Derivative-based analysis for temperature and pressure evolution of dielectric relaxation times in vitrifying liquids

Critical Model Insight into Broadband Dielectric Properties of Neopentyl Glycol (NPG)

This report presents the low-frequency (LF), static, and dynamic dielectric properties of neopentyl glycol (NPG), an orientationally disordered crystal (ODIC)-forming material important for the barocaloric effect applications. High-resolution tests were carried out for 173K Collapse

Key Words

• broadband dielectric spectroscopy
• dielectric constant
• discontinuous phase transitions
• electric conductivity
• glassy dynamics
• low-frequency changes in dielectric permittivity
• neopentyl glycol
• orientationally disordered crystals
• plastic crystals

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MESH Headings Collapse

Grants

• 2022/45/B/ST5/04005 National Science Center

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Affiliation(s)

Aleksandra Drozd-Rzoska
Jakub Kalabiński
Sylwester J. Rzoska Institute of High Pressure Physics Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland

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Steepness-fragility insights from the temperature derivative analysis of dielectric data of a highly nonsymmetric liquid crystal dimer

A comprehensive dynamic analysis of the dielectric relaxation-time data across a broad temperature range for both isotropic and nematic phases has been conducted on the CBO3O.Py liquid crystal dimer, the shorter chain-length compound within the highly nonsymmetric pyrene-based series of liquid crystal dimers (CBOnO.Py, with n ranging from 3 to 11). It was known from another previous study that in the nematic phase, three different relaxation processes contribute to the complex dielectric permittivity depending on the orientation of the alignment axis with respect to the probing electric field direction. The temperature-derivative analysis of the relaxation-time data using different analytic functions reveals that the critical-like description, through the dynamic scaling model, best portrays the relaxation-time data in the nematic phase as the system approaches the glass transition. A single glass transition temperature is obtained which is consistent with thermal stimulated depolarization currents experimental determinations published elsewhere. From temperature-dependent steepness index m(T), the activation-critical model is also considered as a more general analytic function from which the dynamic scaling model is a terminal approximation. Additionally, the critical-like parametrization provides insight into obtaining a universal description of the temperature-dependent steepness index m(T), for all liquid crystal compounds belonging to symmetry-selected glass formers, such as rodlike liquid crystal monomers.

Global population: from Super-Malthus behavior to Doomsday criticality

The report discusses global population changes from the Holocene beginning to 2023, via two Super Malthus (SM) scaling equations. SM-1 is the empowered exponential dependence: P t = P 0 e x p ± t / τ β , and SM-2 is the Malthus-type relation with the time-dependent growth rate r ( t ) or relaxation time τ ( t ) = 1 / r ( t ) : P t = P 0 e x p r t × t = P 0 e x p τ t / t . Population data from a few sources were numerically filtered to obtain a 'smooth' dataset, allowing the distortions-sensitive and derivative-based analysis. The test recalling SM-1 equation revealed the essential transition near the year 1970 (population: ~ 3 billion): from the compressed exponential behavior ( β > 1 ) to the stretched exponential one ( β < 1 ). For SM-2 dependence, linear changes of τ T during the Industrial Revolutions period, since ~ 1700, led to the constrained critical behavior P t = P 0 e x p b ' t / T C - t , whereT C ≈ 2216 is the extrapolated year of the infinite population. The link to the 'hyperbolic' von Foerster Doomsday equation is shown. Results are discussed in the context of complex systems physics, the Weibull distribution in extreme value theory, and significant historic and prehistoric issues revealed by the distortions-sensitive analysis.

