Hydrogen Bonding Slows Down Surface Diffusion of Molecular Glasses

Mechanisms of dissolution and crystallization of amorphous glibenclamide

Amorphous solid dispersions enhance the dissolution and oral bioavailability of poorly water-soluble drugs. However, the link between polymer properties and formulation performance has not been fully clarified yet. We studied the effect of hydroxypropyl cellulose (HPC) polymers molecular weight (Mw) on the storage stability, dissolution kinetics and supersaturation stability of spray-dried amorphous glibenclamide (GLB) formulations. The solid-state stability of amorphous GLB during storage was significantly enhanced by both the 40 kDa (HPC-SSL) and 84 kDa (HPC-L) polymers, regardless of Mw differences. In contrast, HPC-SSL maintained significantly higher aqueous drug concentrations during dissolution, compared to HPC-L (its higher Mw analogue). Dedicated dissolution experiments, in situ optical microscopy and solid-state characterization revealed that aqueous drug concentrations were determined by the interplay between crystallization inhibition, drug ionization, wetting and solubilization effects: (1) HPC prevents surface nucleation, hence inhibiting crystallization, (2) intestinal colloids (bile salts and phospholipids) increase supersaturated drug concentrations via wetting and solubilization effects and (3) pH and drug ionization severely impact the degree of supersaturation. The better performance of the lower Mw HPC-SSL was due to its superior inhibition of surface crystallization during dissolution. These insights into the molecular mechanisms of dissolution and crystallization of amorphous solids provide foundation for rational formulation development.

Correlating Ultrastability with Fragility and Surface Mobility in Vapor Deposited Tetrahedral Glasses

Several experiments on molecular and metallic glasses have shown that the ability of vapor deposition to produce ultrastable glasses is correlated to their structural and thermodynamic properties. Here we investigate the vapor deposition of a class of tetrahedral materials (including silicon and water) via molecular dynamics simulations of the generalized Stillinger-Weber potential. By changing a single parameter that controls the local tetrahedrality, we show that the emergence of ultrastable behavior is correlated with an increase in the fragility of the model. At the same time, while the mobility of the surface compared to the bulk shows only slight changes at low temperature, with increasing the tetrahedrality, it displays a significant enhancement toward the glass transition temperature. Our results point toward a strong connection between bulk dynamics, surface dynamics, and glass-ultrastability ability in this class of materials.

Effect of molecular permanent dipole moment on guest aggregation and exciton quenching in phosphorescent organic light emitting diodes

This study explores the effect of molecular permanent dipole moment (PDM) on aggregation of guest molecules in phosphorescent host-guest organic light-emitting diodes (OLEDs). Through a combination of photoluminescence measurements, high-angle annular dark-field scanning transmission electron microscopy analysis, and an Ising model based physical vapor-deposition simulation, we show that higher PDM of tris[2-phenylpyridinato-C2,N]iridium(III) guest can actually lead to a reduced aggregation relative to tris[bis[2-(2-pyridinyl-N)phenyl-C] (acetylacetonato)iridium(III) when doped into a non-polar host 1,3,5-tris(carbazol-9-yl)benzene. This study further explores the effect of host polarity by using a polar host 3',5'-di(carbazol-9-yl)-[1,1'-biphenyl]-3,5-dicarbonitrile, and it is shown that the polar host leads to reduced guest aggregation. This study provides a comprehensive understanding of the impact of molecular PDM on OLED material efficiency and stability, providing insights for optimizing phosphorescent OLED materials.

Dielectric Study of n-Propanol during Physical Vapor Deposition: No Surface Mobility and No Kinetic Stability

Dielectric relaxation experiments have been performed on n-propanol (NPOH) films during physical vapor deposition at temperatures above and below its glass transition, Tg = 97 K. The results for NPOH are compared with those of analogous experiments on methyl-m-toluate (MMT) and 2-methyltetrahydrofuran (MTHF), with all three deposited at the same reduced temperature, 0.82Tg. While MMT and MTHF display clear signs of a highly mobile surface layer, no such feature is observed for NPOH. The existence of this in situ observed mobile surface layer correlates perfectly with the material's ability to form kinetically stable glasses, as NPOH differs from MMT and MTHF by not displaying kinetic stability.

