Fragility versus activation volume: insight into molecular dynamics of glass-forming hydrogen-bonded liquids.

In this Brief Report we show that key parameters, the fragility m(p) and activation volume ΔV, which characterize the sensitivity of molecular dynamics near the glass transition to temperature and pressure changes, respectively, reflect molecular properties in a nonequivalent way. Our comparative study of fragilities and activation volumes of isomeric pentiols provides evidence that the parameter m(p) can distinguish different H-bonded structures, even if molecular volumes are similar; however, the value of ΔV recognizes mainly the size of a relaxing molecular unit and correlates to the length scale of molecular cooperativity. Thus there is a striking difference in information given by m(p) and ΔV.

Glass transition dynamics of room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate.

The conductivity relaxation dynamics of the room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate ([Si-MIm][BF(4)]) have been studied by broadband conductivity relaxation measurements at ambient pressure and elevated pressures up to 600 MPa. For the first time, several novel features of the dynamics have been found in a room-temperature ionic liquid. In the electric loss modulus M″(f) spectra, a resolved secondary ß-conductivity relaxation appears, and its relaxation time τ(ß) shifts on applying pressure in concert with the relaxation time τ(α) of the primary α-conductivity relaxation. The spectral dispersion of the α-conductivity relaxation, as well as the fractional exponent (1 - n) of the Kohlrausch-Williams-Watts function that fits the spectral dispersion, is invariant to various combinations of pressure and temperature that keep τ(α) constant. Moreover, τ(ß) is unchanged. Thus the three quantities, τ(α), τ(ß), and n, are coinvariant to changes in pressure and temperature. This strong connection to the α-conductivity relaxation shown by the ß-conductivity relaxation in [Si-MIm][BF(4)] indicates that it is the analogue of the Johari-Goldstein ß-relaxation in nonionically conducting glass-formers. The findings have fundamental implications on theoretical interpretation of the conductivity relaxation processes and glass transition in ionic liquids. It is also the first time such a secondary conductivity relaxation or the primitive conductivity relaxation of the coupling model has been fully resolved and identified in M″(f) in any ionically conducting material that we know of.

Note: New feedthrough insulation method for the dielectric spectroscopy under ultrahigh pressure conditions.

We present a new technique to prepare insulated channels for the high pressure cell used in broadband dielectric spectroscopy. The cell resists corrosion or other chemical reactions from the reactive sample liquids. The cell maintains electrical insulation between electrode contacts better than 100 GOmega as well as good mechanical properties in broad temperature range of -80 to +100 degrees C and under extremely high pressures up to 1.8 GPa.

Anomalous behavior of the structural relaxation dispersion function of a carborane-containing siloxane.

Broadband dielectric spectroscopic investigations of a vinyl-terminated carboranylenesiloxane, VCS, were performed at ambient and elevated pressures. At a constant structural relaxation time, results show that the structural relaxation dispersion function of VCS narrows with both increasing pressure and temperature. This narrowing is substantial in the case of pressurization and, consequently, the breakdown of the temperature-pressure superposition rule is observed. The interpretation of this breakdown is presented.

Dielectric properties of two diastereoisomers of the arabinose and their equimolar mixture.

Dielectric relaxation measurements were performed on two enantiomers, D- and L-arabinose and their equimolar mixture, and compared to dielectric data obtained for D-ribose. D-Arabinose differs from d-ribose by having the opposite configuration at C2. This study reveals that both D- and L- of arabinose exhibit alpha-relaxation peaks with the same shape for the same alpha-relaxation time tau(alpha), and the same steepness index for the T(g)-scale T-dependence of tau(alpha). However, the two isomers have slightly different glass transition temperatures T(g)'s, and their secondary gamma-relaxation times also differ slightly from the previously observed gamma-relaxation in D-ribose at the same temperature. However, when samples of both investigated monosaccharides are annealed at higher temperatures, their glass transition temperatures become nearly identical. This is an effect of the mutarotation process, which leads to the formation of pairs of the enantiomers and accordingly they should have the same physical properties. The width of the alpha-relaxation of D- and L-arabinose is broader than that of D-ribose, as reflected by the smaller stretch exponent in the Kohlrausch-Williams-Watts function used to fit the data of the former (beta(KWW)=0.46+/-0.01) than the latter (beta(KWW)=0.55+/-0.01). The width of the alpha-relaxation of racemic mixture of the D- and L-arabinose is slightly broader than that of the pure isomers. While the dielectric loss data of D-ribose in the glassy state at ambient and elevated pressures show an inflexion indicating the presence of the JG beta-relaxation, the data of D- and L-arabinose show no such feature for identification of the supposedly universal JG beta-relaxation. Nevertheless, on comparing the loss spectra of D-arabinose with that of D-ribose, the presence of the JG beta-relaxation in D-arabinose has been rationalized.

Origin of the liquid-liquid phase transition for trans-1,2-dichloroethylene observed by IR spectroscopy.

IR vibrational spectroscopy has been employed to study liquid-liquid phase transitions of trans-1,2-dichloroethylene (TDCE). The temperature dependence of the vibrational frequency, band absorbance, and band broadening has been analyzed in the temperature range of 293-237 K. All peaks show distinct increases in their absorbancies at the temperature of 246.7 K accompanied by a frequency shift and bandwidth changes. Weak H...Cl hydrogen bonding has been suggested as the explanation for the overall behavior in a wide temperature range, whereas the significant changes in the spectrum at 246.7 K are probably due to molecular ordering originating from electrostatic interactions. The transition is well described by the model of the cooperative formation of locally favored structures.

