Calorimetric versus kinetic glass transitions in viscous monohydroxy alcohols

Tuning the dynamic fragility of acrylic polymers by small molecules: the interplay of molecular structures

This report studied changes in the dynamic fragility (m) of poly(butyl methacrylate) (PBMA) by introducing guest hindered phenols capable of forming two or three intermolecular hydrogen bonds (inter-HBs) per molecule with the host polymer. The small molecules effectively decrease the m value, even if they apparently increase the glass transition temperature (T_g) of mixtures. The reduction in m was confirmed by enthalpy relaxation in two aspects: adding the guest molecule leads to a stronger cooling rate dependence of the limiting fictive temperature together with an apparent increase in aging rate of PBMA hybrids at low concentrations. By varying the molecule size and steric hindrance of the hydroxyl group on the hindered phenols, we clarified that m is primarily governed by the strength of inter-HB interactions, while the T_g value of mixtures depends on a combined effect of additive bulkiness and HB interaction. The anomalous dynamics was further rationalized not only by the HB-induced flexibility balance between side groups and backbone, but also by the reduction of cooperative rearranging sizes and alleviation of long-chain connectivity in such HB-driven hybrids.

Unveiling the strong dependence of the α-relaxation dispersion on mixing thermodynamics in binary glass-forming liquids

Structural α-relaxation dispersion in binary molecular glass forming mixtures with distinct mixing enthalpy ΔHmix was investigated using enthalpic and dielectric relaxation measurements across the entire composition range. This study focused on the dependence of the relaxation dispersion on the mixing thermodynamics by determining the non-exponential exponent β, and its composition dependence. The β values determined by the enthalpic and dielectric relaxations agree well. Remarkably, it is found that the systems with positive enthalpy of mixing (exothermic, ΔHmix >0) have positive deviations in the composition dependence of β from the linear averaging of the two β values of the pure components, while negative deviations are observed for the systems with negative enthalpy of mixing (endothermic, ΔHmix <0). Furthermore, the relation between the non-exponential behaviors and entropy of mixing is discussed, revealing that the positive or negative deviation of β in its composition dependence on mixing is accompanied by the same sign of the excess entropy of mixing relative to the ideal one.

Dipole-dipole correlations and the Debye process in the dielectric response of nonassociating glass forming liquids

The nonexponential shape of the α process observed in supercooled liquids is considered as one of the hallmarks of glassy dynamics and has thus been under study for decades, but is still poorly understood. For a polar van der Waals liquid, we show here-in line with a recent theory-that dipole-dipole correlations give rise to an additional process in the dielectric spectrum slightly slower than the α relaxation, which renders the resulting combined peak narrower than observed by other experimental techniques. This is reminiscent of the Debye-process found in monohydroxy alcohols. The additional peak can be suppressed by weakening the dipole-dipole interaction via dilution with a nonpolar solvent.

Entropic Nature of the Debye Relaxation in Glass-Forming Monoalcohols

The dynamics and thermodynamics of the Debye and structural (α) relaxations in isomeric monoalcohols near the glass transition temperature T_g are explored using dielectric and calorimetric techniques. The α relaxation strength at T_g is found to correlate with the heat capacity increment, but no thermal signals can be detected to link to the Debye relaxation. We also observed that the activation energy of the Debye relaxation in monoalcohols is quantitatively correlated with that of the α relaxation at the kinetic T_g, sharing the dynamic behavior of the Rouse modes found in polymers. The experimental results together with the analogy to the Rouse modes in polymers suggest that the Debye process in monoalcohols is an entropic process manifested by the total dipole fluctuation of the supramolecular structures, which is triggered and driven by the α relaxation.

Dynamics of Confined Short-Chain alkanol in MCM-41 by Dielectric Spectroscopy: Effects of matrix and system Treatments and Filling Factor

The dynamics of n-propanol confined in regular MCM-41 matrix with the pore size Dpore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOH medium in the optimally treated MCM-41 system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states.

The Cooling Rate- and Volatility-Dependent Glass-Forming Properties of Organic Aerosols Measured by Broadband Dielectric Spectroscopy