ROY Crystallization on Poly(ethylene) Fibers, a Model for Bed Net Crystallography

Many long-lasting insecticidal bed nets for protection against disease vectors consist of poly(ethylene) fibers in which insecticide is incorporated during manufacture. Insecticide molecules diffuse from within the supersaturated polymers to surfaces where they become bioavailable to insects and often crystallize, a process known as blooming. Recent studies revealed that contact insecticides can be highly polymorphic. Moreover, insecticidal activity is polymorph-dependent, with forms having a higher crystal free energy yielding faster insect knockdown and mortality. Consequently, the crystallographic characterization of insecticide crystals that form on fibers is critical to understanding net function and improving net performance. Structural characterization of insecticide crystals on bed net fiber surfaces, let alone their polymorphs, has been elusive owing to the minute size of the crystals, however. Using the highly polymorphous compound ROY (5-methyl-2-[(2-nitrophenyl)-amino]thiophene-3-carbonitrile) as a proxy for insecticide crystallization, we investigated blooming and crystal formation on the surface of extruded poly(ethylene) fibers containing ROY. The blooming rates, tracked from the time of extrusion, were determined by UV-vis spectroscopy after successive washes. Six crystalline polymorphs (of the 13 known) were observed on poly(ethylene) fiber surfaces, and they were identified and characterized by Raman microscopy, scanning electron microscopy, and 3D electron diffraction. These observations reveal that the crystallization and phase behavior of polymorphs forming on poly(ethylene) fibers is complex and dynamic. The characterization of blooming and microcrystals underscores the importance of bed net crystallography for the optimization of bed net performance.

Crystallization of Amorphous Nimesulide: The Relationship between Crystal Growth Kinetics and Liquid Dynamics

Understanding crystallization and its correlations with liquid dynamics is relevant for developing robust amorphous pharmaceutical solids. Herein, nimesulide, a classical anti-inflammatory agent, was used as a model system for studying the correlations between crystallization kinetics and molecular dynamics. Kinetic parts of crystal growth (u_kin) of nimesulide exhibited a power law dependence upon the liquid viscosity (η) as u_kin~η^-0.61. Bulk molecular diffusivities (D_Bulk) of nimesulide were predicted by a force-level statistical-mechanical model from the α-relaxation times, which revealed the relationship as u_kin~D_bulk^0.65. Bulk crystal growth kinetics of nimesulide in deeply supercooled liquid exhibited a fragility-dependent decoupling from τ_α. The correlations between growth kinetics and α-relaxation times predicted by the Adam-Gibbs-Vogel equation in a glassy state were also explored, for both the freshly made and fully equilibrated glass. These findings are relevant for the in-depth understanding and prediction of the physical stability of amorphous pharmaceutical solids.

Kinetics of Surface Enrichment of a Polymer in a Glass-Forming Molecular Liquid

X-ray photoelectron spectroscopy has been used to measure the surface concentration and the surface enrichment kinetics of a polymer in a glass-forming molecular liquid. As a model, the bulk-miscible system of maltitol-polyvinylpyrrolidone (PVP) was studied. The PVP concentration is significantly higher at the liquid/vapor interface than in the bulk by up to a factor of 170, and the effect increases with its molecular weight. At a freshly created liquid/vapor interface, the concentration of PVP gradually increases from the bulk value at a rate controlled by bulk diffusion. The polymer diffusion coefficient obtained from the kinetics of surface enrichment agrees with that calculated from viscosity and the Stokes-Einstein equation. Our finding allows prediction of the rate at which the surface composition equilibrates in an amorphous material after milling, fracture, and a change in ambient temperature.

