Undergoes Glass Transition Research Articles

Coupled multiphase transport, large deformation and phase transition during rice puffing

Puffing of biomaterials involves mass, momentum and energy transport along with large volumetric expansion of the material. Development of fundamentals-based models that can describe heat and moisture transport, rapid evaporation and large deformations can help understand the factors affecting the puffing processes and optimize them. In this context, salt-assisted puffing of parboiled rice is described. A multiphase porous media model involving heat and mass transfer within the rice kernel undergoing large deformations is developed. The transport model involves different phases and multiple modes of transport. During puffing, intensive heating of rice leads to rapid evaporation of water to vapor resulting in large pressure development. Also, the rice starch undergoes Glass Transition from a rigid, glassy state to a soft, rubbery state. Development of large pressures within a soft matrix results in large volumetric expansion of the kernel causing it to puff. The developed model was validated against moisture changes and volumetric expansion of the rice kernel during the puffing process and good agreement was found. Gas porosity development in puffed rice was determined via 3D reconstruction of micro-CT images of rice puffed at different times which compared favorably well with model predictions. The expansion of the kernel began from the tip of the grain and the model could successfully capture this phenomenon. Expansion ratio, a key quality parameter associated with puffed products, was found to be sensitive to intrinsic permeability and bulk modulus of the solid matrix. The modeling framework for salt-assisted puffing was then extended to the process of gun-puffing (a completely different puffing process) without significant reformulations thus showing the applicability of the framework for a variety of puffing processes. The final expansion after gun-puffing was much higher compared with salt-assisted puffing and was found to be sensitive to the gun opening time.

Using Dielectric Relaxation Spectroscopy to Characterize the Glass Transition Time of Polydextrose.

Dielectric relaxation spectroscopy was used to characterize the glass transition time, tg , of polydextrose, where the glass transition temperature, Tg , and water activity, aw (relative humidity), were held constant during polydextrose relaxation. The tg was determined from a shift in the peak frequency of the imaginary capacitance spectrum with time. It was found that when the peak frequency reaches 30 mHz, polydextrose undergoes glass transition. Glass transition time, tg , is the time for polydextrose to undergo glass transition at a specific Tg and aw . Results lead to a modified state diagram, where Tg is depressed with increasing aw . This curve forms a boundary: (a) below the boundary, polydextrose does not undergo glass transition and (b) above the boundary, polydextrose rapidly undergoes glass transition. As the boundary curve is specified by a tg value, it can assist in the selection of storage conditions. An important point on the boundary curve is at aw = 0, where Tg0 = 115 °C. The methodology can also be used to calculate the stress-relaxation viscosity of polydextrose as a function of Tg and aw , which is important when characterizing the flow properties of polydextrose initially in powder form.

Release mechanism of omega-3 fatty acid in κ-carrageenan/polydextrose undergoing glass transition

A high-solid matrix of κ-carrageenan with polydextrose was developed to entrap α-linolenic acid, which is an omega-3 bioactive compound. Physicochemical analysis of this system utilised modulated DSC, dynamic oscillation in shear, ESEM, FTIR and WAX diffraction. The carbohydrate matrix was conditioned through an extensive temperature range to induce changes in molecular morphology and identify the network glass transition temperature. Thermally induced variation in phase morphology was employed to rationalise transportation patterns of the bioactive compound within the high-solid preparation. Thus, experimental observations using UV–vis spectroscopy modelled diffusion kinetics to document the mobility arresting effect of the vitrifying matrix on the micro-constituent. Within the glass transition region, results argue that free volume theory is the molecular process governing structural relaxation. Further, Less Fickian diffusion follows well the rate of molecular transport of α-linolenic acid as a function of time and temperature of observation in the condensed matrix.

