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.

Anatomy of the dielectric behavior of methyl-m-toluate glasses during and after vapor deposition.

Glassy films of methyl-m-toluate have been vapor deposited onto a substrate equipped with interdigitated electrodes, facilitating in situ dielectric relaxation measurements during and after deposition. Samples of 200 nm thickness have been deposited at rates of 0.1 nm/s at a variety of deposition temperatures between 40 K and Tg = 170 K. With increasing depth below the surface, the dielectric loss changes gradually from a value reflecting a mobile surface layer to that of the kinetically stable glass. The thickness of this more mobile layer varies from below 1 to beyond 10 nm as the deposition temperature is increased, and its average fictive temperature is near Tg for all deposition temperatures. Judged by the dielectric loss, the liquid-like portion of the surface layer exceeds a thickness of 1 nm only for deposition temperatures above 0.8Tg, where near-equilibrium glassy states are obtained. After deposition, the dielectric loss of the material positioned about 5-30 nm below the surface decreases for thousands of seconds of annealing time, whereas the bulk of the film remains unchanged.

Unusual Transformation of Mixed Isomer Decahydroisoquinoline Stable Glasses.

Dielectric relaxation was used to characterize the ability of vapor-deposited mixtures of cis- and trans-decahydroisoquinoline (DHIQ) to form glasses with a high kinetic stability. Vapor-deposited mixtures are technologically relevant, and the effect of mixing on glass stability is a relatively unexplored area. Mixed isomers and pure trans-DHIQ form highly stable glasses that isothermally transform in approximately 104 τα (where τα is the structural relaxation time of the supercooled liquid). Isomeric composition of the glasses does not play a significant role in the maximum kinetic stability of the resulting films. Secondary relaxations in DHIQ are associated with an intramolecular conformational change and are suppressed to a significant extent in highly stable glasses. During isothermal annealing experiments, stable glasses were found to transform initially via a growth front mechanism that transitions to a homogeneous bulk mechanism. Surprisingly, the time dependence of the bulk transformation is different from that reported for other stable glasses and cannot be interpreted in terms of a simple nucleation and growth model.

In situ observation of fast surface dynamics during the vapor-deposition of a stable organic glass.

By measuring the increments of dielectric capacitance (ΔC) and dissipation (Δtan δ) during physical vapor deposition of a 110 nm film of a molecular glass former, we provide direct evidence of the mobile surface layer that is made responsible for the extraordinary properties of vapor deposited glasses. Depositing at a rate of 0.1 nm s-1 onto a substrate at Tdep = 75 K = 0.82Tg, we observe a 2.5 nm thick surface layer with an average relaxation time of 0.1 s, while the glass growing underneath has a high kinetic stability. The level of Δtan δ continues to decrease for thousands of seconds after terminating the deposition process, indicating a slow aging-like increase in packing density near the surface. At very low deposition temperatures, 32 and 42 K, the surface layer thicknesses and mobilities are reduced, as are the kinetic stabilities.

Physical vapor deposition of a polyamorphic system: Triphenyl phosphite.

In situ AC nanocalorimetry and dielectric spectroscopy were used to analyze films of vapor-deposited triphenyl phosphite. The goal of this work was to investigate the properties of vapor-deposited glasses of this known polyamorphic system and to determine which liquid is formed when the glass is heated. We find that triphenyl phosphite forms a kinetically stable glass when prepared at substrate temperatures of 0.75-0.95Tg, where Tg is the glass transition temperature. Regardless of the substrate temperature utilized during deposition of triphenyl phosphite, heating a vapor-deposited glass always forms the ordinary supercooled liquid (liquid 1). The identity of liquid 1 was confirmed by both the calorimetric signal and the shape and position of the dielectric spectra. For the purposes of comparison, the glacial phase of triphenyl phosphite (liquid 2) was prepared by the conventional method of annealing liquid 1. We speculate that these new results and previous work on vapor deposition of other polyamorphic systems can be explained by the free surface structure being similar to one polyamorph even in a temperature regime where the other polyamorph is more thermodynamically stable in the bulk.

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 Tg are explored using dielectric and calorimetric techniques. The α relaxation strength at Tg 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 Tg, 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.

Reorientation Times for Solid-State Electrolyte Solvents and Electrolytes from NMR Spin-Lattice Relaxation Studies.