Glassy relaxation in a de Vries smectic liquid crystal consisting of bent-core molecules

We report experimental investigations of a liquid crystal comprising thiophene-based achiral bent-core banana shaped molecules. The compound exhibits the following phase sequence on cooling: Isotropic (517.4 K), N (514.9 K), de Vries SmA (402 K), SmC. Practically no layer contraction was observed across the SmA to SmC transition, confirming the "de Vries" nature of the SmA phase. Interestingly, the crystallization does not occur on cooling the sample, unlike most other liquid crystals. Instead, the SmC phase undergoes a glass transition at 271 K even at a slow cooling rate. The dielectric spectroscopy studies carried out on the sample reveal the presence of a dielectric mode whose relaxation process is of the Cole-Cole type. The relaxation frequency of the mode was found to drop rapidly with decreasing temperature, confirming the glassy behavior. The variation of relaxation frequency with temperature follows the Vogel-Fulcher-Tammann equation, indicating the fragile glassy nature of the sample. This report identifies a bent-core liquid crystal exhibiting a "de Vries" SmA phase and glassy behavior at lower temperatures.

New paradigm for configurational entropy in glass-forming systems

We show that on cooling towards glass transition configurational entropy exhibits more significant changes than predicted by classic relation. A universal formula according to Kauzmann temperature [Formula: see text] is given: [Formula: see text], where [Formula: see text]. The exponent [Formula: see text] is hypothetically linked to dominated local symmetry. Such a behaviour is coupled to previtreous evolution of heat capacity [Formula: see text] associated with finite temperature singularity. These lead to generalised VFT relation, for which the basic equation is retrieved. For many glass-formers, basic VFT equation may have only an effective meaning. A universal-like reliability of the Stickel operator analysis for detecting dynamic crossover phenomenon is also questioned. Notably, distortions-sensitive and derivative-based analysis focused on previtreous changes of configurational entropy and heat capacity for glycerol, ethanol and liquid crystal is applied.

Dynamics and Pretransitional Effects in C60 Fullerene Nanoparticles and Liquid Crystalline Dodecylcyanobiphenyl (12CB) Hybrid System

The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic-solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the "mixed" ("activated" and "critical") relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau-de Gennes-Ginzburg mesoscopic approach.

‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

Results of dielectric studies in the nematic and isotropic liquid phases of pentylcyanobiphenyl (5CB), a model rod-like liquid crystalline material, are shown. They are based on the discussion of the evolution of dielectric constant ( ε ), its changes under the strong electric field (nonlinear dielectric effect, NDE), and finally, the primary relaxation time. It is shown that changes in ε T and NDE are entirely dominated by the impact of pretransitional fluctuations (pre-nematic and pre-isotropic, respectively) which are associated with the weakly discontinuous character of the isotropic–nematic phase transition. This influence also extends for the low-frequency, ionic species dominated, region. Notable, that the derivative-based and distortions sensitive analysis revealed the tricritical nature of the I-N transition. Although the glass transition in 5CB occurs in the deeply supercooled state at T g ≈ − 68 ° C , the dynamics (changes of the primary relaxation time) follow a previtreous pattern both in the isotropic and in the nematic phase. Finally, the discussion of the ’molecular’ vs. ‘quasi-critical’ characterizations of the isotropic and nematic phases is presented. It shows the evident prevalence of the ‘quasi-critical-picture’, which offers the consistent temperature parameterization in the total tested temperature range.

Activation volume in superpressed glass-formers

In pressurized glass-forming systems, the apparent (changeable) activation volume Va(P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: [Formula: see text], T = const. It is usually assumed that Va(P) = V#(P), where [Formula: see text] or [Formula: see text]. This report shows that Va(P) ≪ V#(P) for P → Pg, where Pg denotes the glass pressure, and the magnitude V#(P) is coupled to the pressure steepness index (the apparent fragility). V#(P) and Va(P) coincides only for the basic Barus dynamics, where Va(P) = Va = const in the given pressure domain, or for P → 0. The simple and non-biased way of determining Va(P) and the relation for its parameterization are proposed. The derived relation resembles Murnaghan - O'Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. The application of the methodology is shown for diisobutyl phthalate (DIIP, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.