Phase-mixing and molecular dynamics in poly(urethane urea) elastomers by solid-state NMR

The dynamical heterogeneity in a series of 4,4′-dicyclohexylmethane diisocyanate-diethyltoluenediamine-poly(tetramethylene oxide) based poly(urethane urea) (PUU) elastomers was studied by solid-state nuclear magnetic resonance (NMR) methods. Extensive phase mixing was evidenced by the 1H wideline signal, which can be approximately fitted by a single exponential model. 13C T1 relaxation time measurements indicate that the hard segments (HS) exhibit some small-amplitude mobility, likely “activated” by neighboring soft segments (SS). Fitting of the time-domain wideline separation (TD-WISE) data was employed to characterize the extent of phase mixing, which revealed that a PUU elastomer contains four fractions: rigid-HS, mobile-HS, rigid-SS, and mobile-SS regions. For a variety of SS MWs, the dynamics and relative portions of rigid vs. mobile fractions among HS were substantially similar, while those for the SS exhibit large contrast. Furthermore, the dynamics in the rigid-SS fraction is at least an order of magnitude slower than that in mobile-SS for all PUUs. Greater phase-mixing substantially lowers the SS mobility, facilitating SS to undergo glass transition at high strain rates, thus can be key to enhancing dynamic mechanical strengthening.

The deposition ice nucleation and immersion freezing potential of amorphous secondary organic aerosol: Pathways for ice and mixed‐phase cloud formation

Secondary organic aerosol (SOA) generated from the oxidation of organic gases are ubiquitous in the atmosphere, but their interaction with water vapor and their ice cloud formation potential at low temperatures remains highly uncertain. We report on onset conditions of water uptake and ice nucleation by amorphous SOA particles generated from the oxidation of naphthalene with OH radicals. Water uptake above 230 K was governed by the oxidation level of the SOA particles expressed as oxygen‐to‐carbon (O/C) ratio, followed by moisture‐induced phase transitions and immersion freezing. For temperatures from 200 to 230 K, SOA particles nucleated ice via deposition mode from supersaturated water vapor independent of O/C ratio at relative humidity with respect to ice (RHice) ∼10–15% below homogeneous ice nucleation limits. The glass transition temperature (Tg) for the amorphous SOA particles was derived as a function of two parameters: (1) relative humidity (RH) with respect to water and (2) oxidation level of the SOA. The data show that particle phase and viscosity govern the particles' response to temperature and RHand provide a straightforward interpretation for the observed different heterogeneous ice nucleation pathways and water uptake by the laboratory‐generated SOA and field‐collected particles. Since SOA particles undergo glass transitions, these observations suggest that atmospheric SOA are potentially important for ice cloud formation and climate.

Calorimetric Study of Glass Transition in Molecular Liquids Consisting of Globular Associates: Dicyclorohexylmethanol and Tricyclohexylmethanol

Heat capacities of liquids and liquid-quenched glasses (LQGs) of dicyclorohexylmethanol (DCHM) and tricyclohexylmethanol (TCHM) were measured by adiabatic calorimetry. Upon cooling the liquid compounds, they undergo glass transitions around 250 and 265 K, respectively. Temperature dependence of the FT-IR spectrum of TCHM liquid showed the gradual formation of dimers in the supercooled state with decreasing temperature. The magnitude of heat capacity jump at glass transition is discussed through a comparison with other low-molecular mass LQG. Combining the present results with previous heat capacity results on crystalline TCHM, residual entropies of LQG and standard thermodynamic quantities are established for both compounds.

Models of thermomechanical behavior of polymeric materials undergoing glass transition

The phenomenological models of the thermomechanical behavior of polymeric materials in a temperature range including the relaxation transition from the highly elastic to the glassy state (vitrification) and the reverse transition (softening) are considered. A model based on the interpretation of the glass transition as a process of gradual increase in intermolecular bonds in the polymer network, “freezing” the current strain with decreasing temperature is developed. A scalar parameter is introduced—the “degree of vitrification,” to establish the quantitative dependence of the relaxation transition completion by temperature. Constitutive relations of thermomechanical behavior of vitrifying polymers in uniaxial and complicated stress states in the “elastic approximation” simplification are obtained. A system of experiments for the identification of the proposed model material functions and constants is formulated and implemented. Analytical model problems are solved, clearly illustrating the mechanism for generation of technological and residual stresses in glass polymers in non-uniform cooling.