Ionic and molecular plastic crystals have been studied recently as solid electrolytes or solvents, but the specific role of molecular reorientation has not been clarified. We use NMR spin-lattice relaxation times (T1 minima) to compare the time scale for magnetic fluctuations in a plastic crystal solvent to the molecular reorientation times, as established by dielectric spectroscopy. We focus on a mixture of succinonitrile and glutaronitrile, in which the rotationally disordered phase is stabilized against crystallization. Reorientation times can then be studied over 13 orders of magnitude, down to the glass transition temperature at 144 K. For each nucleus, 1H and 13C, the most probable magnetic fluctuation time is found to be slightly shorter than the reorientation time, but with practically indistinguishable temperature dependence. This facilitates investigation of the relation of solvent reorientation to ion conductivity relaxation times in ionic conducting systems in which the conductivity swamps the dielectric signature of solvent reorientation.

Relationship between aged and vapor-deposited organic glasses: Secondary relaxations in methyl-m-toluate.

In situ interdigitated electrode broadband dielectric spectroscopy was used to characterize the excess wing relaxations in vapor-deposited and aged glasses of methyl-m-toluate (MMT, Tg = 170 K). MMT displays typical excess wing relaxations in dielectric spectra of its supercooled liquid and glasses. Physical vapor deposition produced glasses with degrees of suppression of the excess wing relaxation that varied systematically with deposition conditions, up to a maximum suppression of more than a factor of 3. The glass deposited at a relatively high temperature, 0.96 Tg (163 K), showed the same amount of suppression as that of a liquid-cooled glass aged to equilibrium at this temperature. The suppression of the excess wing relaxation was strongly correlated with the kinetic stability of the vapor-deposited glasses. Comparisons with aged MMT glasses allowed an estimate of the structural relaxation time of the vapor-deposited glasses. The dependence of the estimated structural relaxation times upon the substrate temperature was found to be stronger than Arrhenius but weaker than Vogel-Fulcher-Tammann dependence predicted from extrapolation of relaxation times in the supercooled liquid. Additionally, this work provides the first example of the separation of primary and secondary relaxations using physical vapor deposition.

Formation of new polymorphs and control of crystallization in molecular glass-formers by electric field.

Static electric fields were observed to modify the crystallization behavior in a simple supercooled liquid, leading to a new crystal polymorph that could not be obtained in the absence of a field, even under high-pressure conditions. Using different thermal protocols and field amplitudes in the range from 40 to 200 kV cm-1, changes in both nucleation and crystal growth rates of 4-vinyl-propylene carbonate (vinyl-PC) are revealed. Remarkably, all field-induced changes in the crystallization behaviour were found to be fully reversible and do not affect dynamics of the tested liquid. Because vinyl-PC is a simple polar molecule, these field induced features are expected to occur in many other materials having permanent dipole moments. Our results highlight the important role of an external electric field as an additional control variable to influence the crystallization tendency of molecular glass-formers, and provide new opportunities in pharmaceutical science or organic electronics.

Nonlinear dielectric features of highly polar glass formers: Derivatives of propylene carbonate.

We have measured the nonlinear dielectric behavior of several highly polar propylene carbonate (PC) derivatives in the vicinity of their glass transition temperatures. Focus is on the effects of a large static electric field on the frequency dependent permittivity and on the cubic susceptibility measured using sinusoidal fields of high amplitude. The case of vinyl-PC shows dielectric saturation as well as an electro-rheological effect, i.e., a field induced increase of dielectric relaxation times, whose magnitude changes linearly with the apparent activation energy. The extent of this shift of the loss profile caused by the field correlates strongly with the peak magnitude of the cubic susceptibility, |χ3|, underlining the notion of a link between the |χ3| "hump" and this electro-rheological behavior. Further support for this picture emerges from the observation that the most polar of these liquids, (S)-(-)-methoxy-PC with εs ≈ 250, lacks both the electro-rheological effect in ε″(ω) and the "hump" typically observed in |χ3(ω)|. The absence of any sensitivity of the dynamics to an electric field is contrary to the expectation that the electro-rheological effect correlates with the field induced entropy change, which is extraordinarily high for this liquid. The results suggest that the dependence of the relaxation time on the electric field is not directly linked to the entropy change.

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 Tdep = 0.6Tg and Tg, where Tg 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 Tdep = Tg display the dielectric behavior of the ordinary supercooled liquid. Films deposited at lower deposition temperatures show a high dielectric loss upon heating toward Tg, which decreases by a factor of about 12 by annealing at Tg = 162 K. This change is consistent with either a drop of the Kirkwood correlation factor, gk, 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 Tg 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 Tg + 40 K.

