Dynamics of aroclor and its modification by dissolved polystyrene

Universal critical-like scaling of dynamic properties in symmetry-selected glass formers

Evidence for a possible general validity of the critical-like behavior of dielectric relaxation time or viscosity tau,eta proportional to (T-T(C))(-phi) with phi-->9 and T(C)<T(g) on approaching glass temperature (T(g)) is shown. This universal behavior is found in various systems where the vitrification is dominated by a selected element of symmetry. The supporting evidence was obtained on the basis of the distortion-sensitive, derivative-based analysis of tau(T) data for a rodlike liquid crystalline compound (E7), orientationally disordered crystals (plastic crystals), a colloidal nanofluid system, polymer melt (polystyrene), oligomeric liquid (EPON 828), and low molecular weight glass formers (glycerol, threitol, sorbitol, and 1-propanol). Results presented explain the puzzling experimental artifacts supporting the dynamical scaling model [R. H. Colby, Phys. Rev. E 61, 1783 (2000); B. M. Erwin, R. H. Colby, J. Non-Cryst. Solids 307-310, 225 (2002)]. It is suggested that spin-glass-like systems may be linked to the discussed pattern.

"Self-concentration" effects on the dynamics of a polychlorinated biphenyl diluted in 1,4-polybutadiene

The mobility of isolated polychlorinated biphenyl (PCB54) in 1,4-polybutadiene (PB) has been investigated by means of broadband dielectric spectroscopy. The aim was to provide new insights about the effect of the environment on the dynamics of PCB54. The authors' results indicate that PCB54 structural dynamics is neither independent of the PB matrix nor slaved to the matrix itself. The authors interpret these results as a consequence of the limited size of cooperatively rearranging regions (CRRs) involved in PCB54 structural relaxation possessing an effective concentration different from the macroscopic one. This implies a non-negligible influence of "self-concentration," already proven for the component segmental dynamics in polymer blends, also in the relaxation of binary mixtures involving low molecular weight glass formers. This allowed the evaluation of the size of CRR, which was about 1 nm for PCB54 in PB. This means that the cooperativity extends over the first shell around PCB54 molecules.

Heterogeneity-related dynamics in isotropic n-pentylcyanobiphenyl

The evolution of the dielectric relaxation time tau(T) and its strong electric field related counterpart tau(fluct).(T) in the isotropic phase of rodlike liquid crystalline n-pentylcyanobiphenyl (5CB) is discussed. For the tau(fluct).(T) a strong pretransitional increase on approaching the virtual critical point located at deltaT = 1.1 K below isotropic nematic clearing temperature T(IN) is shown. The linearized derivative analysis applied to test the evolution of tau(T) revealed a sequence of Vogel-Fulcher-Tammann based dynamical domains, for which the fragility increases enormously on approaching T(IN). The similar analysis focused on testing the "critical" mode-coupling theory behavior showed superior features of such description. A strong asymmetric broadening of the dielectric loss curves is also discussed.

Effect of chemical structure on the isobaric and isochoric fragility in polychlorinated biphenyls

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density rho relative to the effect of temperature T. This result is consistent with the respective magnitudes of the scaling exponent gamma yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho(gamma)/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect which the density has on the relaxation dynamics.