Physical stability and dissolution behaviors of amorphous pharmaceutical solids: Role of surface and interface effects

Amorphous pharmaceutical solids (APS) are single- or multi-component systems in which drugs exist in high-energy states with long-range disordered molecular packing. APSs have become one of the most effective and widely used pharmaceutical delivery approaches for poorly water-soluble drugs in the last several decades. Considerable efforts have been made to investigate the physical stability and dissolution behaviors of APSs, however, the underlying mechanisms remain imperfectly understood. Recent studies reveal that surface and interface properties of APSs could strongly affect the physical stability and dissolution behaviors. This paper provides a comprehensive overview of recent studies focusing on the physical stability and dissolution behaviors of APSs from both surface and interface perspectives. We highlight the role of surface or interface properties in nucleation, crystal growth, phase separation, dissolution, and supersaturation. Meanwhile, the challenges and scope of research on surface and interface properties in the future are also briefly discussed. This review contributes to a better understanding of the surface- and interface-facilitated processes, which will provide more efficient and rational guidance for the design of APSs.

Liquid and Glass Phases of an Alkylguanidinium Sulfonate Hydrogen-Bonded Organic Framework

Glassy phases of framework materials feature unique and tunable properties that are advantageous for gas separation membranes, solid electrolytes, and phase-change memory applications. Here, we report a new guanidinium organosulfonate hydrogen-bonded organic framework (HOF) that melts and vitrifies below 100 °C. In this low-temperature regime, non-covalent interactions between guest molecules and the porous framework become a dominant contributor to the overall stability of the structure, resulting in guest-dependent melting, glass, and recrystallization transitions. Through simulations and X-ray scattering, we show that the local structures of the amorphous liquid and glass phases resemble those of the parent crystalline framework.

Surface diffusion of a glassy discotic organic semiconductor and the surface mobility gradient of molecular glasses

Surface diffusion has been measured in the glass of an organic semiconductor, MTDATA, using the method of surface grating decay. The decay rate was measured as a function of temperature and grating wavelength, and the results indicate that the decay mechanism is viscous flow at high temperatures and surface diffusion at low temperatures. Surface diffusion in MTDATA is enhanced by 4 orders of magnitude relative to bulk diffusion when compared at the glass transition temperature T_g. The result on MTDATA has been analyzed along with the results on other molecular glasses without extensive hydrogen bonds. In total, these systems cover a wide range of molecular geometries from rod-like to quasi-spherical to discotic and their surface diffusion coefficients vary by 9 orders of magnitude. We find that the variation is well explained by the existence of a steep surface mobility gradient and the anchoring of surface molecules at different depths. Quantitative analysis of these results supports a recently proposed double-exponential form for the mobility gradient: log D(T, z) = log D_v(T) + [log D₀ - log D_v(T)]exp(-z/ξ), where D(T, z) is the depth-dependent diffusion coefficient, D_v(T) is the bulk diffusion coefficient, D₀ ≈ 10^-8 m²/s, and ξ ≈ 1.5 nm. Assuming representative bulk diffusion coefficients for these fragile glass formers, the model reproduces the presently known surface diffusion rates within 0.6 decade. Our result provides a general way to predict the surface diffusion rates in molecular glasses.

Surface Diffusion Is Controlled by Bulk Fragility across All Glass Types

Surface diffusion is vastly faster than bulk diffusion in some glasses, but only moderately enhanced in others. We show that this variation is closely linked to bulk fragility, a common measure of how quickly dynamics is excited when a glass is heated to become a liquid. In fragile molecular glasses, surface diffusion can be a factor of 10^{8} faster than bulk diffusion at the glass transition temperature, while in the strong system SiO_{2}, the enhancement is a factor of 10. Between these two extremes lie systems of intermediate fragility, including metallic glasses and amorphous selenium and silicon. This indicates that stronger liquids have greater resistance to dynamic excitation from bulk to surface and enables prediction of surface diffusion, surface crystallization, and formation of stable glasses by vapor deposition.