Polymer matrix ferroelectric composites under pressure: Negative electric capacitance and glassy dynamics

This report presents the results of high-pressure and broadband dielectric spectroscopy studies in polyvinylidene difluoride (PVDF) and barium strontium titanate (BST) microparticles composites (BST/PVDF). It shows that the Arrhenius behaviour for the temperature-related dynamics under atmospheric pressure is coupled to Super-Arrhenius/Super-Barus isothermal pressure changes of the primary relaxation time. Following these results, an explanation of the unique behaviour of the BST/PVDF composite is proposed. Subsequently, it is shown that when approaching the GPa domain the negative electric capacitance phenomenon occurs.

Nanoparticle-controlled glassy dynamics in nematogen-based nanocolloids

Results of broad-band dielectric spectroscopy studies in liquid crystal (pentylcyanobiphenyl, 5CB)-based nanocolloids are presented. They reveal the strong impact of BaTiO_{3} nanoparticles on dynamics and uniaxial ordering. Studies were carried out in an extreme range of temperatures (∼150 K), including the supercooled nematic phase. For the latter, the unique "pretransitional" effect for dielectric constant on approaching solid state is reported. The distortion-sensitive analysis revealed super-Arrhenius dynamics but associated with critical-like behavior. In the isotropic phase, translational-orientational decoupling, unusual for the high temperature dynamic domain, was detected. It can be directly link to heterogeneities-prenematic fluctuations. The model linking the classical Landau-de Gennes approach with Imry-Ma arguments has been developed to discuss experimental results.

Universal behavior of the apparent fragility in ultraslow glass forming systems

Despite decades of studies on the grand problem of the glass transition the question of well-defined universal patterns, including the key problem of the previtreous behavior of the primary (structural) relaxation time, remains elusive. This report shows the universal previtreous behavior of the apparent fragility, i.e. the steepness index mP (T > Tg) = d log10τ(T)/d( Tg/T). It is evidenced that mP(T) = 1(T - T*), for T → Tg and  T*= Tg - Δ T*. Basing on this finding, the new 3-parameter dependence for portraying the previtreous behavior of the primary relaxation time has been derived: τ(T) = CΩ((T - T*)/T)-Ω × [exp((T - T*)/T)]Ω. The universality of obtained relations is evidenced for glass formers belonging to low molecular weight liquids, polymers (melt and solid), plastic crystals, liquid crystals, resins and relaxors. They exhibit clear preferences either for the VFT or for the critical-like descriptions, if recalled already used modeling. The novel relation can obey even above the dynamic crossover temperature, with the power exponent Ω ranging between ~17 (liquid crystals) to ~57 (glycerol), what may indicate the impact of symmetry on the previtreous effect. Finally, the emerging similarity to the behavior in the isotropic phase of nematic liquid crystals is recalled.

Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer

In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic-A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.Py shows three different dielectric relaxation modes and two glass transition (T_{g}) temperatures: The higher T_{g} is due to the freezing of the molecular motions responsible for the relaxation mode with the lowest frequency (μ_{1L}); the lower T_{g} is due to the motions responsible for the two relaxation modes with highest frequencies (μ_{1H} and μ_{2}), which converge just at their corresponding T_{g}. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (μ_{1L} and μ_{1H}) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (μ_{2}) is cooperative just below some crossover temperature. In terms of fragility, at the glass transition, the ensemble (μ_{1H}+μ_{2}) presents a value of the steepness index and μ_{1L} a different one, meaning that fragility is a property intrinsic to the molecular motion itself. Finally, the steepness index seems to have a universal behavior with temperature for the dielectric relaxation modes of liquid crystal dimers, being almost constant at high temperatures and increasing drastically when cooling the compound down to the glass transition from a temperature about 3/4T_{NI}.

