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A broad variety of liquids conform to density scaling: relaxation times expressed as a function of the ratio of temperature to density, the latter raised to a material constant {\gamma}. For atomic liquids interacting only through simple pair potentials, the exponent {\gamma} is very nearly equal to n/3, where n is the steepness of the intermolecular potential, while for molecular liquids having rigid bonds and built using the same interatomic potential, {\gamma}>n/3. We find that for this class of molecular liquids {\gamma}=n/{\delta}, where the parameter {\delta} relates the intermolecular distance to the density along an isomorph (line of approximately constant dynamics and structure). {\delta} depends only on the molecular structure and not the interatomic potential.

Solid propellants are energetic materials used to launch and propel rockets and missiles. Although their history dates to the use of black powder more than two millennia ago, greater performance demands and the need for "insensitive munitions" that are resistant to accidental ignition have driven much research and development over the past half-century. The focus of this review is the material aspects of propellants, rather than their performance, with an emphasis on the polymers that serve as binders for oxidizer particles and as fuel for composite propellants. The prevalent modern binders are discussed along with a discussion of the limitations of state-of-the-art modeling of composite motors., Comment: 24 pages, 7 figures, 3 tables

Recently Sanz et al. [A. Sanz, T. Hecksher, H.W. Hansen, J.C. Dyre, K. Niss, and U.R. Pedersen, Phys. Rev. Lett. 122, 055501 (2019).] reported that the scaling exponent gamma for tetramethyl-tetraphenyl-trisiloxane (DC704) varied with temperature; i.e., was not a material constant. Such a finding is at odds with previously published results on this compound and on more than 100 other liquids and polymers. The result of Sanz et al. comes from their measurement of a pressure dependence of the relaxation time at low temperature that becomes weaker with increasing pressure. Such a result is unphysical and contrary to the behavior of all known liquids. We re-measured this pressure dependence for DC704 and find it to be linear over the studied range. Thus, the conclusion of Sans et al. is incorrect; gamma for DC704 is state-point independent, in accord with other simple liquids and polymers., Comment: 2 pages, 2 figures

Using molecular dynamics simulations we examine the system size dependence of the fast dynamics in two model glass forming liquids, one of them a Lennard-Jones mixture for which cooperative fast relaxation has been reported. We find no indication of a temperature-dependent dynamic length scale characterizing these fast dynamics; the size effects in the short time range are temperature independent, and the consequence of cutting off of long wavelength acoustic modes. In a molecular liquid exhibiting a clear Johari-Goldstein (JG) relaxation, significant size effects are again present both for the vibrational motion and long-time {\alpha} relaxation (only the latter having a significant temperature dependence), but absent for the JG relaxation., Comment: 8 pages, 4 figures

The rate of physical aging of glassy polymethylmethacrylate (PMMA), followed from the change in the secondary relaxation with aging, is found to be independent of the density, the latter controlled by the pressure during glass formation. Thus, the aging behavior of the secondary relaxation is the same whether the glass is more compacted or less dense than the corresponding equilibrium liquid. This equivalence in aging of glasses formed under different pressures indicates that local packing is the dominant variable governing the glassy dynamics. One consequence is that pressure densification yields a reduction in the glass transition temperature. The fact that pressure densification yields different glass structures is at odds with a model for non-associated materials having dynamic properties exhibited by PMMA, such as density scaling of the relaxation time and isochronal superposition of the relaxation dispersion., Comment: 9 pages, 5 figures

The magnitude of the high frequency, static dielectric permittivity is used to determine the density of tetramethyl tetraphenyl trisiloxane, a non-associated glass-forming liquid, as a function of temperature and pressure. We demonstrate that the properties in the glassy state are affected by the pressure applied to the liquid during vitrification. This behavior is normal for hydrogen-bonded liquids and polymers, but unanticipated by models of simple liquids., Comment: 7 pages, 3 figures