Structures of glass-forming liquids by x-ray scattering: Glycerol, xylitol, and D-sorbitol

Synchrotron x-ray scattering has been used to investigate three liquid polyalcohols of different sizes (glycerol, xylitol, and D-sorbitol) from above the glass transition temperatures T_g to below. We focus on two structural orders: the association of the polar OH groups by hydrogen bonds (HBs) and the packing of the non-polar hydrocarbon groups. We find that the two structural orders evolve very differently, reflecting the different natures of bonding. Upon cooling from 400 K, the O⋯O correlation at 2.8 Å increases significantly in all three systems, indicating more HBs, until kinetic arrests at T_g; the increase is well described by an equilibrium between bonded and non-bonded OH with ΔH = 9.1 kJ/mol and ΔS = 13.4 J/mol/K. When heated above T_g, glycerol loses the fewest HBs per OH for a given temperature rise scaled by T_g, followed by xylitol and by D-sorbitol, in the same order the number of OH groups per molecule increases (3, 5, and 6). The pair correlation functions of all three liquids show exponentially damped density modulations of wavelength 4.5 Å, which are associated with the main scattering peak and with the intermolecular C⋯C correlation. In this respect, glycerol is the most ordered with the most persistent density ripples, followed by D-sorbitol and by xylitol. Heating above T_g causes faster damping of the density ripples with the rate of change being the slowest in xylitol, followed by glycerol and by D-sorbitol. Given the different dynamic fragility of the three liquids (glycerol being the strongest and D-sorbitol being the most fragile), we relate our results to the current theories of the structural origin for the difference. We find that the fragility difference is better understood on the basis of the thermal stability of HB clusters than that of the structure associated with the main scattering peak.

Design of a homologous series of molecular glassformers

We design and synthesize a set of homologous organic molecules by taking advantage of facile and tailorable Suzuki cross coupling reactions to produce triarylbenzene derivatives. By adjusting the number and the arrangement of conjugated rings, the identity of heteroatoms, lengths of fluorinated alkyl chains, and other interaction parameters, we create a library of glassformers with a wide range of properties. Measurements of the glass transition temperature (T_g) show a power-law relationship between T_g and molecular weight (M_W), with of the molecules, with an exponent of 0.3 ± 0.1, for T_g values spanning a range of 300-450 K. The trends in indices of refraction and expansion coefficients indicate a general increase in the glass density with M_W, consistent with the trends observed in T_g variations. A notable exception to these trends was observed with the addition of alkyl and fluorinated alkyl groups, which significantly reduced T_g and increased the dynamical fragility (which is otherwise insensitive to M_W). This is an indication of reduced density and increased packing frustrations in these systems, which is also corroborated by the observations of the decreasing index of refraction with increasing length of these groups. These data were used to launch a new database for glassforming materials, glass.apps.sas.upenn.edu.

Glass Dynamics Deep in the Energy Landscape

When a liquid is cooled, progress down the energy landscape is arrested near the glass transition temperature T_g. In principle, lower energy states can be accessed by waiting for further equilibration, but the rough energy landscape of glasses quickly leads to kinetics on geologically slow time scales below T_g. Over the past decade, progress has been made probing deeper into the energy landscape via several techniques. By looking at bulk and surface diffusion, using layered deposition that promotes equilibration, imaging glass surfaces with faster dynamics below T_g, and optically exciting glasses, experiments have moved into a regime of ultrastable, low energy glasses that was difficult to access in the past. At the same time, both simulations and energy landscape theory based on a random first order transition (RFOT) have tackled systems that include surfaces, optical excitation, and interfacial dynamics. Here we review some of the recent experimental work, and how energy landscape theory illuminates glassy dynamics well below the glass transition temperature by making direct connections between configurational entropy, energy landscape barriers, and the resulting dynamics.