Glass transition and crystallization of low-temperature amorphous condensates of the water-propane mixture

Interest in gas hydrates stems from the existence of vast reserves of hydrocarbons on the earth in the form of gas hydrates, prospects for using them as a fuel source, and the possibility of storage and transporta� tion of gas in the gas hydrate state. The known meth� ods of production of gas hydrates are based on rela� tively small deviations from the equilibrium condi� tions of gas dissolution. They require the use of high pressures in laboratory or manufacturing equipment. For example, the pressure corresponding to the for� mation conditions of methane hydrate at temperatures near 0°С amounts to tens of bars. In addition, forma� tion of the crystal hydrate requires dispersion of water, which is afforded by longterm and vigorous stirring of the water-gas mixture. In this work, we have proposed another approach implying that hydrates are formed under conditions far from thermodynamic equilibrium. The approach is based on lowtemperature molecularbeam deposi� tion in a vacuum onto a surface cooled by liquid nitro� gen. During this process, amorphous (glassy) layers of the water-gas mixture are initially formed, which are stabilized by high viscosity. Heating of the amorphous layers is accompanied by avalanchetype nucleation and growth of gas hydrate crystallization sites. This method requires neither high pressures nor stirring of the water-gas system. Its advantage is that it is univer� sal and can be used for producing various gas hydrates. This work deals with studying the glass transition and crystallization of amorphous condensates of the binary water-propane system obtained by lowtem� perature condensation to determine the conditions of crystal hydrate formation. Amorphous solid (glassy) layers of lowmolecular� weight substances can be obtained by molecularbeam deposition onto a cooled surface. At low temperatures, the amorphous state of these substances is stabilized by the high viscosity. Molecularbeam deposition onto a copper substrate cooled by liquid nitrogen affords amorphous layers of water and simple molecular com� pounds (1), as well as of aqueous solutions of organic liquids (2). Cooling rates under these conditions are as high as 10 5 -10 7 K/s. On heating, the condensates undergo glass transition (softening) and subsequent spontaneous crystallization. In the course of the latter, a decisive role for phase transformation is played by homogeneous nucleation. The crystallization of amorphous water-gas condensates can result in the formation of gas hydrates (3-5). Hydrate formation is facilitated by the low chemical affinity of the hydrate� forming substance, as well as by the size and shape of its molecules corresponding to the geometry of cavi� ties in the arising clathrate framework. Among such substances, there are light hydrocarbons of the meth� ane series. Experiments with lowtemperature con� densates of the water-methane mixture showed the possibility of producing massive samples of crystal hydrates with high gas content capable of sustained burning (5).

Modeling Thermal Stress in Asphalt Mixtures Undergoing Glass Transition and Physical Hardening

Asphalt binders have been shown to undergo significant time-dependent stiffening when stored at low temperatures. This physical hardening has a significant effect on the laboratory performance of asphalt binders. However, the importance of isothermal conditioning for asphalt mixtures and its effect on thermal cracking performance have been a subject of significant debate. A theoretical approach accounting for the glass transition and physical hardening in the thermal stress buildup in mixtures was derived from relaxation modulus master curves, the William–Landel–Ferry equation, Boltzmann's superposition principle, and a model describing the isothermal contraction of asphalt as a continuous function of conditioning time and temperature. With the model predictions, it is shown that thermal stress relaxation and stress buildup induced by physical hardening can continuously affect thermal stress throughout the cooling process. The cooling rate also affected the amount of delayed stress buildup that occurred after the temperature had stabilized at isothermal conditions as a result of physical hardening. A relatively simple device was developed and used for verification and support of the thermal stress model. Mixture measurements performed at different cooling rates and isothermal conditions supported the theoretical predictions. The findings clearly show that the effect of physical hardening on stress buildup in mixtures is measurable and important. Therefore, the glass transition of asphalts and their behavior under isothermal conditioning needs to be measured to better predict thermal cracking.