We describe a method to determine whether a material has isomorphs in its thermodynamic phase diagram. Isomorphs are state points for which various properties are invariant in reduced units. Such materials are commonly identified from strong correlation between thermal fluctuations of the potential energy, U, and the virial W, but this identification is not generally applicable to real materials. We show from molecular dynamic simulations of atomic, molecular, and polymeric materials that systems with strong U-W correlation cannot be pressure densified; that is, the density obtained on cooling to the glassy state and releasing the pressure is independent of the pressure applied during cooling., Comment: 9 pages, 3 figures

The dynamics of polypropylene glycol, both neat and attached to silica nanoparticles, were investigated using elastic neutron backscattering and dielectric spectroscopy. The mean square displacement measured by the former is suppressed by the particles at temperatures corresponding to a dielectric secondary relaxation (that involves only a portion of the repeat unit) and the segmental relaxation (glass transition). Despite the suppression of the displacements, the motions are faster in the nanocomposite, primarily due to poorer packing (lower density) at the particle interface. At very low temperatures we discovered a new dynamic process in the polymer. Reflecting its very local nature, this process is unaffected by attachment of the chains to the silica., Comment: 9 pages, 5 figures

We show using molecular dynamics simulations that simple diatomic molecules in the glassy state exhibit only limited participation in the Johari-Goldstein (JG) relaxation process. That is, with sufficient cooling local reorientations are essentially frozen for some molecules, while others continue to change their orientation significantly. Thus, the "islands of mobility" concept is valid for these molecular glass-formers; only near the glass transition temperature does every molecule undergo the JG process. In contrast, for a linear polymer this dichotomy in the distribution of JG relaxation strengths is absent - if any segments are changing their local orientation, all segments are., Comment: 7 pages; 2 figures

The elusive connection between dynamics and local structure in supercooled liquids is an important piece of the puzzle in the unsolved problem of the glass transition. The Johari-Goldstein beta relaxation, ubiquitous in glass-forming liquids, exhibits mean properties that are strongly correlated to the long-time alpha dynamics. However, the former comprises simpler, more localized motion, and thus has perhaps a more straightforward connection to structure. Molecular dynamics simulations were carried out on a two-dimensional, rigid diatomic molecule (the simplest structure exhibiting a distinct beta process) to assess the role of the local liquid structure on both the Johari-Goldstein beta and the alpha relaxation. Although the average properties for these two relaxations are correlated, there is no connection between the beta and alpha properties of a given (single) molecule. The propensity for motion at long times is independent of the rate or strength of a molecule's beta relaxation. The mobility of a molecule averaged over many initial energies, a measure of the influence of structure, was found to be heterogeneous, with clustering at both the beta and alpha timescales. This heterogeneity is less extended spatially for the beta than for the alpha dynamics, as expected; however, the local structure is the more dominant control parameter for the beta prcoess. In the glassy state, the arrangement of neighboring molecules determines entirely the relaxation properties, with no discernible effect from the particle momenta., Comment: 9 pages, 8 figures

We present new viscosity and equation of state (EoS) results extending to high pressures for o-terphenyl, salol, and dibutylphthalate. Using these and data from the literature, we show that the three liquids all conform to density scaling; that is, their reduce viscosities and reorientational relaxation times are a function of the ratio of temperature and density with the latter raised to a constant. Moreover, the functional form of the dependence on this ratio is independent of the experimental probe of the dynamics. This means that there is no decoupling of the viscosities and relaxation times over the measured range of conditions. Previous literature at odds with these results were based on erroneous extrapolations of the EoS or problematic diamond anvil viscosity data. Thus, there are no exceptions to the experimental fact that every non-associated liquid complies with density scaling with an invariant scaling exponent., Comment: 9 pages, 3 figures

The local segmental and global dynamics of a series of polypropylene glycol / silica nanocomposites were studied using rheometry and mechanical and dielectric spectroscopies. The particles cause substantial changes in the rheology, including higher viscosities that become non-Newtonian and the appearance of stress overshoots in the transient shear viscosity. However, no change was observed in the mean relaxation times for either the segmental or normal mode dynamics measured dielectrically. This absence of an effect of the particles is due to masking of the interfacial response by polymer chains remote from the particles. When the unattached polymer was extracted to isolate the interfacial material, very large reductions in the relaxation times were measured. This speeding up of the dynamics is due in part to the reduced density at the interface, presumably a consequence of poorer packing of tethered chains. In addition, binding of the ether oxygens of the polypropylene glycol chains truncates the normal mode, which shifts the corresponding relaxation peak to higher frequencies., Comment: 6 figures