Factors correlating to enhanced surface diffusion in metallic glasses

The enhancement of surface diffusion (D_S) over the bulk (D_V) in metallic glasses (MGs) is well documented and likely to strongly influence the properties of glasses grown by vapor deposition. Here, we use classical molecular dynamics (MD) simulations to identify different factors influencing the enhancement of surface diffusion in MGs. MGs have a simple atomic structure and belong to the category of moderately fragile glasses that undergo pronounced slowdown of bulk dynamics with cooling close to the glass transition temperature (T_g). We observe that D_S exhibits a much more moderate slowdown compared to D_V when approaching T_g, and D_S/D_V at T_g varies by two orders of magnitude among the MGs investigated. We demonstrate that both the surface energy and the fraction of missing bonds for surface atoms show good correlation to D_S/D_V, implying that the loss of nearest neighbors at the surface directly translates into higher mobility, unlike the behavior of network-bonded and hydrogen-bonded organic glasses. Fragility, a measure of the slowdown of bulk dynamics close to T_g, also correlates to D_S/D_V, with more fragile systems having larger surface enhancement of mobility. The deviations observed in the fragility-D_S/D_V relationship are shown to be correlated to the extent of segregation or depletion of the mobile element at the surface. Finally, we explore the relationship between the diffusion pre-exponential factor (D₀) and the activation energy (Q) and compare it to a ln(D₀)-Q correlation previously established for bulk glasses, demonstrating similar correlations from MD as in the experiments and that the surface and bulk have very similar ln(D₀)-Q correlations.

Diffusion-controlled and `diffusionless' crystal growth: relation between liquid dynamics and growth kinetics of griseofulvin

Understanding how liquid dynamics govern crystallization is critical for maintaining the physical stability of amorphous pharmaceutical formulations. In the present study, griseofulvin (GSF), a classic antifungal drug, was used as the model system to investigate the correlations between crystal growth kinetics and liquid dynamics. The temperature dependence of the kinetic part of the bulk crystal growth in a supercooled liquid of GSF was weaker than that of the structural relaxation time τα and scaled as τα −0.69. In the glassy state, GSF exhibited the glass-to-crystal (GC) growth behavior, whose growth rate was too fast to be under the control of the α-relaxation process. Moreover, from the perspective of τα, the GC growth of GSF also satisfied the general condition for GC growth to exist: D/u < 7 pm, where D is the diffusion coefficient and u the speed of crystal growth. Also compared were the fast surface crystal growth rates u s and surface relaxation times τsurface predicted by the random first-order transition theory. Here, the surface crystal growth rate u s of GSF exhibited a power-law dependence upon the surface structural relaxation time: u s ∝ τsurface −0.71, which was similar to that of the bulk growth rate and τα. These findings are important for understanding and predicting the crystallization of amorphous pharmaceutical solids both in the bulk and at the surface.

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.

Physical stability of amorphous pharmaceutical solids: Nucleation, crystal growth, phase separation and effects of the polymers

Compared to their crystalline forms, amorphous pharmaceutical solids present marvelous potential and advantages for effectively improving the oral bioavailability of poorly water-soluble drugs. A central issue in developing amorphous pharmaceutical solids is the stability against crystallization, which is particularly important for maintaining their advantages in solubility and dissolution rate. This review provides a comprehensive overview of recent studies focusing on the physical stability of amorphous pharmaceutical solids affected by nucleation, crystal growth, phase separation and the addition of polymers. Moreover, we highlight the novel technologies and theories in the field of amorphous pharmaceutical solids. Meanwhile, the challenges and strategies in maintaining the physical stability of amorphous pharmaceutical solids are also discussed. With a better understanding of physical stability, the more robust amorphous pharmaceutical formulations with desired pharmaceutical performance would be easier to achieve.

Surface diffusion in glasses of rod-like molecules posaconazole and itraconazole: effect of interfacial molecular alignment and bulk penetration

The method of surface grating decay has been used to measure surface diffusion in the glasses of two rod-like molecules posaconazole (POS) and itraconazole (ITZ). Although structurally similar antifungal medicines, ITZ forms liquid-crystalline phases while POS does not. Surface diffusion in these systems is significantly slower than in the glasses of quasi-spherical molecules of similar volume when compared at the glass transition temperature T_g. Between the two systems, ITZ has slower surface diffusion. These results are explained on the basis of the near-vertical orientation of the rod-like molecules at the surface and their deep penetration into the bulk where mobility is low. For molecular glasses without extensive hydrogen bonds, we find that the surface diffusion coefficient at T_g decreases smoothly with the penetration depth of surface molecules and the trend has the double-exponential form for the surface mobility gradient observed in simulations. This supports the view that these molecular glasses have a similar mobility vs. depth profile and their different surface diffusion rates arise simply from the different depths at which molecules are anchored. Our results also provide support for a previously observed correlation between the rate of surface diffusion and the fragility of the bulk liquid.

