Showing papers by "Sylwester J. Rzoska published in 1997"

The influence of pressure on dielectric relaxation for phthalate derivatives in the supercooled state

Abstract: The dielectric relaxation of phthalate derivatives in the supercooled state has been studied at pressures up to 300 MPa, covering a frequency range from to . The behaviour of the relaxation time is very well reproduced by the relation . The analysis of the absorption curves shows that pressure increases the amplitude of the -relaxation more than the equivalent temperature change. The scaling form presented by Dixon and co-workers for the -process in supercooled liquids is re-examined in the light of pressure - dielectric relaxation data. It was found that Dixon scaling distinguishes isothermal measurements from isobaric measurements, by means of the greater strength of the second relaxation process for the pressure path.

Dielectric relaxation of glass-forming epoxy resin under high pressure

Abstract: Measurements of relaxation time in epoxy resin [poly(bisphenol $A$-co-epichlorohydrin), glycidyl end capped] in the supercooled liquid state have been carried out using dielectric spectroscopy over the frequency range from 10 mHz to 10 MHz as a function of pressure (up to $p=250\mathrm{MPa}$) and as a function of temperature. To our knowledge, this is the first time a two-dimensional surface of relaxation times $\ensuremath{\tau}=\ensuremath{\tau}(T,p)$ in epoxy resin has been plotted. The pressure and the temperature dependence of structural relaxation times are in good agreement with the extended Vogel-Fulcher-Tammann equation. For isobaric data, the fragility of the epoxy system reveals a tendency to decrease at elevated pressures. By using pressure as an additional variable, the influence of pressure on glass transition temperature ${T}_{g}$ has been determined.

High-pressure studies of the low-frequency nonlinear dielectric effect in the isotropic phaseof octyl- and dodecylcyanobiphenyls

Abstract: Results are presented for high-pressure, isothermal studies of the low-frequency nonlinear dielectric effect (LF NDE) in the isotropic phase of n-octylcyanobiphenyl (8CB, isotropic-nematic liquid crystal phase transition) and n-dodecylcyanobiphenyl (12CB, isotropic--smectic-A phase transition). In both cases a linear pressure dependence of reciprocals of LF NDE, with no distortions near the clearing point, was found. This behavior is in agreement with classical relations derived from the Landau--de Gennes model. The pressure dependence (up to 100 MPa) of clearing temperatures and temperatures of extrapolated, hypothetical, continuous phase transitions were determined. For 8CB isothermal pressure and isobaric temperature pretransitional effects have been superimposed on one scaling curve. This makes it possible to investigate the pretransitional effects under high pressure from temperature measurements carried out under atmospheric pressure.