The origin of the dramatic changes in the behavior of liquids as they approach their vitreous state - increases of many orders of magnitude in transport properties and dynamic time scales - is a major unsolved problem in condensed matter. These changes are accompanied by greater dynamic heterogeneity, which refers to both spatial variation and spatial correlation of molecular mobilities. The question is whether the changing dynamics is coupled to this heterogeneity; that is, does the latter cause the former? To address this we carried out the first nonlinear dielectric experiments at elevated hydrostatic pressures on two liquids, to measure the third-order harmonic component of their susceptibilities. We extract from this the number of dynamically correlated molecules for various state points, and find that the dynamic correlation volume for non-associated liquids depends primarily on the relaxation time, sensibly independent of temperature and pressure. We support this result by molecular dynamic simulations showing that the maximum in the four-point dynamic susceptibility of density fluctuations varies less than 10% for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic heterogeneity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials., Comment: revised from version uploaded Oct 3, 2014; now 14 pages, 7 figures

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 table

Molecular dynamics simulations were carried out on a Lennard-Jones binary mixture of rigid (fixed bond length) diatomic molecules. The translational and rotational correlation functions, and the corresponding susceptibilities, exhibit two relaxation processes, the slow structural relaxation (alpha dynamics) and a higher frequency secondary relaxation. The latter is a Johari-Goldstein (JG) process, by its definition of involving all parts of the molecule. It shows several properties characteristic of the JG process - (i) merging with the alpha relaxation at high temperature; (ii) a change in temperature-dependence of the relaxation strength on vitrification; (iii) a separation in frequency from the alpha relaxation that correlates with the breadth of the structural dispersion; and (iv) sensitivity to volume, pressure, and physical aging - that can be used to determine whether a secondary relaxation in a real material is an authentic JG process, rather than trivial motion involving intramolecular degrees of freedom. The latter has no connection to the glass transition, whereas the JG relaxation is closely related to structural relaxation, and thus can provide new insights into the phenomenon., Comment: 12 pages, 10 figures

A current focus in studies of the glass transition is the role of dynamic heterogeneities. Although these efforts may clarify the origin of the spectacular change in properties of liquids approaching vitrification, we point out that a seemingly related phenomenon, thermorheological complexity in polymers, must involve different mechanisms. In particular, as seen from consideration of various properties involving the chain dynamics, averaging over different length and time scales cannot offer a resolution to the problem of thermorheological complexity., Comment: 4 pages; 1 figure

A recent derivation [P.H. Mott and C.M. Roland, Phys. Rev. B 80, 132104 (2009).] of the bounds on Poisson's ratio, v, for linearly elastic materials showed that the conventional lower limit, -1, is wrong, and that v cannot be less than 0.2 for classical elasticity to be valid. This is a significant result, since it is precisely for materials having small values of v that direct measurements are not feasible, so that v must be calculated from other elastic constants. Herein we measure directly Poisson's ratio for four materials, two for which the more restrictive bounds on v apply, and two having values below this limit of 0.2. We find that while the measured v for the former are equivalent to values calculated from the shear and tensile moduli, for two auxetic materials (v < 0), the equations of classical elasticity give inaccurate values of v. This is experimental corroboration that the correct lower limit on Poisson's ratio is 0.2 in order for classical elasticity to apply., Comment: 9 pages, 2 figures