Direct Measurement of Lateral Molecular Diffusivity on the Surface of Supersaturated Amorphous Solid Dispersions by Atomic Force Microscopy

Quantifying molecular surface diffusivity is of broad interest in many different fields of science and technology. In this study, the method of surface grating decay is utilized to investigate the surface diffusion of practical relevant amorphous solid dispersions of indomethacin and the polymeric excipient Soluplus (a polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer) at various polymer concentrations (1-20% w/w). The study confirms that measuring surface diffusivity below the system's glass transition temperature is possible with a simplified atomic force microscopy setup. Results highlight a striking polymer influence on the surface diffusivity of drug molecules at low polymer concentrations and a turnover point to a polymer dominated diffusion at around three percent (w/w) polymer concentration. The surface diffusion measurements further correlate well with the observed increase in physical stability of the system as measured by X-ray powder diffraction. These findings are of vital interest in both the applied use and fundamental understanding of amorphous solid dispersions.

Molecular Dynamics Simulation of the Capillary Leveling of a Glass-Forming Liquid

Motivated by recent experimental studies probing (i) the existence of a mobile layer at the free surface of glasses and (ii) the capillary leveling of polymer nanofilms, we study the evolution of square-wave patterns at the free surface of a generic glass-forming binary Lennard-Jones mixture over a wide temperature range, by means of molecular dynamics simulations. The pattern's amplitude is monitored, and the associated decay rate is extracted. The evolution of the latter as a function of temperature exhibits a crossover between two distinct behaviors, over a temperature range typically bounded by the glass-transition temperature and the mode-coupling critical temperature. Layer-resolved analysis of the film particles' mean-squared displacements further shows that diffusion at the surface is considerably faster than in the bulk, below the glass-transition temperature. The diffusion coefficient of the surface particles is larger than its bulk counterpart by a factor that reaches 10⁵ at the lowest temperature studied. This factor decreases upon heating, in agreement with recent experimental studies.

Exploring the Importance of Surface Diffusion in Stability of Vapor-Deposited Organic Glasses

Stable glasses are formed during physical vapor deposition (PVD), through the surface-mediated equilibration process. Understanding surface relaxation dynamics is important in understanding the details of this process. Direct measurements of the surface relaxation times in molecular glass systems are challenging. As such, surface diffusion measurements have been used in the past as a proxy for the surface relaxation process. In this study, we show that the absence of enhanced surface diffusion is not a reliable predictor of reduced ability to produce stable glasses. To demonstrate, we have prepared stable glasses (SGs) from two structurally similar organic molecules, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (TNB) and 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), with similar density increase and improved kinetic stability as compared to their liquid-quenched (LQ) counterparts. The surface diffusion values of these glasses were measured both in the LQ and SG states below their glass transition temperatures ( T_gs) using gold nanorod probes. While TNB shows enhanced surface diffusion in both SG and LQ states, no significant surface T_g diffusion is observed on the surface of α,α-A within our experimental time scales. However, isothermal dewetting experiments on ultrathin films of both molecules below T_g indicate the existence of enhanced dynamics in ultrathin films for both molecules, indirectly showing the existence of an enhanced mobile surface layer. Both films produce stable glasses, which is another indication for the existence of the mobile surface layer. Our results suggest that lateral surface diffusion may not be a good proxy for enhanced surface relaxation dynamics required to produce stable glasses, and thus, other types of measurements to directly probe the surface relaxation times may be necessary.

Effects of microstructure formation on the stability of vapor-deposited glasses

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse-grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase-separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that, as the tail length is increased, the relaxation times near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase-separated domains on the relevant deposition time scales.

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.

