D. Fragiadakis

On the density scaling of liquid dynamics

Superpositioning of relaxation data as a function of the product variable TV(γ), where T is temperature, V the specific volume, and γ a material constant, is an experimental fact demonstrated for approximately 100 liquids and polymers. Such scaling behavior would result from the intermolecular potential having the form of an inverse power law (IPL), suggesting that an IPL is a good approximation for certain relaxation properties over the relevant range of intermolecular distances. However, the derivation of the scaling property of an IPL liquid is based on reduced quantities, for example, the reduced relaxation time equal to T(1∕2V - 1∕3) times the actual relaxation time. The difference between scaling using reduced rather than unreduced units is negligible in the supercooled regime; however, at higher temperature the difference can be substantial, accounting for the purported breakdown of the scaling and giving rise to different values of the scaling exponent. Only the γ obtained using reduced quantities can be sensibly related to the intermolecular potential.

Insights on the origin of the Debye process in monoalcohols from dielectric spectroscopy under extreme pressure conditions

The dielectric spectra of most simple liquids are characterized by two relaxation processes: (i) the alpha-process, an intense, broad non-Debye relaxation with a non-Arrhenius temperature dependence and (ii) a beta process, evident mainly below the glass transition and having nearly Arrhenius temperature behavior. However, the dielectric spectra of monoalcohols show three processes: two that resemble those of normal liquids and a third very intense Debye peak at lower frequencies, which is non-Arrhenius. Interestingly, this third process is not observed with other techniques such as light scattering and mechanical spectroscopy. There is a disagreement in the literature concerning the nature of this third relaxation. We investigated 2-ethyl-1-hexanol under high pressures (up to approximately 1.4 GPa) over a broad range of temperatures. The Debye process, which is the slowest, is strongly affected by pressure. At higher pressures the relaxation times and intensities of the two non-Arrhenius relaxations become more nearly equal. In light of these results, we propose a modified interpretation of the relaxation processes and their underlying structures in monoalcohols.

Correlation of nonexponentiality with dynamic heterogeneity from four-point dynamic susceptibility χ4(t) and its approximation χT(t)

Various properties of vitrifying liquids are correlated with the dispersity of the dynamics, the latter reflected in the magnitude of the nonexponentiality parameter, β(K), describing the distribution of relaxation times. These properties include the mean relaxation time, τ(α), the fragility, and the dynamic crossover. The correlations with β(K) are observed in both experimental data and the results from molecular dynamics simulations on Lennard-Jones (LJ) type systems. Another, rather obvious property to correlate with β(K) is the dynamic heterogeneity, which can be quantified from the number of molecules, N(c), dynamically correlated over a time span τ(α). For a given LJ system, N(c) can be rigorously calculated and we find that it does indeed correlate with β(K) over a range of thermodynamic conditions. However, the analysis of experimental data for a broad range of real materials, wherein an approximation is required to obtain N(c), reveals the absence of any relationship between N(c) and β(K) among different materials.

Dynamic correlation length scales under isochronal conditions

The origin of the dramatic changes in the behavior of liquids as they approach their vitreous state-increases of many orders of magnitude in dynamic time scales and transport properties-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 are 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 is essentially invariant along isochrones for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic cooperativity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials.

Factors influencing the ballistic impact resistance of elastomer-coated metal substrates

An experimental study was carried out of various factors affecting the ballistic penetration resistance of elastomer/steel bilayers. For blunt penetrators, the contribution of the coating to performance is optimized using the hardest substrates, front surface placement of the elastomer, and (when normalizing by added weight) thin, ca. 2–3 mm, coatings. These results, none of which are predicted by existing models, evince the marked coupling of coating and substrate in the impact response of the bilayer. We also show that nanoparticle fillers have a modest effect on ballistic performance of polyurea coatings, changing the penetration velocity by a few percent or less. This contrasts with the linear dynamic mechanical behavior, which shows much more significant increases in energy absorption due to nano-reinforcement.

Volumetric, dielectric, calorimetric and X-ray studies of smectogenic 10PBO8 at atmospheric and elevated pressures

The synthesis and pressure–volume–temperature (PVT), differential thermal analysis (DTA), dielectric and X-ray diffraction data of 2-(4-octylcarbonyloxyphenyl)-5-decylpyrimidine (10PBO8) are presented. The substance exhibits two crystalline and smectic C (SmC) phases on heating and a SmC–monotropic crystalline smectic B (SmBcr) SmBcr–crystal sequence of phase transitions on cooling. Above ca. 15 MPa, the SmBcr phase becomes enantiotropic (reversible polymorphism). The phase behaviour and molecular dynamics in the liquid crystalline phases are analysed and discussed, with the conformational component of the total entropy for the SmC–isotropic liquid transition estimated. We also calculate from the PVT results the potential parameter characterising the steepness of the interaction potential.

Volumetric study of n-octyloxy-cyanobiphenyl (8OCB)

Volumetric measurements were carried out on n-octyloxy-cyanobiphenyl (8OCB) over temperatures from ambient to 150°C at pressures up to 200 MPa, encompassing the liquid, nematic and smectic liquid crystalline, and crystal states. From the step changes in volume at the transitions, the phase boundaries and the associated activation enthalpies and energies were determined. The results are consistent with prior studies that were limited to a narrower range of thermodynamic conditions. The thermodynamic potential parameter, Γ, found for the isotropic–nematic transition, =3.15, is significantly smaller than the scaling exponent γ (=4.4) that superposes the longitudinal relaxation times in the nematic state. From this non-equivalence, we conclude that the order parameter must vary with pressure along the clearing line. By comparing the phase behaviour of the nematic state for isochoric and isobaric conditions, the relative contribution of volume was determined to be less that the effect of thermal energy on the thermodynamic stability.

A test for the existence of isomorphs in glass-forming materials

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.