Strong correlation between equilibrium fluctuations of the potential energy, U, and the virial, W, is a characteristic of a liquid that implies the presence of certain dynamic properties, such as density scaling of the relaxation times and isochronal superpositioning of the relaxation function. In this work we employ molecular dynamics simulations (mds) on methanol and two variations, lacking hydrogen bonds and a dipole moment, to assess the connection between the correlation of U and W and these dynamic properties. We show, in accord with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre, Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both strong correlations and the expected dynamic behavior. However, for polar liquids this correspondence breaks down - weaker correlation between U and W is not associated with worse conformance to density scaling or isochronal superpositioning. The reason for this is that strong correlation between U and W only requires their proportionality, whereas the expected dynamic behavior depends primarily on constancy of the proportionality constant for all state points. For hydrogen-bonded liquids, neither strong correlation nor adherence to the dynamic properties is observed; however, this nonconformance is not directly related to the concentration of hydrogen bonds, but rather to the greater deviation of the intermolecular potential from an inverse power law (IPL). Only (hypothetical) liquids having interactions governed strictly by an IPL are perfectly correlating and exhibit the consequent dynamic properties over all thermodynamic conditions., Comment: 14 pages, 8 figures

The lower bound usually cited for Poisson's ratio {\nu} is -1, derived from the relationship between {\nu} and the bulk and shear moduli. From consideration of the longitudinal and biaxial moduli, we recently determined that the lower bound on {\nu} for isotropic materials is actually 1/5, a value also consistent with experimental measurements on real materials. Herein we generalize this result, first by analyzing expressions for {\nu} in terms of six common elastic constants, and then by considering arbitrary strains. The results corroborate the prior finding that 1/5 <= {\nu} for linear elasticity to be applicable., Comment: 15 pages, 3 figures, 1 table

Molecular dynamics simulations (mds) were carried out to investigate the reorientational motion of a rigid (fixed bond length), asymmetric diatomic molecule in the liquid and glassy states. In the latter the molecule reorients via large-angle jumps, which we identify with the Johari-Goldstein (JG) dynamics. This relaxation process has a broad distribution of relaxation times, and at least deeply in the glass state, the mobility of a given molecule remains fixed over time; that is, there is no dynamic exchange among molecules. Interestingly, the JG relaxation time for a molecule does not depend on the local density, although the non-ergodicity factor is weakly correlated with the packing efficiency of neighboring molecules. In the liquid state the frequency of the JG process increases significantly, eventually subsuming the slower alpha-relaxation. This evolution of the JG-motion into structural relaxation underlies the correlation of many properties of the JG- and alpha-dynamics., Comment: 12 pages, 6 figures

Recently a new approach to the determination of dynamic correlation lengths, {\xi}, for supercooled liquids, based on the properties of the slow (picosecond) vibrational dynamics, was carried out [L. Hong, V.N. Novikov, and A.P. Sokolov, Phys. Rev. E 83, 061508 (2011)]. Although these vibrational measurements are typically conducted well below the glass transition temperature, Tg, the assumption is that the structure of the liquid is frozen at Tg, so that the {\xi} characterize dynamic heterogeneity in the supercooled liquid state. We compare {\xi} from this method to values calculated using an approximation to the four-point dynamic susceptibility. For 26 different materials we find good correlation between the two measures; moreover, the pressure dependences are consistent within the large experimental error. However, {\xi} from Boson peak measurements above Tg have a different, and unrealistic, temperature dependence., Comment: 10 pages, 3 figures

The two approaches to analyzing the large strain behavior of rubbery networks are phenomenologically, using strain energy functions drawn from continuum mechanics, and molecular models, which apply statistical mechanics to compute the effect of chain orientation on the entropy. The early rubber elasticity models ignored intermolecular interactions, whereas later developments ("constraint models") included the effect of entanglements or steric constraints on the mechanical stress. These constitutive equations for rubber elasticity are compared to experimental results, and the connection of network elasticity to the relaxation behaviour is discussed. For conventional elastomers there is a compromise between stiffness and strength. Different methods to circumvent this limitation are described. Examples are given of the properties obtained with novel network architectures, including interpenetrating networks, double networks, bimodal networks, miscible heterogeneous networks, and deswollen networks., Comment: Chapter 4 of Viscoelastic Behavior of Rubbery Materials (Oxford University Press, 2011)

Constitutive modelling of the stress-strain response of rubbery polymers is described, with an emphasis on the limits to linearity for both neat and filled rubber (the latter due to the Payne effect). Deviations from the Boltzmann superposition principle (time-strain invariance) under reversing strain histories are reviewed, including a discussion of the Mullins effect. The chapter also reviews the application of the stress optic law and the conditions causing its breakdown: internal stress, the glass transition, orientational coupling, and creep recovery. Different empirical rules for nonlinear flow are considered, including the Cox-Merz rule and the relations of Laun and Gleissle., Comment: Chapter 5 of Viscoelastic Behavior of Rubbery Materials (Oxford University Press, 2011)