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, Tα, 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.

Role of quantum fluctuations in structural dynamics of liquids of light molecules.

A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics of supercooled liquids is studied by dielectric spectroscopy. The presented results demonstrate that the liquids, bulk 3-methyl pentane and confined normal and deuterated water, have low glass transition temperature and unusually low for their class of materials steepness of the temperature dependence of structural relaxation (fragility). Although we do not find any signs of tunneling in the structural relaxation of these liquids, their unusually low fragility can be well described by the influence of the quantum fluctuations. Confined water presents an especially interesting case in comparison to the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits a much weaker isotope effect than bulk water, although the effect is still significant. We show that it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zero-point energy upon D/H substitution. The observed difference in the behavior of confined and bulk water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water. Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water behavior is questionable.

Suppression of ß Relaxation in Vapor-Deposited Ultrastable Glasses.

Glassy materials display numerous important properties which relate to the presence and intensity of the secondary (ß) relaxations that dominate the dynamics below the glass transition temperature. However, experimental protocols such as annealing allow little control over the ß relaxation for most glasses. Here we report on the ß relaxation of toluene in highly stable glasses prepared by physical vapor deposition. At conditions that generate the highest kinetic stability, about 70% of the ß relaxation intensity is suppressed, indicating the proximity of this state to the long-sought "ideal glass." While preparing such a state via deposition takes less than an hour, it would require ~3500 years of annealing an ordinary glass to obtain similarly suppressed dynamics.

Dynamics in Supercooled Secondary Amide Mixtures: Dielectric and Hydrogen Bond Specific Spectroscopies.

Alkylacetamide-based model peptides display an intense Debye-type dielectric relaxation. In order to explore the extent to which this feature has to be regarded analogous to that in other supramolecular liquids, notably the monohydroxy alcohols, we applied broadband dielectric, time-dependent solvation, and near-infrared spectroscopies as well as shear rheology and various nuclear magnetic resonance techniques to mixtures of N-methylacetamide (NMA) or N-ethylacetamide (NEA) with N-methylformamide. Compared in the modulus format, dielectric relaxation, solvation dynamics, and mechanical response indicate a common global and local dynamics. The present spin-relaxation measurements reflect motional processes which are significantly faster than the dominant Debye dielectric response, and a similar conclusion is drawn from measurements of the shear viscosity. The NH overtone stretching vibrations reveal a temperature-dependent hydrogen-bond equilibrium that changes its characteristics near temperatures of 325 K. Finally, dielectric low-temperature data recorded for (NEA)0.4(NMF)0.6 mixed with 2-picoline indicate the existence of a critical concentration akin to the situation in various monohydroxy alcohol mixtures.

Unified Criterion for Temperature-Induced and Strain-Driven Glass Transitions in Metallic Glass.

In a model metallic glass, we study the relaxation dynamics in both the linear and the nonlinear response regimes by numerical simulations of dynamical mechanical spectroscopy and analyze the atomic displacement statistics. We find that the primary (α) relaxation always takes place when the most probable atomic displacement reaches a critical fraction (~20%) of the average interatomic distance, irrespective of whether the relaxation is induced by temperature (linear response) or by mechanical strain (nonlinear response). Such a unified scenario, analogous to the well-known Lindemann criterion for crystal melting, provides insight into the structural origin of the strain-induced glass-liquid transition.

Quantum effects in the dynamics of deeply supercooled water.

Despite its simple chemical structure, water remains one of the most puzzling liquids with many anomalies at low temperatures. Combining neutron scattering and dielectric relaxation spectroscopy, we show that quantum fluctuations are not negligible in deeply supercooled water. Our dielectric measurements reveal the anomalously weak temperature dependence of structural relaxation in vapor-deposited water close to the glass transition temperature T(g)∼136K. We demonstrate that this anomalous behavior can be explained well by quantum effects. These results have significant implications for our understanding of water dynamics.

Role of fragility in the formation of highly stable organic glasses.

