thus, we find decreasing g in going from van der Waals fluids to ionic liquids. For strongly H-bonded materials, such as low molecular weight polypropylene glycol and water, the superpositioning fails, due to the non-trivial change of chemical structure (degree of H-bonding) with thermodynamic conditions.Comment: 16 pages 7 figure

Dahl e i limiti dell'elitismo

Il rapporto tra Dahl e l’elitismo è qui affrontato a partire da due diverse prospettive: sono prima analizzati i motivi del confronto critico che l'autore ingaggia con quanti hanno descritto le società democratiche borghesi, e soprattutto la democrazia americana, come regimi gerarchici mascherati da governi democratici, in cui una minoranza al potere è in grado di esercitare un dominio unilaterale e incontrollato sulla maggioranza; sono, quindi, prese in considerazione le ragioni che hanno spinto molti interpreti a collocare la stessa teoria della poliarchia all’interno del c.d. «elitismo democratico»1 – una collocazione che Dahl ha sempre rifiutato, pur riconoscendo l’influenza esercitata sui suoi primi scritti da Joseph A. Schumpeter

Politeness, socialità e carattere in Shaftesbury

Nonostante i suoi legami con la cultura dei country Whigs, Shaftesbury è stato letto per lo più come un autore importante soltanto nell’ambito della morale e dell’estetica. Mai è stato interpretato, almeno fino a tempi recenti, come un pensatore la cui riflessione potesse avere anche una qualche rilevanza politica. È spesso sfuggita – come ha sottolineato nei suoi lavori Lawrence Klein – l’attenzione che Shaftesbury dedica al legame tra forme politiche, cultura e manners. Sussiste, invece, un nesso molto forte tra l’emergere dell’ideale della politeness shaftesburiana e le trasformazioni della polity, oltre che dell’economy, avvenute in Inghilterra tra Sei e Settecento, in particolare in seguito alla seconda rivoluzione inglese, e alla stesura del Bill of Rights del 1689

Disabilità, immaginazione e cittadinanza sessuale

The paper describes and evaluates an initiative recently promoted through the web in Italy: a signatures gathering campaign for formal recognition of professionally trained sexual assistants to disabled people a job that already has proper legal status in several Western countries. I analyze the pros and cons of this kind of answer to the sexual difficulties, and needs, of disabled people, and then move on to the more general issue of disability and what has come to be known as “sexual citizenship”. The contemporary theoretical alliance between critical disability studies, feminist studies, queer theory, critical race theory and postcolonial theory, suggests for us the need for a global cultural revolution: it is necessary, first of all, to understand, and then try to correct, the various, and often unconscious images, associations, and feelings that oppress some groups in our societies. A radical change in view is suggested not just about the image of disabled people, but also for gay men and lesbians, women, people of colour, migrant people, sexual workers, and old people