Glass transitions of secondary organic aerosols (SOA) from liquid/semisolid to solid phase states have important implications for aerosol reactivity, growth, and cloud formation properties. In the present study, glass transition temperatures (T_g) of isoprene SOA components, including isoprene hydroxy hydroperoxide (ISOPOOH), isoprene-derived epoxydiols (IEPOX), 2-methyltetrols, and 2-methyltetrol sulfates, were measured at atmospherically relevant cooling rates (2-10 K/min) by thin film broadband dielectric spectroscopy. The results indicate that 2-methyltetrol sulfates have the highest glass transition temperature, while ISOPOOH has the lowest glass transition temperature. By varying the cooling rate of the same compound from 2 to 10 K/min, the T_g of these compounds increased by 4-5 K. This temperature difference leads to a height difference of 400-800 m in the atmosphere for the corresponding updraft induced cooling rates, assuming a hygroscopicity value (κ) of 0.1 and relative humidity less than 95%. The T_g of the organic compounds was found to be strongly correlated with volatility, and a semiempirical formula between glass transition temperatures and volatility was derived. The Gordon-Taylor equation was applied to calculate the effect of relative humidity (RH) and water content at five mixing ratios on the T_g of organic aerosols. The model shows that T_g could drop by 15-40 K as the RH changes from <5 to 90%, whereas the mixing ratio of water in the particle increases from 0 to 0.5. These results underscore the importance of chemical composition, updraft rates, and water content (RH) in determining the phase states and hygroscopic properties of organic particles.

Mesoscale Aggregates and Dynamic Asymmetry in Ionic Liquids: Evidence from Depolarized Dynamic Light Scattering

Nanoscale structures in ionic liquids (ILs) are usually identified by X-ray or neutron scattering techniques and occur when the alkyl chains of the cations are long enough to show the tendency to segregate into apolar domains. In search of dynamic evidence for these nanostructures, different experimental techniques recently reported bimodal dynamic susceptibility spectra. In all cases, the faster process observed was ascribed to the structural α-relaxation and the slower one to the relaxation of long-lived aggregates. By contrast, we show by depolarized dynamic light scattering (DDLS) experiments on a systematic series of imidazolium-based ILs that the dynamics of the cation and anion are clearly separated for long alkyl chains. Therefore, the observation of a bimodal behavior is not related to any nanostructure but reflects the two-component nature of ILs. Thus, a consistent picture is obtained across different experimental methods, like dielectric and shear mechanical relaxation. Finally, the actual dynamic signature of nanostructures is identified for the first time as a weak feature in some of the DDLS spectra at even lower frequencies.

Peculiar relaxation dynamics of propylene carbonate derivatives

The aim of this work is to analyze in detail the effect of the alkyl chain length on the dynamics of glass-forming propylene carbonate (PC) derivatives. Examined samples are low-molecular weight derivatives of the PC structure, i.e., the 4-alkyl-1,3-dioxolan-2-one series, modified by changing the alkyl substituent from methyl to hexyl. The molecular dynamics (MD) has been analyzed based on experimental data collected from differential scanning calorimetry, broadband dielectric spectroscopy (BDS), X-ray diffraction (XRD), and nuclear magnetic resonance relaxometry measurements as well as MD simulations. The dielectric results show in samples with the propyl- or longer carbon chain the presence of slow Debye-like relaxation with features similar to those found in associative materials. Both XRD and MD reveal differences in the intermolecular structure between PC and 4-butyl-1,3-dioxolan-2-one liquids. Moreover, MD shows that the probability of finding one terminal carbon atom of the side chain of BPC in the vicinity of another carbon atom of the same type is much higher than in the case of PC. It suggests that there is a preference for longer hydrocarbon chains to set themselves close to each other. Consequently, the observed slow-mode peak may be caused by movement of aggregates maintained by van der Waals interactions. Reported herein, findings provide a new insight into the molecular origin of Debye-like relaxation.

Relations between the Structural α-Relaxation and the Johari-Goldstein β-Relaxation in Two Monohydroxyl Alcohols: 1-Propanol and 5-Methyl-2-hexanol

The hydrogen-bonded monohydroxyl alcohols form a large class of glass formers studied more than one hundred years, and still the structure and dynamics have continued to be a research problem. Recent advance suggests a hydrogen-bonded transient supramolecular structure, which is the origin of the Debye relaxation dominating the dielectric loss spectra of many monohydroxyl alcohols. Obscured by the slower Debye relaxation, the structural α-relaxation is either not resolved or showing up as a shoulder and the supposedly universal Johari-Goldstein (JG) β-relaxation is not always observed. Thus, properties of the α-relaxation and the JG β-relaxation as well as the strong connection between the two relaxations generally observed in other classes of glass formers are not commonly known in the monohydroxyl alcohols. Notwithstanding, extremely broadband dielectric relaxation and high-precision light scattering experiments published recently have resolved the α-relaxation and a secondary relaxation in two archetypal monohydroxyl alcohols, 1-propanol and 5-methyl-2-hexanol (5M2H) by Gabriel et al. We analyzed their experimental data and applied the Coupling Model to show that the secondary relaxations in 1-propanol and 5M2H are JG β-relaxations with strong connection to the α-relaxation. The result is novel because it is not known before whether the secondary relaxations of these two monohydroxyl alcohols are JG β-relaxation involving the entire molecule or are intramolecular relaxations. On the basis of this conclusion, we predict that the secondary relaxation is pressure-dependent and the ratio τ_β( T, P)/τ_α( T, P) is invariant to variations of P and T, whereas τ_α( T, P) is maintained constant and provided that the frequency dispersion of the α-relaxation is also constant. The prediction is compared with the dielectric data of 5M2H at elevated pressures. On the basis of the identification of monohydroxyl alcohols as short-chain polymeric liquids by others, an explanation of the stronger T and P dependences of τ_α( T, P) than the Debye relaxation time τ_D( T, P) is given.