In situ dielectric spectroscopy has been used to characterize vapor-deposited glasses of methyl-m-toluate (MMT), an organic glass former with low fragility (m = 60). Deposition near 0.84T(g) produces glasses of very high kinetic stability; these materials are comparable in stability to the most stable glasses produced from more fragile glass formers. Highly stable glasses of MMT, when annealed above T(g), transform into the supercooled liquid by a heterogeneous mechanism. A constant velocity propagating front is initiated at the free surface and controls the transformation of thin films. The transition to a bulk-dominated transformation process occurs at 5 µm, the largest length scale reported for any glass. Contrary to recent conclusions, we find that physical vapor deposition can form highly stable organic glasses across the entire range of liquid fragilities.

Comparison of selected animal observations and management practices used to assess welfare of calves and adult dairy cows on organic and conventional dairy farms.

Differences in adoption of selected practices used in welfare assessment and audit programs were contrasted among organic (ORG; n=192) herds and similarly sized conventional grazing herds (CON-GR; n=36), and conventional nongrazing herds (CON-NG; n=64). Criteria from 3 programs were assessed: American Humane Association Animal Welfare Standards for Dairy Cattle, Farmers Assuring Responsible Management (FARM), and the Canadian Codes of Practice (CCP). Data were collected by trained study personnel during a herd visit and included information about neonatal care, dehorning, pain relief, calf nutrition, weaning, record keeping, use of veterinarians, and animal observations. Associations of management type (ORG, CON-GR, or CON-NG) with adoption of selected practice were assessed. Almost all farms (97%) met criteria suggested for age at weaning but fewer CON-NG farmers weaned calves at ≥5 wk of age compared with ORG and CON-GR farmers. Only 23% of farms met program requirements for use of pain relief during dehorning, and fewer CON-NG farmers used pain relief for calves after dehorning compared with ORG and CON-GR farmers. Calves on ORG farms were fed a greater volume of milk and were weaned at an older age than calves on CON-GR and CON-NG farms. Calves on CON-GR farms were dehorned at a younger age compared with calves on ORG and CON-NG farms. The calving area was shared with lactating cows for a larger proportion of ORG herds compared with conventional herds. About 30% of herds met welfare program criteria for body condition score but only about 20% met criteria for animal hygiene scores. The least proportion of cows with hock lesions was observed on ORG farms. Regular use of veterinarians was infrequent for ORG herds. Results of this study indicate that most of the organic and conventional farms enrolled in this study would have been unlikely to achieve many criteria of audit and assessment programs currently used in the US dairy industry.

Risk factors associated with bulk tank standard plate count, bulk tank coliform count, and the presence of Staphylococcus aureus on organic and conventional dairy farms in the United States.

The purpose of this study was to assess the association of bulk tank milk standard plate counts, bulk tank coliform counts (CC), and the presence of Staphylococcus aureus in bulk tank milk with various management and farm characteristics on organic and conventional dairy farms throughout New York, Wisconsin, and Oregon. Data from size-matched organic farms (n=192), conventional nongrazing farms (n=64), and conventional grazing farms (n=36) were collected at a single visit for each farm. Of the 292 farms visited, 290 bulk tank milk samples were collected. Statistical models were created using data from all herds in the study, as well as exclusively for the organic subset of herds. Because of incomplete data, 267 of 290 herds were analyzed for total herd modeling, and 173 of 190 organic herds were analyzed for the organic herd modeling. Overall, more bulk tanks from organic farms had Staph. aureus cultured from them (62% of organic herds, 42% conventional nongrazing herds, and 43% of conventional grazing herds), whereas fewer organic herds had a high CC, defined as ≥50 cfu/mL, than conventional farms in the study. A high standard plate count (×1,000 cfu/mL) was associated with decreased body condition score of adult cows and decreased milk production in both models. Several variables were significant only in the model created using all herds or only in organic herds. The presence of Staph. aureus in the bulk tank milk was associated with fewer people treating mastitis, increased age of housing, and a higher percentage of cows with 3 or fewer teats in both the organic and total herd models. The Staph. aureus total herd model also showed a relationship with fewer first-lactation animals, higher hock scores, and less use of automatic takeoffs at milking. High bulk tank CC was related to feeding a total mixed ration and using natural service in nonlactating heifers in both models. Overall, attentive management and use of outside resources were useful with regard to CC on organic farms. In all models except the organic CC model, we observed an association with the average reported somatic cell count from 3 mo before the herd visit, indicating that many of the regularly tested milk quality parameters are interconnected. In conclusion, we found that conventional and organic farms are similar in regard to overall herd management, but each grazing system faces unique challenges when managing milk quality.