The dynamics of a large number of liquids and polymers exhibit scaling properties characteristic of a simple repulsive inverse power law (IPL) potential, most notably the superpositioning of relaxation data as a function of the variable TV{\gamma}, where T is temperature, V the specific volume, and {\gamma} a material constant. A related scaling law, TmVm{\Gamma}, with the same exponent {\Gamma}={\gamma}, links the melting temperature Tm and volume Vm of the model IPL liquid; liquid dynamics is then invariant at the melting point. Motivated by a similar invariance of dynamics experimentally observed at transitions of liquid crystals, we determine dynamic and melting point scaling exponents {\gamma} and {\Gamma} for a large number of non-associating liquids. Rigid, spherical molecules containing no polar bonds have {\Gamma}={\gamma}; consequently, the reduced relaxation time, viscosity and diffusion coefficient are each constant along the melting line. For other liquids {\gamma}>{\Gamma} always; i.e., the dynamics is more sensitive to volume than is the melting point, and for these liquids the dynamics at the melting point slows down with increasing Tm (that is, increasing pressure)., Comment: 20 pages, 8 figures, 1 table

We present numerical calculations of a four-point dynamic susceptibility, chi_4(t), for the Kob-Andersen Lennard-Jones mixture as a function of temperature T and density rho. Over a relevant range of T and rho, the full t-dependence of chi_4(t) and thus the maximum in chi_4(t), which is proportional to the dynamic correlation volume, are invariant for state points for which the scaling variable rho^gamma/T is constant. The value of the material constant gamma is the same as that which superposes the relaxation time, tau, of the system versus rho^gamma/T. Thus, the dynamic correlation volume is directly related to tau for any thermodynamic condition in the regime where density scaling holds. Finally, we examine the conditions under which the density scaling properties are related to the existence of strong correlations between pressure and energy fluctuations., Comment: 5 pages, 4 figures, updated references

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: revision

The emergence of nanoscience has increased the importance of experiments able to probe the very local structure of materials, especially for disordered and heterogeneous systems. This is technologically important; for example, the nanoscale structure of glassy polymers has a direct correlation with their macroscopic physical properties. We have discovered how a local, high frequency dynamic process can be used to monitor and even predict macroscopic behavior in glassy polymers. Polyvinylethylenes vitrified by different chemical and thermodynamic pathways exhibit different densities in the glassy state. We find that the rate and amplitude of a high frequency relaxation mode (the Johari-Goldstein process involving local motion of segments of the chain backbone) can either correlate or anti-correlate with the density. This implies that neither the unoccupied (free) volume nor the configurational entropy governs the local dynamics in any general sense. Rather it is the magnitude of the fluctuations in local density that underlie these nm-scale motions. We show how properties of the dynamics and the density fluctuations can both be interpreted in terms of an asymmetric double well potential. Finally, since fluctuations underlie the macroscopic properties, we argue that information about the latter should be obtainable from characterization of the local dynamics., Comment: 14 pages, 4 figures

We use molecular dynamics simulation results on viscous binary Lennard-Jones mixtures to examine the correlation between the potential energy and the virial. In accord with a recent proposal [U. R. Pedersen et. al. Phys. Rev. Lett. 100, 015701 (2008)], the fluctuations in the two quantities are found to be strongly correlated, exhibiting a proportionality constant, Gamma, numerically equal to one-third the slope of an inverse power law approximation to the intermolecular potential function. The correlation is stronger at higher densities, where interatomic separations are in the range where the inverse power law approximation is more accurate. These same liquids conform to thermodynamic scaling of their dynamics, with the scaling exponent equal to Gamma. Thus, the properties of strong correlation between energy and pressure and thermodynamic scaling both reflect the ability of an inverse power law representation of the potential to capture interesting features of the dynamics of dense, highly viscous liquids., Comment: 5 pages, 4 figures; published version, one figure removed

