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Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid
31. Examples of commercial high pressure dielectric equipment are a full dielectric high pressure system from “NOVOCONTROL Technologies GmbH” and high pressure vessels with electrical feed throughs from “Unipress Equipment.”
51.L. A. Roed, “Isochronal superposition - an experimental test of a prediction from the isomorph theory,” Master’s thesis, NSM, IMFUFA, Physics, Roskilde University, Denmark, 2012.
56.D. Gundermann, “Testing predictions of the isomorph theory by experiment,” Ph.D. thesis,Roskilde University, DNRF Centre “Glass & Time”, IMFUFA, NSM, 2013).
65. The longitudinal volume specific heat cl is related to the isochoric specific heat (cv) by where KS and KT respectively are the adiabatic and the isothermal bulk moduli, and G is the shear modulus. At state points where relaxation occurs G will differ from 0, which means that cl and cp differs. However, the difference in characteristic timescale is very small (see supplementary material of Ref. 54).
66.B. Igarashi, T. Christensen, E. H. Larsen, N. B. Olsen, I. H. Pedersen, T. Rasmussen, and J. C. Dyre, Rev. Sci. Instrum. 79, 045106 (2008).
71.K. Niss and B. Jakobsen, “Dielectric and shear mechanical relaxation in glass forming liquids — a thorough analysis and experimental test of the DiMarzio-Bishop model,” Master thesis, Roskilde University (2003), text nr. 424 in “Tekster fra IMFUFA.”
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The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependences of the characteristic time scale associated with the specific heat is compared to the equivalent time scale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two time scales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying “inner clock.” Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measuredliquid, comply to the isomorph theory.
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