Fractional Debye-Stokes-Einstein behaviour in an ultraviscous nanocolloid: glycerol and silver nanoparticles

One of the major features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This paper presents studies of this phenomenon in glycerol, an accepted molecular glass former, concentrating on the impact of two exogenic factors: high pressures (P) up to the extreme 1.5 GPa and silver (Ag) nanoparticles (NPs). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relationship: σ(T,P)(τ(T,P))(S) = const, linking DC electric conductivity (σ) and primary (alpha, structural) relaxation time (τα). In glycerol and its nanocolloid (glycerol + Ag NPs) at atmospheric pressure only negligible decoupling (S ∼ 1) was detected. However, in the compressed nanocolloid, a well-defined transformation (at P = 1.2 GPa) from S ∼ 1 to the very strongly decoupled dynamics (S ∼ 0.5) occurred. For comparison, in pressurized 'pure' glycerol the stretched shift from S ∼ 1 to S ∼ 0.7 took place. This paper also presents the general discussion of FDSE behavior in ultraviscous liquids, including the new link between the FDSE exponent, fragility and the apparent activation enthalpy and volume.

Fragility and basic process energies in vitrifying systems

The concept of 'fragility' constitutes a central point of the glass transition science serving as the 'universal' metric linking previtreous dynamics of qualitatively distinct systems. Finding the fundamental meaning of fragility is the 'condicio sine qua' for reaching the long expected conceptual breakthrough in this domain. This report shows that fragility is determined by the ratio between two fundamental process energies, viz. the activation enthalpy and activation energy. The reasoning, avoiding any underlying physical model, is supported by the experimental evidence ranging from low molecular weight liquids and polymers to plastic crystals and liquid crystals. All these lead to the new general scaling plot for dynamics of arbitrary glass former. The limited adequacy of broadly used so far semi-empirical relationships between fragility and the activation energy is shown. Results presented remain valid for an arbitrary complex system and collective phenomena if their dynamics is described by the general super-Arrhenius relation.

Divergent dynamics and the Kauzmann temperature in glass forming systems

In the last decade the challenging analysis of previtreous behavior of relaxation time (τ(T)) in ultraviscous low molecular weight liquids led to the conceptual shift of the glass transition physics toward theories not predicting a “finite-temperature” divergence. This “breakthrough” experimental finding was strengthened by the discovery that “dynamic” (i.e. from τ(T) fitting) and “thermodynamic” estimations of the “ideal glass” (Kauzmann) temperature do not match, what in fact questioned its existence. In this report, due to the novel way of analysis based on the transformation of τ(T) experimental data to the activation energy temperature index form, the clear prevalence of the “finite-temperature” divergence is proved. The obtained “dynamic” singular temperatures clearly coincide with “thermodynamic” estimations of the Kauzmann temperature, thus solving also the second mystery. The comprehensive picture was obtained due to the analysis of 55 experimental data-sets, ranging from low molecular weight liquids and polymers to liquid crystal and plastic crystals.

Distortion-sensitive insight into the pretransitional behavior of 4-n-octyloxy-4'-cyanobiphenyl (8OCB)

Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4'-cyanobiphenyl (8OCB) with isotropic (I)-nematic (N)-smectic A (SmA)-crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I-N and N-SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid-fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τ(c)(T-T(C))(-φ). In the immediate vicinity of the I-N transition a novel anomaly has been found: Δτ is proportional to 1/(T - T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the 'background behavior'. Experimental results are confronted with the comprehensive Landau-de Gennes theory based modeling.

A universal description of ultraslow glass dynamics

The dynamics of glass is of importance in materials science but its nature has not yet been fully understood. Here we report that a verification of the temperature dependencies of the primary relaxation time or viscosity in the ultraslowing/ultraviscous domain of glass-forming systems can be carried out via the analysis of the inverse of the Dyre-Olsen temperature index. The subsequent analysis of experimental data indicates the possibility of the self-consistent description of glass-forming low-molecular-weight liquids, polymers, liquid crystals, orientationally disordered crystals and Ising spin-glass-like systems, as well as the prevalence of equations associated with the 'finite temperature divergence'. All these lead to a new formula for the configurational entropy in glass-forming systems. Furthermore, a link to the dominated local symmetry for a given glass former is identified here. Results obtained show a new relationship between the glass transition and critical phenomena.