Sugar Crystallization and Glass Transition as Destabilizing Factors of Protein-Stabilized Emulsions

Temperature cycling across the glass transition of the aqueous phase of oil-in-water emulsions stabilized by whey protein isolate was considered as a possible factor affecting stability. Emulsions were formulated with an aqueous phase containing 80% (w/w) fructose, fructose:glucose 1:1 or glucose, in order to prepare a glass forming aqueous phase with sugar concentration corresponding to that of the unfrozen phase of the maximally freeze-concentrated solutions. This allowed thermal cycling across the glass transition in the absence of the formation of ice crystals. Emulsion stability was studied using differential scanning calorimetry, dynamic light scattering and by visual analysis of the morphology of the systems. Emulsified systems undergoing glass transition cycles of the aqueous phase did not show destabilization of the dispersed (crystallized) lipid phase. Sugar crystallization in the aqueous phase, which occurred when glucose systems were stored above the Tg, led to emulsion breakdown. In this study, the formation of a glassy structure in the continuous aqueous phase preserved the interfacial structure of WPI, thus protecting the dispersed lipid phase from destabilization. On the contrary, glucose crystallization caused disruption of the interfacial membrane structure and loss of integrity of the interface which resulted in extensive lipid phase destabilization.

New methods to assess water diffusion in amorphous matrices during storage and drying

Water diffusion coefficient is an important parameter for food processing and stability, strongly influenced by the composition and the physical state of the matrix (amorphous glassy or rubbery/crystalline). In this study, two different methods to assess water diffusion in amorphous matrices were compared. First, an automatic sorption device was used to measure the water sorption kinetics of powders of maltodextrin DE21 and DE47 with defined shapes when equilibrating at different air relative humidity (0–60% at 25°C). Calculation of water diffusion coefficients was done according to Crank’s and Weibull’s models, and it was not possible when powders undergo glass transition. In the second part of the study a thin film dryer was used to assess the drying kinetics of aqueous solutions of maltodextrin (DE 6-12-29-40). The Yamamoto’s model allowed calculating water diffusion as a function of water content and temperature, above and below glass transition temperature.

The Use of Surface Active Compounds as Additives in Spray Drying

Yields of spray drying pure lactose (10% w/w) solutions, as well as lactose–protein mixtures with casein and whey protein isolate (WPI) concentrations between 0.5 and 50% w/w of the total solids, were measured. Proteins were found to improve spray-drying yields more significantly at higher inlet air temperatures. Scanning electron micrograph (SEM) images showed that adding proteins to lactose solutions caused surface morphology changes on the products. X-ray photoelectron spectroscopy (XPS) analysis showed that the surface concentrations of proteins were higher than their bulk concentrations. Modulated differential scanning calorimetry (MDSC) measurements showed that the mixtures undergo glass transitions at similar temperatures to pure lactose.

Crystallization and vitrification of cryoprotectants studied by Raman scattering, Brillouin scattering and THz-TDS

We examine one of the best candidates of cryoprotectant, ethylene glycol aqueous solutions by using Raman and Brillouin scattering and Terahertz time domain spectroscopy. The molecular structure can be determined by Raman scattering and the EG aqueous solutions were investigated to characterize the conformations of molecular structure. The intermediate concentrations of EG solutions easily undergo glass transitions, while the crystallization occurs in the solutions of low and high concentrations of EG. The structural configuration of EG in a crystalline phase in pure EG shows only gauche OCCO form of EG which is the lowest energy conformers, while gauche and trans forms exist in a liquid phase. The dynamical properties from gigahertz to terahertz frequency range are investigated by using Raman scattering, Brillouin scattering and terahertz time domain spectroscopy (THz-TDS). In the present frequency range, the relaxation and vibrational modes coexist. It is shown that the combination of giga to terahertz spectroscopy is a powerful tool to provide information on fast dynamics of bioprotective solutions.

