Comment on "Correlations between isobaric and isochoric fragilities and thermodynamical scaling exponent for glass-forming liquids"

Testing density scaling in nanopore-confinement for hydrogen-bonded liquid dipropylene glycol

Recently, it has been demonstrated that the glassy dynamics of the molecular liquids and polymers confined at the nanoscale level might satisfy the density scaling law (ρ ^γ /T) with the same value of the scaling exponent, γ, as that determined from the high-pressure studies of the bulk material. In this work, we have tested the validity of this interesting experimental finding for strongly hydrogen-bonded molecular liquid, dipropylene glycol (DPG), which is known to violate the ρ ^γ /T scaling rule in the supercooled liquid bulk state. The results of the independent dielectric relaxation studies carried out on increased pressure and in nanopores, have led to an important finding that when the density change induced by geometrical confinement is not very large, DPG can still obey the density scaling law with the same value of the scaling exponent as that found for the bulk sample. In this way, we confirm that the information obtained from the universal density scaling approach applied to nanoscale confined systems is somehow consistent with the macroscopic ones and that in both cases the same fundamental rules governs the glass-transition dynamics.

In search of invariants for viscous liquids in the density scaling regime: investigations of dynamic and thermodynamic moduli

In this paper, we report the nontrivial results of our investigations of dynamic and thermodynamic moduli in search of invariants for viscous liquids in the density scaling regime by using selected supercooled van der Waals liquids as representative materials. Previously, the dynamic modulus M_p-T (defined in the pressure-temperature representation by the ratio of isobaric activation energy and activation volume) as well as the ratio B_T/M_p-T (where B_T is the thermodynamic modulus defined as the inverse isothermal compressibility) have been suggested as some kinds of material constants. We have established that they are not valid in the explored wide range of temperatures T over a dozen decades of structural relaxation times τ. The temperature dependences of M_p-T and B_T/M_p-T have been elucidated by comparison with the well-known measure of the relative contribution of temperature and density fluctuations to molecular dynamics near the glass transition, i.e., the ratio of isochoric and isobaric activation energies. Then, we have implemented an idea to transform the definition of the dynamic modulus M_p-T from the p-T representation to the V-T one. This idea relied on the disentanglement of combined temperature and density fluctuations involved in isobaric parameters and has resulted in finding an invariant for viscous liquids in the density scaling regime, which is the ratio of thermodynamic and dynamic moduli, B_T/M_V-T. In this way, we have constituted a characteristic of thermodynamics and molecular dynamics, which remains unchanged in the supercooled liquid state for a given material, the molecular dynamics of which obeys the power density scaling law.

Reply to "Comment on 'Correlations between isobaric and isochoric fragilities and thermodynamical scaling exponent for glass-forming liquids' "

Our recent paper [A. Grzybowski, K. Grzybowska, J. Zioło, and M. Paluch, Phys. Rev. E 74, 041503 (2006)] tested the correlations between isobaric and isochoric fragilities, m_{P} and m_{V} , as well as the scaling exponent gamma under elevated pressure conditions. The preceding paper [R. Casalini and C. M. Roland, Phys. Rev. E 76, 013501 (2007)] is a Comment by the authors of the correlations originally determined for atmospheric pressure. In this Reply we present our point of view on criticisms contained in the Comment. We clarify and maintain our previously drawn conclusions.