Deviations of dynamic parameters characterizing enthalpic and dielectric relaxations in glass forming alkyl phosphates

In our recent study [T. Wu et al., J. Chem. Phys. 147, 134501 (2017)], an alkyl phosphate glass former was studied and it suggested that the enthalpy relaxation involving the motions of all parts of the molecule is global, while the dielectric relaxation detects the local rotation of the polar core. In this work, we study a series of trialkyl phosphates using calorimetric and dielectric measurements over a wide temperature range. The results indicate a departure of the dielectric fragility indexes from the enthalpic ones as the length of the branch chain increases in the trialkyl phosphates. The Kirkwood correlation factor (g _k ) is found to coincide at ∼0.6 at glass transition temperature (T _g ) from triethyl phosphate to tributyl phosphate, indicating a similar structural alignment. The enthalpic relaxation serving as the more fundamental relaxation relevant to the structural relaxation is confirmed. Strikingly, we observed the relation of T _g to the chain length in alkyl phosphates, revealing a minimum T _g behavior, and its explanation assists in the understanding of the glass transition in relation to the structure of the glass-formers.

Organic Glasses with Tunable Liquid-Crystalline Order

Liquid crystals (LCs) are known to undergo rapid ordering transitions with virtually no hysteresis. We report a remarkable counterexample, itraconazole, where the nematic to smectic transition is avoided at a cooling rate exceeding 20  K/s. The smectic order trapped in a glass is the order reached by the equilibrium liquid before the kinetic arrest of the end-over-end molecular rotation. This is attributed to the fact that smectic ordering requires orientational ordering and suggests a general condition for preparing organic glasses with tunable LC order for electronic applications.

Modifying hydrogen-bonded structures by physical vapor deposition: 4-methyl-3-heptanol

We prepared films of 4-methyl-3-heptanol by vapor depositing onto substrates held at temperatures between T_dep = 0.6T_g and T_g, where T_g is the glass transition temperature. Using deposition rates between 0.9 and 6.0 nm/s, we prepared films about 5 μm thick and measured the dielectric properties via an interdigitated electrode cell onto which films were deposited. Samples prepared at T_dep = T_g display the dielectric behavior of the ordinary supercooled liquid. Films deposited at lower deposition temperatures show a high dielectric loss upon heating toward T_g, which decreases by a factor of about 12 by annealing at T_g = 162 K. This change is consistent with either a drop of the Kirkwood correlation factor, g_k, by a factor of about 10, or an increase in the dielectric relaxation times, both being indicative of changes toward ring-like hydrogen-bonded structure characteristic of the ordinary liquid. We rationalize the high dielectric relaxation amplitude in the vapor deposited glass by suggesting that depositions at low temperature provide insufficient time for molecules to form ring-like supramolecular structures for which dipole moments cancel. Surprisingly, above T_g of the ordinary liquid, these vapor deposited films fail to completely recover the dielectric properties of the liquid obtained by supercooling. Instead, the dielectric relaxation remains slower and its amplitude much higher than that of the equilibrium liquid state, indicative of a structure that differs from the equilibrium liquid up to at least T_g + 40 K.

Structural and Dielectric Relaxations in Vitreous and Liquid State of Monohydroxy Alcohol at High Pressure

2-Ethyl-1-hexanol monoalcohol is a well-known molecular glassformer, which for a long time attracts attention of researchers. As in all other monohydroxy alcohols, its dielectric relaxation reveals two distinct relaxation processes attributed to the structural relaxation and another more intense process, which gives rise to a low-frequency Debye-like relaxation. In this monoalcohol, the frequency separation between these two processes reaches an extremely high value of 3 orders of magnitude, which makes this substance a rather convenient object for studies of mechanisms (supposedly common to all monoalcohols) leading to vitrification of this type of liquids. In this work, we apply two experimental techniques, dielectric spectroscopy and ultrasonic measurements (in both longitudinal and transverse polarizations) at high pressure, to study interference between different relaxation mechanisms occurring in this liquid, which could shed light on both structural and dielectric relaxation processes observed in a supercooled liquid and a glass state. Application of high pressure in this case leads to the simplification of the frequency spectrum of dielectric relaxation, where only one asymmetric feature is observed. Nonetheless, the maximum attenuation of the longitudinal wave in ultrasonic experiments at high pressure is observed at temperatures ≈50 K above the corresponding temperature for the transverse wave. This might indicate different mechanisms of structural relaxation in shear and bulk elasticities in this liquid.

