Slow dynamics of salol: a pressure- and temperature-dependent light scattering study

Temperature and pressure dependence of the alpha relaxation in ortho-terphenyl

Molecular dynamics (MD) simulations of ortho-terphenyl using an all-atom model with the optimized potentials for liquid simulations (OPLS) force field were performed both in the high temperature Arrhenian region and at lower temperatures that include the onset of the super-Arrhenian region. From the MD simulations, the internal energy of both the equilibrium liquid and crystal was determined from 300 K to 600 K and at pressures from 0.1 MPa to 1 GPa. The translational and rotational diffusivities were also determined at these temperatures and pressures for the equilibrium liquid. It is shown that within a small offset, the excess internal energy Ū_x from the MD simulations is consistent with the experimentally determined excess internal energy reported earlier [Caruthers and Medvedev, Phys. Rev. Mater. 2, 055604, (2018)]. The MD mobility data {including extremely long-time 1 atm simulations from the study by Eastwood et al. [J. Phys. Chem. B 117, 12898, (2013)]} were combined with experimental data to form a unified dataset, where it was shown that in both the high temperature Arrhenian region and the lower temperature super-Arrhenian region, the mobility is a linear function of 1/Ū_x(T,p), albeit with different proportionality constants. The transition between the Arrhenian and super-Arrhenian regions is relatively sharp at a critical internal energy Ū_x ^α. The 1/Ū_x(T,p) model is able to describe the mobility data over nearly 16 orders-of-magnitude. Other excess thermodynamic properties such as excess enthalpy and excess entropy (i.e., the Adam-Gibbs model) are unable to unify the pressure dependence of the mobility.

A universal modified van der Waals equation of state. Part I: Polymer and mineral glass formers

PVT data of glass formers (minerals and polymers) published in the literature are re-analyzed. All the polymer glass formers (PS, PVAc, PVME, PMMA, POMS, PBMA, PVC, PE, PP, PMPS, PMTS, PPG) present two main properties which have never been noted: a) the isobars P(V) have a fan structure characterized by the two parameters T* and V*; b) the isotherms verify the principle of temperature-pressure superposition for P < P*. From these properties we show that the Equation Of State (EOS) can be put on a modified van der Waals form (VW-EOS), (V - V*) = (V0 - V*)P*/(P + P*). The characteristic pressure P* and the covolume V* are T and P independent. In polymer glass formers P* and V* have same values in the α (melt) and β (glass) domains. The characteristic temperatures T* deduced from the Fan Structure of the Isobar (FSIb) above and below T(g) are different. The characteristic temperature T*(α) of the melt state is found near the Vogel temperature T0 for linear polymers and more than 100 K below T0 for atactic polymers (with pendent groups). This difference in atactic polymers (and in some low molecular weight compounds) is explained by the importance of the β motions due to the pendent groups. The independence of T0 on P is discussed. A modified VFT equation (analogous to the compensation law and Meyer-Neldel rule) giving the relaxation time τ of the α motions as a function of P and T is proposed. The fan structure of the isotherm logτ versus P is explained. It is shown that organic non-polymeric liquids (C6H12, C6H14, DHIQ, OTP, Glycerol, Salol, PDE, DGEBA), mineral glass (SiO2, Se, GeSe4, GeSe2, GeO2, As2O3 and two metallic glasses (LaCe and CaAl alloys) verify this VW-EOS with similar accuracy. The relation P* = B 0/γ B among the characteristic pressure P*, the zero-pressure modulus B0 and the Slatter-Grüneisen anharmonicity parameter γ(B0 deduced from the VW-EOS, is observed in all the glass formers.

Pressure coefficient of the glass transition temperature in the thermodynamic scaling regime

We report that the pressure coefficient of the glass transition temperature, dT(g)/dp, which is commonly used to determine the pressure sensitivity of the glass transition temperature T(g), can be predicted in the thermodynamic scaling regime. We show that the equation derived from the isochronal condition combined with the well-known scaling, TV(γ) = const, predicts successfully values of dT(g)/dp for a variety of glass-forming systems, including van der Waals liquids, polymers, and ionic liquids.

Connection between dynamics and thermodynamics of liquids on the melting line

The dynamics of a large number of liquids and polymers exhibit scaling properties characteristic of a simple repulsive inverse power-law potential, most notably the superpositioning of relaxation data as a function of the variable TV^{γ}, where T is temperature, V the specific volume, and γ a material constant. A related scaling law T{m}V{m}{Γ}, with the same exponent Γ = γ, links the melting temperature T{m} and volume V{m} 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 γ and Γ for a large number of nonassociating liquids. Rigid, spherical molecules containing no polar bonds have Γ = γ; consequently, the reduced relaxation time, viscosity, and diffusion coefficient are each constant along the melting line. For other liquids γ > Γ always; that is, 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 T{m} (that is, increasing pressure).

The role of nucleation in vitrification of supercooled liquids

Low-frequency Raman and differential scanning calorimetry (DSC) investigations were carried out during a structural transformation of supercooled liquid salol (phenyl salicylate) in a wide temperature range. DSC experiments indicate that in the supercooled liquid salol at temperature ~40 K above the glass transition temperature metastable nuclei start to form. During subsequent cooling the nuclei become an important element of the glass structure, and thereby are considered as a measure of the intermediate range order in this glass. It was shown that the crystalline structure of the metastable nuclei differ from that of the stable nuclei. Low-frequency Raman spectra of the glassy salol show a broad band in the spectral range from 14.5 to 17.2 cm(-1); the so called 'Boson peak', which can be interpreted in terms of its relationship to the formation of structured clusters, with typical sizes in the nanometer range (critical radii).

Cauchy relation in relaxing liquids

The Cauchy-like relation M(infinity) = A + BG(infinity) has recently been found to hold for the high frequency limit values of the longitudinal modulus M(infinity) and transverse modulus G(infinity) of viscoelastic liquids, with B approximately 3 in all the investigated systems. The Brillouin scattering results here reported for curing epoxy systems and thermal glass formers give evidence for the validity of a Cauchy-like relation M(') = A + BG(') for the real part of the elastic moduli measured at finite frequencies. Our results suggest as well the validity of a pure Cauchy relation DeltaM = 3 DeltaG for the relaxation strengths of longitudinal and shear moduli in relaxing liquids.

A high pressure cell for dynamic light scattering up to 2 kbars with conservation of plane of polarization

We report on a high pressure cell with six optical windows which can be used up to 2 kbars for laser light scattering applications at scattering angles of 45 degrees , 90 degrees , and 135 degrees of liquid samples in a temperature range between -20 and 150 degrees C. The pressure transmitting medium is compressed nitrogen. The window material used is SF57 NSK, a glass with an extremely low stress optical coefficient in the order of about 10(-5) which allows thus to maintain the plane of polarization even under the action of high pressure. In order to demonstrate the functioning of the cell we show Rayleigh-Brillouin spectra of poly(methylphenylsiloxane) at different polarizations and pressures.