The manner in which the intermolecular potential u(r) governs structural relaxation in liquids is a long standing problem in condensed matter physics. Herein we show that diffusion coefficients for simulated Lennard-Jones m-6 liquids (8

By analyzing the experimental data for various glass-forming liquids and polymers, we find that non-exponentiality $\beta$ and the relaxation time $\tau$ are uniquely related: $\log(\tau)$ is an approximately linear function of $1/\beta$, followed by a crossover to a higher linear slope. We rationalize the observed relationship using a recently developed approach, in which the problem of the glass transition is discussed as the elasticity problem.

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. Ngai

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 figures

The recently discovered scaling law for the relaxation times, tau=f(T,V^g), where T is temperature and V the specific volume, is derived by a revision of the entropy model of the glass transition dynamics originally proposed by Avramov [I. Avramov, J. Non-Cryst. Solids 262, 258 (2000).]. In this modification the entropy is calculated by an alternative route, while retaining the approximation that the heat capacity is constant with T and P. The resulting expression for the variation of the relaxation time with T and V is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover. From this analysis, which is valid for any model in which the relaxation time is a function of the entropy. we find that the scaling exponent g can be identified with the Gruneisen constant., Comment: 24 pages, 7 figures

The experimental fact that relaxation times, tau, of supercooled liquids and polymers are uniquely defined by the quantity TV^g, where T is temperature, V specific volume, and g a material constant, leads to a number of interpretations and predictions concerning the dynamics of vitrification. Herein we examine means to determine the scaling exponent g apart from the usual superpositioning of relaxation data. If the intermolecular potential can be approximated by an inverse power law, as implied by the TV^g scaling, various equations are derived relating g to the Gruneisen parameter and to a common expression for the pressure derivative of the glass temperature. In addition, without assumptions, g can be obtained directly from pressure-volume-temperature data. These methods for determining g from molecular or thermodynamic properties are useful because they enable the P- and V-dependences of tau to be obtained, and thereby various analyses of the dynamics to be explored, without the need to carry out relaxation measurements beyond ambient pressure., Comment: 13 pages, 4 figures proceedings of 5th IDMRCS, Lille, France, July 2005

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 figures

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 figures

The fragilities (Tg-normalized temperature dependence of alpha-relaxation times) of 33 glass-forming liquids and polymers are compared for isobaric, mP, and isochoric, mV, conditions. We find that the two quantities are linearly correlated, mP = (37 +/- 3) + (0.84 +/- 0.5)mV. This result has obvious important consequences, since the ratio mV/mP is a measure of the relative degree to which temperature and density control the dynamics, Moreover, we show that the fragility itself is a consequence of the relative interplay of temperature and density effects near Tg. Specifically, strong behavior reflects a substantial contribution from density (jammed dynamics), while the relaxation of fragile liquids is more thermally-activated. Drawing on a scaling law, a physical interpretation of this result in terms of the intermolecular potential is offered., Comment: 15 pages 4 figures

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density, rho, relative to the effect of temperature, T. This result is consistent with the respective magnitudes of the scaling exponent, gamma, yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho^gamma/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect density has on the relaxation dynamics., Comment: 20 pages 5 figures

Dielectric spectroscopy measurements over a broad range of temperature and pressure were carried out on poly(oxybutylene) (POB), a type-A polymer (dielectrically-active normal mode). There are three dynamic processes appearing at lower frequency, the normal and segmental relaxation modes, and a conductivity arising from ionic impurities. In combination with pressure-volume-temperature measurements, the dielectric data were used to assess the respective roles of thermal energy and density in controlling the relaxation times and their variation with T and P. We find that the local segmental and the global relaxation times are both a single function of the product of the temperature times the specific volume, with the latter raised to the power of 2.65. The fact that this scaling exponent is the same for both modes indicates they are governed by the same local friction coefficient, an idea common to most models of polymer dynamics. Nevertheless, near Tg, their temperature dependences diverge. The magnitude of the scaling exponent reflects the relatively weak effect of density on the relaxation times. This is usual for polymers, as the intramolecular bonding, and thus interactions between directly bonded segments, are only weakly sensitive to pressure. This insensitivity also means that the chain end-to-end distance is invariant to P, conferring a near pressure-independence of the (density-normalized) normal mode dielectric strength., Comment: 30 pages, 14 figures