Relaxation dynamics in the strong chalcogenide glass-former of Ge22Se78

The enthalpy relaxation is performed in the glassy Ge₂₂Se₇₈ to understand the dynamic behaviors. The structure of the glass is examined by X-ray diffraction and Raman spectra. The dynamic parameters such as the fragility, stretching exponent and non-linear factor are determined. A low fragility of m = 27 is exhibited for the chalcogenide, however, the stretching exponent is found not to have a larger value. The enthalpy relaxation spectra are constructed for various glass formers, and a relationship between the fragility and the symmetry of the spectra is demonstrated. The dynamic results are used to evaluate the structure of the Ge₂₂Se₇₈ glass.

Glass formability in medium-sized molecular systems/pharmaceuticals. I. Thermodynamics vs. kinetics

Scrutinizing critical thermodynamic and kinetic factors for glass formation and the glass stability of materials would benefit the screening of the glass formers for the industry of glassy materials. The present work aims at elucidating the factors that contribute to the glass formation by investigating medium-sized molecules of pharmaceuticals. Glass transition related thermodynamics and kinetics are performed on the pharmaceuticals using calorimetric, dielectric, and viscosity measurements. The characteristic thermodynamic and kinetic parameters of glass transition are found to reproduce the relations established for small-molecule glass formers. The systematic comparison of the thermodynamic and kinetic contributions to glass formation reveals that the melting-point viscosity is the crucial quantity for the glass formation. Of more interest is the finding of a rough correlation between the melting-point viscosity and the entropy of fusion normalized by the number of beads of the pharmaceuticals, suggesting the thermodynamics can partly manifest its contribution to glass formation via kinetics.

Revisiting the glass transition and dynamics of supercooled benzene by calorimetric studies

The glass transition and dynamics of benzene are studied in binary mixtures of benzene with five glass forming liquids, which can be divided into three groups: (a) o-terphenyl and m-xylene, (b) N-butyl methacrylate, and (c) N,N-dimethylpropionamide and N,N-diethylformamide to represent the weak, moderate, and strong interactions with benzene. The enthalpies of mixing, ΔH(mix), for the benzene mixtures are measured to show positive or negative signs, with which the validity of the extrapolations of the glass transition temperature T(g) to the benzene-rich regions is examined. The extrapolations for the T(g) data in the mixtures are found to converge around the point of 142 K, producing T(g) of pure benzene. The fragility m of benzene is also evaluated by extrapolating the results of the mixtures, and a fragility m ∼ 80 is yielded. The obtained T(g) and m values for benzene allow for the construction of the activation plot in the deeply supercooled region. The poor glass formability of benzene is found to result from the high melting point, which in turn leads to low viscosity in the supercooled liquid.

Dynamics of glass-forming liquids. XVII. Dielectric relaxation and intermolecular association in a series of isomeric octyl alcohols

It is well established that many mono-hydroxy alcohols show an extra relaxation process of the Debye type in addition to the signatures of primary and secondary structural relaxations, which is observed only in dielectric spectroscopy and related techniques. In order to gain further insight into the nature of this Debye peak, we study the linear and nonlinear dielectric behavior of a series of isomeric octyl alcohols and of mixtures of n-propanol with one of the octanols. These samples display systematic variations of the Debye peak intensity and concomitant changes in the Kirkwood correlation factor gK from 0.1 to 4, indicative of different equilibrium constants, K(c∕r), that characterize the populations of non-polar ring and polar open chain structures. For cases where K(c∕r) is not too far from unity, we find that a high electric field shifts K(c∕r) towards more chains, and that the accompanying change in the end-to-end vector of hydrogen-bond connected structures occurs on the Debye time scale. The results suggest that gK is correlated with the spectral separation of the Debye and primary structural peaks, as both features depend on steric hindrance of chain flexibility or bond rotation barriers and on average chain lengths. Based on the complex dynamics of supercooled mono-hydroxy alcohols with three relaxation peaks that cover many orders of magnitude in frequency, it is argued that a frequency dependent gK may be required for assessing the average orientational correlations within hydrogen-bonded structures correctly.