The liquid-glass and liquid-liquid transitions of TPP at elevated pressure.

We studied the polyamorphism of the liquid triphenyl phosphite by means of broadband dielectric spectroscopy at ambient and elevated (p=500 MPa) pressures. The effect of pressure on fragility, liquid-liquid phase transition, and its kinetics is discussed in relation to the two-order-parameter model proposed by Tanaka. The experimental evidence in support of this model is presented.

Anomalous temperature behavior of nonlinear dielectric effect in supercooled nitrobenzene.

Studies of the nonlinear dielectric effect (NDE), describing changes of the dielectric permittivity induced by a strong electric field, in normal and supercooled liquid nitrobenzene are presented. An unusual increase of the stationary NDE, portrayed by the critical-like relation approximately (T-T+)(-1), was obtained. Nitrobenzene samples solidified at TS approximately T+ +2 K , approximately 10 K below the reported melting temperature Tm approximately 278 K . The anomalous increase of the NDE coincided with a slow relaxation process, detected in time-resolved measurements. The results presented offer a reinterpretation of the classical description by Piekara and Piekara [A. Piekara and B. Piekara, C. R. Hebd. Seances Acad. Sci. 203, 852 (1936)] of a positive sign contribution to the NDE in liquids. It is suggested that the obtained anomaly may be associated with the appearance of local quasinematic structures. This is supported by a speculative link to a general model for liquid-liquid transitions [H. Tanaka, Phys. Rev. E 62, 6968 (2000)] and a phenomenological model originally developed for the self-focusing of laser beams [J. Hanus, Phys. Rev. 178, 420 (1969)]. The case of the isotropic-nematic transition in liquid crystalline materials is also recalled. The NDE results reported here are related neither to the glass transition phenomenon nor to the recently developed concept of a second liquid-liquid transition.

Additive property of secondary relaxation processes in di-n-octyl and di-isooctyl phthalates: signature of non-Johari-Goldstein relaxation.

Broadband dielectric spectroscopy was used to study relaxation dynamics of supercooled di-n-octyl phthalate, di-isooctyl phthalate, and their mixtures. Additionally, low temperature measurements were performed to investigate the nature of the secondary relaxation processes in both glass formers. The authors found that the secondary relaxation observed in the mixture is the additive sum of the secondary relaxations of the two components. This experimental evidence indicates that these secondary relaxation processes are intramolecular in origin, and they are non-Johari-Goldstein secondary relaxations.

Complex dynamics of supercooling n-butylcyanobiphenyl (4CB).

Results of broadband dielectric spectroscopy (BDS) studies, for a remarkable temperature range (130 K), in supercooled rodlike liquid crystalline n-butylcyanobiphenyl (4CB) are shown. They are supplemented by static nonlinear dielectric effect (NDE) measurements in 4CB and 3CB (n-propylcyanobiphenyl), giving unequivocal estimations of the hypothetical isotropic-nematic (I-N) continuous phase transition temperature T* and of the discontinuity of this transition deltaT. The distribution of relaxation times becomes strongly non-Debye on approaching the I-N transition and follows the Debye pattern for T > T(I-N) + 75 K. The temperature evolution of relaxation times [tau(T)] and dc conductivity [sigma(T)] remain non-Arrhenius in the tested temperature range. Their descriptions via the Vogel-Fulcher-Tammann (VFT), Cohen-Grest (CG), and mode coupling theory (MCT) based relations are discussed. The derivative analysis of tau(T) and sigma(T) experimental data enabled the detection of dynamic crossovers and the insight into the behavior of the apparent activation energy which follows the relations E(A)(tau),E(A)(sigma) (sigma) is proportional to (T/T-T0)2, with a small distortion in the immediate vicinity of the I-N transition. The glassy dynamics coexist with the critical-like behavior of temperature dependences of the static dielectric permittivity, maximum of the loss curves and strong pretransitional NDE pretransitional anomaly. These results indicate the significant role of prenematic fluctuations-heterogeneities in the isotropic, fluidlike, surrounding regarding both static and dynamic properties.

Mode coupling behavior in glass-forming liquid crystalline isopentylcyanobiphenyl.

Linear and nonlinear dielectric measurements of liquid crystalline chiral isopentylcyanobiphenyl (5*CB) and n -pentylcyanobiphenyl (5CB), combined with viscosity eta (T) data, are presented. The 5*CB compound glassifies on cooling in the cholesteric phase whereas 5CB crystallizes in the nematic phase. In both compounds the temperature evolution of dielectric relaxation times, the dc conductivity, and the viscosity are well described by the "critical-like" description from mode coupling theory (MCT). However, for 5*CB a unique coincidence of the MCT "critical" temperature and extrapolated temperature of the hypothetical continuous isotropic-cholesteric (T*) phase transition was found. The temperature dependence of the strong electric-field-induced changes of the dielectric permittivity exhibits a strong anomaly in the direction of negative values on approaching T* , not observed up to now. The anomaly is described by the susceptibility-related critical exponent gamma=1 . The divergence of the "nonlinear" dielectric relaxation follows a power dependence described by the exponent y=1 . This paper recalls the recent discussions on the glassy dynamics of a "hard-ellipsoid" liquid and the possible relationship between the glass transition, critical phenomena, and isotropic-nematic transition.