Disentangling molecular motions involved in the glass transition of a twist-bend nematic liquid crystal through dielectric studies

Broadband dielectric spectroscopy spanning frequencies from 10(-2) to 1.9 × 10(9) Hz has been used to study the molecular orientational dynamics of the glass-forming liquid crystal 1",7"-bis (4-cyanobiphenyl-4'-yl)heptane (CB7CB) over a wide temperature range of the twist-bend nematic phase. In such a mesophase two different relaxation processes have been observed, as expected theoretically, to contribute to the imaginary part of the complex dielectric permittivity. For measurements on aligned samples, the processes contribute to the dielectric response to different extents depending on the orientation of the alignment axis (parallel or perpendicular) with respect to the probing electric field direction. The low-frequency relaxation mode (denoted by μ(1)) is attributed to a flip-flop motion of the dipolar groups parallel to the director. The high-frequency relaxation mode (denoted by μ(2)) is associated with precessional motions of the dipolar groups about the director. The μ(1)-and μ(2)-modes are predominant in the parallel and perpendicular alignments, respectively. Relaxation times for both modes in the different alignments have been obtained over a wide temperature range down to near the glass transition temperature. Different analytic functions used to characterize the temperature dependence of the relaxation times of the two modes are considered. Among them, the critical-like description via the dynamic scaling model seems to give not only quite good numerical fittings, but also provides a consistent physical picture of the orientational dynamics on approaching the glass transition.

Glassy dynamics in the isotropic phase of a smectogenic liquid crystalline compound

The temperature evolution of the primary relaxation time in the isotropic phase of 4-cyano-4'-tetradecylbiphenyl (14CB) above the isotropic-smectic A (I-SmA) transition is discussed. Based on the enthalpy space and distortion-sensitive analysis, the prevalence of the mode coupling theory (MCT) "critical" and "glassy" dynamics is shown. The obtained singular dependence is related to the MCT critical temperature located approximately 48 K below the clearing (I-SmA) temperature. However, a weak but detectable distortion in the immediate vicinity of the transition occurs. It is also shown that the value of the fragile strength coefficient D(T) is characteristic of a very fragile glassy liquid whereas the steepness index m is typical of a strong one. Both magnitudes anomalously change on approaching the I-SmA phase transition. The static permittivity shows the pretransitional effect linked to the temperature of the hypothetical continuous phase transition located approximately 10.2 K below the I-SmA transition.

Enthalpy space analysis of the evolution of the primary relaxation time in ultraslowing systems

For decades the Vogel-Fulcher-Tammann equation has dominated the description of dynamics of the non-Arrhenius behavior in glass forming systems. Recently, this dominance has been questioned. Hecksher et al. [Nat. Phys. 4, 737 (2008)], Elmatad et al. [J. Phys. Chem. B 113, 5563 (2009)], and Mauro et al. [Proc. Natl. Acad. Sci. U.S.A. 106, 19780 (2009)] indicated superiority of several equations showing no divergence at a finite (nonzero) temperature. This paper shows distortion-sensitive and derivative based empirical analysis of the validity of leading equations for portraying the previtreous evolution of primary relaxation time.

Evidence for critical-like behavior in ultraslowing glass-forming systems

Glass transition constitutes one of main problems of condensed matter physics and material engineering that remains unsolved. The common acceptance of the Vogel-Fulcher-Tammann (VFT) equation for portraying the primary relaxation time or shear viscosity indicated a possible phase transition, hidden below the glass transition temperature (T(g)). Recently Hecksher [Nat. Phys. 4, 737 (2008)] delivered strong empirical arguments that VFT description lacks a direct experimental basis and thus theories not predicting a dynamic divergence should be focused on. We present clear evidence for a superiority of critical-like divergent equation τ(T)=τ(0)(T-T(C))(-ϕ) and T(C)<T(g) for liquid crystalline (LC) glass formers and orientationally disordered crystals (ODIC). Such dependence was already known for spin-glasslike systems and the dynamical scaling model, although the latter was hardly explored so far. The pressure-related behavior is also discussed. Results obtained support arguments for the suggested direct link between critical phenomena and vitrification [Tanaka, Nat. Mater. 9, 324 (2010)]. LCs and ODICs may be considered as simple experimental model systems for the structural glass formers group.