Dynamics of supercooled liquid and plastic crystalline ethanol: Dielectric relaxation and AC nanocalorimetry distinguish structural α- and Debye relaxation processes

Physical vapor deposition has been used to prepare glasses of ethanol. Upon heating, the glasses transformed into the supercooled liquid phase and then crystallized into the plastic crystal phase. The dynamic glass transition of the supercooled liquid is successfully measured by AC nanocalorimetry, and preliminary results for the plastic crystal are obtained. The frequency dependences of these dynamic glass transitions observed by AC nanocalorimetry are in disagreement with conclusions from previously published dielectric spectra of ethanol. Existing dielectric loss spectra have been carefully re-evaluated considering a Debye peak, which is a typical feature in the dielectric loss spectra of monohydroxy alcohols. The re-evaluated dielectric fits reveal a prominent dielectric Debye peak, a smaller and asymmetrically broadened peak, which is identified as the signature of the structural α-relaxation and a Johari-Goldstein secondary relaxation process. This new assignment of the dielectric processes is supported by the observation that the AC nanocalorimetry dynamic glass transition temperature, T_α, coincides with the dielectric structural α-relaxation process rather than the Debye process. The combined results from dielectric spectroscopy and AC nanocalorimetry on the plastic crystal of ethanol suggest the occurrence of a Debye process also in the plastic crystal phase.

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.

Influence of Hydrogen Bonding on the Surface Diffusion of Molecular Glasses: Comparison of Three Triazines

Surface grating decay measurements have been performed on three closely related molecular glasses to study the effect of intermolecular hydrogen bonds on surface diffusion. The three molecules are derivatives of bis(3,5-dimethyl-phenylamino)-1,3,5-triazine and differ only in the functional group R at the 2-position, with R being C₂H₅, OCH₃, and NHCH₃, and referred to as "Et", "OMe", and "NHMe", respectively. Of the three molecules, NHMe forms more extensive intermolecular hydrogen bonds than Et and OMe and was found to have slower surface diffusion. For Et and OMe, surface diffusion is so fast that it replaces viscous flow as the mechanism of surface grating decay as temperature is lowered. In contrast, no such transition was observed for NHMe under the same conditions, indicating significantly slower surface diffusion. This result is consistent with the previous finding that extensive intermolecular hydrogen bonds slow down surface diffusion in molecular glasses and is attributed to the persistence of hydrogen bonds even in the surface environment. This result is also consistent with the lower stability of the vapor-deposited glass of NHMe relative to those of Et and OMe and supports the view that surface mobility controls the stability of vapor-deposited glasses.

Decoupling of surface diffusion and relaxation dynamics of molecular glasses

Tobacco mosaic virus is used as a probe to measure surface diffusion of ultrathin films of N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) (12 nm [Formula: see text] 53 nm, where [Formula: see text] is the film thickness) at various temperatures below the glass transition temperature, [Formula: see text], of all films. As the film thickness is decreased, [Formula: see text] decreases rapidly and the average film dynamics are enhanced by 6-14 orders of magnitude. We show that the surface diffusion is invariant of the film thickness decrease and the resulting enhanced overall mobility. The values of the surface diffusion coefficient and its temperature dependence are invariant of film thickness and are the same as the corresponding bulk values ([Formula: see text]400 nm). For the thinnest films ([Formula: see text]20 nm), the effective activation energy for rearrangement (temperature dependence of relaxation times) becomes smaller than the activation energy for surface diffusion. These results suggest that the fast surface diffusion is decoupled from film relaxation dynamics and is a solely free surface property.

Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses

Surface diffusion of molecular glasses is found to be orders of magnitude faster than bulk diffusion, with a stronger dependence on the molecular size and intermolecular interactions. In this study, we investigate the effect of variations in bulk dynamics on the surface diffusion of molecular glasses. Using the tobacco mosaic virus as a probe particle, we measure the surface diffusion on glasses of the same composition but with orders of magnitude of variations in bulk relaxation dynamics, produced by physical vapor deposition, physical aging, and liquid quenching. The bulk fictive temperatures of these glasses span over 35 K, indicating 13 to 20 orders of magnitude changes in bulk relaxation times. However, the surface diffusion coefficients on these glasses are measured to be identical at two temperatures below the bulk glass transition temperature T_{g}. These results suggest that surface diffusion has no dependence on the bulk relaxation dynamics when measured below T_{g}.