Effect of compression on the relationship between viscosity and dielectric relaxation time in hydrogen-bonded primary alcohols

High pressure viscosity and dielectric measurements were carried out on two monohydroxy alcohols, 2-ethyl-1-hexanol and 5-methyl-2-hexanol, at room temperature. Analysis of the dielectric relaxation times versus viscosity revealed the breakdown of the Einstein-Debye relation above some characteristic pressure. The failure of the Einstein-Debye relation is a manifestation of pressure induced changes of supramolecular hydrogen bonded structures which occur in these liquids.

Watching hydrogen-bonded structures in an alcohol convert from rings to chains

In hydrogen-bonded liquids including monohydroxy alcohols, the prominent Debye process that often dominates the dielectric relaxation behavior is associated with hydrogen bonding, but its microscopic origin has remained unclear to date. High electric field impedance spectroscopy on 5-methyl-3-heptanol reveals a field-induced change in the Kirkwood-Fröhlich correlation factor g(K), viewed as evidence for an electric field driven conversion from ring- to chain-type hydrogen-bonded structures. The concomitant rearrangement of the chain structure is observed to occur on the time scale of the Debye process, suggesting that the Debye peak of monohydroxy alcohols originates from a fluctuation of the net dipole moment via g(K) of the chain structures on a time scale that is largely controlled by viscosity.

Dynamics of glass-forming liquids. XIV. A search for ultraslow dielectric relaxation in glycerol

A recent dielectric study of various polyalcohols reported on the general occurrence of an ultraslow process with Debye type character in hydrogen bonded liquids [R. Bergman, H. Jansson, and J. Swenson, J. Chem. Phys. 132, 044504 (2010)], whereas previous work suggested that such behavior is specific to monoalcohols only. Clarifying this issue is highly relevant for assessing models aimed at rationalizing these modes that are slower than the primary structural relaxation and associated with a single time constant. To this end, the dielectric relaxation of glycerol is measured at different electrode distances with high accuracy. In this manner, electrode polarization can be separated from the dielectric signals intrinsic in the supercooled liquid. In the frequency range below the loss peak frequency omega(max) of the alpha-process, only dc-conductivity is required to understand the dielectric properties of supercooled glycerol within a margin of epsilon(") approximately +/-0.1 and thus no indication of an ultraslow peak is found. More quantitatively, any dielectric Debye like mode located around 10(-5)omega(max) would need to have an amplitude smaller than 0.4% of that of the primary dielectric process to be consistent with the present findings, in contrast to previous claims of >50%.

Molecular dynamics changes induced by solvent in 2-ethyl-1-hexanol

Apart from other classes of materials, supramolecular structures may exist in H-bonded liquids due to the existence of hydrogen bonding. The dynamics of these structures remains one of the most exciting topics of interest of modern science because of its crucial meaning for the behavior of water and its participation in biological processes. A special group of these liquids form monohydroxy alcohols due to their similarity to water, their ability to vitrification, and the existence of the Debye relaxation process in dielectric loss spectra reflecting the dynamics of H-bond structures. Dynamics of these structures can be studied by changes of thermodynamic conditions, by immersion of the liquid into the constraint geometry, and by dilution in a nonassociated solvent. Herein we studied the behavior of relaxation dynamics of mixtures of 2-ethyl-1-hexanol with bromobutane using broadband dielectric spectroscopy. Analysis of the results exhibits the existence of crossover in temperature dependence of static permittivity of the Debye process at some particular temperature T(c). This temperature shifts to lower values with increasing concentration of bromobutane. Moreover, below some "critical" concentration of alcohol in the mixture the shape of the Debye process loses exponentiality and the temperature dependence of relaxation times starts to change. This change was illuminated based on the analysis of the steepness index. For the lowest concentration, the value of this parameter becomes the same as the value of the steepness index of faster relaxation, called process II, of pure alcohol at ambient pressure. The observed change in relaxation dynamics with lowering concentration of alcohol is astonishingly similar to the behavior observed in the same material at elevated pressure. A possible origin of these similarities is also discussed.

Communication: Thermodynamic scaling of the Debye process in primary alcohols

The molecular dynamics of hydrogen-bonded liquids usually does not satisfy the thermodynamic scaling. However, very recently, two opposite conclusions about validity of thermodynamical scaling in monohydroxy alcohol, 2-ethyl-1-hexanol, were presented by Reiser et al. [J. Chem. Phys. 132, 181101 (2010)] and Fragiadakis et al. [J. Chem. Phys. 132, 144505 (2010)]. In this communication we present new experimental results that can explain this ostensible contradiction.