Master curves of the relaxation time, tau, or viscosity, eta, versus T^-1V^-x, where T is temperature, V the specific volume, and x a material constant, are used to deduce the effect of pressure on the dynamic crossover and the fragility. The crossover is determined from the change in slope of derivative plots of the relaxation times or viscosities. We confirm our previous findings that the value of tau or eta at the crossover is independent of both T and P; that is, the dynamic crossover is associated with a characteristic value of the relaxation time. Previous determinations were limited to liquids having crossovers occurring at large values of tau (> 10 microsec), whereas by interpolating within T^-1V^-x space, we extend the analysis to smaller values of the crossover time. Using the superpositioned data, the dynamic crossover can be observed in isochoric data, where it is found that the relaxation time at the crossover for constant volume is equivalent to the value obtained under (the more usual) condition of constant pressure. Similarly, from the scaling analysis, isobaric relaxation times at high pressure are deduced from experimental measurements at atmospheric pressure. We find for all glass-formers studied, that the fragility (normalized temperature dependence of tau or eta) is a decreasing function of pressure. This conclusion is less subject to uncertainties in the measurements than published determinations of the pressure coefficient of fragility. Finally, we show that an empirical function, having the form of the Cohen-Grest relation but without connection to any free volume model, parameterizes the master curves, and accurately describes the data over all measured conditions., Comment: 33 pages, 12 figures

Dielectric measurements of the alpha-relaxation time were carried out on a mixture of ortho-terphenyl (OTP) with ortho-phenylphenol (OPP), over a range of temperatures at two pressures, 0.1 and 28.8 MPa. These are the same conditions for which heat capacity, thermal expansivity, and compressibility measurements were reported by Takahara et al. [S. Takahara, M. Ishikawa, O. Yamamuro, and T. Matsuo, Journal of Physical Chemistry B 103 (16), 3288 (1999).] for the same mixture. From the combined dynamic and thermodynamic data, we determine that density and temperature govern to an equivalent degree the variation of the relaxation times with temperature. Over the measured range, the dependence of the relaxation times on configurational entropy is in accord with the Adam-Gibbs model, and this dependence is invariant to pressure. Consistent with the implied connection between relaxation and thermodynamic properties, the kinetic and thermodynamic fragilities are found to have the same pressure independence. In comparing the relaxation properties of the mixture to those of neat OTP, density effects are stronger in the former, perhaps suggestive of less efficient packing.

Published dielectric relaxation measurements on polypropylene glycol and 1,4-polyisoprene are analyzed to determine the relative effect that thermal energy and volume have on the temperature dependence of the normal mode relaxation times, and compare this to their effect on the temperature dependence of the local segmental relaxation times. We find that for both polymers at temperatures well above Tg, both relaxation modes are governed more by thermal energy than by volume, although the latter's contribution is not negligible. Such a result is consistent with an assumption underlying models for polymer viscoelasticity, such as the Rouse and tube models, that the friction coefficient governing motions over large length scales can be identified with the local segmental friction coefficient. We also show that relaxation data for both the segmental and the normal mode superimpose, when expressed as a function of the product of the temperature and the volume, the latter raised to a power. This scaling form arises from an inverse power form for the intermolecular potential. The value of the exponent on the volume for these two polymers indicates a relatively "soft" potential., Comment: 15 pages, 3 figures

Classification of glass-forming liquids based on the dramatic change in their properties upon approach to the glassy state is appealing, since this is the most conspicuous and often-studied aspect of the glass transition. Herein, we show that a generalized scaling, log tau proportional to T^(-1)V^(-gamma), where gamma is a material-constant, yields superpositioning for ten glass-formers, encompassing van der Waals molecules, associated liquids, and polymers. The exponent gamma reflects the degree to which volume, rather than thermal energy, governs the temperature and pressure dependence of the relaxation times., Comment: 12 page, 4 figures