Successful Cryopreservation of Mouse Oocytes by Using Low Concentrations of Trehalose and Dimethylsulfoxide1

Sugars such as trehalose, sucrose, and glucose are effectively used by a variety of animals (e.g., brine shrimp, tardigrades, some frogs, and insects), as well as by bacteria, yeasts, and plant seeds to survive freezing and extreme drying. The objective of this study was to examine the potential application of sugars to mammalian oocyte cryopreservation. To this end, we used trehalose, a nonreducing disaccharide, and mouse metaphase II oocytes as models. Our experiments show that extracellular trehalose alone affords some protection at high subzero temperatures (e.g., -15 degrees C), which diminishes with further cooling of the oocytes to -30 degrees C and below. When present both intracellularly and extracellularly, trehalose dramatically improves the cryosurvival with increasing extracellular concentrations to 0.5 M, even after cooling to -196 degrees C. Furthermore, the combination of intracellular and extracellular trehalose with small amounts of a conventional penetrating cryoprotectant (i.e., 0.5 M dimethylsulfoxide) provide high survival, fertilization, and embryonic development rates statistically similar to untreated controls. When transferred to foster mothers, cryopreserved oocytes give rise to healthy offspring showing the proof of principle. Our experiments with differential scanning calorimetry indicate that when cooled using the same cryopreservation protocol, the mixture of 0.5 M trehalose and cryopreservation medium undergoes glass transition at high subzero temperatures, which further substantiates the use of sugars as intracellular and extracellular cryoprotectants. Taken together, our results are in agreement with the survival schemes in nature and demonstrate the successful use of sugars in cryopreservation of mammalian oocytes.

Role of Glass-Transition on Fluid Transport in Porous Food Materials

Many food materials such as pasta, potatoes, starch, corn, soybeans, food gels etc. exhibit glass transition in the range of industrial temperature and fluid content values during transport processes such as drying, sorption, cooking, frying etc. Glass transition plays a critical role in affecting the fluid distribution profiles inside the food and average rate of fluid uptake by the whole food matrix. Yet this phenomenon is not included in most modeling studies that are primarily based upon Fick's law of diffusion. The purpose of this communication is to elucidate that Fick's law is valid only when the food is in glassy or rubbery states. Near glass-transition, the relaxation time of polymers is of the order of diffusion time. Thus, time-dependent conformational changes in food biopolymers add an extra stress term to fluid flow. Keeping track of this stress term is also the key to predict quality changes (related to food texture, physical state and crust formation) in foods via modeling. Based upon the average moisture and temperature at a given instant, even if the entire food matrix exists in a specific mechanical state in an average sense, its different sub-regions may be glassy, rubbery or in glass transition state. For example, during sorption, as a fluid front penetrates through a glassy food matrix, regions ahead of the front will be glassy. Regions in the proximity of the moving front would be undergoing glass transition and the regions already saturated by the fluid would be in a rubbery state. In the proximity of the moving front, where glass-transition is taking place, the fluid flow behavior is non-Fickian. During drying of biopolymers these regions will be in reverse order with the outer surface in a glassy state, center in a rubbery state and the middle region in transition state. Here a continuum mechanics based modeling approach is presented that can be used to predict fluid profiles and normalized stress profiles in all three states (glassy, rubbery and transition).

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30–50°C, relative humidity: 0–80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.

Model for cage formation in colloidal suspension of laponite

In this paper we investigate glass transition in aqueous suspension of synthetic hectorite clay, laponite. We believe that upon dispersing laponite clay in water, the system comprises of clusters (agglomerates) of laponite dispersed in the same. Subsequent osmotic swelling of these clusters leads to an increase in their volume fraction. We propose that this phenomenon is responsible for slowing down of the overall dynamics of the system. As clusters fill up the space, the system undergoes glass transition. Along with the mode coupling theory, the proposed mechanism rightly captures various characteristic features of the system in the ergodic regime as it approaches glass transition.