Scaling the dynamics of orientationally disordered mixed crystals

The evolution of the primary relaxation time of orientationally disordered (OD) mixed crystals [(CH(3))(2)C(CH(2)OH)(2)](1-X)[(CH(3))C(CH(2)OH)(3)](X), with 0 < X < or = 0.5, on approaching the glass temperature (T(g)) is discussed. The application of the distortion-sensitive, derivative-based procedure revealed a limited adequacy of the Vogel-Fulcher-Tammann parametrization and a superiority of the critical-like description tau proportional to (T - T(C))(-phi(') ), phi(') = 9-11.5, and T(C) approximately T(g) - 10 K. Basing on these results as well as that of Drozd-Rzoska et al. [J. Chem. Phys. 129, 184509 (2008)] the question arises whether such behavior may be suggested as the optimal universal pattern for dynamics in vitrifying OD crystals (plastic crystals). The obtained behavior is in fair agreement with the dynamic scaling model (DSM) [R. H. Colby, Phys. Rev. E 61, 1783 (2000)], originally proposed for vitrifying molecular liquids and polymers. The application of DSM made it possible to estimate the size of the cooperatively rearranging regions ("heterogeneities") in OD phases near T(g).

Universal pattern for the distribution of relaxation times in the isotropic phase of liquid crystalline n-cyanobiphenyls

A universal pattern emerging from the analysis of the distribution of relaxation times in the isotropic phase of liquid crystalline n-alkylcyanobiphenyls (nCB) from 4CB to 14CB is presented. The increase of the length of nCB molecules causes the high-frequency (short-time) branch of the primary relaxation loss curve to approach the form epsilon(f>fpeak) approximately omega(-n), with n-->1/2 in frequency (omega=2pif) or approximately sqrt[t] in time on cooling toward the isotropic-mesophase "clearing" phase-transition temperature (TC). Recently, such behavior was suggested as a hypothetical universal pattern for diverse glass forming organic liquids on approaching the glass temperature [A. I. Nielsen, T. Christensen, B. Jakobsen, K. Niss, N. B. Olsen, R. Richert, and J. C. Dyre, J. Chem. Phys. 130, 154508 (2009)]. The isotropic phase of rodlike liquid crystalline compounds is considered to be an important experimental model system for studying glassy dynamics since it constitutes a link with the model fluid of hard ellipsoids of revolution.

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.

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.

On the glass temperature under extreme pressures

The application of a modified Simon-Glatzel-type relation [Z. Anorg. Allg. Chem. 178, 309 (1929)] for the pressure evolution of the glass temperature is presented, namely, Tg(P)=Tg0[1+DeltaP/(pi+Pg0)]1/bexp[-(DeltaP/c)], where (Tg0,Pg0) are the reference temperature and pressure, DeltaP=P-Pg0, -pi is the negative pressure asymptote, b is the power exponent, and c is the damping pressure coefficient. The discussion is based on the experimental Tg(P) data for magmatic silicate melt albite, polymeric liquid crystal P8, and glycerol. The latter data are taken from Cook et al. [J. Chem. Phys. 100, 5178 (1994)] and from the authors' dielectric relaxation time (tau(P)) measurements, which employs the novel pressure counterpart of the Vogel-Fulcher-Tammann equation: tau(P)=tau0P exp[DPDeltaP/(P0-P)], where DeltaP=P-PSL (PSL is the stability limit hidden under negative pressure), P0 is the estimation of the ideal glass pressure, and D(P) is the isothermal fragility strength coefficient. Results obtained suggest the hypothetical maximum of the Tg(P) curve, which can be estimated due to the application of the supporting derivative-based analysis. A hypothetical common description of glass formers characterized by dTg/dP>0 and dTg/dP<0 coefficients is suggested. Finally, the hypothetical link between molecular and colloidal glass formers is recalled.