Mg-Cu-Zn-Y-Zr bulk metallic glassy composite with high strength and plasticity

Mg 65Cu 20Zn 5Y 9Zr 1 bulk metallic glass matrix composite with a diameter of 2 mm was produced by copper mold casting. Upon cooling the Mg 65Cu 20Zn 5Y 9Zr 1 melt, Mg 2Cu acicular crystalline phase precipitates uniformly with a size of about 20 μm long and 1 μm thick while the remaining melt undergoes glass transition. Room temperature compression tests revealed that the high fracture strength up to 830 MPa and the plastic strain of 2.4% before failure are obtained for the Mg-based bulk metallic glass matrix composite. The formation of the Mg 2Cu phase was proposed to contribute to high strength and plastic deformation of the material.

Amorphous Diphenylaminofluorene-Functionalized Iridium Complexes for High-Efficiency Electrophosphorescent Light-Emitting Diodes

Two new phosphorescent iridium(III) cyclometalated complexes, [Ir(DPA-Flpy)3] (1) and [Ir(DPA-Flpy)2(acac)] (2) ((DPA-Flpy)H = (9,9-diethyl-7-pyridinylfluoren-2-yl)diphenylamine, Hacac = acetylacetone), have been synthesized and characterized. The incorporation of electron-donating diphenylamino groups to the fluorene skeleton is found to increase the highest occupied molecular orbital (HOMO) levels and add hole-transporting ability to the phosphorescent center. Both complexes are highly amorphous and morphologically stable solids and undergo glass transitions at 160 and 153 °C, respectively. These iridium phosphors emit bright yellow to orange light at room temperature with relatively short lifetimes (< 1 μs) in both solution and the solid state. Organic light-emitting diodes (OLEDs) fabricated using 1 and 2 as phosphorescent dopant emitters constructed with a multilayer configuration show very high efficiencies. The homoleptic iridium complex 1 is shown to be a more efficient electrophosphor than the heteroleptic congener 2. Efficient electrophosphorescence with a maximum external quantum efficiency close to 10 % ph/el (photons per electron), corresponding to a luminance efficiency of ∼ 30 cd A–1 and a power efficiency of ∼ 21 lm W–1, is obtained by using 5 wt.-% 1 as the guest dopant.

Theory of fluidity of liquids, glass transition, and melting

This is a presentation of a rigorous theory of fluidity of liquids, glass transition and melting of solids in the frame of an asymmetric double well potential model. Potential wells are doubled time to time by the local density fluctuations caused by the thermal longitudinal waves. The average frequency of doubling of potential wells is equal to the frequency of the most energetic waves which obey a law similar to Wein’s displacement law in black body radiation. Based on the equilibrium thermodynamic theory of fluctuations and the displacement law, a law of linear pre-diffusion mean-square displacement of particles in a solid is derived: the mean-square displacement of molecules within their potential wells increases linearly with temperature. It is shown that when this is broken-down (where the mean-square displacement at a certain temperature rapidly changes its slope as a function of temperature) glass devitrifies and crystal melts, and all possible solid–liquid transitions of a substance occur at the same critical mean-square displacement: any solid ( not only crystals) transforms into liquid when the mean-square displacement, as a fraction of the average intermolecular distance, acquires a certain universal critical value – the same for different substances. It is proved that molecules in a liquid perform specific Brownian motion. The average jump distance is a function of temperature and it is much smaller than the nearest intermolecular distances. At a certain temperature, shown to be the Kauzmann temperature, the average jump distance of Brownian motion becomes equal to zero: the super-cooled liquid undergoes glass transition. The transition was proven to be a phase transition of the fourth order: the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Molecular mobility, diffusion and viscosity are obtained as functions of temperature.